New nuclear structure data after fission: The g.s. of 136Sb

Nuclei in the neutron-rich region beyond 132Sn have been produced recently by various experiments using fission. Using isomer and -decay studies nuclear structure data has been collected on the orbital evolution and collectivity in the region with both the increase of proton and neutron numbers. Examples on particular questions related to the g.s. of the A=136 odd-odd 136Sb nucleus and its heavier neighbours are given in the scope of expectations by shell-model theory.


Introduction
Exotic nuclei beyond the 132 Sn double-shell closure are influenced by both the Sn superfluidity and the evolving collectivity only few nucleons away. Toward even more neutron-rich nuclei, especially at intermediate mass number, interplay between singleparticle and collective particle-hole excitations starts to compete. In some cases with the extreme addition of neutrons also other effects as the formation of neutron skin [1], stabilization as sub-shell gaps [2], and/or orbital crossings [3] may be expected. The knowledge of nuclear ingredients is especially interesting beyond 132 Sn and little is known on how the excitation modes develop with the addition of both protons and neutrons [4][5][6][7]. Analogy to the nuclei beyond 208 Pb [8] could be used close to the doubly-shell core, based on the dominant role of the pairing correlations in its vicinity. Little data is available for more than two-proton, four neutron systems [9] and it is unclear whether other components mix in the configuration of states preferentially when neutron rather than proton pair is added into the system. It is a challenge for the today's realistic shell model calculations, that work well for the species around the doubly-closed core, to constrain the effective interaction between the nucleons for such very exotic region with extreme N, thus any experimental data is crucial input to theory to allow a global description and valuable theoretical predictions.

Motivation
Several nuclei in the region draw attention due to various inconsistencies beyond N=82 as the fast B(E2) drop for 136 Te with respect to 134 Sn, 136 Sn [10,11]. This was suggested to appear due to the excess of neutrons and strong neutron-proton exchange asymmetry, leading to dramatic drop in the E(2 1 + ) state in 138 Te (and 140 Xe) [12]. The first excited 5/2 + state in 135 Sb was found to be drastically lower, instead of increasing in energy as for 133 Sb [13,14]. It was expected that the repulsion between νf 7/2 and πd 5/2 orbitals appears stronger due to a reduced pairing with respect e.g. of the nearly degenerate πd 5/2 and πg 7/2 single particle orbitals, found to cross at an extreme N=89 for 140 Sb [15]. Orbital evolution between N=82 and N=90 is still unknown and is given emphasis in recent studies. For example, the ground state spin of 136 Sb is unfixed with several possibilities suggested, ranging from (0 -, 1and 2 -) depending on the theoretical interpretation on the very few existing experimental results [8,16]. In particular, in an earlier β-decay experiment a good agreement was achieved with spin/parity of (1 -) with πg 7/2 νf 7/2 3 configuration, while (0 -) was ruled out due to the relatively strong first forbidden unique β to the 0 + of 136 Te but also to unobserved branches. According to [17], the (2 -) state should be very low in energy and possibly a g.s., as the assumption for spin/parity of (1 -) for the g.s. does not allow to theoretically reproduce the experimental spectra of both 134,135 Sb. We have initiated a new β−decay study to clarify this question linking to the heavier Sb. For example we have proposed (3 -) spin assignment to the g.s. of 138 Sb [18] due to a strong -decay branch to the (2 + ) states in the 138 Te daughter nucleus build on the g 7/2 f 7/2 5 configuration, in variance to the theoretical (4 -) [17,19]. The g 7/2 -d 5/2 orbital inversion we have proposed in [15] together with the observation of the first 2 + , 4 + states in 140 Te lead us to most probable (3 -) assignment build on the same configuration for the g.s. of 140 Sb [20].

Experimental studies 3.1 Fission to populate the 132 Sn region
Recently, we have approached the region of nuclei in several measurements following fission of 238 U on 9 Be target at high energies of 350 MeV/n, within the EURICA project [21][22], thermal n-induced fission on 235 U/ 241 Pu targets using prompt-decay spectroscopy within the EXILL/FATIMA campaigns [23,24], as well as in -decay spectroscopy at Lohengrin. These complementary studies on several nuclei in the region show coherent picture and possible interpretation of the new data beyond 132 Sn. Hereafter, we describe first outcomes for the A=136 nuclei performed at the ILL.

