Andreyev, A N

Barton, C

Bastin, B

Bentley, M

Bildstein, V

Bree, N

Butler, P A

Bürger, A

Cederkäll, J

Clement, E

Davinson, T

Delahaye, P

Ekström, A

Fahlander, C

Gernhäuser, R

Grahn, T

Greenlees, P T

Guttormsen, M

Görgen, A

Habs, D

Herzberg, R-D

Huyse, M

Ivanov, O

Iwanicki, J

Jenkins, D G

Julin, R

Juutinen, S

Korten, W

Kröll, Th

Krücken, R

Larsen, A C

Lieb, K-P

Ljungvall, J

Morgan, T

Napiorkowski, P

Nara Singh, B S

Nyhus, H T

Obertelli, A

O’Donnell, D

Page, R D

Pakarinen, J

Petts, A

Peura, P

Rahkila, P

Rainovski, G

Robinson, A P

Scheck, M

Schwerdtfeger, W

Siem, S

Simpson, J

Srebrny, J

Sulignano, B

Syed, N U H

Theisen. Ch

Thirolf, P G

Toft, H

Van de Walle, J

Van Duppen, P

Wadsworth, R

Wenander, F

Wimmer, K

Woods, P

Wrzosek, K

Zielinska, M

CERN Document Server

CERN-INTC-2008-025/INTC-P-24621/04/2008Measurements of competing structures inneutron-deficient Pb isotopes by employingCoulomb excitationCERN-ISOLDE (J. Cederka¨ll, P. Delahaye, J. Van de Walle, F. Wenander)STFC Daresbury Laboratory (D. O’Donnell, J. Simpson)University of Edinburgh (T. Davinson, P. Woods)GANIL (E. Clement)University of Go¨ttingen (K.-P. Lieb)University of Jyva¨skyla¨ (P.T. Greenlees, R. Julin, S. Juutinen, P. Peura, P. Rahkila)KU Leuven (A.N. Andreyev, B. Bastin, N. Bree, M. Huyse, O. Ivanov, P. Van Dup-pen)University of Liverpool (P.A. Butler, T. Grahn, R.-D. Herzberg, R.D. Page, J. Pakari-nen, A. Petts, M. Scheck)LMU Mu¨nchen (P.G. Thirolf, D. Habs, W. Schwerdtfeger, T. Morgan)University of Lund (A. Ekstro¨m, C. Fahlander)TU Mu¨nchen (V. Bildstein, R. Gernha¨user, Th. Kro¨ll, R. Kru¨cken, K. Wimmer)University of Oslo (M. Guttormsen, A.C. Larsen, S. Siem, N.U.H. Syed, H.T. Ny-hus, A. Bu¨rger, H. Toft)CEA-Saclay (A. Go¨rgen, W. Korten, J. Ljungvall, A. Obertelli, B. Sulignano, Ch. Theisen)University of Sofia (G. Rainovski)HIL University of Warsaw (J. Iwanicki, P. Napiorkowski, J. Srebrny, K. Wrzosek,M. Zielinska)University of York (M. Bentley, C. Barton, D.G. Jenkins, B.S. Nara Singh, A.P. Robin-son, R. Wadsworth)Spokespersons: T. Grahn, J. Pakarinen (Liverpool)ISOLDE contact: J. Van de WalleAbstractA Coulomb excitation measurement to study the shape coexistence andquadrupole collectivity of the low-lying levels in neutron-deficient Pb nu-clei is proposed. Even-mass 188−192Pb nuclei will be post-accelerated atREX-ISOLDE in order to measure matrix elements for the lowest lyingtransitions. In combination with results obtained in previous lifetime mea-surements, this will allow the sign of the quadrupole deformation parameterto be extracted for the first time for 188Pb. In addition, collectivity of low-lying 2+ states will be probed for 190,192Pb.IntroductionThe interplay between single-particle motion, collectivity, and pairing in light Pbnuclei is manifested as a rich gamut of coexisting nuclear shapes and exotic ex-citations [1, 2, 3, 4, 5]. One of the goals of modern nuclear physics research isto understand the origin of these structures and their relation to the fundamentalinteractions between the nuclear constituents. These subjects can be investigatedparticularly well in the Pb isotopes close to neutron mid-shell, where a relativelysmall proton shell gap, together with a large valence neutron space, provides fertileground for studies of shape transitions within a small energy range. In α-decay stud-ies, the first two excited states of the mid-shell nucleus 186Pb were observed to be0+ states [6]. On the basis of α-decay hindrance factors, the 0+2 state was associatedwith mainly pi(2p − 2h) configuration, whereas the 0+3 state was associated with api(4p − 4h) configuration. Consequently, together with the spherical ground state[7], the three 0+ states with largely different structures establish a unique shape-triplet in 186Pb. Very recently, collective rotational bands built on these states wereobserved in in-beam γ-ray measurement [8].A complementary view of these 0+ states is provided by mean-field methods inwhich each intruder minimum of the potential energy surface is associated with adifferent collective shape. The first calculations of quadrupole potential energy sur-faces were performed within the Strutinsky approach [9, 10, 11]. The existence of aspherical ground state with low-lying oblate and prolate minima has been confirmedby self-consistent mean-field approaches based on effective Skyrme [12, 13, 14] andGogny [15] interactions. In a truncated shell-model approach, these oblate and pro-late mean-field configurations can be associated with pi(2p − 2h) and pi(4p − 4h)excitations, respectively, forming a unique system of three different shapes. Al-though much experimental effort has been