PHYSICAL REVIEW C 80, 037301 (2009) The 11/2− [505] neutron extruder orbital in 159 Sm W. Urban,1,2 J. A. Pinston,3 G. S. Simpson,3 A. G. Smith,4 J. F. Smith,4 T. Rza˛ca-Urban,2 and I. Ahmad5 1 Institut Laue-Langevin, 6 rue J. Horowitz, F-38042 Grenoble, France Faculty of Physics, University of Warsaw, ul. Hoża 69, PL-00-681 Warsaw, Poland 3 LPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, F-38026 Grenoble Cedex, France 4 Department of Physics and Astronomy, The University of Manchester, M13 9PL Manchester, United Kingdom 5 Argonne National Laboratory, Argonne, Illinois 60439, USA (Received 23 June 2009; published 8 September 2009) 2 Excited states in 159 Sm, populated following the spontaneous fission of 252 Cf have been studied by means of γ spectroscopy, using the Gammasphere array. In 159 Sm we have identified an isomeric level with a half-life of T1/2 = 116(8) ns at an excitation energy of 1276.8 keV and observed a rotational band on top of this isomer. On the basis of the observed properties of the isomer and the band on top of it we propose that the 1276.8-keV level in 159 Sm corresponds to the 11/2− [505] neutron extruder configuration. The new excitation scheme of 159 Sm is compared to quasiparticle rotor model calculations. DOI: 10.1103/PhysRevC.80.037301 PACS number(s): 21.10.Tg, 23.20.Lv, 25.85.Ca, 27.70.+q In our investigation of the onset of deformation in the A ∼ 100 region [1] we discussed the role of the 9/2+ [404] neutron extruder orbital in shaping the sudden onset of quadrupole deformation around N = 59. A mechanism creating strongly deformed bands based on an extruder orbital was first employed in the A ∼ 150 region, where deformed bands with 11/2− bandheads were observed in odd-N nuclei around N = 90 [2]. It was proposed that one of the two neutrons occupying the spherical-driving, 11/2− [505] extruder orbital rising from below the N = 82 shell gap drops into the deformation-driving, low- orbital originating from the i13/2 intruder orbital, increasing the deformation of the core in an odd-N nucleus. Consequently, one observes a strongly deformed 11/2− band based on the 11/2− [505] orbital. A proper study of this phenomenon requires an observation of extruder orbitals in a wide range of nuclei. However, the 9/2+ [404] orbital was observed directly in only three nuclei, 99 Zr [3], 97 Sr [4,5], and 101 Zr [1]. In contrast, several 11/2− [505] bands were found in the A ∼ 150 region. It seems, therefore, that the A ∼ 150 region is better suited for a systematic study of extruder orbitals. The systematics suggests that new cases of 11/2− [505] bands may be found in neutron-rich Sm isotopes. An additional motivation for the present work was to clear some inconsistencies concerning isomeric states reported recently in neutron-rich neodymium isotopes [6]. We have searched for 11/2− [505] bands in the neutronrich nuclei of the A ∼ 150 region using the data from a measurement of high-fold coincidences between γ rays following spontaneous fission of 252 Cf, performed with the Gammasphere array of anti-Compton spectrometers at Argonne National Laboratory. The electronic time windows defining the coincidence events allowed measurement of times over a range of 900 ns, counted from the “start” given by the “Master Gate” signal (see Ref. [7] for more details on the experiment). Coincidence events were sorted into three-dimensional histograms with a “parabolic” energy calibration Eγ = a0 + a1 × n + a2 × n2 , where a0 = 0.45 keV, 0556-2813/2009/80(3)/037301(4) 037301-1 a1 = 0.666700 keV/n, a2 = 0.000155235 keV/n2 , and n is a channel number. Because of its unique and high K value, an extruder orbital is