Answer to Question #13001 Submitted to "Ask the Experts"

I am helping my grandson learn more about radiation and for this reason I purchased a cheap Arduino DIY Geiger Mueller (GM) detector on eBay. It came from China. The tube is of type J305. The circuit only seems to produce a maximum of 220 V which is less than the recommended voltage of 400 V but it seems to work okay at about 150 V. It counts background consistently and when an americium source from an old smoke detector is put near the tube, the count rate increases.

Now for the question: When we take it outside on a sunny day at about noon, the count rate increases enormously when the tube is aimed at the sun. If it is then put in a box or shielded from sunlight, the count rate drops back to background. Also, in late afternoon, the count rate is quite low when the tube is pointed at the sun. Why is this?

My first suspicion regarding the apparent response of your detector system to sunlight is that some ultraviolet (UV) light of sufficient energy is entering through the glass envelope of the J205 GM tube and producing some ionization in the gas within the tube.

GM detectors operate on the principle that the initial gas ionization from an incoming photon or other ionizing particle is amplified tremendously by a gas multiplication effect in which electrons accelerated toward the anode collide with neutral gas molecules and produce further ionization, resulting in an avalanche of charge representing a large, easily detectable pulse. Because of this, even a very small amount of ionization in the gas will lead to a detectable pulse. It is well-known that UV light is capable of ionizing typical GM gases, leading to detectable pulses. The glass walls of the tube will eliminate a large part of the ionizing UVB rays that impinge on it, but it takes very little getting through to initiate a detectable event.

Your experiment with shielding the detector from light is consistent with what we might expect. Regarding your experiment with the ²⁴¹Am source from the smoke detector, I will note simply for your and your grandson's information, the smoke detector relies on the alpha particle emission from the source to function. The alpha radiation, however, is not able to penetrate the thickness of glass of the GM tube, and any increase in reading you see as you bring the source close to the detector is most likely a response to low level gamma radiation emitted during the decay of ²⁴¹Am.

I do find it a bit surprising that your detector is able to operate at the low voltage of 150 V that you mention, especially since the specifications that I found indicated the starting voltage for the tube is 350 V. You might want to check your voltmeter to make sure it is giving accurate readings.

I'm pleased that you are taking the time and effort to expand your grandson's education, especially as it is directed to improving his understanding of ionizing radiation and its detection. Best wishes in your endeavors. It sounds as if you're having fun!

George Chabot, CHP, PHD