IN recent clinical radioactive tracer studies using sodium-24, phosphorus-32 and iodine-131, it was found desirable to have a reliable and convenient radioactive standard, of similar beta- and gamma-ray energy, easily reproducible, of negligible radioactive decay and with adequate carrier protection against exchange reactions upon the glass walls of Geiger–Muller liquid counters1. The radioactive isotope, potassium-40 of normally occurring potassium, is present to one part in 9,000 and has a half-life of 12.7 ± 0.5 × 108 years2 with a beta-ray spectrum of maximum energy 1.38 MeV. and a gamma-ray of energy 1.5 MeV.
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CHALMERS, T. Calibration of Geiger-Müller Liquid Counters by the Radioactivity of Potassium-40. Nature 168, 870 (1951). https://doi.org/10.1038/168870a0
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