Fine structure in the α decay of high-spin isomers in 155 Lu and 156 Hf

© 2018 American Physical Society Publisher's PDF Parr, E.; Page, R. D.; Joss, D. T.; Ali, F. A.; Auranen, Kalle; Capponi, L.; Grahn, Tuomas; Greenlees, Paul; Henderson, J.; Herzan, Andrej; Jakobsson, Ulrika; Julin, Rauno; Juutinen, Sakari; Konki, Joonas; Labiche, M.; Leino, Matti; Mason, P. J. R.; McPeake, C.; O’Donnell, D.; Pakarinen, Janne; Papadakis, Philippos; Partanen, Jari; Peura, Pauli; Rahkila, Panu; Revill, J. P.; Ruotsalainen, Panu; Sandzelius, Mikael; Sarén, Jan; Scholey, Catherine; Simpson, J.; Smith, J. F.; Smolen, M.; Sorri, Juha; Stolze, Sanna; Thornthwaite, A.; Uusitalo, Juha

An experimental observable that has not previously been utilized to study these states, however, is α-decay fine structure.The study of fine structure provides α-decay reduced hindrance factors (proportional to the inverse of the reduced decay widths), which are a measure of the overlap of the initial and final nuclear wave functions in an α-decay process; these then indicate the similarities of configurations of the initial and final states.The comparison of reduced hindrance factors to different levels in product nuclei from the same initial state can also, therefore, provide evidence for the similarity, or otherwise, of these final states.Additionally, α-decay finestructure studies are useful in constructing, or confirming, level schemes populated in product nuclei.
The main experimental challenge in populating states in N = 82 nuclei via α decay is the large excitation energies of their s > 1 states; which have minimum excitation energies of around 1.5 MeV.The reduction in Q α leads to a dramatic drop in α branching ratios to the states.A possible solution to this problem is to search for α-decaying branches from highenergy isomeric states.Although the reduction in Q α is the same, the higher energies of the possible α decays populating excited states allows these branches to compete with those to the ground states.This phenomenon has previously been observed in the region above 208 Pb.In that region there have been five examples of nuclei whereby a high-energy isomeric state has been observed to α decay to a state with E excitation 1.5 MeV; specifically those from 211 Po [13,14], 212 Po [14], 214 Ra [15], 216 Ra [16], and 217 Pa [17].
This paper presents the results of a study of the α-decay fine structure populating excited states in the N = 82 nuclei 151 Tm and 152 Yb from the high-spin isomers in 155 Lu (J π = 25/2 − ) and 156 Hf (J π = 8 + ), respectively.This is the first time α-decay fine structure to states with seniority s > 1 configurations in N = 82 isotones above 146 Gd has been reported.Previously only the α decay to single-proton states in odd isotones has been observed [18][19][20][21][22].It is also the first report of states with E excitation 1.5 MeV being populated following α decay in a different region to that just above 208 Pb.

A. Excited states in 151
69 Tm and 152 70 Yb Excited states in 151 Tm were first studied using γ -ray spectroscopy following the decay of a J π = 27/2 − , T 1/2 = 470(50) ns isomer [7].Four γ -ray transitions were observed, and from intensity comparisons were determined to have stretched E2 multipolarity.This allowed for the π (h 11/2 ) 5 , s = 3, multiplet sequence to be established.A subsequent investigation identified the γ rays emitted promptly following the production of 151 Tm via fusion evaporation, as well as those from the decay of the isomer [10].The initial level scheme below the isomer was confirmed, as well as the sequence of three positive-parity states described in Sec.I. Due to the low statistics some of these positive-parity states could only be placed tentatively in the work of Ref. [10].
The excited states in 152 Yb were first investigated by studying prompt γ rays, as well as those emitted following the decay of a J π = 10 + , T 1/2 = 39(5) µs isomer [10].A cascade of five γ rays was used to identify levels from the π (h 11/2 ) 6 , s = 2, multiplet sequence, as well as the three negative-parity states.A further investigation was carried out detecting γ rays and conversion electrons emitted following the decay of the isomer in 152 Yb [9].From this work, the multipolarities of all the transitions were determined, allowing for a firm assignment of all energies, spins, and parities of the levels.The lowest three transitions were also observed following the β decay of 152 Lu [23].High-spin isomers in 155 Lu and 156 Hf were first observed via their α decays to the ground states of 151 Tm and 152 Yb, respectively [24].The decay half-lives and α-particle energies were measured to be 2.7(3) ms and 7408 (10) keV for 155 Lu and 0.52 (16) ms and 7804 (15) keV for 156 Hf.Although identified as decaying isomeric states with excitation energies between ∼2 and 3 MeV, they were not, at the time, attributed to specific nuclei.Subsequent discussion, however, assigned them as states in 155 Lu and 156 Hf in Refs.[25,26]; the latter reference also giving new values of E α = 7379(15) keV and T 1/2 = 2.60 (7) ms for the decay from the isomer in 155 Lu.Finally, the α decays from both of the isomers were studied and reported in Ref. [21].Values of E α = 7390(5) keV, T 1/2 = 2.71(3) ms and E α = 7782(4) keV, T 1/2 = 0.52(1) ms were given for the α decays from the 155 Lu and 156 Hf isomers, respectively, and the mass assignments were confirmed using A/q recoil separation.No other α-decay branch or decay mode has been reported from either isomeric state.

