The rate of emission of alpha particles from radium

The number of disintegrations taking place per second in a gram of radium is a consatant of great importance in quantitative radioactive work, and an accurate knowledge of its value is accordingly essential. This constant, which is usually denoted by Z, has been the subject of numerous researches, but the results, even of comparatively recent measurements, have been disappointingly inconsistent. A discussion of the methods employed and the results obtained has been recently given in a paper by Braddick and Cave,* so that it is unnecessary to discuss them at length here. The most direct way of measuring Z is to determine the number of α-particles emitted per second by a known mass of radium: this may be done either (a ) by measuring the charge carried by the particles emitted within a known solid angle, or (b ) by directly counting the number of particles emitted within a known very small solid angle. The most recent measurements by the “total charge” method are those of Jedrzejowski and of Braddick and Cave. The former used sources of radium (B + C) prepared in a special way and measured the charge by a quartz piezo-electrique; a value for Z of 3·50 × 1010 was obtained. Braddick and Cave, using active deposit sources, and measuring the charge by the Townsend compensation method obtained a value of 3·69 × 1010 . The discrepancy between these two results obtained by similar methods remains at present unexplained.