Presentation on theme: "NE 301 - Introduction to Nuclear Science Spring 2012 Classroom Session 8: Radiation Interaction with Matter Non-Charged Radiation Mass Attenuation Tables."— Presentation transcript:
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
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
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
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
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
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
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
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 )
Attenuation of Uncollided Radiation 9 Beam with intensity I, interacting with shield (1-D)
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
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
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)
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
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
Clicker solution 16
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
Solution 18 Watch out for sign in exponential
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
20 Pair Production Compton Scattering The Photoelectric Effect
Example: Photon Interactions for Pb Energy High Intermediate Low Pair Production Compton Scattering Photoelectric Effect
22 : Gammas
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?
Problem with Photons 100 mCi 38 Cl, water tank 50 cm dia. What is the uncollided -flux at the surface of the tank?
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
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
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?
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
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.
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!