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Geiger Counter

The installed Geiger counter measures only activity (counts per minute (CPM) convertible to Becquerel). As the energy of the individual impacts is not measured by the device, the dose cannot be measured. Therefore, we use an approximation to convert from counts per minute to mSv/a in order to obtain an approximate value.

The most critical approximation concerns the nature of the radiation. This is related to the source of the isotope. For our approximation, we choose the Cs-137 isotope. For this isotope, there are 6.5E-14 mSv/h per Bq/m3 (Facilia AB, Rodolfo Avila, July 2006http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/37/101/37101600.pdf). We also know that the are 0.0167 Bq per CPM and that there are 8765.76 hours in a year. The volume of our Geiger counter is 0.00002325.

We can therefore calculate the following relation between CPM and mSv/a:

mSv/a/CPM = 6.5E-14*8765.76/0.00002325/60=4.08E-7

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