Beyond 132 Sn Examples of new data on exotic neutron-rich Te isotopes from ﬁssion and β -decay

. Exotic nuclei beyond the 132 Sn double shell-closure have both single-particle and collective particle-hole excitations and are expected to have competing excitation patterns from both type of excitations together with possible structural changes. We are, therefore, studying the region in the close vicinity beyond 132 Sn and further with the neutron increase with experimental methods such as induced ﬁssion and β decay. A short overview of this knowledge will be given together with examples of newly obtained data at preliminary stage


Introduction and Motivation
Exotic nuclei beyond the 132 Sn double shell-closure are influenced by both the Sn superfluity and the evolving collectivity only a few nucleons away, involving predominantly protons from the lower-lying πg 7/2 and πd 5/2 orbitals and the neutrons in the ν f 7/2 , νp 3/2 and νh 9/2 orbitals beyond, respectively, Z=50, N=82 closed shells.For neutron-rich nuclei, for example at intermediate mass number A∼136, the interplay between single-particle and collective particle-hole excitations [1,2] is evident in midshell ν f 7/2 .On the other hand at the end of the ν f 7/2 shell possible sub-shell gap with respect to νp 3/2 has been suggested [3].With the extreme addition of neutrons but also protons additional effects are expected such as the formation of neutron skin [4], orbital crossings between πg 7/2 and πd 5/2 orbitals [5,6] and possible quickly evolving deformation [7].
The knowledge of experimental nuclear ingredients is especially interesting beyond 132 Sn as little is known on how the excitation modes develop with the addition of both protons and neutrons for the Sb, Te, I nuclei.Therefore, systematic prompt and decay studies can be such a sensitive probe for their structure [8,9].Aiming at more global picture and understanding this barely explored neutronrich portion of the nuclear chart, we have performed several investigations, recently.
We have produced the nuclei of interest following fission such as relativistic 238 U on 9 Be in inverse kinematics, thermal neutron-induced fission on 241 Pu and 235 U or fast neutron-induced fission on 238 U and 232 Th and in β-decay of fission products in several recent γ-ray spectroscopy projects [8][9][10].Consistent data analysis allows to access various spins and excitation energies and to provide complementary data, better understanding, as well as a new and indispensable input to theory.Examples from some of * e-mail: radomira.lozeva@csnsm.in2p3.frthese studies on isotopes with A∼136 will be briefly presented along with a short discussion of the new data.Detailed description and further details will be accordingly published in dedicated articles [11,12].

New data on 134 Te from fission
With only two valence protons outside the doubly-magic 132 Sn, a long-lived isomeric J = 6 state emerges in 134 Te based on the πg 2 7/2 proton configuration.Below the 6 + 1 isomer, a short-lived 4 1 + isomer with T 1/2 = 1.28(10) ns has been observed [13].The nucleus of interest has been produced in a fast neutron-induced fission experiment and its de-excitation measured with a hybrid array consisting of HpGe and LaBr 3 (Ce) scintillation detectors [10].
Due to its short half-life, the 4 + 1 state is not measurable with HpGe detectors, but delayed LaBr 3 (Ce), after tagging the 134 Te nucleus can be utilized for measuring this state.In the left panel of Fig. 1, the LaBr 3 (Ce) energy projection can be seen after gating on the 1279 keV 2 + 1 → 0 + 1 transition and several transitions above the 6 + 1 isomer.Both, 115 and 297 keV transitions feeding and de-populating the 4 + 1 state are visible.Furthermore, the LaBr 3 (Ce) projection after an additional LaBr 3 (Ce) gate on 115 keV is shown.The time difference spectrum, illustrated in the right panel of Fig. 1 has been fitted with an exponential decay curve plus constant background to obtain the half-life, T 1/2 .A value of T 1/2 = 1.3(3) ns has been obtained, in accordance with the literature value of 1.36 (11) ns [14].This measurement demonstrates the feasibility of measuring ns and sub-ns lifetimes with this experiment and is employed toward more neutron rich Te isotopes of interest.

New data on 136 Te from fission
The neutron rich 136 Te has two valence protons and neutrons outside the doubly magic 132 Sn and is of major importance to study the onset of collectivity beyond the 132   core.Excited states in 136 Te have been populated in fast neutron-induced fission and its γ-rays detected using the previously described (see Section 2) combination of HpGe and LaBr 3 (Ce) scintillation detectors.Figure 2 shows an example for the 136 Te nucleus.In the left panel of Fig. 2, the energy projection after applying a clean HpGe gate on the 750 keV, 8 + 1 → 6 + 1 transition in 136 Te is presented.All the transitions below the 6 + 1 are clearly visible in both LaBr 3 (Ce) and HpGe energy projections.Utilizing the superior energy resolution of the HpGe detectors one can conclude that the peaks of interest show almost no contribution from other contaminants.From measuring time differences between the labeled transitions, lifetimes of the respective states are deduced.
In the right panel of Fig. 2 an energy matrix is shown to demonstrate the HpGe -LaBr 3 (Ce) coincidence between the 8 + 1 → 6 + 1 and 6 + 1 → 4 + 1 transition in 136 Te.The number of coincidence counts amounts to about 10 3 which, scaled by efficiencies for the LaBr 3 (Ce), is reasonable to measure the lifetime of the 6 + 1 state in 136 Te.This data result will be presented in a forthcoming article [12].

