How much is bad?  Where I live, I calculated (using the EPA calculator at http://www.epa.gov/radiation/students/calculate.html ) that my annual radiation dose from background radiation, etc, is about 350 mR, or approximately 1 mR per day. In trying to asses my risk in keeping this collection of compasses, I have attempted to determine my approximate annual exposure above and beyond this background level.

I tested my compasses using a GM-10 detector from www.blackcatsystems.com. It is not a scientific instrument, and is not calibrated, so my figures are very rough.  According to the manufacturer’s website, the readings in counts per minute (CPM) from the GM-10 can be roughly calculated into milliroentgens per hour (mR/Hr) by dividing by 1000. A compass which emits 2500 CPM would cause an exposure of 2.5 mR/Hr.

Radium emits mostly alpha particles, and only a tiny fraction of beta and gamma particles. However, I can’t assume that the luminescent paint always contains radium. There were other elements known to be used in luminescent paint, which may be stronger beta and gamma emitters.  I measure the compasses at various distances, with different materials interposed between the source and the counter.  Gamma particles can penetrate much better than alpha or beta particles, but so far I’m finding that the CPM falls off drastically with distance, and for almost all of the items, is at near background levels after a foot of distance.  This leads me to believe that all of my luminous painted compasses are simple alpha emitters.

The compasses are being stored in my office, in a closed display case which is several feet away from me during my work. I have used my detector and have been unable to detect any elevated count levels coming from the display case from the position of my desk.

As far as I can tell, my annual exposure is not being elevated by having these compasses in my office.