[Gate-users] Energy spectra of coincidences is inconsistent

[Artur Meinusch]

Hello,
The coincidences sorter assigns the singles and their respective
energies in the temporal order by of deposition in the scintillator.
In a cylindrical PET the energy1-photon is not necessarily registered
always in the same module/rsector!

To understand the spectra from your simulations consider the following:
In the case that one of the two "back-to-back" photons escapes without
an interaction, it is still possible, that the other photon triggers a
coincidence! This occurs when the first interaction of this single
photon is a backscatter (large Compton-angle, deposited energy @
up to (2/3)*m_e*c^2), stored in the “energy1”-branch and the
backscattered photon deposits its energy in the second (opposing)
scintillator. In this case the second deposited energy of up to
(1/3)*me*c^2 will be stored in the "energy2”-branch.

The first deposited energy will be stored in the "energy1"-Branch of
"Coincidences”, i.e. @~340keV (@Compton-Edge).
The backscattered photon has an energy peak @~170keV (energy2).

This happens only if the coincidence window is larger than the time
required by the backscattered photon to arrive the other scintillator.

To get “identical” spectra you have to sort the energies (energie1 &
energie2) by rsectorID.

best regards,

Artur Meinusch


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