Isomeric Lifetime Measurement in the Neutron-rich 189 Ta

. Isomeric states of the neutron-rich isotope Ta 116 73189 were populated via fragmentation of a primary beam of 208 Pb ions at 1 GeV/u impinging on a 9 Be target at GSI, Darmstadt, Germany. The isotopes of interest were separated, identified and delivered to the DESPEC setup. Two isomers were deduced in 189 Ta 116 and their lifetimes were measured based on the γ -ray time distributions.


Introduction
Neutron-rich nuclei in the A ~ 190 mass region exhibit a variety of nuclear structural properties.For instance, * Corresponding author: s.alhomaidhi@gsi.dewell deformed prolate, triaxial, oblate and spherical shapes (at N = 126) could be characterized by the properties of their respective ground-state shapes [1][2][3][4].The evolution from prolate to oblate nuclei passing through the γ-soft nuclei have been described as the prolate-oblate phase transitional systems [5].In oddmass systems, the shape of the atomic nucleus is influenced by the addition of an unpaired particle or hole.A study of odd-A neutron-rich Rhenium isotopes (Z =75) in the A ~ 190 region indicated increasing γ deformation and the evolution of triaxiality with increasing neutron number [6].The 187 Ta114 nucleus was studied recently using multinucleon transfer (MNT) reactions by Walker et al.The γ rays depopulating the isomeric states of 187 Ta114 were associated with a perturbed rotational band, showing the prolate-oblate shape transition effect [7].
However, with increasing neutron number, it becomes difficult to synthesize and select isotopes using MNT reactions and the isotope separation method.To overcome this obstacle, projectile fragmentation reactions at relativistic energies proved to be an efficient method for populating states in neutron-rich isotopes.For 189 Ta116, two data sets were obtained from the RISING campaigns in 2006 and 2007, which reported different isomeric half-lives of T1/2 = 1.6(2) μs [8] and T1/2 = 0.58(22) μs [9], respectively.The involvement of two different isomeric states were assumed to explain the discrepancy in half-lives.
The 189 Ta116 nucleus and other neutron-rich nuclei in the A~190 region were populated for the study of shape evolution, using the projectile fragmentation reactions at GSI Helmholtzzentrium für Schwerionenforschung.The spectroscopic study was carried out using the DEcay SPECtroscopy (DESPEC) [10] setup within the FAIR Phase-0 campaign in March 2021.

Experimental Details
The nuclei of interest were produced by the fragmentation of a 1 GeV/u primary beam of 208 Pb, impinging on a 2.7 g/cm 2 9 Be target.The primary beam was delivered by the UNILAC and SIS-18 synchrotron with a beam intensity of up to 10 9 ions/s.The fragments were separated and identified on an event-by-event basis in the FRagment Separator (FRS) [11], operated in the standard achromatic mode with an Al degrader placed in its intermediate focal plane.The FRS was tuned to transmit fully-stripped 190 Ta117 ions in the central trajectory of the FRS to the final focal plane.A summary of the FRS setting is shown in Table 1.A series of detectors at the intermediate and final focal planes of the FRS were used for the secondary beam identification.The time of flight (ToF) was determined by measuring the time difference between the two scintillation detectors placed at the intermediate and the final focal planes of the FRS.Time projection chambers (TPCs) were used for position measurements.Two multi-sampling ionization chambers (MUSICs), placed at the final focal plane, were used for the energy loss (ΔE) measurements.
An Al degrader was used to decelerate the ions arriving at the final focal plane and allowed for their implantation into an active stopper, the Advanced Implantation Detector Array (AIDA) [12].AIDA consisted of 3 layers of (8 x 8) cm 2 Double-Sided Silicon Strip Detectors (DSSSDs), for the identification of the implanted ions and the subsequent β decays.Two fast plastic scintillators were mounted upstream and downstream of AIDA with 10 mm distance for the βdecay timing.The downstream plastic was also used for vetoing light ions passing through the detectors.Surrounding AIDA, two high-purity Ge cluster detectors of the EUROBALL array [13], each with 7 segments, and 36 LaBr3(Ce) detectors of the FAst TIMing Array (FATIMA) [14] were used to detect γ radiation.The LaBr3(Ce) detectors were used for fasttiming spectroscopy, while HPGe detectors provided precise energy information.
All the subsystems of the DESPEC setup were triggered independently, in which the White Rabbit common time clock with ~ 1 ns precision was employed to correlate the events (for more details about the setup, the reader is referred to Ref. [10]).

