Prof. J. K. Goswamy

UIET, Panjab University

Chandigarh

NEUTRON PHYSICS

OVERVIEW

Road to Discovery of Neutron.

Neutron Sources.

Passage of Neutrons through Matter.

Detection of Neutrons.

Neutron Activation Analysis.

ROAD TO NEUTRON DISCOVERY

In 1898, J.J. Thomson proposed

that the atom is basically a

spherical cloud of positively

charged matter with electrons

embedded in it like the seeds in a

watermelon.

This was a static model of atom

with intrinsic electrostatic

instability.

It failed to explain the energy

levels of the atom.

The Nucleus: Discovery

THOMSON MODEL

Rutherford Experiments

Rutherford, a former student of Thomson,

performed experiments with the scattering of alpha-

particles from thin metal foils.

The scattered alpha-particles were detected

through tiny light flashes produced by them on ZnS

screen.

Most of the alpha-particles travelled without

deflection through the foil.

Small fraction suffered deflection through a

large angle (upto 90o).

Very few alpha particles were deflected back.

Angular Distribution of Scattered -particles

Angular distribution of alpha-particles from gold foils.

Most of the α-particles pass through foil with

deflection less than 8o indicating that atom

predominantly has empty space.

The large angle deflections (~90o) suffered by

small fractions of α-particles, indicated that the

positive charge in atom was concentrated in a

very small volume at the centre of atom.

The very small fraction of backscattered α-

particles was possible as the central core

accounted for nearly whole mass of atom.

From angular distribution of scattered -

particles Rutherford concluded existence of

positively charged core of atom then called

nucleus.

The size of the nucleus was much smaller (10-

14m) than size of the atom (10-10m) .

Electron-Proton Theory

After the Rutherford model of atom, the nucleus was

postulated to be constituted by electrons and protons.

Nucleus (A, Z) = A Protons + (A-Z) Electrons

Drawbacks

Ground state spin of most of the nuclei could not be

reproduced.

Magnetic moments of nuclei were predicted to be much

higher than the observed values.

Discovery of Neutron

The process of discovery of neutrons started with the

capture of α-particles by 9Be and the reaction was

supposed to be 9Be(α,γ)13C.

Bothe and Becker, through absorption of gamma-rays

in lead, estimated the photon energy to be 7MeV.

Later Curie and Joliot showed that emitted gamma-

rays could knock out protons from paraffin and other

hydrogenous materials. They estimated the energy of

the emitted photon to be 55MeV.

Chadwick performed a series of experiments to

study recoil energy of different nuclei stuck by

gamma rays emitted in this reaction. It was

concluded that photon energy depended on

nuclei which recoiled due to photon impact. This

was surprising and not acceptable.

Chadwick removed this anomaly by the

hypothesis that emitted particle in this reaction is

actually not gamma ray but an electrically neutral

particle with mass nearly same as of proton. This

discovered particle was called neutron.

o Rutherford model suggests that the atomic

mass is nearly equal to the mass of the nucleus,

which contains +ve charged particles called

protons.

o The number of protons is equal to the number of

electrons, often called atomic number Z of atom.

o For light nuclei, the atomic mass is

approximately twice the mass of protons and

this ratio is more in case of heavier nuclei.

o This discrepancy was resolved in 1932 by

James Chadwick who discovered neutron of

mass nearly equal to that of proton.

o A nucleus is made up of protons and neutrons:

A = N + Z

Neutron Proton Theory

Mass of neutron (1.6748 x 10-27kg)

is slightly more than proton.

Neutron is uncharged but has an

internal structure.

Spin of neutron is h/4π

Due to internal structure and spin, its

magnetic moment of -1.91µN.

Free neutron undergoes β-decay

with a half life of 12.5 minutes as

pn

SOURCES OF NEUTRONS

Neutron Sources

Pure isotopic sources of neutrons do not exist as no

radioactive decay process causes emission of neutrons.

Neutron Sources

Fission Sources

Isotopic (,n) Sources

Photo neutron Sources

Other Sources

Spontaneous Fission Sources

Many trans-uranium nuclei have high spontaneous fission probability. The

products of spontaneous fission process are:

Heavy fission products.

