Systematics of the K  = 8 - isomeric decay in the N=74 isotones: a study of the lighter 128 Xe isotone

1. Systematics of the K = 8- isomeric decay in the N=74 isotones: a study of the lighter 128Xe isotone †School of Engineering, University of Brighton Brighton, BN2 4GJ, UK Nico Orce Alison Bruce email: j.n.o.gonzalez@bton.ac.uk Acknowledgements A. Emmanouilidis†, L. Frankland†, A.P. Byrne1, G.D. Dracoulis1, J. Hazel1, T. Kibedi1, P.M. Walker2, Z. Podolyak2, M. Camaaño2, H. El-Masri2, C. Pearson2, C. Wheldon3 and D.M. Cullen4 1Department of Nuclear Physics, The Australian National University, Canberra, ACT 0200, Australia 2Department of Physics, University of Surrey, Guildford, GU2 5XH, UK 3Department of Physics, University of Liverpool, Liverpool, L69 7ZE, UK 4Department of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK

2. Introduction • For prolate deformations around  0.22 at neutron number N=74 the neutron orbitals 7/2+[404] and 9/2-[514] are close to the Fermi surface and may combine to give a K=8- state • This K=8- state decays by an E1 transition to the 8+ member of the yrast band and has been found to be isomeric in 138Gd, 136Sm, 134Nd, 132Ce and 130Ba with half-lifes ranging from ~10-6 to ~10-3 s.

3. What is interesting? •The observed E1 transition from the K = 8- isomeric state to the 8+ state of the yrast band (assumed to have K=0) is seven times K-forbidden and should be rigorously hindered. However, such an E1 transition has been observed in 138Gd, 136Sm, 134Nd, 132Ce and 130Ba but not yet in 128Xe. • Validity of the K quantum number: as the proton number approaches the Z=50 closed shell the K quantum number is not expected to be such a good quantum number, allowing possible K-mixing. Therefore, the observed lifetimes should decrease. This is not observed experimentally in the N=74 isotones, where 132CeZ=58 and 130BaZ=56 isomeric states have much longer lifetimes (see above). • Indeed, these E1 decays are observed to have greater reduced hindrance values ( f= (T1/2 / T1/2w )1/ , where  = K-) as the proton number decreases (see right), which may indicate some collective or pairing effects. • This contradiction with that which is theoretically predicted has been explained by A.M. Bruce et al. (Phys. Rev. C, 1997. volume 55, p.620.) on the basis of a two-band mixing calculation.

4. Previous work on 128Xe • Goettig’s experiment • Lonnroth’s experiment • T1/2 = 63(12)ns isomeric state observed in 128Xe. Only two states observed above the isomeric state (L.Goettig et al., Nucl. Phys. A, 1980, V357, 109) • K= 8-two quasineutron configuration is supported by a measurement of the g-factor. Once more, only two states observed above the isomeric state (T.Lonnroth et al., Z. Phys. A, 1984, V317, 215)

5. Nucleus PACE2 (%) Measured(%) 128Xe 73.6 51.7 127Xe 16.5 35.9 126Te 0.5 12.4 Experimental details • High spin states were populated using the fusion-evaporation reaction124Sn(9Be,5n) 128Xe at a beam energy of 58 MeV • The bunched and chopped beam (107ns on, 15x107ns off) was incident on the 3mg/cm2 124Sn target, sufficient to stop most of the recoiling nuclei. • The experiment was run for five days at the ANU (Canberra, Australia) and emitted -rays were detected using the CAESAR array • The total projection of the - matrix shows that 128Xe was indeed the main channel produced in the reaction

6. Proposed level scheme for 128Xe • 44 new -rays and 32 new excited levels have been proposed • yrast band extended to 22 • Study of the angular distribution still in progress • New information about the nuclear structure above the isomeric state is extremely significant since it may provide information about the shape symmetry of the nucleus in the isomeric state.In fact, no “nice rotational band” is observed above the isomeric state which might indicate that the nucleus is not axially deformed and K is not a good number.

7. Data analysis • Analysis of the delayed-delayed matrix: gate on the 6+ to 4+ yrast transition (704 keV) • Analysis of the long early-delayed matrix: gate on the 328 keV transition which feeds the isomeric state No link between the K= 8- isomeric state and the yrast band has been observed in 128Xe

8. “OK, this is what happens…” • The deformation of the N=74 isotones decreases as the proton number approaches the Z=50 closed shell. Therefore, due to the increasing energy gap observed between the two neutron orbitals 7/2+[404] and 9/2-[514], more energy is needed in 128Xe than in the heavier N=74 isotones to combine the two-quasineutron orbitals to give a K=8- state • Due to this increase in energy, for the first time in the N=74 isotone region the K=8-isomeric state in 128Xe is above the 7- and 6- states of a negative parity sideband. Therefore, it preferentially decays to these negative parity states since the reduced transition probability for the M1 transition is twice as large as for the competing E1 transition