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Originally posted by Alchemy2

Hi gang!

I don't work for GQ Electronics, so what I am about to say is what I see as a scientist (M.Sc. Physical Chemistry) with significant instrumentation and analytical chemical experience.

This device is an instrument. OK, what does this mean. In this case the unit measures counts that are triggered in the GM tube internally inside the unit (or external for those who have modified a unit or have a remote logger unit with external input). The internal GM (Geiger-Muller) tube is set to yield a response to X-ray, beta and gamma ray energies (see manual for energy windows for respective radiation energies that are detected). Most standard GM tubes are about 20% efficient, which means that about 1/5 counts is triggering the unit.

In order to get an idea of the environment one is measuring, one needs a good (1-2 days minimum) sampling of the BACKGROUND natural radioactivity in your area. This is certainly where this unit shines - because the radiation field is relatively uniform and orientation of the tube should have no or very little effect to the number of counts (CPM). It is NORMAL to see variance in the background, because natural background is truly random, and will vary over a range. Example, here in Winnipeg Canada, I see usually 20CPM +/- 8CPM.

Rain events show elevated CPM with windows open in the house. Some natural precipitated nuclides will raise the background slightly. Temperature and pressure variances between inside and outside affect the radon concentrations present in the air naturally, and this will affect the background as well since 222Rn gas decays to 210Bi and 210Pb, both beta emitters, which attach to fine particulates in the air (dust).

Bear in mind that the uSv/h dose rates are approximate, since the only way to truly measure these is to evaluate under lab conditions the energy of the ionizing radiation and the degree of absorption of this energy by tissues, which vary with type. Since this has been evaluated over the years, the numbers seen here are a good estimate only. If you want to use this device as a dosimeter of sorts, you need to clear the total counts, then note the times of start and finish of the counts, and evaluate an average CPM value. the default is 200 CPM = 1.0uSv/h set in the unit. Take your average CPM / 200 to get uSv/h average dose rate, and total CPM /200 for total dose in uSv.

When measuring a sample to test for radioactivity, remember that source geometry and tube orientation and distance have HUGE effects to the result. Make sure that the background is subtracted from the reading to get the contribution due to the sample being tested. Bear in mind that if you see a variance of say 8-10CPM like me in the normal background, any small CPM changes may be within this limit and may not be valid, or are below the detection limit since one cannot truly say for certainty that the very small increase in CPM is truly due to the sample.

Be careful when checking food. The closest this tube can get to the sample is ~1cm, or where case contact occurs. Very weak emitters like bananas with 40 potassium (40K) and low natural abundance of this nuclide may not be detected easily.

I urge those who want to play with this unit for measuring a variety of items, and samples to get both a general source of beta and gamma (like a uranium or pitchblende ore sample) and a calibrated Cs-137, Co-60, or Sr-90 source that is treated like a virtual point source to calibrate their unit to determine overall efficiency of the unit to the activity of a known source.

Experiment with the ore sample, and you'll see geometry of the source and tube and orientation and distance affect the CPMs over background a lot! Also, if you want to measure activity, remember that the Bq is a disintegration per second, so divide your CPM by 60 to get CPS.

1uCi = 37000Bq, by the way...

That's it for now.

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