Open Access JournalWe present information on the excited states in the prolate-deformed, neutron-rich nuclei 165;167Tb100;102. The nuclei of interest were synthesized following in-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN, Japan. The exotic nuclei were separated and identified event-by-event using the BigRIPS separator, with discrete energy gamma-ray decays from isomeric states with half-lives in the _s regime measured using the EURICA gamma-ray spectrometer. Metastable-state decays are identified in 165Tb and 167Tb and interpreted as arising from hindered E1 decay from the 7/2-[523] single quasi-proton Nilsson configuration to rotational states built on the 3/2-[411] single quasi-proton ground state. These data correspond to the first spectroscopic information in the heaviest, odd-A terbium isotopes reported to date and provide information on proton Nilsson configurations which reside close to the Fermi surface as the 170Dy doubly-midshell nucleus is approached.postprin

Alharbi, T

Baba, H

Berry, TA

Browne, F

Bruce, AM

Carroll, RJ

Catford, WN

Chae, KY

Dombradi, Z

Doornenbal, P

Estrade, A

Fukuda, N

Griffin, C

Gurgi, LA

Ideguchi, E

Inabe, N

Isobe, T

Kanaoka, H

Kanaya, S

Kojouharov, I

Kondev, FG

Korkulu, Z

Kubo, T

Kubono, S

Kurz, N

Kuti, I

Lalkovski, S

Lane, GJ

Lee, CS

Lee, EJ

Lee, P

Liu, HL

Liu, J

Lorusso, G

Lotay, G

Moon, CB

Nishimura, S

Nishizuka, I

Nita, CR

Odahara, A

Patel, Z

Phong, VH

Podolyák, Z

Regan, PH

Roberts, OJ

Sakurai, H

Schaffner, H

Shand, CM

Sumikama, T

Suzuki, H

Söderström, PA

Takeda, H

Terashima, S

Vajta, Z

Valiente-Dobón, JJ

Walker, PM

Watanabe, H

Wu, J

Xu, FR

Xu, Z

Yagi, A

Zhang, GX

Acta Physica Polonica B

We present information on the excited states in the prolate-deformed, neutron-rich nuclei 165;167Tb100;102. The nuclei of interest were synthesized following in-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN, Japan. The exotic nuclei were separated and identified event-by-event using the BigRIPS separator, with discrete energy gamma-ray decays from isomeric states with half-lives in the _s regime measured using the EURICA gamma-ray spectrometer. Metastable-state decays are identified in 165Tb and 167Tb and interpreted as arising from hindered E1 decay from the 7/2-[523] single quasi-proton Nilsson configuration to rotational states built on the 3/2-[411] single quasi-proton ground state. These data correspond to the first spectroscopic information in the heaviest, odd-A terbium isotopes reported to date and provide information on proton Nilsson configurations which reside close to the Fermi surface as the 170Dy doubly-midshell nucleus is approached.postprin

