Spectroscopy of Neutron-Deficient Nuclei Near the Z = 82 Closed Shell via Symmetric Fusion Reactions

In-beam and decay-spectroscopy studies of neutron-deficient nuclei near the Z=82 shell closure were carried out using the Fragment Mass Analyzer (FMA) and the Gammasphere array, in conjunction with symmetric fusion reactions and the Recoil Decay Tagging (RDT) technique. The primary motivation was to study properties of 179Tl and 180Tl, and their daughter, and grand-daughter isotopes. For the first time, in-beam structures associated with 179Tl and 180Tl were observed, as well as γ rays associated with the 180Tl α decay. No long-lived isomer was identified in 180Tl, in contrast with the known systematics for the heavier odd-odd Tl isotopes.


Introduction
Systematic nuclear structure studies of proton-rich Au, Hg, Tl, and Pb nuclei are important in order to elucidate their shape evolution with neutron number from wellstudied deformed minima at mid-shell to the near spherical ground states observed for nuclei near the proton drip line [1].In addition, detailed knowledge of level and decay properties of those nuclei is also relevant for a better understanding of rare decay modes in this region, such as electron-capture delayed fission [2,3].
Over the last several years, we have performed a number of experiments using the Gammasphere spectrometer and the Fragment Mass Analyzer (FMA) at ATLAS aimed at measuring properties of proton-rich nuclei in this region (see, for example, Refs.[4,5] and references therein).In those studies, the use of the FMA was essential in order to differentiate evaporation residues associated with the nucleus of interest from both the large fission background, dominating the reaction cross section, and evaporation residues produced in other reaction channels.The use of symmetric fusion reactions at bombarding energies near the Coulomb barrier proved beneficial in performing these studies.On the one hand, the large, negative Q values for such reactions [6] result in a relatively low excitation energy of the compound system.As a consequence, the fission competition is reduced and the large fragmentation of the evaporation-residue cross sections is a e-mail: kondev@anl.govb present address: NNDC, Brookhaven National Laboratory c present address: Nuclear Science Division, Lawrence Berkeley National Laboratory significantly suppressed, since only a few reaction channels are energetically possible.On the other hand, the enhancement of the fusion cross sections in the sub-barrier region [7] results in relatively large cross sections (up to tens of millibarns) for one-and two-particle evaporation channels, hereby enabling detailed spectroscopy studies to be carried out.
Here, we present new results from spectroscopy studies of the odd-odd 180 Tl nuclide.The results on 179 Tl are presented elsewhere [8].The 180 Tl nuclide was discovered by Lazarev et al. [2] via the observation of EC/β + −delayed fission decays and new results on this decay mode were recently reported by Andreyev et al. [3].Its main decay mode (94(4) %) is EC/β + decay, which was recently studied by Elseviers et al. [9].Results from α-decay spectroscopy of 180 Tl were reported by Toth et al. [10], but the decay scheme remains incomplete, partially owing to the small α-decay branch (b α = 6(4) %).Information on excited structures in 180 Tl is also scarce and only three γ rays with energies of 89(1) keV, 124(1) keV and 449 (1) keV were assigned to this nuclide in α-decay studies of 184,184m Bi [11].

Experimental Details
The 180 Tl isotope was produced in the 1n channel of the symmetric fusion reaction of 89 Y ions with 92  (>95%) and self-supported with a thickness of approximately 510 µg/cm 2 .Prompt γ rays were detected with the Gammasphere array [12] consisting of 100 large volume escapesuppressed Ge detectors.
The evaporation residues were transported through the Fragment Mass Analyzer (FMA) [13] and were dispersed according to their massto-charge (m/q) ratio.A position-sensitive parallel grid avalanche counter (PGAC), located at the FMA focal plane, provided the required m/q information and the time of arrival of the recoils.The latter were subsequently implanted into a 160 × 160 strips (∼142 µm thick) doublesided silicon strip detector (DSSD).Each event in the DSSD was time stamped and identified as being either an implant or a charged-particle decay, depending on the coincidence or anti-coincidence with a signal from the PGAC.An array consisting of four CLOVER Ge detectors surrounded the DSSD.It was used to detect γ rays in coincidence with α-particle decays.
The 180 Tl residues and the corresponding prompt γ rays were isolated from the dominant background originating from scattered beam, fission products, and deexcitations in neighboring isotopes produced in other reaction channels, by placing coincidence gates in the offline analysis on (i) the time of flight of the evaporation residues from the target to the focal plane, (ii) the PGAC positions corresponding to three charge states (q = 31, 32, and 33) of ions with the appropriate A = 180 mass focus, and (iii) the two-dimensional histogram of the energy of recoils measured in the DSSD vs. the time of flight from the PGAC to the DSSD.The data were then sorted in coincidence histograms, gated in various ways on the energy and time information from the DSSD.