β- and γ-activities of fission products.

Prompt fast neutrons.

These sources are usually encapsulated in a sufficiently thick container

so that only fast neutrons and gamma rays escape from the source.

252Cf Half life = 2.65 years.

Modes of decay: >90% α-decay and <10% spontaneous fission.

Neutron yield = 0.116 neutrons/second per Becquerel of activity.

Intense yield of 2.3 million neutrons/second per microgram of the sample.

Energy Distribution: 0.5 -10 MeV.

Spectrum of Neutrons

Type of Neutrons Energy Range

Thermal Neutrons 0.025 eV-0.5 eV

Epithermal Neutrons 0.5 eV-100 keV

Fast Neutrons 100 keV-25 MeV

Radio-Isotope (α, n) Sources

• These are small self-contained

neutron sources obtained by mixing

an α-emitting source with Be like

elements.

• Usually the actinide elements are α-

emitters and form stable alloy with

beryllium. Sources are prepared

through metallurgical process.

• The α-particles, emitted by actinide,

interact with Be nuclei within alloy

without much loss of energy.

Source

Half life

Eα

Yield

239Pu/9Be 24000y 5.14MeV 65 npm

241Am/9Be 433y 5.48MeV 82 npm

238Pu/9Be 87.4y 5.48MeV 79 npm

Photo-Neutron Sources Some radio-isotopes, which are γ-ray

emitters, produce neutrons when

combined with appropriate target

material.

The gamma-rays produced in a

radioactive decay, are absorbed by the

target nucleus thereby getting excited

sufficiently to emit neutron.

Two commonly used reactions for

producing photo-neutrons are:

9Be(γ, n)8Be Eγ>1.666MeV

2H(γ, n)1H Eγ>2.226MeV

Relatively mono-energetic neutrons are

emitted.

emitter

Aluminum Encapsulation

Neutron Emitting target

Accelerator Based Neutron Sources

Deutron Induced Reactions are source of neutrons

2He(2He, n)3He 2H(3H, n)4He

These reactions are possible through artificially

accelerated particles. As coulomb barrier of light target

nuclei for incident deutrons is low so it can be overcome

through small acceleration.

Charged particle Induced reactions yielding neutrons are

9Be(p,n) 7Li(p,n) 3H(p,n)

Neutron Generators

3H(d,n)4He Deutrons are accelerated to 200 kV

14MeV neutrons in reactions: (n,p), (n,α), (n,2n))

Neutron yields: 1011/s/mA, Neutron flux: 109/cm2/s

Research Reactors

Thermal power: 100 kW-10 MW

Thermal neutron flux: 1012-1014 n/cm2 s

INTERACTION OF NEUTRONS

Interaction of Neutrons

Neutrons

Slow Neutrons

Elastic scattering resulting in moderation of neutron energy.

Cause (n,p), (n,r) reactions.

Fast Neutrons

Elastic scattering causing recoil of secondary radiations.

Inelastic scattering causing excitation of absorber nuclei.

Neutrons are uncharged particles and

can travel large distance without

interacting with absorber’s atoms.

Neutrons interact with the nuclei of the

absorber atoms in which they may (a)

Disappear resulting in production of

secondary radiations or (b) their energy

or direction is changed significantly.

Neutron Flux Attenuation If a neutron beam passes through a slab of material, it suffers attenuation

through scattering as well as absorption by the material nuclei.

Absorption of Neutrons

o Direct Nuclear Reaction: Neutrons interact with matter via direct nuclear reaction.

The probability of reaction process depends upon the energy of neutrons and the

nature of target nuclei.

o Compound Nuclear Reaction: Fast neutrons get captured to form a compound

nucleus which has excitation energy equal to the sum of neutron’s kinetic and

binding energy of nucleus. This energy is subsequently released in the form of

reaction products, gamma-rays and neutrons.

Scattering of Neutrons

o Secondary Radiation Production: Neutron may get scattered and portion of its

energy is transferred to the recoiling nucleus.

o Moderation: Slow neutrons suffer multiple scattering to slow down to thermal

energies often called moderation.

absc

DETECTION OF NEUTRONS

Principle of Neutron Detection

A neutron detector does not record the presence of neutron

directly but responds through secondary radiation (charged

particles or gamma rays) which are emitted due to neutron

induced nuclear reaction in the detector medium.

