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A study of the characteristics of the pressure ionization chamber at the National Bureau of Standards for X-rays up to 400 kv has been discussed by Taylor, Singer, and Charlton [I]2. The results of their study showed that it was possible to measure accurately the dosage rates of high-voltage X-rays in accordance with the international definition of the roentgen. Taylor and Singer [2] also used this chamber to measure the ionization of the gamma radiation from a radium source and determined a value for the emission constant of radium. At that time it was not feasible to investigate completely the chamber response with quantum energies in the million-volt region. However, Taylor and Singer believed their measurements were accurate to within ±0.4 percent. Radioactive materials of sufficient radioactivity intensity and particularly suited for use with the pressure chamber have become available only recently. The study of the characteristics of the chamber with isotopes having gamma radiation in the energy regions of 0.6 and 1.3 Mev was undertaken to determine the reliability of the chamber for measuring the dosage rates produced by radiation energies up to 1.3 Mev. An extrapolation chamber and a Victoreen thimble chamber were also tested so that their readings could be compared with that of the pressure chamber. Cesium-137 and cobalt-60 isotopes were obtained from the Oak Ridge National Laboratory for use with the chamber. The cesium isotope emits a gamma ray of 0.6614 Mev [3], and cobalt-60 emits 2 quanta in cascade per disintegration with energies of 1.1715 and 1.3316 Mev [4]. The activities of the cesium and cobalt samples were about 8 and 1.5 curies, respectively.

Measurement of cesium-137 and cobalt-60 gamma radiation with a pressure ionization chamber