Increasing diode count doesn't improve sensitivity (TODO: check with comparator and counter).
Scintillator crystal is a MUST to improve sensitivity to be able to register natural radiation photons.
I highly recommend replacing BPW34 with a bigger diode with more surface area. For example, FirstSensor is manufacturing PIN diodes with a build-in scintillator.
Probably losing some gain to LMC662 bandwidth.
Add a guard ring to the second stage on the PCB.
Improve sensitivity
Add comparator and counter
Sound or light effects
Improve SNR
Increase gain
Spectrometry
