Photovoltaic cells are generally made with a PIN structure, with a large undoped intrinsic region between the P and N doped regions. I think that such a PIN diode, when back-biased (the normal case with a photovoltaic cell) will conduct like the avalanche tube of a Geiger counter when exposed to gamma rays or some other kinds of radioactivity.
There are cheaper PIN diodes available than solar cells, but for detecting radioactivity, a large total area is advantageous.
opengeiger.de has an open design for a PIN-diode-based Geiger counter using a low-capacitance PIN diode.
Probably the most practical radioactivity sources for testing such a Geiger counter are radioactive fluorescent light starters (now, thankfully, becoming obsolete) and supermarket potassium chloride, which emits mostly β particles and some gamma rays.