Multiparticle Configurations in N = 84 Isotones Located at the Proton Drip Line

PHYSICAL REVIEW C 71, 054319 (2005) Multiparticle configurations in N = 84 isotones located at the proton drip line D. Seweryniak,1,2 J. Uusitalo,1 P. Bhattacharyya,3 M. P. Carpenter,1 J. A. Cizewski,1,4 K. Y. Ding,4,∗ C. N. Davids,1 N. Fotiades,4,† R. V. F. Janssens,1 T. Lauritsen,1 C. J. Lister,1 A. O. Macchiavelli,5 D. Nisius,1 P. Reiter,1 W. B. Walters,2 and P. J. Woods6 1Argonne National Laboratory, Argonne, Illinois 60439, USA 2University of Maryland, College Park, Maryland 20742, USA 3Purdue University, West Lafayette, Indiana 47907, USA 4Rutgers University, New Brunswick, New Jersey 08903, USA 5Lawrence Berkeley Laboratory, California 94720, USA 6University of Edinburgh, Edinburgh, EH9 3JZ United Kingdom (Received 11 February 2005; published 26 May 2005) = 156 157 158 Excited states in the proton-rich N 84 isotones 72Hf84, 73Ta84,and 74W84 were observed using 102Pd(58Ni, xp2n) reactions at 270 MeV. γ rays were detected with the Gammasphere array of Compton- suppressed Ge detectors coupled with the Argonne fragment mass analyzer and were assigned to individual reaction channels using the recoil-decay tagging method. Prompt γ -ray cascades were associated with the α + 156 157 + decay of both the ground state and the 8 isomeric state in Hf, the h11/2 state in Ta, and the 8 isomeric state in 158W. The level schemes constructed for 156Hf,157Ta, and 158W are compared with these of lighter N = 84 isotones and are discussed within the framework of the shell model. DOI: 10.1103/PhysRevC.71.054319 PACS number(s): 23.20.Lv, 27.70.+q I. INTRODUCTION Another interesting aspect of nuclei with N 82 and Z 64 is the close proximity of the proton drip line. A number of The structure of nuclei close to the N = 82 neutron shell proton emitters have been identified in this region in a series closure, above the Z = 64 line, have for a long time been the of experiments performed at the ATLAS accelerator using the aim of numerous studies. Interest in this region of the chart Argonne Fragment Mass Analyzer [4]. Every odd-Z element of nuclides was motivated by the discovery that 146Gd can 64 82 between Ta and Bi has been found to have at least one isotope be regarded as a doubly magic nucleus [1]. This has been decaying by proton emission. In addition, many nuclei in this attributed to the presence of a large energy gap between the region α decay (see Ref. [5] and references therein). g , d proton orbitals, which are filled in 146Gd, and the 7/2 5/2 Studies of proton-rich nuclei close to N 82, with Z>64 remaining proton orbitals of the Z = 50–82 major shell (i.e., are hampered by very small production cross sections, because h ,s , and d ). The immediate neighbors of 146Gd can be 11/2 1/2 3/2 of the lack of suitable stable target and beam combinations, produced with relatively large cross sections using heavy-ion strong competition from fission, and fragmentation of the induced fusion-evaporation reactions. Based on a large body 154 fusion-evaporation reactions. The 72Hf82 nucleus is the of data, single-particle energies and two-body matrix elements = 146 heaviest N 82 isotone with known excited states. The decay with respect to the Gd core were deduced in Ref. [2]. Several + of its 10 isomer has been observed using the Daresbury Recoil shell-model calculations using these parameters successfully Separator [6]. Excited states of even-even N = 84 isotones reproduce excited states observed in nuclei with more than two 154 = 146 through 70Yb84 [7] and odd-Z,N 84 isotones through valence nucleons outside the Gd core [2,3]. 153 = 146 69Tm84 [8] have been investigated in a series of in-beam The N 82 isotones heavier than 64Gd82 are an ideal ground for studying proton-proton residual interactions. Yrast experiments summarized in Ref. [3]. In the present work, prompt γ -ray cascades correlated with states in these nuclei can be accurately described using 156 157 158 n α lines assigned previously to Hf, Ta, and Wwere the (πh11/2) configuration space. Conversely, the N = 84 148 identified using the recoil-decay tagging (RDT) method. This isotones heavier than 64Gd84 are crucial for determining both the interaction between neutrons occupying the lowest f is a continuation of an earlier measurement carried out with the 7/2 AYEBALL Ge array [9]. The results on excited states in 155Yb and h9/2 orbitals, and the interaction between these neutrons 155,156,157 and protons gradually filling the h orbital. In addition, and Lu from the present experiment have already 11/2 been reported in Ref. [10]. The experimental setup and the the i13/2 neutron orbital plays a significant role at higher excitation energies, where