First observation of excited states in Hg17595

First observation of excited states in 175 Hg95 D. O’Donnell,1 J. Simpson,1 C. Scholey,2 T. Bäck,3 P.T. Greenlees,2 U. Jakobsson,2 P. Jones,2 D.T. Joss,4 D.S. Judson,4 R. Julin,2 S. Juutinen,2 S. Ketelhut,2 M. Labiche,1 M. Leino,2 M. Nyman,2 R.D. Page,4 P. Peura,2 P. Rahkila,2 P. Ruotsalainen,2 M. Sandzelius,2, 3 P.J. Sapple,4 J. Sarén,2 J. Thomson,4 J. Uusitalo,2 and H.V. Watkins4 2 1 STFC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, United Kingdom. Department of Physics, University of Jyväskylä, PO Box 35, FI-40014, Jyväskylä, Finland. 3 Department of Physics, Royal Institute of Technology, Stockholm SE-10691, Sweden. 4 Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, United Kingdom. (Dated: May 21, 2009) Excited states of the neutron-deficient nucleus 175 Hg, populated using fusion-evaporation reactions, are reported for the first time. The spin and parity of the ground state has been determined to be I π = 7/2− through measurements of the α decay to the daughter nucleus 171 Pt. A structure based on an isomeric state (T1/2 = 0.34(3) µs) with I π = 13/2+ and its decay path to the ground state have been established. The observed structures are interpreted in terms of single-particle configurations and the trends of coexisting shapes in neighboring nuclei are discussed. PACS numbers: 23.20.Lv, 21.10.Tg, 23.20.Nx, 23.60.+e In this work the evolution of these shapes and the structure of the Hg isotopes towards the N = 82 shell closure is augmented with a study of 175 Hg, which has 21 fewer neutron than the lightest stable isotope. Oddmass nuclei are particularly useful in allowing the active single-particle orbitals to be identified. Studies of the odd-A Hg nuclei 177 Hg [2] and 179 Hg [4, 5] have revealed the coexistence of three distinct shapes and have identified the single-particle orbits thought to be responsible. In the present study of 175 Hg, excitations based on singleneutron states have been identified for the first time. Previous studies of 175 Hg have been limited to the decay of the ground state by means of α decay to its daughter 171 Pt [9–12] and, more recently, population and decay of its ground state via the proton decay of 176 Tl [13]. The reason for the lack of detailed spectroscopic information for 175 Hg is the difficulty in its production. Using fusion-evaporation reactions, the optimum mechanism for populating such exotic nuclei is via neutron evaporation. However, in this extremely neutron-deficient region, these channels are very unfavorable, with the evaporation of charged particles and fission dominating the 2.5 175 10 Counts/10 keV Au 2.0 175 x 30 1.5 Pt 174 Hg Pt 172 0.5 0.0 Au 176 1.0 174 Hg 175 5 For thousands of years the element 80 Hg has proved to be a scientific curiosity. This interest has not been lost on the atomic nuclei of Hg in which some of the most widely varying nuclear shape phenomena have been observed. In the case of the neutron-deficient isotopes [1–7], evidence has been found for shape coexistence in which different shapes compete for the lowest energy structure in the same nucleus. Spherical shapes, associated with the close proximity to the Z = 82 shell gap, in addition to oblate and prolate shapes resulting from particle-hole excitations to high-Ω and low-Ω intruder orbitals, respectively, compete for the yrast structure of neutron-deficient Hg nuclei. The extent of the competition between these configurations, particularly the oblate and prolate shapes, is observed to change as the neutron number varies [8]. Pt 6000 6500 7000 Energy (keV) FIG. 1: Spectrum showing all α particles observed within 50 ms of the implantation of a recoil in the DSSDs. cross section. Nonetheless, the highly selective recoildecay tagging (RDT) technique [14–16] has proved successful in identifying the excited states of many neutrondeficient nuclei in the vicinity of the Z = 82 spherical shell closure [17]. RDT involves the exploitation of high-efficiency γ-ray detector arrays, which are coupled to recoil separators