physics questions

[BraggPeak]

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on the 2012 raphex there is a quesiton that says: for 6 MV x-rays, arrange the following tissues in order of INCREASING absorbed dose: muscle, fat, bone

The answer according to them is: bone, muscle, fat and they say it's because the # electons/gram is greatest for fat (high hydrogen)

i thought compton was based on e- density, which is higher for bone:

another question from 2012 raphes: which interaction predominates for portal images from a 6 MV machine?

answer per raphex: compton with some photoelectric

isn't the answer compton with some pair production???

 

[Gfunk6]

To answer the second question:

Photoelectric effect is predominant at < 30 KeV
Compton is predominant at > 30 KeV and < 25 MeV

While pair production is present at < 5 MeV it does not predominate until > 25 MeV.

Therefore Compton > Photoelectric > Pair production at 6 MeV.

 

[BraggPeak]

According to Khan, there is no photoelectric at 6 MV.

 

[radiaterMike]

According to Khan, there is no photoelectric at 6 MV.

Not sure.... But maybe the trick is that a 6 MV beam has an average energy of ~2 MV. 6 MV is the max energy.

 

[Gfunk6]

According to Khan, there is no photoelectric at 6 MV.

Perhaps after the primary beam is attenuated, you get more photoelectric interactions from secondary photons?

 

[Pointless]

Its a bad question. Our physics instructor laughed at it when we brought it to him. As for the first one, my understanding is that electron density is highest in fat (most amount of hydrogen), and I believe this is reflected in the f-factor tables at that energy.

 

[Werg]

I am not an expert in this, but my understanding is that absorbed dose "susceptibility" is really about electron excitation energies, rather than simply electron/mass densities.

For that reason, fat would receive the highest absorbed dose, even though muscle has a higher electron/mass density, but a higher mean excitation energy. That is because, proportionally, fat has a lot more carbon (low ionization energy), whereas muscle has a lot more oxygen (higher ionization energy). The proportional hydrogen content is about the same for both.

If you really wanted to predict things from first-ish principles, you'd probably need a lot of info about the energies of the bonds in the constituent molecules of the materials, etc. Way outside my realm of knowledge (and interest).