# Determining Gamma Ray energy from spectrum

## Homework Statement

(Preface: I am not a physicist)

The pulse height spectrum of a radioactive source known to emit high energy photons was measured using a small detector. Three distinct peaks were observed at heights of 7.38, 6.49 and 5.60 V, along with a continuous portion of the spectrum.

i) Describe the processes involved in the origin of these peaks.
ii) What is the initial gamma-ray energy?

## The Attempt at a Solution

The detector is specified as small, so singular photon reactions are presumed.

From my very limited knowledge, i assumed the largest peak to be the photopeak/ full energy peak, and perhaps the other peaks were related to double escape from pair production or a compton peak from backscatter. As mentioned above, I am not a physicist, so be gentle!

Paul Colby
Gold Member
I see no way to solve this problem without knowing more about the detector. What are you studying at the moment in your class?

we have studied photon interactions and a small amount on generic rules for detectors, this is a question from a previous exam that i cannot understand.

Paul Colby
Gold Member
Well, the problem statement is incomplete and there is nothing I can do about it. Typically semiconductor detectors are "small" and work by collecting charge deposited (as particle-hole pairs). The energy deposited is proportional to the energy it takes to generate particle-hole pairs. For larger energy gammas > MeV type energies a typical detector might be a NaI crystal used as a scintillator. In this case one used phototubes to count photons which are proportional to the deposited energy. Just giving you the voltage of the pulses out of the amplifier alone tells you nothing of the deposited energy.