Population of yrast states in191Os using deep-inelastic reactions

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS G: NUCLEAR AND PARTICLE PHYSICS J. Phys. G: Nucl. Part. Phys. 31 (2005) S1891–S1894 doi:10.1088/0954-3899/31/10/095 Population of yrast states in 191 Os using deep-inelastic reactions G A Jones1, Zs Podolyák1, P M Walker1, P H Regan1, G de Angelis2, M Axiotis2, D Bazzacco3, P G Bizzeti4, F Brandolini3, R Broda5, D Bucurescu6, E Farnea3, W Gelletly1, A Gadea2, M Ionescu-Bujor6, A Iordachescu6, Th Kröll2, S D Langdown1, S Lunardi3, N Marginean2, T Martinez2, N H Medina7, B Quintana8, B Rubio9, C A Ur3, J J Valiente-Dobón1, S J Williams1 and Y H Zhang10 1 Department of Physics, University of Surrey, Guildford, GU2 7HX, UK INFN, Legnaro National Laboratories, Legnaro, Italy 3 Dipartimento di Fisica and INFN, Padova, Italy 4 Dipartimento di Fisica and INFN, Firenze, Italy 5 Niewodniczanski Institute of Nuclear Physics, Krakow, Poland 6 Institute of Physics and Nuclear Engineering, Bucharest, Romania 7 Instituto de Fı́sica, Universidade de São Paulo, São Paulo, Brazil 8 University of Salamanca, Spain 9 Instituto de Fı́sica Corpuscolar, Valéncia, Spain 10 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China 2 E-mail: [email protected] Received 18 March 2005 Published 12 September 2005 Online at stacks.iop.org/JPhysG/31/S1891 Abstract Several nuclei in the A ∼ 190 region have been studied following deep-inelastic reactions using a 460 MeV 82 Se projectile impinging upon a thick 192 Os target. The GASP array (at the Legnaro National Laboratory in Italy) was used to measure the resulting γ -decays. The previously reported near-yrast structure of 191 Os is extended to a t 12 = 61 ns isomer, at an energy of 2640 keV. + Branching ratios for I = 1 and I = 2 transitions in the K π = 11 band 2 have been measured, giving |(g − g )/Q | = 0.022(3) and 0.024(7) for K R 0
19 +  + and states respectively. These are consistent transitions from the 17 2 2 with the theoretical calculation for the proposed ν11/2+ [615] configuration of the band. Nilsson plus BCS calculations reveal that the isomer is likely to have a {ν11/2+ [615] ⊗ π 11/2− [505] ⊗ π 9/2− [514]} configuration with + J π = K π = 31 . This yields an implied reduced hindrance of fν = 1.9, 2 in accordance with empirical systematics of K isomers in the A ∼ 180–190 region. 0954-3899/05/101891+04$30.00 © 2005 IOP Publishing Ltd Printed in the UK S1891 S1892 G A Jones et al 1. Introduction The A ∼ 190 region reveals many facets of nuclear structure including K-isomerism [1], γ -softness [2] and oblate–prolate shape transition [3]. However the spectroscopy of neutronrich nuclei in this mass region at high spins is somewhat sparse. Using a variety of reaction techniques, such as projectile fragmentation [4] or deep-inelastic multi-nucleon transfer reactions [3], combined with powerful detector arrays, it is now possible to access nuclei in this region. Here we report new results for 191 Os. 2. Experimental details and analysis A broad swathe of nuclei in the A ∼ 190 region was populated using a deep-inelastic reaction between a 82 Se projectile incident upon an isotopically enriched (97.8%), 192 Os (50 mg cm−2 ) target backed with 0.2 mm of 181 Ta. The subsequent γ -decays were detected using the GASP array [5], in configuration I, at the Laboratori Nazionali di Legnaro, Italy. The array consists of 40 high purity Compton suppressed germanium detectors combining to a 3% absolute photo-peak efficiency at 1332 keV, and an 80 element BGO ball acting as a multiplicity filter. Triggering events of multiplicity 3 Ge + 2 BGO and 2 Ge + 2 BGO detectors were recorded to magnetic tape for approximately 3 days of beam time each. The experiment has yielded an abundance of new spectroscopic information in both the projectile-like [6, 7] and target-like [7, 8] species. Due to the lack of particle identification, coincidences in unknown binary partner nuclei and a large number of γ -rays with comparable energies in the populated regions, it can be very difficult to obtain precise spectroscopic information in a given nucleus. It is necessary to have a knowledge of low lying near-yrast transitions in the nucleus which can be used as gates in γ -ray coincidence techniques to determine weaker transitions between higher-spin states. Low-spin yrast transitions from case of 191 Os. The previously reported level Garrett et al [9] were used for this
purpose 
in31the + 19 + structure has been extended from 2 to 2 in the current work. The most intense transitions for 191 Os are shown in the spectrum of figure 1(a), corresponding to the proposed level scheme in figure 1(b). The 61 ns isomer identified of the isomer by Valiente-Dobón et al [10] has been populated in this experiment. The
