link to lecture transcript - UT-H GSBS Medical Physics Class Site

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At least for a while it’s very flat and then it starts trailing off. So we drop two orders of magnitude for five orders of magnitude of energy and we say the Compton scatter coefficient is essentially independent of energy. 8
What about the dependence of the Compton coefficient on atomic number Z? The model we are using is a free electron gas. We are looking at the probability of an interaction per electron. We are not talking about the nucleus. Nor are we talking about the size of the nucleus or the number of electrons in the atom. Consequently, the electronic coefficient for Compton scatter is essentially independent of Z, and, as we have seen previously, it is also roughly independent of photon energy. So what is it dependent on? About the only quantity the linear attenuation coefficient is going to be dependent on is density. The mass attenuation coefficient is going to be essentially independent of everything. Consequently, when Compton scatter is the predominant interaction, the linear attenuation is going to be linearly dependent on the density. AAnother th way of f saying i this thi is i “Equal “E l masses attenuate tt t equal l amounts.” t ” That’s Th t’ actually t ll a very good d thing; thi and d the reason why that’s good results from the fact that radiation dose is defined to be energy absorbed per unit mass. If we look at the absorption of energy in one gram of bone versus one gram of soft tissue with the same incident photon fluence, we find that one gram of bone absorbs approximately the same amount of energy as one gram of soft tissue. Equal masses absorb equal amounts. How is that different from a situation in which the photoelectric effect is the predominant interaction? Remember that for the photoelectric effect, absorption is going to be very strongly dependent upon atomic number. So high-Z materials absorb a greater fraction of photons than do low-Z materials. This difference in absorption gives rise to contrast in diagnostic imaging and an enhancement of dose to bone in low-energy radiation therapy therapy. This was a problem in the pre-megavoltage pre megavoltage era when we were treating patients with 100 kVp or 250 kVp x-rays. Because there was so much photoelectric interaction, for a given amount of radiation, bone would receive a much higher dose than soft tissue. When we’re in the Compton regime, bone receives the same dose as soft tissue, and that’s good. Now, from the point of view of imaging, Compton interaction is not good because we don’t have as much contrast. Recall that equal masses absorb equal amounts, so that because bone is a little denser than soft tissue, we get a little bit of contrast between bone and soft tissue. From the point of view of shielding, the Compton interaction is very good because equal masses absorb equal amounts. So an equal mass of lead is as good a shield as an equal mass of concrete. Now if you were a builder what would you rather work with, lead or concrete? Concrete is a standard building material; builders know how to pour concrete. They need to make sure there aren’t any air bubbles or anything like that. It is easy to pour four feet of concrete. Consequently when we shield a radiation therapy facility we are going to be looking at conventional building materials to do our shielding; we just use thick walls. Several inches of lead shielding may be just as effective a shield as a few feet of concrete, but the lead is much harder to work with and builders don’t like to use it. 9
Page 1 and 2: Today, we continue our talk about C
Page 3 and 4: We previously derived an equation f
Page 5 and 6: So σ 0 is the classical cross sect
Page 7: Let’s look at this graph. Notice
Page 11 and 12: The energy transferred to the elect
Page 13 and 14: If we plot the energy transfer coef
Page 15 and 16: The energy transfer coefficient has
Page 17 and 18: What about the scatter coefficient?
Page 19 and 20: Using this equation we can now plot
Page 21 and 22: This probability is represented by
Page 23 and 24: This figure is a plot taken from Jo
Page 25 and 26: We can always calculate what the ma
Page 27 and 28: Remember dσ/dE is relatively const
Page 29 and 30: Now we also look at the graph and s
Page 31 and 32: First, let’s recall that the atte
Page 33 and 34: What we now want to find out is how
Page 35 and 36: So far, our approximations have bee
Page 37 and 38: These values were tabulated by Hubb
Page 39 and 40: So what is the effect of binding en
Page 41 and 42: At higher energies there is no diff
Page 43 and 44: Note that this graph is for very lo
Page 45: The fraction of energy transferred

link to lecture transcript - UT-H GSBS Medical Physics Class Site

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