Chapter 5 Flashcards
Define Fluence
Particles per unit area (in some predetermine amount of time)
Define Flux
Particles per unit time (in some predefined area)
Define HVL
Thickness of specified material to reduce fluence to half
Why is the first HVL usually smaller than subsequent?
Concrete How HVL e changes with energy?
Pb How HVL e Changes with energy?
the first layer filters a lot of low energy photons in the polyenergetic beam
Concrete TVL2 < TVL1 , high attenuation but big scatter just as water (increased low energy component of the beam has lower TVL) decrease TVL with depth.
Narrow Beam even in concrete TVL2 >TVL1 (beam hardening is the main effect not scatter).
For Pb, TVL 2> TVL1, in Pb Pair production is dominant.
Types of interactions
Compton, photoelectric, pair production (2-kinds) Raliegh, atomic interactions
What is the energy of a photoelectron
hv-Eb
Binding energy of a k shell photoelectron in Pb. What does is resonance?
- Spikes are absorption edges and correspond to the binding energies of L and K shells (15 and 88 keV for Pb).
- Resonance occurs when the photon has an energy that just equals the binding energuy of the L shell, significantly increasing the probability of photoelectric absorption
- Beyond the L Shell resonance the probability of absorption decreases ~ 1/E3 until the next resonance at the K absorption edge.
- Absorption edge of water is very low (~0.05 keV)
Where do jumps in photoelectric attenuation as a function of energy come from?
when a new shell becomes available the attenuation spikes then gradually decreases as the energy increases until the beam has enough energy to interact with the next level of shell
What is the dependence of compton scatter probability on Z?
How can you estimate the Shielding based on Compton for two different materials?
- Compton tends to happen with essentially free electrons so it is more or less independent of Z
- Compton effect depends almost exclusively on the electron density which is simply the number of electrons per grams * the physical density
- The number of Electrons per gram is approximately the same for all materials except hydrogen
- Linear attenuation properties can be estimated for two different materials based on the ratio of their difference in physical density (g/cm3) .
Given lead and water, which has the higher mass attenuation coefficient?
Lead is higher in pair production and photoelectric range about equal in the compton range
Compton equations: hv’ in terms of hv theta (e- angle) in terms of phi (photon angle) E (KE of e-) in terms of hv alpha in answer is hv/moc2
Two main neutron interactions
recoiling protons nuclear disintegration
Which absorbs more neutron energy - muscle or fat?
Fat - it has more hydrogen for interaction
Pair Production
If the energy of the photon is greater than 1.02 MeV, the photon may interact with matter through the mechanism of pair production. The photon interacts trongly with the electromagnetic field of an atomic nucleus and gives up all its energy in the process of creating a pair consisting of a negative electron (e-) and a positive electron (e+). The total kinetic energy available for the electron–positron pair is given by (hn - 1.02) MeV. The particles tend to be emitted in the forward direction relative to the incident photon.
Annihilation Radiation
The positron created as a result of pair production as an electron does, namely by ionization, excitation, and bremsstrahlung. Near the end of its range, the slowly moving positron combines with one of the free electrons in its vicinity to give rise to two annihilation photons, each having 0.51 MeV energy. Because momentum is conserved in the process, the two photons are ejected in opposite directions.
The total mass attenuation coefficient (μ/r)