energy + state of the yrast is determined to be 2640 keV, de-excited by a 453 keV γ -ray to the 27 2 ν11/2+ [615] band. Transitions above and below the isomer were studied using a variety of time-restricted, asymmetric γ –γ –γ and γ –γ –time cubes. This helped to clarify the structure depopulating the isomer, and has enabled the identification of candidates for yrast transitions with higher spin. Gating conditions placed upon these cubes have revealed γ -rays above the isomer, the most intense of which have energies of 254, 335, 496 and 603 keV. Each of these transitions is individually coincident with delayed transitions in 191 Os (a sum spectrum is shown in figure 1(a)). However, the weak intensities of these transitions have made it difficult to establish their sequence, and the level scheme above the isomer does not figure explicitly in this report. Tentative spin assignments have been made based upon the continuation of the strongly + band. DCO and angular distribution analysis could not determine coupled rotational 11 2 unambiguously the multipolarities of any of the transitions. The 330 and 219 keV transitions are marked as tentative due to their weak intensities. 3. Discussion Despite the low intensities of the intraband M1 transitions, I = 1 and I = 2 transition branching ratios were measured using γ -ray coincidences above and below the states to Population of yrast states in 191 Os using deep-inelastic reactions 657 (27/2 ) 2187 415 657 1772 (25/2 ) 0 200 300 2640 61 4 ns 478 20 549 453 456 349 (a) (29/2 , 31/2 ) 453 415 40 369 60 γd{176, 349, 456, 549, 453}; γp {254, 335, 496, 603} 176/177 Counts per 1 keV 80 S1893 400 500 Energy (keV) 700 600 (23/2 ) (330) 1342 (21/2 ) 402 765 303 354 638 602 191 7/2 940 (19/2 ) 411 285 235 549 1200 (219) 1081 981 259 478 17/2 15/2 (b) 1530 572 359 456 722 198 369 524 13/2 171 349 353 11/2 177 176 11/2+[615] 176 191Os 9/2 0 Figure 1. (a) Gamma-ray spectrum highlighting the transitions in 191 Os. The spectrum was created using an asymmetric cube demanding that two ‘delayed’ γ ’s arrive within 35 ns of each other and between 15 and 100 ns after a ‘prompt’ γ -ray. The energies of the ‘prompt’, γp , and ‘delayed’, γd , transitions summed to project the second ‘delayed’ coincidence are listed in brackets. (b) Level scheme for 191 Os showing transitions below the 61 ns isomer, as deduced in the current work. 10 178 W Ta 191 Os 184 Os 183 Re 182 W 179 fν Hindered Unhindered 1 0 20 40 60 80 100 120 140 160 180 200 Nn.Np Figure 2. Truncated systematics showing the reduced hindrance fν , of E2 isomeric transitions, in different nuclei from the A ∼ 180–190 region, as a function of the product of valence neutrons Nn , and protons Np , adapted from Walker et al [14, 15]. Filled symbols refer to K = 10 transitions, and open symbols have K = 6. be measured. Using standard procedures, following for example
[12], experimental values + + and 19 states, respectively. are |(gK − gR )/Q0 | = 0.022(3) and 0.024(7) from the 17 2 2 Theoretical estimates for |(gK − gR )/Q0 | were calculated using KgK = i (g  + g ), with a quenching factor of 0.6 for the intrinsic g-factors, gR = 0.35 and Q0 = 4.7 eb, taken as a mean of values for 190 Os and 192 Os [13]. Calculations for the yrast ν11/2+ [615] configuration S1894 G A Jones et al yield a value of |(gK − gR )/Q0 | = 0.019, which is in good agreement with the experimental values. New spin and energy information enables us to make comparisons with the Nilsson plus BCS models [11] to determine the configuration of the 61 ns isomer. Results show that the isomer is likely to be the product of a π(h11/2 )2 excitation with configuration + {ν11/2+ [615] ⊗ π 11/2− [505] ⊗ π 9/2− [514]}, combining to J π = K π = 31 . This 2 spin assignment would imply that the 453 keV γ -ray, de-exciting the isomer, is a stretched E2 transition. With these assumptions we can infer that the transition from the isomer has K = 10, and is thus 8 times K forbidden (ν = 8), corresponding to a reduced hindrance of
γ 1 γ fν = T 1 T 1Weis ν = 1.9, where T 1 is the partial γ -ray half-life and T 1Weis is the Weisskopf 2 2 2 2 single-particle estimate. Although of very small magnitude, the implied value of reduced hindrance for 191 Os fits very well with an empirical logarithmic dependence of fν on the product of valence neutron and proton numbers for E2 transitions de-exciting 2 and 3-quasiparticle isomers with K > 4 [14–16], shown in figure 2. It is worth noting that the extra datum that 191 Os provides to the plot corresponds with the linearity that approaches fν = 1 for closed-shell nuclei. This is consistent with the expectation that nearly magic nuclei are independent of effects from the K quantum number. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Walker P M and Dracoulis G 1999 Nature 699 35 Casten R F and Cizewski J A 1987 Phys. Lett. B 185 293 Wheldon C et al 2001 Phys. Rev. C 63 011304 Podolyák Zs et al 2000 Phys. Lett. B 491 225 Rossi Alvarez C 1993 Nucl. Phys. News 3 3 Zhang Y H et al 2004 Phys. Rev. C 70 024301 Podolyák Zs et al 2004 Int. J. Mod. Phys. E 13 123 Jones G A et al 2005 Acta Phys. Polon. B 36 1323 Garrett P E et al 1998 Phys. Rev. C 58 3734 Valiente-Dobón J J et al 2004 Phys. Rev. C 69 024316 Jain K et al 1995 Nucl. Phys. A 591 61 Regan P H et al 1995 Nucl. Phys. A 586 351 Raman S et al 2001 At. Nucl. Data Tables 78 1 Walker P M and Schiffer K 1991 Z. Phys. A 338 239 Walker P M 1990 J. Phys. G: Nucl. Part. Phys. 16 L233 Regan P H et al 2002 Phys. Rev. C 65 037302