Question

For something like Gamma Ray Spectroscopy, what do taking photo peak measurements give us? What does the Count vs Channel plot tell us?

Answer 1

Main purpose of Gamma ray spectroscopy is to measure energy of gamma ray emitted.

Since we can not measure energy of gamma ray directly we employ the methods such as spectroscopy.

Detection of gamma rays is primarily dependent on causing gamma ray photon to undergo an interaction that transfers all or part of the photon energy to an electron in the absorbing material.

For a detector to serve as Gamma ray spectrometer it must behave as a conversion medium for gamma rays who have reasonable probability of interacting to yield fast electrons and to detect these fast electrons it should also act as a conventinal detector.

Gamma ray photons interact primarily through follwing processes :

1) Compton Scattering

Predominant for low energy gamma rays (100 KeV)

2) Photoelectric Effect

Predominant for high energy gamma rays(5-10 MeV)

3) Pair production

Most probable over the range of energies between above two process

2) Photo-Electric Effect

a) In photoelectric effect when gamma rays are incident on material, photoelectrons are created.These photoelectrons mainly comes from K-shell with kinetic energy.

  

Here B.E. is binding energy of electrons whic is of the order of few Kev for most of the materials.

Thus we can safely say that whatever incident energy of gamma photon had,now it is kinetic energy of photo electron.

b) In this process it is also possible that one or more low energy electrons are created corresponding to absorption of the original binding energy of the photoelectron.

And thus we can say that if nothing escapes from the detector ,then sum of the kinetic energies of th eletrons that are created must equal to the original energy of the gamma ray photon.

c) In a plot of count vs Channel

Counts indicates no. of photoelectric effects that took place

Channel in short tells us kinetic energy of the photo elctrons.

while photopeak tells us the gamma ray energy.

Consider if mono energetic gamma rays are being emiited then after doing gamma ray spectroscopy photoelectron kinetic energy will nearly equal to the gamma rays involved.

And if monoenergetic gamma rays are involved our Counts Vs Channel plot will simply look like a delta function(this is nothing but a photo peak).