Test of parity violation in 207Pb by nuclear orientation

A measurement of parity nonconserving (PNC) asymmetries in the scattering of low energy polarized neutrons from 232Th showed large asymmetries, most of them having the same sign. Several theoretical calculations require that either there is a significant enhancement of PNC effects in heavy nuclei, or there is a misunderstanding of the reaction mechanism involved in these experiments. Study of PNC matrix elements in a heavy nucleus like 207Pb by a different method can differentiate between these two possibilities. The 1.063MeV, 13/2+ to 5/2-γ transition in 207Pb can have an open-quotes irregularclose quotes E4 multipole, due to a 5/2+ admixture into the 5/2- state. The measurement of forward-backward asymmetry of this γ-ray from the decay of polarized 207Bi nuclei will give the magnitude of the PNC effect. The forward-backward asymmetry in this line might be 3x10-4. In preliminary experiments, a 207Bi radioactive source dissolved in a Ni foil and cooled to 7mK showed a 50% parity allowed 0 degrees enhancement for this γ-ray. Ge detectors placed at 0 degrees and 180 degrees with respect to an external magnetic field will be used to measure the asymmetry