Interaction processes

Question 1

What is the process of coherent scatter?

Answer 1

The photon passes a bound electron. The electron absorbs the recoil and begins to oscillate. The electron the re-emits the photon at the same frequency as the photon but at a different angle. It conserves energy.

Question 2

What is the mass absorption coefficient of coherent scatter proportional to?

Answer 2

Z/(hv0)^2

Question 3

What proportion of interactions in tissue does coherent scatter account for?

Answer 3

a few %

Question 4

What is the process of pair production?

Answer 4

A photon interacts with the electromagnetic field which surrounds the nucleus. All of the energy is used to make an electron, positron pair. The energy is shared between the particles, most commonly with a 20/80% split.
The electron and positron then interact by excitation, bremsstrahlung production, and finally annihilation. Annihilation produces two 0.511MeV photons in opposite directions.

Question 5

What is the equation for the angle which the electron and positron are emitted at in pair production?

Answer 5

theta = mc^2/E

Question 6

What is the mass absorption coefficient of pair production proportional to?

Answer 6

Energy (minimum of 1.02MeV), Z, and atomic mass

Question 7

What is the process of triplet production?

Answer 7

The photon interacts with the field around an electron, so a positron, electron, and the original electron are produced. Minimum energy 2.04MeV.

Question 8

What is the process of photo nuclear interactions?

Answer 8

The energy of the photon is greater than the binding energy of the nucleus. The photon is therefore absorbed by the nucleus and either a proton or neutron is produced, making the nucleus radioactive.

Question 9

What is the probability of photo nuclear interactions

Answer 9

Low outside 12-24MeV range, ~5% in 15-20MeV sweet spot.

Question 10

What problems are caused by photo nuclear interactions?

Answer 10

There is induced radioactivity in the linac, the air needs to be changed ragularly to remove the radioactive material, and low Z material needs to be put in the maze to trap neutrons.

Question 11

What are the relative importances of PE, compton, and pair

Answer 11

PE dominant @ low enery
Compton dominant @ 0.03-30MV range
Pair dominant @ >30MV