Experiments on the transmutation of elements by protons

Extract

In the pioneer experiments of Cockcroft and Walton on the transformation of elements by bombardment with swift protons, it was found that easily measurable disintegration effects could be observed in certain elements for comparatively small accelerating voltages—of the order of 100,000 volts. Their experimental arrangement was specially devised for the use of much higher voltages. The proton stream from the discharge tube passed through a long evacuated path before reaching the target, and spread over a considerable area. Proton currents of the order of 1 microampere were usually employed. In order to study these transformation effects at still lower voltages we con­structed a special discharge tube which would give us a more intense and con­centrated stream of protons, of the order of 100 microamperes, using an accelerating voltage not greater than 250,000 volts. The stream of charged particles generated by the discharge through hydrogen was analysed into its components by means of a magnetic field, so that the effects due to the particles H⁺ and H₂⁺ could be separately determined. The stream of protons fell on a target of about 1 sq. cm. in area and the solid angle over which particles passed into the detecting chamber was much increased. In these ways it was found possible to obtain for examination at least 1000 times the number of disintegration particles at any voltage as in the original apparatus of Cockcroft and Walton. This large increase in the number of particles available for counting made it possible to examine the disintegration of such active elements as lithium and boron at low voltages and in the form of very thin films. In addition, it was not difficult to obtain intense but narrow streams of particles in order to analyse with more detail the disintegration as regards both range and velocity of the expelled particles.