[Gate-users] Missing 80 keV energy peak of 224Ra decay without collimator

[Monika Kvassheim]

Hi all,

I have a simulation with a SPECT camera and alpha emitter 224Ra (using an ion source). I am comparing the energy spectra with experimental results. With both the medium energy collimator and the high energy collimator the spectra are consistent with experimental results. When I run the simulation without a collimator however, the 80 keV peak completely dissappears and I just get a large 240 keV peak. The experimental spectrum without collimator has an 80 keV peak about twice as large as the 240 keV peak. Now I wonder if the well matched 80 keV peak with collimators is really all coming from the atomic deexcitation in the collimator, when it's supposed to be from the emission of 80 keV photons in the 224Ra decay chain. I have included /gate/physics/addProcess RadioactiveDecay in my macro.

Am I somehow getting a wrong emission spectrum from 224Ra or is there anything else that can cause an error like this?

Many thanks,
Monika Kvassheim
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