and focal plane detector systems. By correlating characteristic radiation (α particles, protons or γ rays), observed at the focal plane, with fusionevaporation residues and prompt γ radiation emitted at the target position, the study of excited states of nuclei produced with cross sections down to a few tens of nanobarns is possible [18]. The RDT technique has been utilized in order to study excited states of 175 Hg following the reaction 92 Mo(86 Sr17+ ,3n) at a beam energy of 403 MeV. The beam, provided by the K130 cyclotron of the University of Jyväskylä, bombarded a self-supporting 600 µg/cm2 92 Mo target (of 98% isotopic enrichment), which was lo- 2 cated at the center of the jurogam γ-ray detector array that consisted of 43 escape-suppressed Ge detectors [19]. The recoiling fusion-evaporation residues (recoils) were transported to the focal plane of the ritu He-filled magnetic separator [20]. The great spectrometer, which consisted of a multi-wire proportional chamber (MWPC), two double-sided silicon strip detectors (DSSDs), a planar Ge detector, a clover Ge detector and an array of Si PIN diodes, was located at the focal plane. This spectrometer facilitated the spectroscopy of α particles, delayed γ rays and conversion electrons following the decay of the implanted recoils. The MWPC provided energyloss and time-of-flight information (in conjunction with the DSSDs) and allowed the recoils of interest to be distinguished from the background of scattered beam and radioactive decays. The Total Data Readout system [21] was employed in order to record time-stamped events observed in each of the constituent detectors. In this way it was possible in the offline analysis to correlate events with a characteristic α decay and unambiguously assign any prompt and delayed γ rays or conversion electrons to the decay of a specific nucleus. The data were sorted using the Grain software package [22]. Conditions were set such that only those events in which an implanted recoil into the DSSDs was followed by the detection of an α particle within 50 ms were accepted. The resulting α-particle energy spectrum is shown in Fig. 1. A total of 25,000 recoil-α(175 Hg) coincidences were collected and the cross section for the production of 175 Hg was estimated to be ∼ 1.5 µb. The α particles associated with the decay of 175 Hg [10] were observed at 6913(5) keV. The time elapsed between the implantation of a recoil in the DSSDs and the detection of a 175 Hg α particle was also measured, resulting in a half-life for 175 Hg of 10(1) ms. This value has been corrected for the ∼ 400 Hz average implantation rate, observed across the entire area of the DSSDs, in accordance with the method of Leino et al. [23]. This value, while not as precise as that of Rowe et al. [12] (10.8(4) ms), is consistent with all prior measurements [9–11]. The ground-state spin and parity of 175 Hg were determined through an analysis of the α-decay properties. The Rasmussen [24] formalism for calculating reduced α-decay widths was utilized and assuming a branching ratio of 100% and ∆l = 0 emission, a reduced width of δ 2 = 54(5) keV is obtained. This value indicates that there is no change in spin and parity involved in the α decay of 175 Hg. The ground-state spins and parities of the α-decay chain partners 163 W, 167 Os and 171 Pt have been established as 7/2− [25] and the present measurement therefore establishes the ground state of 175 Hg to also be I π = 7/2− . This measurement is consistent with the observations of Kettunen et al. [13] in which the ground state of 175 Hg was populated via a ∆ℓ = 0 proton decay of the 176 Tl ground state leading to the assignment of I π = 7/2− or 9/2− for the 175 Hg ground state. Fig. 2(a) shows all γ rays observed at the focal plane that have been correlated with the α decay of 175 Hg following within 50 ms of the implantation of a recoil. Three distinct peaks have been observed at energies of 71, 80 and 414 keV. The 80 and 414 keV transitions are in prompt coincidence at the focal plane. Using the PIN-diode detectors