| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
We buy the cotton pads from the supermarket - their intended use is
for wiping makeup off users' faces - and they are around 7 cm in
diameter and 5 to 10 mm thick, uncompressed. They fit nicely over the
end of the tube of the vacuum cleaner, with a metal gauze to stop them
disappearing down the tube. After filtering for 5 minutes we put the pad
on the RM80 geiger tube and the count rate jumps to around 10x the
background, from around 20 per minute to 200, very consistently, every
time we do it, year after year. Definitely beta, not gamma, as verified
by shielding with a thin aluminium plate. We're sitting on top of the
huge limestone mass of the Karst here near Trieste, so the geology is
not specially favourable to radon emission, although supposedly one can
get local hot spots where a deep fissure vents radon from under the
limestone, but checks carried out some years ago by the state health
service, measuring radon levels by counting the alpha pits in the bit of
plastic exposed to air for a few months, showed nothing unusual. What we
do for our lab is to get the decay curve of the radiation from the
cotton pad and show that it is consistent with a mix of Pb 214 and Bi
214, which have half lives of around 30 min and 20 min respectively.
Could it be that the difference between your observations and those of
the RM80 users is due to the fact that the RM80 has a much bigger
diameter than the typical GM tube, so that it's capable of catching many
more betas?
Mark
UWC Adriatic
Trieste, Italy