For slow and thermal neutrons commonly employed

reactions on light nuclei are

(n, p) (n, α) (n, fission)

For fast neutrons of several MeV energy, the scattering off a

light target nuclei can give enough energy to the recoiling

nucleus for detection as secondary radiation.

Slow Neutron Detectors Boron Fluoride Proportional Counter

The isotope 10B is commonly used in the form of BF3 gas inside a proportional

counter. This gas serves both as Target for nuclear reaction and Counter fill gas.

The neutron causes the reaction 10B(n,α)7Li.

The outgoing particle and recoiling nucleus cause ionizations in the detector

gas.

These ionization serve as a signal for neutron detection.

Count rates are proportional to neutron density at the detector.

3He Proportional Counter

3He acts are target as well as counter fill gas.

This utilizes the reaction 3He(n,p)3H.

Reaction cross-section is high but energy of outgoing particles is low.

Fission Counters

The fission cross-sections of 233U, 235U and 239Pu are relatively

large at low neutron energies and thus these materials can be

used.

The detectors using these materials yield much larger output pulse

amplitude than any other detector used for slow neutrons.

These detectors are mostly in the form of ionization chamber with

its inner surface coated with fissile material.

Self Powered Detectors

In these detectors, materials having high cross-section for neutron

capture are used which subsequently emit β- or γ-rays.

The β-decay current following neutron capture determines the

neutron flux.

Fast Neutron Detectors

These neutrons can be detected using the conversion process

in which fast neutron collides with target nucleus and causes it

to recoil. The recoiling nucleus is detected as signal for neutron.

Most commonly used target for the fast neutron detection are

abundant in hydrogen, which offer the advantage that fast

neutrons can transfer whole of their energy to protons. Such

detectors are capable to measure incident neutron’s energy.

Certain detectors like BF3 proportional counter, coated with thick

wax, are used for fast neutron detection. The incident neutrons

are moderated by wax before they enter detector.

Neutron Activation Analysis

G. Hevesy (Hungary) H. Levi (Denmark)

Various Activation Techniques

Activation is general technique to transform element(s)

constituting a sample to radioactivity and subsequently

measure its nature, quantity and profile of distribution

through radioactive decay.

o Charged Particle Activation Analysis (CPAA)

o Photon Activation Analysis (PAA)

o Neutron Activation Analysis (NAA)

Neutron Activation Analysis ( G. Hevesy and H. Levi in 1936)

Multi-elemental technique which can detect up to 74

elements in gases, liquids and solid mixtures. C, H, N, O

and Si do not activate well.

Neutron irradiation of the sample causes radioactivity

formation. The subsequent decay is studied for

determining nature and concentration of elements.

Can determine concentration and profiles of elements

at ppm and ppb levels using Physical or Radiochemical

Techniques

The chemical form and physical state of the elements

do not influence the activation and decay process.

Neutron Activation Analysis

Nondestructive (Instrumental) NAA keeps the

resulting radioactive sample intact.

Destructive (Radiochemical) NAA results in

chemical decomposition of the radioactive sample

and the elements are chemically separated.

NAA: Principle & Detection

Hit source with neutrons.

Source becomes radioactive.

Decays in predictable ways.

Irradiated samples are analyzed by

gamma-ray spectrometry.

Detect the gamma-rays with gas

detector, scintillators, semiconductors.

Some Elements of Interest

Arsenic

Chromium

Selenium

Chlorine

Mercury

Magnesium

Applications of NAA

Environmental Studies

o Migration of pollutants in ecosystems.

o Air pollution studies.

Biotechnology

o Medicine

o Development of new pharmaceuticals.

o Impurities in industrial products and foods

o Hazardous material at dumps

Material Science

o High purity materials,

o Nanoparticles.

o Trace elements in archeological remains or objects of national heritage.

Advantages of NAA

Small sample sizes (.1mL or .001gm).

Non-destructive.

Can analyze multiple element samples.

Doesn’t need chemical treatment.

High sensitivity, high precision.