Experiments on Parity Nonconservation in Nuclear Forces. I. Gamma Transitions inTa181andLu175

We report measurements of the circular polarization of $\gamma$ rays from unpolarized sources of ${\mathrm{Hf}}^{181}$ and ${\mathrm{Yb}}^{475}$ of 100-1000-Ci strength, using a Compton polarimeter in a forward-scattering geometry. Our technique consisted of integrating the total detector current and digitizing the charge. Several aspects of this novel method are discussed, such as the role of the switching pattern in optimizing the signal-to-noise ratio. All possible causes of false asymmetries are investigated. Control experiments are carried out with ${\mathrm{Ru}}^{103}$ sources. The contribution to the observed circular polarization due to bremsstrahlung has been calculated, as well as determined experimentally. Our results for the circular polarization are $P_{\gamma }=(-3.9\mathrm{}\pm 1.2)\times {10}^{-6\mathrm{}}$ for the 482-keV transition in ${\mathrm{Ta}}^{181}$, and $P_{\gamma }=(6.3\mathrm{}\pm 1.0)\times {10}^{-5\mathrm{}}$ for the 396-keV transition in ${\mathrm{Lu}}^{175}$. These findings confirm the existence of a parity-nonconserving term in the nuclear Hamiltonian.