Fast-neutron transmission through a polarized holmium target

The observation of the nuclear deformation effect, that is the dependence of the neutron total cross-section of deformed nuclei on the degree of nuclear orientation, is particularly convenient on 165Ho because this nuclide is isotopically pure, strongly deformed, and relat ively easy to orient by means of the hypcrfine ferromagnetic interaction at low temperature. Among the works on this subject (1-6), we mention that one of MARSHAK et al. (6), who measured the effect in the neutron energy range from 2 to 135 McV and explained its oscillatory nature with a semi-empirical model based on the consideration of the socalled (~ nuclear Ramsauer effect ~) and also in terms of the optical model by an adiabatic coupled-channel calculation. A theoretical calculation of the effect has been presented also in the work of FASOLI et al. (7), by using the optical-model parameters obtained from a measurement of fast-neutron total and differential elastic-scattering crosssection. A fairly good agreement has been found with the experimental results. The nuclear deformation is expected to play an important role also in other types of interaction of neutrons with oriented targets. With the aim of extending this field of investigation we constructed a polarizcd holmium target (s), and as preliminary work, described here, we performed a new measurement of the deformation effect of the neutron total cross-section in the energy interval from 0.6 to 4.0 MeV. The longitudinal orientation required to observe the effect has been achieved by cooling a polycristalline homium sample 11 cm long with a diameter of 1.8 cm, to (0.31 -40.10) ~ in a magnetic field of 40 T obtained by means of a superconducting solenoid. The nuclear-orientation degree has been evaluated to be intermediate between