00 Month 2010 | NewScientist | 17

CosmiC raysFrank Close

INSTANTEXPERT

26

6 October 2012 | NewScientist | iii

Every moment of every day, the Earth is bombarded with particles from outer space. Known as cosmic rays, they have revolutionised our understanding of matter at the subatomic scale.

Radiation fRom above

VF

Hes

s so

Ciet

y/eC

Ho

pHys

iCs/

sCH

loss

pö

llau

/au

stri

a

Radioactivity has fascinated scientists ever since its discovery in 1896. its ability to ionise air made it easy to detect and led to a surprising discovery: even when no radioactive source was present, detectors revealed the presence of some other radiation that was ionising the air. this radiation even showed up out at sea, with no radioactive rocks in sight. furthermore, it was very powerful, able to penetrate shielding around the lab apparatus. another source of unknown rays must exist, and of immense penetrating power – but where?

the first clues came when theodor Wulf, a physicist and Jesuit priest, ascended the eiffel tower and found more radiation up there than he expected. He surmised that these were rays of extraterrestrial origin, and suggested ascending to great heights in balloons as a way of testing the idea. but the spirit of adventure seemed to desert him, and it was left to others, notably the austrian physicist victor Hess, to take the risk.

Hess made 10 ascents in 1911 and 1912 and found that the intensity of the rays increased rapidly above 1000

tHe disCoVery oF CosmiC rays

Victor Hess took to the skies in a balloon and discovered cosmic rays

When a cosmic ray shoots down through the upper atmosphere, its collisions with atoms in the air generate an avalanche of particles (see diagram, left). most of the particle shower is absorbed before reaching the earth’s surface. Whereas each square centimetre of the upper atmosphere is hit by about 20 particles every second, on average, at sea level only a feeble drizzle remains: a mere 1 per minute.

Particles such as pions, and others like them that respond to the strong nuclear force, tend either to be absorbed or to decay into electrons, muons, photons and neutrinos, which penetrate further. the electrically charged particles go on to create their own showers of electrons, positrons and photons. the total energy of the initial cosmic ray is thus shared among ever more constituents, most of which never reach the ground.

anatomy oF a CosmiC ray sHower one exception are muons, which

are essentially heavy electrons. they can punch through the atom-filled atmosphere and reach ground level, even penetrating the soil. if you visit a science exhibition and see a spark chamber recording the passage of cosmic rays, it is most likely muons that are triggering it.

neutrinos penetrate the most, often passing right through the earth to fly out the other side. as you read this, neutrinos from cosmic rays hitting the far side of the planet are emerging from beneath your feet and zipping through your body. Yet more

metres. at an altitude of 5000 metres, their intensity was some five times greater than at sea level. Hess concluded that a powerful radiation originates in outer space and enters the earth’s atmosphere, diminishing in intensity as it passes through the air.

it is Hess who is traditionally credited with the discovery of cosmic rays, for which he won the nobel prize in physics in 1936. the evocative name “cosmic rays” was coined by US physicist Robert millikan in 1925.

initially millikan had doubted Hess’s claims, but this changed in the 1920s when millikan made measurements of his own. He invented an electrometer whose readings were recorded on moving film. this enabled the apparatus to be lofted on unmanned balloons, and extended the measurements to very high altitudes. by 1926 millikan was convinced of the existence of cosmic rays, even going so far as to claim the discovery for himself.

are showering down over your head.in very extreme cases, an incoming

cosmic ray particle may have 10 million times the energy of the beams at our most powerful particle accelerator, the Large Hadron Collider at CeRn, near Geneva, Switzerland. Such energy can spawn millions of secondary particles and this shower can spread out over several kilometres. even so, the shower preserves the overall direction of the main thrust. by measuring the relative arrival times of particles at several widely separated locations, it is possible to determine the direction of the primary cosmic ray to within a few degrees.

ii | NewScientist | 6 October 2012

in 1927, dmitry Skobeltsyn of the Leningrad Physicotechnical institute in the Soviet Union was studying radioactivity using a cloud chamber, a device that makes the invisible visible. Cloud chambers are sealed vessels filled with water vapour on the cusp of condensing. When a charged particle such as an electron zips through, it ionises the vapour and this causes water

droplets to form along the trail. Skobeltsyn was using his cloud chamber with

a powerful magnet, which steered the relatively slow-moving electrons in tight circles. He noticed that some trails were nearly straight, though, showing that they had very high momentum far beyond those from any source known at the time. Unknowingly, he had become the first person to directly observe the trails of cosmic rays.

three years later at the California institute of technology in Pasadena, Robert millikan’s student, Carl anderson, built a cloud chamber surrounded by very powerful magnets specifically to study cosmic rays. much to his surprise, he found some trails that curved as if they were electrons with positive electric charge. anderson had discovered the positron, the antimatter analogue of the electron, which had been predicted earlier by british theoretical physicist Paul dirac.