put into investigating light Pb nuclei, theinformation obtained is still rather scarce. It remains a challenge for both theoreti-cal and experimental studies to obtain a consistent and detailed description of theobserved phenomena.We propose to carry out the investigations of nuclear collectivity and mixingof the low-lying states in neutron-deficient Pb nuclei, namely even-mass isotopes188−192Pb, employing the REX-ISOLDE facility. In Fig. 1, the level energy sys-tematics of Pb isotopes is shown. The pi(2p − 2h) configuration, associated withan oblate shape, intrudes down in energies close to the spherical ground state whenapproaching the neutron mid-shell at N = 104 and becomes the first excited state atA = 194. The onset of prolate deformation, mainly associated with the pi(4p− 4h)configuration, can be seen around A = 190. The prolate states form the yrast bandat A = 188. Thus, light Pb isotopes provide a unique laboratory to study competingstructures of different shapes around 1 MeV.In order to establish a complete picture of shape coexistence in this region,the knowledge of transition probabilities from nuclear states assigned with differ-ent shapes is essential. Transition probabilities are very sensitive to the details ofa nuclear wave function and, consequently, information about nuclear shape andconfiguration mixing can be inferred. So far, collectivity of γ-ray transitions orig-180 182 184 186 188 190 192 194 196 198 200 202 204Mass number05001000150020002500Energy [keV]98 100 102 104 106 108 110 112 114 116 118 120 122Neutron numberOblatePb isotopesEnergy [keV]ProlateEnergy [keV]Spherical0+0+2+4+6+ 2+2+4+4+8+6+0+6+8+10+Mid−shellFigure 1: Level energy systematics for even-even Pb isotopes adapted from Ref. [8].inating from prolate states in 186,188Pb has been established by in-beam lifetimemeasurements carried out at the University of Jyva¨skyla¨ [16]. While in-beam life-time experiments probe mainly yrast states, with Coulomb excitation the populationof non-yrast states becomes feasible and, enables a comprehensive study of coexist-ing shapes and their mixing in these nuclei. In addition, heavier 190,192Pb isotopescannot be probed in in-beam lifetime measurement due to presence of relativelylong-lived isomeric states. Hence, Coulomb excitation measurements would also en-able the collectivity of spherical states to be probed in these neutron-deficient Pbisotopes, for which the data are conspicuously lacking.Proposed experimentRadioactive beams of neutron-deficient Pb isotopes will be produced using the UCxprimary target and RILIS laser ion source [17]. The use of RILIS will allow Pbbeams to be purified from isobaric contaminants. The required charge state toaccelerate Pb beams up to 2.95 MeV/u will be obtained with REX-EBIS chargebreeder. Accelerated Pb beams will be delivered to MINIBALL target positionwhere Pb nuclei will be Coulomb excited using the secondary 112Cd target (E2+= 617.4 keV). MINIBALL Ge-detector array, with a photopeak efficiency of ≈ 7%for 1.3 MeV γ rays, will detect γ rays de-exciting levels under investigation. Bothscattered projectiles and target recoils will be detected using the annular doublesided silicon strip detector positioned on the beam axis after the secondary target.The selectivity of RILIS laser ion source will provide rather pure Pb beams,although in Ref. [17], it was observed that surface ionised Tl and Fr contaminantsremain. Fr contaminants will only present a problem at A > 206 [17], but the levelof Tl contaminants remains unknown for the proposed Pb beams. Thus, a separatetest run is required to ascertain the isobaric purity of neutron-deficient Pb beams.Count rate estimate and beam time requestThe experiment IS452 carried out in 2007 successfully demonstrated that heavy ra-dioactive beams, such as 184Hg can be exploited for Coulomb excitation studies atREX-ISOLDE. In that experiment, 184Hg beam intensity on a target was observedto be 3000 pps. However, due to technical problems in that experiment, the trans-mission efficiency of REX was reduced (0.19% instead of 1%). The reported primaryyield for 184Hg is 1.3× 108µC−1.The yield of the radioactive Pb beams for the proposed experiment would besimilar to that in experiment IS452. In Table 1 number of counts for the transitionsfrom the first 2+ state to the ground state in even-mass 188−192Pb nuclei have beenestimated based on the measured ISOLDE primary yields. The reduced matrixelement for 188Pb has been extracted from the experimental lifetimes reported inRef. [16]. For 190,192Pb the matrix element has been assumed to be similar tothat in 188Pb. The MINIBALL γ-ray detection efficiency and REX transmissionefficiency have been assumed