Nand in applying that knowledge in useful ways

N A T U R E A T T H E F E M T O - S C A L E 1

This advance is largely the result of technological

breakthroughs in developing equipment for nuclear

physics experiments. Until recently, nuclear scientists

had to be content with conducting experiments on

stable nuclei, of which there are only about 300. In

the past decade, however, we have learned how to

build high-energy facilities for producing short-lived,

radioactive nuclei. With these new beams of unstable

nuclei we can make and study many thousands of

exotic nuclear species – most of which have never

existed before, or are created fleetingly only in the hot

interiors of stars. These will tell us more about the

structure of matter and how it evolved in the Universe.

Radioactive beams also offer exciting

opportunities for new medical procedures, and for

applications in other areas of research and industry.

Europe has been at the forefront of this endeavour,

with pioneering technology developed at CERN

(ISOLDE), and the world’s first dedicated facility built

at CRC’s CYCLONE in Louvain-la-Neuve, Belgium. New,

second-generation facilities are now planned or being

built in a number of European laboratories. They will

enable European scientists to remain at the forefront

of these new developments for the next decade.

High-energy experiments are also being designed

to study not only the structure and behaviour of

exotic nuclei but also other, related short-lived

subatomic particles, some of which may have existed

in the very early Universe. Such studies throw light

on the fundamental forces that hold matter

together. Another complementary area of nuclear

study involves carrying out sensitive experiments on

nuclei and nuclear constituents at low energies,

which may point the way to a deeper understanding

of the physics of the cosmos.

All these nuclear experiments are underpinned by

advances in theory which, thanks to developments in

high-performance computing, continue apace.

Nuclear research is very much a science of the

future. The aim of this booklet is to illustrate its

scientific and technological potential in the 21st

century through highlights of work carried out in

laboratories and institutions across Europe.

FINUPHYFrontiers in Nuclear Physics

uclear science is entering a new era of discovery inunderstanding how Nature works at the most basic leveland in applying that knowledge in useful waysN

forewordG

SI

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You can navigate this PDF brochure by selecting the desired article on the contents page (p.3). Click in the surrounding blue area (outside this note) direct link.

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1

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16

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Foreword

Journey into the heart of matter

A world-leading programme of nuclear research

The complex nucleus

Nuclear physics in the Universe

Exploiting the nucleus

From nuclear liquid to quark soup

Nuclei and the universal forces

What holds nuclei together?

Producing exotic nuclei

Why become a nuclear physicist?

Contacts

About FINUPHY

INTRODUCTION

THE EUROPEAN DIMENSION

NUCLEAR STRUCTURE

NUCLEAR ASTROPHYSICS

APPLICATIONS

PHASES OF NUCLEAR MATTER

FUNDAMENTAL INTERACTIONS

QCD

RADIOACTIVE ION BEAMS

EDUCATION AND TRAININGco

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4 N A T U R E A T T H E F E M T O - S C A L E

Nuclear science now and the future Physicists soon realised that the electrically charged

subatomic particles they had uncovered, such as

electrons and protons, could be accelerated to higher

energies in electric fields, and fired at target materials

so that they actually penetrated the nucleus and

broke it up. In this way they discovered a whole new

zoo of subatomic particles, such as muons, pions and

kaons, which were stable only for short amounts of

time. Some of them were first discovered in cosmic

rays coming from space – giving clues to the

existence of violent processes in the Universe.

Over the past 50 years, such high-energy physics

experiments have slowly unravelled the fundamental

building blocks of matter and their interactions,

while advances in quantum theory have produced a

cohesive description of them, as described on p.22.

We now know that protons and neutrons (as well as

some of the other particles discovered) are made of

elementary particles called quarks held together by

the strong force (p.24). Theorists also have some

powerful ideas as to how these building blocks came

into existence when the Universe was born, and how

they were built up into the elements in stars (p.12).

Today, nuclear physics laboratories across Europe

carry out vigorous programmes to investigate the

properties of particles at different energy scales, and

to test ideas about stellar nucleosynthetic processes.

The arrangements of protons and neutrons in

nuclei remain an intriguing, but still poorly

understood area of nuclear science. Nuclei exhibit

incredibly complex behaviour as a result of the forces

holding them together (p.8). They represent one of the

richest quantum systems in Nature, demonstrating

many aspects of structure and behaviour mirrored at

larger scales; nuclei can behave like atoms, clusters of

atoms, molecules, quantum liquids and even

superconductors. Indeed, they can be regarded as

miniature laboratories for testing broad ideas about

the way Nature organises itself.

A vast variety of nuclei is possible with widely

differing proportions of protons and neutrons. Most

of them are very unstable. However, research into

their structure has tremendous potential – not just in

understanding nuclear forces, astrophysical processes

The starting point was the truly visionary work

carried out in European laboratories on the structure

of the atom in the early years of the 20th century.

The discovery of the electron, and work on

radioactivity and transmutation (see box), provided

the first hints of a coherent organisation of

fundamental particles that explained the existence

of the elements. But the key discovery that unlocked

the door to modern science was made by Ernest

Rutherford at the University of Manchester in the UK,

when he showed that one form of radioactivity –

alpha particles – could be sharply deflected by a thin

gold sheet. He correctly deduced that these particles

were occasionally hitting the dense, positively

charged cores of the gold atoms and being bounced

back. The concept of the atomic nucleus was born.

With the idea of the atom as consisting of a

positively-charged nucleus surrounded by orbiting

negative electrons came the notion that electrons

must occupy states differing by fixed units of energy

called quanta, which could be emitted or absorbed

as electromagnetic radiation. This tremendous

conceptual leap led to the development of one of the

main cornerstones of modern scientific thinking –

quantum theory – now used to explain interactions

between the microscopic building blocks of matter –

whether subatomic particles, atoms or molecules.

It offers a profound, if perplexing, insight into the

nature of reality.

Rutherford and others later showed that nuclei

were made of smaller units, protons and neutrons,

which explained fully the relationship of elements in

the Periodic Table as differing in the numbers of

protons they possess, and the existence of isotopes

as variants of elements with differing numbers of

neutrons. This deeper perception of matter ultimately

underpins all modern chemistry and biology.

t is only during the past 100 years that we haveachieved any real understanding of matter andthe Universe at a fundamental level

I N T R O D U C T I O N ::

I

heart ofmatterJourney into the

The ISOLDE target

for generating

radioactive beams

Alpha particle emission

CE

RN


and quantum concepts better, but also in exploring

future technological applications. Nuclei and

nuclear processes are already successfully applied

in analysis and in medical therapies, but the vast

energy trapped in nuclei has been exploited only

in the crudest of ways through uranium fission.

A better understanding of the nucleus through

innovative experiments could lead to new sources

of safe, clean energy and other environmentally

significant technologies (p.16).

Nanotechnology – the ability to manipulate

matter at a scale of one-billionth of a metre (atoms)

is expected to have an enormous impact on

human progress. It may be that ‘femtotechnology’

(a femtometre is one million billionth of a metre) –

the scale of the nucleus – will have even more impact

in the future.

Radioactivity – messagesfrom the nucleusNuclear physics really started with an observationby Henry Becquerel in 1896, that ‘emanations’from a uranium salt fogged a photographic plate.This radioactivity was investigated further byMarie and Pierre Curie who isolated two newradioactive elements, polonium and radium. Itwas Rutherford who showed that there werethree types of radioactivity – alpha, beta andgamma radiation. Together with Frederick Soddy,his studies of radioactivity in thorium revealed forthe first time that an element could transmuteinto another element by spitting out particles inthe form of radiation; elements were not soelementary after all. Alpha particles turned out tobe helium ions, while beta rays are electrons.Gamma-rays are very high energyelectromagnetic radiation. Their characteristicenergies measured in experiments today giveclues to the quantum states in nuclei. Rutherford’searly experiments firing alpha particles at variouselements led to the discovery of the proton andthus helped to elucidate the nuclear constituents.

Becquerel’s first evidence

of radioactivity

Preparing radioisotopes for medical use

Part of the COSY

accelerator at FZJ

Ernest Rutherford

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Today, European nuclear research is spread over a

network of large and medium-sized regional

facilities, and university departments. Because

experiments usually involve attaining very high

energies, the equipment – accelerators, detectors and

so on – is physically large, involves incredibly complex

engineering requiring multidisciplinary teams of

people, and so is expensive. As a result, even the

smaller laboratories are becoming increasingly

international in nature. One of the main tasks of the

European nuclear physics community is to develop a

coordinated research strategy that makes the best

use of current and proposed facilities. The large

international and the smaller, national laboratories

have complementary roles in providing a balanced

framework of experimental programmes for

supporting creative research.

The most famous European high-energy physics

laboratory, now regarded as a world facility, is CERN,

the European Organisation for Nuclear Research,

T H E E U R O P E A N D I M E N S I O N ::

A world-leading E urope has a noble heritage in nuclear

physics that started with the discoveries ofradioactivity and nuclear structure

Part of the AGOR accelerator

at KVI in the Netherlands

The accelerator

at JYFL in Finland


based in Geneva. As well as building the world’s

highest-energy particle machine, the Large Hadron

Collider (LHC) to investigate a new realm of

fundamental physics, CERN carries out a number of

lower energy nuclear physics experiments. The

electron-proton collider HERA, investigating the

structure of the proton, has established the DESY

laboratory in Hamburg as an international laboratory.

Germany also has another major laboratory devoted

to nuclear physics, GSI in Darmstadt, which has

ambitious plans to upgrade and extend its

accelerator complex so as to open up new frontiers in

nuclear science. France and Italy also host large

international facilities carrying out many

experiments – respectively, GANIL in Caen and the

GRAN SASSO underground laboratory near Rome. All

these research centres have made major frontier

discoveries in nuclear physics, as the pages in this

booklet show.

The role of medium-sized facilities While certain experiments require the cutting-edge

facilities and infrastructure of a large laboratory, the

medium-sized, nationally-based research institutes*play a crucial role within the research framework.

These laboratories are able to focus on specialised

experiments perhaps requiring particular kinds of

particle beams and detectors. Because the

equipment is less costly to operate, very precise data

can be obtained by running the experiment for a

long time. It may also be advantageous to develop

and test new detectors and experimental techniques

first in university laboratories. The instrumentation

can then be shared by several facilities across Europe.

The national laboratories can readily offer students

the necessary broad education in both physics and

engineering. Finally, many of these laboratories have

close relationships with local industry, and have their

own spin-out companies offering, for example,

isotopes and cyclotrons for medical applications,

microfabrication facilities and radiation-hazard

assessment services.

Most nuclear physics research centres are involved

in technology transfer, for example, developing novel

semiconductor devices that may find use in medical

and industrial equipment. Many European facilities

now run particle-beam cancer therapy programmes

which may take up a substantial proportion of

accelerator time. The public is thus benefiting from

nuclear research in a very direct way.

Finally, the nuclear physics community is aware of

the importance of communicating to the public the

significance of its research findings. Many laboratories

organise regular public outreach and education

activities, including open days and school visits.

