Parity violation in compound nuclei: experimental methods and recent results

The TRIPLE Collaboration studies of space-parity symmetry in the compound n ucleus show numerous examples of parity violation in Br, Rh, Pd., Ag, Cd, I n, Sn, Sb, I, Cs, Xe, La, U, and Th. The longitudinal cross section asymmet ries have measured values, in the range of 10(-3)-10(-1) for neutron energi es, from several eV up to 300-2000 eV, depending on the target. The high de nsity of states leads to enhancement of the parity violation by factors as large as 10(6) relative to parity violation in pp scattering. The high degr ee of complexity of the levels permits the use of statistical methods for d etermination of the root mean square weak matrix element M for each nucleus . This report is focused on the experimental results of the TRIPLE Collabor ation studies. Parity violation has been observed in 75 resonances of 18 nu clides. The experimental data and analysis are presented for each nuclide s tudied. A nonstatistical anomaly (the sign correlation effect) was observed in thorium. Statistical analysis techniques were developed and successfull y applied to determine the rms weak matrix elements and the weak spreading widths Gamma (w). The value of Gamma (w) obtained from our analysis is abou t 1.8 x 10(-7) eV, which is in qualitative agreement with theoretical expec tations. The individual weak spreading widths are consistent with a constan t or slowly varying mass dependence and there is evidence for local fluctua tions. (C) 2001 Elsevier Science B.V. All rights reserved.