Fieldwork 4 Additional Thoughts: ずくなしの冷や水

2014年06月15日

Fieldwork 4 Additional Thoughts

April 27th 2014

Fieldwork 4 Additional Thoughts

I found out through my fieldwork trips that there is a danger of beta particles in my surrounding area. What does this mean in everyday life and what could we do to avoid them? My concerns are especially for small children who are close to the ground hence closer to the source of radiation.

1. Risks for young children:
Irradiation from gamma rays is inversely proportional to the distance from the source. To avoid gamma irradiation one simply has to move away as far as possible from the source. Beta radiation have a considerably shorter traveling distance than gamma rays, which means that exposure to beta radiation increases dramatically if you get very close to the source.

Do not use push-chairs in areas susceptible to have high beta fallout! Holding children in your arms or carrying them piggyback would involve much less irradiation because the child would be further away from the sources on the ground.

The average beta energy that Sr 90 emits as it undergoes beta decay is 195.8 keV and its average range in air is about 33 cm. The distance between a child in a push-chair and the ground would only be a little more than 30 cm. There are beta particles that have higher energy than Sr 90 and therefore travel further as well. But the principle remains the same: the higher you move from the ground the safer for the child.

2. Risks for the eyes:
Looking for beta sources involves high risk and isn’t easy. It was by pure coincidence that I found the extremely hot spot described in “Fieldwork 4”. I searched in other places since but haven’t found any.

Unless the detector is close to touching distance with the ground it is difficult to determine the energy of the beta radiation. Otherwise the radiation either doesn’t reach the height or is mixed with gamma rays when it hits the sensor.

And if you try to detect beta sources on the ground with a hand held dosimeter, you will receive beta radiation in your eyes whilst reading the monitor.

Beta radiation does serious damages to the eyes. Let’s suppose that there are 10.000 cpm of beta radiation. The detector is then catching 167 radiation “hits” per second. But even those 167 radiation “hits” are only a part of what the source is emitting. And it might be your eyes that are being hit by some of that remaining radiation.

I hear that there is a lot of eye damage reported in the northern part of Miyagi prefecture. Perhaps it is related to a high amount of beta fallout. I also remember a case where a person ended up with damage to the eyes after repeatedly measuring contaminated spots in the Kanto region.

3. Using a bicycle will reduce the risk:
In areas where high beta fallout is suspected you are more likely to avoid irradiation by using a bike instead of walking. It is even better if you can bike and carry the child in a bike seat at the same time.

4. Risk of burns:
Among the damage that beta radiation causes are burns. Although I haven’t heard of very high strontium fallout so far, the dose rate will rise if there is deposit of biggish particles.

5. US survey:
According to a survey of soil samples conducted by the US, there is 113 Bq/kg of strontium 89 in Kumagaya city (Saitama prefecture). If you multiply this by the coefficient 65 to get the dose rate per m2, you reach the extremely large figure of 7.345 Bq/m2. I tried to compare this figure to the hot spot I found.

The conversion of count rates into Bq is not simple as it is specific to the state of contamination, the distribution of the sources at the surface and the sensitivity of the instrument you use.

However, according to the Fukushima Technology Centre, the surface contamination can be roughly converted as follows:

Bq/cm2=cpm÷60÷instrument efficiency÷source efficiency÷effective detection window area



If the sensitivity of the dosimeter is low, the count rate will be much less than if you measure with a sensitive instrument.

So far I don’t have a reliable means of converting Bq into cpm.


Related articles:
Fieldwork 2
Fieldwork 4

posted by ZUKUNASHI at 12:02| Comment(0) | 福島原発事故
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