it contributes efficiently to the total results are described in Sec. II. In Sec. III the level schemes obtained for 156Hf, 157Ta, and 158W are compared with lighter spin because of its large intrinsic angular momentum. The = 3− state in 146Gd is situated at only 1597 keV. As a result, N 84 isotones and with shell-model calculations. many excited states contain also a sizable octupole phonon component. II. EXPERIMENTAL RESULTS ∗Present address: Telecordia Technology, Piscataway, NJ 08854. A 270-MeV 58Ni beam from the ATLAS accelerator †Present address: Los Alamos Laboratory, Los Alamos, NM 87545. was used to bombard a 1 mg/cm2 102Pd target, enriched to 0556-2813/2005/71(5)/054319(9)/$23.00 054319-1 ©2005 The American Physical Society D. SEWERYNIAK et al. PHYSICAL REVIEW C 71, 054319 (2005) (a) (b) FIG. 1. The energy spectrum of α particles detected in the DSSD. FIG. 2. γ -ray singles spectra correlated with (a) the ground state α decay of 156Hf and (b) the α decay of the 8+ isomeric state in 156Hf. 69%, to populate excited states in light N = 84 isotones via xp2n fusion-evaporation channels. γ rays were detected Singles γ -ray spectra tagged by the 156Hf α lines are with the Gammasphere [11] array of Compton-suppressed presented in Fig. 2. The energies, intensities, and angular distri- ◦ ◦ HPGe detectors, in coincidence with the Argonne fragment bution ratios R = Iγ (≈180 )/Iγ (≈90 )forγ rays correlated mass analyzer (FMA) [12]. Detected γ rays were assigned with the 156Hf α decays are given in Table I. The spectrum to individual reaction channels using the RDT method [13]. associated with the ground-state 156Hf decay is dominated Recoiling nuclei were dispersed in the FMA according by three strong transitions. γ rays in coincidence with the to their mass-to-charge state ratio and implanted into a 415-, 728-, and 857-keV transitions are shown in Fig. 3. The 60-µm-thick, 16 × 16 mm2, 48 × 48-strip, double-sided sil- three γ rays are mutually coincident. Their angular distribution icon strip detector (DSSD) placed behind the focal plane. ratios are consistent with a stretched quadrupole character and + + + + The subsequent characteristic α decays, observed in the same they are proposed to form a 6 → 4 → 2 → 0 cascade. DSSD pixel as the implantation, allowed for the identification According to Fig. 3, the weaker 636- and 597-keV transitions of the implanted residues and, thus, of the γ rays detected at feed the ground-state band at spin 6¯h and 4¯h, respectively. The the target position in coincidence with the residues. 857-keV coincidence gate reveals a 597-keV transition, which + The energy spectrum of α particles collected in the DSSD is proposed to feed the 2 state. during the experiment is shown in Fig. 1. The α spectrum is The spectrum correlated with the α particles emitted from rather complex. Observed α lines were assigned to individual the isomeric state is more complex, indicating fragmentation of A/Q values in the FMA and were identified based on their the decay paths. γ rays in coincidence with the three strongest energies and half-lives. The lines associated with the 155Lu, tramsitions at 919, 800, and 586 keV are shown in Fig. 4. 156Hf, 157Ta, and 158W nuclei are labeled in Fig. 1. These α These transitions are in mutual coincidence. Their angular lines represent about 1% of the entire spectrum. distribution ratios are consistent with stretched quadrupole + + + γ -ray transitions correlated with individual α lines were multipolarity and spins 10 ,12 , and 14 are assigned to the ordered on the basis of γ -γ coincidence relationships, as well 2896-, 3696-, and 4282-keV states, respectively. The 919-keV as energy and intensity balance. γ -ray multipolarities were transition was placed at the bottom of the cascade because obtained from angular distributions. A maximum spin change it is in coincidence with the 311-keV transition, which is was assumed for most of the observed γ -ray transitions, as is not present in the 800- and 586-keV coincidence gates (see often the case in heavy-ion fusion evaporation reactions. Fig. 4). Based on the γγ coincidence relations, a side structure is proposed, which feeds into the yrast band at spin 8¯h and 10¯h. A. The 156Hf nucleus The spin and parity assignments for the side band are discussed below. A level scheme for 156Hf is presented in Fig. 5, where The ground state of 156Hf has an half life of 23 ms and only the strongest transitions were placed because of limited is known to decay predominantly by emitting an α particle γγ statistics. with an energy of 5.87 MeV. Another α line with an energy of 7.78 MeV and a half-life of 0.52 ms has been assigned to the decay of an isomeric state in 156Hf [5].

Load more