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Neutron Physics• Discovery of neutron– Experiments by Bothe and Becker

– Curie and Joliot– James CHADWICK

• Properties of the Neutron– Electrically neutral

– Free neutron is unstable against β decay, T1/2 is 10.6 min

– Its mass is slightly large than proton (1.2934 MeV)

– Possess magnetic diploe moment -1.91340 μN

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Neutron Sources

• Alpha-Beryllium Source– Radium-Beryllium– Polonium-Beryllium (138 d)– Plutonium-Beryllium (86 y)– Americium-Beryllium (458 y)

• Photo neutron Sources– Gamma rays from 24Na (15 h)– Gamma rays from 124Sb (60 d)

• Spontaneous Fission– Californium-252 (2.65 y)

• Nuclear Reactions– 3H(d, n)4He (17.6 MeV)– 9Be(α, n)12C (5.7 MeV)– 7Li(p, n)7Be (-1.6 MeV)– 2H(d, n)3He (3.3 MeV)

• Reactor Sources

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Neutron Absorption and Moderation

• Neutron are produced as fast neutrons in a reactor as a result of chain fission

• Fast neutrons disappear through many reactions like (n, p), (n, α), (n, 2n), ...

• The neutron loss their energy by making collisions (elastic and inelastc scattering) with the nuclei of moderating medium

• Thermal neutrons are removed primarily through (n, γ) reaction

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Neutron Absorption and Moderation (Cont.)

Neutron attenuation

• Neutron are produced as fast neutrons in a reactor as a result of chain fission

• When these neutron pass through a material, some them are removed from the beam by scattering or absorption

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0

t

xt

dI I Ndx

I I e

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Neutron Absorption and Moderation (Cont.)

• In case of elastic scattering, the ratio of energy of scattered neutron to that of incident is:

• The values of E’/E are uniformly distributed between minimum and maximum values.

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• Elastics Scattering

2

2

' 1 2 cos

1

E A A

E A

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Neutron Absorption and Moderation (Cont.)

Characteristics of Elastics Scattering

• For neutron energy of about 10 MeV and below, the scattering is s wave scattering and is independent of θ.

• Maximum Energy Loss occurs for θ = 180◦

• The ratio of energy of scattered neutron to and the incident neutron for the case of maximum energy transferred is called collision Parameter

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2

min

' 1

1

E A

AE

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Neutron Absorption and Moderation (Cont.)

Quantitative Calculations of Energy Loss

• Each neutron scatter many times, so repeated calculations are needed for energy loss

• In second collision, the neutrons are no longer monoenergetic

• This effect is counted for by dividing the energy spectrum into small intervals of ΔE

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0 0.2 0.4 0.6 0.8 1.0E’/E

∆EN(E)

0 0.2 0.40.6 0.81.0

E’/E

N(E)

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Neutron Absorption and Moderation (Cont.)

Quantitative Calculations of Energy Loss

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log' av

E

E

2

2

( 1)log

1 2 cos

Ad

A A

d

2( 1) 11 log

2 1

A A

A A

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Neutron Absorption and Moderation (Cont.)

Energy decrement after n collisions

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log' av

E

E

'log logE E

'log lognE E n

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Neutron Absorption and Moderation (Cont.)

Moderating properties of various nuclei

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Nucleus ξ N (for thermalization)

1H 1.00 18

2H 0.725 25

4He 0.425 43

12C 0.158 110

238U 0.0084 2200

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Neutron Absorption and Moderation (Cont.)

• Maxwellian Distribution of thermal neutrons

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1/2 /

3/2

2( ) E kTnf E dE E e dE

kT