More than 700,000 thorium C and C′ alpha-particle tracks have been photographed with a stereoscopic camera in a Wilson chamber filled with a mixture of hydrogen sulphide and hydrogen. A range-velocity curve for sulphur recoil atoms has been constructed by plotting the measured ranges (reduced to standard air) against the calculated velocities for 60 selected alpha-particle sulphur collisions. This curve is discussed and compared with the results of other workers. A possible relationship has been noted between the range-velocity curves of atoms having similar electronic configurations. No collisions indicating possible sulphur disintegrations were observed. Several low energy collisions were noted which were apparently lacking in coplanarity, but this effect is explained as being due to small deflections suffered by the recoil nucleus shortly after the collision. The calculated distances of closest approach are compared with the radius of the sulphur nucleus as given by Pollard and Brasefield. The methods of measurement and calculation employed have been checked by applying them to collisions with protons.

• Received 20 July 1936

DOI:https://doi.org/10.1103/PhysRev.50.726