Answer to Question #9862 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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


The people at this website have been measuring radiation in Japan.

They claim to have measured over 330 cps and 2.5 nSv s-1 at one place. This sounds to me like a fairly normal reading in microsieverts, but a high reading in CPM.

Can you explain what this means? Is the high CPM reading potentially dangerous? Do you think these people are using a sound technique or is there something they are missing?


The quoted reading of 2.5 nSv s-1 is quite a lot higher than “normal" external radiation levels, in fact about a factor of 100 higher.

The“Inspector" survey meter that the individuals were using I believe yields about 21 cps per nSv s-1. The 2.5 nSv s-1 field would then produce an expected count rate of 52.5 cps. The fact that 330 cps was observed  (presumably when the detector was held close to a surface) would imply the presence of detectable contamination on the surface. Assuming the meter had been calibrated and was functioning properly, we may conclude that the area in Japan where the measurements were made does appear to have been contaminated with radioactive material. Assuming that the radioactivity was the result of releases from the Fukushima nuclear site, we could judge that the likely radionuclides responsible for the elevated readings were 137Cs, 134Cs, and possibly 131I. The cited external radiation levels are not so high as to require immediate evacuation by the individuals involved, but are high enough to restrict extended access to the area.

A dose rate of 2.5 nSv s-1 would yield an annual dose to an individual present continuously in the area of about 80 mSv, a dose about 60 percent higher than the annual limit of 50 mSv currently used in the United States for occupationally exposed workers and about four times the International Commission on Radiological Protection recommended average annual limit of 20 mSv for radiation workers in nonemergency situations.

The reading of 330 cps is more difficult to interpret in terms of potential for exposure via inhalation and ingestion of the radioactive contamination because the conditions of the measurement, such as placement/location of the detector, the nature of the surface and the extent of penetration of contaminants into the surface, and the potential for resuspension in air are not fully known. If we assume that the detector was placed in near contact with the surface and that the activity was primarily on the surface and consisted of the beta/gamma emitters mentioned above, and that the counting efficiency was about 0.10 counts per disintegration, we would estimate an activity of 3.3 x 103  Bq below the detector. For an active area of the pancake-style detector of 15 cm2, the activity per unit area would be about 220 Bq cm-2. The potential for resuspension of surface activity into the air depends on a number of factors, but for this situation, where there is very little pedestrian or vehicle traffic, the resuspension factor (air concentration divided by surface area concentration) would probably not exceed about 10-8 cm-1. Using this figure, we would then predict an air concentration of about 2.2 x 10-6 Bq cm-3. For comparison, this concentration is about three times less than the 10 CFR 20 App. B Table 2 concentration limit (for any of the three radionuclides noted above, the Table 2 value is 7.4 x 10-6 Bq cm-3), which relates to public exposure and represents the concentration limit allowed at the boundary of an operating Nuclear Regulatory Commission-licensed facility. The implication is that the inhalation dose consequences of the quoted surface contamination level are not of great concern, at least for the assumptions applied.

The measurements you have cited do appear to confirm the known fact that radioactivity from the Fukushima site has been dispersed beyond the site boundaries and that the potential for excessive exposure has warranted the evacuation of the public from some areas. Certainly, the incidents that have occurred in Japan have been catastrophic, and there will be many ongoing concerns for many more months and possibly years. The ultimate cleanup of the reactor site and areas beyond the site is a daunting task. We must be aware that such circumstances often precipitate various statements, actions, conclusions, and recommendations on the parts of many groups and individuals that add no useful information to resolve problems but simply increase fear and uncertainty among the public, sometimes with the intention of promoting a personal agenda. The reporting of accurate radiation levels and contamination levels is essential to a useful assessment of the potential risks associated with living and working in various affected areas.

The individuals who made the measurements cited in your question apparently belong to the Safecast group, which was initiated shortly after the earthquake and Tsunami. As I understand it, the group includes numerous volunteers who have been using their own instruments to make radiation measurements in areas beyond the affected Japanese nuclear plants. While such measurements can be effectively used to provide a broader picture of the distribution of contamination beyond the Fukushima reactor site boundary, there is also the danger that some individuals may use improperly designed or improperly calibrated instruments that might result in false readings and even that some individuals might be motivated to report false readings in order to promote their own agenda. I have no evidence that such has been the case with the Safecast group, and I hope that if any such instances occur, they will be rare. For purposes of our assessment here, I have assumed that the reported readings are valid and based on acceptably accurate measurements.

Thanks for the question. I hope the discussion has been helpful.

George Chabot, PhD, CHP

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