H- \ " DoE- 'The submitted manuacnpt ha* bean author* 1 by » contractor of the u S Government under contract No. 0£- AC05-84Ofl21400. Accordingly, the U.S. Government retain a nonezduaiv* royalty-free fcanaa to pubfafi or reproduce the pubtshed form of tha contribution, or etow otheri to do «o. for U.S. Government purposes.' Deformation Studies in the Extremely Neutron-Deficient Praseodymium, Neodymium and Promethium Isotopes
J. Breitenbach(1)» R. A. Braga(2\ J. L WoodO), P. B. Semmes(3) and J. KormickiW
(1) Georgia Institute of Technology, School of Physics, Atlanta, GA 30332 <2) Georgia Institute of Technology, School of Chemistry, Atlanta, GA 30332 (3) Tennessee Technological University, Physics Department, Cookeville, TN 38505 W Vanderbilt University, Physics Department, Nashville, TN 37235
UMiU Abstract Several experiments were performed at the UNISOR isotope separator facility at HHIRF at the Oak Ridge National Laboratory on the p+/EC decay of neutron-deficient rare 1 earth isotopes. Data for the three decay chains 37gu_^137sm_>137pin_>137j^(ij 1 1 35sm-» 35pm_>135Nd and ^Sn^^pm^^^ were obtained consisting of multiscaled spectra of 7 rays, X rays, and conversion electrons, as well as yyt, X-yt, eyt and eXt
coincidences. Gamma rays associated with the decay of 133Sm and 133pm were observed for the first time. The decay of a new low-spin (1/2,3/2) isomenc state, with a half life around 70 sec was seen in 133Nd. Systematics and particle-rotor calculations are discussed.
1. Introduction
Nuclei with atomic numbers Z>50 and N<82 are expected to exhibit strong deformations far from the closed shells. The portion of the nuclear chart containing this area is shown in Fig. 1. Theoretical predictions by Leander and Moller [1] yield £2 deformation values greater then 0.28 for those nuclei shaded in Fig. 1. Even-even nuclei located to the left of the dotted boundary in Fig. 1 exhibit E(4+)/E(2+)>3.0 and are considered to be well- deformed rotors. This investigation concentrates on the study of low-spin states of nuclei within and bordering the deformed region with Z£58 and N£78. This is a relatively recently established region of deformation and little is known about low-spin states in this region. Extensive in-beam studies have been made in this region, but such studies observe mainly high-spin states. The goal of this work is to establish detailed level schemes, to identify possible Nilsson bands, and to extract deformation parameters. 133PrTW133Nd_>133Pr
N = 66 68 70 72 74 76 78
Fig. 1. Region of interest on the nuclear chart; investigated isotopes are indicated by arrows and half-lives;
asterisks: compound nuclei; shaded area: nuclei with a predicted e2 deformation greater than 0.28; dotted boundary: nuclei to the left exhibit experimental energy ratios E(4+)/E(2+)>3.0; solid diagonal line: odd- proton drip line
2. Experimental procedure
All the experiments discussed in this work were performed at the University Isotope Separator at Oak Ridge (UNISOR) which is operated on-line to the tandem electrostatic accelerator of the Holifield Heavy Ion Research Facility (HHIRF) at the Oak Ridge
National Laboratory. Mass-separated samples of 13?Eu, 137sm ^jj 137pm were produced by the 92Mo(48Ti,xpxn) reactions. Sources of 135Sm, 135»133Pm and 133Nd were produced via the ^Mo(^^Ti,xpxn) reactions. The three investigated decay chains are depicted in Fig. 1. The target consisted of a 97.37% enriched 92Mo foil with a thickness of 2.81mg/cm2. Beam energies for the 46,48*n projectiles provided by the HHIRF accelerator ranged from 216-246 MeV with intensities of 30-50 pnA. Multiscaled spectra of y rays, X rays, and conversion electrons, as well as yyt, Xyt, eyt and eXt coincidences were obtained. The new thermal ion source which has been constructed following a design [2] provided by Kirchner of GSI, Darmstadt gave high yields for rare-earth elements.