to be 10% and 1%, respectively. The low-lying levelstructure of the even-mass 188−192Pb is illustrated in Fig. 2 showing also the knownlevel lifetimes.Table 1: Counts obtained in one day (3 shifts) of beam time for the 2+1 → 0+g.s.transition for each nucleus. Reduced matrix elements have been extracted fromlifetimes measured in Ref. [16]. For 190,192Pb, matrix elements have been assumedto be similar to that in 188Pb. Primary yields have been taken from the ISOLDEdatabase.Nucleus ISOLDE Beam intensity on 〈0||Eˆ2||2〉 (eb) γ-ray yieldyield (µC−1) MINIBALL target (pps)188Pb 1.7× 106 1.7× 104 0.475 550190Pb 2.3× 107 2.3× 105 0.475 7400192Pb 4.0× 107 4.0× 105 0.475 10700The principal goal of the proposed experiment is to determine the diagonalmatrix element for the lowest 2+ state in 188Pb as the level lifetimes of the low-lying yrast states have recently been measured [16]. This will allow the sign ofthe quadrupole deformation parameter to be extracted from the proposed Coulombexcitation measurements. For 190,192Pb, lifetimes of the lowest 2+ states remain un-known. However, despite the lack of level lifetime information, B(E2) values for the0 02 7244 10642 9534 13152 7740 04 12290 6582 11630 02 8544 13560 7682 12380 58872434095336245577438911638545021238592251384 4695−12 ps15.9(10) ps188 190 192Pb Pb PbFigure 2: Partial level scheme showing observed γ-ray transitions below 1.4 MeV forthe even-mass 188−192Pb. Transitions feeding the excited 0+ states in 188,190Pb havenot been observed. This figure is reproduced from Refs. [8, 18] and level lifetimesare taken from Ref. [16].2+1 → 0+g.s. transitions can be obtained and, thus, level of collectivity of these tran-sitions can be deduced. In 190,192Pb, intruder structures are expected to lie higher inenergy, hence the proposed measurement will elucidate the collectivity of assumedspherical states in these nuclei. It is anticipated that the cross section to populatelow-lying non-yrast states is an order of magnitude lower than that for the yraststates. However, in 192Pb a low B(E2; 2+2 → 0+2 )/B(E2; 2+2 → 0+1 ) = 5(4) ratio withrather large error bars has been observed [18]. This has been explained by strongmixing of the 2+1 and 2+2 states. Together with relatively large ISOLDE primaryyield this fact may allow the population of the 2+2 state in192Pb.For the proposed programme, the following experimental procedure is suggested:1. Pb beam purity measurement (3 shifts) to test the feasibility of the proposedexperiment.2. Provided that the purity of required Pb beams is sufficient, 11 shifts of beamtime are required for the γ-ray yield measurement of 188Pb. For the measure-ments of 190,192Pb, 2 shifts for each nucleus are required. A further 3 shifts arerequired for setting up the REX facility.The beam time request is summarised in Table 2.Table 2: A summary of the beam time request.Beam Target Ion source ShiftsBeam purity test UCx RILIS 3REX set-up UCx RILIS 3188Pb UCx RILIS 11190Pb UCx RILIS 2192Pb UCx RILIS 2Total 21It should be noted that the proposed programme would be a forerunner for acampaign at HIE-ISOLDE where the higher energy will allow going a step further inthe multi-step Coulomb excitation. In addition, the proposed experiment will shedmore light on capability of the REX-ISOLDE + MINIBALL facility to probe evenlighter Pb isotopes, namely 186Pb.References[1] J.L. Wood et al., Phys. Rep. 215, 101 (1992).[2] P. Van Duppen et al., Phys. Rev. Lett. 52, 1974 (1984).[3] P. Van Duppen et al., J. Phys. G, 16, 441 (1990).[4] K. Heyde, J. Schietse, and C. De Coster, Phys. Rev. C 44, 2216 (1991).[5] R. Julin, K. Helariutta, and M. Muikku, J. Phys. G, 27, R109 (2001).[6] A.N. Andreyev et al., Nature (London) 405, 430 (2000).[7] H. De Witte et al., Phys. Rev. Lett. 98, 112502 (2007).[8] J. Pakarinen et al., Phys. Rev. C 75, 014302 (2007).[9] F.R. May et al., Phys. Lett. B, 68, 113 (1977).[10] R. Bengtson and W. Nazarewicz, Z. Phys. A, 334, 269 (1989).[11] W. Nazarewicz, Phys. Lett. B, 305, 195 (1993).[12] N. Tajima et al., Nucl. Phys. A, 551, 409 (1993).[13] T. Duguet et al., Phys. Lett. B, 559, 201 (2003).[14] M. Bender et al., Phys. Rev. C, 69, 064303 (2004).[15] R.R. Rodriguez-Guzman et al., Phys. Rev. C, 69, 054319 (2004).[16] T. Grahn et al., Phys. Rev. Lett. 97, 062501 (2006).[17] U. Ko¨ster et al., Nucl. Instr. and Meth. in Phys. Res. B 204, 347 (2003).[18] P. Van Duppen et al., Phys. Rev. C 35, 1861 (1987).

Measurements of competing structures in neutron-deficient Pb isotopes by employing Coulomb excitation

Measurements of competing structures in neutron-deficient Pb isotopes by employing Coulomb excitation

Abstract

Similar works

Full text

Available Versions

CERN Document Server