Nuclear Lecture 01

8/8/2019 Nuclear Lecture 01

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Nuclear Physics and Radiation Detectors

P4H 424 Course, Candlemas 2004Nuclear Physics Lecture 1

Dr Ralf Kaiser

Room 514, Department of Physics and Astronomy

University of Glasgow

[email protected]

P4H 424: Nuclear Physics Lecture 1 p.1/2
http://www.physics.gla.ac.uk/http://www.physics.gla.ac.uk/~kaiserhttp://www.physics.gla.ac.uk/~kaiserhttp://www.physics.gla.ac.uk/


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Summary

Course Overview

Global Properties of Nuclei

http://www.physics.gla.ac.uk/~kaiser/

http://www.physics.gla.ac.uk/~kaiser/http://www.physics.gla.ac.uk/~kaiser/


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Nuclear Physics

Historically, nuclear physics can be seen as the child ofchemistry and atomic physics and in turn as the parent ofparticle physics and one of the parents of medical physics.

When hearing the word nuclear most people will think of twothings: nuclear bombs (aka WMDs) and nuclear reactors. Bothare not exactly popular these days.

Thanks to bombs and reactors nuclear physics was probablythe part of science with the biggest impact on politics in the20th century. Just think of the entire cold war. The Manhattanproject was probably the most high-profile science project of

the 20th century, with a large number of future Nobel-prizewinners involved.

In cultural relevance it is possibly rivalled by the moon-landing -

another technological spin-off of World-War II, and inevery-day-relevance by electronics.



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Nuclear Physics

Todays mainstream nuclear physics research has very little to dowith bombs and reactors. Current research topics include:

Hadron structure: the structure of the nucleon and of hadronsin general.

Hadron spectroscopy: the search for glueballs, hybrids andmultiquark states. (Maybe youve heard about the recent

evidence for pentaquarks.)

Heavy ion physics: quark-gluon plasma, a new phase of matter

Nuclear Astrophysics: understanding stars, super-novae etc.

Still, it is necessary to understand the main results of classicalnuclear physics before one enters current research and certaintopics therefore have to be part of a nuclear physics course.



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Course Overview

Lecture Topics1 basic properties of nuclei

spectroscopy and scattering

2,3,4 the nucleusnuclear models, geometric shapes of nuclei

electron scattering

5 nucleon-nucleon interactionsthe deuteron, nucleon-nucleon scattering, Yukawa potential

6,7,8 the nucleon

elastic scattering, deep inelastic scattering

form factors, structure functions

quark model, mesons and baryons, hadron physics

9 reactors and bombs

10 modern topics in nuclear physics



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Books and Reference Material

The books that I mainly used in preparing the present course are

K.Krane, Introductory Nuclear Physics, Wiley

B.Povh, K.Rith, C.Scholz, F.Zetsche, Particles and Nuclei,Springer

In addition Im using the lectures of two colleagues, D.Ireland(Glasgow) and M.Dueren (Giessen) as input. Im also using theweb as a source of pictures etc.

As this is the first time Im teaching the present course, the materialwill develop from week to week. The slides will be made available

on the web from week to week as well. If you find typos (and therewill be some) or mistakes (dito) please let me know.

As this is not a basic course, I will assume that some basics arealready known. If I assume too much, or too little, please also let

me know.



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The Nucleus

Nucleus

Nucleus

Protonsand Neutrons 208

Pb Nucl.

10-10

m

Na-Atom

Atom

Proton

10-14

m

10-15

m

0

0.3

0

3.0

0

3.0

Quark

Proton

[eV]

[MeV]

[GeV]

Atoms consist of a nucleus and anelectron shell.

A nucleus consists of nucleons:

protons and neutrons. As the massof a nucleon is about 2000 timesthe mass of an electron the nucleuscarries practically all the mass of an

atom.A nucleon consists of 3 quarks (andgluons).

1 fm (femtometer, Fermi) = 10

mis the typical length scale of nuclearphysics

1 MeV (Mega-electron volt) =

1.602

J is the typical energyscale of nuclear physics



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Nuclear Theory and Experiment

Atomic physics has a single consistent theory -Quantumelectrodynamics (QED). This is unfortunately not truefor nuclear physics: There is a fundamental theory of thestrong interaction - Quantumchromodynamics (QCD) - but it

describes the interactions between quarks, not nucleons.The energies involved in nuclear decays are of the order of1-10 MeV, less than 0.1 % of the mass of the nucleus. As aresult non-relativistic QM can be used to describe the nucleus.

This is not true for the study of the structure of the nucleon,where the incident beam energy in a scattering experimentmay be 100 times the proton mass equivalent.

Both nuclei and nucleons are complex systems involving manyconstituents. The theories and models that describe them aretherefore often phenomenological in nature and nuclear

physics is rather led by experiment than by theory.



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Nuclear Physics Experiments

Nuclear physics experiments can be classified as scattering orspectroscopic experiments (the same holds true for hadronphysics).

