How to calibrate a homemade Scintillation Counter?

[WarpedWatch]

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.

 

[Sideways]

Calibration's a tricky subject - not easy to explain on a forum. There's a lot of good books out there; Leo's book has a few chapters on PMT/scintillation counters with a how-to approach; I'd recommend it.

https://www.amazon.com/dp/3540572805/?tag=pfamazon01-20


If you have a good sized scintillator, you should "see" cosmic rays quite frequently (on the order of several-second intervals, at least). I hear salt substitute (potassium chloride) makes a fair beta source, if radioactive samples are out of the question. (Not sure about alphas.)

What you used to calibrate, though, will depend on what you intend to use it for.

 

[WarpedWatch]

Sideways,
Yes, I've heard the same thing about salt - except it was water softener salt, supposed to have lots of Potassium 40 in it. How comforting to know!

Recently, I found out that radon at 4 pico Curies per liter equates to about only 8 disintegrations per minute, so unless the ZnS(Ag) gives a nice big pulse of photons, radon might not show up at all amidst the background of cosmic rays, etc.

Somebody suggested I try attenuating LEDs. They give fairly narrow wavelengths and some apparently have very fast frequency response, so maybe I can pulse them and use them as some kind of reference point?

thanks for your inputs,
Mark

 

[Vanadium 50]

As sideways said, this is not a simple thing to do, and your efficiency will depend on what you are looking at. I would expect very low efficiency for alphas, largely because it's hard for them to penetrate the wrapping.

Most efficiency measurements involve three devices, and you only have one.

 

[jennyhicks]

I'd always use someone for calibration for peace of mind like http://www.lambda-cal.co.uk

 

[herbamatic]

Perhaps you can use a smoke detector? The ones with americium 241 in them, they have a known amount of radiation, though intensity is highly dependant on distance from the source. It decays to alpha and gamma-- I worked on a project were we used Am241 as the xray source for an xray spectrometer. They will have the activity printed on the smoke detector, you would just have to compensate for decay over time.
Also, yes, "salt" can be radioactive its due to potassium 40. It is present in a known amount in the Earth's crust (0.0118 %). There are gamma ray spectrometers that are used to determine a potassium percent based on the gamma emmisions (they also do U, Th, see for instance this device: http://www.radiationsolutions.ca/index.php?id=78). So i would think if you knew the amount of K, you should be able to work out the activity. Just get ahold of a known amount of KCl. Also read here:
http://www.rerowland.com/K40.html
Don't forget, you will likely need several calibration points, especially if the linearity of the response is not great.
I don't think you can use the granite around you, the concentrations of K, U, Th in them vary quite widely, so unless you know the concentration of those elements in that granite, you will not be able to precisely calibrate
I am just about to build my own scinc meter. I can't wait.