Answer to Question #11784 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation

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

Q

According to Radiation Network, readings across the country have doubled since 2011. Can you tell me which radioactive nuclides have contributed to these readings? Using the counts per minute (cpm) units reported on the Radiation Network website, how would we extrapolate these results to microsieverts per hour (µSv h-1)? In Omaha, I read an average of 35–50 µSv h-1 on granite countertops, and on some foods I read 100 µSv h-1.

A

I have checked out the website for the Radiation Network, and I cannot confirm your information that the readings shown over time have doubled in the period from 2011 to the present. The data displayed are real-time measurements, and I could not identify any connection to investigate historical results. I also am not familiar with any other independent sources that have identified a doubling in the intensity of the external radiation background over the past five or six years. I will make a few comments and then attempt to address your other questions.

As is noted on the cited web page, the network responsible for collecting the real-time data consists of a relatively large number of individuals who possess Geiger-Mueller (GM) instruments that feed count-rate information to the website. While I cannot offer any personal endorsement of the technical details of how the website operates or of the capabilities of any of the participants in the monitoring process, I believe that the intentions associated with this operation are laudable and worthwhile. It uses information from many individuals at many locations throughout the country/world to provide representative data as to the external radiation levels at those locations. It has the important intention of being able to identify significant changes in radiation levels that might occur as a consequence of possible human activity or natural variations.

As the author(s) of the website describes, a variety of different instruments are used by different individuals. Thus the results are from instruments with different radiation sensitivities, so that the absolute count rates that are reported at one location are not intended to be compared directly with those from a different location unless it is known that the instruments being used at the two locations are the same models with the same response characteristics. What is intended to be evaluated then is not the absolute count rate at one location compared to that at another location, but rather the sequential count rates from a single instrument at a single location. To accomplish the latter in a meaningful and reliable manner, it is necessary that the proper operation of a given instrument at a given location be confirmed on a regular basis. For most professional users of portable radiation monitoring instruments, there is a generally accepted requirement (usually enforced by regulating agencies) that such instruments should be calibrated at least once annually. Additionally, it is highly recommended that each such instrument should be subjected to a performance check at least once prior to use on any given day. The performance check typically involves exposing the detector to a small check source at a fixed and reproducible location with respect to the detector so that consistency of performance can be confirmed.

It would be difficult to confirm whether such measures are being taken by all of the individuals reporting measurements on the cited network, and this could lead to some uncertainty regarding accuracy of some recorded data. As you may know, a number of other factors can affect the readings obtained with a given instrument. These include geographical factors, considering especially the presence and extent of naturally occurring radioactivity in the earth; the physical topography of the land; the elevation of the measurement location, with higher elevations being subject to higher levels of cosmic radiation; and the presence of any large bodies of water close to the measurement location, which tend to shield some of the terrestrial radiation. Local perturbing influences can include the presence of notable amounts of natural radioactivity, as is often associated with granite used as structural walls and foundations in some buildings and decorative granite and other earth-derived products popular as countertops and floor tiles in many homes.

While I cannot validate an increase in the background count rate by a factor of two, we can posit the likely causes of the radiation responses that are generally observed on the affected GM instruments. First, there are interactions from secondary radiations produced by cosmic rays interacting in earth's atmosphere. This component can vary noticeably with elevation and also with latitude (cosmic charged particles find it easier to penetrate the atmosphere near the magnetic poles of the earth where they can follow the magnetic field lines toward the earth rather than being shielded heavily by the magnetic field as they would be if they entered at lower latitudes). The most likely terrestrial radiations that affect response would be the gamma radiation for the naturally occurring decay products in the uranium and thorium decay series. Typical and dominant gamma emitters in this category are lead-214 (214Pb) and bismuth-214 (214Bi), decay progeny in the uranium-238 (238U) decay series. There are small amounts of man-made radioactive nuclides also present in the earth, most notably cesium-137 (137Cs), most of which has come from past atmospheric bomb testing. Nuclear accidents at Three Mile Island, Chernobyl, and Fukushima released some radioactivity; relatively large amounts were released in the latter two cases, but only tiny amounts (insufficient to produce any increased GM readings) have been reported in precipitation and other environmental media in the United States.

The conversion from cpm to µSv h-1 can be reasonably approximated if you know what manufacturer and model instrument and associated sensitivity (or the specific gamma response) was used to obtain the count rate. For example, most GM instruments that use a typical pancake probe (rather flat, cylindrical tube with window area of 15 to 16 square centimeters [cm2]) have a sensitivity of about 300 to 350 cpm per µSv h-1 when exposed to gamma radiation; if such an instrument yielded a count rate of 70 cpm, we would estimate the dose equivalent rate to be about (70 cpm)(1 µSv h-1/350 cpm) = 0.2 µSv h-1. Such conversions assume that the instrument has been properly calibrated for the energies of gamma rays being measured and that the reading was not affected by any other radiations (such as beta radiation that could be observed if a detector was close to a granite surface or some other potential source of beta or alpha radiation, if the detector was sensitive to such radiation).

You mention a reading of about 35 µSv h-1 associated with a measurement on granite. If you performed this measurement with a thin-window detector designed to allow entry of beta (and possibly alpha) radiation, the reading in µSv h-1 would not be meaningful because most of the reading may have been due to the beta radiation from uranium and/or thorium progeny decay, and the instrument was most likely calibrated for gamma radiation response. The gamma contribution to the reading would not likely exceed about 0.1 µSv h-1 above typical background. You also cite an elevated reading of about 100 µSv h-1 on some foods. To my knowledge, most elevated readings of foods in this country have been associated with the uptake of significant potassium by vegetation. A tiny fraction of all natural potassium is the radioactive potassium-40, a beta-gamma emitting radionuclide. If one measures some high-potassium foods (some seaweeds being worth mentioning), readings significantly above background may be noted. Again, measurements with a thin-window GM detector will yield an overresponse to the beta radiation from potassium-40 decay. The value of 100 µSv h-1 that you cite seems unusually high, and I am not able to explain its cause without more information about the specific food, its origin, and the calibration and use of the radiation detection instrument.

George Chabot, CHP, PhD

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