*JYFL Finland; KVI The Netherlands; TSL Sweden; FZJ Germany;CRC Belgium; IReS France; LNL, LNF and ECT Italy

programme of nuclear research

The accelerator

hall at FZJ in

Germany

The multidetector

ICARE used for

nuclear reaction

studies at IReS


of the neutron dripline has so far been explored. There

is a vast territory of nuclei with high numbers of

neutrons which is still terra incognita. At the top end

of the nuclear map exist possible islands of superheavy

nuclei which are expected to be relatively stable.

Why is it important to make and study the broad

range of nuclei possible? First, nuclei with specific

combinations of protons and neutrons allow us to

test theories of nuclear structure (see box) and

fundamental interactions; secondly, their properties

may uncover the pathways by which the elements

are created astrophysically, and finally, many isotopes

have potentially significant applications – in analysis,

medical treatment and perhaps most excitingly,

though more speculatively, in safe energy production

and storage (p.16).

To create the nuclei we want to study requires a

wide range of experimental techniques. First,

particles such as protons, neutrons and various

nuclei, accelerated to moderately high energies, are

projected against a target. This produces new nuclei

in several ways – neutrons or protons may flow

between the target and the projectile nuclei, or the

nuclei may fuse into heavier species or break up into

lighter ones. Depending on the energy of the beam and

the target material, specific nuclei can be selected and

then guided by electromagnetic fields to a detector.

As physicists begin to explore out into the

unstable frontiers of the nuclear landscape, the first

important measurements to make are those of mass,

lifetime and mode of decay, which give information

on the stability and the binding energy of the

nucleus. Nuclei are often prepared in high-energy

states, perhaps set spinning by glancing collisions.

The energy spectra of the gamma-rays emitted as

the nucleus returns to its ground state, or particles

emitted as it decays, provide vital information on its

structure and shape.

The limits of nuclear stabilityDuring the past few years, the neutron dripline has

been mapped as far as fluorine-31 (9 protons and 22

neutrons, neon-34 (10 protons and 24 neutrons) and

sodium-37 (11 protons and 26 neutrons). What is

remarkable is that the dripline jumps from oxygen-24

Nuclei are well-defined, yet somewhat mysterious

systems containing up to a couple of hundred

protons and neutrons (collectively called nucleons)

held together by nuclear forces. Nuclei are capable of

immense variation and complexity due to the subtle

interplay of these forces.

The stable nuclei making up the elements we are

familiar with in everyday life represent just a small

fraction of those that can exist. Other nuclei with

vastly varying proportions of protons and neutrons

can be created in experiments, and play a vital role in

the synthesis of elements in stars (p.12): at least

6000 different proton-neutron combinations are

possible. These can be plotted across a ‘landscape’ of

protons and neutrons (see diagram). The chart

reveals a long ‘valley of stability’ rising diagonally,

which is inhabited by families of stable nuclei; north

and south of the valley, unstable nuclei with widening

ratios of proton and neutron numbers survive for a

time. Out on the wild frontiers of these regions,

nuclei live dangerously – the proton-neutron ratios

are so extreme that the nucleons leak out. These

‘driplines’ are a very active area of study. We know

where the proton dripline is but only the lower part

N U C L E A R S T R U C T U R E

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The nuclear landscape

PROT

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proton dripline

neutron dripline

r-process path

rp-process path

unknown nuclei

known nuclei

stable nuclei

::

complexnucleusThe

ost of the Universe that we can see is made of nuclei – the dense coresof atoms only about one million millionth of a centimetre acrossM


(8 protons, 16 neutrons) to fluorine-31 with six more

neutrons. Now, the eight protons of oxygen represent

a stable, ‘magic number’ (see box) so that adding

an extra proton requires a fair amount of energy;

nevertheless that one extra proton orbiting the

closed eight-proton shell binds six extra neutrons –

giving us important insights into the subtleties of

the interactions between the two sets of nucleons.

:: HALOS AND SKINSThe complexity of these interactions are revealed in

light nuclei close to the neutron dripline in which the

excess neutrons are loosely bound, forming a halo

around the central core. The best known are helium-6

and lithium-11, which have been studied at a number

of European laboratories. In these nuclei, two

neutrons sit far outside the respective helium-4 and

lithium-9 cores, each forming a mutually attractive

three-body system but which any separate pair of

the constituent bodies does not bind. The

system is called Borromean after the

heraldic sign of the Italian princes of

Borromeo of three interlocked rings

which fall apart when one ring is

removed. So far, several neutron halo systems have

been discovered, the largest being carbon-19.

Bound quartets of neutrons may even exist on

their own (tetraneutrons) – recent results at GANIL

have yet to be confirmed. However, hydrogen-5 and

hydrogen-7 containing one proton, and four and six

neutrons respectively have recently been made. Studies

of these extraordinary neutron-rich species may

throw light on the structure of neutron stars (p.12).

In heavier neutron-rich nuclei, a mantle of

neutrons can envelop the nuclear core to form a low-

density neutron ‘skin’. Such systems are expected to

show complex behaviour – an experimental

challenge for the future. The neutron dripline seems

to be much further out than originally thought and

embraces some fascinating phenomena. Protons

might also form halos in proton-rich nuclei and will

be investigated in the future. One important recent

discovery are nuclei teetering on the proton dripline

which fall apart by spitting out two protons at a

time, such as oxygen-12 and iron-45.

Nuclear modelsThe nucleus is a self-organised, many-body quantumsystem which interacts through the strong, weak andelectromagnetic forces (p.22), so that developing asingle theoretical description is extremely challenging.At the moment, there are several approaches whichwork well for different types of nuclear species. Oneimportant characteristic, first noticed in the 1940s,was that certain proton-neutron combinations areparticularly stable. These ‘magic numbers’ (2, 8, 20, 28,50, 82...) can be explained by the so-called shell model,in which each nucleon moves in an orbit held by acentral force calculated from the average effects of allthe nucleons. The nucleons build up in shells accordingto quantum mechanical principles, as for electrons inatoms; the magic numbers represent fully occupied, orclosed stable shells of protons or neutrons. Anynucleons orbiting beyond the outermost closed shellbehave as ‘valence’ nucleons (single particles), whichcan be excited to higher quantum states. The shellmodel works well for most light nuclei and those withnumbers of neutrons and protons near a closed shell.

Another model, successful for many nuclei,describes the nucleons as pairing up into nuclearbuilding blocks called ‘bosons’, which are characterisedby particular quantum properties also found in thepairs of electrons responsible for superconductivity.Changes in structure can then be characterised interms of the pairs being excited or breaking up.

For heavy nuclei, and those with nucleons not neara magic number, the interactions are best described byanother class of models in which the nucleons aretreated collectively as a liquid drop, held together bysurface tension. When excited, the nucleons all movetogether causing the nucleus to vibrate or rotate, oreven change shape. Many nuclear species showcomplex behaviour involving interplay betweencollective and single-particle or boson excitations.

Investigating the intricate changes in structuretriggered by adding or removing a nucleon, changing aproton for a neutron (a parameter called isospin), orexciting the nucleus to high energies can be used totest these models.

>>

Equipment used in

experiments to study

heavy nuclei at GSI

The protons

(green) and

neutrons (pink)

in a nucleus

Shell model

Liquid drop

model


at GSI and GANIL have coaxed into existence two

unusual doubly-magic nuclei, tin-100 with 50

protons and neutrons, and nickel-48 with 28 protons

and only 20 neutrons. By comparing the behaviour of

these unstable, neutron-starved species with their

close neighbours on the nuclear map, theorists can

test their models of nuclear structure.

:: MIRROR NUCLEITin-100 and nickel-48 are fascinating for another

reason. Although quantum physics prefers the

number of protons and neutrons to be equal, the

electromagnetic force pushes the protons apart,

which is why increasing numbers of neutrons are

needed to hold together the heavier nuclei. Tin-100

does in fact have equal numbers of protons and

neutrons so is rather special; the protons and

neutrons are probably paired off. In the case of

nickel-48, if the numbers of protons and neutrons

are swapped around, then its ‘mirror’ nucleus

calcium-48 is created which is both doubly-magic

and stable. Such a mirror pair can be used to probe

the competing effects of proton and neutron

interactions within the nucleus.

:: NUCLEAR SHAPEAlthough magic nuclei tend to be spherical, just a

small change in energy or number of particles can

cause the nucleons to reorganise, adopting a

radically different shape to achieve stability. Recently,

using the accelerator at GSI, European researchers

uncovered a triple nuclear shape-shifter, lead-186.

This isotope has a closed shell of 82 protons which

likes to be ball-shaped; however, when the shell is

broken by exciting a pair of protons, the nucleus

quickly settles into either a pumpkin (oblate) or

melon (prolate) shape. Similarly, just adding, say, a

pair of neutrons to a medium-mass magic nucleus

can cause a drastic change in physique.

:: SHAKE, RATTLE AND ROLLAs nuclei get heavier, nucleons start to lose their

individuality and become more fluid-like. New types

of surprising phenomena appear as the nuclei are

made to vibrate and rotate very quickly, and these

:: CLUSTERS AND NUCLEAR MOLECULESLight nuclei can also form unusual configurations

comprising loosely bound clusters of alpha particles

(two neutrons and two protons), and these continue

to be a theorist’s paradise. Carbon-12 and oxygen-16

can be thought of as clusters of three and four alpha

particles respectively. Such structures are highly

deformed, the most extreme being excited

magnesium-24 – possibly a polymeric chain of six

alpha particles. Other, even more exotic structures

are thought to exist akin to atomic molecules held

together by covalent bonds, for example, silicon-28

made up of an oxygen-carbon conglomerate and

beryllium-9 which can be thought of as two alphas

with a neutron in shared orbits around them. Such

ideas await further spectroscopic exploration.

:: THE SUPERHEAVIESPerhaps the most exotic nuclei, at least to the

general public, are the heaviest at the top end of the

nuclear map. These are entirely artificial and usually

short-lived. Nevertheless, theorists have predicted an

island of stability between element 114 (the isotope

with 184 neutrons is doubly magic so is expected to

be stable) and 126. Much effort in Europe has been

put into reaching this island using carefully planned

fusion reactions. So far, 112 has been made (at GSI)

and there have been sightings of elements of 114 and

116 (at Dubna in Russia). Further work will require

much more intense beams of both stable and

unstable heavy nuclei. In the meantime, researchers

continue to analyse the chemistry of the superheavy

elements made so far, relying on just a few atoms!

Recently, an international team at GSI investigated

the chemical properties of hassium (element 108),

showing that it behaved similarly to Group 8

elements in the Periodic Table.

