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

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:


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. Please see the link to the Alibaba website. 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, PHD

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 13 August 2019. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.