of the great spectrometer [26] it was possible to measure directly the internal conversion of the 414 keV transition. A conversion electron energy spectrum, correlated with the α decay of 175 Hg, is shown in Fig. 2(b). Two peaks were identified and associated with internal conversion electrons competing with the 414 keV γ-ray transition. The lowerenergy peak was identified as corresponding to the emission of electrons from the atomic K shell while the higherenergy peak is related to emissions from the L and M shells. A comparison of the number of observed 414 keV γ rays and the number of conversion electrons yields internal conversion coefficients (ICCs) of αK = 0.36(11) and α(L+M) = 0.10(3). These values correspond well with theoretical values of 0.38 and 0.10 calculated using BrIcc [27], assuming the 414 keV γ ray is of M2 multipolarity. The ICCs corresponding to other multipolarities are inconsistent with the experimental values. In addition to the direct measurements performed using the PIN-diode detectors, the K conversion coefficient was obtained through a comparison of the number of observed 414 keV γ rays and Kα X rays. Note the Kα X rays can only arise from conversion of the 414 keV transition since the binding energy is 83 keV [28]. An αK of 0.45(10) was determined using this method, which is consistent with both the PIN-diode result and the theoretical prediction for an M2 multipolarity. The inset of Fig. 2(a) shows the distribution of measurements of the time interval between the implantation of a recoil and the detection of a 414 keV γ ray in one of the focal plane Ge detectors. A least-squares fit to the data yielded a half-life of 0.34(3) µs. Weisskopf estimates of the lifetime for a 414 keV M2 decay are consistent with a decay from this isomeric state. The Kα and Kβ X-ray lines in the case of Hg are reported [28] to have energies of 71 and 80 keV, respectively. Accordingly, the lowest energy of the lines in Fig. 2(a) is associated with the Kα X-ray line. The second photopeak will, in part, be associated with the Kβ transition, which has an intensity of ∼25% of the Kα (Hg) transition [28]. However, the efficiency-corrected intensity measurements show that the 71 keV peak has an intensity 73(13)% of the 80 keV peak. This leads to the conclusion that an 80 keV transition must also result from the γ decay of an excited state of 175 Hg. It was not possible to measure directly an ICC for the 80 keV transition since the apparatus was not sensitive to such low energy electrons. However, the efficiencycorrected intensity of the 414 keV γ-ray transition is approximately three times that of the 80 keV γ ray. At the focal plane, all of the feeding of the first excited state is via the 414 keV transition depopulating the isomeric state. Therefore, the difference in efficiency-corrected γray intensity must be due to internal conversion. Us- 250 80 60 (a) (a) 687 50 10 150 Kα 100 Kβ 0 0 50 1 0.0 0.5 414 50 200 400 1.0 20 600 800 Counts/5 keV 1000 (b) 414-(L+M) 40 30 20 651 30 K α 1.5 Time (µs) 414-K 614 728 80 843 10 0 10 (b) 8 6 10 0 0 708 40 Counts/keV Counts/keV 200 Counts/0.25 µs 3 4 100 200 300 400 500 2 Energy (keV) FIG. 2: (a) Spectrum showing delayed γ rays correlated with 175 Hg α decays. The inset shows the distribution of time intervals between the implantation of a recoil and the detection of a 414 keV γ ray. The dashed line is the result of a least-squares fit to the data. (b) 175 Hg α-tagged conversion electron spectrum as observed with the PIN-diode detectors. ing BrIcc [27], total ICCs were calculated yielding values of 0.156 for an 80 keV E1 transition, 2.74 for M1 and 14.12 if the transition was E2 in nature. Higher multipolarities result in higher ICCs. The measured value of Iγ (414 keV)/Iγ (80 keV) ∼ 3 is only consistent with the 80 keV transition having M1 multipolarity. Figure 3(a), shows all prompt γ rays observed in jurogam and correlated with the α decay of 175 Hg. Seven clear photopeaks are observed in addition to the characteristic Hg Kα X-ray line. The transition at 80 keV is