Investigating nuclear structure and shape:: MAGIC NUCLEIA key area of study are the magic nuclei with stable,

filled shells of neutrons or protons. Particularly

intriguing are doubly-magic nuclei with full shells of

both protons and neutrons tightly held together. In

the past few years, researchers working with facilities

N U C L E A R S T R U C T U R E ::

>>

Magnesium-24

can be regarded

as a cluster of six

alpha particles

Nulear structure can be

studied by creating exotic

nuclei containing a kaon, for

example. Such experiments

are carried out using the

DEAR apparatus at FNL


offer important insights into structure. Centrifugal

and Coriolis forces may induce elongated shapes

with axial proportions of 2 to 1 – called

superdeformation – revealed by the emission of a

characteristic gamma-ray cascade, or nuclei may

start to wobble as in the case of lutetium-163. Theory

predicts even more extreme ‘hyperdeformed’ nuclei

with an axis ratio of 3 to 1, and researchers are

searching for them among barium, xenon and tin

isotopes. Another topic of intense interest are the

collective oscillations of nucleons called giant

resonances. They also provide information on

structural changes in excited nuclei, and throw light

on what happens when a nucleus is squeezed.

:: EXOTIC NUCLEI A completely different approach to probing nuclear

structure is to introduce an ‘impurity’ into the

nucleus. Protons and neutrons are made up of two

kinds of quark – ‘up’ and ‘down’ (p.22). Four other

quarks exist in Nature, and so a neutron can be

replaced with an exotic nucleon containing another

quark flavour, such as a lambda (Λ). This has a

strange quark as well as an up and a down quark.

Recently experimenters converted lithium-7 (with

three protons and four neutrons) into lithium-6-Λ.

The effect was extraordinary: the nucleus shrank in

size by 20 per cent as the lambda became more

tightly bound than the outer neutron and proton.

The need for advanced facilitiesTo continue the exciting work highlighted here will

require new advanced facilities for generating a wide

range of nuclei far from stability (p.26) not possible

at the moment – whether to explore the driplines,

probe unusual or significant proton-neutron

combinations, or try to make new elements that have

never existed before. There is still a lot to learn in this

fertile field of scientific exploration and we can

expect many more surprises.

3025201510β2cos(γ+30)

β2 sin(γ+30)

En

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Potential energy surface for 186Pb

5200

-200

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3

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186Pb

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Similar nuclei can adopt quite

different shapes to achieve stability.

Experiments at JYFL have been

probing such shape coexistence in

heavy nuclei close to the proton

dripline (left), while work at GSI

(below) has uncovered an isotope of

lead which has three different

shapes depending on energy

Researchers at JYFL measure the masses

of rare neutron-rich heavy nuclei


fewer neutrinos (elementary particles produced in

nuclear reactions) than predicted theoretically. One

suggestion was that, on their way to the Earth, the

solar neutrinos were changing, or oscillating into

other kinds of neutrino (p.22) not detectable by the

instruments then used. Recent measurements made

at the Sudbury Neutrino Observatory in Canada

confirm the existence of these neutrino oscillations –

a fundamental phenomenon which looks to have

momentous implications for particle physics theory.

In the meantime, our standard nuclear model of the

Sun remains reassuringly intact.

OBSERVATIONS AND MEASUREMENTSNot all the elements are made in stars like the Sun.

To explain the abundances of elements we see today,

physicists have proposed a series of complex but

coherent networks of nucleosynthetic reactions

thought to operate in certain astrophysical

environments such as red giants and supernovae.

These abundances can now be checked in much

more detail against spectroscopic observations

available from powerful telescopes such as the

Hubble Space Telescope and the Very Large

Telescopes of the European Southern Observatory.

The new generation of X-ray and gamma-ray

telescopes, including the European Space Agency’s

XMM-Newton and Integral, will provide important

new data on element-building and distribution in the

Universe. Integral, in particular, will be able to home

in on the explosive processes in supernovae thought

One of the deepest questions on which humans have

always pondered is how did the world and ourselves

come into existence. Clearly our planetary

environment, and the life it supports, have been

greatly shaped by the constituent proportions of

elements. We know that the first, lightest elements,

hydrogen, helium and some lithium, were created in

primordial processes just after the Big Bang – the

observed proportions indeed support those predicted

theoretically. All the heavier elements were and are

still being made by nuclear reactions in stars.

Unravelling the pathways by which elements are

synthesised is a key factor in understanding the early

Universe, the evolution of galaxies and stars, and the

development of planetary systems, such as our Solar

System, which are hospitable to life. Of particular

relevance to us, of course, are the mechanisms

responsible for building up the elements necessary

for our existence.

SOLAR NUCLEAR PHYSICSAnother issue gaining increasing interest, in which

nuclear physics plays a part, are the effects of the

Sun’s inherent variability on the Earth. The Sun is very

much an average star, and its energy output upon

which we depend is driven by those very reactions in

which the elements are made, so understanding

their role in solar dynamics is of clear importance.

Solar nuclear physics has also come under scrutiny

for another reason. In recent decades, researchers

discovered that the Sun appeared to be emitting

N U C L E A R A S T R O P H Y S I C S ::

Probing the nuclear processes that drive the engines of stars is now oneof the most important areas of nuclear physics research

Nuclear physicsin the Universe

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Gamma-ray emission across the

Universe from aluminium-26,

a signature isotope of

element-building in stars

The heavier elements are

synthesised in dying stars

(see the Cat's Eye planetary

nebula, above), and in

supernovae (1987A, below)

to be responsible for creating the heaviest elements.

Meteorites also act as cosmic probes. Most of them

are left-over debris from the construction of the Solar

System, and thus provide pristine ‘fossil’ evidence, in

the form of elemental abundances, on stellar processes.

Another crucially important approach is to study

nuclei and nuclear reactions of astrophysical

relevance in the laboratory using particle and nuclear

beams. In this way, it’s possible to determine

significant parameters of a nucleus such as mass,

lifetime and the characteristic excited states, or the

likelihood of a particular reaction. Nucleosynthetic

pathways may involve intermediate unstable nuclei

with extreme ratios of protons and neutrons which

are far away from the valley of stability on the

nuclear map (p.8), and these can be studied with

radioactive beams. Another aim is to simulate the

real physical conditions found in stars, perhaps using

high-intensity beams at low energies.

Cooking up the elementsAlthough we now have quite robust models for the

various reaction pathways that occur during a star’s

evolution, observational and experimental evidence

is still sparse. Many of the standard reactions are

notoriously difficult to measure. Nuclear astrophysics

is clearly a burgeoning area where many discoveries

are waiting to be made.

:: GENTLY HEATFor most of their lives, stars produce energy by

fusing hydrogen into helium by the pp chain in

which helium-4 is effectively produced from four

protons. In second-generation stars like the Sun, the

transformation of hydrogen to helium can also be

catalysed by carbon-12 (the CNO cycle). We still don’t

know the relative importance of the two mechanisms;

a recent suggestion is that the CNO cycle could be

responsible for up to half the energy production in

the Sun. Measuring the fusion rates is extremely

difficult because they occur at relatively

low energies with low probabilities

(because of the mutually repulsive effects

of the positively charged nuclei).

However, these reactions have recently

been carried out underground (to avoid

interfering background radiation) in the

LUNA laboratory at the Gran Sasso facility

in Italy using a dedicated accelerator.

Other important reactions are the fusion of

helium-3 and helium-4 into beryllium-7 and its

conversion to boron-8 by the capture of a proton. The

latter reaction is largely responsible for neutrinos

emitted from the Sun whose flux measurements led

to the confirmation of neutrino oscillations

mentioned earlier, so understanding fully the

neutrino-generating process is vital to further study

of this newly discovered phenomenon.

In stars more massive than the Sun, there are still

many uncertainties about the rates of the reactions

that follow the onset of helium burning. One of the

most significant reactions is the fusion of helium-4

and carbon-12 to oxygen-16, partly because it

influences all the future stages of nuclear burning

and the final core collapse. Its rate also determines

the abundances of the two foundation stones of life

– carbon and oxygen. Much more work is needed on

this very important reaction. Another significant

uncertainty to investigate is the rate of conversion of

magnesium-25 to aluminium-26. The latter isotope is

quite long-lived and provides a useful marker across

the Galaxy for the distribution of

nucleosynthetic processes (p.12) with

implications for our understanding

of stellar evolution and the origin

of planets. >>

Equipment used at

CYCLONE in Louvain-la-Neuve

to study nuclear reactions of

astrophysical importance

The blast wave from a

supernova explosion

(Cygnus Loop nebula)

ground state which has a half-life of only a month,

thus changing the effective half-life by 15 orders of

magnitude. These kinds of changes have implications

for radioactive ‘clocks’ used to measure astrophysical

ages. The slow decay of lutetium-176 to hafnium-176

is a perfect clock in the laboratory (the principle is

the same as for carbon-dating) and since lutetium-176

is produced only by the s-process, it could be used to

determine the age of s-elements. Again at the high

temperatures in red giants, the isotope can decay via

an alternative shorter-lived state.

At high stellar temperatures, virtually all the

electrons are stripped off atoms, which also reduces

the half-life since nuclear electrons can escape more

easily, and this also has to be taken into account in

isotope-dating. An intriguing additional effect was

recently discovered at GSI in the decay of some

isotopes such as rhenium-187 to osmium-187 (used

to measure the age of the Universe). When atomic

electrons are stripped off, the emitted electron

remains tightly bound in close orbit around the atom

which further modifies the half-life.

:: ALLOW TO BOILThe s-process cannot produce all the heaviest

elements. Instead, another process – the rapid

neutron capture, or r-process – is thought to produce

elements heavier than iron all the way up to uranium

and beyond. The free neutrons must be in great

abundance and stick to nuclei in quick succession

before they have a chance to decay by beta emission,

or be knocked loose by gamma-rays. In this way

elements are rapidly built up (in seconds) through

neutron-rich isotopes far from the valley of stability.

The process may get held-up when especially stable,

‘magic’ nuclei (p.9) are reached – so-called waiting

points – and these regulate its rate. The effects of

these waiting-point nuclei are indeed reflected in the

relative abundances of the heavy elements such as

strontium-88, barium-138 and lead-208.

Unravelling the r-process requires an

understanding of nuclear structure in heavy,

neutron-rich species, and this depends on measuring

lifetimes, masses and spectroscopic properties. The

beta-decays of about 30 neutron-rich nuclei were

:: SIMMER AND STIRAbout half the elements heavier than iron are made

in red giants, starting with iron ‘seed’ nuclei. A free

neutron is captured by a nucleus and decays into a

proton, emitting an electron (beta particle) at the

same time, to give the next heavier element. This

neutron-capture process is extremely slow compared

with the beta decay, and so is called the slow

neutron-capture, or s-process. Successive neutron

captures produce elements all the way up to lead,

following the valley of stability in the nuclear

landscape on p.8. Some 30 elements are produced

only by the s-process. One source of evidence for the

s-process abundances are meteorites which contain

grains with small amounts of elements thought to

originate from the atmospheres of red giants. High-

precision measurements of ratios of specific isotopes

in meteorites can then be compared with theoretical

predictions and experimental results.

Although we have a good, general model of the

s-process and the accompanying mixing of material

in red giants, there are still uncertainties and

anomalies. The two main sources of the

neutrons for the s-process are a reaction in

which carbon-13 captures a helium

nucleus (alpha particle), then spits out a

neutron to form oxygen-16, and a

similar process converting neon-22

into magnesium-25. Their reaction

rates still need to be determined

more accurately, as do the rates of

many s-process reactions. Recently

the neutron-capture rate for the

rarest stable nucleus in Nature,

tantalum-180, was measured.