HKU Scholars Hub

Title Isomer Spectroscopy of Neutron-rich 165,167TbAuthor(s)Gurgi, LA; Regan, PH; Söderström, PA; Watanabe, H; Walker,PM; Podolyák, Z; Nishimura, S; Berry, TA; Doornenbal, P;Lorusso, G; Isobe, T; Baba, H; Xu, Z; Sakurai, H; Sumikama, T;Catford, WN; Bruce, AM; Browne, F; Lane, GJ; Kondev, FG;Odahara, A; Wu, J; Liu, HL; Xu, FR; Korkulu, Z; Lee, P; Liu, J;Phong, VH; Yagi, A; Zhang, GX; Alharbi, T; Carroll, RJ; Chae, KY;Dombradi, Z; Estrade, A; Fukuda, N; Griffin, C; Ideguchi, E;Inabe, N; Kanaoka, H; Kojouharov, I; Kubo, T; Kubono, S; Kurz,N; Kuti, I; Lalkovski, S; Lee, EJ; Lee, CS; Lotay, G; Moon, CB;Nishizuka, I; Nita, CR; Patel, Z; Roberts, OJ; Schaffner, H; Shand,CM; Suzuki, H; Takeda, H; Terashima, S; Vajta, Z; Kanaya, S;Valiente-Dobón, JJCitationThe 2016 Zakopane Conference on Nuclear Physics Extremes ofthe Nuclear Landscape, Krakow, Poland, 28 August-4 September2016. In Acta Physica Polonica Series B, 2017, v. 48 n. 3, p. 601-608Issued Date 2017URL http://hdl.handle.net/10722/241032Rights This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.Vol. 48 (2017) ACTA PHYSICA POLONICA B No 3ISOMER SPECTROSCOPY OF NEUTRON-RICH165,167Tb∗L.A. Gurgia, P.H. Regana,b, P.-A. Söderströmc, H. Watanabed,eP.M. Walkera, Zs. Podolyáka, S. Nishimurac, T.A. Berrya, P. DoornenbalcG. Lorussoa,b,c, T. Isobec, H. Babac, Z.Y. Xuf,g, H. Sakuraic,h, T. Sumikamac,iW.N. Catforda, A.M. Brucej, F. Brownej, G.J. Lanek, F.G. KondevlA. Odaharam, J. Wuc,n, H.L. Liuo, F.R. Xun, Z. Korkuluc,p, P. Leeq, J.J. LiufV.H. Phongc,r, A. Yagim, G.X. Zhango, T. Alharbis, R.J. CarrollaK.Y. Chaet, Zs. Dombradip, A. Estradeg,u, N. Fukudac, C. GriffinuE. Ideguchim,v, N. Inabec, H. Kanaokam, I. Kojouharovw, T. KubocS. Kubonoc, N. Kurzw, I. Kutip, S. Lalkovskia, E.J. Leet, C.S. LeeqG. Lotaya, C.B. Moonx, I. Nishizukai, C.R. Nitaj,y, Z. Patela, O.J. RobertszH. Schaffnerw, C.M. Shanda, H. Suzukic, H. Takedac, S. TerashimaeZs. Vajtap, S. Kanayam, J.J. Valiente-DobónαaDepartment of Physics, University of Surrey, Guildford, UKbNational Physical Laboratory, Teddington, UKcRIKEN Nishina Center, 21 Hirosawa, Wako-shi, Saitama, JapandInternational Research Center for Nuclei and Particles in the Cosmos, Beihang UniversityBeijing, ChinaeSchool of Physics and Nuclear Energy Engineering, Beihang University, Beijing, ChinafDepartment of Physics, the University of Hong Kong, Pokfulam Road, Hong KonggKU Leuven, Instituut voor Kern- en Stralingsfysica, Leuven, BelgiumhDepartment of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, JapaniDepartment of Physics, Tohoku University, Aoba, Sendai, Miyagi, JapanjSchool of Computing, Engineering and Mathematics, University of Brighton, UKkDepartment of Nuclear Physics, R.S.P.E., Australian National UniversityCanberra, A.C.T.02000, AustralialNuclear Engineering Division, Argonne National Laboratory, Argonne, Illinois, USAmDep. of Physics, Osaka Uni., Machikaneyama-machi 1-1, Osaka, Toyonaka, JapannSchool of Physics, Peking University, Beijing, ChinaoDepartment of Applied Physics, School of Science, Xin Jiaotong University, ChinapInstitute for Nuclear Research, Hungarian Academy of SciencesP.O. Box 51, Debrecen, HungaryqDepartment of Physics, Chung-Ang University, Seoul, Republic of KorearVNU Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, VietnamsDepartment of Physics, College of Science in Zulfi, Almajmaah UniversityP.O. Box 1712, 11932, Saudi ArabiatDepartment of Physics, Sungkyunkwan University, Suwon, KoreauSchool of Physics and Astronomy, University of Edinburgh, Edinburgh, UKvResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka, JapanwGSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, GermanyxHoseo University, Asan, Chungnam, KoreayHoria Hulubei National Institute of Physics and Nuclear Engineering, (IFIN-HH)Bucharest, RomaniazSchool of Physics, University College Dublin, Belfield, Dublin 4, IrelandαInstituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Legnaro, Italy(Received January 17, 2017)∗ Presented at the Zakopane Conference on Nuclear Physics “Extremes of the NuclearLandscape”, Zakopane, Poland, August 28–September 4, 2016.(601)602 L.A. Gurgi et al.We present information on the excited states in the prolate-deformed,neutron-rich nuclei 165,167Tb100,102. The nuclei of interest were synthesisedfollowing in-flight fission of a 345 MeV per nucleon 238U primary beam ona 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN,Japan. The exotic nuclei were separated and identified event-by-event us-ing the BigRIPS separator, with discrete energy gamma-ray decays fromisomeric states with half-lives in the µs regime measured using the EURICAgamma-ray spectrometer. Metastable-state decays are identified in 165Tband 167Tb and interpreted as arising from hindered E1 decay from the72−[523] single quasi-proton Nilsson configuration to rotational states builton the 32−[411] single quasi-proton ground state. These data correspond tothe first spectroscopic information in the heaviest, odd-A terbium isotopesreported to date and provide information on proton Nilsson configurationswhich reside close to the Fermi surface as the 170Dy doubly-midshell nucleusis approached.DOI:10.5506/APhysPolB.48.6011. IntroductionThe single-particle structure of states in quadrupole-deformed nuclei inthe vicinity of the 170Dy valence maximum nucleus can be used as a measureof the magnitude of the (prolate) deformation in these systems [1, 2]. Theterbium isotopes (Z = 65) correspond to a single proton–hole in the 170Dydeformed core [3] and, therefore, their structure can be used to probe theevolution of competing, prolate-deformed single-particle structures in thisregion of the Segré chart. Prior to this work, 163Tb was the heaviest isotopeof this element for which excited state information had been reported [4] andthe systematics of the lighter odd-A Tb isotopes 161,163Tb are consistentwith ground states built on the same 32+[411] Nilsson single quasi-protonconfiguration [1, 2, 4, 5]. Here, we report on the decay of isomeric states inthe neutron-rich systems 165Tb