The military Radiac Set, AN/PDR-56 is a portable scintillation type instrument used for detection of alpha contamination. The system includes a large and small interchangeable probe with a probe extension. This system is being phased out by the US Air Force, so new probes are becoming widely available in the surplus market.
The “x-ray” probe for the AN/PDR-56 uses a CaF2(Eu) scintillator/photomultiplier combination to detect the 14-21 keV gammas from Pu-239. The x-ray probe is an assembly which includes the amplifier-discriminator circuits integral to the phototube scintillator housing. The discriminator is a single channel analyzer adjusted to detect Pu-239 gamma rays.
I modified the probe by replacing the replace the CaF2(Eu) by a more conventional NaI(Tl), giving the probe a much wider detection energy range (100keV to 1.3MeV gamma). The following pdf file gives detailed instructions for the modification, as well as on how to trim the electronics to expand the discrimination window: dt590_to_nai
The probe works great! Using a 1″ dia x 2″ long NaI(Tl) crystal and setting the discrimination floor at 100keV, I have a background count of ~800 CPM. I get 10,000 CPM from a 10uCi Cs-137 checksource from 2′ away.
Click here for the PDR-56F scintillation radiac set’s repair manual.