Photodiode scintillation detector for gamma spectroscopy
You may consider using an avalanche photodiode so very weak radiation could be detectable. 

Avalanche photodiodes generate current, so basically a single photon generates thousands of electrons and makes it easier to measure the number of particles hitting the detector. There are ready devices that use single-photon detection avanalche diodes, but I suspect they are quite pricey. 

The other thing is that: 


  "Americium-241 decays mainly via alpha decay, with a weak gamma ray
  byproduct."[1]


Alpha rays are basically 4|2 He and penetrate very tiny distances. Am is a bad gamma ray source, so you may want to use other sources of radiation to detect gamma rays.

[1]https://en.wikipedia.org/wiki/Americium-241#Decay