In this case we have only considered the energy emitted from a single product independently of the successive products which may arise from it. Radium, for example, may be considered a radio-active product which slowly breaks up and gives rise to four subsequent α ray products. The total heat emission from one gram of radium and products is thus about five times the above amount, or 4 × 109 gram calories.
The total emission of energy from radium is discussed later in section 266 from a slightly different point of view.
252. Number of ions produced by an α particle. In the first edition of this book it was calculated by several independent methods that 1 gram of radium emitted about 1011 α particles per second. Since the actual number has later been determined by measuring the charge carried by the α rays (section 93) we can, conversely, use this number to determine with more certainty some of the constants whose values were assumed in the original calculation.
For example, the total number of ions produced by an α particle in the gas can readily be determined. The method employed is as follows. 0·484 mgr. of radium bromide was dissolved in water and then spread uniformly over an aluminium plate. After evaporation, the saturation ionization current, due to the radium at its minimum activity, was found to be 8·4 × 10-8 ampere. The plates of the testing vessel were sufficiently far apart to absorb all the α rays in the gas. The number of α particles expelled per second into the gas was found experimentally to be 8·7 × 106. Taking the charge on an ion as 1·13 × 10-19 coulombs (section 36), the total number of ions produced per second in the gas was 7·5 × 1011. Thus each α particle on an average produced 86,000 ions in the gas before it was absorbed.
Now Bragg (section 104) has shown that the α particles from radium at its minimum activity are stopped in about 3 cms. of air. The results obtained by him indicate that the ionization of the particles per cm. of path is less near the radium than some distance away. Assuming, however, as a first approximation that the ionization is uniform