III. EXPERIMENTAL DETAILS
The results presented in this paper were obtained from an experiment performed at the Accelerator Laboratory of the University of Jyvaskyla, Finland.The 155 Lu and 156 Hf nuclei were produced by a fusion-evaporation reaction using a 58 Ni beam incident on a 106 Cd target for around 292 hours.The 58 Ni beam had energy of 318 MeV with an average intensity of ∼6.4 particle nA.The target was a self-supporting 106 Cd target of thickness 0.975 mg cm −2 .The fusion-evaporation products were separated from other reaction products and unreacted beam ions using the RITU gas-filled recoil separator [29,30].They were then implanted into two double-sided silicon-strip detectors (DSSDs), which are part of the GREAT spectrometer [31], located at a focal plane of RITU.The two DSSDs each consisted of 40 horizontal and 60 vertical strips giving a total of 4800 individual pixels.An array of 28 silicon PIN diode detectors were located upstream from the DSSDs positioned to detect charged particles emitted out of the DSSDs.An array of three HPGe clover detectors surrounding the DSSDs was used to detect γ and x rays emitted by decaying implanted nuclei.These detectors were placed at θ = 90 • to the central path of the recoils, on either side and above the DSSDs.Downstream of the DSSDs, within the vacuum chamber of GREAT, was a double-sided germanium strip detector.This was used to detect predominantly low-energy γ rays and x rays emitted following nuclear decays.At the entrance of GREAT was a multiwire proportional counter (MWPC).This was used to measure the energy loss of incoming recoils which, along with the time-of-flight from the MWPC to the DSSDs, enabled the selection of desired recoils over incoming unreacted beam or other reaction products.For the temporal correlation of the detector signals each was time stamped in units of 10 ns [32].

IV. DATA ANALYSIS
The data analysis was performed using the GRAIN software [33], which was developed for use with data acquired by the Total Data Readout system [32].The DSSDs were calibrated using α particles emitted by implanted evaporation residues, or those in their decay chains, produced during the experiment.The α particles used were from 150 Dy and 158m W [E α = 8286( 7) keV] [35].The branching ratios of the studied α decays of interest in 155 Lu and 156 Hf were small, therefore analysis of coincidences between α particles detected in the DSSDs and γ rays, emitted following the population of excited states in daughter nuclei, detected in the focal-plane clover-detector array was needed to identify them.The absolute efficiency for the detection of γ rays in the focal-plane clover-detector array was determined using GEANT4 Monte Carlo simulations.
Candidates for decays from fusion-evaporation products were identified as signals in the DSSDs, which did not have coincident MWPC signals.As the recoiling nuclei were implanted close to the surface of the DSSDs a significant proportion (∼40%) of the α particles were emitted out of the detectors, therefore depositing only a fraction of their energy.Some of these escaping α particles were then detected in the PIN-diode detectors.The background signals in the DSSDs produced by the partial energy deposition of the escaping α particles could, therefore, be reduced to some extent by vetoing potential α particles with a coincident PIN signal.Possible α decays were also correlated with a preceding recoil implantation in the same pixel of the DSSD.The incoming recoils were identified by gating on their characteristic energy loss in the MWPC and their time-of-flight from the MWPC to the DSSD.The time between the recoil and the decay was required to be up to 8.2 ms to identify α decays from 155 Lu(25/2 − ) (T 1/2 = 2.7 ms) and up to 1.5 ms for those from 156 Hf (8 + ) (T 1/2 = 0.52 ms).