Positrons are not some peculiar extraterrestrial phenomenon. Patrick blackett and Giuseppe occhialini proved this in 1932 when they found that positrons made up roughly half of the particles produced when a cosmic ray struck a metal plate inside a cloud chamber, with the other half electrons.

anderson and another colleague later went on to discover that some trails penetrated the cloud chamber much further than electrons, and did not create such showers. these were due to a particle that seemed like a heavy version of the electron. this was the discovery of the muon.

the positron and muon were the first of a series of discoveries that showed earth-bound physics had sampled only a small part of nature’s rich pageant. by the 1950s cosmic rays had revealed yet more new particles that existing theories could not explain.

the pion was discovered in 1947. this, at least, had been predicted, whereas a family of “strange” particles had not. the year marked an explosion of strange-particle finds, with the kaon, lambda, xi and sigma being discovered over the next six years.

in order to understand their nature, physicists built particle accelerators, which in effect simulated the interactions of cosmic rays, but under controlled conditions. So the discovery of cosmic rays led to the modern field of high-energy particle physics.

partiCle explosion

NeutronNucleusPositive pionNegative pionProtonPhotonNeutrinoNegative muonPositive muonElectronPositronNeutral pion

1

-1

2

-2

4

6

8

10

20

30

40

50

100

200

300

400

Height (km)

Cosmic showerAn incoming cosmic ray initiates a shower of particles in the upper atmosphere. Most of them are absorbed before they reach the ground

PRIMARY COSMIC RAYS (mostly protons, also

some nuclei)

STRATOSPHERE

OXYGEN OR NITROGEN NUCLEUS

COSMIC RAY DETECTORS

UNDERGROUND DETECTOR (e.g. Ice Cube)

INTERNATIONAL SPACE STATION

(ALTITUDE 350 KM)

COMMERCIAL AIRCRAFT

(10 KM)

SEA LEVEL

abo

Ve:

Go

ron

wy

tud

or

Jon

es, u

niV

ersi

ty

oF

birm

inGH

am

/spl

, baC

kGro

un

d: b

ettm

an

n/C

orb

is

6 October 2012 | NewScientist | v

A century after their discovery, we still have a relatively poor understanding of where cosmic rays come from and how they reach Earth. Despite this, cosmic rays have revealed examples of matter previously unknown, such as the muon, positron and the group known as strange particles. The hunt is now on for dark matter and other possible exotic forms, known to nature but not yet to us.

CoSmiC RaY eniGmaS

wHere do tHey Come From?

Antarctic ice is studded with detectors making up the IceCube experiment

Awesome energyCosmic ray protons hitting Earth have energies for surpassing those we can produce in accelerators

Energy (electron volts)1 10 105 1010 1015 1020

Most energetic cosmic ray

Theoretical energy limit (GZK limit)

Protons at Large Hadron Collider

Rest-mass energy of proton

SOU

RCE:

PAR

TICL

E DA

TA G

ROU

P

Cosmic ray is a catch-all name for radiation of extraterrestrial origin. We now know that over 95 per cent of the “primary” cosmic rays that crash into earth’s upper atmosphere are high-energy protons, with helium and other nuclei making up the remainder. Primary cosmic rays originate in deep space, but where exactly remains a mystery. theories

favour huge explosions, such as supernovae and gamma ray bursts, or supermassive black holes.

one reason why it is hard to identify the source of primary cosmic rays is that the vast majority of them are charged particles and so they are deflected by the magnetic fields that thread through space. by the time they arrive at earth they come randomly from all over the sky.