and 167Tb, and compare the deduced levelschemes with long-standing predictions of which quasi-proton orbitals arefavoured in this well-deformed region of the nuclear chart.2. Experimental details and resultsThe nuclei of interest were created following production via the projec-tile fission mechanism at the Radioactive Isotope Beam Factory (RIBF) [6],RIKEN, Japan. The production synthesis used collisions between a 238Uprimary beam of the energy of 350 MeV/u on a 2 mm thick Be targetwith a typical on-target beam current of 10 pnA. The secondary cocktailbeam of in-flight fission fragments was separated using the BigRIPS Sepa-rator and the ZeroDegree spectrometer [7, 8] at RIBF. Specific radionuclideIsomer Spectroscopy of Neutron-rich 165,167Tb 603species could be tagged on an event-by-event basis using measured parame-ters of magnetic rigidity (Bρ), Time-of-Flight (ToF) and energy loss (∆E)for the ions as they passed through the separator [9, 10]. The primary fis-sion residues were subsequently stopped in the Wide-range Active SiliconStrip Stopper Array (WAS3ABI) [11] which allowed direct detection of theimplanted heavy ion and also subsequent position correlated β-decay eventsfrom the same ion. The WAS3ABI stopper was surrounded by the EuroballRIKEN Cluster Array (EURICA) which allowed the detection of the delayedgamma rays emitted following the decay of heavy-ion correlated isomeric orbeta-delayed events. In this experiment, EURICA consisted of 84 coaxialHigh-Purity Germanium (HPGe) detectors, arranged in an array of 12 × 7element CLUSTER detector modules, complemented by 18 LaBr3(Ce) de-tectors for fast-timing measurements [12–14].The current work reports on data from two distinct magnetic rigidity(Bρ) settings: one centred on the transmission of 170Dy ions, which ran for13.5 hours [3]; and a second one centred on the transmission of 172Dy [15]for 45 hours. The particle-identification (PID) plots for both settings areshown in figure 1. These PID plots show ions which are transmitted both asfully-stripped (Q = Z) and also as hydrogen-like (Q = Z − 1) species. Bygating on defined isotopes, gamma-ray energies measured in the EURICAspectrometer were correlated, event-by-event with isomeric decays. Figure 2shows the EURICA gamma-ray spectra gated on isomer-delayed decays from165Tb and 167Tb in the current work.2.54 2.56 2.58 2.6 2.62 2.64Z62646668Tb167 Tb165Dy170Tb168Tb169Gd167Eu162A/Q2.54 2.56 2.58 2.6 2.62 2.64Z62646668Tb166 Tb167 Tb166 Tb167Ho173 Dy168Ho171 Dy172 Tb168Fig. 1. (Colour on-line) Particle identification plots obtained from the two BigRIPSsettings used in this work. (Upper) PID for the first setting centred on 170Dy,identifying the H-like 165Tb and fully-stripped 167Tb ions: (Lower) PID for thesecond setting centred on 172Dy, identifying fully-stripped (grey/red circle) andH-like (black/blue circle) 167Tb events.604 L.A. Gurgi et al.Counts / keV02468101214161820 = 44 αK = 51βK76ab152 55Counts 0510sµ=0.82(14)1/2T76 keVTime (ns)0 2000 4000Counts 0510 sµ=0.8(1)1/2T152 keV(a)024681012141618 = 44 αK 53 = 51βK73147 0 5000 10000Counts 02468sµ=2.2(6)1/2T73 keVTime (ns)0 5000 10000Counts 051015s µ=2.3(4)1/2T147 keVd(b)Energy (keV)0 50 100 150 200 250 300Counts / keV0510152025303540 = 44αK = 51βK73 147530 5000 10000 15000Counts 0246 sµ=2.7(7)1/2T73 keVTime (ns)0 10000Counts 0102030 sµ=2.2(3)1/2T 147 keV(c)Energy (keV)0 50 100 150 200 250 300020406080100120 = 44αK = 51βK53 731470 5000 10000 15000Counts 010203040sµ=2.0(2)1/2T73 keV Time (ns)0 10000Counts 02040 sµ=2.1(3)1/2T 147 keV(d)Fig. 2. Ion-gated, delayed HPGe gamma-ray spectra for EURICA showing transi-tions associated with the isomeric decays in (a) H-like 165Tb and (b) fully-stripped167Tb from the 170Dy centred setting; and (c) fully-stripped and (d) H-like 167Tbions in the 172Dy centred setting. The gamma-ray coincidence time condition usedto generate these spectra were 400 ns to 2 µs after the 165Tb implantation inWAS3ABI and 200 ns to 5 µs for 167Tb. The insets in each spectrum show theprojected time distribution after of the measured gamma-ray transitions followingimplanation, fitted to a single-component exponential decay.The spectrum shown in figure 2 (a) identifies gamma rays following theisomeric decay in 165Tb. Discrete energy peaks are evident at 76 and 152 keVand a smaller peak is noted at 55 keV; this latter peak can be resolvedfrom the observed characteristic terbium K X-ray peaks at 44 and 51 keV.Contaminant peaks, labelled “a” and “b”, are also noted in this spectrumfrom overlaps with misidentified 168Tb [16] and 170Dy [3] ions. The timedistributions for the 76 keV and 152 keV transitions yields fitted values of0.82(14) µs and 0.8(1) µs, implying that both of these transition energiesare associated with the decay of a single isomeric state. The weighted meanof these two measurements gives a half-life value for the isomeric state in165Tb of T1/2 = 0.81(8) µs.The delayed gamma-ray spectra associated with fully stripped andH-like 167Tb ions are also shown in figure 2. Discrete energy peaks at 53, 73and 147 keV are evident, together with the expected K X-rays from terbium,which are assumed to arise from the competing conversion electron decayprocess. The time distributions for the measurement of the 73 and 147 keVIsomer Spectroscopy of Neutron-rich 165,167Tb 605lines relative to the implantation time are shown for the three different 167Tbidentification positions, with a weighted mean of these six independent half-life distributions of T1/2 = 2.1(1) µs.Figure 3 shows the gamma-ray coincidence spectra associated with gateson transitions identified in the isomeric decay of 165Tb and 167Tb. Gamma-ray coincidences between these identified transitions were investigated for167Tb. These show that the 73 keV transition is a self-coincident doubletwith additional coincidences with the Tb K X-rays. This suggests that oneof the members of the 73 keV doublet has a relatively large internal elec-tron conversion coefficient and perhaps that the two members of the doublethave different multipolarities. The