I've been doing a bit of reading on other solid state detectors and have decided that it's a great idea to detect muons rather than gamma rays. Secondary showers consist mainly of electrons, photons, and muons. Unlike photons, muons are easily able to pass through thick metal shielding. This means I can detect only muons by adding thick metal around the pin diode. (I was going to add some shielding anyway to prevent electromagnetic interference. I realize now that this simultaneously decreases gamma ray sensitivity in a somewhat unpredictable way.) The benefit of detecting muons rather than gamma rays is that the node will detect less terrestrial background radiation, which is important if I intend for the node to be used in somewhat populated areas.
At this point I'm not sure what kind of material or thickness is needed for the shield, nor am I totally positive that detecting only muons is the right way to go. Thankfully, my detector design is flexible enough that I can continue working on the electronics and software while simultaneously working on the particle physics aspect.
Coincidence detection might also be something to look into. This is where two PIN diodes (or hopefully array of diode) have shielding in between them, and a circuit is used to record only events where the top detector get a pulse a certain amount of time before the bottom detector. This should reduce terrestrial radiation interference even more.