Most water softener tablets are sodium chloride (common salt). The more expensive ones are potassium chloride, which is slightly radioactive due to the naturally occurring K-40 isotope (comprising approximately 1 in 7000 potassium atoms). The local Ace Hardware had these bags stacked near the entrance such that everyone who entered the store received a free low-level radiation dose.

Potassium-40 decays 89.3% of the time by beta emission with an average of 508 keV and a maximum of 1.31 MeV. 10.7% of the time it decays by electron capture with the emission of a 1.461 MeV gamma ray. It is this gamma ray which the PM1703M detects. The energy response curve is used to adjust the cesium-calibrated reading for the 1.461 MeV energy level of K-40. Subtracting the background of 12 µR/hr from the peak reading of 50 µR/hr yields a net increse of 38 µR/hr. After correcting for the K-40 energy level (multiplying by 2.2), the energy-compensated gamma dose rate is 84 µR/hr above background.

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