3. Results
Gamma rays associated with the decay of *33Sm and ^33Pm were observed for the first time. Gamma rays with energies of 156.8 keV and 369.6 keV exhibiting a half life of 3.7±0.7 sec were identified with the 133sm decay. Earlier half-life measurements were made on delayed proton emission by Bogdanov et al. [3] with a reponed half life of 3.2
sec. The collection time of 25 sec was optimized for the 133pju dtcay (Ti/2=12sec). Gamma rays belonging to the 133pm decay and to die 70 sec 133jsjd decay were readily distinguished. A second independent isotope identification was done by observing gamma- ray coincidences with die characteristic X rays. Because the sources were mass selected, a Z identification specifies the isotope uniquely. Fig 2. shows the yrays coincident with the Nd and Pr Xrays. A distinction between y rays belonging to the Pm and Nd decays was achieved. The following yrays were assigned to the 133Pm decay (energies in keV, relative intensities in brackets): 45(26), 118(30), 139(7), 170(8), 176(46), 180(100), 192(12), 200(11), 218(9), 225(51), 245(13), 266(11), 270(56), 272(63), 291(76), 299(18), 315(6), 319(9), 340(5), 382(13), 408(15), 414(10), 426(8), 487(7), 546(6), 565(7), 587(11), 614(5), 646(5), 722(7), 813(18).
Tentative level schemes have been constructed for 133,135Nd, 135,137pra and
133pr# Our data on the 133Nd decay confirms the work of 133pr by Liang et al. [4]. Many transitions between the low-spin levels of the in-beam work by Hildingsson et al. [5] were observed. A new low-spin isomeric level in 133Nd is seen to decay very strongly to the 402 keV level of ^Pr. This decay has not been reported previously. While our analysis supports the earlier spin assignments made by Liang et al., the ground-state spin assignment of 3/2 is inconsistent with the 5/2 assignment from the atomic beam magnetic resonance measurement of Ekstrom et al. [6]. Further measurements may be required to
clarify this discrepancy. The preliminary analysis on the *35pm and 135j^d isotopes agree with the low-level structures reported by Vierinen et al. [7] and Korthelathi et a!. [8]. Based on the systematics, and the fact that the low-spin levels in 135j,jd are strongly
populated, positive parity low-spin assignments instead of the high-spins for the 135pm would be preferred. A spin sequence of 3/2+,5/2+,5/2+ and 7/2+ would be-consistent with the existing systematics. In Fig. 3, level systematics are shown for the N=76 and N=74 Pm band and Pr isotopes. With the exception of ^^Ptn, all the nuclei show a decoupled h11/2" structure with spin ordering 11/2", 7/2", 15/2", 9/2"> and 13/2". The first four positive parity states show a pattern of 3/2+, 5/2+\5/2+' and 7/2+.
4. Particle-triaxial rotor calculations with a deformed Woods-Saxon potential
Calculations with the Lund code [9] have been performed for the Pm isotopes. Standard parameters were used as much as possible and no attempt was made to fit actual data. The deformation input parameters P, y, |34 were taken from the predictions by Kern et al. [10]. The moment of inertia was estimated from the ground-state band in the even-even neighbors. The pairing gap and Fermi energy were calculated by the program. Fig. 4 summarizes the results for ^7pn^ Tne negative parity states originate from the spherical GRHMfliGRTE ON 36 GRHMfl 380.
320.
u
5 160. 120.
60 .
600 650 700 750 800 650 900 950 1000 CHRNNELS GRHMRiGRTE ON 37x GRNHR 170.
ISO.
190- noj
50 .