Results and Discussion
Figure 1 provides a first-generation α-decay spectrum produced with the requirement that a A=180 recoil was im- planted in the same pixel where a decay event was detected.The main line at E α1 =6119(5) keV corresponds to the decay of 180 Hg, which is produced in the 1p reaction channel.The higher-energy lines at E α1 =6291 (10), 6367 (10), 6490 (10) and 6558 (10) keV are associated with decays of 180 Tl (1n reaction channel).All of them were found to be correlated with E α2 =6294(10) keV and 5747 (10) keV α lines, as illustrated in Fig. 2. The former line is associated with the ground-state decay of the daughter nuclide, 176 Au [14], while the latter is identified as belonging to 176 Pt [15], produced in the EC/β + decay of 176 Au.An α-decay branching intensity of b α =75(8) % was deduced for the ground-state decay of 176 Au.It is worth noting that no correlations with the previously-known α lines associated with the decay of a high-spin isomer in 176 Au [14] were observed in the present work.This implies that only a single α-decaying state exists in 180 Tl.The half-life of 180 Tl was determined in the present work to be T 1/2 =1.1(2) s, in agreement with values reported in earlier studies [3,9,10].A somewhat shorter value of 0.70(+12-9) s was reported by Lazarev et al. [2] from fission activity measurements.We have also measured a value of T 1/2 =1.2(4) s for the ground state of 176 Au, in agreement with the earlier published results [14,16].In addition, values of T 1/2 =2.60(1) s and 5.87(2) s were measured in the present work for 180 Hg and 176 Pt, respectively.Sample γ-ray spectra detected in the CLOVER array are presented in Fig. 3.The E α1 =6367 keV αdecay line is observed to be in coincidence with only the 204.8(5)-keV γ ray, while lines with energies of 69.9(5), 109.5 (5), 204.8(5), and 210.7 (5) keV are in coincidence with E α1 =6291 keV.It is worth noting that 205-and 210-keV γ rays were observed in the previous in-beam studies of 176 Au [14] and both were found to be correlated with the ground-state α decay.Based on the available α − γ coincidence information, a partial decay scheme of 180 Tl was constructed in the present work, as shown in Fig. 4. It should be noted, however, that we were not able to place the 109.5-keVγ ray, as well as several other weak transitions, in the decay scheme.
Using the Recoil Decay Tagging method, we searched for in-beam γ rays correlated with the 180 Tl α decays.The statistics of resulting spectra were very low and we were able only to tentatively establish the 276-, 323-, 555-, and 1141-keV γ rays as possible candidates.We were unable to confirm any of the three γ rays that were reported in the α-decay studies of 184,184m Bi [11].This may be a consequence of the different level-population pattern in α decay, which is sensitive to the spin and configuration of the parent state that are presently unknown.
The neutron-deficient, odd − odd Tl nuclei with N ≥101 are known to have a low-spin, I π =2 − , ground state and an I π =7 + spin-trap isomer [17,18].Their structures can be interpreted as resulting from the coupling of the π1/2 + (s 1/2 ) proton orbital, which is associated with the ground state of all even − N Tl with the ν3/2 − (p 3/2 ) and ν13/2 + (i 13/2 ) neutron orbitals, assigned to the ground and isomeric states in the odd − N Pb nuclei, respectively [17,18].It is worth noting that the systematic trend of the excitation energies of the I π =7 + isomers as a function of neutron number is very similar to that for the ν13/2 + (i 13/2 ) isomers in the odd − N Pb isotopes.It has been shown for the first time by Carpenter et al. [4] that, at N≤99, the structure of the Pb isotopes changes significantly and that the ν9/2 − (h 9/2 ) orbital becomes the ground state in 181 Pb.As a consequence, the ground state of 180 Tl (N=99) can be assigned I π =5 − with the π1/2 + (s 1/2 ) ⊗ ν9/2 − (h 9/2 ) configuration.The spin assignment is supported by the observed direct populations of I=4, 5, and 6 levels in the 180 Hg daughter isotope, following EC/β + decay of 180 Tl [9].Changes in the neutron single-particle structure near N=99 can also explain the absence of a long-lived isomeric state in 180 Tl.The excitation energy of the ν13/2 + (i 13/2 ) level in 181 Pb is unknown.However, if one extrapolates the known energies for this orbital from the heavier 183 Pb (N=101), 185 Pb (N=103), and 187 Pb (N=105) nuclei [18] towards N=99, then the I π =7 + , π1/2 + (s 1/2 ) ⊗ ν13/2 + (i 13/2 ) state in 180 Tl can be estimated to be located at ∼130 keV and it can decay via an M2 transition to the I π =5 − ground state.A half-life of ∼5 µs can be expected for the I π =7 + state using B(M2)=0.20(3)W.u., deduced from the decay of the I π =13/2 + isomer in 179 Hg (N=99) [19], and a total electron conversion coefficient of α T ∼27.4 [20] for a 130-keV, M2 transition in 180 Tl.This value is much shorter than the expected partial α and EC/β + decays of 20 s and 1.3 s, respectively, deduced from the known halflife and branching ratios for the 180 Tl ground state.Therefore, no α and/or EC/β + decays are expected from this I π =7 + state, which would rather de-excite via γ rays and conversion electrons.

Figure 2 .
Figure 2. Energy spectrum of second-generation α-decay events produced by gating on E α1 =6367 keV.

Figure 3 .
Figure 3. Gamma-ray spectra detected in the CLOVER array in coincidence with a) E α1 =6291 keV and b) E α1 =6367 keV decays.

Figure 4 .
Figure 4. Partial α-decay scheme of 180 Tl deduced in the present work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Article available at http://www.epj-conferences.orgorhttp://dx.doi.org/10.1051/epjconf/20136301013Figure1.Energy spectrum of first-generation α-decay events with a requirement that the decay occurred within 5 s of a mass A=180 implant.