Up to now, I have indicated that I have found no contamination in my rainwater, from a couple of previous tests. This changed, however, on May 10th when we received an unseasonable 1/4" rainfall here in Prescott, Arizona. The family car had been outdoors in the rain, so I wiped an otherwise clean, but wet window with a paper towel, brought that inside, and laid it on a paper plate, which itself was sitting on a wooden table. First, I attempted a momentary scan with my Digilert 100, a "standard" tubed Geiger counter, aiming its thin mica end window directly at the wet towel, slowly surveying the surface area. But the momentary scan did not indicate any perceptible radioactivity.
So in the interest of thoroughness, I resorted to a 20 minute count, using the Total/Timer mode of the Digilert, fixing the position of the instrument, with the end window oriented directly at the water-soaked towel just 1/4" away. At the end of the test, the instrument had accumulated 514 radiation counts, or about 26 CPM (Counts per Minute). Then I created a scientific control, conducting a similar 20 minute count on an identical paper towel moistened with tap water this time, sitting on an identical paper plate, and in the same position on the same wooden table. The accumulated count was 407, or about 20 CPM.
If that comparison wasn't enough to indicate slight radioactivity from the rainwater, I looked at my logged average of indoor "environmental" radiation for the previous 20 minutes before disconnecting my Digilert from the computer to scan the water, and wouldn't you know it, but that was also 20 CPM. So short of an even longer timed count, this analysis almost certainly revealed that the captured rainwater was emitting 6 CPM of radiation. What are the lessons here?
When determining whether or not an object or environment is radioactive, always start with a momentary scan of a few seconds. If the substance is obviously radioactive, you will know it right away, and then can spend the rest of your time on other matters. In the real life example above, even though the rainwater was emitting radiation, in order for me to notice that from a momentary scan, I would have had to hear or see one extra radiation count every 10 seconds (1 minute divided by the 6 CPM radiation emission), a feat too great for this human being.
If a momentary scan does not reveal perceptible radioactivity, and if you want to be thorough, only then would you typically resort to a timed count, and preferably using a radiation detector that automates that process.
Back to the rainwater, though. The question is, "Was the radiation in the rainwater from Fukushima?" I have received input from two scientists indicating that there is a natural explanation for radioactive rainwater. In one explanation, the atmosphere includes radon gas among the other gaseous elements, and radon "daughters" are brought to the earth's surface by rainfall. These radioactive particles include alpha and beta emitters often with short half lives, so the same scan of rainwater done hours or days or weeks later may no longer reveal any radioactivity.
The same scientist, from a recent trip to Japan, has indicated that there are "hot spots" of radioactivity in the soil at different locations there, presumably from localized rainfall. In those cases, it would make sense that most of the the radiation in the rainwater was from Fukushima. To say the same thing about rainwater here in Prescott, Arizona would require more study, including analysis or baselines of rainfall from pre-Fukushima days, which I do not have.
A passing thought - we have a vegetable garden, and I have often wondered how natural rainwater seems to exert a magical, therapeutic effect on the plants, as compared to irrigation water despite its natural mineral content. Could it dare be that a little, naturally-occurring radiation in the rainwater is a good thing?
