Answer to Question #10281 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

Q

I have a 4ec Geiger-Mueller (GM) counter by SE International. I have noticed that the needle will jump up close to 100 cpm (representing equivalent dose rate of about μSv 1 h-1) occasionally on the X1 setting. Normally it stays around zero. Should this be a cause for concern? I am in the melting-of-precious-metals industry and we do use a Fisher x-ray fluorescence machine (XRF). The reason I am concerned is I haven’t noticed it jump up to this level before, but this is the first time I have really let the GM counter go for about 10 minutes, so it could have been doing this the whole time. I had a customer yesterday who had material (slag from melting precious metals) that, when we put on the x ray, did show some strontium and zirconium. I know that these two can potentially be radioactive or can be completely harmless as well depending on which type it is. My question is, how can I be assured that the radiation levels are fine in my store, and what type of level is dangerous on the GM counter? The counter will beep frequently, but not exceed 100 cpm. I assume this to be background radiation, but do not want to assume.

A

We should begin by noting that typical background levels that one might expect to measure using an instrument of the type you mention range from about 0.05 µSv h-1 to 0.2µSv h-1 depending on where you are and what types of structures are in the vicinity. As is discussed below, routine background readings as high as 1 µSv h-1 would be very unlikely. While sustained readings at this level are sufficiently above background that they would warrant investigation to determine the cause, they are not high enough to produce any negative health effects. It is true that zirconium is often associated with naturally occurring radioactive materials, and if the detector were placed close to such material you may see an elevated reading, but I would expect the reading to be sustained as long as the material was maintained close by. The fact that you are observing these elevated readings as occasional spikes of short duration makes me suspect that these are spurious signals probably not associated with ionizing radiation, and you need have no concern for any negative health consequences. I shall attempt to provide some additional rationale for this view.

I have not used the specific GM instrument that you have identified, but I have used many others. Typically, GM detector-based instruments, as well as other pulse-measuring detector types that are used to measure background radiation levels, will show a considerable amount of variability in their readings. Much of this variability is a natural result of the normal statistical fluctuations that are part of the detection and measurement process.

For an analog ratemeter of the type used in the 4ec instrument that you mention, the statistical uncertainty (represented by the relative standard deviation in the count rate) varies inversely as the square root of the count rate and the meter time constant. I could not find a specification of the time constant for the instrument in question on the manufacturer’s website, but if we assume a relatively fast time constant of 4 seconds and a background count rate of 0.17 counts per second, or 10 cpm (which is the count rate we would expect for the 4ec detector when the background dose rate is 0.1 μSv h-1 and the response is 100 cpm per μSv h-1), the relative standard deviation (standard deviation divided by count rate) would be about 0.87. Most all of the observed count rates would be expected to fall within about three standard deviations of the average count rate, implying that if the usual background is 0.1 μSv h-1, yielding a count rate of 10 cpm, three standard deviations added to the usual background rate would be 10 cpm + (3 x 0.87 x 10 cpm) = 36 cpm. We might then conclude that a true reading of 100 cpm would be a very unlikely event when measuring typical background.

There are some relatively rare natural events, such as some unusual cosmic ray bursts, that might produce fluctuations that large. However, I am more inclined to think that the larger-than-expected transients that you appear to be observing relatively frequently may be associated with an unintended and erratic behavior of the 4ec instrument or other nonionizing radiation influencing factors. Larger-than-expected swings of the meter needle may be associated with a simple inertial effect in which a smaller transient gives the needle movement a boost, and inertia carries it beyond the expected range. Other electronic design characteristics or faults that might result in spurious signals could also be associated with the unexpected transients.

A common environmental influence that can produce notable false readings is radiofrequency (RF) radiation. RF radiation may induce electrical currents in the instrument and produce false readings. There are a variety of sources that can produce such RF that can affect readings on a typical GM instrument. These include microwave radiation, RF from some cell phones and other electronic devices, high-voltage switches, and certain motors and other electric appliances/apparatus. RF pulses may be of short duration, depending on the source, and could appear as spikes on the GM meter. If you have such sources in your work area, you should be aware of whether any are operating when you observe the effect on the GM detector.

Naturally, the GM detector would also be sensitive to the x radiation from the Fisher XRF instrument. I assume the XRF unit is of the handheld type and probably uses a radionuclide source, such as 109Cd, as the excitation source. These units produce relatively low levels of gamma radiation, but certainly sufficient that you might see a response from scattered radiation if the GM was in proximity to the XRF unit when it was operating. From what you say, it does not sound as if the XRF instrument is the source of the elevated readings on the GM instrument.

From your description, it sounds as if the GM instrument is working properly, with the possible exception of these transient events, and if this is the case you may not want to invest more money in having the instrument checked out and/or possibly repaired, especially if you are able to identify the likely cause of the transient readings. One item that is important for you to acknowledge is that portable instruments used for dose measurements, such as the SE International instrument of concern, should be calibrated annually to ensure that the instrument is responding properly. The next time you have the instrument 0calibrated, if you are still experiencing regular elevated transient readings, you might discuss this with the calibration facility staff; such facilities often offer repair services and they may be able to take a closer look at the instrument to see if they can identify any problem. Of course, if the instrument is still under original warranty, and the sporadic readings continue, you should contact the manufacturer to get some resolution. It may be worth contacting the manufacturer anyhow to ascertain whether your experience with the instrument is common among other users.

Your question as to what level on the instrument represents a "dangerous" level is somewhat subjective, but we can offer some perspective.

The natural background radiation dose (including radon exposure) that most people in this country experience amounts to about 3,000 μSv per year. The average individual also receives about an additional 3,000 μSv per year from the application of medical diagnostic radiation. Radiation workers are allowed to receive additional doses up to 50 mSv per year from their work with radiation sources (although most receive much less). There are several occupied locations in the world where external radiation doses from natural background levels are much higher than in the United States,some more than 100 times higher, leading to individual annual doses of tens of mSv per year. No ill effects or long-term disease have been associated with exposures at any of these levels.The Health Physics Society, in Position Statement PS010-2, recommends that we should not attempt to assign any risk estimates for annual doses less than 50 mSv. Given all these considerations, my personal recommendation would be that we treat the value of 50 mSv in a year as a level that has at least some potential for causing harm. This is not to say that we should not attempt to identify the causes for and to mitigate radiation levels that are lower than this, but simply that levels below this are probably not going to produce any measurable health impact.

I realize that the above may be more than you required or desired, but your question raises points I think worthy of the discussion. I hope you are comfortable with the answer.

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 4 June 2012. 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.