V. RESULTS
The properties of α decays identified in the present study are given in Table I.The table gives the following information: the α-particle energies; the α-decay branching ratios; the reduced decay widths; reduced hindrance factors of the decays calculated as described in Sec.VI; the spins, parities, and energies of the states populated in the daughter nuclei; and the total Q values of the decays, which is the sum of the Q value of the α decay and the excitation energy of the final state.Figure 1 shows the states in 151 Tm and 152 Yb populated following the α decays of 155 Lu(25/2 − ) and 156 Hf (8 + ) reported here, as well as those from the 155 Lu and 156 Hf ground states.
To confirm that the α decays identified are from 155 Lu(25/2 − ) and 156 Hf (8 + ), the total Q values of the decays, are compared with those for the α decays which populate the respective ground states.Figure 2(a) shows α-particle energies measured in the DSSDs which were identified with a recoil implantation in the same pixel up to 8.2 ms preceding them.From this spectrum α particles were measured with energies E α = 7383(4) keV from 155 Lu(25/2 − ) and E α = 7775(5) keV from 156 Hf (8 + ).These values are consistent with those previously reported in Refs.[21,[24][25][26] and as they were seen only in coincidence with background γ rays they are assumed to populate the ground states of the daughter nuclei.Also, to help identify α decays from the 155 Lu(25/2 − ) and 156 Hf (8 + ) isomers the decay times for the αγ coincidences from each of the α-decaying groups are compared with those from the decays to the ground states of the daughter nuclei; shown in Fig. 3.By plotting the decay time on a logarithmic scale a distribution of universal shape with a peak value at the mean lifetime is produced, as detailed in Ref. [36].The random correlation component, corresponding to a recoil-implantation lifetime per DSSD pixel of around 1.5 s, is also visible.5521 (8) keV and γ rays with E γ = 415 keV are highlighted; the projection of the coincident γ rays is shown in Fig. 4(b).Previously, a level has been tentatively assigned at 1905 keV with J π = (19/2 + ) in 151 Tm, which decays to the (15/2 + ) level via the emission of a 415-keV γ ray [10].It is therefore proposed that the α decay associated with these coincidences directly populates this (19/2 + ) level in 151 Tm from the 155 Lu(25/2 − ) isomeric state; this also confirms the positioning of a level at 1905 keV.The DSSD spectra in coincidence with the 415-and 1490-keV γ rays are given in Fig. 2(b) and 2(c), respectively.As expected, the 5521(8)-keV α particle is seen in coincidence with both of these γ rays.The prominent 155 Lu(25/2 − ) 7383-keV contaminant peak in Fig. 2(b) is the result of random coincidences due to the high intensity of Compton-scattered 511-keV electron-positron annihilation γ rays over the 415-keV peak.The total decay Q T value of 7572 (8) keV is consistent with the Q value of 7578(4) keV for the α decay to the ground state of 151 Tm. Figure 3(a) shows the decay times of the αγ coincidences with γ -ray energy 415 keV, which are proposed to populate the (19/2 + ) state.The distribution is in excellent agreement with that from the α decays to the ground state.The large long-lived component is again caused by the background of Compton-scattered 511-keV γ rays.

E α = 5928 keV
Coincidences between α particles with E α = 5928(5) keV and γ rays with E γ = 1490 keV are highlighted in Fig. 4(a), with the projected energies of the γ rays given in Fig. 4(c) and α particles in Fig. 2(c).These coincidences appear on the Q[ 155 Lu(25/2 − ) → 151 Tm(11/2 − )] line.A (15/2 + ) state has previously been observed in 151 Tm at 1490 keV, which decays via γ -ray emission directly to the ground state [10].It is therefore proposed that these coincidences are associated with the population, and subsequent decay, of this (15/2 + ) state via the α decay of 155 Lu(25/2 − ).The total Q T value of the decay is 7575(5) keV, which is consistent with the Q = 7578(4) keV value for the α decay to the ground state.The distribution of decay times of these coincidences, shown in Fig. 3(b), are also consistent with the distribution of the α decays to the ground state.

