Recoil Isomer Tagging on Proton-Rich Odd–Odd N=77 Isotones 142Tb and 144Ho

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Recoil Isomer Tagging on Proton-Rich Odd–Odd N=77 Isotones 142Tb and 144Ho © 2001 Jagellonian University Published version Scholey, C.; Cullen, D.M.; Paul, E.S.; Bentlby, M.A.; Boston, A.J.; Bruce, A.M.; Butler, P.A.; Caamaño, M.; Dorvaux, O.; Enqvist, T.; Fox, C.; Garcés, Narro J.; Greenlees, P.T.; Helariutta, K.; Jones, P.M.; Joss, D.T.; Julin, R.; Juutinen, S.; Kankaanpää, H.; Kettunen, H.; Kuusiniemi, P.; Leino, M.; Muikku, M.; Nieminen, P.; Rahkila, P.; Scraggs, H.C.; Shepherd, S.L.; Stezowski, O.; Uusitalo, J.; Walker, P.M. Scholey, C., Cullen, D.M., Paul, E.S., Bentlby, M.A., Boston, A.J., Bruce, A.M., Butler, P.A., Caamaño, M., Dorvaux, O., Enqvist, T., Fox, C., Garcés, N. J., Greenlees, P.T., Helariutta, K., Jones, P.M., Joss, D.T., Julin, R., Juutinen, S., Kankaanpää, H., . . . Walker, P.M. (2001). Recoil Isomer Tagging on Proton-Rich Odd–Odd N=77 Isotones 142Tb and 144Ho. Acta Physica Polonica B, 32(3), 1005-1008. https://www.actaphys.uj.edu.pl/fulltext?series=Reg&vol=32&page=1005 2001 Vol. 32 (2001) ACTA PHYSICA POLONICA B No 3 RECOIL ISOMER TAGGING ON PROTON-RICH ODDODD N = 77 ISOTONES 142 Tb AND 144 Ho  C. Sholey a , D.M. Cullen a y , E.S. Paul a , M.A. Bentley d A.J. Boston a , A.M. Brue b , P.A. Butler a , M. Caamaño  O. Dorvaux e z , T. Enqvist a x , C. Fox a , J. Garés Narro  P.T. Greenlees e , K. Helariutta e x , P.M. Jones e , D.T. Joss d R. Julin e , S. Juutinen e , H. Kankaanpää e , H. Kettunen e P. Kuusiniemi e , M. Leino e , M. Muikku e , P. Nieminen e P. Rahkila e , H.C. Sraggs a , S.L. Shepherd a , O. Stezowski a { J. Uusitalo e , and P.M. Walker  a Oliver Lodge Lab oratory, Department of Physis, University of Liverp o ol Liverp o ol L69 7ZE, UK b Sho ol of Engineering, University of Brighton, Brighton, BN2 4GJ, UK  Department of Physis, University of Surrey, Guildford, Surrey, GU2 7XH, UK d Sho ol of Sienes, Staordshire University, Stoke-on-Trent, ST4 2DE, UK e Department of Physis, University of Jyväskylä P.O. Box 35, FIN-40351, Jyväskylä, Finland (Reeived Otober 30, 2000) The isomeri struture of the N = 77 isotones 142 65 Tb and 144 67 Ho have b een studied with the 92 Mo( 54 Fe, xpn ) fusion evap oration reation at the University of Jyväskylä. The Jurospehere I I germanium array was employed in onjuntion with the RITU gas lled reoil separator. The feeding and deay of a 500(20) ns isomeri state in 144 Ho has b een established for the rst time together with states built up on the known 15  s isomer in 142 Tb. The b ehavior of these new strutures ab ove the isomers suggest that they are low deformation ongurations whih display signs of triaxiality. PACS numb ers: 21.10.Re, 23.20.g, 23.20.Lv, 27.60.+j  Presented at the XXXV Zakopane Sho ol of Physis Trends in Nulear Physis, Zakopane, Poland, Septemb er 513, 2000. y Present address: Department of Physis and Astronomy, University of Manhester, Manhester, M13 9PL, UK. z Present address: Institut de Reherhes Subatomique, CNRS-IN2P3, 23 Rue du Lo ess, BP 28, F67037 Strasb ourg, Frane. x Present address: G.S.I., Plankstr. 1, D-64291 Darmstadt, Germany. { Present address: Institut de Physique Nuléaire de Lyon, IN2P3/CNRS, Université C. Bernard Lyon-1, F-69622 Villeurbanne, Cedex, Frane. (1005) 1006 C. Sholey et al. 1. Intro dution The N = 77 isotones with A  140 lie b etween two regions whih have b een relatively well studied. The more proton-rih region has b een studied reently due to the interest in proton emission. Even though these nulei are diult to aess, their study has b een p ossible as the emission of a proton an b e used as a tag whih inreases the hannel seletion signiantly. On the other hand slightly more neutron-rih nulei are easily aessible through fusion evap oration reations. The nulei in the area disussed here have small ross-setions in omparison to the more neutron-rih nulei p opulated and as they do not proton deay no suh partile emission tag is available. In this ase some other form of hannel seletion is required and as there are a wealth of isomers in this region they an b e utilised as a signature with whih to tag the nulei. The reoil-isomer tagging tehnique was rst employed at the University of Jyväskylä to establish the rotational band ab ove the K  = 8  isomer in 138 Gd [1℄ and proved to b e very