2012/01/26

Amateur Food Testing and Evidence of Post-Decontamination Recontamination

My Terra-P dosimeter is capable of giving estimations of "surface contamination" of beta emitting radionuclides, in addition to taking readings of gamma radiation. This means that it can give a consumer some assurance about the level of cesium 137 contamination in food. The beta particles are measured as "beta flux density." The unit of measurement is particles/square centimeter.minute, and the manufacturer's recommended limit for food consumption is 0.020.

Bananas are famous for being a common food that everyone eats without concern, but which also contain a measurable amount of naturally occurring radioactive potassium. I took this as a standard of comparison. Bananas are also good for comparison because they are imported. The reading on the latest batch of bananas was 0.003 - about 7 times lower than the recommended limit of 0.020. The bananas we had last week measured 0.007.

mikan 0.001
Next I measured some other food from our kitchen, all of it purchased in Chiba, Japan. All of the levels were very close to that found in the banana.

yogurt 0.001
domestic pork - 0.001
imported pork (if you can trust the label) - 0.002
dog food - 0.002
backyard soil - 0.004
For other comparison, I measured the soil and the concrete patio in our small backyard. It was just slightly higher than the food.

backyard patio - 0.004
the scene in August 2011
The really interesting comparison was with the hotspot I found near our house last summer. This is on a pedestrian path, in place where rain washes sand down from higher ground and collects it at the curb side. Last summer I found the collected sand was giving off a gamma dose of 0.50~0.60 microsieverts / hour, while the common readings on open ground were about 0.14 microsieverts/hour (pre-disaster background radiation used to be 0.05~0.07). I "volunteered" to clean this up (chronicled in an earlier post), partly as a good deed for the community, but also for selfish reasons of just wanting this stuff to not blow in the wind near our house. We complained to city hall, and some parks and recreation workers reluctantly came around to haul away what I had put in bags, even though at the time officials told us they had not yet worked out a plan for where to store such waste. 

gamma dose rate of the hotspot 0.62 microsieverts/hour,
January 26, 2012
beta flux density of the hotspot 0.107
At the time, I had naively thought that this solution would last, but recently I found that sand and soil have collected in the same spot again, and it is just as contaminated as before. Last summer I didn't measure beta flux density, but this time I did, just to compare this soil with the food in our kitchen. The soil gave a reading of 0.107, 35 times as high as the banana, five times above the safety limit. The gamma dose rate on the soil today was 0.62 microsieverts/hour. The only good thing about this finding is knowing that this is soil in a mini hotspot at a curbside. No one is going to grow food in this. The same cannot be said of the highly contaminated rice paddies 200 hundred kilometers north of here, where farmers, lacking government compensation or restrictions, intend to plant rice for the coming season.

What is better than my small-scale amateur measurements is the monitoring done by Greenpeace Japan. Such independent, non-government monitoring is an essential part of establishing food safety. The large supermarket chain AEON has eagerly got on board with Greenpeace, seeking its stamp of approval for adopting high standards of monitoring the food sold in its stores. 

3 comments:

  1. A couple of questions about beta flux density measurement : (1) are you removing the geiger tube back tray and (2) are you covering the geiger counter with any plastic to avoid contamination from adhering to the counter ?

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  2. Sorry for the delayed response. Yes, I remove the back tray when doing beta flux density measurement, and I usually cover it in plastic when doing other measurements.

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  3. This is a new idea for me, I have not careful solar radiation until an upcoming evaporation project we are installing a net pyrometer to estimate evaporation. dosimeter badges.I would like to potentially use this data for other purposes and will lookthrough some of these papers. It is interesting, thankyou.

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