coincidence data and energy sum consid-erations imply that the 147 keV decay transition provides a parallel decaypath from the isomeric state in 167Tb which competes with a cascade oftwo, mutually coincident transitions of energy 73 keV. Both of these com-peting decay paths appear to populate the first excited state at the energyof 53 keV. The internal conversion coefficients and intensity balances (seeTable I) for the proposed level scheme imply an E1 multipolarity for the147 keV transiton (α(E1:147 keV) = 0.1067) and an M1 decay for the 53keV transition (α(M1:53 keV) = 13.5). The 73 keV doublet is interpreted asbeing an E1 transition (α(E1:73 keV) = 0.694) followed by a predominantlyM1 in-cascade decay (α(M1: 73 keV) = 5.44). For 165Tb, the statistics arepoor, but essentially the same behaviour is evident, in this case, two 76 keVtransitions in parallel with a 152 keV transition. The tentative level schemesassociated with the isomeric decays in 165Tb and 167Tb as proposed in thecurrent work are shown in figure 4.Counts / keV-2-1012 76 Gated on 76 keV(a)Energy (keV)0 20 40 60 80 100 120 140 160 180 200Counts / keV01234 αKβKGated on 152 keV(b)-4-20246  αK βK73 Gated on 73 keV(c)Energy (keV)0 20 40 60 80 100 120 140 160 180 200-202468αK Gated on 147 keV(d)Fig. 3. Left: Gamma-ray coincidence spectra gated on the (a) 76 keV and(b) 152 keV transitions associated with the isomeric decay of 165Tb. Right:Gamma-ray coincidence spectra gated on the (c) 73 keV and (d) 147 keV tran-sitions associated with the isomeric decay of 167Tb observed in the current work.606 L.A. Gurgi et al.TABLE ISingle quasi-particle isomers in 165,167Tb decaying via E1 and M1 transitions forfirst setting.Nucleus Eγ [keV] Ipii Irel αtot Irel(αtot + 1)165Tb(H) 152(2)E1(72−) 1.9(3) 0.0975(14) 2.12(27)76(2)E1(72+)0.72(10) 0.624(9) 1.17(16)76(2)M1(52+)0.20(3) 4.84(7) 1.17(18)55(4)M1(32+)0.21(3) 12.28(18) 2.78(44)167Tb(F) 147(4)E1(72−) 1.2(2) 0.1067(15) 1.33(18)73(4)E1(72+)0.48(6) 0.694(10) 0.81(11)73(4)M1(52+)0.13(2) 5.44(8) 0.83(18)53(4)M1(32+)0.21(4) 13.51(19) 3.05(59)H → Hydrogen-Like; F → Fully-StrippedFig. 4. Tentative level schemes of 165Tb and 165Tb populated in the isomeric decaysreported in the current work.From the reported systematics of lighter, prolate-deformed odd-A Tbisotopes, 161Tb and 163Tb [4, 5], the likely ground state spin/parities for165,7Tb are Ipi = 32+ for both. These are associated with the occupationof the 32+[411] orbital and consistent with the predictions of equilibriumdeformation calculations for axially-symmetric, prolate-deformed nuclei asreported by Jain et al. [1] and Nazarewicz et al. [2]. The other favoured,odd-proton configurations predicted to lie close to the Fermi surface forZ = 65 are associated with the 52+[413], 72−[523] and 92+[404] Nilsson de-formed single-particle states. The isomeric nature of the observed statesand deduced partial level schemes for 165Tb and 167Tb are consistent withhindered decays from an isomeric state to rotational states built on the(likely) positive parity 32+ ground state. The most probable candidate forIsomer Spectroscopy of Neutron-rich 165,167Tb 607the isomeric states would then be the pi 72−[523] orbital which would requirea (naturally) hindered E1 decay to the positive parity states built on theground state configuration.3. ConclusionsIn-flight fission of 238U has been used to synthesise the neutron-richdeformed nuclei 165Tb and 167Tb. The first published information on ex-cited states in these systems is presented in the current work followingisomer-delayed gamma-ray spectroscopic analysis. In both systems, one-quasi-proton isomeric states with Ipi = (72−) states are identified in theseprolate deformed nuclei with decay half-lives of T1/2 = 0.81(8) µs and T1/2 =2.1(1) µs in 165Tb and 167Tb respectively. The deduced level schemes areconsistent with E1 multipolarity decays from these metastable states builton the 72−[523] one quasi-proton states, which decay through rotationally-coupled states built on the deformed pi 32+[411] ground state configuration.The authors are indebted to the facility crews who provided the beamsat RIBF, operated by RIKEN Nishina Center and CNS, University of Tokyo,the EUROBALL Owners Committee for the loan of germanium detectors,the PreSpec Collaboration for the use of the readout electronics. Part ofthe WAS3ABi was supported by the Rare Isotope Science Project which isfunded by MSIP and NRF of Korea. This work is supported by the UKScience and Technology Facilities Council (STFC); the UK National Mea-surement Office (NMO, P.H.R.); JSPS KAKENHI grant Nos. 24740188,25247045, and 25287065; the U.S. Department of Energy, Office of Sci-ence, Office of Nuclear Physics under contract No. DE-AC02-06CH11357(F.G.K.); NRF Korea grants Nos. 2009-0093817 and 2013R1A1A2063017(C.S.L.), 2015R1D1A1A01056918, 2016R1A5A1013277 and 2016K1A3A7A09005579 (K.Y.C.); the Hungarian Research Fund OTKA contractNo. K100835 and Science Foundation Ireland under grant No. 12/IP/1288(O.J.R.).REFERENCES[1] A.K. Jain et al., Rev. Mod. Phys. 62, 393 (1990).[2] W. Nazarewicz, M.A. Riley, J.D. Garrett, Nucl. Phys. A 512, 61 (1990).[3] P.A. Söderström et al., Phys. Lett. B 762, 404 (2016).[4] C.W. Reich, Nucl. Data Sheets 111, 1211 (2010).[5] C.W. Reich, Nucl. Data Sheets 112, 2497 (2011).608 L.A. Gurgi et al.[6] Y. Yano, Nucl. Instrum. Methods Phys. Res. B 261, 1009 (2007).[7] T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97 (2003).[8] T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003 (2012).[9] K.-H. Schmidt et al., Nucl. Instrum. Methods Phys. Res. A 260, 287 (1987).[10] H. Suzuki et al., Nucl. Instrum. Methods Phys. Res. B 317, 756 (2013).[11] S. Nishimura, Prog. Theor. Exp. Phys. 2012, 03C006 (2012).[12] P.-A. Söderström et al., Nucl. Instrum. Methods Phys. Res. B 317, 649(2013).[13] Z. Patel et al., RIKEN Accel. Prog. Rep. 47, (2014).[14] F. Browne et al., Phys. Lett. B 750, 448 (2015).[15] H. Watanabe et al., Phys. Lett. B 760, 641 (2016).[16] L.A. Gurgi et al., submitted to Radiation Physics and Chemistry (2016).