likely to occur as a K isomer, as observed in several nuclei in A ∼ 100 and A ∼ 150 regions. This property helps when looking for “extruder bands,” especially in neutron-rich nuclei produced in the fission process. We have searched for possible delayed transitions in 159 Sm using triple-γ coincidences between two prompt γ rays and one delayed γ ray, sorted into a so-called ppd cube, where the prompt time window extended from −10 to +10 ns, relative to the “0” time given by the Master Gate signal and the delayed time window extended from 40 to 210 ns past 0 time. We gated on the 707.1- and 1123.0-keV prompt lines of 90 Kr, the most abundant fission-fragment partner to 159 Sm in spontaneous fission of 252 Cf. The resulting double-gated spectrum is shown in Fig. 1, where one can see delayed lines of 158 Sm at 1039.4 and 781.2 keV [6], delayed lines of 156 Sm at 880.4 and 1147.7 keV [6], and a delayed line at 869.5 keV, which was reported in Ref. [6] as an isomeric transition in 154 Nd (see Fig. 6 in Ref. [6]). However, our recent study of 154 Nd [8] has shown that there is no such delayed line in this nucleus. According to the present analysis the 869.5-keV line belongs to a samarium isotope. In Fig. 2(a) we show a spectrum doubly gated in the ddp cube, where triple events consisting of two delayed-γ signals and one prompt-γ signal were sorted with time windows as described above. The first gate was set on the 707.1-keV prompt line of 90 Kr and the second gate on the 869.7-keV delayed line. In the resulting delayed spectrum one sees lines at 163.7 and 243.4 keV. The 163-243-870-keV cascade was reported in Ref. [6] as belonging to the 154 Nd nucleus. In 154 Nd there is a transition of 162.4 keV but no transition of 243 keV [8]. The 163.7-keV line seen in Fig. 2(a) has the same energy as the first transition in the 5/2− [523] band of 159 Sm, reported by Hwang et al. [9], who also reported the 243.4-keV transition in the ground-state band of 159 Sm. Therefore we conclude that the 163.7-243.4869.7-keV cascade belongs to 159 Sm. In our study of 154 Nd ©2009 The American Physical Society PHYSICAL REVIEW C 80, 037301 (2009) BRIEF REPORTS 1039.5 Gate 707−1123 keV 20 〈A(Kr) 1147.7 880.4 869.7 781.3 40 92 〈 Counts 60 91 158.9(3) 90 89.65(15) 89 0 1000 1100 1200 154 1300 FIG. 1. Fragment of the coincidence spectrum double gated in the ppd cube on 707.1- and 1123.0-keV prompt-γ lines of 90 Kr, as observed in fission of 252 Cf. Energies of γ lines are labeled in keV. See text for details about the ppd cube. 100 0 163.7 600 800 (b) Gate 869 − (163+243) keV 200 400 600 Channels 800 1000 707.1 400 769.0 775.3 200 200 655.6 Counts (a) Gate 869 − 707 keV 80 142.7 159.2 175.8 191.8 207.9 223.9 238.5 255.0 270 302.7 335.1 367.5 400.5 432.0 462.4 494.0 525.0 Counts 243.4 [8] we observed a 248-keV line in this nucleus instead of 243-keV line. The 248-keV line was also reported in Ref. [10] as transition in the ground-state cascade of 154 Nd. In Fig. 2(b) we show a spectrum doubly gated on the ddp cube with the first gate on the 869.7-keV delayed line and the second gate on both the 163.7- and 243.4-keV delayed lines. In the spectrum one observes prompt-γ lines, which are in coincidence with this delayed cascade. There are several new lines between 140 and 600 keV and known transitions of ground-state bands in 88 Kr, 90 Kr, and 92 Kr at 775.3, 707.1, and 769.0 keV, respectively [11]. The information form Fig. 2(b) can be used to identify the samarium isotope, to which the 869-keV line belongs, employing the mass-correlation technique proposed in Ref. [12] and used in dozens of cases (see, e.g., Refs. [8,11]. Taking γ intensities of the