Particles of long range from polonium

With the discovery of new groups of α-rays of long range from radium C and thorium C, the question of the emission of small groups of long-range particles from all radioactive substances, which emits a group of α-rays, naturally presented itself. Polonium, which emits a group of α-rays of range 3∙93 cm. in air at 15° C. and a pressure of 760 mm. of mercury, was at once considered a substance that might be investigated with advantage. The method of investigation was that previously employed in the examination of the α-rays from the above sources. A source was placed at a known distance from a zinc sulphide screen and the numbers of scintillations were observed, by means of a specially designed microscope with a large field of view and of high light-collecting power, when mica sheets of different stopping powers were placed between the source and screen. The source was screwed in the end of a horizontal brass rod, which was attached to the carrier of a reading microscope, so that it could be adjusted in definite positions. By plotting the observed numbers of scintillations (corrected, of course, for any contamination or natural effect of the screen) against the equivalent air gap between the source and screen, the ranges of any long-range particles, if present in sufficient number, could be found.