Measurements of parity violation in neutron-nucleus reactions

摘要

In this talk I describe a new generation of experiments studying the weak interaction between nucleons. Measurements of the effect of this interaction are few in number and the significance of the observed effects are generally small. It is well known that the weak interaction violates parity. This was first experimentally established by C. S. Wu through measurement of an asymmetry of electrons emitted in the beta-decay of polarized (sup 60)Co. The measured asymmetry was large because beta decay is a weak interaction process. For a process in which the strong interaction can contribute, we expect much smaller asymmetries, of order 10(sup (minus)7). In the work I will describe here we study the effects of the weak interaction through the signal of the parity violation associated with that interaction. There are two basic classes of experiment used to detect parity violation. The first relies on the measurement of a cross section or width that would vanish if parity were conserved. One example of this type of experiment in nuclear physics is the decay of an unnatural parity state to a 0+ nucleus and an (alpha)-particle. Such measurements have been made for two nuclei: (sup 16)O(2(sup (minus))) -> (sup 12) C(g.s) + (alpha) and (sup 20)N(var epsilon)(1(sup +)) -> (sup 16) O(g.s) + (alpha). Parity-violating widths as small as of 10(sup (minus)10) eV have been measured in these experiments. The second class of experiments involves a measurement of pseudo-scalar observables which are odd under parity inversion. These involve correlations between spin and linear angular momenta, for example circular polarization of (gamma)-rays ((sigma)(sub (gamma)) (center dot) (kappa)(sub (gamma))) or longitudinal analyzing power ((sigma)(sub p) (center dot) (kappa)(sub p)). 20 refs., 6 figs.