Tantalum-180 is an example of

where the laboratory-measured

half-life may be drastically

changed by the stellar

environment. This isotope normally

sits in an unusual long-lived high-

energy state lasting a thousand

million million years; but experiments

show that at red-giant temperatures, it

is most likely pushed down into the

N U C L E A R A S T R O P H Y S I C S ::

e+ν

ν e+

4 11H 4

2He 3 4

2He 12

6C 12

6C + 4

2He 16

8O

Nuclear fusion reactions at the heart of the Sun

Protons: red;Neutrons: blue

>>

Detector arrays of the

Sudbury Neutrino

Observatory to detect

solar neutrinos



recently measured at the ISOLDE facility including

those of waiting-point nuclei, cadmium-30 and

silver-129. GSI has measured a large number of

masses of short-lived neutron-rich nuclei and hopes

to measure hundreds more in the future. It is not yet

possible to measure directly rates of neutron capture

of these highly unstable nuclei.

So far, our current understanding of the r-process

is supported by accurate measurement of the

abundances of r-process elements in very old stars,

in the Sun and in meteorites, but we still don’t know

for sure where it takes place. Clearly, it has to be an

explosive event, and the most likely site is the hot

wind, driven by neutrinos, gusting off the surface of a

neutron star newly-formed in a supernova explosion.

Neutrinos themselves may also trigger

nucleosynthetic reactions.

Re-heatIn binary systems, the transfer of matter from one

star to another causes violent nuclear reactions

(see box) leading to a different set of abundances.

The hydrogen that accumulates ignites via the ppand CNO cycles which is followed by the rapid

capture of protons (the rp-process). These reactions

scramble up the proton-rich side of the valley of

stability to isotopes around mass 40. Pioneering

experiments on these reactions are carried out using

radioactive beams at CYCLONE. If the accreting star is

a neutron star, then proton capture goes further,

resulting in nuclei up to mass 100.

There is clearly strong interplay between nuclear

astrophysics and nuclear structure studies, and with

the advent of new radioactive beam facilities we can

expect exciting developments in this area.

Star life Although most of the matter in theUniverse is invisible, and of anuncertain nature, a large part of thatwhich we can observe consists of largeglowing spheres held together bygravity – stars. These objects mostlycomprise hydrogen and helium withsmaller amounts of other elements,and are hot and dense enough totrigger nuclear ‘burning’ – the fusion ofthe lighter elements into heavier ones.The energy liberated creates pressure,with the result that a star’s life is anongoing tussle between this outwardpressure and gravity pulling matter tothe star’s centre.

Stars start by fusing hydrogen intohelium which settles into the stellarcore and eventually starts burning aswell. The heat from the burning,shrinking core causes the star to swellinto a red giant (which will happen to

the Sun in another five billion years orso). Stars at least eight times moremassive than the Sun carry on burning,starting with carbon and oxygen, goingthrough, neon and magnesium, siliconand sulfur, creating elements withmasses up to iron. The result is a starwith onion-like layers of elementssurrounding an iron core. Smallamounts of heavier elements are alsoproduced in the inner layers.

When a medium-sized star like theSun has used up all its helium, itshrinks under gravity and settles to adense ‘white dwarf’ while blowing offits outer gaseous envelope to form adiffuse shell called a planetary nebula.Much heavier stars end their lives morespectacularly as supernovae. Theirdense iron cores collapse after thenuclear fuel has run out. The outerlayers fall onto the core and bounce offagain, creating a gigantic shock wave

that rapidly travels outward inducingfurther nuclear reactions to create theheaviest elements. The exploding starbecomes brighter than a million Sunsas it spills its guts into the surroundingspace. Left behind is a compact object –a neutron star or a black hole.

If a dead star is part of a binarysystem, then its nuclear life might besporadically resurrected by suckinghydrogen-rich material from its stellarcompanion. In the case of a typicalwhite dwarf, the accreted mattertriggers a nova explosion and thehydrogen ignites leading to furthernuclear burning. A more massive whitedwarf might destroy itself in asupernova type I explosion, ejecting allthe products of nuclear burning. If thestellar remnant is a neutron star, thenaccretion may lead to thermonuclearrunaway on its surface, resulting inperiodic bursts of X-rays.

Part of GSI’s accelerator system

used to study neutron-rich nuclei

important in nucleosynthesis


NUCLEAR FISSION AND TRANSMUTATIONTwo of the main problems associated with current

nuclear-energy production are those of long-lived

radioactive waste (transuranium elements, plutonium,

neptunium, americium and curium, and fission

products such as iodine-129 and technetium-99),

and nuclear proliferation. The next generation

of advanced reactors is being designed to be

considerably safer and to produce much less waste.

They can also utilise plutonium and other waste

components as fuel, and could be deployed in

combination with an approach that is gaining

interest: that of transmutation.

Here, the principle is to create a source of

neutrons by firing a proton beam from a high-power

accelerator at a heavy-metal target. The neutrons

knocked out of the target material (a process called

spallation) would then react with waste products in

a surrounding assembly, either being absorbed by

them or causing them to fission, thus converting the

long-lived nuclides into stable or short-lived isotopes.

An important plus is that the process produces more

energy than it consumes, so could also be used to

generate electricity.

SUBCRITICAL NUCLEAR REACTORSSuch so-called accelerator-driven systems (ADS)

could further be deployed in a novel nuclear reactor

burning the element thorium (plentiful in the Earth’s

crust) instead of uranium. Thorium doesn’t readily

fission, but when bombarded with neutrons, it is

transformed into uranium-233 which also fissions.

Significantly, the reaction is subcritical so can be

switched off, and it also produces less waste. The idea

has been developed in Europe by Carlo Rubbia at

CERN, who calls it the Energy Amplifier.

Research into the feasibility of ADS is still at a very

early stage but there are a number of EU-supported

programmes across Europe looking at the neutron

production process (for example, at GANIL and CERN)

and neutron-induced fission (using the neutron beam

facility at The Svedburg Laboratory in Sweden).

Much of our advanced technology derives from

exploiting the complex behaviour of atoms. However,

we have hardly begun to utilise the rich and intricate

properties of nuclei for the benefit of society.

Nevertheless, nuclei and their components,

protons and neutrons – and nuclear processes such

as radioactive decay – are already vital analytical

tools for laboratories and industry, and have also

become an essential part of many medical treatments.

More controversial is whether we can safely

harness the powerful forces binding the nucleus for

energy production. Regrettably, most people

associate nuclear physics studies with just one

nuclear reaction – the chain-reaction fission of

uranium-235, the basis of the atomic bomb. Today’s

nuclear-power generation, which depends on the

same reaction, produces one-third of the EU’s

electricity (2000 figures) but is regarded by many as

environmentally hazardous and uneconomic.

However, some exciting work is going on in European

laboratories to study novel methods of producing

safer, cheaper nuclear energy.

Safe nuclear energyMuch of the world’s energy comes from another

environmentally hazardous chain-reaction – chemical

combustion (mostly of oil and gas). Alternative

sources of energy such as wind and solar power are

not likely to satisfy future energy demands, whereas

harvesting the huge amount of energy locked up in

the nucleus would secure the world’s energy needs

for many thousands of years.

A P P L I C A T I O N S ::

N UCLEAR PHYSICS HAS THE POTENTIAL TO OFFER NEW TYPES OF 21st-CENTURYTECHNOLOGY FOR ENERGY PRODUCTION, INDUSTRY AND HEALTHCARE

Law

ren

ce B

erk

ele

y L

ab

ora

tory

Inertial confinement

fusion could be the route

to clean energy

Fo

rsch

un

gsz

en

tru

m J

üli

ch

PET images of the

brain using iodine-124

(left) and fluorine-18 as

positron-emitters

Exploitingthe nucleusThe energy amplifier –

a route to subcritical

nuclear energy being

developed at CERN


NUCLEAR FUSION Another nuclear reaction being studied for energy

production is that mimicking the main nuclear

process powering stars – nuclear fusion (p.12).

Isotopes of hydrogen – deuterium and tritium –

combine to form helium and energy-carrying

neutrons at extremely high pressures and

temperatures. The process is very clean, uses a readily

available fuel and produces little waste. One way of

achieving the right conditions for fusion is to fire a

high-power laser, or a beam of light or heavy ions,

at an encapsulated solid pellet of the isotopes. The

intense radiation heats and compresses the fuel so

much that fusion occurs. Work is being carried out on

the heavy ion approach by the HIDIF collaboration

(European Study Group on Heavy Ion Driven Inertial

Fusion) using facilities at GSI, and a large laser facility

is being constructed in France.

ENERGY STORAGEHigh-energy lasers also offer intriguing ways of

tapping into the nucleus’s rich energy stores. Some

nuclear species are readily excited into higher energy

states that are long-lived (nuclear isomers). Indeed,

the naturally occurring form of tantalum-180 is a

stable isomer. If this energy could be released in the

form of gamma-rays by ‘tickling’ the isomer with a

laser, then we would have the basis of a nuclear

battery or even a gamma-ray laser. There is much

potential in investigating these and other more

subtle aspects of nuclear behaviour.

The nucleus as microscope and cameraEver since radioactivity was discovered, the nucleus

has been used as a probe to analyse materials, and

to obtain images of objects otherwise too difficult

to investigate. Today, there are a huge variety of

techniques based on nuclear properties used in areas

as diverse as industry, medicine, environmental

protection and archaeology.

NUCLEAR MAGNETIC RESONANCE AND MRIOne of the most powerful analytical techniques to

come out of nuclear physics exploits the magnetic

properties of certain nuclei – hydrogen (single

proton) or phosphorus-31 have magnetic moments

(they act like bar magnets) which can be oriented by

magnetic and radio-frequency fields. The interaction

is influenced by the nucleus’s chemical environment

so offering a clever method for analysing chemical

structure. Nuclear magnetic resonance (NMR) is now

an indispensable tool for chemists and molecular

biologists alike. Over the past 20 years, the technique

has been successfully taken further to construct

images of regions inside the body, in particular the

brain, the heart and lungs, and tumours. This is

known as magnetic resonance imaging, or MRI, and

MRI scanners are now found in most major hospitals.

RADIOACTIVE PROBESFor many years, radioactive isotopes have been

applied in the clinic as diagnostic tools, and also as

tracers in biological studies following, for example,

metabolic pathways or the uptake of drugs. They

are similarly employed to monitor waste and

pollutants in the environment. Extremely small

amounts of radioactive and stable isotopes can then

be analysed using the technique of accelerator mass

spectrometry (AMS) in which samples are converted

into a beam of ions. The different kinds of ions are

separated by magnetic and electric fields, and their

signature masses measured. The sensitivity of AMS,

whereby single atoms can be detected, has opened

up a much broader use of radioactive labelling in

medical diagnosis and in measuring environmental

pollution. One of the most successful uses of AMS is

in carbon-dating. Instead of relying on measuring the

radioactivity of carbon-14, the ratio of carbon-14 to

stable carbon-12 can be obtained directly by counting

the carbon atoms.

