Die Winkelverteilung bei der Photospaltung des Thoriums

Nuclear emulsions impregnated with thoriumnitrate solution were exposed to the 15·8 MeV bremsstrahlung of the Tübingen betatron. Fission tracks were identified by ionization and range, and their angular distribution relative to the incomingγ-rays was measured. As the origin of fission was unknown, the distributions obtained extend from 0? to 90? only. All tracks with a dip angle less than 50? were accepted, giving a total of 1726 events. The preponderance of fragment emission under 90? is pronounced even in the distribution of projected angles. After some calculation and correction also the distribution of true, spatial angles could be obtained from the data. Although a three constants formula gave the best fit to the distribution observed, the quadrupole term was found not to be statistically significant. We therefore conclude the angular distribution to be of pure dipole form with an anisotropyα=0·55± 0·12. This is in agreement with previous counter experiments. The anisotropy is interpreted in terms of the A. Bohr model, some of the implications of which are discussed. Extension of the measurements to 180? might be feasible by utilizing the ionization properties of fission tracks near the origin.