Answer to Question #12984 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

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

Q

I have a Geiger-Mueller (GM)-based gamma monitor (range: 0.01 to 100 mR/h), which normally shows a background of 0.01–0.02 mR/h. What would be the minimum detectable radiation level of the instrument over and above the background? Do I need any additional information to calculate the same?

A

The determination of the minimum detectable level that you desire depends on a number of factors. An important one is what type of instrument you are using—in particular, is this GM monitor an analog type that displays the exposure rate based on the signal output voltage that results from the radiation field measurement, or is it a digital type of monitor which accumulates counts over predetermined intervals and uses these values to convert to exposure rate? The distinction between these two types is important because the standard deviation in the reading is determined differently for each, and such deviation in the background rate will determine the detection limit. I shall briefly deal with these. Other considerations relate to the count rate sensitivity of the detector and to what quantitative criteria you intend to use to specify the minimum detectable level. I shall attempt to go through an example case to demonstrate a possible approach.

As shown by Knoll (2010) page 646, if the detector is an analog type, the expected relative standard deviation in the voltage signal is given by:

σV/V = 1/(2rRC)0.5,

where r is the count rate, and RC is the time constant for the GM detector. You should know or be able to find the value of the time constant being used and the relationship between count rate and exposure rate. As an example, for a conversion rate of 2,500 counts per minute (cpm)  per mR-h-1, a background rate of 0. 015 mR h-1 would yield a count rate of 37.5 cpm. If the time constant were a relatively long 10 seconds (0.167 min), we would calculate a value of σV/V of 0.283, which when applied to the count rate of 37.5 cpm would yield an absolute standard deviation of (0.283)(37.5 cpm) = 10.6 cpm. Depending on what your decision level was for establishing the minimum detection level, you could use this value as appropriate. People commonly use a net value of three times the standard deviation in the background as an acceptable minimum detection limit. For this case such a value would be 31.8 cpm (above background) or a gross rate of 69.3 cpm, which would translate to a gross exposure rate of 0.028 mR h-1, about twice the assumed background level.

If the instrument is a digital monitor the approach would be somewhat different since such devices operate by accumulating counts for a fixed time interval and then displaying the related exposure rate. A question dealing with a closely related topic has been answered earlier, and I shall not now go through it. I refer you to previous question Q8910, especially paragraphs five through seven of the answer.

I hope this satisfies your needs.

George Chabot, CHP, PhD

Reference

Knoll GJ. Radiation detection and measurement. 4th ed. New York: Wiley; 2010.

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 3 July 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.