A Quest for Neutral-Current-Enhanced Parity Mixing in ¹⁸F

A measurement of the circular polarization of the gamma rays from the 1.081- MeV-to-0.0-MeV transition of the nucleus 18F has been made in order to search for possible effects of neutral weak currents . Excited 18F nuclei were created in a flowing water target by a 4.0 MeV 3He beam via the reaction 16O(3He,p) 18F*. Pulse-height spectra were accumulated for approximately 1500 hours of running time in two Ge(Li) detectors which viewed the target through transmission-style Compton polarimeters. Additional measurements were made to calibrate and check the apparatus. A careful analysis of the data leads to a 1.081 MeV gamma ray circular polarization of (-0.7 ± 2.0) x 10-3. This value is consistent with zero, and smaller than but not inconsistent with the most recent predictions based on the Weinberg-Salam theory of neutral currents.