Im not sure what type of use a radiation detector would be used for in the paranormal but just looked on E-Bay and they have some used ones for like $15 to $30 dollars. More advanced ones run up to $200.00.
Like MJ's site says it detects Alpha, Beta, and Gamma Rays which are radiation. Do you think spirits give off these types of radiation? I guess if you got one it would be interesting to find out.