Neutron-rich 89,91Rb nuclei populated as fission products in heavy-ion reactions have been studied with the Gammasphere array. The previously known level schemes have been extended to higher excitation energies and spins. Spin and parity assignments were based on angular correlation analyses. A value of T1/2 = 8(2) ns was extracted for the isomeric g9/2 state in 89Rb

Broda, R.

Carpenter, M.P.

Fornal, B.

Hoteling, N.

Janssens, R.V.F.

Królas, W.

Pawłat, T.

Walters, W.B.

Wrzesiński, J.

Zhu, S.

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Vol. 40 (2009) ACTA PHYSICA POLONICA B No 3EXPERIMENTAL STUDYOF NEUTRON-RICH NUCLEI 89Rb AND 91Rb∗T. Pawłat, R. Broda, B. Fornal, W. Królas, J. WrzesińskiH. Niewodniczański Institute of Nuclear Physics, Kraków, 31-342, PolandR.V.F. Janssens, S. Zhu, M.P. CarpenterPhysics Division, Argonne National Laboratory, Illinois 60439, USAW.B. Walters, N. HotelingUniversity of Maryland, College Park, Maryland 20742, USA(Received December 15, 2008)Neutron-rich 89,91Rb nuclei populated as fission products in heavy-ionreactions have been studied with the Gammasphere array. The previouslyknown level schemes have been extended to higher excitation energies andspins. Spin and parity assignments were based on angular correlation anal-yses. A value of T1/2 = 8(2) ns was extracted for the isomeric g9/2 state in89Rb.PACS numbers: 23.20.Lv, 25.70.Jj, 27.50.+e, 27.60.+j1. IntroductionNeutron-rich nuclei with masses A ≈ 100 exhibit a shape transition fromspherical to strongly prolate deformed ground states. Sr and Zr isotopes withneutron number between N = 50 and N = 60 have been extensively studiedvia different experiments. Evidence of structural changes in these nuclei hasbeen reported [1,2]. Some experimental information is also available on themedium and high-spin states of the Kr isotopes [3]. Much less is knownabout the structure of odd-Z nuclei in this region, e.g. Y and Rb isotopes.We used the data obtained in a series of experiments performed recently atthe Argonne National Laboratory to investigate high spin states in the 89Rband 91Rb isotopes.∗ Presented at the Zakopane Conference on Nuclear Physics, September 1–7, 2008,Zakopane, Poland.(629)630 T. Pawłat et al.2. ExperimentsThe nuclei 89,91Rb were produced as fragments of the target-like prod-ucts in three experiments which were carried out at the Argonne NationalLaboratory using beams from the Argonne Tandem-Linac Accelerator Sys-tem (ATLAS). For the 48Ca + 208Pb, 48Ca + 238U and 64Ni + 238U collidingsystems the beam energies of 305, 330 and 430 MeV were selected, respec-tively. These correspond to values approx. 20–25% above the Coulomb bar-riers. In all experiments thick, 50 mg/cm2 targets were used. High statisticsgamma-ray coincidence data were collected with the Gammasphere arraywhich consisted of 100 Compton-suppressed Ge detectors. The collected en-ergy and timing information served to sort the data into appropriate cubes,as triple gamma coincidences were required to provide satisfactory selectiv-ity. Coincidence events were also analysed with respect to the angle betweenthe detectors and multipolarities were extracted following a gamma-gammaangular correlation analysis.3. ResultsThe 89Rb nucleus was previously investigated using β− decay and the(α, p) reaction in which low-spin states were populated [4]. Recently, thehigher lying yrast structure above the 9/2+ state was established with multi-nucleon transfer reactions [5]. The present quality of the tripple-gamma co-incidence data allowed us to add 5 new transitions above the known ones.In a double-gate set on the previously known 221 and 974 keV γ rays deex-citing the 1195 keV level, three strong γ lines have been observed identicalwith those already observed in a PRISMA-CLARA experiment [5]. In tripleγ-coincidence spectra obtained by setting double gates on those three γrays, five new transitions were observed and subsequently placed in the levelscheme. The coincidence analyses performed with data from the three exper-iments provided essentially identical results. The 89Rb level scheme deducedfrom the present work is shown in Fig. 1 (left). The resulting angular cor-relation coefficients for the 974–221 keV sequence (a2 = −0.16(4)) supporta quadrupole-dipole