Answer to Question #12760 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

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

Q

We purchased a professional survey meter with wand (Eberline SPA-3 NaI scintillation device) calibrated to counts per minute (cpm) with 137Cs gamma radiation. I understand this is the equipment that professionals use in the field. The cpm reads consistently between 3k and 10k cpm any time the meter is turned on. We feel nauseous with headaches, etc. Can you provide me with a basic example of the calculation for what this means: 

  1. At the lowest intensity for counts to establish a baseline vs. average vs. high radiation intensity?
  2. Average for human absorption based on cpm?

Sorry if the question uses imprecise language, but want to get this evaluation right.

A

I will note up front that I do not find that the readings that you cite are indicative of anything outside the norm of expected background readings. I shall attempt to address your specific questions and concern.

The NaI detector that you describe is a 5 cm x 5 cm cylindrical detector, which has a high sensitivity to gamma radiation. You note that it is an instrument that is used by professionals. This statement is true; its primary strength lies in its high sensitivity, which makes it a good choice as an instrument to search for lost or otherwise hidden gamma-emitting sources or for looking for small changes in gamma radiation intensity. Its high sensitivity is the reason behind the relatively high count rates (compared to other types of commonly used detectors) you observe even for normal background radiation. It is frequently not a highly effective detector for measuring radiation dose to people except in well-defined fields for which the photon energy dependence is known. This is because the NaI detector has a much higher atomic number than does soft tissue, and this greatly affects the likelihood of gamma radiation interactions, which depend strongly on atomic number as a function of gamma ray energy. Interpretation of radiation dose rate is subject to uncertainty unless one is measuring a gamma field with the same energy distribution as was used for calibration of the instrument.

The instrument that you are using has a manufacturer-cited response to 137Cs gamma rays (energy is 662 keV) of 3.1 x 105 cpm per C kg-1 h-1; this would translate to a count rate per unit equivalent dose rate of about 1.2 x 105 cpm per µSv h-1 at the same energy. Typical background equivalent dose rates in much of the United States from penetrating radiation range from about 0.05 µSv h-1 to about 0.20 µSv h-1 with most likely readings between about 0.05 and 0.10 µSv h-1. Significant variations can occur, depending on the nature of the surroundings. For example, the presence of earth-derived building products, such as granite, often containing significant amounts of uranium and thorium that produce decay products that emit measurable gamma radiation, may noticeably elevate the apparent background count rate. Other structural materials may reduce dose rates by shielding some of the gamma radiation. Even weather changes may affect the background count rate. This can often be seen when gamma radiation levels near the surface of the earth may increase for a time during and after a rainfall that washes radioactive products of radon decay out of the air. Additionally, people living at higher elevations typically experience increased dose rates because of the increase in cosmic radiation at higher elevations.

While we cannot interpret exactly the significance of the values that you have observed with your NaI instrument, we can make a reasonable judgment, based on the cited response of the detector and the typical background dose rates—e.g., for background dose rates of 0.05 µSv h-1, 0.10 µSv h-1, and 0.20 µSv h-1 and the response of 1.2 x 105 cpm per µSv h-1 we would expect respective count rates of 6,000 cpm, 12,000 cpm, and 24,000 cpm. If you are inside a structure that provides some radiation shielding or in an area of low natural radioactivity observed count rates could be lower; in other areas of higher naturally occurring radiation, dose rates could be higher. A detector may be heavily shielded by natural materials in some situations. One example is when measurements are made when in a boat on a body of water. The water below the boat may shield the detector very effectively from terrestrial radiation, and the readings may be due almost entirely to cosmic radiation, leading to values at times that may be well less than one half of the readings obtained over land.

The values that you cite of between 3,000 cpm and 10,000 cpm are consistent with what I would expect if I took an instrument such as yours and wandered around a bit, making measurements of background radiation at a variety of locations. As such, I find no reason for undue concern on your part. It is difficult to define a minimum likely value, since conditions can vary dramatically, but for most likely cases I would not expect count rate much below the 3000 cpm that you have observed.

The conversions from dose rates to count rates that we made here are not exact because of the dependence of response on energy, and the natural background gamma radiation includes gamma ray energies both lower and higher than that from 137Cs. The expected count rates may be somewhat different from the values we projected, but I am confident that the values you have measured are consistent with reasonable expectations for normal background measurements with the NaI-based instrument that you used.

I do not have any reason to believe that the ill feeling that you have reported bears any relationship to radiation levels. If such feelings continue I would recommend that you consult a physician to identify other likely sources. I hope that you are soon feeling better.

George Chabot, CHP, 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.
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