I'm trying to build a basic detector for x-ray or low energy gamma spectroscopy. I have a big photodiode with a CsI(Tl) scintillation crystal glued on top (Hamamatsu S8559 for x-ray detection ) and want to detect for example gamma radiation from a ~1uCi Am-241 source.
To do spectroscopy and being able to discern the pulses by energy I would first have to be able to measure single pulses coming from radiation hitting the detector. So I tried to calculate the charge of these pulses:
The CsI(Tl) scintillation crystal  has a light yield of 54 photons/keV and Am-241 decays partly via gamma decay with an energy of 59.541keV, therefore I calculate the number of 550nm photons produced by the crystal after being hit with a gamma photon to be 54 photons/keV * 59.541keV = 3215.2 photons.
Quantum efficiency has been calculated to be about 0.586 for my photodiode at 550nm, so I would expect 3215.2 photons * 0.586 = 1884.1 electrons being generated by the photodiode, which equals a charge of about 0.3 femtocoulombs.
This seems really low, so my questions are:
I realize this is not the physics stackexchange, but can someone check if my calculations are realistic?
If this is really the charge I have to detect, how would I do this? Should I use a charge amplifier circuit with a tiny feedback capacitance, for example with an LMC662? 
Is this photodiode + crystal combination even usable for this application or would I have to use a silicon photomultiplier?
Sadly my photodiode has a huge junction capacitance of over 500pF even with the highest allowed reverse bias. Can I use photodiode bootstrapping to reduce the capacitance? 
I found a circuit that seems close to what I need in , but it uses a photodiode without scintillation crystal that is also much lower capacitance than mine, so I'm not sure it would work.