E α = 5937 keV
A small number of coincidences between α particles with E α = 5937 (15) keV and γ rays with E γ = 1478 keV are highlighted in Fig. 4(a), with the projection of γ rays given in Fig. 4(c).These coincidences appear on the Q[ 155 Lu(25/2 − ) → 151 Tm(11/2 − )] line.A 15/2 − state has previously been observed in 151 Tm at 1478 keV, which decays via γ -ray emission directly to the ground state [7,10].Although there are only a small number of coincidences, the clean α-particle energy in coincidence with the 1478-keV γ rays, shown in Fig. 2(d), gives a total decay Q T value of 7573 (15) keV.As this is consistent with the Q value of the α decay to the ground state of 7578(4) keV it is proposed that the coincidences are associated with the population of the 15/2 − state at 1478 keV in 151 Tm.Further evidence is also provided for this assignment by agreement of the decay times of the four αγ coincidence events with the distribution from the α decays to the ground state, shown in Fig. 3(c).
B. 156 Hf (8 + ) → 152 Yb α-decay fine structure Figure 5 shows αγ coincidences gated for α decays from 156 Hf (8 + ) (as detailed in Sec.IV).Strong contaminant coincidences from the α-decay fine structure of 155 Lu(25/2 − ), discussed previously, are highlighted in a dashed circle and labeled in brackets.The α particles from the 156 Hf (8 + ) isomers were identified with the help of the diagonal line shown on the αγ -coincidence spectrum.The line represents a constant Q T value for the sum of the α-decay Q value, calculated from the α-particle energy, and the γ -ray energy.It is equal to the Q value between the 156 Hf (8 + ) isomeric state and the 152 Yb ground state, Q[ 156 Hf (8 + ) → 152 Yb(0 + )].

E α = 6274 keV
Coincidences between α particles with E α = 6274 (15) keV and γ rays with E γ = 1531 keV are highlighted in Fig. 5(a).Figure 5(b) shows the projection of γ rays in coincidence with 6274-keV α particles (as well as those of 5942 keV to be discussed in the next section).These appear on the Q[ 156 Hf (8 + ) → 152 Yb(0 + )] line and the 2 + 1 state in 152 Yb has previously been identified 1531 keV above the 0 + ground state [9,10,23].The coincidences are therefore proposed to derive from the α decay of 156 Hf (8 + ) to the 2 + 1 state in 152 Yb.The Q T value of 7971 (15) keV is consistent with the value of 7980 (5) keV for the α decay to the ground state.Also, the decay times, shown in Fig. 3(d), compare well with the distribution for the decays to the ground state of 152 Yb. ).The diagonal line on (a) represents a constant energy for the sum of the α-decay Q value, calculated from the α-particle energy, and the γ -ray energy; the energy represents that between the 156 Hf (8 + ) isomeric state and the ground state of 152 Yb.The αγ coincidences identified from 156 Hf (8 + ) are circled with contaminant coincidences from 155 Lu(25/2 − ) also labeled.Also shown are the γ -ray energies in coincidence with (b) the 5942(15)or 6274(15)-keV α particles and (c) the α-particle energies in coincidence with the 1531-keV γ rays.

E α = 5942 keV
The DSSD energies in coincidence with the 1531-keV γ rays are shown in Fig. 5(c).Along with the counts associated with the population of the 2 + 1 state there is a cluster of three counts with an energy of 5942 (15) keV.Comparison of the decay times of these three coincidences with the distribution for the decay of 156 Hf (8 + ) to the ground state of 152 Yb, in Fig. 3(e), shows them to be consistent; implying they could be produced by the decay of 156 Hf (8 + ).If these counts are assumed to be associated with the α decay that populates the 3 − state in 152 Yb at 1890 keV [9,10,23], which decay via a cascade of 359and 1531-keV transitions, then the total Q value would be 7989 (15) keV for the decay.This is consistent with the value of 7980 (5) keV for the α decay to the ground state.It is therefore proposed that the coincidences are associated with the α decay of 156 Hf (8 + ) to the 3 − state in 152 Yb.No coincidences were observed between α particles with 5942 keV and 359-keV, 3 − → 2 + , γ rays.Considering the low statistics of the αγ coincidences between 5942(15)-keV α particles and 1531-keV γ rays, only a small number, if any, of these counts would be expected.As the γ -ray energy lies in the Compton continuum produced by the 511-keV background γ ray, small numbers of these αγ would be difficult to identify; this can be seen in Figs.4(a) and 4(b).

