The Multichannel Analyzer and it's supporting software is probably the most difficult part of the project. Open designs for them are fairly lacking and commercial products are fairly expensive. While not demanding, care will need be taken when dealing with high voltages and nanosecond length signals. Signals will be fed to the analog front end where they will amplified and where the protection circuitry is located. Then they will fed into an ADC connected to an FPGA, which will handle all the signal processing. This is all controlled by a microcontroller which will also communicate with the host.
For the PMT, I've gone with the XP5312, a surplus PMT that was manufactured for medical imaging. I haven't been able to find any documentation for the support circuitry on the back but I'm done reversing it and seems usable for my purpose. It seems to be setup for the a grounded cathode which is how i was going to configure it anyway. The SMB connector on the back is also going to be pretty useful.
For the scintillator I've settled on BC-412, a specially made plastic scintillator. The choice was mostly down to cost and availability. While not exactly optimal, it's significantly cheaper than the alternatives while having satisfactory performance. It also seems to be slightly neutron sensitive according to it's documentation, which could be interesting down the road.
When radioactive elements decay energy is released, some in the form of gamma radiation. The amount of energy is dependent on the element. This allows us to determine the composition of the radioactive element by measuring the energy of the gamma ray. This is can be done using gamma scintillators, materials that when struck by gamma rays emit visible light, the amount of which is proportional to the energy of the beginning gamma ray. The amount of light is relatively small and must be amplified, in this case using a photomultiplier tube, which does pretty much what it says on the can. This signal is then sent to multichannel analyzer where it's measured and stored.