DOI 10.1140/epja/i2007-10534-y Regular Article – Experimental Physics Eur. Phys. J. A 35, 167–170 (2008) T HE EUROPEAN P HYSICAL JOURNAL A Lifetime measurement of the six-quasiparticle isomer in 140 Nd and evidence for an isomer above the 19/2 + state in 139 Nd M. Ferraton a , R. Bourgain, C.M. Petrache, D. Verney, F. Ibrahim, N. de S´ er´ eville, S. Franchoo, M. Lebois, C. Phan Viet, L. Sagui, I. Stefan, J.F. Clavelin, and M. Vilmay Institut de Physique Nucl´ eaire, IN2P3-CNRS and Universit´ e Paris-Sud XI, F-91406 Orsay Cedex, France Received: 21 November 2007 / Revised: 21 January 2008 Published online: 11 March 2008 – c  Societ`a Italiana di Fisica / Springer-Verlag 2008 Communicated by R. Kr¨ ucken Abstract. The lifetime of the recently discovered six-quasiparticle (6-qp) isomer in 140 Nd has been mea- sured using the 126 Te( 18 O, 4n) reaction and the pulsed-beam technique at the Institut de Physique Nucl´ eaire (IPN) Orsay. The deduced lifetime of the 6-qp isomer in 140 Nd of 1.23(7) μs supports the 20 + spin-parity assignment to the isomeric state which is based on a spherical configuration that coexists with the triaxial bands observed in this spin range. Evidence for delayed components for the transitions below the 19/2 + state in 139 Nd was observed, with an apparent half-life of 272(4) ns. PACS. 27.60.+j 90 <A< 149 – 21.60.-n Nuclear structure models and methods – 21.10.Tg Lifetimes, widths 1 Introduction The nuclei around the N = 82 shell closure are a fertile field of spectroscopic investigations both at low and high spins: at low spins the presence of isomers based on simple particle-hole configurations helps to establish the active quasiparticle configurations in a specific nucleus and test the suitability of various nuclear potentials [1], whereas at high spins the combined contribution of neutron holes and proton particles drives the nuclear shape toward a stable triaxial shape with γ ≈ +30 ◦ [2, 3]. New experimental data were recently published for the 140 Nd nucleus, which has two neutron holes with respect to the N = 82 shell closure and therefore presents several isomeric states, with configurations involving up to six quasiparticles [4]. In the odd-even 139 Nd, the existence of an isomeric state was reported long time ago [5], but its position in the level scheme and the lifetime could not be established. In this article we report on the lifetime measurement in a pulsed-beam experiment of the 20 + isomeric state in 140 Nd and on the apparent half-life of the transitions below the 19/2 + state in 139 Nd. 2 Experimental details High-spin states in 140 Nd and 139 Nd have been populated in the reaction 126 Te( 18 O, x n) with a 75 MeV 18 O beam a e-mail: ferraton@ipno.in2p3.fr delivered by the Tandem accelerator of IPN Orsay. The 126 Te target with a thickness of 400 μg/cm 2 was deposited on a 10mg/cm 2 thick Au backing. Gamma-ray coinci- dences were measured with one Clover and three coax- ial Ge detectors with BGO Compton-suppression shields. The four detectors were positioned in the horizontal plane, around the reaction chamber, at angles of ±45 ◦ , and ±135 ◦ with respect to the beam axis. The reaction channel leading to 139 Nd is ( 18 O, 5n) and to 140 Nd is ( 18 O, 4n), with nearly equal cross sections of around 200 mb as calculated with CASCADE [6]. The re- coiling residual nuclei were stopped by the thick Au back- ing in the center of the array to measure the isomeric decay. The beam pulsing was realized using a chopper- buncher system which produced Gaussian beam pulses with FWHM of 1.8 ns and FWTM of 5 ns. A repetition rate of 10 μs was chosen between the beam pulses, a time interval sufficient to study isomers with lifetimes in the range from tens of nanoseconds to several microseconds. We have measured single γ -rays and their detection time with respect to the beam pulse to deduce the lifetime of the isomeric states. Events were written on disk employing the NARVAL program. The data were collected using the Orsay acquisi- tion system based on COMET-6X cards [7], designed to be used as high-resolution ADCs. A COMET-6X card con- sists of a 32 bits 40 MHz Digital Signal Processor (DSP) which realizes part of the data processing in situ, pro- ducing an event composed of the absolute time coded on