Parity violation in the y-decay of polarized g3T~ nuclei in the 17 /2- isomeric state

We report on a determination of the parity nonconserving (PNC) matrix element in the bound parity doublet f--y’ of “‘Tc. The experiment was carried out at the GSI, Darmstadt and LNL, Legnaro laboratories. The recoil-mass-separated radioactive beam of ‘“Tc nuclei in the yisomer, following a fusion-evaporation reaction, was polarized by the tilted-foil method and the resulting O”-180” y asymmetry with respect to the induced polarization direction was measured by two large-volume Ge detectors. The measured y asymmetry of 3-u significance, A, = 8.4( 2.7) . 10m4, yields a matrix element of I( ~IHFncj y)I = 0.59( 19) (25) meV. This experimental result is compared to microscopic calculations based on the DDH “best value” interaction for the nuclear weak Hamiltonian. We discuss our results and their significance with respect to the existing data regarding PNC effects in bound nuclear systems. I’4C.S. 2l.lO.Ky: 2l.lO.P~: 23.40.H~ The $!isomeric level of 9”Tc has a y’ partner at a separation of only 300 eV [ 1,2]. The isomer decays to a 7’ state through a mixed M2/E3 transition. Any contribution of the opposite parity !$’ state would lead to an E2 admixture to the transition whose intrinsic transition matrix element is larger by a factor of N 1000 than M2 and E3. For a polarized sample of the -____ ’ Permanent address: Department of Particle Physics, Weizmann Institute of Science, Rehovot, Israel. isomer, the mixed parity component in the transition introduces a PI (cos 13) term in the angular distribution 0370.2693/96/$12.00 @ 1996 Elsevier Science B.V. All rights reserved .SSD/O370-2693(95)01596-5 26 hf. Huss et rd. / Physiw Letters B 371 (1996) 25-33 with respect to the direction of polarization, yielding a O”-I 80’ asymmetry A, of the decay y-rays. The detection of this small asymmetry and its implication for establishing and understanding PNC in a bound nuclear system is the subject of this publication. Parity nonconservation originates at the most fundamental level from the weak currents mediated by the exchange of the W and Z bosons. PNC in nuclei was the first place where the nonstrangeness-changing sector of the standard weak-interaction model was observed [3]. Nuclear PNC is usually interpreted in terms of the underlying nucleon-nucleon (NN) PNC interaction. Theoretical “best value” strengths of NN PNC interactions were established by Desplanques, Donoghue and and Holstein (DDH) [4] and their hadronic model dependence has been investigated [ 4,5]. In order to experimentally establish the strengths of the isoscalar (IS), isovector (IV) and isotensor (IT) components of the NN PNC interaction, it is important to compare results from NN scattering with those from nuclear bound states. The interpretation of the results for the nuclear bound states depends upon accurate nuclear-structure calculations. For a few nuclei, such as lsF and 19F, much of the nuclear-structure uncertainty can be removed by comparison to the analogue axial-charge beta decays [ 3 3, and in general for light nuclei large-scale shell-model calculations can he carried out [ 3,6]. The upper limit for the empirical strength of the IV component found from light nuclei [6] is a factor of three smaller than the DDH “best value” based upon the SU( 3) quark model plus penguin-diagram corrections. This is surprising because it is this component which was predicted to be enhanced by the neutral current, Z-boson, contribution which is unique to the nonstrangeness changing PNC [4]; never-the-less, it is still within the range allowed hy hadronic models [ 41. There are a number of cases, such as in ‘*‘Hf [ 71, where nuclear PNC has been observed, but where the interpretation is limited by the uncertainties in the nuclear wave functions [ 31. Recently, large PNC effects in low-energy neutron scattering have been observed, and the interpretation of the results for these compound nucleus states has focused on the nuclear manybody “enhancement” factor [ 81. All bound-state cases studied to date are thought to he dominated by the “one-body” nuclear mechanism which originates from a coherent average of the NN PNC interaction over all core nucleons and which yields single-particle matrix elements (t,jlV&If 1 ,j) on the order of one eV The case of 9”Tc presents a unique situation. The structure of the nuclei with 50 neutrons (N = 50) including 9’Tc are well described by shell-model wave functions [9,10], and thus reasonable nuclearstructure calculations can be carried out, although not at the same level of completness as those for light nuclei. It is different from the cases studied in light nuclei because it is a high-spin doublet and because there is (in the simplest model) no “one-body” component it is entirely two-body. In particular, we will show below that the 93Tc PNC is particularly sensitive to the isotensor component of the NN PNC potential. A full description of the essentials of the experiments, including the method of tilted-foil polarization of separated isomer beams can be found in Ref. [ I I 1. We briefly summarize the salient points. The energy level spectrum of 9”Tc is shown in Fig. I. The close proximity of the uand Ef 2 2 levels favors a mixof wave if parity maelement q-1 1 7’) is finite. For the Tstate, the parity mixed wave function leads to an E2 admixture in the predominant M2/E3 gamma transition to the 9’ level. The sensitivity to the parity mixing is strongly enhanced due to the intrinsically more intense E2 strength. The parity admixture yields an A 1 PI (cos 0) term in the angular distribution of the gamma-rays with respect to the direction of polarization [ 21. The measured anisotropy coefficient is given by: A, = A 1 Ql fip, where Qi is the geometrical attenuation coefficient due to the finite size of the Ge’s and p/ = h. Following the derivation in Ref. [ ] references the matrix is by: