A Microcalorimetric Determination of the Mean Energy ofβ-Rays from Radium E

A very precise microcalorimetric method of determining the mean energy of Ra E β-rays is described. The method involves an adiabatic change of temperature of a small calorimeter whose heat capacity is measured by determining the change in temperature produced by several strong sources of polonium. With the aid of Mme. P. Curie's absolute apparatus, the absolute value of the polonium content in these sources as well as in the Ra E samples after the decay of the parent substance could be determined with an accuracy of from 1 to 2 percent. The average value of the β-ray mean energy of Ra E deduced from these experiments is 320,000 ±{}5000 ev. The measurements made in air gave a mean energy of 323.6±{}3.5 kev, or slightly higher than those carried out in nitrogen (317.6±{}2.5 kev). It has been pointed out that the most probable value is nearer the lower limit. The results have been compared with those derived from other calorimetric determinations as well as with the values calculated from β-ray spectra of Ra E established by different authors.