assumed to be the decay of the first excited state, which is also populated by the decay of the isomeric state. The fact that this state decays via a prompt transition supports the earlier argument that the 80 keV decay is a dipole since Weisskopf estimates suggest this transition would have a half-life of 0.4 ps, 0.9 ps or 0.2 µs if the multipolarity were E1, M1 or E2, respectively. A lack of sufficient prompt γ-ray statistics prevented a γ-γ analysis. However, coincidences between prompt events and those observed at the focal plane permitted a level scheme for 175 Hg to be proposed. Figure 3(b) shows 175 Hg α-correlated prompt transitions observed in delayed coincidence with either an 80 keV or 414 keV 0 0 200 400 600 800 1000 1200 Eγ (keV) FIG. 3: (a) Energy spectrum of prompt γ rays, correlated with the α decay of 175 Hg. (b) Same as (a) but with added condition in which only those transitions observed in coincidence with delayed γ rays of 80 or 414 keV or associated conversion electrons are shown. delayed γ ray or associated conversion electrons. Transitions of energy 614, 687 and 728 keV are common to both Figs. 3(a) and 3(b). Accordingly, these decays were placed in the level scheme, shown in Fig. 4, as feeding the isomeric state and have been ordered based on their efficiency-corrected intensities. The γ rays feeding the isomer are assumed to be of stretched E2 character such that the band is tentatively observed to a spin of 25/2. The prompt 651 keV transition is the most intense transition that is not observed in coincidence with events at the focal plane. This transition is therefore assumed to bypass the 13/2+ isomer and is assigned to directly feed the first excited state. A lack of statistics prevented the location of the 708 and 843 keV γ-ray transitions, seen in Fig. 3(a), to be fitted into the 175 Hg level scheme. Kondev et al. [4] established the ground-state spin and parity of 179 Hg as 7/2− and argued that this state is likely to result from the unpaired neutron occupying K = 7/2 f7/2 or h9/2 orbitals. It was also suggested that the ground state must be near-spherical (β2 < 0.15) in order for such orbitals to lie close to the Fermi surface. 4 (25/2 ) Moment of inertia (h /MeV) 2523 (21/2 ) 2 614 1909 728 (17/2 ) (13/2 ) 687 731 13/2 651 414 7/2 9/2 80 0 80 1181 s FIG. 4: Proposed level scheme for 175 Hg as deduced in the present study. Arrow widths are proportional to the efficiency-corrected intensities of the transitions while unfilled regions indicate the extent of internal conversion. These measurements and arguments were subsequently confirmed by Jenkins et al. [5]. The same configurations were assumed to constitute the tentative 7/2− ground state of 177 Hg [2]. In addition, total Routhian surface calculations [2] predicted a weakly-deformed minimum at β2 ≈ 0.1 to correspond to the ground state of 177 Hg. The ground state of 175 Hg and the first excited state at 9/2− are most likely the f7/2 and h9/2 single-particle configurations, respectively. In both 177 Hg and 179 Hg the first excited state is also reported as having I π = 9/2− . This state and connected higher-lying states were assumed to arise from weakly-deformed near spherical excitations [2, 4]. In 175 Hg the tentative transition at 651 keV, which bypasses the isomeric level, is assumed to have a similar origin. The present work establishes that the isomeric state at 494 keV has I π = 13/2+ , which is consistent with observations of heavier odd-A Hg nuclei [2, 5] in which this state is associated with the coupling of an i13/2 neutron to an oblate deformed core. This state in 175 Hg is also most likely to correspond to the unpaired neutron occupying the i13/2 state. The deduced B(M2) of 0.16(1) W.u. suggests the decay of the isomeric state is hindered by a factor of six compared with single particle estimate. In previous studies of nuclei in this region, such as Refs. [5, 29], where hindrances of this magnitude have been measured, they have been attributed to changes in shape. In the case of 183 Tl, the large hindrance of 