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3/2003 Rev 1 I.3.7 – slide 1 of 23 Part I Review of Fundamentals Module 3Interaction of Radiation with Matter Session 7Neutron Interactions Module I.3.7 IAEA Post Graduate Educational Course Radiation Protection and Safety of Radiation Sources

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3/2003 Rev 1 I.3.7 – slide 2 of 23 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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3/2003 Rev 1 I.3.7 – slide 3 of 23 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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3/2003 Rev 1 I.3.7 – slide 4 of 23 Overview Mechanisms of neutron interactions with matter will be discussed Health physics significance of neutron interactions will be discussed

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3/2003 Rev 1 I.3.7 – slide 5 of 23 Charged Particle Interactions Ionizing radiation is divided into two categories: Directly Ionizing alpha and beta Indirectly Ionizing photons and neutrons

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3/2003 Rev 1 I.3.7 – slide 6 of 23 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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3/2003 Rev 1 I.3.7 – slide 7 of 23 Neutron Energy Categories Name/TitleEnergy Cold 0 < eV Thermal eV Epithermal < 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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3/2003 Rev 1 I.3.7 – slide 8 of 23 Radiative Capture 1 n + 1 H 2 H + 1 H(n, ) 2 H Slow Neutron Interactions

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3/2003 Rev 1 I.3.7 – slide 9 of 23 Radiative Capture 1 n Cd 114 Cd + 113 Cd(n, ) 114 Cd Slow Neutron Interactions

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3/2003 Rev 1 I.3.7 – slide 10 of 23 Charged Particle Emission 1 n + 10 B 7 Li + 4 He 10 B(n, ) 7 Li Slow Neutron Interactions

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3/2003 Rev 1 I.3.7 – slide 11 of 23 Charged Particle Emission 1 n + 14 N 14 C + p 14 N(n, p) 14 C Slow Neutron Interactions

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3/2003 Rev 1 I.3.7 – slide 12 of 23 Fission 1 n U fission products available for more fission Slow Neutron Interactions

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3/2003 Rev 1 I.3.7 – slide 13 of 23 Fission

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3/2003 Rev 1 I.3.7 – slide 14 of 23 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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3/2003 Rev 1 I.3.7 – slide 15 of 23 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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3/2003 Rev 1 I.3.7 – slide 16 of 23 Elastic Scattering

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3/2003 Rev 1 I.3.7 – slide 17 of 23 Inelastic Scattering

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3/2003 Rev 1 I.3.7 – slide 18 of 23 Neutron Cross Sections Probability that neutron will interact with a given material Unit is “barn” where 1 barn = cm 2 Leads to concepts of “microscopic” and “macroscopic” cross sections

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3/2003 Rev 1 I.3.7 – slide 19 of 23 Microscopic Cross Section Sum of separate cross sections for all processes which may occur with a given atom. Unit is cm 2 total = scatter + capture + fission

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3/2003 Rev 1 I.3.7 – slide 20 of 23 Macroscopic Cross Section Product of microscopic cross section and the total number of atoms per cm 3 in the material. Unit is cm -1 total = N total where N = number of atoms/cm 3

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3/2003 Rev 1 I.3.7 – slide 21 of 23 Neutron Removal by an Absorber I = I 0 e where I = neutrons passing through the absorber I 0 = neutrons incident on the absorber N total = total macroscopic cross section x = absorber thickness - N total x

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3/2003 Rev 1 I.3.7 – slide 22 of 23 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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3/2003 Rev 1 I.3.7 – slide 23 of 23 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, 6 th 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 (8 th Edition, 1999 update), Wiley, New York (1999)

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