High-spin states above the isomers in neutron-rich iodine nuclei near N=82

Abstract

Excited states of neutron rich Iodine isotopes 130−134 I above the high spin isomers have been identified using prompt-delayed γ-ray spectroscopy. The Iodine isotopes have been produced as fission fragments of fusion-fission and transfer induced fission of 9 Be(238 U, f) at a beam energy of 6.2 MeV/u. The complete (A, Z) identification was obtained using the large acceptance magnetic spectrometer VAMOS++. The AGATA γ-ray tracking array was used to detect the prompt γ rays while the delayed γ rays (in the time range of 100 ns to 200 µs) from the isomeric states were identified by the EXOGAM segmented Clover detectors, placed behind the focal plane of VAMOS++ spectrometer. The high spin states above the (8 −) isomers in 130,132 I have been populated for the first time and a new isomer in 132 I has been identified. New γ-ray transition has also been assigned to the level structure of 134 I. Prompt transitions above the 19/2 − isomer have been identified in 131,133 I, for the first time. The level structures are interpreted in terms of the systematics of odd-Z nuclei above Z = 50 shell closure and large scale shell model calculations.

Gamma-ray spectroscopy of fission fragments with state-of-the-art techniques

“…In yttrium nuclei, deformed structures are found to appear at high excitation energy already in 96 Y (N = 57) [147,148], and 98 Y becomes and extraordinary example of shape coexistence, with a spherical ground state and two low-lying isomeric states (i.e., below 500 keV), with prolate deformation [149]. A similar scenario of shape coexistence is also observed in 99 Zr [141], while systematic investigation of heavier 104,105,106 Zr isotopes, performed with EXOGAM+VAMOS in GANIL, point to a rather smooth structure change characterized by collective (prolate) excitations with some degree of triaxiality [146].…”

“…Establishing the nature of the triaxiality in the Mo region is, therefore, not straightforward and it is still an open question. Complementary investigations in the neighboring even-even 104,108 Mo nuclei seemed to point to axially symmetric nuclei [227,234], while experimental level energies and γ -decay patterns of the odd systems 103,105,107 Mo were better reproduced by simple particle-rotor calculations, assuming that these nuclei have an asymmetric shape [233,235,236], as shown in Fig. 29a.…”

“…The difference is attributed to the properties of the p-n interaction [103]. (Adapted from [101,103]) Prompt-delayed γ -ray spectroscopy, with very selective setups such as AGATA+ VAMOS++ with EXOGAM detectors at the focal plane, have also been instrumental to locate excited states in neutron-rich 130−134 I, above high-spin isomers (8 − and 19/2 − in even and odd systems, respectively) [104]. In particular, comparisons with largescale shell model calculations show that the structure of the excited negative-parity states is dominated by the three proton particles, outside Z = 50, in the π g 7/2 , π d 5/2 and π h 11/2 orbitals, and by the three neutron holes in the νd 3/2 , νs 1/2 , νh 11/2 , while all other orbitals remain relatively inactive (cfr.…”

“…In these kinds of experiments attention needs to be paid to the mechanical stability of target and degrader, due to thermal expansion caused by the beam. The isotopic identification 104 Mo [387] provided by the separator together with the Doppler correction capabilities of the γ spectrometer are essential in these studies.…”

“…Bottom: Examples of curves of the ratio f p = N p /(N p + N f ) (N p and N f being the intensity of the partially-and fully-shifted components of the γ -ray lineshape) versus the distance between the source and the degrader foil. Experimental data refer to yrast states in104 Mo[387]…”