Answer to Question #10531 Submitted to "Ask the Experts"

I recently purchased a Geiger-Mueller (GM) detector and have a question about "calibration." I was told that normally the manufacturer does not provide National Institute of Standards and Technology (NIST) source calibration unless specifically requested, but my use is as a "hobbyist" (rock collector mostly) so I'm mainly interested in doing simple surveys—such as "is this rock safe to put on my shelf or did I accidentally pick up a piece of pitchblende or some such?”

If I base my surveys on simple  counts per minute (cpm) for a longer period of time to set a "background" and any rock I want to test just compare to the "background" based on cpm, is a "calibration" with a NIST source really needed? 

Also, does calibration merely provide the correlation between cpm and dosage units or does calibration of a GM counter fundamentally affect the "counting" capabilities (i.e., does calibration alter the cpm or does it just relate cpm to dosage metrics)?

GM detectors are certainly among the most commonly used ionizing radiation detectors and have found rather widespread use among members of the public concerned about radiation levels and radioactivity contamination in the environment. If the interest of the user is in defining radiation dose rates associated with gamma radiation, then a proper calibration, and recalibration at regular intervals, usually at least annually, using sources traceable to an accepted laboratory or agency such as NIST, is necessary. Such calibrations are often carried out with a standard gamma-emitting source such as ¹³⁷Cs and they do, as you have inferred, establish a relationship between count rate, e.g., cpm, and dose rate, e.g., µSv h^-1. When a new such instrument is purchased, the manufacturer typically provides an initial such calibration that is reliable. If the calibration is in terms of dose rate, the detector yields a fixed count rate per unit dose rate (this assumes that the gamma radiation response does not change significantly with photon energy).

If the GM detector is equipped with a thin window and is to be used for quantitative assessments of surface contamination by alpha- or beta-emitters, special calibrations may be required to make correlations between count rates and surface contamination levels of radionuclides of interest. Even then, quantitative assessments may be prone to significant errors when the surface contamination characteristics and measurement protocols (e.g., specific radionuclide(s) present, presence of any attenuating materials or physical irregularities on the surface of interest, distance of the detector from the surface) are variable and/or different from those used in the calibration process. If the instrument being used was calibrated to read dose rate from a gamma-emitting source, and the detector is measuring alpha and/or beta radiation from a source, one cannot reliably interpret what the reading means on a quantitative basis, since the calibration does not apply to these measurement conditions.

For your interests, I believe you can comfortably perform routine comparative measurements in the fashion that you describe. If you have a rock that you know to be free of significant radioactive contamination and use that as your “background reference source,” then you certainly can make measurements of other rocks or materials and make judgments about the relative significance of radioactivity present. When measuring radiations from typical naturally occurring materials in rock using a thin-window detector that is sensitive to beta and possibly alpha radiation as well as gamma radiation, it is often difficult to evaluate the significance of the count rate in terms of radioactivity content. The responses to the different radiations are very different and very variable, depending on the radionuclide content and the proximity of the radioactivity to the surface of the rock being measured. One way to reduce some of this uncertainty is to cover the face of the thin window with about 1 cm of a common plastic, such as Lucite (Plexiglas) so that the detector response is only to penetrating gamma radiation. For some rocks that have alpha/beta emitters at the surface this techniques may greatly reduce the reading on the detector since the GM is much less sensitive to gamma radiation than it is to the particulate alpha and beta radiations.

If you are not performing routine calibrations, however, it is a good idea to have a check source that you can use to assess the operability of the instrument. A check source can be a low-activity source purchased for this purpose or it can be a rock or other sample that you have that contains sufficient activity to produce a distinct reading on the instrument. It is important that the source that you use be stable in radioactive content and physical characteristics (e.g., the radioactivity is not decaying sufficiently to reduce the apparent activity and pieces are not flaking off the surface) and that the source be held in the same orientation to and at the same distance from the detector each time it is used. As long as the instrument is performing properly, the readings obtained using the check source should be constant within normal statistical fluctuations. It is a good idea to use the check source at least once at the start of any series of measurements that you might be planning. If you find that the check source readings are different from what you have experienced in the past, this might be a sign that the instrument is failing, and some service may be required.

I wish you well in your rock hunting.

George Chabot, PhD, CHP