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**NE 301 - 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 – 1st Demo Session MCA Gamma Spectroscopy identification of isotopes NAA of samples

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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

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**Radiation Interaction with Matter**

Five Basic Ways: Ionization Kinetic energy transfer Molecular and atomic excitation Nuclear reactions Radiative processes

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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 R

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**Stochastic (Probabilistic) With an electron or a nucleus **

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

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**Flux or Intensity Flux is usually for neutrons (n)**

Intensity is usually for photons (’s) Target Beam Density of particles in the beam Velocity of beam particles

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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 (Bi)

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**Attenuation of Uncollided Radiation**

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 ) Notice Different Units: is measured in cm-1 is measured in barns 1 barn = cm2 Constant of Proportionality or Microscopic Cross-Section

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**A beam of neutrons is normally incident on a slab 20 cm thick**

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? 0.01 cm-1 0.1 cm-1 1 cm-1 10 cm-1

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**Attenuation of Uncollided Radiation**

Beams of particles: with intensity I0, interacting with shield (1-D) Point sources: Isotropic source emitting Sp particles per unit time

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**Related Concepts Mean Free Path (mfp or ):**

Average distance a particle travels before an interaction Half-thickness (x1/2) of the slab? Thickness of slab that will decrease uncollided flux by half Similar concepts to mean-life and half-life

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**10 and 6.9 cm 20 and 13.8 cm 116 and 80 cm 1000 and 693 cm**

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 x1/2 for the slab? 10 and 6.9 cm 20 and 13.8 cm 116 and 80 cm 1000 and 693 cm

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Clicker solution

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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) 1.7e8 n/cm2s 1.7e5 n/cm2s 1.5e5 n/cm2s 8e3 n/cm2s 2e3 n/cm2s

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Solution Watch out for sign in exponential

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**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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**Pair Production Compton Scattering The Photoelectric Effect**

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**Example: Photon Interactions for Pb**

Low Intermediate High Energy Photoelectric Effect Compton Scattering Pair Production

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: Gammas

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Problem with Photons 100 mCi source of 38Cl 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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**100 mCi 38Cl, water tank 50 cm dia.**

Problem with Photons 100 mCi 38Cl, 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 μt Linear Scattering Coefficient μs Macroscopic Fission Cross-section Σf, μf for neutrons

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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 DO NOT add microscopic cross-sections

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**In natural uranium (=19. 21 g/cm3), 0. 720% of the atoms are 235U, 0**

In natural uranium (=19.21 g/cm3), 0.720% of the atoms are 235U, % are 234U, and the remainder 238U. From the data in Table C.1. What is the total linear interaction coefficient (macroscopic cross section) for a thermal neutron in natural uranium? 0.24 cm-1 cm-1 238U: 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: rad = 1 Gy rad = Radiation Absorbed Man Dose rate = dose/time

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**Kerma (Approx. dose for neutrons)**

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/cm2 s] Notice Difference tr(E)/ =mass interaction coefficient (table C3) E = particle energy [MeV] = flux [particles/cm2 s] Engineering Equations – PLEASE Watch out for units!

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