Structure of neutron-rich Br and Nb nuclei populated in fission experiments

Spectroscopic information on exotic Br and Nb nuclei, with odd and even masses around A=100, respectively, was obtained by analyzing fission fragments data collected with AGATA+VAMOS++ at GANIL and the FIPPS spectrometer at ILL. The complementarity of these two state-of-the art setups has been used, for the first time, to investigate the structure of these neutron-rich fission fragments via gamma-ray spectroscopy. Details on the two fission experiments will be given, as well as examples of high-resolution gamma-ray spectra used for the reconstruction of the level schemes. The active fission target, used in the neutron-induced fission experiment at ILL, allowed for high-statistics prompt-delayed coincidences for the analysis of a new isomeric state in 100Nb. Evidence for a spherical isomeric state in 100Nb will be shown and discussed within the systematic of neighbouring nuclei.


Introduction
The structure of neutron-rich nuclei has been investigated for many years with gamma-ray spectroscopy techniques [1].The region around mass 100 is of particular interest for drastic nuclear shape transitions, as in Zr isotopes [2].In this work, odd nuclei have been studied, in particular Br and Nb isotopes.The neutron-rich Nb nuclei are also placed in the region of shape transitions [2].In this work, the bands above isomeric states were studied, in comparison with systematics in neighbouring nuclei [3,4,5].The Br isotopes were studied to complete the missing spectroscopic information and they were used as a test for nuclear models in this region, above the doubly magic 78 Ni.Experimental details are given in section 2, the 85 Br "test case" is shown in section 3 and the example of 100 Nb is discussed in section 4.

Experimental setup
The first data set used for this work was obtained using thermal-neutron induced fission on 233,235 U at the FIPPS (FIssion Product Prompt gamma-ray Spectrometer) instrument at the Institut Laue-Langevin (ILL) [6].Neutrons were collimated to obtain at the target position a flux of about 5 × 10 7 n s −1 cm −2 with a 15 mm diameter.Around the target, 16 HPGe clover detectors were placed, 8 of them equipped with Compton shields (loan from IFIN-HH).The geometry of the array allowed angular correlations to be performed to determine the spins of excited states.The thermal-neutron induced fission experiment was performed in 2018 with a scintillator-based active fission target [7], consisting in an actinide material dissolved in a liquid scintillator.A pulse shape discrimination (PSD) analysis separated the fission events from the β and α ones.The selection of gamma rays produced in the fission events was thus possible.By selecting the fission events by PSD, the gamma rays coming from the β-decay of the fission fragments were suppressed.Further details on the setup and its performance can be found in Ref. [7].
A second data set was used for this work from an experiment in GANIL in 2015 with AGATA+VAMOS++ [8,9].The neutron-rich nuclei were populated by a fusion-fission reaction with a 238 U beam (6.2 MeV/u and 1 pnA) impinging on a 9 Be target.VAMOS++ was used to identify, on an event by event basis, the mass and the atomic number of the produced nuclei.Only prompt gamma rays were detected in AGATA.Due to the inverse kinematics of the reaction, a gamma-ray emission after an isomeric decay will occur downstream from the target position, resulting in low detection efficiency for the AGATA array.
The two data sets were used in a complementary way, e.g., by exploiting the mass and nuclear charge identification in VAMOS++ for selectivity, while the FIPPS data set was used to study multiple gamma-ray coincidences, isomeric states lifetimes, and angular correlations.
3. Neutron-rich Br: the "test case" of 85 Br In this paper, 85 Br will be shown as a "test case" for the analysis with the two data sets.This isotope was previously studied in β-decay [10] and in fusion-fission reactions [11].In Fig. 1 the level scheme is shown, with the transitions widths corresponding to the intensities obtained from the AGATA+VAMOS++ data set.The Doppler-corrected gamma-ray spectrum in Fig. 2 was obtained in coincidence with the identified 85 Br in VAMOS++.The FIPPS data were analysed performing a triple gamma coincidence.In Fig. 3 an example of doubly gated spectrum is shown, where 85 Br transitions, 147 La fission partner transitions, and a contamination coming from 143 Ba and Kr are labelled.
The same analysis was done for 87,89,91,93 Br isotopes and the results are detailed in [12,13], together with performed shell model calculations.
4. Neutron-rich Nb: the case of 100 Nb New spectroscopic information was obtained for the 100,102,104 Nb isotopes [14], by performing the same analysis described in the previous section.In this paper, the lifetime measurement of an isomeric state in 100 Nb will be discussed.The 492-keV level (Fig. 4) was identified as an isomeric state because the depopulating 124-keV transition was consistently reduced in AGATA+VAMOS++ data.Therefore, the FIPPS active target data were used to perform a lifetime measurement for this state with the delayed-coincidence method.In normal prompt gamma-ray coincidences, matrices are built within a 0-150 ns time window.For this analysis a different time window was set to study only the events depopulating the isomeric state.Different sets of delayed cubes (i.e.triple gamma coincidence built with delayed time windows) were built with non-overlapping time intervals every 20 ns.Gates on the depopulating transitions (67, 91, 102, and 106 keV) were set and the intensity of the 124-keV transition was evaluated.Fig. 5 shows the 124-keV transition intensity as a function of the time intervals used, together with the linear interpolation computed to obtain the lifetime of the 492-keV state.The same analysis was done also with 40 ns intervals, which increased the statistic leading to smaller errors, but it reduced the points in the linear fit.Additionally, data with a 233 U target were available,  Doppler corrected gamma-ray spectrum obtained for 85 Br in the AGATA+VAMOS++ experiment, taken from [12].Intensities are shown in black relative to the strongest transition.The obtained lifetime value is 50(3) ns (preliminary).Prompt-delayed coincidences were thus performed and the level scheme above the isomer was studied [14,15].
A systematic study was performed for the N=59 isotones leading to a tentative spin assignment of 7 + for the isomeric state since the 124-keV transition is assumed to be a stretched E2 based on the systematics in the neighbouring nuclei [3,4].Additionally, a spherical behaviour can be tentatively assigned to the band above the isomer, while below the isomer a rotationallike band is found.This scenario is supported by the comparison with 98 Y [3] and 96 Rb [4], both of which have a µs isomeric state.

Conclusion and perspectives
The complementarity of AGATA+VAMOS++ and FIPPS setups has been discussed in the context of the study of neutron-rich fission fragments, in particular in the 85 Br and 100 Nb nuclei.The data analysis is still ongoing, with the aim of extracting spectroscopic information for different isotopic chains.Angular correlation analyses will performed to extract nuclear excited states spins.Finally, an analysis is in progress to determine ps and sub-ps lifetimes in Zr and Nb nuclei with a Doppler attenuation method in the FIPPS active target data.

Figure 1 .
Figure 1.Partial 85 Br level scheme obtained in this work.The arrows widths are proportional to the observed experimental intensities.Spin and parity are taken from[11].

Figure 2 .
Figure 2.Doppler corrected gamma-ray spectrum obtained for 85 Br in the AGATA+VAMOS++ experiment, taken from[12].Intensities are shown in black relative to the strongest transition.

Figure 3 .
Figure 3. Doubly gated gamma-ray spectrum obtained with FIPPS active target data, taken from [12].The labels indicate the isotopes identified.

Figure 5 .
Figure 5. Variation of the 124-keV transition intensity as a function of the time intervals used in the data sorting.The linear fit computed to obtain the lifetime of the 492-keV level of 100 Nb is shown, from data with a 235 U target, taken from [14].