The Radiation of the Active Deposit from Radium through a Vacuum

Abstract

WHEN the radium emanation is transformed into radium A, the process is accompanied by the emission of α particles with a velocity of 1.70 × 10⁹ centimetres per second (Rutherford, Phil. Mag., October, 1906). The portion of the atom from which the α 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 the α particle is projected. If we further consider that the mass of the 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 centimetres 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. These particles should, in fact, constitute a type of very easily absorbed radiation. It has been the object of some experiments which we have recently performed to demonstrate directly the existence of this radiation.