On 09/30/2014 9:37 AM, dpb wrote:
....

You'd have difficulty in discerning its presence at any distance from
natural background radiation without very good measurement technique.

The 241Am isotope used is primarily an alpha emitter and the range of
alpha particles is only a few inches in free air plus they can't even
penetrate a single sheet of paper owing to their size and charge
(they're a He atom w/o the two electrons so have +2 proton charge).

There's a plethora of gammas, but the dominant is only about 60 keV
which is pretty weak and the overall source intensity of a typical
detector source is only about one microcurie, and the exposure as long
as you don't remove the source from the device would be less than about
1/100 of a millirem per year.

To put that in context, average background in the US is about 300 mrem/yr.
....

BTW, to get the above I took the easy way out and used the calculator at
http://www.radprocalculator.com/Gamma.aspx

Selecting Am-241 Gamma (the alpha as noted above can be discounted
entirely as a contributor as long as the source is in the device), and
an estimate of 10-ft average distance from a 1-uCi source, the dose rate
is ~1.5E-6 mrem/hr. Multiplying by 24*364 and assuming an occupancy of
1/2 time in a given room, the annual dose works out at ~0.0066 mrem
which I rounded to 0.01 mrem. This is still an upper estimate as
there's no shielding at all from the source containment material nor the
materials in the detector between the source itself and the exterior;
only the 1/r-squared geometric factor is included.

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