The scattering of fast β -particles by mercury nuclei

An account of the scattering of fastβ -particles emitted by Ra E, with energies lying in the region of 1mV by nitrogen nuclei has been given in a previous paper (Champion 1936). Using an expansion chamber it was found that the scattering between 20 and 180° was in fair agreement with a formula deduced by Mott (1932) on wave mechanics. The present paper gives an account of similar experiments carried out with mercury nuclei. Apart from very large deviations from Mott's formula, other phenomena of an interesting nature are observed. A standard semi-automatic expansion chamber of rubber piston type was used and a magnetic field of 400 oersteds was supplied by a pair of Helmholtz coils in the usual way. Illumination was provided by tow groups of four 100 V 100 W semicircular filament lamps, flashed on a 200 V D. C. circuit for a fraction of a second. The lamps were silvered on the surface behind the filament and covered with black Chinese lacquer, except for a slit some 3 mm. wide in the direction of the chamber. They were run continuously at dull-red heat, the resistance being short circuited at the instant of flash. Used in this way, each lamp lasted for more than a thousand photographs. Mercury was introduced into the expansion chamber in the form of mercury dimethyl vapour. The mercury dimethyl, which is liquid at room temperature, was contained in a U tube which communicated through a tap with a hole in the top of the chamber. In order to make sure that the chamber was kept saturated with vapour, opportunity was frequently provided for fresh vapour to be drawn into the chamber, by opening the communicating tap and operating the chamber piston before the commencement of each run of fifty photographs.