Clues come from cosmic rays’ collisions with gas in interstellar space, which create electrically neutral pions, gamma rays and neutrinos. all three are unaffected by magnetic fields but gamma rays preserve their direction of travel best. High-energy gamma rays have been spotted coming from remnants of supernovae by naSa’s fermi space telescope. Young supernovae seem to possess both the strongest magnetic fields and emit the highest-energy cosmic rays. these observations support enrico fermi’s suggestion, made in 1949, that cosmic rays get their high energies as a result of interactions between particles and wandering magnetic fields.

the latest theory for the source of the biggest hitting cosmic rays – with energies of around 100 million teraelectronvolts (tev) – is supermassive black holes at the centres of galaxies. the previous candidate, gamma ray bursts, which are believed to arise from the collapse of massive stars to black holes, looks less likely thanks to observations from the iceCube neutrino observatory at the South Pole. it was thought that these catastrophic collapses would spew out protons and accelerate them to vast energies. these protons would interact with the gamma rays, producing pions and high-energy neutrinos. iceCube has been looking for these energetic neutrinos but has so far found none, suggesting there must be a different source for 100 million tev cosmic rays.

Awesome energyCosmic ray protons hitting Earth have energies for surpassing those we can produce in accelerators

Energy (electron volts)1 10 105 1010 1015 1020

Most energetic cosmic ray

Theoretical energy limit (GZK limit)

Protons at Large Hadron Collider

Rest-mass energy of proton

SOU

RCE:

PAR

TICL

E DA

TA G

ROU

P

iv | NewScientist | 6 October 2012

How nature accelerates particles to extreme energies far beyond anything possible with purpose-built accelerators has long been one of the main questions in astrophysics. the Large Hadron Collider at CeRn is our most powerful accelerator, smashing proton beams each carrying 4 teraelectronvolts of energy. Cosmic rays can reach 100 million tev (see chart, below right). although the flux of particles arriving at earth with energy exceeding that is small, such particles pervade the cosmos and so in total represent a huge amount of energy.

Cosmic ray protons with these extreme energies may be the result of acceleration over long distances, perhaps involving several stages, rather than being the result of a single dramatic event. the strong magnetic fields in young supernovae, for example, can keep the highest-energy particles in the remnant’s shock wave long enough to speed them to the energies observed.

However, theory says that there should be a rapid fall off in the number of protons with energies above 50 million tev coming from distant sources. that’s because the cosmos is filled with photons left over from the big bang, known as the cosmic microwave background. Protons of very high energies are unable to travel more than about 150 million light years

extreme enerGies

antimatter and matter were created equally from the energy of the big bang, theory has it. When british physicist Paul dirac predicted antimatter, he remarked that there could be as many stars made of antimatter as of matter, if the laws of nature are the same for both. individual particles of antimatter have indeed been seen in cosmic rays and created in experiments at particle accelerators. but there is no evidence for antimatter in bulk in our galaxy, nor throughout the observable universe.

if anti-stars had exploded in the cosmos, we would expect some examples of anti-nuclei to appear in cosmic rays. So far nothing even as complicated as a nucleus of anti-helium has been found.

Perhaps earth’s atmosphere is confounding us. to find out, physicists have built an experiment called the alpha magnetic Spectrometer and mounted it on the international Space Station (see picture, below). amS records thousands of cosmic rays each second. being above the atmosphere, the particles are from primary cosmic rays and give information about the make-up of the wider universe. amS is searching for antimatter and other exotic objects, including the constituents of mysterious dark matter.

dark matter is an enigma. Cosmologists have inferred the existence of this invisible stuff, but we know nothing about its constituents other than that they must be massive and electrically neutral. if such

particles exist, then they are new to science. dark matter outweighs the visible matter in the universe by a ratio of about 4 to 1, and so one may expect that examples may be present in cosmic rays.

the challenge is to detect “dark particles”. they do not feel the forces that other particles do and will give themselves away, like H. G. Wells’s invisible man – by jostling the crowd. deep underground, shielded from the majority of cosmic rays, supersensitive detectors seek evidence of a dark particle hitting an atom and making its nucleus recoil. this is exceedingly difficult to detect, and it is essential to reduce natural radioactive backgrounds to a minimum to give any genuine signal a chance of showing up.

antimatter and dark matter

before interacting with these photons. Such interactions produce pions and lead to a loss of energy. in effect, the universe is not transparent to ultra-high-energy protons, so they cannot penetrate space and reach earth as cosmic rays. this is known as the GZK cut-off, after Kenneth Greisen, Georgy Zatsepin and vadim Kuz’min, who first pointed out that this phenomenon should occur.

even so, experiments occasionally claim to spot cosmic rays with energies that exceed this limit. one possibility is that these particles have been produced within 150 million light years of us and hence their numbers have been dimmed somewhat but not entirely cut off. While this cannot be ruled out, no obvious sources have been identified within that radius.