In a scattering experiment, a beam of particles with known

energy and momentum is directed towards the object to bestudied (the target). The achievable resolution is determinedby the de-Broglie-wavelength

of the particles. Nuclear

radii can be measured with electron beams of about 10

eV,proton radii with 10

eV.

The term spectroscopy is used to describe those experimentswhich determine the decay products of excited states. In this

way, one can study the properties of the excited states as wellas the interactions between the constituents. States can bedifferent nuclids or in hadron physics different mesons orbaryons. The energies required to produce excited states are

similar to those for scattering experiments.



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Nuclids

A nuclid is a specific combination of a number of protons andneutrons.

is the complete symbol for a nuclid, but the information is

redundant and

is sufficient.

X is the chemical symbol of the element

Z is the atomic number, giving the number of protons in thenucleus (and electrons in the shell)

N is the number of neutrons

A = Z + N is the mass numberNuclids with the same atomic number Z are called isotopes,same A isobars, same N isotones (isos (gr.) - the same).



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Nuclid Chart

Y

X

W

Y

X

W

A

Z N

A1

Z N1 Z

A+1

XN+1

Z1 Z1

Z+1 Z+1 Z+1

N1

N1

A2

A

N

N

A1

A+1

W

Y

N+1

N+1

Z1

A

Z

N

Y

X

W

V V V V

A+2

A4

Z2 N2

n

p +

nuclids can be put onto achart, not unlike a periodictable for nuclear physics

typically the chart plots

Z vs N

the different radioactivedecays can easily be

connected with movement inthe chart, e.g. -decaycorresponds to two-left,two-down.

this allows to visualise entiredecay chains in an effectivefashion

it also allows to visualiseother properties, e.g. lifetimeor date of first detectionP4H 424: Nuclear Physics Lecture 1 p.11/2


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Nuclid Chart - Lifetime



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Nuclid Chart - Historical



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Nuclear Masses

Atomic masses (actually, ionic masses) can be determinedwith high precision using mass spectrometers. Because theelectron mass is know very precisely this allows to determinethe mass of the concerned nucleus.

Mass spectrometers use a combination of electric andmagnetic fields to measure the Q/M ratio and thus the mass M.

In an electric field the radius of curvature of the ion trajectory is

proportional to the kinetic energy:

In a magnetic field the radius of curvature is proportional to themomentum:



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Nuclear Masses - Mass Spectrometer

Detector

Ion source



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Nuclear Masses

By careful design of the magnetic fields, ions with identicalQ/M ratios are focused at a point at the end of thespectrometer, where a detector can be placed.

Modern mass spectrometers often use a more complicated but

also more elegant arrangement of magnetic quadrupoles andoscillating electric fields (quadrupole mass spectrometer).

The mass reference is not the proton or the hydrogen atom,

but the isotope

C. Carbon and its many compounds arealways present in a spectrometer and are well suited for amass calibration.

An atomic mass unit is therefore defined as 1/12 of the massof the

C nuclid:

For comparison, the proton mass is 938.272 MeV/c

.P4H 424: Nuclear Physics Lecture 1 p.16/2


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Nuclear Abundance

Mass number A

Abundance[Si=106]

One application ofnuclear massspectroscopy is the studyof relative isotope

abundances in the solarsystem. (see figure,normalised to Si).

They are generally the

same throughout thesolar system.

deuterium and helium :

fusion in the first minutesafter the big bang, nuclei

up to

Fe : stars, heaviernuclei in supernovae.


N l Ab d E l


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Nuclear Abundance - Example

Counts

Mass number A

Top: Mass spectrum ofxenon isotopes, found ina 2.7 billion year oldgneiss sample from a drill

core on the Kolapeninsula.

Bottom: xenon isotopespectrum as they occur inthe atmosphere.

The Xe in the gneisswere produced by sponta-

neous fission of uranium.(K.Schfer, MPI Heidel-berg).


N l Bi di E


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Nuclear Binding Energy

The binding energy

of a nucleus is the difference in in mass

energy between a nucleus

and its constituent Z protons

and N neutrons:

where

is the atomic mass of

. The binding energy is

determined from atomic masses, since they can be measured

much more precisely than nuclear masses.Grouping the Z proton and electron masses into Z neutralhydrogen atoms, we can re-write this as:

With the masses generally given in atomic mass units, it isconvenient to include the conversion factor in , thus

= 931.481 MeV/u.


N l Bi di E


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N l Bi di E


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The first obvious feature of the B/A vs A plot is that it the curveis relatively constant, with the exception of very light nuclei.The average binding energy of most nuclei is about 8 MeV pernucleon.

Second, the curve reaches a peak around A=60, to be preciseat

. This suggests that light nuclei, below

can gain

energy by fusion into heavier nuclei. Heavy nuclei above

can release energy by fission into lighter nuclei.

This is already the basic argument why only nuclids up to

can be formed in normal stars.

More about the shape of this curve a little later, when we studythe semi-empirical mass formula.

Last, but not least,

and

appear to be off the curve. Wewill get back to this when we study the shell model.


Nuclear Lecture 01

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