1. 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

2. 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

3. 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

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

5. 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

6. 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

7. 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

8. 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)

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

10. 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

11. 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

13. 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

14. 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

15. 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

16. Clicker solution

17. 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

18. Solution
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

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

22. : Gammas

23. 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?

24. 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?

27. Linear Coefficients – Macroscopic Cross Sections
Linear Absorption Coefficient μt
Linear Scattering Coefficient μs
Macroscopic Fission Cross-section
Σf, μf for neutrons

28. Neutrons:

30. 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

31. 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?

32. 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

33. 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.

34. 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!