Average Energy Lost by Sulfur-35 Beta Rays per Ion Pair Produced in Air

The work described in this paper was initiated more than ten years ago, but unequivocal results could not be obtained-in the case of S35-until recently. The success is due to a cooperative endeavor between the National Bureau of Standards and our laboratory, which has gone on for a number of years. A method for the absolute counting of charged particles emitted by a radioactive nuclide, by measuring the rate of negative lost by a source suspended in a large vacuum tank, was developed in this laboratory many years ago (1). Since the system was large enough to ensure complete absorption of the P-rays of S35 in air at atmospheric pressure, it was thought desirable to check the accuracy of the method in the following way: The number of 3-particles leaving the source per unit time was determined by measuring the current in vacuo (1). After the tank was filled with air, the total ionization current was measured. This represents a certain number of ion pairs per unit time, which, multiplied by Wa, gives the energy absorbed by the air per unit time. It should be equal to the number of /-particles leaving the source per unit time multiplied by their average energy. After the average energy per disintegration of S35 was calculated from its spectrum and certain corrections made for absorption in the source structure, good agreement was found when a value of 32.5 ev (2) was used for Wa. Since this was the accepted value at that time, it was concluded that the method was satisfactory for the determination of the disintegration rate of S35. A simpler loss of charge counting method was developed soon thereafter (3), and the results obtained with S35 agreed closely with those of the vacuum tank method. Subsequent intercomparisons with the Bureau of Standards using p32 (4) agreed within 1 %. The large 1 Presented in summary at the meeting of Radiological Society of North America, December 1955.