Remarkable in vivo images of the body can be

reconstructed by detecting the distribution of

radioactivity. Positron emission tomography (PET) is a

sensitive technique of growing importance employed

to study blood flow and metabolic activity especially

in the brain. Positrons (positively charged electrons)

emitted from an injected radioisotope such as

fluorine-18 reveal their position when they annihilate

to release pairs of gamma-rays which are detected by

a camera. A similar technique, single photon emission >>

Making measurements related

to the transmutation of nuclear

waste at TSL in Sweden


nucleus of a target atom so that X-rays are emitted.

This technique – particle induced X-ray emission

(PIXE) – offers a nondestructive way of analysing, for

example, pigments in paintings. Two other techniques,

nuclear reaction analysis (NRA) and particle-induced

gamma-ray emission (PIGE), rely on an ion beam

exciting – or reacting with – a nucleus to emit

gamma-rays. Radioactive ion beams, such as those

produced at the ISOLDE facility at CERN, can also be

used to study materials. When the ions become

implanted on a target surface, their pattern of decay

is perturbed by the surrounding atoms, so revealing

the nature of their immediate environment.

Creating new materialsHigh-energy ion beams also modify materials in

interesting ways. Ions can be implanted into surfaces,

or they may push their way through a material

drilling a nanometre-sized track which can then be

etched away. In this way, new microstructures can

be created, for example, arrays of micropores in

membranes for filtration or drug release, or

semiconductor devices using the tracks as a template.

Curing cancerEvery year, more than 1 million people in the

European Union are diagnosed with cancer, and

while surgery, chemotherapy and conventional

radiotherapy with X-rays cure about 50 per cent of

patients, a large proportion of tumorous cancers are

rather resistant to radiation, or are sited in locations,

such as the head and neck, which are difficult to

target without damaging surrounding tissue. An

alternative is to irradiate tumours with nuclear

beams – neutrons, protons or light nuclei. Neutrons

have been employed for a number of years but are

less interesting because they are not very

penetrating and tend to scatter into surrounding

tissue. Proton and ion beams, however, have the

advantage of depositing all their energy near the end

of their range, so delivering the maximum possible

dose while sparing traversed and surrounding tissue.

Because the ions are electrically charged, they can be

focused with magnetic fields into a thin pencil beam

of variable penetration depth, which can then be

computed tomography (SPECT), relies on radiotracers

that emit a single gamma-ray to follow tracer

distribution in the body and reconstruct an image.

THE POWER OF NEUTRONSOne of the main constituents of the nucleus, the

neutron, offers a powerful probe of atomic and

molecular structure. Neutron beams, generated

either in a nuclear reactor or by spallation, behave as

waves as well as particles, and can scatter off arrays

of atoms to create a diffraction pattern similar to

that from X-rays. Neutron scattering provides

complementary information to X-rays, and has

become a vital tool in molecular biology and

materials studies. With major neutron facilities at the

Institut Laue Langevin in Grenoble, France, and at the

Rutherford Appleton Laboratory in the UK, Europe

leads the world in neutron experiments. Portable

neutron emitters have also been developed which

can detect explosive materials, for example, in

landmines and terrorist bombs.

SEEING INSIDE MATERIALS WITH ION BEAMS Atoms stripped of some of their electrons and

accelerated to form an ion beam is another way of

delving just below the surface of a material, and

analysing its composition and structure. There’s now

a whole range of techniques using fine ion beams,

which are used to investigate thin-film structures for

integrated circuits, corrosion, and delicate

archaeological artefacts (see box). The ions (usually

hydrogen or helium) may bounce off atoms allowing

their mass to be measured, or they may lose a

characteristic amount of energy that indicates the

composition of the target material. These ion-beam

techniques include Rutherford backscattering (RBS)

and medium-energy ion scattering (MEIS). The ion

beam may excite electron energy levels close to the

A P P L I C A T I O N S ::

>>

Microscopic copper

needles made using

tracks fabricated by ion

beams at GSI

PS

I

Some 200 patients

have been treated so

far with GSI’s novel

ion beam therapy

Ion-track irradiation for the

preparation of nano-wires (right)

and testing electronic devices (far

right) at TSL; the SPECT technique

used to image a pancreatic

tumour (inset below)


drawn back and forth just across the tumour volume,

while avoiding the surrounding tissue. A 3D image of

the tumour is first prepared from a CT scan and used

to program the precise dose distribution. So far,

about 30,000 patents around the world have

undergone proton therapy, which works well for

large tumours unsuitable for X-ray treatment.

NEW ION THERAPYFor deep-seated tumours resistant to X-rays and

protons, carbon-ion beams promise to be the most

effective. Ion beams are three times as damaging to

tumour-DNA than protons or X-rays, allowing little

chance of cell repair. This also means that far fewer

treatment sessions are needed. An additional

advantage is that carbon ions also undergo nuclear

reactions at their target site to form positron-

emitting carbon-11 and oxygen-15. This offers the

opportunity of using simultaneous 3D PET imaging

as a treatment planning tool. GSI is currently carrying

out clinical ion therapy trials with promising results.

Head and neck cancers are particularly suitable for

treatment, but the Laboratory is hoping to make the

equipment responsive enough to compensate for

movement – from breathing, for example.

A TREATMENT FOR LIVER CANCERFinally, another ingenious nuclear therapy exploits

the reaction between boron-10 and low-energy

neutrons to give lithium-7 and an extremely lethal

alpha particle. The combined range of nuclear

products roughly corresponds to the size of a cell so

the dose is very localised. This is the basis of boron

neutron capture therapy (BNCT): boron-10 is

delivered to the target site by a tumour-seeking

boron compound, and the tumour area is then

irradiated with neutrons. Although early trials were

not very successful, the development of new

chemicals and treatment regimes has resurrected

interest in this approach. BNCT is especially suitable

for treating metastases and diffuse tumours.

Recently, a research group in Pavia, Italy treated a

patient with liver metastases by first removing the

liver, irradiating it and then putting it back into the

patient. The patient is now fully cured!

Nuclear science in art and archeology Science and art forge a rewarding relationship when exploringour cultural heritage. Ion-beam analysis is now commonly usedby museums, art galleries and archeological laboratories to dateand establish the provenance of all kinds of artefacts.

For instance, Italian researchers employed PIXE to analyse thecomposition of inks on Galileo’s handwritten, undated notes onastronomy. They were then compared with his dated documentssuch as domestic bills, so helping to understand the evolution ofhis ideas during his life.

The Louvre has its own ion-beam accelerator (AccelerateurGrand Louvre pour l’Analyse Elementaire, AGLAE), and recentlyshowed that one Renaissance portrait in its collection containedchromium and lead pigments introduced only after 1850, thusinferring that the painting was probably a forgery.

The laboratory has also been analysing the crowns and jewelsof the Visigoth kings found at Guarrazar in Spain, in particular todiscover the provenance of their magnificent emeralds. UsingPIGE, and by comparing the emeralds with those mined all overthe world, the researchers showed that they actually came fromthe Austrian Alps, and not from Egypt or central Asia aspreviously thought.

One of the most intriguing archeological finds of recentyears was the discovery of a frozen body high in the Ötztal Alpsnear the Austrian-Tyrol border. It was soon realised that this‘Iceman’ was thousands of years old; in fact, AMS carbon-14measurements carried out in Zurich and Oxford established thathe lived about 4500 years ago.

The Louvre has its own

accelerator (main) for

studying artefacts (top).

It was to used ascertain

the provenance of the

crown jewels of the

Visigoth kings (inset left)

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P H A S E S O F N U C L E A R M A T T E R ::

further, and may be significant for understanding

how neutron stars form in collapsing supernovae.

As the temperature rises, the nucleons are excited

into higher states of energy called baryon

resonances, and new particles form – mesons (which

contain two quarks as opposed to three in baryons

like protons and neutrons, p.24).

At much higher energies, a transition to a new

type of matter happens. The hadrons (baryons and

mesons) dissolve into a ‘soup’ of quarks and gluons –

the quark-gluon plasma which is thought to have

existed in the earliest moments of creation. This

transition is predicted to occur at densities 10 times

that of ordinary nuclear matter, and at a critical

temperature of about 170 MeV, which is 130,000 times

hotter than the Sun’s interior. In this same region,

theorists believe that the quarks also undergo another

transition when they lose their mass through a

process called chiral symmetry restoration (p.24).

Another intriguing area of the phase diagram is

the area of high densities and low temperatures.

Here, theorists predict that, instead of forming

colourless baryons and mesons, quarks pair up so

that their spins cancel out – in the same way that

electrons do in a typical superconducting material.

For this reason, the new phase of matter is called a

colour superconductor. This cold, dense medium may

actually exist in the heart of neutron stars. And

between the hadron-vapour state, the quark-gluon

plasma and the colour superconducting state, there

may be a critical point where the phases meet.

Exploring nuclear phasesNuclear physicists investigate the various parts of

the phase diagram, using both computer simulations

based on the theories and models described on p.8,

and experiments which compress the nucleus at high

energies. This compression is achieved by colliding

nuclei under a wide variety of conditions, for example,

by firing beams of heavy ions at a nuclear target.

Careful analysis of how they break up and the energies

of the particles will reveal various kinds of information

– on the flow of particles, whether or not the nuclei

are easily compressed, or on particle distributions that

may be a signature of an imminent phase change.

Just as water changes phase from a liquid to a gas at

a certain temperature and pressure, so does nuclear

matter. Studies of nuclear behaviour over ranges of

different temperatures and pressures – or densities –

are a major endeavour in nuclear physics. They

provide information needed to interpret the results

of nuclear experiments such as those colliding heavy

ions, and also to understand the behaviour of matter

under the most extreme conditions in the Universe –

just after the Big Bang when matter was at its

densest and hottest, and in supernova explosions

and neutron stars.

The phase diagramAs for everyday materials, nuclear phases are

characterised by an equation of state (EOS), and can

be plotted on a graph of temperature (energy)

against pressure (density) – the phase diagram, as

shown opposite.

At low temperatures and normal densities, the

nucleus can be regarded as a drop of liquid. At

temperatures of around 20 megaelectronvolts (MeV)

and at two or three times the normal nuclear

density, the liquid bound-state

of protons and neutrons in a

nucleus undergoes a phase

change, boiling to a gas of

freely moving nucleons. In fact,

protons and neutrons can be

regarded as separate liquid

phases, giving rise to a gas

with different proton and

neutron concentrations. This

complicates the phase diagram

What happens when nuclei are heated to hightemperatures or compressed to high densities?

t=-16.34 fm/c

>

A lead-lead

collision used

to create the

conditions for

de-confining

quarks at CERN

Part of the ALICE

experiment being built

at CERN to detect a

quark-gluon plasma

nuclear liquidto quark soupFrom


NUCLEI ON THE BOILBeams of moderate energies of around 100 MeV will

gently heat up and compress the nucleus. At higher

energies the nucleus vaporises. At intermediate

energies, it may suddenly expand, become unstable

and break up into fragments of all sizes. Studies of

this multifragmentation pattern can be used to

ascertain the heat capacity over a range of energies.