Isomer Spectroscopy of Neutron-rich 165,167Tb

L.A. Gurgi

P.H. Regan

P.-A. Söderström

H. Watanabe

P.M. Walker

Zs. Podolyák

S. Nishimura

T.A. Berry

P. Doornenbal

G. Lorusso

T. Isobe

H. Baba

Z.Y. Xu

H. Sakurai

T. Sumikama

W.N. Catford

A.M. Bruce

F. Browne

G.J. Lane

F.G. Kondev

A. Odahara

J. Wu

H.L. Liu

F.R. Xu

Z. Korkulu

P. Lee

J.J. Liu

V.H. Phong

A. Yagi

G.X. Zhang

T. Alharbi

R.J. Carroll

K.Y. Chae

Zs. Dombradi

A. Estrade

N. Fukuda

C. Griffin

E. Ideguchi

N. Inabe

H. Kanaoka

I. Kojouharov

T. Kubo

S. Kubono

N. Kurz

I. Kuti

S. Lalkovski

E.J. Lee

C.S. Lee

G. Lotay

C.B. Moon

I. Nishizuka

C.R. Nita

Z. Patel

O.J. Roberts

H. Schaffner

C.M. Shand

H. Suzuki

H. Takeda

S. Terashima

Zs. Vajta

S. Kanaya

J.J. Valiente-Dobón

Crossref

Isomer Spectroscopy of Neutron-rich $^{165,167}$Tb

We present information on the excited states in the prolate-deformed, neutron-rich nuclei 165;167Tb100;102. The nuclei of interest were synthesised following in-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN, Japan. The exotic nuclei were separated and identified event-by-event using the BigRIPS separator, with discrete energy gamma-ray decays from isomeric states with half-lives in the s regime measured using the EURICA gamma-ray spectrometer. Metastable-state decays are identified in 165Tb and 167Tb and interpreted as arising from hindered E1 decay from the 7 2 [523] single quasi-proton Nilsson configuration to rotational states built on the 3 2 [411] single quasi-proton ground state. These data correspond to the first spectroscopic information in the heaviest, odd-A terbium isotopes reported to date and provide information on proton Nilsson configurations which reside close to the Fermi surface as the 170Dy doubly-midshell nucleus is approached

