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IAEA Interaction of radiation with matter - 5 Neutrons Day 2 – Lecture 5 1.

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Presentation on theme: "IAEA Interaction of radiation with matter - 5 Neutrons Day 2 – Lecture 5 1."— Presentation transcript:

1 IAEA Interaction of radiation with matter - 5 Neutrons Day 2 – Lecture 5 1

2 IAEA Objective To discuss about Neutron interaction, radiative capture, fission, neutron cross sections and neutron activation analysis 2

3 IAEA Content  Neutron interaction mechanisms  Neutron energy categories  Radiative capture  Charged particle emission  Fission  Elastic and inelastic scattering  Neutron cross sections and removal  Neutron activation analysis 3

4 IAEA Charged Particle Interactions Ionizing radiation is divided into two categories: Directly Ionizing alpha and beta Indirectly Ionizing photons and neutrons Gamma rays and neutrons release charged particles in matter which are themselves directly ionizing. 4

5 IAEA Neutron Interactions  Neutrons have no charge and thus are indirectly ionizing  Always a “mixed” field of neutrons and gamma rays  Biological effects of neutrons are strongly energy dependent  Neutrons are arbitrarily divided into “slow” (thermal) or “fast” (energies of 1 MeV and above) 5

6 IAEA Neutron Interactions  All neutrons are “born fast” and then lose energy as they interact with matter  The chief interaction mechanisms of neutrons are scattering and capture (followed by emission of a photon or another charged particle from the absorber nucleus). 6

7 IAEA Radiative Capture 1 n + 1 H  2 H +  1 H(n,  ) 2 H Slow Neutron Interactions This reaction is important in neutron dosimetry and shielding. Hydrogen is a component of human tissue, so this reaction will produce radiation dose to humans. Also, this reaction makes materials containing hydrogen (concrete, water, parrafin, polyethylene, etc.) good shields for neutrons. 7

8 IAEA Radiative Capture 1 n Cd  114 Cd +  113 Cd(n,  ) 114 Cd Slow Neutron Interactions This reaction is important in neutron shielding and is also used as the principal reaction for some neutron detectors. 8

9 IAEA Charged Particle Emission 1 n + 10 B  7 Li + 4 He 10 B(n,  ) 7 Li Slow Neutron Interactions This is why boron controls are used in nuclear power reactors, since it tends to reduce the number of neutrons present and therefore helps control the fission process. 9

10 IAEA Charged Particle Emission 1 n + 14 N  14 C + p 14 N(n, p) 14 C Slow Neutron Interactions The energy of the proton released in this reaction is 0.6 MeV. 10

11 IAEA Fission 1 n U  fission products available for more fission Slow Neutron Interactions the mean number of neutrons released per fission for U- 235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.” 11

12 IAEA 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, wax, concrete  Accounts for about 80% of fast neutron dose to tissue 12

13 IAEA Elastic Scattering In collision with protons, neutrons lose half their energy on average. 13

14 IAEA Fast Neutron Interactions Inelastic scattering – neutrons interact with particles of much greater mass (e.g. iron) (analogy of ping pong ball striking bowling ball) For fast neutrons of energies of about 1 MeV, inelastic scattering can become appreciable. Inelastic scattering occurs primarily with high-Z absorbers. 14

15 IAEA Inelastic Scattering The neutron is captured, then re-emitted by the target nucleus together with the gamma photon. It has lesser energy 15

16 IAEA Fast Neutron Interactions Why do you think fast neutrons interaction is not significant from a tissue dose point of view? 16

17 IAEA Neutron Cross Sections  Probability that neutron will interact with a given material  Unit is “barn” where 1 barn = cm 2 17

18 IAEA 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 18

19 IAEA 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 Note the macroscopic cross section is used for neutrons, in place of the linear or mass absorption coefficients that are used in photon attenuation. 19

20 IAEA 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 20

21 IAEA Neutron activation analysis (NAA) NAA is a process used for determining the concentrations of trace elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The study spectra of the emissions of the radioactive sample allow s the identification of the element. 21

22 IAEA 23 Na (n,  ) 24 Na Neutron Capture in 23 Na 22

23 IAEA Examples of Importance of Neutron Activation  Production of isotopes (for example 60 Co, 192 Ir, etc.)  Co + n → Co → Ni + e + gamma rays  Accident dosimetry (for example 24 Na in blood)  Crime detection in forensic medicine (for example Napoleon’s hair) Neutron irradiation hair on Napoleon's head was examined for their arsenic content. Naturally occurring arsenic is composed of one stable isotope 75 As. 23

24 IAEA  Activation analysis for measurement of trace elements  Activation products in a reactor are major sources of radiation exposure to workers (for example 60 Co)  They can expose the public (for example direct gamma radiation from 16 N in steam in BWR is produced by neutron irradiation of water ) Examples of Importance of Neutron Activation 24

25 IAEA  Determination of fast neutron radiation component at Hiroshima  Accelerator mass spectrometry of 63 Ni (half-life = 100 years) produced by fast neutron  activation of copper in building materials  Reaction is 63 Cu (n, p) 63 Ni Examples of Importance of Neutron Activation 25

26 IAEA 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 and neutron activation were described 26

27 IAEA Where to Get More Information  Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)  International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002) 27


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