character, which is consistent with the 9/2+ assignmentof the 1195 keV level. Similarly, the analysis of the 809–836–1192 keV cas-cade indicates quadrupole-quadrupole character of all transitions involved(a2 = 0.15(8) and a2 = 0.17(5) respectively). Thus, assignments of 13/2+,17/2+ and 21/2+ are adopted for the states at 2005, 2841 and 4034 keV,consistent with previous suggestions [5]. The levels at 5328 and 5606 keVare tentatively assigned 23/2+ and 25/2+, as predicted in shell model cal-culations discussed by the authors of Ref. [5]. The established level schemeallowed to analyse the timing parameter in delayed coincidences betweengamma transitions above and below the 9/2+ state. This enabled the deter-mination of the half-life of T1/2 = 8(2) ns for the 9/2+ isomeric state. Thislevel predominantly decays by an M2 transition.Experimental Study of Neutron-Rich Nuclei 89Rb and 91Rb 63191Rb5437108.9 108.90721.91134.31841.02979.43574.63878.74098.24572.05299.56238.9931.01195.50586.0997.42004.62840.64033.65327.75605.96699.66704.87391.3221.03/2-5/2-(7/2)(7/2)+9/213/2+17/2+(21/2 )+(23/2 )+(25/2 )+89Rb52371294.1 1572.31371.9 1098.9686.51193.0836.0809.1198.0931.0411.5221.0586.0997.4776.5974.4345.0710.0264.48(2)ns17ns1025.3412.4613.03/2-5/2-+9/213/2+17/2+(21/2 )+939.4727.5473.8304.1595.2 1118.81138.4706.7Fig. 1. Level schemes of 89Rb and 91Rb.The 91Rb nucleus has been studied previously in β decay, and therefore,only low-spin states were known prior to our study including the 17 ns 9/2+isomeric state [6]. In the present work the assignment of γ lines to 91Rb isbased on coincidence spectra obtained with double gates set on transitionsbelow this 9/2+ isomer at 1134 keV and confirmed by examination of delayedcoincidences across the isomer. This analysis allowed to identify eight newγ transitions. Their relative intensities and coincidence relations were usedto place them in the level scheme of Fig. 1 (right). A γ–γ angular correlationanalysis settles the quadrupole character of the 707–1138–1119 keV cascade(a2 = 0.18(3) and a2 = 0.09(6), correspondingly). Consequently, spinsand parities 13/2+, 17/2+, 21/2+ are adopted for the states at 1841, 2979,4098 keV, respectively. The statistics was too poor to obtain any significantanisotropy results for other transitions.Fig. 2 shows the evolution of the yrast states observed in odd-A Rbisotopes. This systematics shows that the yrast transitions identified in thiswork follow smoothly the trend observed in lighter nuclei. The evolutionalso supports the assignment of 9/2+ to the 1134 keV state rather than the7/2+ proposed in Ref. [6].632 T. Pawłat et al.100020003000400050006000Rel. energy [ keV ]29/2+25/2+21/2+17/2+15/2+11/2+13/2+81Rb9/2+ 9/2+ 7.8 ms 9/2+ 1.0 sm 9/2+ 6 ns83Rb 85Rb 87Rb 89Rb 91Rb13/2+ 13/2+ 13/2+13/2+13/2+9/2+ 8 ns 9/2+ 17 nsN = 5011/2+17/2+15/2+17/2+15/2+17/2+17/2+17/2+11/2+15/2+21/2+21/2+21/2+19/2+21/2+21/2+23/2+23/2+23/2+25/2+25/2+29/2+29/2+23/2+25/2+0Fig. 2. Systematics of yrast states observed in odd-A Rb isotopes.4. ConclusionsThe 89,91Rb isotopes have been produced in three different reactions. Thealready known high-spin level scheme of 89Rb has been improved and thehalf-life of the 9/2+ isomer was determined. High-spin states in 91Rb havebeen identified for the first time. Spin-parity assignments for the strongesttransitions were obtained from a γ–γ correlation analysis. These new resultsextend the experimental data systematics of Rb isotopes and will be helpfulto further develop the shell model for nuclei with N > 50 and Z < 38.This work is supported by the Polish Ministry of Science, Grants No1P03B05929 and NN202103333, and by the U.S. Dept. of Energy, Office ofNuclear Physics under contract DE-AC02-06CH11357.REFERENCES[1] E.A. Stefanowa et al., Phys. Rev. C65, 034323 (2002).[2] D. Pantelica et al., Phys. Rev. C72, 024304 (2005).[3] T. Rząca-Urban et al., Eur. Phys. J. A9, 165 (2000).[4] B. Singh, Nucl. Data Sheets, 85, 1 (1998).[5] D. Bucurescu et al., Phys. Rev. C76, 064301 (2007).[6] K. Sistemich et al., Z. Phys. A325, 139 (1986).View publication stats

Experimental study of neutron-rich nuclei 89Rb and 91Rb

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Experimental study of neutron-rich nuclei 89Rb and 91Rb

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