the evidence for such ultra-high-energy cosmic rays remains controversial. determining the energy of cosmic rays up to and beyond the GZK limit is one of the outstanding challenges in the field.

na

sa

baCk

Gro

un

d: iC

e Cu

be/n

sF, r

iGH

t: n

sF

neutrinos emitted by the sun pass through the earth continuously. these solar neutrinos are traditionally not regarded as cosmic rays. Collisions of cosmic rays in the upper atmosphere spawn neutrinos similar to solar ones but also produce neutrinos of other flavours. by comparing these cosmic neutrinos with their solar counterparts, we have gained fundamental insights into the nature of these enigmatic particles. the findings have revealed that neutrinos have mass. the challenge for physicists is to explain why.

CosmiC neutrinos

6 October 2012 | NewScientist | vii

Particles from outer space have taught us about the inner workings of the subatomic world and promise to solve other puzzles on a cosmic scale. Closer to home, however, we have learned to harness cosmic rays in ingenious ways. They are helping us to unravel secrets of our past. Yet, ultimately, they may limit our future ability to explore the solar system.

CoSmiC RaYS and Life

time Capsulesover the aeons, cosmic rays have left an imprint in the antarctic snow from which we can determine the history of the sun and possibly our climate.

Primary cosmic rays from deep space have to penetrate the sun’s solar wind before being caught by the earth’s magnetic field and hitting the upper atmosphere. So the solar wind acts like a protective shield, called the heliosphere, which fills the solar system and deflects cosmic rays.

” Cosmic rays may affect cloud formation and, through this mechanism, earth’s climate”

Ice cores hold a record of the isotopes made by cosmic rays

However the sun’s activity goes through cycles and this influences the intensity of cosmic rays arriving at earth. When the sun is quiet, the heliosphere is weaker, which enables more cosmic rays to penetrate the solar system and collide with atoms in the earth’s atmosphere. the collision between incoming cosmic-ray protons and atmospheric oxygen nuclei leads to nuclear transmutations and, in particular, to two isotopes of beryllium: beryllium-7 and beryllium-10. the momentum of the primary ray is transferred to the beryllium isotopes which fall to earth. any of these isotopes landing in antarctica are deposited in the snow, “footprints” which accumulate in layers over the centuries.

beryllium-10 has a half-life of about 1.4 million years and decays to boron-10, while beryllium-7’s half-life is a mere 53 days before decaying to lithium-7. the ratio between the two isotopes in the ice today reveals how long has elapsed since they were formed in the atmosphere. by drilling ice cores and gathering samples at various depths in the antarctic, it may be possible to measure the concentration of beryllium isotopes, and deduce how solar activity has varied over thousands of years.

many scientists suspect that whereas solar activity has had only a minor role in climate change over the last century, it may have played a much more significant role over many centuries. the beryllium footprints left by cosmic rays may reveal if there is indeed a relationship between solar activity and climate change on earth.

the idea that there is a correlation between solar activity and the earth’s climate has been around for nearly half a century, but it has never been totally convincing. in recent years, Henrik Svensmark at the danish national Space institute in Copenhagen and colleagues have proposed that solar variations modulate the cosmic ray intensity at the earth, which in turn may affect cloud formation and, through this mechanism, climate. an experiment at the CeRn laboratory near Geneva, Switzerland, is now attempting to test this hypothesis.

Cosmic-ray protons passing through the atmosphere can ionise volatile compounds, leading to airborne droplets, or aerosols. these droplets may then become the seeds for clouds. While these basic facts are generally agreed, it is not known whether cosmic rays play a significant role in cloud formation on a large scale.

CeRn physicist Jasper Kirkby is leading an attempt to study the effects that cosmic rays may have on atmospheric chemistry. the experiment is called Cosmics Leaving oUtdoor droplets (CLoUd, see picture, above left). it consists of a custom-built chamber filled with ultra-pure air and laced with the molecules that are believed to seed clouds, such as water vapour, sulphur dioxide, ozone and ammonia. Protons are then fired into the chamber in an attempt to simulate collisions between cosmic rays and the real atmosphere.

after this irradiation, the CLoUd team samples the “atmosphere” in their apparatus to see what effect the protons have had. the experiment is ongoing, but early results suggest that collisions between high-energy protons and the CLoUd atmosphere lead to the copious production of nanometre-scale

Cloud in a bottle

droplets. these are far too small to serve as seeds for clouds, however.

