The high-spin yrast structure of the 210Bi nucleus was investigated using -ray coincidence spectroscopy following deep-inelastic reactions in the 208Pb+208Pb system. Cascades of rays following the decay of a new isomer were identified. Spin-parity assignments to the states known from previous studies as well as to newly located excitations were made based on the measured angular distributions of rays combined with a transition conversion coefficient analysis

Bowry, M.

Broda, R.

Bunce, M.

Carpenter, M.P.

Chiara, C.J.

Cieplicka, N.

Deo, A.Y.

Dracoulis, G.

Fornal, B.

Gelletly, W.

Hoffman, C.R.

Janssens, R.V.F.

Kempley, R.

Kondev, F.G.

Królas, W.

Lane, G.

Lauritsen, T.

Maier, K.H.

Pawlat, T.

Podolyák, Zs.

Reed, M.

Regan, P.

Rodriguez-Triguero, C.

Szpak, B.

Walker, P.

Wilson, E.

Wrzesinski, J.

Zhu, S.

Carolina Digital Repository

Vol. 45 (2014) ACTA PHYSICA POLONICA B No 2ANGULAR DISTRIBUTIONS OF γ RAYS FROM 210BiPRODUCED IN 208Pb+208Pb DEEP-INELASTICREACTIONS∗N. Cieplicka, B. Fornal, K.H. Maier, B. Szpak, R. BrodaW. Królas, T. Pawłat, J. WrzesińskiThe H. Niewodniczański Institute of Nuclear Physics, Polish Academy of SciencesRadzikowskiego 152, 31-342 Kraków, PolandR.V.F. Janssens, M.P. Carpenter, C.J. Chiara, C.R. HoffmanF.G. Kondev, T. Lauritsen, S. ZhuPhysics Division, Argonne National Laboratory, Argonne, IL 60439, USAZs. Podolyák, M. Bowry, M. Bunce, W. Gelletly, R. KempleyM. Reed, P. Regan, P. Walker, E. WilsonDepartment of Physics, University of Surrey, Guilford, GU2 7XH, UKA.Y. DeoDepartment of Physics, University of Massachusetts, Lowell, MA 01854, USAG. Dracoulis, G. LaneDepartment of Nuclear Physics, Australian National University, CanberraACT 0200, AustraliaC. Rodriguez-TrigueroUniversity of Brighton, Brighton, BN2 4GL, UK(Received November 22, 2013)The high-spin yrast structure of the 210Bi nucleus was investigated us-ing γ-ray coincidence spectroscopy following deep-inelastic reactions in the208Pb+208Pb system. Cascades of γ rays following the decay of a newisomer were identified. Spin-parity assignments to the states known fromprevious studies as well as to newly located excitations were made basedon the measured angular distributions of γ rays combined with a transitionconversion coefficient analysis.DOI:10.5506/APhysPolB.45.205PACS numbers: 27.80.+w, 23.20.Lv, 23.20.En, 21.60.Cs∗ Presented at the XXXIII Mazurian Lakes Conference on Physics, Piaski, Poland,September 1–7, 2013.(205)206 N. Cieplicka et al.1. IntroductionThe 210Bi nucleus is a one-proton, one-neutron system with respect tothe doubly-magic 208Pb core. Its ground state has spin-parity Jπ = 1− andbelongs to a fully-identified multiplet of states that originate from πh9/2νg9/2couplings [1]. The member of this multiplet with maximum spin, Jπ = 9−,lies at 271 keV in excitation energy and is a long-lived, α-decaying isomer(T1/2 = 3.04×106 y). Proton–neutron multiplets πh9/2νi11/2 and πf7/2νg9/2have been identified in this nucleus as well [2]. The known yrast structureof 210Bi also includes states arising from core excitations. They were tracedup to 4595 keV [3] and an angular momentum of approx. 16 ~, althoughspin-parity assignments to those levels are uncertain.In this contribution, we report on an effort to extend information onthe high-spin structure of 210Bi. Such structure may serve as a testingground for shell-model calculations which involve the promotion of protonsand neutrons across the Z = 82 and N = 126 shell gaps.2. ExperimentIn the experiment, which was performed at the Argonne National Lab-oratory, USA, a 208Pb beam (1446 MeV) from the Argonne Tandem LinearAccelerator System (ATLAS) was sent on a thick 208Pb target and induceddeep-inelastic reactions in which neutron-rich nuclei were excited up to rel-atively high spins. One of these intense products was 210Bi. The beam,coming in bursts of ∼ 0.3 ns time width, was pulsed with a 412 ns repe-tition rate to provide a clean separation between prompt and isomeric de-cays. Gamma rays were measured with Gammasphere — an array of 101Compton-suppressed Ge detectors. Coincidence data were collected with anevent trigger requiring three or more γ rays to be measured in coincidencewithin a 2 µs time window. The trigger signal was correlated in time withthe first coincident γ ray. Each event contained energy and timing infor-mation for all Ge detectors which fired. In the analysis, conditions were seton the time parameter in order to obtain various versions of prompt anddelayed (for γ rays emitted between beam bursts) γ-coincidence matricesand cubes.3. Data analysis and results3.1. Construction of the level schemeInformation about the structure lying above the α-decaying isomer at271 keV was a starting point in the search for the high-spin states in 210Bi.We analyzed the spectra (which covered an energy range up to 4 MeV) ofprompt and delayed γ rays by requiring coincidence relationships with theAngular Distributions of γ Rays from 210Bi Produced in 208Pb+208Pb . . . 