Gurgi, Laila

Regan, Patrick

Söderström, P-A

Watanabe, H

Walker, Philip

Podolyak, Zsolt

Nishimura, S

Berry, Thomas

Doornenbal, P

Lorusso, G

Isobe, T

Baba, H

Xu, ZY

Sakurai, H

Sumikama, T

Catford, Wilton

Bruce, AM

Browne, F

Lane, GJ

Kondev, FG

Odahara, A

Wu, J

Liu, HL

Xu, FR

Korkulu, Z

Lee, P

Liu, JJ

Phong, VH

Yagi, A

Zhang, GX

Alharbi, T

Carroll, RJ

Chae, KY

Dombradi, Z

Estrade, A

Fukuda, N

Griffin, C

Ideguchi, E

Inabe, N

Kanaoka, H

Kojouharov, I

Kubo, T

Kubono, S

Kurz, N

Kuti, I

Lalkovski, S

Lee, EJ

Lee, CS

Lotay, Gavin

Moon, CB

Nishizuka, I

Nita, CR

Patel, Z

Roberts, OJ

Schaffner, H

Shand, Callum

Suzuki, H

Takeda, H

Terashima, S

Vajta, Z

Kanaya, S

Valiente-Dobón, J-J

University of Surrey

Vol. 48 (2017) ACTA PHYSICA POLONICA B No 3ISOMER SPECTROSCOPY OF NEUTRON-RICH165,167Tb∗L.A. Gurgia, P.H. Regana,b, P.-A. Söderströmc, H. Watanabed,eP.M. Walkera, Zs. Podolyáka, S. Nishimurac, T.A. Berrya, P. DoornenbalcG. Lorussoa,b,c, T. Isobec, H. Babac, Z.Y. Xuf,g, H. Sakuraic,h, T. Sumikamac,iW.N. Catforda, A.M. Brucej, F. Brownej, G.J. Lanek, F.G. KondevlA. Odaharam, J. Wuc,n, H.L. Liuo, F.R. Xun, Z. Korkuluc,p, P. Leeq, J.J. LiufV.H. Phongc,r, A. Yagim, G.X. Zhango, T. Alharbis, R.J. CarrollaK.Y. Chaet, Zs. Dombradip, A. Estradeg,u, N. Fukudac, C. GriffinuE. Ideguchim,v, N. Inabec, H. Kanaokam, I. Kojouharovw, T. KubocS. Kubonoc, N. Kurzw, I. Kutip, S. Lalkovskia, E.J. Leet, C.S. LeeqG. Lotaya, C.B. Moonx, I. Nishizukai, C.R. Nitaj,y, Z. Patela, O.J. RobertszH. Schaffnerw, C.M. Shanda, H. Suzukic, H. Takedac, S. TerashimaeZs. Vajtap, S. Kanayam, J.J. Valiente-DobónαaDepartment of Physics, University of Surrey, Guildford, UKbNational Physical Laboratory, Teddington, UKcRIKEN Nishina Center, 21 Hirosawa, Wako-shi, Saitama, JapandInternational Research Center for Nuclei and Particles in the Cosmos, Beihang UniversityBeijing, ChinaeSchool of Physics and Nuclear Energy Engineering, Beihang University, Beijing, ChinafDepartment of Physics, the University of Hong Kong, Pokfulam Road, Hong KonggKU Leuven, Instituut voor Kern- en Stralingsfysica, Leuven, BelgiumhDepartment of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, JapaniDepartment of Physics, Tohoku University, Aoba, Sendai, Miyagi, JapanjSchool of Computing, Engineering and Mathematics, University of Brighton, UKkDepartment of Nuclear Physics, R.S.P.E., Australian National UniversityCanberra, A.C.T.02000, AustralialNuclear Engineering Division, Argonne National Laboratory, Argonne, Illinois, USAmDep. of Physics, Osaka Uni., Machikaneyama-machi 1-1, Osaka, Toyonaka, JapannSchool of Physics, Peking University, Beijing, ChinaoDepartment of Applied Physics, School of Science, Xin Jiaotong University, ChinapInstitute for Nuclear Research, Hungarian Academy of SciencesP.O. Box 51, Debrecen, HungaryqDepartment of Physics, Chung-Ang University, Seoul, Republic of KorearVNU Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, VietnamsDepartment of Physics, College of Science in Zulfi, Almajmaah UniversityP.O. Box 1712, 11932, Saudi ArabiatDepartment of Physics, Sungkyunkwan University, Suwon, KoreauSchool of Physics and Astronomy, University of Edinburgh, Edinburgh, UKvResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka, JapanwGSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, GermanyxHoseo University, Asan, Chungnam, KoreayHoria Hulubei National Institute of Physics and Nuclear Engineering, (IFIN-HH)Bucharest, RomaniazSchool of Physics, University College Dublin, Belfield, Dublin 4, IrelandαInstituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Legnaro, Italy(Received January 17, 2017)∗ Presented at the Zakopane Conference on Nuclear Physics “Extremes of the NuclearLandscape”, Zakopane, Poland, August 28–September 4, 2016.(601)602 L.A. Gurgi et al.We present information on the excited states in the prolate-deformed,neutron-rich nuclei 165,167Tb100,102. The nuclei of interest were synthesisedfollowing in-flight fission of a 345 MeV per nucleon 238U primary beam ona 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN,Japan. The exotic nuclei were separated and identified event-by-event us-ing the BigRIPS separator, with discrete energy gamma-ray decays fromisomeric states with half-lives in the µs regime measured using the EURICAgamma-ray spectrometer. Metastable-state decays are identified in 165Tband 167Tb and interpreted as arising from hindered E1 decay from the72−[523] single quasi-proton Nilsson configuration to rotational states builton the 32−[411] single quasi-proton ground state. These data correspond tothe first spectroscopic information in the heaviest, odd-A terbium isotopesreported to date and provide information on proton Nilsson configurationswhich reside close to the Fermi surface as the 170Dy doubly-midshell nucleusis approached.DOI:10.5506/APhysPolB.48.6011. IntroductionThe single-particle structure of states in quadrupole-deformed nuclei inthe vicinity of the 170Dy valence maximum nucleus can be used as a measureof the magnitude of the (prolate) deformation in these systems [1, 2]. Theterbium isotopes (Z = 65) correspond to a single proton–hole in the 170Dydeformed core [3] and, therefore, their structure can be used to probe theevolution of competing, prolate-deformed single-particle structures in thisregion of the Segré chart. Prior to this work, 163Tb was the heaviest isotopeof this element for which excited state information had been reported [4] andthe systematics of the lighter odd-A Tb isotopes 161,163Tb are consistentwith ground states built on the same 32+[411] Nilsson single quasi-protonconfiguration [1, 2, 4, 5]. Here, we report on the decay of isomeric states inthe neutron-rich systems 165Tb and 167Tb, and compare the deduced levelschemes with long-standing predictions of which quasi-proton orbitals arefavoured in this well-deformed region of the nuclear chart.2. Experimental details and resultsThe nuclei of interest were created