Angular distributions in photofission of uranium

Abstract

The angular distributions in photofission of natural uranium have been measured. Bremsstrahlung with maximum energies of 10 and 20 MeV and monoenergetic gamma radiation of 6.1, 6.9 and 7.1 MeV energy were used as photon sources. The fission fragments were detected in two ways. One method was to irradiate nuclear emulsions containing uranium. In the other method a scintilation counter was used to detect the fission fragments. The first method is more tedious but has a considerably better angular resolution. The two methods give results which agree very well. The following results have been obtained: The angular distribution at 6.1 MeV is of the form 0.07+sin 2θ+0.37 sin 2 2θ. At 6.9 MeV the distribution 1+0.70 sin 2θ+0.05 sin 2θ was obtained. The distribution at 7.1 MeV is similar but somewhat more isotropic. The angular distribution obtained by 10 MeV bremsstrahlung is of the form 1+0.85 sin 2θ+0.13 sin 2 2θ. Irradiation with 20 MeV bremsstrahlung gives an isotropic angular distribution. The results of the present work are compared with the theory of A. Bohr. They agree very well with this theory. It is also possible to use these results to separate the photonuclear absorption at 6 MeV into a dipole and a quadrupole part. The quadrupole absorption in particular is discussed and compared with theoretical estimates.