# What is a photopeak?

I'm currently studying for an exam, where I'm looking into decays and gamma radiation. I then have a diagram where there are stuff like "Compton scattering", "Single Escape Peaks", "Double Escape Peaks" which all are calculated if I know the value of the photopeak. And all these three are actually quite well explained

But, nothing in my notes actually tells me what the photopeak is :/

So could anyone explain, just briefly, what the photopeak is/means, at least in this regard.

Thanks in advance :)

• The photopeak is the peak formed by the case where the gamma ray deposits all of its energy in the detector. This can happen because the gamma interacts once through the photoelectric effect. It can also happen if the gamma initially interacts through Compton scattering or pair production, but the secondary particles then are also completely absorbed.
– user4552
Jun 23, 2013 at 16:02

## 2 Answers

[Oops, posted this as a comment, but meant it to be an answer.]

The photopeak is the peak formed by the case where the gamma ray deposits all of its energy in the detector. This can happen because the gamma interacts once through the photoelectric effect. It can also happen if the gamma initially interacts through Compton scattering or pair production, but the secondary particles then are also completely absorbed.

• So is it usually the highest peak, or peaks ? 'Cause I'm thinking that it is possible to be more than one depending on what you are detecting ? Jun 23, 2013 at 16:50
• @DenverDang Compared to the three other things you listed, the photopeak will have the highest energy, since everything else only captures part of the original energy. It won't necessarily have the largest area or highest height. Of course there could be a coincidence peak, where two gammas are fully absorbed at the same time - this will occur at double the photopeak energy and will be stronger in detectors with worse time resolution.
– user10851
Jun 23, 2013 at 19:22
• @ChrisWhite So if I look at this spectrum: speedy.sh/BPDSx/spektre-2.pdf it is not wrong to conclude that the three peaks in the top spectrum is actually photopeaks from the decay of Mn-56 to Fe-56 ? Or do I actually have to calculate "the three other things" ,which I have done in the bottom spectrum (Just a zoomed view of the top spectrum), to be totally sure which is which ? Jun 23, 2013 at 19:52

A peak which is formed due to the complete absorption of photon energy in the detector is called "photo peak",