following production via the projec-tile fission mechanism at the Radioactive Isotope Beam Factory (RIBF) [6],RIKEN, Japan. The production synthesis used collisions between a 238Uprimary beam of the energy of 350 MeV/u on a 2 mm thick Be targetwith a typical on-target beam current of 10 pnA. The secondary cocktailbeam of in-flight fission fragments was separated using the BigRIPS Sepa-rator and the ZeroDegree spectrometer [7, 8] at RIBF. Specific radionuclideIsomer Spectroscopy of Neutron-rich 165,167Tb 603species could be tagged on an event-by-event basis using measured parame-ters of magnetic rigidity (Bρ), Time-of-Flight (ToF) and energy loss (∆E)for the ions as they passed through the separator [9, 10]. The primary fis-sion residues were subsequently stopped in the Wide-range Active SiliconStrip Stopper Array (WAS3ABI) [11] which allowed direct detection of theimplanted heavy ion and also subsequent position correlated β-decay eventsfrom the same ion. The WAS3ABI stopper was surrounded by the EuroballRIKEN Cluster Array (EURICA) which allowed the detection of the delayedgamma rays emitted following the decay of heavy-ion correlated isomeric orbeta-delayed events. In this experiment, EURICA consisted of 84 coaxialHigh-Purity Germanium (HPGe) detectors, arranged in an array of 12 × 7element CLUSTER detector modules, complemented by 18 LaBr3(Ce) de-tectors for fast-timing measurements [12–14].The current work reports on data from two distinct magnetic rigidity(Bρ) settings: one centred on the transmission of 170Dy ions, which ran for13.5 hours [3]; and a second one centred on the transmission of 172Dy [15]for 45 hours. The particle-identification (PID) plots for both settings areshown in figure 1. These PID plots show ions which are transmitted both asfully-stripped (Q = Z) and also as hydrogen-like (Q = Z − 1) species. Bygating on defined isotopes, gamma-ray energies measured in the EURICAspectrometer were correlated, event-by-event with isomeric decays. Figure 2shows the EURICA gamma-ray spectra gated on isomer-delayed decays from165Tb and 167Tb in the current work.2.54 2.56 2.58 2.6 2.62 2.64Z62646668Tb167Tb165Dy170Tb168Tb169Gd167Eu162A/Q2.54 2.56 2.58 2.6 2.62 2.64Z62646668Tb166 Tb167Tb166 Tb167Ho173 Dy168Ho171Dy172Tb168Fig. 1. (Colour on-line) Particle identification plots obtained from the two BigRIPSsettings used in this work. (Upper) PID for the first setting centred on 170Dy,identifying the H-like 165Tb and fully-stripped 167Tb ions: (Lower) PID for thesecond setting centred on 172Dy, identifying fully-stripped (grey/red circle) andH-like (black/blue circle) 167Tb events.604 L.A. Gurgi et al.Counts / keV02468101214161820 = 44 αK = 51βK76ab152 55Counts 0510sµ=0.82(14)1/2T76 keVTime (ns)0 2000 4000Counts 0510 sµ=0.8(1)1/2T152 keV(a)024681012141618 = 44 αK53 = 51βK73147 0 5000 10000Counts 02468sµ=2.2(6)1/2T73 keVTime (ns)0 5000 10000Counts 051015s µ=2.3(4)1/2T147 keVd(b)Energy (keV)0 50 100 150 200 250 300Counts / keV0510152025303540 = 44αK = 51βK73 147530 5000 10000 15000Counts 0246 sµ=2.7(7)1/2T73 keVTime (ns)0 10000Counts 0102030 sµ=2.2(3)1/2T147 keV(c)Energy (keV)0 50 100 150 200 250 300020406080100120 = 44αK = 51βK53 731470 5000 10000 15000Counts 010203040sµ=2.0(2)1/2T73 keV Time (ns)0 10000Counts 02040 sµ=2.1(3)1/2T147 keV(d)Fig. 2. Ion-gated, delayed HPGe gamma-ray spectra for EURICA showing transi-tions associated with the isomeric decays in (a) H-like 165Tb and (b) fully-stripped167Tb from the 170Dy centred setting; and (c) fully-stripped and (d) H-like 167Tbions in the 172Dy centred setting. The gamma-ray coincidence time condition usedto generate these spectra were 400 ns to 2 µs after the 165Tb implantation inWAS3ABI and 200 ns to 5 µs for 167Tb. The insets in each spectrum show theprojected time distribution after of the measured gamma-ray transitions followingimplanation, fitted to a single-component exponential decay.The spectrum shown in figure 2 (a) identifies gamma rays following theisomeric decay in 165Tb. Discrete energy peaks are evident at 76 and 152 keVand a smaller peak is noted at 55 keV; this latter peak can be resolvedfrom the observed characteristic terbium K X-ray peaks at 44 and 51 keV.Contaminant peaks, labelled “a” and “b”, are also noted in this spectrumfrom overlaps with misidentified 168Tb [16] and 170Dy [3] ions. The timedistributions for the 76 keV and 152 keV transitions yields fitted values of0.82(14) µs and 0.8(1) µs, implying that both of these transition energiesare associated with the decay of a single isomeric state. The weighted meanof these two measurements gives a half-life value for the isomeric state in165Tb of T1/2 = 0.81(8) µs.The delayed gamma-ray spectra associated with fully stripped andH-like 167Tb ions are also shown in figure 2. Discrete energy peaks at 53, 73and 147 keV are evident, together with the expected K X-rays from terbium,which are assumed to arise from the competing conversion electron decayprocess. The time distributions for the measurement of the 73 and 147 keVIsomer Spectroscopy of Neutron-rich 165,167Tb 605lines relative to the implantation time are shown for the three different 167Tbidentification positions, with a weighted mean of these six independent half-life distributions of T1/2 = 2.1(1) µs.Figure 3 shows the gamma-ray coincidence spectra associated with gateson transitions identified in the isomeric decay of 165Tb and 167Tb. Gamma-ray coincidences between these identified transitions were investigated for167Tb. These show that the 73 keV transition is a self-coincident doubletwith additional coincidences with the Tb K X-rays. This suggests that oneof the members of the 73 keV doublet has a relatively large internal elec-tron conversion coefficient and perhaps that the two members of the doublethave different multipolarities. The coincidence data and energy sum consid-erations imply that the 147 keV decay transition provides a parallel decaypath from the isomeric state in 167Tb which competes with