30 -
to. 600 sso 700 790 800 850 900 950 1000 CHANNELS Bg. 2. Identification of gammarays in the A=133 mass chain through Nd and Pr X-ray coincidences. Top: Gammarays in coincidence with Pr Xrays; Bottom: Gammirays in coincidence with Nd Xfcays; m hj 1/2* j-sheil where the decoupled band is built mainly on the [532]5/2 Nilsson orbital. The positive parity levels are mainly based on Nilsson states [411J3/2 and [413J5/2 which originate from the ds/2» g7/2 spherical j-shells. Rotational bands built on these Nilsson orbitals have distinct Ml properties which can be used to identify them. In Fig. 4 the calculated bandhead spins and moments and B(M1)/B(E2) ratios in the rotational bands are shown. Comparison of the predicted with the experimental level schemes indicate |3 deformations of 0.20 to 0.25 for the investigated nuclei. 11/2"1• 694+e 9/2+ 623+e 9/2+ 586+e 13/2" 559
9/2+ 469+e
9/2- 380 15/2* 338 15/2" 286
201+e 7/2+ 181+5 in* 5/2+ 182+e 7/2* 163 5/2+ 104+5 5/2+ 55+5 7/2- e 5/2+ 13+e 3/2+ 5 3/2+ e 11/2- 0 11/2" 0 I35p 137Pm
9/2- 799 11/2+ 777 15/2- 731 11/2+ 702 9/2+ 688 9/2+ 619 9/2- 551 9/2+ 517 7/2- 543 15/2- 502 9/2+ 476 U)2- 358 KM pa•— 105us
7/2- 295 245 1 r 7/2+ 225 7/2+ 11/2- 192 5/2+ 166 r—• 5/2+ 206 ABMR 5/2+ E3 r \ 5/2+ 61 5/2+ 41 ^ 3/2+ 0 3/2+ 0
/ 133p r / 135Pr Fig 3. Level systematics for the N=76 and N=74 Pm and Pr isotopes 137 gvf Pm Calculations: W-S potential 0=0.202 7*=-100° P4=-0.013
A = 1.004 MeV 1000 I2 X = -2.966 MeV E2+ = 0.310 MeV 3/2+ 885 9/24 873 1/2* 157 9/2+ 778 (Q = 0) 5/2- 740 3/2- 704 1/2[411J 13/2- 709 (d3/2)
500 7/2+ 493 9/2- 482
5/2+ 402 3/2+ 330 5/2+ 349 15/2- 333 Q=.+0.94b Q = +0.70b BE2 = 0.43 li = +105 H* 5/2(413] 3/2{4111 7/2- 129 (d5/2) Q=-1.29 b ^ = +6.66^ H/2->
Rg. 4 Predicted level structure for
Acknowledgments
We would like to thank the UNISOR and HHIRF staff and especially K. Carter for the help during those experiments. We are also grateful for the collaboration with R. Kirchner (GSI). This work was supported in part by US Department of energy Grant DE-FG05- 87ER 40330 (Georgia Tech), and contract AC05-76ORO 0033 (UNISOR).
References
[1] G. A. Leander and P. M6iler, Phys. Leu. llflfi. (1982) 17 [2] R. Kirchner et ai., NucL Instr. Meth. J&. (1981) 295-305 [3] D. D.BogdanovetaL, NucL Phys. A225. (1977) 229 [4] C. F. Liang et al.. Phvs. Rev. C40. (1989) 2796 [5] L.Hildingssonetal., Phys. Rev. £21(1988) 985 [6] C Ekstrflm et al., Nud. Phys. A12& (1972) 178 17} K. S. Vierinen et al., Nucl. Phys. ^422. (1989) 1-28 [8] M. O. KonhelaUii et al., Z. Physik A332. (1989) 229-230 [9] I. Ragnarsson and P. Semmes, Hyi»rfine Inter., 42 (1988) 425 [10] B. D. Kern et al., Phys. Rev. £26, (1987) 1514 DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or othenvise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.