New isomer in ⁹⁶Y marking the onset of deformation at N = 57

¹ Institute of Nuclear Physics, PAN - PL-31-342 Kraków, Poland
² Dipartimento di Fisica, Università degli Studi di Milano - I-20133 Milano, Italy
³ INFN, Sezione di Milano - via Celoria 16, I-20133, Milano, Italy
⁴ Horia Hulubei National Institute of Physics and Nuclear Engineering - Bucharest, 077125, Romania
⁵ ILL - 71 Avenue des Martyrs, F-38042 Grenoble CEDEX 9, France
⁶ GANIL - BP 55027, F-14076 Caen CEDEX 5, France
⁷ IKP, University of Cologne - Zülpicher Str. 77, D-50937 Köln, Germany
⁸ LPSC - 53 Avenue des Martyrs, F-38026 Grenoble, France
⁹ INFN, Sezione di Padova - Padova, Italy
¹⁰ ELI-NP - Magurele-Bucharest, Romania
¹¹ Faculty of Physics, University of Warsaw - ul. Pasteura 5, PL-02-093 Warsaw, Poland
¹² SCEM, University of Brighton - Lewes Road, Brighton BN2 4GJ, UK
¹³ Grupo de Fsica Nuclear, Universidad Complutense, CEI Moncloa - 28040 Madrid, Spain
¹⁴ CEA, Centre de Saclay, IRFU - F-91191 Gif-sur-Yvette, France
¹⁵ Institut fr Kernphysik, TU Darmstadt - Schlossgartenstrasse 9, D-64289 Darmstadt, Germany
¹⁶ Faculty of Physics, University of Sofia - 5 James Bourchier Blvd, 1164 Sofia, Bulgaria
¹⁷ INFN, Sezione di Firenze - Firenze, Italy
¹⁸ Dipartimento di Fisica, Università degli Studi di Padova - I-35131 Padova, Italy
¹⁹ INFN, Laboratori Nazionali di Legnaro - I-35020 Padova, Italy
²⁰ Acoustics and Ionizing Radiation Division, National Physical Laboratory Teddington, Middlesex, TW11 0LW, UK

Received: 26 January 2017
Accepted: 13 February 2017

Abstract

The level scheme of ⁹⁶Y was significantly extended and a new 201 ns isomer was located at 1655 keV excitation energy, with spin-parity assignment of 5^± or 6⁻. The isomer decays to spherical low-spin structure by transitions with large hindrance and is fed by a short cascade which resembles the beginning of a rotational band. This is in analogy with the feeding and decay pattern of the 4⁻ isomer in ⁹⁸Y, here confirmed, by lifetime analysis, as a bandhead of a rotational structure with sizable deformation. It is suggested that the new isomer in ⁹⁶Y arises from a shape change between deformed and spherical configurations, which indicates the appearance of deformation already at N = 57 in the yttrium chain. The experimental findings for ⁹⁶Y are strengthened by theoretical calculations based on the complex Excited Vampir model.