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Part I Review of Fundamentals

Module 3 Interaction of Radiation with Matter

Session 7 Neutron Interactions

Module I.3.7

IAEA Post Graduate Educational CourseRadiation Protection and Safety of Radiation Sources

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Introduction

Neutron interactions with matter will be discussed

Students will learn about neutron interaction mechanisms, neutron energy categories, the concept of neutron cross section, and neutron removal from a beam

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Content

Neutron interaction mechanisms Neutron energy categories Radiative capture Charged particle emission Fission Elastic and inelastic scattering Neutron cross sections and removal

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Overview

Mechanisms of neutron interactions with matter will be discussed

Health physics significance of neutron interactions will be discussed

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Charged Particle Interactions

Ionizing radiation is divided into two categories:

Directly Ionizing

alpha and beta

Indirectly Ionizing

photons and neutrons

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Neutron Interactions

Neutrons have no charge and thus are indirectly ionizing

There is always a “mixed” field of neutrons and gamma rays

The biological effects of neutrons are strongly energy dependent

Neutrons are arbitrarily divided into “slow” (thermal) or “fast” (energies of 1 MeV and above)

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Neutron Energy Categories

Name/Title EnergyCold 0 < 0.025 eV

Thermal 0.025 eV

Epithermal 0.025 < 0.4 eV

Cadmium 0.4 0.6 eV

Epicadmium 0.6 < 1 eV

Slow 1 10 eV

Resonance 10 < 300 eV

Intermediate 300 eV 1 MeV

Fast 1 < 20 MeV

Relativistic > 20 MeV

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Radiative Capture

1n + 1H 2H + 1H(n, )2H

Slow Neutron Interactions

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Radiative Capture

1n + 113Cd 114Cd + 113Cd(n, ) 114Cd

Slow Neutron Interactions

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Charged Particle Emission

1n + 10B 7Li + 4He

10B(n,) 7Li

Slow Neutron Interactions

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Charged Particle Emission

1n +14N 14C + p

14N(n, p)14C

Slow Neutron Interactions

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Fission

1n + 235U fission products

available for more fission

Slow Neutron Interactions

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Fission

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Fast Neutron Interactions

Elastic scattering - neutrons interact with particles of approximately the same mass such as protons (billiard ball analogy)

Occurs in materials rich in hydrogen such as water, paraffin wax, concrete

Accounts for about 80% of fast neutron dose to tissue

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Fast Neutron Interactions

Inelastic scattering – neutrons interact with particles of much greater mass (e.g. iron nuclei)

(analogy of ping pong ball striking bowling ball)

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Elastic Scattering

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Inelastic Scattering

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Neutron Cross Sections

Probability that neutron will interact with a given material

Unit is “barn” where 1 barn = 10-24 cm2

Leads to concepts of “microscopic” and “macroscopic” cross sections

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Microscopic Cross Section

Sum of separate cross sections for all processes which may occur with a given atom. Unit is cm2

total = scatter + capture + fission

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Macroscopic Cross Section

Product of microscopic cross section and the total number of atoms per cm3 in the material. Unit is cm-1

total = N total

where N = number of atoms/cm3

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Neutron Removal by an Absorber

I = I0 e

where

I = neutrons passing through the absorberI0 = neutrons incident on the absorberN total = total macroscopic cross sectionx = absorber thickness

- N total x

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Summary

Neutron interactions with matter were discussed

Neutron interaction mechanisms, neutron energy categories, the concept of neutron cross section, and neutron removal from a beam were described

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Where to Get More Information

Cember, H., Johnson, T. E., Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2008)

Martin, A., Harbison, S. A., Beach, K., Cole, P., An Introduction to Radiation Protection, 6th Edition, Hodder Arnold, London (2012)

Attix, F. H., Introduction to Radiological Physics and Radiation Dosimetry, Wiley and Sons, Chichester (1986)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999)