A recent experiment at GANIL colliding a xenon

beam on a tin target revealed a negative heat

capacity (latent heat of vaporisation) which was the

first sign of a nuclear liquid-gas transition.

HOT HADRONSBeam energies of a few hundred MeV reach

temperatures and pressures that convert nuclei into

a hot, dense gas of excited hadrons. New hadrons

such as pions and kaons (mesons containing a

strange quark) are created, and their energy patterns

give some insight into nuclear compressibility

relevant to the creation of black holes from neutron

stars. Recent experiments at GSI using gold-gold

collisions have been using kaons as probes of this

high-density state.

Theory shows further interesting kaon behaviour.

The kaons and antikaons produced behave slightly

differently: positive kaons are slightly repelled by the

other nucleons, while the negative antikaons are

attracted, respectively increasing and reducing their

effective masses. This may explain why neutron stars

are always less than two solar masses. A neutron star

is 10 per cent protons and electrons, but it is thought

that the electrons are converted into the lighter

negative kaons. Unlike electrons, kaons can all exist

in the same quantum energy state, so condense out

thus reducing the repulsive pressure between particles

in the neutron star. This means that gravity can more

easily take over at lower masses than expected to

pull in the matter and create a black hole.

Changes in the expected masses of mesons at

very high energies are also signatures of the next

impending phase change when chiral symmetry is

restored: quarks no longer interact with virtual

quark-antiquark pairs in the vacuum and lose their

mass. A short-lived high-energy version of the pion

called the rho heralds this phase change as its mass

drops and becomes less certain. An experiment at

GSI has been measuring the mass of the rho by

detecting the pairs of electrons and positrons

emitted when the rho decays.

RECREATING EMBRYONIC MATTERAn important aim of nuclear physics is to understand

how primordial matter created in the Big Bang

condensed into hadrons. Experiments using colliders

aim to recreate these conditions, and investigate the

high-energy region when quarks are no longer

trapped in their hadron prisons. Experiments using

the Super Proton Synchrotron (SPS) at CERN to collide

lead nuclei have been exploring the high-density

fireball-state that forms prior to the transition to

the free quark-gluon phase. A key signature of the

transition is the production of the heavier mesons:

theory predicts that as the quark-gluon plasma cools

and condenses, an avalanche of mesons containing

strange quarks is produced, while the formation

of those containing the lighter charmed quarks –

J/Psi particles – remain blocked by the sea of still

de-confined quarks and gluons. CERN experiments,

indeed, observed an enhancement of strangeness

and a reduction of charm, thus catching the first

glimpses of a quark-gluon plasma.

Nudging even closer to the transition-energy

range should reveal even heavier mesons made from

bottom quarks. These will be seen in the Large

Hadron Collider, which will start up at CERN in 2007.

An experiment called ALICE, again colliding beams

of lead ions but at incredibly high energies of

5.5 teraelectronvolts per beam should give the

clearest picture yet of a quark-gluon plasma.

t=-00.34 fm/c t= 07.66 fm/c t= 16.46 fm/c t= 31.66 fm/c t= 182.06 fm/c

> > > > >The phase changes during a high-energy

collision of uranium nuclei

A phase diagram showing

the behaviour of hadrons

at different pressures and

temperatures. At low

temperatures, hadrons

(protons and neutrons)

condense into everyday

nuclei. At higher

temperatures, the nuclei

vaporise into various kinds

of hadrons, eventually

dissolving into separate

quarks and gluons at

extreme temperatures and

pressures. At lower

temperatures and high

pressures, an unusual

superconducting ‘colour’

state forms that might

exist in neutron stars

About 10 times normal nuclear density

Atomic nuclei

170

100

Hadron gas

Quark-gluon plasma

EARLY UNIVERSE

DENSITY

TEM

PERA

TURE

(M

eV)

250

Colour superconductor

Neutron stars

H. W

eb

er/

J. W

. Go

eth

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niv

ers

ity

, Fra

nk

furt


Mysterious neutrinosThe strange properties of neutrinos are now

uncovering physics beyond the SM. These ghostly

particles are emitted, alongside electrons, in nuclear

reactions when a neutron is converted into a proton

via the weak interaction – radioactive beta-decay. The

Sun emits vast numbers of electron-neutrinos as a

result of this process. However, physicists noted that

far fewer solar neutrinos were being detected than

expected from solar theory (p.12). One explanation

was that they were ‘oscillating’ into the other kinds

of neutrino, which were not detected. This can

happen only if neutrinos have mass; however, the SM

was established assuming they would be massless.

Recent solar neutrino observations have indeed

confirmed the existence of oscillations. A major task

of the next generation of neutrino experiments will

be to measure their masses so as to probe the

theoretical implications further.

:: MEASURING NEUTRINO MASS DIRECTLYOne approach is to try to measure the mass of the

neutrino directly by analysing the energies of the

particles emitted in the beta-decay of tritium

(hydrogen with two additional neutrons) to give

helium-3, an electron and an antineutrino. The

electron may carry away almost, but not quite, all the

energy if the neutrino has mass, and this tiny effect

can be measured from the electron’s energy spectrum.

Such experiments in Troitzk, Russia, and Mainz,

Germany, are being followed up with an international

project, the Karlsruhe Tritium Neutrino experiment

(KATRIN) which is 10 times more sensitive.

A very rare kind of beta-decay in which a nucleus

emits two electrons but no neutrinos is also being

investigated by several experiments based in

underground laboratories in France (Fréjus

Underground Laboratory) and Italy (Gran Sasso

National Laboratory). Neutrinoless double beta-decay

can happen only if the neutrino is its own

antiparticle (the emitted neutrinos then cancel each

other out), for which the neutrino must have mass.

By studying the subtle behaviour of nuclei we can

find out more about the basic properties of the

matter and energy that make up the Universe.

The building blocks of the UniverseOver the past 50 years, physicists have pieced

together a description, called the Standard Model

(SM) based on quantum theory, of the building

blocks of matter and known forces by which they

interact (see box). Although a wonderfully successful

framework for predicting particle interactions, the

SM is not complete: it doesn’t predict the masses of

particles and many other properties that have to be

put in ‘by hand’; it doesn’t explain why there are just

three particle families – and it doesn’t include the

fourth fundamental force, gravity.

To take the SM further, theorists rely on the

underlying mathematical principle of symmetry. A

powerful symmetry would be that which unifies all

the forces into one description – Grand Unification.

Particle theorists believe that when the Universe

came into existence in the Big Bang there was one

symmetrical ‘superforce’ which then broke up into

the forces we see today, as the Universe cooled. So

far, the SM unifies only the electromagnetic and

weak forces as the electroweak interaction. There are

now a number of ‘Grand Unification Theories’ going

beyond the SM, which need to be tested

experimentally. One way is to look for subtle nuclear

behaviour that deviates from SM predictions.

F U N D A M E N T A L I N T E R A C T I O N S ::

e know that nuclei are composed of protons and neutronswhich themselves are made up of more fundamental particlesheld together by a combination of powerful interactions

universal and the Nuclei

W

The cell used at

the Institut Laue

Langevin (above)

to store ultra-cold

neutrons while

measuring their

electric dipole

moment, which is

significant in

understanding

fundamental

interactions

The KATRIN experiment

to measure the mass of

the electron neutrino


Strange behaviour of quarksBeta-decays and rare decays also give us information

about quarks, leptons and yet-unknown forces that

may throw light on beyond-the-SM physics, and on

how symmetries broke in the early Universe. Quark

flavours, like neutrinos, can also ‘mix’ but according

to the SM, the total mixing must be a zero sum – an

effect called unitarity. Already small deviations from

unitarity have been discovered in experiments at the

Institut Laue Langevin in France measuring the decay

and the lifetime of the neutron (free neutrons

change into protons via beta-decay). Similar precise

experiments in laboratories around Europe are also

probing the decay of mesons (these contain just two

quarks) such as pions and other more exotic particles.

Looking into a mirrorA set of symmetries that are a sensitive probe of

the SM and its extensions describe what happens

when certain particle properties are reflected as

though in a mirror. There’s the charge mirror (C)

which changes particles into antiparticles of opposite

charge, the parity mirror (P) which changes the spin,

or handedness, of a particle, and the time (T) mirror

which reverses a particle interaction or process,

like rewinding a video.

Surprisingly, these mirrors don’t work perfectly.

For instance, electrons emitted in the beta-decay of

cobalt-60 always spin in the same direction even

when the spin of the cobalt nucleus is reversed.

Cracks in the C and P mirrors (CP-violation) also

appear in the decay of certain exotic mesons – the

kaon and B-meson. Electroweak theory within the

SM does, in fact, predict CP-violation.

:: SOME EXPERIMENTAL EXAMPLES■ Parity violation is seen in rare electronic

transitions in atoms. Although atoms are mostly

ruled by electromagnetism, the weak force makes

itself felt through the neutral Z particle which allows

a transition between states with the same spin,

otherwise forbidden. Precise measurements of this

transition in well-understood atoms such as

caesium-133 and other heavy isotopes will look for

tell-tale signs of beyond-SM phenomena.

■ The weak interaction responsible for beta-decay

recognises only left-handedness. More sensitive

measurements in other isotopes of different atoms

and of beta-decays may uncover traces of right-

handed effects that would have existed when the

forces were unified.

■ Violation of time-reversal symmetry is observed in

the beta-decay of neutrons and nuclei, and while a

small amount of T-violation is tolerated by the SM,

a large effect would indicate the need for a new

particle model.

■ Connected to CP and T-violation is the existence

of permanent electric dipole moments (EDMs) in

fundamental particles, nuclei and atoms: EDMs are

forbidden by P, T and CP symmetries, but might be

essential to explain the predominance of matter over

antimatter in the Universe. Laboratories worldwide

are actively searching for these EDMs.

These kinds of experiments typify the kinds of

precise measurements that can be made using

nuclear particles at low and medium energies. They

are playing a vital role in furthering our knowledge

of fundamental interactions.

The Standard ModelThe current picture describes matter as consisting of sixtypes, or ‘flavours’ of quark – called up, down, charm, strange,bottom and top – and six very light particles, or leptons – theelectron, muon, and tau – and their three neutrino partners.The 12 particles are divided into three families of increasingmass, each containing two quarks and two leptons. Eachparticle also has an antiparticle of opposite electric charge.

