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NE Introduction to Nuclear Science Spring 2012 Classroom Session 8: Radiation Interaction with Matter Non-Charged Radiation Mass Attenuation Tables and Use Absorbed Dose (D), Kerma (K) Gray (Gy) = 100 rad Dose Calculations Analysis of Gamma Information (NAA) Chemical Effects of Nuclear Reactions

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Reminder Load TurningPoint Reset slides Load List Homework #2 due February 9 Next Tuesday February 14 – 1 st Demo Session MCA Gamma Spectroscopy identification of isotopes NAA of samples 2

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Ionizing Radiation: Electromagnetic Spectrum Each radiation have a characteristic, i.e.: Infrared: Chemical bond vibrations (Raman, IR spectroscopy) Visible: external electron orbitals, plasmas, surface interactions UV: chemical bonds, fluorecense, organic compounds (conjugated bonds) X-rays: internal electron transitions (K-shell) Gamma-rays: nuclear transitions Neutrons mK, can be used to test metal lattices for example) Ionizing Radiation Ionizing

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Radiation Interaction with Matter Five Basic Ways: 1. Ionization 2. Kinetic energy transfer 3. Molecular and atomic excitation 4. Nuclear reactions 5. Radiative processes 4

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Radiation from Decay Processes Charged Directly ionizing (interaction with e - ’s) β’s, α’s, p + ’s, fission fragments, etc. Coulomb interaction – short range of travel Fast moving charged particles It can be completely stopped Uncharged Indirectly ionizing (low prob. of interaction – more penetrating) , X-Rays, UV, neutrons No coulomb interaction – long range of travel Exponential shielding, it cannot be completely stopped 5 R

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Neutral Interactions Stochastic (Probabilistic) With an electron or a nucleus Can be scattering – elastic or inelastic Can be absorptive It is still a collision: Flux of particles is important 6

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Flux or Intensity Flux is usually for neutrons (n) Intensity is usually for photons ( ’s) 7 Velocity of beam particles Density of particles in the beam Beam Target

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Attenuation of Uncollided Radiation How do we calculate the change in the flux of (uncollided) particles as it moves through the slab? Uncollided radiation is a simplification. In reality not every collided photon/neutron is lost and there are buildup factors (B i )

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Attenuation of Uncollided Radiation 9 Beam with intensity I, interacting with shield (1-D)

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Microscopic and Macroscopic Cross Sections Sigma-N = Linear Attenuation Coefficient or Macroscopic Cross Section ( or ) 10 Notice Different Units: is measured in cm -1 is measured in barns 1 barn = cm 2 Constant of Proportionality or Microscopic Cross-Section

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A beam of neutrons is normally incident on a slab 20 cm thick. The intensity of neutrons transmitted through the slab without interactions is found to be 13% of the incident intensity. What is the total interaction coefficient t for the slab material? cm cm cm cm -1

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Attenuation of Uncollided Radiation 13 Point sources: Isotropic source emitting Sp particles per unit time Beams of particles: with intensity I 0, interacting with shield (1-D)

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Related Concepts Mean Free Path (mfp or ): Average distance a particle travels before an interaction Half-thickness (x 1/2 ) of the slab? Thickness of slab that will decrease uncollided flux by half 14 Similar concepts to mean-life and half-life

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It is found that 35% of a beam of neutrons undergo collisions as they travel across a 50 cm slab. What is the mfp and x 1/2 for the slab? and 6.9 cm and 13.8 cm and 80 cm and 693 cm

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Clicker solution 16

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What is the intensity of uncollided neutrons near a 1m diameter water tank containing a 1Ci source? (assume t =0.1 cm -1 ) e8 n/cm 2 s e5 n/cm 2 s e5 n/cm 2 s 4. 8e3 n/cm 2 s 5. 2e3 n/cm 2 s

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Solution 18 Watch out for sign in exponential

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19 Photon Interactions - tables Photon energies: 10 eV < E < 20 MeV IMPORTANT radiation shielding design For this energy range, 1. Photoelectric Effect 2. Pair Production 3. Compton Scattering

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20 Pair Production Compton Scattering The Photoelectric Effect

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Example: Photon Interactions for Pb Energy High Intermediate Low Pair Production Compton Scattering Photoelectric Effect

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22 : Gammas

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Problem with Photons 100 mCi source of 38 Cl is placed at the center of a tank of water 50 cm in diameter What is the uncollided -flux at the surface of the tank?

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Problem with Photons 100 mCi 38 Cl, water tank 50 cm dia. What is the uncollided -flux at the surface of the tank?

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Linear Coefficients – Macroscopic Cross Sections Linear Absorption Coefficient μ t Linear Scattering Coefficient μ s Macroscopic Fission Cross-section Σ f, μ f for neutrons 27

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28 Neutrons:

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For homogeneous mixes of any type Valid for any cross section type (fission, total, etc) Valid for chemical compounds as well 30 DO NOT add microscopic cross-sections

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In natural uranium ( =19.21 g/cm 3 ), 0.720% of the atoms are 235 U, % are 234 U, and the remainder 238 U. From the data in Table C What is the total linear interaction coefficient (macroscopic cross section) for a thermal neutron in natural uranium? cm cm U: 0.59 cm -1 Who dominates?

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Absorbed Dose, D (Gray, rad) Energy absorbed per kilogram of matter (J/kg) Gray: 1 Gy = 1 J/kg The traditional unit: Rad: 100 rad = 1 Gy rad = Radiation Absorbed Man Dose rate = dose/time

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Kerma (Approx. dose for neutrons) Kerma Kinetic Energy of Radiation absorbed per unit MAss For uncharged radiation Kerma is easier to calculate than dose for neutrons Kerma and Dose: same for low energy Kerma over-estimates dose at high energy No account for “Bremsstrahlung” radiation loses.

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Calculating Dose Rate and Kerma Rate en (E)/ =mass interaction coefficient (table C3) E = particle energy [MeV] = flux [particles/cm 2 s] tr (E)/ =mass interaction coefficient (table C3) E = particle energy [MeV] = flux [particles/cm 2 s] Notice Difference Engineering Equations – PLEASE Watch out for units!

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