VI. DISCUSSION: α-DECAY REDUCED HINDRANCE FACTORS
Table I and Fig. 1 give the reduced hindrance factors (HFs) for each of the α decays observed.These are found from the reduced decay widths, δ 2 , calculated using the method prescribed by Rasmussen [37], with the lowest permissible spin change for each α decay considered.The reduced hindrance factors have been taken as the inverse of these reduced decay widths, scaled so that HF( 212 Po → 208 Pb) = 1 [where δ 2 ( 212 Po → 208 Pb) = 71.4keV].Figure 6 shows the reduced hindrance factors of all of the α decays observed from 155 Lu(25/2 − ) and 156 Hf (8 + ), as well as those from their ground states.Populated states with analogous configurations in 151 Tm and 152 Yb have the same symbols.
It can be seen that the hindrance factors to states in 151 Tm and 152 Yb, which have been previously assigned with analogous configurations are comparable.This appears to corroborate the assignments.Comparing the hindrance factors to the daughter ground states (circles) from both the ground and isomeric states of the decaying nuclei, there is roughly an order of magnitude increase for the decays from the isomers.The hindrance of an α decay is determined by both the difference in nuclear structure of the initial and final states and also the pairing of the decaying state; this having a large influence on the α-particle preformation factor [38].In this case, the increase in HFs may be attributed to the weakening of pairing correlations produced by the ν(f 7/2 h 9/2 ) configuration of the isomeric states compared with the fully paired ν(f 7/2 ) 2 ground states.
For α decays from the isomeric states there is again roughly an order of magnitude increase for the hindrance factors to the first π (h 11/2 ) 5 (6) , s = 3(2) multiplet excitations with 15/2 − (2 + ) in 151 Tm( 152 Yb) (triangles) compared with those to the s = 1(0) ground states.This increase may be explained by nuclear-structure considerations due to the rearrangement of the h 11/2 protons required to form the first multiplet excitation.More surprising perhaps, when considering the α decays from 155 Lu(25/2 − ), is that the hindrances to the 15/2 + and 19/2 + states are very similar.As they have been assigned with different structures, a π (h 11/2 d −1 5/2 ) octupole excitation (15/2 + ) (square) and a π (h 11/2 s 1/2 ) proton excitation (19/2 + ) (cross), different hindrances may be expected to be observed to each of them.However, it may be the case that the populated states are both similarly different so as to produce comparably hindered α decays.The hindrance of the decay from 156 Hf (8 + ) to the π (h 11/2 d −1 5/2 ) (square) state in 152 Yb is somewhat uncertain due to low statistics.However, it is consistent with that of the analogous octupole state in 151 Tm.
Recent theoretical attempts have been made to quantify the reduction of pairing in multiquasiparticle isomers, which causes an increase in α-decay hindrance from these states compared with those from ground states [38,39].However, the effects of nuclear structure and pairing changes are difficult to deconvolute.Experimental data for the fine structure in α decay from isomeric states in this region, combined with those from nuclei around 208 Pb, could prove helpful in determining the effects of reduced pairing on α-decay hindrances.

VII. SUMMARY AND FUTURE WORK
The α-decay fine structure of high-spin isomers in 155 Lu(25/2 − ) and 156 Hf (8 + ) has been studied using αγcoincidence analysis.Three new α decays from 155 Lu(25/2 − ) and two from 156 Hf (8 + ) have been identified, which populate states in the N = 82 isotones 151 Tm and 152 Yb.This has allowed confirmation of the previously tentative level at 1905 keV assigned with J π = (19/2 + ).The populated states had previously been interpreted as various proton seniority s > 1 structures, which are well described by the shell model.An analysis of the hindrance factors of the α decays populating these states was consistent with the structural assignments previously made.This is the first report of states with such high energies (E excitation 1.5 MeV) being populated following α decay outside the region above 208 Pb.As well as providing a challenge for theorists to describe these α-decay branches in both regions there is also scope for further experimental investigation in nuclei above 146 Gd.For example, another α-decaying highenergy spin-trap isomer in the N = 84 isotone chain is known to exist in 158 W [26], and significant branches populating states in 154 Hf could be observed.Additionally, a hint of a high-energy α-decaying isomeric state was reported in 157 Ta [21], but the apparent similarity of its α-decay energy and half-life to that of the α decay from 156 Hf (8 + ) has meant this has not been possible to confirm.The observation of α-decay branches from this isomer to known excited states in 153 Lu would provide confirmation of its existence.