Everyday protons andneutrons comprise threequarks – two ups and adown, and two downsand an up, respectively.The Standard Model alsoincludes three of thefour fundamental forces,the electromagneticforce, and the weak andstrong interactions.These are carried by particles calledintermediate vectorbosons – respectively,the photon, the W and Zparticles and the gluon.

u c td s b

up

νe νµ ντe-neutrino µ-neutrino τ-neutrino

eelectron

µmuon

τtau

charm top

down strange bottom

lept

ons

quar

ks

w± z°photon

ggluon

?graviton

F E R M I O N S

STRONGWEAK

ELECTROWEAK

ELECTRO-MAGNETIC GRAVITY

γB O S O N S

forc

e ca

rrie

rs

forces

Preparing components for

the NEMO-3 neutrinoless

double beta-decay

experiment in the Fréjus

Underground Laboratory

in France


they interact with themselves! Yet a further level of

complexity comes from quantum theory which

allows ‘virtual’ particles to pop in and out of

existence from the vacuum. So, even though we

think of nucleons as being made of the lightest,

up and down quarks (as well as interacting gluons),

there are also contributions from virtual quark-

antiquark pairs. The result is that it’s very hard to get

a clear picture of protons and neutrons – let alone a

nucleus – in terms of fundamental interactions.

COMPUTATIONAL STRATEGIES AND MODELSFor this reason, theorists have developed a series of

theoretical pictures that portray what is going on at

a particular scale – and therefore energy – from

about one-tenth of a femtometre (the scale of

quarks) to 1 femtometre (the scale of nucleons). For

instance, one increasingly successful approach, which

rides on the back of the huge numerical power of

supercomputers, regards quarks and gluons as

interacting points and links on a lattice in space-

time. At lower energies, and thus longer scales, this

approach becomes increasingly difficult because the

interactions are so strong and complex. Other, more

approximate pictures are used to describe

interactions between the hadrons as involving clouds

of virtual pions. Eventually, theorists want to link up

the models into a complete QCD theory that will be

able to explain the confinement of quarks into

hadrons, how the pion clouds arise and hadrons

interact, and predict important properties such as

the masses and spins of the proton and neutron. In

this way, we will be able to explain the generation of

nuclear structure.

Inside the protonThese ideas, of course, need to be explored in

experiments. One of the most important ways is to

look at the quarks inside a hadron such as a proton,

by smashing it with another particle. In fact, this was

how evidence for quarks was first uncovered in such

‘deep inelastic scattering’ (DIS) experiments at the

Stanford Linear Accelerator Laboratory in California.

This work has been carried on using the HERA

How exactly does the strong force act to cause

quarks and gluons to ‘condense’ into protons and

neutrons, which then arrange themselves into

nuclei of everyday matter?

QCD, the theory of the strong forceTo start to answer this question, theorists have

developed a mathematical description of the strong

force called quantum chromodynamics (QCD). It is

what is called a quantum field theory similar to that

of electromagnetism describing the behaviour of the

electron and light – quantum electrodynamics (QED).

While QED is well understood, explaining electronic

phenomena in terms of positive and negative electric

charges, QCD requires another kind of charge called

colour, which can have three (rather than two) values

and which have, rather confusingly been named red,

green and blue. The attractive colour charges are

powerful, and actually increase with distance, so that

quarks are always tightly bound into colourless

entities called hadrons. These are either three-quark

systems (combinations of red, green and blue) such

as protons and neutrons, or quark-antiquark pairs (in

which the colours cancel out). The latter are known

as mesons of which the lightest is the pion.

However, the workings of the strong force are

more complicated because the gluons, which flit

between the quarks, also have colour charge, and

Q C D ::

N uclei are ultimately made of fundamental particles called quarks.These point-like particles prefer to cling together in twos and threes,tightly held by the strong force mediated by the exchange of gluons

nuclei together?What holds

u u

d

The spin structure of

a proton – quarks,

gluons and virtual

quarks all play a part

The COMPASS

experiment at CERN to

study the role of gluons

in the spin of a proton


collider machine (colliding

electrons with protons) at

the DESY Laboratory in

Hamburg. The aim is to map

the distributions of the

quarks, gluons and virtual

particles inside the proton.

WHERE DOES THE SPIN OF THEPROTON COME FROM?One particular problem being investigated is the spin

of the proton or neutron. Surprisingly, the quarks in

the proton account for only a fraction of its spin. One

experiment at DESY, called HERMES, has been using

polarised electrons and target nuclei (with spins all

in the same direction) to investigate spin. The results

seem to confirm that the proton’s spin is largely

made up of contributions from gluons. Further

experiments at DESY and CERN are also looking at

how the spins move in relation to the overall

direction of hadron spin.

GLUONS GET TOGETHERBecause gluons interact with each other, QCD

predicts that they can gather together to form

gluon-rich exotic entities called glueballs. Over the

past decade, experiments testing this idea with

beams of protons and antiprotons at CERN suggest

that there may be a whole spectrum of glueballs.

These will be investigated further with antiproton

beams made in the new High Energy Storage Ring

(HESR) to be built at GSI. Other possibilities are more

complex hybrids of quarks and gluons to form quark

‘molecules’. Just recently several laboratories

reported discovering a bound system of five quarks –

a ‘pentaquark’.

WHERE DOES THE MASS OF THE PROTON COME FROM?Quarks and gluons on their own can be considered as

simple, non-interacting massless points, so how are

the masses of quarks, when confined in hadrons,

generated? QCD predicts that, below a certain

energy, they acquire mass by

interacting with strong gluon

fields and with quarks and

antiquarks condensing out of vacuum

– a process called chiral symmetry

breaking. This idea is being tested in various ways.

One approach is to study how heavier quarks (which

give simpler information than light quarks) bind with

each other. Mesons consisting of charmed quark-

antiquark pairs, known as charmonium, will be made

in abundance with HESR, and will provide a unique

testing-ground for this and other QCD predictions.

This research will complement observations of the

signatures produced by heavy mesons in high-energy

experiments designed to liberate quarks completely

from their gluon chains. The ALICE experiment using

the Large Hadron Collider being built at CERN will

collide heavy ions at high energies to create a soup of

free quarks and gluons (p.20).

Applying QCD to nucleiThe effective force between nucleons in nuclei are

currently described in terms of pion exchange, but a

long-term aim of nuclear theorists is to predict

nucleon interactions at a more basic level

of QCD theory. This will give us a better

understanding of nuclear structure (p.8)

and the build-up of elements in stars

(p.12). Unique information about the

strong force can be obtained by studying

‘hypernuclei’ containing a strange or

charmed quark. These are just some of the

exciting experiments planned to test QCD

at lower energies at which the strong

force is least understood.

The consequences of QCD at low energies are

studied in a variety of experiments that probe

the properties of hadrons, for example, the

KLOE experiment (left) using the DAΦNE

electron-positron accelerator complex (inset) at

FNL in Italy and the MAMI electron accelerator

(below) in Mainz, Germany

Detection of glueballs in

the Crystal Barrel

Experiment at CERN

Fe

de

rici

/ L

NF

IN

FN


arrangement of the nucleons they contain. Theories

of nuclear structure (p.9) can be tested by preparing

nuclear species with extreme ratios of protons and

neutrons that are teetering on the edge of existence

(the neutron and proton driplines), or are as heavy

as possible. Unstable nuclei with particular

configurations, such as equal numbers of protons

and neutrons, may also reveal some of the more

complex aspects of nuclear forces.

:: ASTROPHYSICSConditions in stars are very different from those on

Earth; their hot, turbulent environments host the

nuclear reactions that build up the elements. The

reaction pathways are thought to involve highly

unstable nuclei – perhaps very neutron-rich, or even

proton-rich (p.12). Radioactive beams are the ideal

experimental tool for testing current ideas about

nucleosynthetic pathways.

:: FUNDAMENTAL FORCESOne way of probing the fundamental forces that

govern matter is to measure the subtleties of decay

processes in various nuclei, such as beta-decay (p.22).

Radioactive beams can provide pure sources of useful

unstable nuclei, which can then be stored and

studied in special magnetic ‘traps’.

:: APPLICATIONS A rewarding use of beams of radioactive ions is as

diagnostic and imaging tools in a range of areas such

as medicine and electronics (p.16). Cancer therapy

with radioactive-ion beams is an exciting new area.

Radioactive-beam generation is also key to

developing new methods of destroying nuclear

waste by transmutation – a new approach that could

transform society’s view of nuclear physics.

R A D I O A C T I V E I O N B E A M S ::

exoticnucleiProducing

The previous pages have shown how our

understanding of nuclei has developed by testing

their behaviour under extreme conditions. For example,

many experiments involve observing nuclei with

unusual ratios of protons and neutrons, or subjecting

nuclear systems to high temperatures and pressures.

Until a few years ago, experiments were mostly limited

to studies of nuclei that were stable or nearly so.

Today, nuclear physics studies, as well as applications,

are being dramatically extended by facilities that

generate unstable nuclei in intense beams.

Why we need radioactive beams :: NUCLEAR STRUCTURE The nuclear map on p.8 shows that while only a

limited number of stable nuclei exist naturally on

Earth, a much larger number can survive for varying

amounts of time depending on the numbers and

urope is developing a new generation of radioactiveion beams that will lead to a deeper understandingof Nature and new technologies E

GANIL’s SPIRAL accelerator

(above) and the source for

producing exotic ions and

the VAMOS detector (inset)

M. D

ésa

un

ay

/ G

AN

IL


How to produce radioactive beamsThe first dedicated source of accelerated radioactive

beams was at CYCLONE in Louvain-la-Neuve,

Belgium. Using innovative technologies, the

laboratory started in 1989 with a nitrogen-13 beam

producing oxygen-14, important in astrophysical

processes. The underlying method to produce the

isotopes was developed at CERN, with the ISOLDE

facility, which has delivered more than 600 isotopes

of 70 elements in the past 30 years. A new

radioactive beam facility at GANIL, France called

SPIRAL started in 2001.

These facilities employ the ISOL (isotope

separation online) method to produce the beams.

Particles such as protons or ions are accelerated

(for example, in a cyclotron) and directed at a thick

target of suitable material where they produce new

radioactive atomic species. These are then ionised

and separated according to mass by various electro-

magnetic devices. The selected isotope is then re-

accelerated and directed towards the experiment.

There is an alternative approach called the

in-flight method in which an energetic beam of

heavy ions undergoes fragmentation or fission while

passing through a thin target. The radioactive ions

produced are then separated and kept in a storage

ring for experiments. GSI in Germany uses its heavy-

ion accelerator complex to produce radioactive

beams in this way.

The two systems are considered to be

complementary. The ISOL method generates pure,

high-intensity, high-quality beams of isotopes near

stability, which are suitable for a very wide range of

applications – from measurements of astrophysical

importance to sensitive low-energy experiments and

medical applications. The in-flight method produces

less intense beams of nuclei at higher energies. In

theory, it can be used to produce isotopes of any

element. It is a faster method so is suitable for

producing very short-lived nuclei, either as single

beams, or as cocktails of several similar nuclei; their

masses and other properties can be measured

simultaneously in the storage ring. Single beams can

also be prepared for collider experiments.

The radioactive beam layout

at CYCLONE, Louvain-la-Neuve

ISOL method In-flight method

Accelerator

Thickproduction

target

Thinproduction

target

Accelerator

Fragmentseparator

Storage ring

Experiments

Ion source

Isotopeseparator

Postaccelerator

Experiments

The two main methods for

producing radioactive ion beams

Radioactivelaboratory

ISOLDE CERN

Robot1-1.4 GeV protons

GPS

HRS

REX-ISOLDE

New extension

Experimental hall

Controlroom

>>


Future European facilities Europe has a long tradition and leadership in

radioactive beam production, and its nuclear physics

community is now planning the next generation of

facilities. The aim is to generate much more intense

beams and improve detector technology. Post-

accelerated radioactive beams at ISOLDE (REX)

at CERN and SPIRAL at GANIL now offer new

opportunities in terms of secondary-beam energies

and intensities for nuclei with light-to-medium masses.

