Answer to Question #12128 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Measurements and Reporting

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

Q

I am preparing to set up a family home on Phangan Island off Thailand. The site is on S-type/ilmenite igneous granite in a low-lying, flood-prone area about 300 meters (m) from a white sand beach.

I would like help interpreting background readings of 170-190 counts per minute (cpm) on the ground and about 150 cpm at waist level. The detector is a Mazur-PRM 9000 meter with a pancake Geiger-Mueller (GM) tube.

Another person's readings were: at Thongsala (an old Chinese town), 20-36 cpm; on the flat back roads between Bantai and Madeua Ban, 30-70 cpm; at an older landfill quarry near Phaeng waterfall, 120-180 cpm; on a rock face, 220-245 cpm; on new dirt roads at the back of Bantai, 70-100 cpm; at a one-year-old landfill quarry in the Bantai hills behind Wat Kow Tham, 210-280 cpm; at the same quarry on the rock face at ground level, 300-472 cpm. His detector is an Inspector EXP.

At any one time can we estimate the percentage of alpha, beta, and gamma radiation that constitutes the background readings so we can distinguish what's in air or soil? What does the GM detector's efficiency mean for actual activity in becquerels?

A

The bottom line is that the count rates you mention (170-190 cpm and as high as 472 cpm) are not nearly high enough to be a health concern. Now let me give you a little background.

Granite (especially the lighter-colored rock) is high in potassium (K), uranium (U), and thorium (Th)—as well as the U and Th decay series. This is due to the geochemistry of these elements—they're larger ions that tend to be preferentially partitioned into the melt in a magma chamber, meaning that the minerals that crystallize last have the highest concentrations. Over time, the great majority of these elements have ended up in the upper crust and there's very little left in the mantle (not much in the oceanic crust either).

By and large, beach sand is comprised of quartz, which has virtually no radioactive elements. Feldspar makes up some sands—it might have some K. And the ilmenite you mention is notorious for having elevated levels of U and Th—some of the highest natural radiation levels are on beaches with ilmenite sands. I should hasten to add that these materials don't seem to affect the health of those living in those areas, but they are "hot" compared to the average.

With regard to the instrument you're using, I have to admit I'm not familiar with this exact instrument, but I have used a lot of pancake-type GM detectors. They are not very good at measuring radiation dose rate, but they do a good job of measuring count rate—with some caveats. The biggest caveat is that count rate, especially when you're measuring natural radiation, has very little to do with dose rate or the risk posed by a particular area.

The reason for this is that dose rate—and the dose one receives—is a measure of energy deposition, and GM detectors cannot differentiate between high- and low-energy gammas. Most GMs are calibrated against intermediate-energy gammas (like those from cesium-137 [137Cs]), so they will underestimate the dose from high-energy and will overestimate the dose from low-energy gammas. While potassium-40 (40K) emits a high-energy gamma, most of the radioactive nuclides from U and Th emit low-energy gammas. Thus, a high count rate from rocks with high U and Th concentrations doesn't really mean a correspondingly high dose rate. The only way to know the dose rate is to use a detector that is independent of the radiation energy—an energy-compensated GM or (better yet) an ion chamber or pressurized ion chamber. Just as one example: at a contaminated site in New York City, the dose rates measured by a GM-type instrument were higher by a factor of two to three compared to dose rates measured by a pressurized ion chamber at the same location. This correction factor can be even higher if the number of lower-energy gammas is higher.

In all honesty, it's not possible to give any sort of precise assessment of expected radiation dose in the areas you mention because of this; any sort of safety determination depends on dose rate and not on count rate. Having said that, I sincerely doubt that this area would pose you any risk. I have visited a naturally high dose-rate area in Iran that is the highest dose-rate area in the world; our studies showed that people living in this area do not suffer higher levels of cancer or birth defects and do not have shorter lifespans than people in other areas of Iran. Similar studies have been performed on populations living in Brazil, India, and China (as well as in the United States)—there is no instance I am aware of in which the people living in high background-radiation areas fare worse than those with similar social and economic circumstances. In fact in the United States, cancer rates are actually lower in the areas with higher natural radiation levels compared to lower-dose areas (although I have to add that there are so many confounding factors that we can't make any claim for cause and effect here). Anyhow, even though I don't know the actual radiation dose rates in the areas you mention, I feel fairly confident they are not high enough to cause health concerns. To repeat, the count rates you mention (170-190 cpm and as high as 472 cpm) are not nearly high enough to be a health concern.

P. Andrew Karam, PhD, CHP

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