ground-state band transitions in 88,90,92 Kr isotopes as weights, we calculated a weighted average Kr 0 158 160 A(Sm) Channel number 40 156 1000 FIG. 2. Coincidence spectra double gated in the ddp cube (a) on the 707.1-keV prompt line of 90 Kr and a new 869.7-keV delayed-γ line and (b) on the 896.7-keV delayed line on one d axis and the 163.7- and 243.4-keV delayed lines on the other d axis, as observed in fission of 252 Cf. Energies of γ lines are labeled in keV. See text for details about the ddp cube. FIG. 3. Mass correlation diagram for samarium and krypton isotopes produced following the spontaneous fission of 252 Cf. See text for further explanation. mass of 89.65(15) in coincidence with the 869.7-(163.7 + 243.4)-keV cascade. Analogous average masses were calculated for the Kr isotopes seen in coincidence with spectra doubly gated on the ground-state bands in 154,156,158,160 Sm, as shown in Fig. 3. There is a clear correlation between the masses of the Sm isotopes and their Kr fission-fragment partners. A straight-line fit through the data points in Fig. 3 can be used as a mass calibration to determine the mass of the samarium isotope, to which the 869.7-keV delayed line belongs. For a krypton isotope mass of 89.65(15) this calibration gives an average samarium isotope mass of 158.9(3), indicating that the 869.7-keV delayed line belongs to 159 Sm. The 869.7-keV delayed line feeding the 407.1-keV level in 159 Sm defines an isomeric level in this nucleus at 1276.8-keV. The half-life of the 1276.8-keV isomer in 159 Sm was obtained from fitting the time spectrum corresponding to the 869.7-keV isomeric transition. The spectrum was cut from the pgt cube where we sorted along the p axis γ signals registered within the prompt time window, along the g axis γ signals with no time condition, and along the t axis time values corresponding to the g-axis γ signals. The t-axis range was from −50 to +900 ns relative to the 0 time. In the pgt cube we set prompt-γ gates on the 142.7-, 159.2-, and 175.8-keV lines feeding the isomer and we set the g-axis gate on the 869.7-keV line. The resulting time spectrum for the 869.7-keV isomeric transition is shown in Fig. 4. Fitting an exponentialplus-constant-background function to this spectrum gave a half-life of 115(10) ns for the 1276.8-keV isomer. New prompt lines seen in Fig. 2(b) between 140 and 600 keV belong to a band build on top of the 1276.8-keV isomer. Further gating on these lines allowed the construction of a band on top of this isomer, as shown in Fig. 5, displaying the level scheme of 159 Sm, as obtained in this work. The ground state of 159 Sm has spin and parity 5/2− and is interpreted as a 5/2− [523] neutron configuration [13]. The first excited 7/2− state of the 5/2− [523] band was proposed at 71.8 keV in β − -decay studies of 159 Pm [13,14]. Higher energy levels of the ground-state band were proposed in Ref. [9], but without spin assignments. The ground-state band of 159 Sm seen in our work agrees with that reported in Ref. [9]. Because of its similarity with analogous bands in 161 Gd and 163 Dy, 037301-2 PHYSICAL REVIEW C 80, 037301 (2009) BRIEF REPORTS Gate 869.7 keV 200 100 60 40 20 60 140 100 Channel number FIG. 4. Time spectrum gated on the 869.7-keV isomeric transition in the pgt cube. The dashed line represents a fit of an exponential decay plus a constant background. The time calibration is 4.4 ns per channel. See text for further explanation. 