How a pair spectrometer works?

[ShayanJ]

I need to know how a pair spectrometer works but I can't find a book that explains it in detail. They say that it has three crystals located in a line and the beam is collimated on the central crystal and somehow this signal is only analyzed when there are simultaneous signals from the other two crystals. But I don't understand what signals are these! Are they from the pair-producted electron and positron? But if this is the case, the beam collimated at the central crystal should have been diminished and can't reach the central crystal because it gave its energy to the electron-positron pair!
I really don't understand how this device works. I'll appreciate any explanations.
Thanks

 

[mfb]

They say

Who is "they"?
Which beam of what?

It is hard to answer a specific question if your description is so vague.

I would guess you have a beam of photons, electrons or positrons that can lead to pair production, and either the produced pair or the annihilation of positron and electron can lead to signals in the other two crystals - but that is just a guess.

 

[ShayanJ]

I figured it out. Thanks.