18 reported for the decay of the 13/2+ isomer, was attributed to the decay of the prolate state to an oblate 9/2− level. The hindrance of 5 reported in the case of 179 Hg [5], was associated with an oblate (13/2+ ) to near-spherical (9/2− ) change of shape. The similar hindrance observed in 175 Hg is also consistent with evidence for shape coexistence in which a weakly-deformed oblate configuration 179 Hg 60 178 Hg 50 177 Hg 176 Hg 40 175 Hg 30 174 Hg 20 10 0 494 T1/2 = 0.34(3) 70 0.4 0.5 0.6 0.7 0.8 Eγ (MeV) FIG. 5: The kinematic moment of inertia as a function of γ-ray energy for the yrast sequences in Hg nuclei with 94 ≤ N ≤ 99. For odd-A isotopes, the spin of the yrast band head, 13/2, has been subtracted. Data other than those reported here have been extracted from Refs. [1–4, 17]. competes with spherical configurations at low spin. Figure 5 shows the kinematic moment of inertia plotted as a function of γ-ray energy for the yrast sequences of even- and odd-mass Hg nuclei in the vicinity of 175 Hg. In the case of the odd-A isotopes, the spin of the yrast band head (13/2) has been subtracted from each of the states in order to allow a more direct comparison with the even-A counterparts. For A > 176, the initial rapid gain in moment of inertia followed by a gradual gain as a function of γ-ray energy was interpreted in previous studies [1, 2] in terms of a change of shape. This shape change was associated with the crossing of the low-lying oblate i13/2 band by an excited prolate configuration [1, 2]. Systematics suggest that the excitation energy at which the prolate band is observed to cross the oblate band in odd-A Hg nuclei is minimized at N = 101 and increases roughly parabolically away from this minimum. In 179 Hg99 the crossing was observed to occur 854 keV [4] above the 13/2+ isomeric state while in 177 Hg97 the energy increased to 1623 keV [2]. This trend suggests that the yrast states of 175 Hg have not been observed to sufficiently high energy or spin to observe the crossing and hence the prolate structure. Indeed, based on the parabolic dependence, the crossing of the two configurations might be expected at ≈ 2.5 MeV above the 494 keV isomeric state. The similarities observed in Fig. 5 between 175 Hg and its immediate even-even neighbors (174 Hg and 176 Hg) suggest 175 Hg can be treated equally as an i13/2 neutron or neutron-hole weakly-coupled as a spectator to the 174 Hg or 176 Hg cores. To summarize, selective RDT techniques have allowed the decay of excited states of 175 Hg to be measured for the first time. Measurements of the α-decay properties to the daughter nucleus 171 Pt have allowed the spin and parity of the 175 Hg ground state to be established as I π = 7/2− . In addition, an isomeric state (T1/2 = 0.34(3) µs) 5 at an excitation energy of 494 keV has been observed and established to be I π = 13/2+ . A band based on the isomeric state, which may have an oblate deformation, and a near-spherical configuration observed to bypass the isomer have been identified. The crossing of the oblate configuration by an excited prolate band, as reported in the heavier odd-A Hg isotopes, has not been observed and will require further investigation to higher spin. The authors would like to express their gratitude to the support staff of the Accelerator Laboratory at the University of Jyväskylä and thank Paul Morrall of Dares- bury Laboratory for preparation of the Mo targets. Financial support has been provided by the UK Science and Technology Facilities Council (STFC) and by the EU 6th Framework Programme “Integrating Infrastructure Initiative - Transnational Access”, Contract Number: 506065 (EURONS) and by the Academy of Finland under the Finnish Center of Excellence Programme 2006-2011 (Nuclear and Accelerator Based Physics Programme at JYFL). PTG acknowledges the support of the Academy of Finland, contract number 111965. [1] M. Muikku, J. F. Cocks, K. Helariutta, P. Jones, R. Julin, S. Juutinen, H. Kankaanpää, H. Kettunen, P. Kuusiniemi, M. 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