3142 270 2872 525 255 2616.5 494.0 238.5 2378.0 2128.8 462.4 223.9 (29/2 – ) 2154.1 432.0 207.9 1946.2 400.5 536.7 191.8 367.5 1592.1 (25/2 – ) 1754.5 1578.6 335.1 159.2 302.7 468.1 1124.0 1276.8 (21/2 – ) 1419.5 – (11/2 ) T 1/2 = 115(10) ns 142.7 395.9 728.1 (17/2 – ) Sm Gd Dy 175.8 869.7 Eexc (11/2[505]) [MeV] Counts 400 this band can be interpreted as the favored branch of the ν5/2− [523] configuration. Therefore we tentatively assign spins to this band as shown in Fig. 5. The unfavored branch, including the (7/2− ) level at 71.8-keV, was not reported in Ref. [9] and was also not observed in the present work. At neutron number N = 97 the Fermi level approaches the 5/2− [523] and 7/2+ [633] Nilsson levels, which are crossed by the 11/2− [505] extruder at a deformation of ǫ
0.3. Therefore, one expects three deformed near-yrast configurations in 159 Sm. We observe the 5/2− [523] configuration as the ground-state band and it is likely that the 1276.8-keV isomer has a 11/2− [505] neutron configuration. This hypothesis is supported by the systematic behavior of excitation energies of the 11/2− [505] neutron level, shown in Fig. 6. To verify the proposed configuration assignments and to check where to expect the 7/2+ [633] Nilsson level, we have performed quasiparticle rotor model calculations for 159 Sm using the codes GAMPN, ASYRMO, and PROBI [16]. In the calculations we used a deformation of ǫ2 = 0.34 for the positive-parity levels and ǫ2 = 0.315 for the negative-parity levels, an inertia parameter of a = 23.3 keV, and a Coriolis attenuation parameter of ξ = 0.70. Standard values for the κ and µ parameters of the ls and l 2 terms, were used [17]. To calculate the γ -decay pattern we took a collective g factor for the core of gR = Z/A and an effective value of the free neutron g factor of gseff = gsfree . More information on such calculations can be found in Refs. [18,19]. A comparison between the experimental and calculated energies of the excited states in 159 Sm, as obtained in the present work, is presented in Fig. 7. The overall reproduction of the experimental scheme is good. Intraband-transition energies for the 5/2− [523] band, which appears in the calculations as the ground-state configuration, are reproduced to within 5 keV. The position of the 11/2− [505] bandhead is calculated 1.0 ( ) ( ) 321.0 407.1 (13/2 – ) 243.4 163.7 g.s. 0.0 (9/2 – ) 87 163.7 5/2 – 159 Sm FIG. 5. Partial level scheme of 159 Sm obtained in the present work. 89 91 93 95 Neutron number 97 FIG. 6. Excitation energies of 11/2− [505] neutron levels in Sm, Gd, and Dy isotopes. The data are taken from the database [15]. Data points in parentheses are from systematics. Lines are drawn only to guide the eye. 037301-3 PHYSICAL REVIEW C 80, 037301 (2009) BRIEF REPORTS 159 5/2[532] band 1124 21/2 1127 21/2 Sm 11/2[505] band 2534 23/2 2378 2284 724 17/2 402 13/2 728 17/2 407 21/2 2154 Level (keV) 163.7(2) 271th 407.1(3) 2056 19/2 1946 1850 17/2 1754 1665 15/2 1578 9/2 163 9/2 1503 13/2 1419 70 0 7/2 5/2 72 0 7/2 5/2 1362 11/2 1276 exp. th. 11/2 exp. FIG. 7. Comparison of the experimental and calculated energies of excited states in 159 Sm, as obtained in the present work. Errors for the experimental level energies are less than 1 keV. at 1362 keV, only 95 keV above the experimental 11/2− level at 1276 keV. Again the intraband transition energies are reproduced well, though the experimental band is slightly more compressed than the calculated one. Experimental γ -ray branching ratios in 159 Sm, defined as Rexp = Iγ (I = 1)/Iγ (I = 2), observed in this work, are compared to the calculated values in Table I. The calculations reproduce well the experimentally observed feature that branching ratios in the ground-state band are much lower than those in the 11/2− band. Low branchings from the favored to the unfavored level in the ground-state band hinder the observation