-decay fission products at Lohengrin
Experimental study was performed at the Lohengrin spectrometer on A=136 fission fragments produced in 235 U(n th ,f) reaction and transported to the focal plane behind a focusing magnet using the technique described in [25]. An experimental setup was built using the -decay station of plastic detectors in 4 geometry from the LOENIE -delayed neutron detector [26]. The box was placed around a vacuum chamber that supported a movable tape, used to evacuate the radioactivity from the implanted ions. The duty cycle was adapted to the t 1/2 of the isotopes of interest e.g. the A=136 Sb ions. Emitted -rays for selected -decay event were detected by two Clover detectors (with Anti-Compton (AC) shields) and one standard coaxial HpGe detector in close geometry (see Fig. 1). In addition, LaBr 3 (Ce) detectors were added to the -detection to scan lifetimes of the populated states in the sub-ns time range. Further experimental details can be found elsewhere [27].

Preliminary results
The experimental data on A=136 was collected for several charge states (Q=21,25,30) in order to obtain good ratio between A=136 nuclei of interest and contaminants with A/Q accepted by the system. The resulting spectrum for the best peak/contaminant selection corresponding to Q=21 is shown in Fig. 2. -rays following the -disintegration of the mother nucleus 136 Sb (t 1/2 of 0.923 (14)

Identification
After proper energy and efficiency calibration all observed -rays and their intensities were attributed to the different isobars based on their time behaviour (see Fig. 3). Despite the optimized time cycle, the -rays belonging to the 136 Sb 136 Te decay were weaker than the 136 Te 136 I decay. This is due to both, the lower fission yield of 136 Sb and the stronger g.s. to g.s. transition in the decay of the latter. We note that no (e.g. long-lived) isomers in these nuclei contribute to the observed transitions. This is not the case for 136 I 136 Xe decay, where we have detected -rays from the (6 -) 46.9(10) s, state as identified in [28,29]. The P n branch (16.3% [28]) for 136 Sb was taken care of the analysis [27]. Fig. 3. Decay schemes observed in the experiment, spin/parities are adopted from [28], Q values from [30]. The strong g.s. to g.s. feeding between 136 Sb and 136 Te is based on the deduced intensities (see text). New information for 136 Te is in bold, the (6 -) decay transitions in 136 Xe are in bold.

Discussion
The low intensities of transitions following the 136 Sb 136 Te decay observed in our study support the fact that most of the decay proceeds through the g.s. of 136 Te, which is in agreement with the earlier conclusions of Hoff et al. [16]. This results in the most probable spin/parity assignment of (1 -) for the g.s. of 136 Sb. Our deduced logft values (for 136 Te in Fig. 3) using [31] support this conclusion. For example, assuming the (2 -) spin/parity proposed in [17] for the g.s. of 136 Sb, would increase the feeding to the first and second 2 + states in 136 Te, with similar arguments for the weakly fed 4 + state. In addition, we have not observed other strong branches beyond 1.6 MeV excitation energy, also in agreement with [16]. Taking into account the logft values, one can conclude that the -decay proceeds by transitions of first forbidden type, the strong transition that is preferred in the Sb case g 7/2 f 7/2 3 g 7/2 2 f 7/2 2 was indeed expected in [16,29,18,20,32]. The (2 -) alternative would imply an exceptionally fast logft of 8.2 first forbidden unique transition [16] to g.s. of 136 Te, and has not been observed here. The suggested mixture between πg 7/2 νf 7/2 3 and πh 9/2 νg 9/2 [17,19] for the g.s. of 136 Sb could also be rejected as both orbitals manifest at higherexcitation energy [8,9,15,20]. Any strong configuration mixings will affect the states in 136 Te by pushing them e.g. towards lower energies in comparison to the corresponding twoproton and two-neutron systems. The total number of states below 2.5 MeV should not be appreciably different from the total number found in these systems which is not seen here and in [16].
One can note that the g.s. spin/parity of the neighbouring 134 Sb was identified as (0 -) with a (1 -) state close-lying in energy [33]. This was also debated by Sarkar et al. in [17], where both states were proposed as yrast traps based on shell-model calculations using both empirical and realistic interactions. The agreement found for the (0 -) assignment was possible using the realistic interaction [19]. For the g.s. of 136 Sb, except fitting to the assignments in the analog Pb-chain [8] for 212 Bi with g.s. spin/parity (1 -) [16], a supporting argument for (1 -) can be given following its isomeric decay study [9]. The multipolarities of all transitions we have observed following the (6 -) sub-s isomer agree indeed better with (1 -) assignment, as supported by our shell-model calculations [9,15].
It is interesting to note that the decay of the 136 Te 136 I also supports strong g.s. to g.s. feeding with spin/parity of (1 -) for 136 I [28]. New data on this issue will be discussed in a forthcoming article [27].