FIG. 1 .Figure 4 FIG. 2 .
FIG. 1. Level schemes of151 Tm and 152 Yb populated following the α decays of the155 Lu J π = 25/2 − isomer and ground state and the 156 Hf J π = 8 + isomer and ground state, respectively.The spins, parities, and energies of each level are given along with the energies of the transitions.For each α decay the α-particle energy and reduced hindrance factors are given from the results of the present work and the state populated is also indicated.The configurations that have previously been assigned to each state (see text for details) are shown.A. 155 Lu(25/2 − ) → 151 Tm α-decay fine structure Figure 4 shows αγ coincidences gated for α decays from 155 Lu(25/2 − ) (as detailed in Sec.IV).Spectra of α-particle energies in coincidence with the three γ rays identified from the deexcitation of states in 151 Tm are shown separately in Figs.2(b)-2(d).The α particles from 155 Lu(25/2 − ) were identified with the help of the diagonal lines shown on the αγ -coincidence spectrum in Fig. 4(a).The lines represent a constant Q T value when summing the γ -ray energy and the α-decay Q value.They represent the Q T values between 155 Lu(25/2 − ) and the 151 Tm ground state, Q[ 155 Lu(25/2 − ) → 151 Tm(11/2 − )] (dashed line), and J π = (15/2 + ) state 1490 keV above the ground state (as reported in Ref.[10]), Q[ 155 Lu(25/2 − ) → 151 Tm(15/2 + )] (dot-dashed line).

1 .FIG. 3 .
FIG. 3. Decay time for the α decays identified from 155 Lu(25/2 − ) to the (a) (19/2 + ), (b) (15/2 + ), and (c) 15/2 − states in 151 Tm and from 156 Hf (8 + ) to the (d) 2 + and (e) 3 − states in 152 Yb.Also shown as a dashed line on each panel is the distribution of the decay times from the respective isomer to the ground state.These have been scaled for comparison with the weaker branches.

FIG. 4 .
FIG.4.Energies of coincident α particles and γ rays measured following the decay of 155 Lu(25/2 − ).The diagonal lines on (a) represent a constant energy for the sum of the α-decay Q value, calculated from the α-particle energy, and the γ -ray energy; the energies represented are those between the 155 Lu(25/2 − ) isomeric state and both the ground state (dashed line) and excited state at 1490 keV (dot-dashed line) in 151 Tm.The αγ coincidences identified are circled and the γ -ray projections in coincidence with (b) the 5521(8)-keV and (c) the 5928(5)-or 5937(15)-keV α particles are shown.

FIG. 5 .
FIG.5.Energies of coincident α particles and γ rays measured following the decay of 156 Hf (8 + ).The diagonal line on (a) represents a constant energy for the sum of the α-decay Q value, calculated from the α-particle energy, and the γ -ray energy; the energy represents that between the 156 Hf (8 + ) isomeric state and the ground state of 152 Yb.The αγ coincidences identified from 156 Hf (8 + ) are circled with contaminant coincidences from 155 Lu(25/2 − ) also labeled.Also shown are the γ -ray energies in coincidence with (b) the 5942(15)or 6274(15)-keV α particles and (c) the α-particle energies in coincidence with the 1531-keV γ rays.

TABLE I .
α-particle energies, E α , branching ratios, b α , reduced decay widths, δ 2 , and reduced hindrance factors, HF, of α decays from 155 Lu(25/2 − ) and 156 Hf (8 + ) to final states with J π f at energies E f in 151 Tm and 152 Yb.Total decay Q values, Q T , are given by FIG.6.Reduced hindrance factors of the α decays from the 155 Lu(25/2 − ) and 156 Hf (8 + ) isomers (except where labeled) to states in 151 Tm and 152 Yb, respectively, with J π shown on the x axis.The configurations assigned to each of the states populated is indicated and analogous states in 151 Tm and 152 Yb have the same symbols.