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Prof. J. K. Goswamy
UIET, Panjab University
Chandigarh
NEUTRON PHYSICS
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OVERVIEW
Road to Discovery of Neutron.
Neutron Sources.
Passage of Neutrons through Matter.
Detection of Neutrons.
Neutron Activation Analysis.
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ROAD TO NEUTRON DISCOVERY
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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
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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.
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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) .
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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.
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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.
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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.
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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
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SOURCES OF NEUTRONS
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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
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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
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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
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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
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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
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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)
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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
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INTERACTION OF NEUTRONS
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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.
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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
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DETECTION OF NEUTRONS
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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.
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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.
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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.
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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.
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Neutron Activation Analysis
G. Hevesy (Hungary) H. Levi (Denmark)
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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)
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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.
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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.
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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.
http://www.answers.com/topic/naa6-png
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Some Elements of Interest
Arsenic
Chromium
Selenium
Chlorine
Mercury
Magnesium
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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.
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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.