How to calibrate a homemade Scintillation Counter??? Greetings, I have recently built a homemade scintillation counter using a surplus photomultiplier tube (PMT) and it seems to work very well in the photon counting mode. Problem is, I would like to get some idea of how efficient the overall system is, how does one of its set thresholds relate to actual number of photons and/or energy level (color) of the photons it is counting. I don't have any radioactive sources and, if possible, I would like to avoid having to buy or store any. The rocks around where I live, however, are granite, which I'm sure has plenty of trace elements in it. One idea I've considered: I know that the basement where I work has a radon problem, that its radon level is slightly above what is considered acceptable without abatement. (I think the acceptable limit is something like 4 picoCuries/liter???). Radon breaks down into alpha particles, I think with an energy of about 5 MeV. So I thought about using a ZnS(Ag) screen to detect the alphas. The ZnS(Ag) screen is supposed to emit light that peaks at 450 nm. Its emission drops off at 330 nm on the UV end, but I'm not sure about the red end of its emission spectrum. So my question is: could radon at the levels in my basement workspace provide a crude calibration source? If I make background measurements with no ZnS(Ag) screen and then take measurements with the screen and compare the differences in spectral output, could I reasonably assume that any peak I might see is from the decay of radon in the air around the screen??? Another idea I considered: perhaps cosmic ray background could provide a peak location if I have the right scintillator??? any comments or suggestions would be greatly appreciated, thanks, Mark PS: A few details on the construction of the scintillation counter: The design is fairly simple. It is the calibration aspect that is bothering me, yet I do not expect to get high resolution with this device, just some ballpark numbers. Because the PMT is a surplus module, I can't find its exact tube type, so I don't really know its characteristics (response to color, etc). The PMT outputs across a resistor to a high speed comparator whose threshold is adjusted every few seconds by a digital pot. The digital pot is controlled by a microprocessor, which also records the counts onto an SD card for later analysis. Thanks.