New data on 136 Te from β decay of 136 Sb
The β-decay data of 136 Sb to 136 Te, accessing the lowspin states which are not populated in fission, is extremely scarce [14].It gives very important information not only on the ground state spin/parity and thus its properties, but also on specific type first excited states, such as the 2 + 1 , 2 + 2 , 2 + 3 etc.Such measurement has been performed using β-decay of A =136 fission products after the thermal neutron-induced fission of 235 U and detected using a system of clover HpGe, coaxial HpGe and LaBr 3 (Ce) detectors in combination with β-decay detectors and a tape station.
Well adapted to the lifetime of the 136 Sb nucleus [14], the duty cycle of the system allowed the short-lived daughter to be well separated from the grand-daughter decays 1 → 0 + transition in 136 Te (red) and an additional anti-coincidence gate on the 1313 keV transition 2 + 1 → 0 + in 136 Xe (green).New transitions e.g.candidates belonging to 136 Te are in black.
Te→I and I→Xe with the help of a chopper system.In addition, to clean-up from the strongly produced granddaughter activities anti-coincidence gates from the granddaughter nucleus are applied.Furthermore, long-lived activities in 136 I grand-daughter are also subtracted from the time window.This is demonstrated in Fig. 3, where several new candidates for the level scheme of 136 Te are shown.The detailed level scheme will be given together with all newly extracted logft values in a forthcoming article [11].

Discussion
The new data allows to verify and expand our current knowledge for these mid-shell nuclei (A∼136) with respect to the ν f 7/2 orbital nuclei, which is the lowest-lying neutron orbital beyond 132 Sn.The collected new information allows multiple coincidence relations to be established and used determine the position in the level scheme of new, or verify previously known γ transitions.
In addition, several lifetime measurements have been possible in the data analysis.Added to the new γ-ray information these provide new and important ingredients to compare with shell-model theory.The current understanding of the region with reasonably slow development of collectivity at mid-shell, expected to increase with the increase of the valence particles, can now be reexamined, especially for states which have not been populated in previous measurements.From preliminary view, the new data reasonably well agrees with the expectations from theory, however only the very detailed comparison will allow specific conclusions to be drawn [11,12].The experimental ingredients such as the transition rates obtained from the lifetime measurements allow better tuning of the transition matrix elements.Such data is valuable when testing the nucleon-nucleon interaction for the region beyond 132 Sn and when predicting nucleon or two nucleon excita-tions e.g.type πg 2 7/2 and ν f 2 7/2 in the currently out of reach A>140 region.
In the data on the Sb β decay, for the first time a very large Q β window has been experimentally scanned.This allows the population of many new low-spin states at high excitation energy.This, respectively, provides a field for more detailed comparison to shell-model, particularly on the strength of the first-forbidden transitions beyond 132 Sn.New ingredients in understanding the role of first-forbidden transitions with respect to the Gamow-Teller strength can now be analysed in details, especially as it is open in 136 I [8], but not seen in the Te chain [14].Thus, combining both data sets with our previous knowledge in the region, new transitions, new excitation energies and extension of the level schemes, with new spin/parity etc. contributes importantly to the structure studies of the populated states and their behaviour beyond 132 Sn.

Figure 2 .
Figure 2. Left: Prompt energy projection after applying a HpGe gate on the 750 keV transition in 136 Te.The LaBr 3 (Ce) energy spectrum is shown in red and the HpGe spectrum in blue.The strongest transitions from 136 Te are labeled with their respective energies.Right: LaBr 3 (Ce) -HpGe energy matrix after gating on the two lowest transitions in 136 Te.The zoomed area corresponds to the 750 -352 keV coincidence.

Figure 3 .
Figure3.Short-lived energy projection corresponding to the decay of 136 Sb using HpGe detectors and gate on 606 2 + 1 → 0 + transition in 136 Te (red) and an additional anti-coincidence gate on the 1313 keV transition 2 + 1 → 0 + in 136 Xe (green).New transitions e.g.candidates belonging to136 Te are in black.
Sn Left: LaBr 3 (Ce) energy projection after applying a HpGe gate on several transitions in 134 Te (red) and an additional LaBr 3 (Ce) gate on 115 keV (blue).Right: Time difference of the 297 -115 keV cascade to measure the half-life of the 4 + 1 state in134Te.The distribution was fitted using an exponential decay (red) plus a constant background (black).