Gamma-ray polarization sensitivity of the Gammasphere segmented germanium detectors

We present a technique for operating the Gammasphere segmented Ge detectors as γ-ray polarimeters. Using γ-rays of known polarization, we have measured the polarization sensitivity, Q(Eγ), of these detectors in the energy range Eγ=0.4–1.4 MeV. The experimentally obtained value of Q ranges from 5% at 415 keV to 4% at 1368 keV. The magnitude and energy dependence of Q(Eγ) has also been determined theoretically by means of a Monte Carlo simulation, and the agreement between experiment and theory is within 20% over the energy range measured. In order to investigate the potential for operating Gammasphere as a γ-ray polarimeter in a high background environment (one which is typical of high-spin nuclear structure studies), we also discuss data that we have analyzed from the 176Yb ( 26Mg, 5n)197Pb reaction at E=135 MeV. Although the polarimeter performance of Gammasphere is seen to be relatively poor, we are still able to demonstrate, by way of γ-ray polarization measurements, that “Shears Band 1” in 197Pb should have negative parity.