Fieldwork 2: ずくなしの冷や水


Fieldwork 2

When the wind blows…
Still shocked from the result of my previous fieldwork, I made some improvements to my equipment so I could carry out my survey more easily. I put the detector in a cloth wine bag so I wouldn't need to squat down to hold it close to the ground each time. I first thought of fastening the detector to a stick on a wheel but I thought that the shaking and bumping might just be too much for the poor old thing. For the main body that has the monitor I shall look for a compact and sturdy bag, perhaps something like a tool bag that I can carry on my shoulders.

I went to a nearby park and measured the air dose rate. Maximum 60 cpm. As I am measuring while moving around, there is no correlation between the readings of the Air Counter (stick shaped gamma ray dosimeter) and the survey meter in the bag.

I found out that when I measured gamma and beta rays together, generally the counts were higher on permeable asphalt paving than on grass although sometimes on grass you can find hot places.

When the wind blew the counts went up. One gust brought an extra 5-10 cpm (as compared to 20 cpm on my last fieldwork). If the reading was at 20 cpm before the wind and after it goes up by another 20 cpm, it means that the input has doubled for the time constant of 30 sec. In other words, an increase of 10 cpm recorded over a 30 sec period means that there would have been a single burst of 300 counts over a very short time (1-3 sec).

It is only an instant rise and drops down straight away. But considering those figures perhaps I shouldn't ignore radioactive particles that the wind brings. If they come in a gaseous state they wouldn't settle so wouldn't leave any proof on the ground. However much there could be in the wind, it wouldn't raise the base of the air dose rate of that place. But the danger is there.

Some physically sensitive people mention on the Internet that the alarm of their dosimeter often goes off. If it's a sensitive one that needs little time to get the readings, it will certainly set off the alarm when it suddenly receives 5cps on an average 0.2cps basis. That's how these dosimeters work. But on the public monitoring posts with a time constant of 10 min, it wouldn't show anything at all.

Another hot spot: 150 cpm
So, my survey meter is still sensitive enough to sense these instant differences. Contented to witness that my survey meter is not senile yet (although it's an oldie just like myself) I went from the park to execute some more fieldwork.

I went to a spot that I had been suspecting to be radioactively hot and placed the end of the tube-shaped detector on the ground. There was only a thin plastic bag and a roughly woven textile between the detector and the ground.

The reading shot up. The range was 0-300 cpm. Time constant:10 sec.

I raised the detector by 50 cm.

The same thing happened over and over again. When I stepped away by 40-50 cm from the spot and took a measurement it didn't show as high but if I went back to that spot it rose again. The photo below shows the 4th time I took a measurement. The detector was laid directly on the ground. I had never had such a high constant reading when measuring the air.

I lifted the detector by 50cm.

I think that it is quite consistent.

This is what the soil looked like.

What is it?
Now what was the particle that emitted such a high dose rate? Two hypotheses are possible:

1. There is a lump of cesium 137. If it is a dot source it is natural that the dose rate drops considerably when you move away from it. But would it drop so much just by moving 50 cm away? Even if my survey meter is old and not so sensitive any more?
2. There is a concentration of beta radiation sources. A beta source in this location could only be from the Fukushima Daiichi Power Plant. Pre-Chernobyl there wasn't residential land here. So could it possibly be strontium?

Then I remembered that there was a mention on Twitter in 2011 by someone who had measured the filter of his indoor air purifier. It said that when he measured with his dosimeter that detects both beta and gamma rays, he detected 4 times as many beta rays than gamma ones.

This observation backs up my experience of having 40 cpm at 50 cm above ground and 150 cpm on the ground. The ratio is roughly 1:4. What is it that's there on the ground? 3 years after the accident at Fukushima Daiichi Power Plant, the short-lived particles that were deposited there would have disappeared already. What is it that's still there?

Although it is difficult to determine, I presume that it is strontium. It is more prudent to assume that there is strontium there and try and protect yourself. Whatever the radiation source may be, my two field trips taught me that there are beta radiation sources everywhere. It's just a difference of concentration. And that sometimes, it can be tremendously high.


April 27th, 2014 Follow up

1. I put a u-shaped resin shield over the end of the detector and measured again with it lying directly on the ground. Time constant: 30 sec. Reading: 45 cpm.

2. Then I measured this time without the shield. Time constant: 30 sec. Reading: 65-95 cpm. I raised the sensor about 3 cm from the ground so as to protect the mica window of the detector from contamination by the particles on the ground. Perhaps that explains the relatively low reading.

So I confirmed that the 0.8 mm thick polyvinyl chloride resin shield nearly shuts off the emission. I also confirmed that a few cm of distance are enough to get a considerably lower reading. This is probably due to the low permeability of the beta rays in the air and that the effective surface of the mica window reduces as you increase distance from the source.

Which means, to avoid damages from beta rays you need to move away as many cm away as possible from the source. Even 1 cm can make a difference. In other words, if taken internally, the damage done by beta rays is likely to be huge.

(first written on April 15th 2014, addition made on April 27th 2014)

posted by ZUKUNASHI at 13:32| Comment(0) | 福島原発事故
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