The expulsion of radio-active matter in the radium transformations

When the radium emanation is transformed into radium A, the process is accompanied by the emission ofα -particles with a velocity of 1·70 x 109 cm. per second. The portion of the atom from which theα -particle has been emitted, which constitutes the radium A, must therefore be subjected to considerable shock and recoil in a direction opposite to that in which theα -particle is projected. If we further consider that the mass of theα -particle is 4 (H = 1), and that of the active deposit of the order 100, it follows that at the moment of its formation this product must be travelling with a velocity of the order 107 cm. per second. In ordinary circumstances, when the emanation is mixed with air at atmospheric pressure, the radium A particle will possess only sufficient energy to permit it to travel a fraction of a millimetre before being stopped by collision with air molecules. On the other hand, at very low pressures, these particles should travel considerable distances without being stopped by the rarefied air, and come to rest on the enclosure containing the emanation. Since the formation of radium B from radium A is also accompanied by the expulsion of anα -particle, it might also be expected that, at the moment of its formation, the recoil of the radium B atom would cause it to travel an appreciable distance through an evacuated space. There is some evidence that radium B can escape from a surface which has been exposed to the radium emanation and which is therefore coated with a film of active deposit. Now since the volatilisation point of radium B is above 600°C., it seems unlikely that this phenomenon can be due to the volatility of radium B at ordinary temperatures as was at first suggested, and it may well be that the radium B leaves the surface as the result of the recoil when formed from radium A by the expulsion of anα -particle as has been suggested by Rutherford.