I have a fast-neutron scintillator which also is sensitive to gammas (much less so, though). When I connect it to a SiPM instead of a PMT, the gamma pulses (obtained from performing pulse shape discirmination) are almost negligible compared to neutron pulses. Under what circumstances could it be possible that a SiPM responds differently to scintillation light from neutrons vs gammas?
One possible reason is the time it takes the particle to dump energy in the scintillator. This will affect the rise time of the pulse. The neutrons must scatter off a nucleus to be detected while a gamma-ray interacts with the atoms in the scintillator which are much larger than nuclei of course.