Detectors

Question 1

What would you want to use for a photon PDD and why?

Answer 1

Ionisation chamber:
High sensitivity, small energy dependence in response to photon beam, long term stability.
Small volume ionisation chamber allows for good spatial resolution.

Question 2

What would be considerations of using a diode for photon measurements?

Answer 2

Show energy dependent response at large depth and edges of beams.
Can use energy compensated diodes (with high density material on back). These have high directional dependence, but can be mounted with front facing the beam.
Still recommended that you compare with ionisation chamber measurements in standard conditions before you use them for commissioning.

Question 3

What would you use for electron PDD and why?

Answer 3

Can use an ionisation chamber, but these are high dependent on electron energy, and so you need a stopping power ratio correction. If using ionisation chamber, should be parallel plate to minimise cavity perturbation effects.
Diodes response to electron energy is practically energy independent because of small variation with energy of water to silicon stopping power ratio. Still want to compare with ionisation chamber.

Question 4

What would you want to use for photon profile measurements?

Answer 4

Diode provides good spatial resolution, would be good option. Small volume ionisation chamber would also be acceptable.
Need to use photon diode, as diodes have some energy dependence at field edges and at depth.