The EXCYT (Exotics with Cyclotron and Tandem)

project at Catania will use a tandem post-accelerator.

Another approach being developed at the University

of Munich (MAFF) will exploit the high flux of

neutrons from a new research nuclear reactor in

Garching to fission uranium-235 into neutron-rich

fragments. These are then ionised, separated and

cooled, and then accelerated in a linear accelerator.

Further ahead, two major European projects are in

progress. One will be sited at GSI. It will produce

secondary beams, through the fragmentation of

primary ion beams of all the elements up to

uranium, which are 100 times more intense than is

possible at the moment. New-generation storage

and cooler rings will offer unique world-wide

experimental opportunities especially for the

shortest-lived nuclei.

The other large project is EURISOL, the next-

generation ISOL facility with the most intense,

high quality, post-accelerated secondary beams ever

produced in the energy range up to 100 MeV. This

challenging project is a joint undertaking of many

laboratories. Planned intermediate facilities, SPIRAL II

at GANIL and SPES at INFN-LNL Legnaro, together

with further upgrades of the REX-ISOLDE facility,

represent demonstration steps towards the

construction of EURISOL.

In this way, the European science community will

have access to a network of advanced radioactive

beam facilities that will cover the wide range of

experiments needed to push back the frontiers not

only in nuclear physics research but also in other

areas that depend upon the science and technology

that radioactive beams can deliver.

R A D I O A C T I V E I O N B E A M S ::

Detectors for radioactive beamsSince radioactive beams are not as intense as stable ones, highly efficientdetectors are needed to gather experimental results. A range of detectorsis being developed for the next generation of radioactive beam facilities.Amongst the most important are those that detect gamma-rays. Themost efficient are crystals of the semiconducting material germanium,which are arranged in position-sensitive spherical arrays around theexperiment. Individual detectors are often built by internationalcollaborations and shared between several laboratories.

The EXOGAM detector

at GANIL (inset)

The DEMON neutron detector

(below) is shared among several

exotic-ion beam facilities

GA

NIL


nuclear physics gave her a golden opportunity to do

just that. “I preferred nuclear physics because it was

related to what I was interested in: philosophical

questions such as what the Universe is built of, but

also applications,” says Konstanze. GSI’s radiobiology

department offered her the chance to work on its

heavy-ion tumour therapy project (p.18). Konstanze’s

project involved measuring the radiation dose

contributed by fast neutrons when a beam of

carbon-12 ions is used to treat a tumour. Using a

volume of water as a ‘phantom’ target, she could

measure the production rates and energy

distributions of neutrons generated by nuclear

fragmentation – a classical nuclear physics

investigation. She also compared the light particles

produced during the treatment of patients with

those from the phantom target. The results showed

that the dose contribution from fast neutrons was

less than 1 per cent of the total dose applied to the

tumour volume, so did not affect the success of the

carbon-12-ion tumour therapy. Konstanze hopes to

continue working in medical physics, either in

academia or in the clinic. “The great satisfaction is

knowing that I am helping people as well as carrying

out research,” she says.

:: Richard Woolliscroft, from York University in the

UK, decided to do nuclear physics because he wanted

the opportunity to work at a fundamental level,

where he could carry out original measurements and

calculations that would be used to test and expand

on the known models. “The main benefit was being

able to set my work standards at a high level. I am

Many young people go into physics because they are

fascinated by questions such as: how did the

Universe evolve and what is it comprised of? Nuclear

science not only aims to answer these questions but

also covers a very wide range of applications, from

medical therapies and energy production to art

conservation. Because it touches on a wide range of

disciplines – for example, particle physics, astrophysics,

condensed matter physics, engineering, computing,

and biology – PhD students have the opportunity to

widen their scientific expertise. Another advantage is

that nuclear research generally involves well-defined

experiments that can be completed within the time-

span of a doctorate. At the same time, students get

the chance to travel to international laboratories

and work within interdisciplinary (often multi-

institutional and multi-national) teams.

The result is that young nuclear physicists develop

an excellent range of transferrable skills:

■ General problem-solving abilities needed in

management and industry;

■ Mathematical and computer skills which can be

applied in the financial and commercial sectors;

■ Engineering expertise in areas such as vacuum

technology, semiconductors and data-processing;

■ The ability to work in a team, and the experience

and skills required to work in a multidisciplinary,

international environment, which is essential in

business today.

:: Konstanze Gunzert-Marx wanted to combine

her physics studies at the Technical University of

Darmstadt in Germany with biology, and a PhD in

Why become a nuclear physicist?N uclear physics offers an attractive

research area for graduate students

>>

E D U C A T I O N A N D T R A I N I N G::

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:: Jean-Sébastien Graulich carried out his PhD at

Louvain-la-Neuve in Belgium, using the radioactive

beam facility there to study nuclear reactions

thought to happen in explosive stellar

nucleosynthesis, in particular on the conversion of

fluorine-18 to oxygen-15. Afterwards he went to CERN

to work on a new detector for the HARP experiment

there, which will study hadron production in

preparation for the construction of a neutrino

factory. “My PhD in nuclear physics built up a good

basic knowledge in physics, and the ability to work

within international collaboration. It also gave me

the experience of working on a smaller-scale project

from the very beginning up to the end,” says Jean-

Sébastien. He then spent a year working in a

company developing digital X-ray imaging systems

for medical and industrial applications. He is now

back at CERN, working again on detector

development and production.

:: Zoran Radivojevic found that the broad skills

developed through his PhD research on nuclear

structure and neutron detectors were invaluable

to the electronics industry. Working at the Finnish

Accelerator Laboratory in Jyväskylä (JYFL) and in

collaboration with laboratories across Europe, he

worked on the design of a wide range of detection

systems. Part of the research for his PhD thesis

involved developing computer simulations for

describing the transport of neutrons in different

materials to optimise the detection processes and

detector design. In 2001, Zoran started work on novel

silicon detectors for the ALICE experiment at CERN

(p.21). The flexible miniaturised technology involved

has proved to be attractive to the commercial

microelectronics industry, and Zoran has now

joined Nokia Research, carrying out simulations to

optimise mobile phone design. He also organises

collaborations between several large industrial

semiconductor laboratories and universities in

experimental and simulation research. “This kind of

job demands a multi-skilled background in physics,

computation and organisational capabilities,”

says Zoran.

also appreciative of the time spent at international

conferences and laboratories, meeting other

physicists and improving my team-working and

communication skills,” says Richard. After his PhD,

Richard used the experience gained of programming

and data-handling, in going to work as a scientific

programmer for a biotechnology company. He

worked with biologists and chemists, finding that

his scientific background allowed him to understand

rapidly the particular needs of the company and the

scientists he worked with. More recently he has

returned to the field of physics and now works as a

developer of data acquisition software at the new UK

synchrotron facility, Diamond, which will be used

largely for applications in biology and chemistry.

E D U C A T I O N A N D T R A I N I N G ::

Public awareness of nuclear scienceMany people associate nuclear science just with the negative aspects ofnuclear weapons and nuclear waste, but as the highlights in this booklet haveshown, nuclear research is not only concerned with the most exciting scientificendeavours – that of understanding Nature at the deepest levels, but alsounderpins some important applications.

The European nuclear physics community aims to improve public awarenessof these studies – through open days at laboratories and teaching projects forschools, and via an outreach programme, Public Awareness of Nuclear Science(PANS). The PANS team has a travelling exhibition on radioactivity, andsupported the publication of a unique coffee-table book, Nucleus – A Trip to theHeart of Matter, written by Ray Mackintosh, Jim Al-Khalili, Bjorn Jonson andTeresa Pena. PANS also has an EU-funded project, NUPEX, to produce a web-based resource on nuclear physics for schools. It will cover many aspects fromfundamental physics and astrophysics to nuclear medicine. Further informationabout PANS can be obtained at www.pans-info.org

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>>

Konstanze Gunzert-Marx

Jean-Sébastien Graulich

Zoran Radivojevic

Richard Woolliscroft

Accelerator Laboratory (JYFL)JyväskyläFinland

Kernfysisch Versneller Instituut(KVI)GroningenThe Netherlands

The Svedberg Laboratory (TSL)UppsalaSweden

Forschungszentrum Jülich (FZJ)JülichGermany

Centre de Recherches du Cyclotron(CRC)Louvain-la-NeuveBelgium

Grand Accélérateur Nationald'Ions Lourds (GANIL)CaenFrance

Gesellschaft fürSchwerionenforschung (GSI)DarmstadtGermany

FINUPHYCoordinator Professor Jean VervierTel +32-10-473273 (secretary)Fax +32-10-452183E-mail [email protected] www.finuphy.org

P A R T I C I P A T I N G L A B O R A T O R I E S

Institut de RecherchesSubatomiques (IReS)StrasbourgFrance

ISOLDE collaborationCERNGenèveSwitzerland

European Center for TheoreticalStudies in Nuclear Physics andRelated Areas (ECT)TrentoItaly

Laboratori Nazionali di Legnaro(LNL)PadovaItaly

Laboratori Nazionali di Frascati(LNF)FrascatiItaly


contacts

GANIL

ISOLDEIReS

GSIFZJ

KVI

LNF

LNLECT

TSL

JYFL

CRCGANIL

ISOLDEIReS

GSIFZJ

KVI

LNF

LNLECT

TSL

JYFL

CRC

This booklet was produced byFINUPHY, an InfrastructureCollaboration Network supportedby the EU Fifth FrameworkProgramme. The Networkoperated from 1 October 2000 to30 September 2004.

Editor Professor Brian Fulton,University of York

E-mail [email protected] Nina HallE-mail [email protected] Pete Hodkinson, SpacedE-mail [email protected]

Further information and copies of thebooklet can be obtained through DieterMüller at GSI, Darmstadt in Germany.Dieter MüllerGesellschaft für SchwerionenforschungPlanckstraße 164291 DarmstadtGermanyE-mail [email protected]


FINUPHYAbout

rontiers In NUclear PHYsics (FINUPHY) is an

Infrastructure Cooperation Network supported by

the European Commission under its Fifth Framework

Programme(FP5). It includes representatives from 12

research institutes in nuclear physics.

FINUPHY aims to promote collaboration and

coordination between the research institutes

through regular round table meetings, and through

joint scientific and technological activities and

studies. It also promotes information on advances in

research and technical development programmes in

nuclear physics.

FINUPHY is particularly interested in education and

public outreach, and supports the PANS (Public

Awareness of Nuclear Science) activity, p.30. This

booklet is a further contribution by FINUPHY to

communicate to a wider audience the wide-ranging

and exciting programme of nuclear science carried

out in Europe.

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