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IAEA Interaction of Radiation with Matter - 6 Neutron Shielding Day 2 – Lecture 6 1.

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Presentation on theme: "IAEA Interaction of Radiation with Matter - 6 Neutron Shielding Day 2 – Lecture 6 1."— Presentation transcript:

1 IAEA Interaction of Radiation with Matter - 6 Neutron Shielding Day 2 – Lecture 6 1

2 IAEA Objective To learn about the principles of neutron shielding (as well as associated gamma radiation) To discuss health physics significance of neutron sources and shielding 2

3 IAEA Content Neutron energy categories Neutron shielding principles Shielding materials Gamma emission from neutron shields Common neutron sources 3

4 IAEA Neutron Energy Categories Neutron Name/TitleEnergy (eV) Cold Neutrons0 < 0.025 Thermal Neutrons0.025 Epithermal Neutrons0.025 < 0.4 Cadmium Neutrons0.4 < 0.6 Epicadmium Neutrons0.6 < 1 Slow Neutrons1 < 10 Resonance Neutrons10 < 300 Intermediate Neutrons300 < 1,000,000 Fast Neutrons1,000,000 < 20,000,000 Relativistic Neutrons>20,000,000 4

5 IAEA Neutron Shielding Complicated and not straightforward Choice of shielding is strongly dependent on neutron energy Neutrons are shielded with combinations of high-Z, low-Z and absorber materials In all cases, shielding materials must account for induced gamma rays 5

6 IAEA Neutron Shielding For very fast neutrons with energies > 10 MeV use inelastic scattering (high-Z materials) For fast neutrons with energies > 1 MeV, use elastic scattering (moderation with H) For slow neutrons use absorbers 6

7 IAEA Shielding Very Fast Neutrons Use shielding materials in this order  High Z (for example Pb or iron) or concrete - inelastic scattering for initial slowing down  Followed by low Z (H) (for example polymers, water, concrete) – elastic scattering 7

8 IAEA Shielding Fast Neutrons Use low-Z materials (with H) (for example polymers, water, concrete) Takes advantage of elastic scattering with hydrogen 8

9 IAEA Fast Neutron Scattering From Heavy Nuclei 9

10 IAEA Fast Neutron Scattering From Hydrogen Nuclei 10

11 IAEA Shielding Slow Neutrons Preferentially use absorbers to capture slow neutrons without gamma emission Use absorbers such as B or Li utilize (n,  ) reaction without emission of capture gamma rays Second choice is hydrogenous materials using 1 H (n,  ) 2 H reaction (emits a very energetic 2.23 MeV gamma ray which must be shielded against) 11

12 IAEA Shielding Neutrons of Mixed Energies Slow neutrons down, then absorb Use low Z materials – elastic scattering to moderate neutron energies Use absorber materials to remove moderated neutrons Use high Z materials to shield against the induced x and gamma rays 12

13 IAEA Gamma Radiation Produced by Neutron Shielding Secondary gamma rays arise mostly from capture of thermal neutrons Neutron inelastic scattering also contributes somewhat Secondary gamma rays are less important for higher neutron energies 13

14 IAEA Neutron Capture Gamma Rays for Selected Materials TargetThermal Neutron Cross Section (barns) Highest Energy Gamma Ray (MeV) Al0.2357.724 B-1038370.478 Cd24509.046 C-120.00344.95 H0.3322.23 Si0.16010.599 N-140.07510.833

15 IAEA Material Compositions for Common Neutron Shields MaterialElements Contained Atoms/cm 3 (x10 -21 ) Borated polyethylene (8% B 4 C by weight) H C B-10 B-11 76.8 39.2 0.658 2.67 Water HOHO 66.9 33.45 Concrete H C Al Si 13.75 45.87 1.743 20.15

16 IAEA Neutron Sources SourceReactionAverage Neutron Energy (MeV) ReactorFissionFission spectrum 24 Na + Be ( ,n) 0.83 Ra + Be ( ,n) 5.0 Po + Be ( ,n) 4.0 252 CfSpontaneous fission Fission spectrum Pu + Be ( ,n) 4.0

17 IAEA Shielding of Po-Be Neutrons MaterialHalf-Thickness (cm) Paraffin6.6 Water5.4 12% Borax in Water5.3 Brass4.9 Steel (cold roll)4.9 Lead6.8 Aluminum7.8

18 IAEA Summary Principles of neutron shielding were discussed Students learned about neutron energy categories, shielding principles, commonly used shielding materials, associated gamma radiation, and common neutron sources 18

19 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) 19

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