While this is an important first step, it says little so far about a possible cosmic-ray effect on clouds and climate.

CLoUd will continue to take data, probably for at least five years, and the apparatus will be refined as researchers learn more about the impact of protons in its atmosphere. they are also planning experiments with larger aerosol particles in CLoUd’s atmosphere, which they hope will eventually produce artificial clouds large enough to study. this will hopefully settle the question one way or the other, says Kirkby.

the technique of radiocarbon dating is used widely in archaeology and is possible thanks to cosmic ray collisions in the atmosphere. the collisions produce a number of unstable isotopes, such as carbon-14, which has a half-life of 5730 years. Were it not for cosmic rays replenishing carbon-14 in the atmosphere, this isotope would long ago have disappeared from earth. When plants die or are consumed by humans or animals, their accumulation of carbon-14 stops. over time, the concentration of this isotope falls. Comparing the relative amounts of carbon-14 and stable isotopes in an archaeological artefact allows its age to be determined.

RADIOCARbON DATINg

When a cosmic ray passes through an electronic circuit, its energy may cause a transient error to occur. this can be a problem for the electronics found in satellites, spacecraft and even aircraft. a cosmic ray may have caused the flight-control system of a Qantas flight to malfunction in 2008. the aircraft plunged hundreds of metres, causing injuries to many of those on board, but was landed safely.

Software systems in aircraft have since been redesigned to average out sudden power spikes of the kind that a cosmic ray might induce. a cosmic ray is believed to have caused a malfunction two years ago aboard the voyager 2 spacecraft, which was launched in 1977.

ElECTRONIC DAMAgE

the accumulation of radiation from exposure to cosmic rays can ionise molecules, leading to adverse health effects. Cosmic rays are a significant fraction of the radiation exposure of humans, about 15 per cent of the total background at sea level. this rises rapidly with altitude, however, and in high places such as denver and mexico City can be up to five times as high.

airline crews, who fly long-distance high-altitude routes regularly, may receive double the exposure to ionising radiation. above the atmosphere, the ubiquitous presence of cosmic rays is a major impediment to human space exploration. in a round trip flight to mars, crew members would be exposed to accumulated radiation at the limits of what radiological studies regard as safe.

bIOlOgICAl DAMAgE

Denver’s high altitude means greater exposure to cosmic rays

vi | NewScientist | 6 October 2012

baCk

Gro

un

d: p

an

or

am

iC im

aGes

/Get

ty,

riG

Ht:

ka

ren

ka

smau

ski /

Get

ty,

to

p: d

aVe

sto

Ck

Co

tto

n C

ou

lso

n/n

Gs

viii | NewScientist | 6 October 2012

a GoLden aGe Our exploration of cosmic rays 70 years ago led to today’s experiments at colossal, high-energy particle accelerators. Now, the large Hadron Collider at CERN enables collisions at about 10 teraelectronvolts, but this is only halfway between the heat energy of a summer’s day and the extreme energy found in the moment after the big bang. We would dearly love to explore the very extreme energy range, because theory tells us that’s where quantum mechanics and gravity meld into one “theory of everything”. There is no conceivable

way to reproduce it in terrestrial laboratories, so if we are to explore such extremes it is likely that cosmic rays will lead the way. As a result, cosmic rays seem likely to define the high-energy frontier for the foreseeable future. Meanwhile, the development of large-scale neutrino detectors is opening up a new window on the universe. Using cosmic rays as a novel form of astronomy, revealing events deep in space and far back in time, is an exciting field. A century after their discovery, cosmic rays are entering a golden age.

fURtHeR ReadinG

Neutrino by Frank Close (oxford university press, 2010)

Antimatter by Frank Close (oup, 2009)

Particle Physics: A very short introduction by Frank Close (oup, 2004)

The Particle Odyssey: A journey to the heart of matter by Frank Close, michael marten and Christine sutton (oup, 2004)

Cosmocopia 1.usa.gov/orudoe

“on its Centenary, Celebrating a ride that advanced physics” by bill breisky, The New York Times, 7 august 2012 (nyti.ms/oxh30v)

“a discovery of cosmic proportions” CERN Courier, July/august 2012 (bit.ly/Qaq9uw)

Cover image Goronwy tudor Jones, university of birmingham/science photo library

Frank Close is a professor of physics at the university of oxford

frank Close NEXTINSTANTEXPERTPeter Norvig

aRTiFiCial iNTElligENCE

3 November