Answer to Question #10421 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
I came across an issue which is causing me an accuracy problem in measuring for low-level contamination. I used a 44-9 Geiger-Müller (GM) pancake probe which is an alpha-beta-gamma type detector. I noticed an odd reading of 125 to 175 counts per minute (cpm) which was in close proximity to a common plastic sandwich bag. My background count is about 50 cpm with this equipment in this area. I thought it was something on or in the bag, but I could pretty much repeat this outcome with another “new” bag. The thing I noticed was that the bags had a pretty heavy static electric charge on them. This was enough to lift the hair on my forearm up as I moved the bags within an inch of it. I am assuming that the static field penetrates the mica surface of the detector and is enough to react the same as an alpha radiation would. This is the first time I realized this was possible, as I haven't read any accounts of this before. Could I assume that a detector that was not alpha capable would have shielded this out? Is this a pretty common occurrence with some plastics or hard-rubber materials in a low-humidity environment?
Your observations and preliminary conclusions I believe are correct. Static charges that build up on insulating surfaces often represent the cause of high-induced voltages, often thousands to tens of thousands of volts, as most of us can confirm from our experiences of transferring charge to ourselves through friction by walking on synthetic carpet materials in dry weather and then touching a conductor, which leads to a mild electric shock often accompanied by an actual spark (that could be viewed in dim light) as our finger approaches the conductor. The plastic bags were clearly charged, as you described, and probably held the potential for delivering a few millijoules of energy.
In general, the flow of current from a statically charged body proceeds from an object of high electrical resistance (good insulator) to an object of lower electrical resistance, and this is what likely occurred as the bag was brought near the detector. The energy transfer could easily be sufficient to yield some ionization of the gas and/or excitation that produced ultraviolet photons capable of promoting ionization. Depending on the construction materials and design characteristics of a radiation detector, the static electricity can also affect other components, especially semiconductors that are often quite sensitive. Such events can lead to false readings and/or damage to the instrument. Your inference that static electricity effects can be shielded against is generally true. Using a good electrical conductor to surround sensitive components and carry charge away is often effective.
The type of interference you observed is common enough that an American National Standards Institute (ANSI) standard has been written that provides recommendations for some radiation detection instruments to perform within acceptable allowances in specified electrostatic fields. The testing requirements are specified in ANSI N42.17A, American National Standard for Performance Specifications for Health Physics Instrumentation-Portable Instrumentation for Use in Normal Environmental Conditions (1989 revised 2003).
In addition to the more publicized suffocation hazard and antigreen impact of plastic bags, I guess you’ve discovered another negative aspect of their use. Since the GM detector you refer to is a fairly common one, I expect other users may have encountered, knowingly or not, similar effects. Thanks for the question.
George Chabot, PhD