Experimental limit for the circular polarization of the 1081 keV gamma ray inF18

A measurement of the circular polarization $P_{\gamma }$ of the 1081 keV γ-ray in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ is reported. The result is $P_{\gamma }$=(2.7±5.7)×${10}^{\mathrm{-}4}$. The reaction used was ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${(}^3$He,p${)}^{18}$F with a 3.4 MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ beam of an intensity of 7 μA impinging on a windowless water target. The circular polarization was measured with a four-prong transmission-type Compton polarimeter. The measurement of the analyzing power of the polarimeter is presented. Gamma rays were detected by means of 130 ${\mathrm{cm}}^3$ p-type germanium detectors placed behind the prongs. Four independent data acquisition channels were used: pile-up losses at a counting rate of 60 kcps were kept lower than 15%, owing to the use of trapezoidal shapers. Systematic effects of instrumental origin are discussed and found to give negligible contributions with respect to the statistical uncertainty. The calculation of the asymmetries is performed using different estimators; an extensive comparison of the various methods is presented. The result of the experiment, when averaged with those of the other experiments on ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ carried out until now, gives a grand average $P_{\gamma }$=(1.2±3.9)×${10}^{\mathrm{-}4}$. This result is compared with recent theoretical estimates.