Nuclear-orientation measurement of parity admixture in the 501-keV gamma transition inHfm180

Abstract

The effect of the parity nonconserving nuclear force on the 501-keV y transition of 80Hf has been confirmed by a remeasurement of the angular distribution asymmetry of the radiation emitted by~H f~polarized at temperatures down to 16 mK. A difference of 1% was observed between the p-ray intensities parallel and antiparall. el to the nuclear polarization direction. When the results of the present study are averaged with previous nuclear polarization experiments and compared with the results of circular polarization experiments, it is deduced that the parity mixing can be uniquely assigned to an irregular &2 multiple, rather than to a higher order irregular multiple such as M3. Assuming a vanishing M3 component, the present study yieMs~= (E2)/(M2) = -0.026+ 0.002, and the global average from all measurements is e= -0.030 + 0.002.

Confirmation of parity violation in the γ decay of180Hfm

“…It was first observed in gamma ray circular polarization experiments [1,2]. Using the technique of low temperature nuclear orientation (LTNO), the reported result is an apparently well established effect, a mixing ratio ǫ(E2/M2) = -0.030 (2), of a magnitude more than fifteen times the experimental error [3,4,5,6]. Other LTNO measurements are listed in [7].…”

“…Full descriptions of the formalism of the nuclear orientation experimental method and of the previous measurements on 180 Hf m are given in Refs. [3,4,5,6,7]. Here only the essential details relevant to the present work and its analysis are presented.…”

“…The gamma radiation from such a polarized source exhibits a strongly anisotropic angular distribution. If the nuclear states between which the gamma decay occurs are eigenstates of parity, the theoretical distribution contains only even terms in cosine theta, measured from the axis of polarization, and is given by the expression [6]…”

The on-line low temperature nuclear orientation facility NICOLE

“…Because of some experimental uncertainties in the original measurement, the experiment was repeated at the NICOLE facility [20]. The mass-separated molecular 180m HfF 3 + beam from the ISOLDE mass separator [21] was implanted into an iron foil maintained at mK temperatures, producing higher degrees of polarization than were achieved in previous studies [22]. The value found for the irregular E2/M2 mixing ratio ε=−0.0324 (16), is in close agreement with the previous published average value ε=−0.030(2) [22], in full confirmation of the presence of E2 admixture in the 501 keV transition.…”

“…The mass-separated molecular 180m HfF 3 + beam from the ISOLDE mass separator [21] was implanted into an iron foil maintained at mK temperatures, producing higher degrees of polarization than were achieved in previous studies [22]. The value found for the irregular E2/M2 mixing ratio ε=−0.0324 (16), is in close agreement with the previous published average value ε=−0.030(2) [22], in full confirmation of the presence of E2 admixture in the 501 keV transition. The temperature dependence of the forward-backward asymmetry has been measured over a more extended range of nuclear polarization than previously possible, giving further evidence for parity mixing of the 8 − and 8 + levels and the deduced E2/M2 mixing ratio.…”