suessful. This subsequent exp eriment has enabled the feeding and deay of a new 500(20) ns isomeri state in 144 Ho to b e observed, together with states built up on the known 15  s isomer in 142 Tb. Suh isomers are a result of the highj states whih involve the intruding h 11 = 2 proton orbit. An additional fator is that the last protons o upy the lower part of the shell and tend to favour olletive prolate deformation (  = 0 Æ ), where as the neutrons lying in the upp er part of the h 11 = 2 shell tend to b e of a olletive oblate shap e (  =  60 Æ ). The data presented here give an insight to the single-partile ongurations of these isomers and demonstrate how the omp eting fators disussed ab ove give rise to triaxial nulei. 2. Reoil-isomer tagging The exp erimental setup onsisted of the Jurosphere I I germanium array oupled with the RITU gas lled separator [2℄. At the fo al plane of RITU was a MWPC (Multi-Wire Prop ortional Counter) [3℄ and a silion strip detetor. Surrounding the fo al plane detetor hamb er was a small array of ve germanium detetors. The reoil-isomer tagging tehnique orrelates prompt and delayed events aross isomeri states. This is ahieved by deteting  -ray transitions at either end of RITU [2℄ orresp onding to the target p osition and fo al plane resp etively. Delayed  -rays are deteted within a 32  s time gate after the implantation of a reoil into the silion detetor. The nulei were p opulated using the reation 92 Mo( 54 Fe, pxn ) at b eam energies of 226 and 236 MeV delivered by the K130 ylotron at the University of Jyväskylä, Finland. During the exp eriment, a total of 6  10 6 reoil gated prompt events were orrelated with delayed events in  5 days at a b eam energy of 236 MeV and 4  10 5 events in 1 day at 226 MeV. A omplete desription of the exp erimental detail an b e found in Ref. [4℄. Reoil Isomer Tagging on Proton-Rih OddOdd N=77 Isotones ::: 1007 3. 142 Tb and 144 Ho The main  rays in a 1030  s time gate were 37, 137, 165, and 303 keV Fig. 1(b). These are known transitions in 142 Tb b elow the 15  s isomer [5℄. (a) (d) (b) (c) Fig. 1. Gamma-ray sp etra: (a)  prompt 142 Tb, (b)  delayed 142 Tb, ()  prompt 144 Ho, (d)  delayed 144 Ho. (a) and () are pro dued by gating on the  rays in (b) and (d) resp etively and vie versa. By gating on this band and setting a time gate omparable to the isomer lifetime we were able to establish the deays ab ove the isomer shown in Fig. 1(a) and onstrut the level sheme shown in Fig. 2. When a short time gate of  100 ns was set on the delayed  -rays the main transitions were 56, 61, 148 and 209 keV with X-rays of 47 and 53 keV shown in Fig. 1(d). The oinident X-rays onrmed that these  -rays b elong to a holmium isotop e and the relative intensities of these p eaks at the two dierent b eam energies allow them to b e rmly assigned to 144 Ho. An isomeri state was identied and measured to have t 1 = 2 = 500(20) ns. Prompt (see Fig. 2()) and delayed events were then orrelated to identify the bands ab ove the isomer shown in Fig. 2. Theoretial Total Routhian Surfae (TRS) [6, 7℄ and Wo o dSaxon Cranked Shell Mo del (CSM) [8, 9℄ alulations have b een p erformed for this data. The TRS alulations gave a deformation of  2 = 0 : 192 ,  4 =  0 : 22 and  =  30 Æ whih represents a triaxial shap e. This large  -deformation is resp onsible for the large energy splitting of the single-partile orbitals, whih is manifested as the signature split bands ab ove the isomers shown in Fig. 2. The alulations suggest that the single partile ongurations for these nulei are as follows  h 11 = 2   ( h 11 = 2 ; f 7 = 2 ) or  h 11 = 2   ( s 1 = 2 ; d 3 = 2 ) . Unfortunately the low statistis for this data do es not p ermit the spins and ongurations of these bands to b e unambiguously assigned. 1008 C. Sholey et al. (6 ) (7 ) (8 ) (9 ) (11 ) (13 ) (15 ) (17 ) (10 ) (12 ) (14 ) (16 ) (19 ) 148 56 60 209 348 299 502 724 856 885 363 567 762 868 430 468 813 0.5(2) s 144Ho (7 ) (5 ) (8 ) (2 ) 1 (3 ) (9 ) (11 ) (13 ) (15 ) (10 ) (12 ) (14 ) 37 303 165 137 84 219 208 98 182 68 212 143 557 760 877 367 628 190 439 538 15 s 142Tb 0.3 s Fig. 2. The level sheme of 142 Tb from the present work and [5℄ and the newly established level sheme for 144 Ho. In summary we have identied bands built up on the known isomeri state in 142 Tb and observed exited states in 144 Ho. 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