[ RadSafe ] FW: Reporter's question about lower limits ofdetection

Steven Dapra sjd at swcp.com
Tue Aug 9 20:03:19 CDT 2011

Aug. 9

         Let's all take a deep breath. . . .  Forget about filters, 
their efficiency, miles driven, and all the rest of it.

         Below, Chris Busby says, "The [car] filters showed between 
1.2 and 3Bq per cu metre of Cs-137."  Right or wrong, let's stick with that.

         Take such a filter and fasten it to the wall three feet 
above the floor.  Ten feet from that filter, have reference man sit 
in a chair facing the filter 24 h per d, for 365 d.  At the end of 
the year, how much radiation exposure will reference man have 
received from the 1.2 to 3Bq per cubic meter of Cs-137 that is in or 
on the filter?

         We need a specific figure, or at least a range within three 
sigma.  I am assuming that it is possible to make such a 
calculation.  If the answer is at or less than normal background (say 
400 mrem), I would have to say that this entire argument about car 
filters is a waste of time.

         Can such a calculation be made?

Steven Dapra

At 06:39 AM 8/7/2011, you wrote:

>Dear Radsafers,
>It was my intention to show that the levels in Japan in air were not 
>trivial. I do know this because I have measured it in several car 
>filters for which the engine size is known and the number of km 
>driven after the incident is known. The efficiency of the filters is 
>assumed to be 50% but this is not known for sure although I have 
>asked the manufacturers. The filters showed between 1.2 and 3Bq per 
>cu metre of Cs-137. This can be compared with the attached data from 
>Harwell. The results were from my lab and also from Harwell who we 
>paid to do the analysis.
>I am interested to learn that the levels were higher in the USA 
>during the atmospheric tests than in the UK: Stewart Farber says 
>100mBq/m3. Probably because the US is where many of the tests were done.
>But my argument was about Japan, not levels in the USA. I am quite 
>aware that the levels in the USA were far smaller, as we would 
>expect. But i see a maximum of 0.116pCi is 4.29mBq/m3 The average is 
>0.7mBq/m3. Compare with the graph attached. But I think it depends 
>on where you live, doesnt it? And what other stuff comes along as a passenger.


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