of the unfavored levels in this band. The decay of the 1276.8-keV isomer in 159 Sm via a single transition differs from decays of 11/2− [505] bands in other nuclei, many of which have branches to positive-parity states. The position of the 7/2+ [633] bandhead is calculated to be 391 keV above the 5/2− [523] ground state. We could not see any sign of this band. It is possible that it is located higher [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Rexp Rth Level (keV) Rexp Rth – – – 0.15 0.06 0.02 1578.6(5) 1754.5(5) – 3.3(7) 2.1(4) – 15.6 7.1 – 13/2 160 th. TABLE I. Experimental and calculated γ -ray branching ratios, Rexp and Rth , for levels in the 11/2− band of 159 Sm, as obtained in the present work. The ratios are defined as R = Iγ (I = 1)/Iγ (I = 2). The 271th -keV level is the calculated 11/2− member of the ground-state band. Errors for the experimental level energies are given in parentheses. W. Urban et al., Eur. Phys. J. A 22, 241 (2004). P. Kleinheinz et al., Phys. Rev. Lett. 32, 68 (1974). W. Urban et al., Eur. Phys. J. A 16, 11 (2003). J. K. Hwang et al., Phys. Rev. C 67, 054304 (2003). A. Złomaniec et al., Phys. Rev. C 72, 067302 (2005). C. Gautherin et al., Eur. Phys. J. A 1, 391 (1998). D. Patel et al., J. Phys. G 28, 649 (2002). G. S. Simpson et al., Phys. Rev. C 80, 024304 (2009). J. K. Hwang et al., Phys. Rev. C 78, 017303 (2008). X. Q. Zhang et al., Phys. Rev. C 57, 2040 (1998). than the calculations predict. This would explain the absence of any E1 decays of the 11/2− [505] isomer in 159 Sm. The calculations predict in 159 Sm four decay branches from the 11/2− [505] level to levels in the ground-state band of which we observe only the decay to the 13/2− level. In our data we do not see the 1113-keV delayed line reported in Ref. [6] as another isomeric decay branch. The calculated transitions to the 11/2− and 9/2− levels of the ground-state band are the dominant branches in the decay of the 11/2− [505] isomer in 159 Sm. Consequently, the calculated half-life of the 11/2− [505] isomer of about 4 ns, is significantly shorter than the observed one, whereas the partial half-life for the 11/2− → 13/2− decay is 43 ns, in fair agreement with the observation. We cannot presently explain the above discrepancy but would like to point to two facts. First, the decay to the 13/2− level of the ground-state band has been seen so far only in 149 Sm [20] and 159 Sm, where the excitation energy of the 11/2− [505] level is high enough for this decay to compete with other decays. Second, the 11/2− [505] band in 159 Sm is the most deformed and rigid of all the 11/2− [505] bands observed in the region. This indicates that the 11/2− band in 159 Sm is a purer 11/2− [505] configuration than in other nuclei. Therefore, higher K forbiddeness may be associated with decays of this configuration in 159 Sm than in 149 Sm. Consequently, in 149 Sm other decays are also observed, whereas in 159 Sm one observes a decay to the 13/2− level of the ground-state band only, for which the K number is probably not as well defined as for the lower-spin levels. [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] 037301-4 T. Rza˛ca-Urban et al., Eur. Phys. J. A 9, 165 (2000). M. C. A. Hotchkis et al., Nucl. Phys. A530, 111 (1991). R. G. Helmer, Nucl. Data Sheets 99, 483 (2003). S. Ichikawa et al., Phys. Rev. C 71, 067302 (2005). ENSDF database at www.nndc.bnl.gov. P. Semmes and I. Ragnarsson, 1991 (unpublished). T. Bengtsson and I. Ragnarsson, Nucl. Phys. A436, 14 (1985). S. E. Larsson et al., Nucl. Phys. A307, 189 (1978). J. A. Pinston et al., Phys. Rev. C 74, 064304 (2006). P. Kleinheinz et al. Nucl. Phys. A283, 189 (1977).