a cascade oftwo, mutually coincident transitions of energy 73 keV. Both of these com-peting decay paths appear to populate the first excited state at the energyof 53 keV. The internal conversion coefficients and intensity balances (seeTable I) for the proposed level scheme imply an E1 multipolarity for the147 keV transiton (α(E1:147 keV) = 0.1067) and an M1 decay for the 53keV transition (α(M1:53 keV) = 13.5). The 73 keV doublet is interpreted asbeing an E1 transition (α(E1:73 keV) = 0.694) followed by a predominantlyM1 in-cascade decay (α(M1: 73 keV) = 5.44). For 165Tb, the statistics arepoor, but essentially the same behaviour is evident, in this case, two 76 keVtransitions in parallel with a 152 keV transition. The tentative level schemesassociated with the isomeric decays in 165Tb and 167Tb as proposed in thecurrent work are shown in figure 4.Counts / keV-2-101276 Gated on 76 keV(a)Energy (keV)0 20 40 60 80 100 120 140 160 180 200Counts / keV01234 αKβKGated on 152 keV(b)-4-20246  αKβK73 Gated on 73 keV(c)Energy (keV)0 20 40 60 80 100 120 140 160 180 200-202468αKGated on 147 keV(d)Fig. 3. Left: Gamma-ray coincidence spectra gated on the (a) 76 keV and(b) 152 keV transitions associated with the isomeric decay of 165Tb. Right:Gamma-ray coincidence spectra gated on the (c) 73 keV and (d) 147 keV tran-sitions associated with the isomeric decay of 167Tb observed in the current work.606 L.A. Gurgi et al.TABLE ISingle quasi-particle isomers in 165,167Tb decaying via E1 and M1 transitions forfirst setting.Nucleus Eγ [keV] Iπi Irel αtot Irel(αtot + 1)165Tb(H) 152(2)E1(72−)1.9(3) 0.0975(14) 2.12(27)76(2)E1(72+)0.72(10) 0.624(9) 1.17(16)76(2)M1(52+)0.20(3) 4.84(7) 1.17(18)55(4)M1(32+)0.21(3) 12.28(18) 2.78(44)167Tb(F) 147(4)E1(72−)1.2(2) 0.1067(15) 1.33(18)73(4)E1(72+)0.48(6) 0.694(10) 0.81(11)73(4)M1(52+)0.13(2) 5.44(8) 0.83(18)53(4)M1(32+)0.21(4) 13.51(19) 3.05(59)H → Hydrogen-Like; F → Fully-StrippedFig. 4. Tentative level schemes of 165Tb and 165Tb populated in the isomeric decaysreported in the current work.From the reported systematics of lighter, prolate-deformed odd-A Tbisotopes, 161Tb and 163Tb [4, 5], the likely ground state spin/parities for165,7Tb are Iπ = 32+ for both. These are associated with the occupationof the 32+[411] orbital and consistent with the predictions of equilibriumdeformation calculations for axially-symmetric, prolate-deformed nuclei asreported by Jain et al. [1] and Nazarewicz et al. [2]. The other favoured,odd-proton configurations predicted to lie close to the Fermi surface forZ = 65 are associated with the 52+[413], 72−[523] and 92+[404] Nilsson de-formed single-particle states. The isomeric nature of the observed statesand deduced partial level schemes for 165Tb and 167Tb are consistent withhindered decays from an isomeric state to rotational states built on the(likely) positive parity 32+ ground state. The most probable candidate forIsomer Spectroscopy of Neutron-rich 165,167Tb 607the isomeric states would then be the π 72−[523] orbital which would requirea (naturally) hindered E1 decay to the positive parity states built on theground state configuration.3. ConclusionsIn-flight fission of 238U has been used to synthesise the neutron-richdeformed nuclei 165Tb and 167Tb. The first published information on ex-cited states in these systems is presented in the current work followingisomer-delayed gamma-ray spectroscopic analysis. In both systems, one-quasi-proton isomeric states with Iπ = (72−) states are identified in theseprolate deformed nuclei with decay half-lives of T1/2 = 0.81(8) µs and T1/2 =2.1(1) µs in 165Tb and 167Tb respectively. The deduced level schemes areconsistent with E1 multipolarity decays from these metastable states builton the 72−[523] one quasi-proton states, which decay through rotationally-coupled states built on the deformed π 32+[411] ground state configuration.The authors are indebted to the facility crews who provided the beamsat RIBF, operated by RIKEN Nishina Center and CNS, University of Tokyo,the EUROBALL Owners Committee for the loan of germanium detectors,the PreSpec Collaboration for the use of the readout electronics. Part ofthe WAS3ABi was supported by the Rare Isotope Science Project which isfunded by MSIP and NRF of Korea. This work is supported by the UKScience and Technology Facilities Council (STFC); the UK National Mea-surement Office (NMO, P.H.R.); JSPS KAKENHI grant Nos. 24740188,25247045, and 25287065; the U.S. Department of Energy, Office of Sci-ence, Office of Nuclear Physics under contract No. DE-AC02-06CH11357(F.G.K.); NRF Korea grants Nos. 2009-0093817 and 2013R1A1A2063017(C.S.L.), 2015R1D1A1A01056918, 2016R1A5A1013277 and 2016K1A3A7A09005579 (K.Y.C.); the Hungarian Research Fund OTKA contractNo. K100835 and Science Foundation Ireland under grant No. 12/IP/1288(O.J.R.).REFERENCES[1] A.K. Jain et al., Rev. Mod. Phys. 62, 393 (1990).[2] W. Nazarewicz, M.A. Riley, J.D. Garrett, Nucl. Phys. A 512, 61 (1990).[3] P.A. Söderström et al., Phys. Lett. B 762, 404 (2016).[4] C.W. Reich, Nucl. Data Sheets 111, 1211 (2010).[5] C.W. Reich, Nucl. Data Sheets 112, 2497 (2011).608 L.A. Gurgi et al.[6] Y. Yano, Nucl. Instrum. Methods Phys. Res. B 261, 1009 (2007).[7] T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97 (2003).[8] T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003 (2012).[9] K.-H. Schmidt et al., Nucl. Instrum. Methods Phys. Res. A 260, 287 (1987).[10] H. Suzuki et al., Nucl. Instrum. Methods Phys. Res. B 317, 756 (2013).[11] S. Nishimura, Prog. Theor. Exp. Phys. 2012, 03C006 (2012).[12] P.-A. Söderström et al., Nucl. Instrum. Methods Phys. Res. B 317, 649(2013).[13] Z. Patel et al., RIKEN Accel. Prog. Rep. 47, (2014).[14] F. Browne et al., Phys. Lett. B 750, 448 (2015).[15] H. Watanabe et al., Phys. Lett. B 760, 641 (2016).[16] L.A. Gurgi et al., submitted to Radiation Physics and Chemistry (2016).

Isomer Spectroscopy of Neutron-rich 165,167Tb

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