Analysis and Results
As the particle identification is based on the mass-overcharge ratio (A/Q) as a function of the atomic number (Z), the charge states of the ions traversing the FRS must be selected first.Previous studies in the A ~ 190 mass region have shown how the different charge states can overlap in the particle ID plot [8,9].In particular, isotopes with atomic mass equal to (A -3), Z and charge (Q = Z -1), have comparable magnetic rigidity to fullystripped ions (A, Q = Z).Fully-stripped ions transmitted through the FRS can pick up an electron when they interact with matter and detectors at the intermediate focal plane.In order to overcome this, the energy loss ΔEdeg in the degrader in the intermediate focal plane can be deduced from the difference in the magnetic rigidity, Bρ, between the first and the second halves of the FRS.The charge states of the ions were determined by correlating the energy loss, ΔEdeg, with the atomic number (Z) [15].
As shown in Fig. 1, two main isotopic regions (1, 2) in the two-dimensional plot can be seen, one is for the H-like ions from Platinum (Z = 78) to Osmium (Z = 76), and the second region corresponds to the fully-stripped ions from Rhenium (Z = 75) to Hafnium (Z = 72).The isotopes from Z = 76 to 78 acquire an electron at the mid-focal plane and retain it to the final focal plane.After the particle identification, the fully-stripped ions (Q = Z), shown in Fig. 2, were used for the γspectroscopic analysis.The presence of the delayed 292 keV γ-ray transition in the 188 Ta115 isotope [16], confirms the validity of the used method.The γ rays decaying from the isomeric states in 189 Ta were detected within a time widow of 10 µs for HPGe clusters and 7 µs for the LaBr3(Ce) detectors after the implantation.The presence of previously-reported 83, 134, 154, 199, 246, 283, 389 and 481 keV γ-ray transitions have been confirmed in the present data set.As shown in Fig. 3, the delayed γ-ray transitions spectrum, where the energy peaks are labelled, as recorded in HPGe detectors.The 154, 199, 246, 283, 389 and 481 keV γ-ray transitions were found to be in coincidence within a ~ 10 ns time window of the γ-γ-ΔT cube.The 134 keV transition is in coincidence with the other transitions mentioned above, when the ΔT range is long (10 ns < ΔT < 5 µs).The time difference between transitions for example: ΔT (154 keV -134 keV) > 10 ns suggests that the first isomer is decaying from a higher-lying energy state via a 134 keV γ-ray, feeding the second isomer which then depopulates to the ground state via other γray transitions.
With a single-component exponential decay fit to the time difference distribution of the 134 keV transition with respect to the implant time, the half-life of the first isomer was estimated to be T1/2 ~ 200 ns.For the second isomer, fitting to the exponential decay of the sum of the time difference distributions of the 154, 283, 389 and 481 keV γ-ray transitions yields a half-life of T1/2 ~ 1.2 µs.A comparison with theoretical calculations, interpretation of the isomeric states and a suggestion for the level scheme for the neutron-rich 189 Ta116 isotope will be reported elsewhere [17].

Conclusions
The isomeric states of 189 Ta116 were produced by fragmentation reactions at the GSI accelerator facility, Darmstadt.The study of γ rays via spectroscopy using the DESPEC setup shows that two isomers are present in this isotope.The half-life of the shorter isomer was estimated to be T1/2 ~ 200 ns, and T1/2 ~ 1.2 µs for the longer one.The γ-γ-ΔT analysis has shown that the shorter isomer is decaying from a higher-lying energy state and feeding the second isomer.

Fig. 3 .
Fig. 3. Delayed energy spectrum of the γ rays known in 189 Ta as registered in HPGe detectors.In the previous measurements by Alkhomashi et al. and Steer et al., one isomeric state was identified.The half-life of the isomer was reported to be T1/2 = 1.6(2) µs and T1/2 = 0.58(22) µs in [8] and [9], respectively.In this experiment, two isomeric states have been identified.The γ-ray emission time was registered with respect to the time of implantation.The time behaviour of the 134 keV transition, as shown in Fig. 4, shows a shorter decay constant in comparison with time behaviour of the 154, 283, 389 and 481 keV transitions.

Fig. 4 .
Fig. 4. (color online) The energy-time matrix for the LaBr3(Ce) detectors.The 134 keV γ-ray transition is indicated by the black arrow.

Table 1 .
Summary of FRS setting parameters.