207known 210Bi yrast transitions. As a first step, double gating in the off-beamcube on every pair of lines at 151, 398, 653, 744, 1252, 1403, and 1514 keVwas made. The resulting summed spectrum, displayed in Fig. 1, showed newtransitions belonging to cascades deexciting an unknown isomeric state atsome higher excitation. However, the exact placement of this isomer couldnot be determined. A reason for this difficulty could be the presence inthe deexcitation cascades of low-energy transitions which, due to their highinternal conversion and low detection efficiency, remained unobserved. Onlya rough estimate of the half-life of the new isomer could be obtained as10÷100 ns.Fig. 1. Representative γ-ray off-beam coincidence spectrum for 210Bi showing γ rayscoincident with any of the double gates set on the 151-, 398-, 653-, 744-, 1252-,1403-, 1514-keV transitions.Further analysis involved also the in-beam γ-ray coincidence relation-ships. The detailed investigation of spectra double gated on the pairs oftransitions confirmed the level scheme obtained above and resulted in theaddition of new prompt lines of 371, 217, 296, 518, 783, and 663 keV, whichcould be used to establish levels at 4966, 5183, 5479, 5749, and 5997 keV.The ordering of these γ rays was determined by analyzing their in-beam tooff-beam intensity ratio, which should decrease monotonically with excita-tion energy, independently of the branching ratio and detector efficiency. Inparticular, the existence of a 4087-keV state was confirmed by the observa-tion of a parallel 1360-keV branch connecting it with the level at 2726 keV.3.2. Angular distributions of γ raysIn an earlier work [4], we have shown that the spin of deep-inelasticreaction products (in the vicinity of the heavy reaction partner) exhibits an208 N. Cieplicka et al.appreciable alignment in the plane perpendicular to the beam direction. Thisis the same situation as in fusion–evaporation reactions and, consequently,the same, well known, angular distribution formalism [5] can be applied forthe transition multipolarity analysis.Fig. 2. (Color online) Examples of angular distribution functions (measured pointsand fitted curves) obtained for strong transitions (a). Yrast structure of 210Bicompared with the results of a shell-model calculation (b). Transitions markedby gray/red arrows were identified in present studies. The width of the arrowsindicates the observed intensity.Each detector in Gammasphere is associated with a pair of angles (θ,φ)with respect to the beam direction. This allows to divide detectors into sixrings around beam axis with average values of θ: 17.3◦, 35.5◦, 52.8◦, 69.8◦,79.9◦, 90.0◦ with equivalent rings at backward angles. The number of doubleγ-coincidences that was acquired during the 208Pb + 208Pb experiment wassufficient to carry out an analysis of the angular distribution for the strongestlines in 210Bi. The gates were set on transitions in in-beam coincidencematrices and γ-ray yields detected in a specific ring were obtained. Thenormalized intensity of a transition can be expressed as a function of thedetection angle, parametrized by A2 and A4 coefficients of the Legendrepolynomials Pn(cos θ)W (θ) = A0[1 +A2P2(cos θ) +A4P4(cos θ)] . (3.1)Angular Distributions of γ Rays from 210Bi Produced in 208Pb+208Pb . . . 209The angular distribution function depends on the γ-ray multipolarity (in-cluding the possibility of multipole mixing), spin-parity of the initial andfinal states as well as on the angular momentum alignment of the nucleusresulting from the reaction.At first, we obtained angular distributions for pure transitions: 653 keV(11+ → 10−) of E1 and 1403 keV (14− → 11+) of E3 character (displayed inFig. 2 (a)) in order to calculate the attenuation coefficient characteristic ofa particular reaction. The value resulting from this analysis is 0.6(1) for theattenuation of the A2 coefficient. This determines also the attenuation ofthe A4 coefficient as 0.2(1) [6], which is in agreement with the experimentalresult: 0.1(1). We also observed that the attenuation coefficients do notchange significantly for the remaining transitions.The angular distribution obtained for the 744-keV line is also presentedin Fig. 2 (a). The negative A2 = −0.20 value indicates its pure ∆I = 1character. Investigation of the parallel 175–1821 keV cascade brought in-formation, firstly, on the conversion coefficient of the 175-keV transition,α = 0.70(7), which determines its E2 character, and, secondly, on the an-gular distribution of the 1821-keV line which is consistent with its M1+E2character. These findings allowed to assign Jπ = 15+ to a 3470-keV stateand, as an implication, E1 multipolarity to the 744-keV transition. Simi-larly, for other strong transitions, by combining information obtained fromangular distributions with conversion coefficients extracted from the tran-sition intensity balance, we assigned spin-parities to the previously knownlevels and some of the newly established states (Fig. 2 (b)).The comparison of the experimental results with theoretical calculationsusing the modified Kuo–Herling interaction is presented in Fig. 2 (b) for thestates involving promotion of a proton or a neutron to higher-lying orbitalswithin the valence configuration space. The maximum available spin-parityvalue, Jπ = 14−, results from the πi13/2νj15/2 coupling. To describe higher-lying states, the calculations must include excitations of the core.4. ConclusionsWe were able to establish spins and parities of previously identified yraststates and extend the spectroscopic information to higher excitation energyin the one-proton, one-neutron 210Bi nucleus. Spin and parity assignmentsfor known and some of the newly identified states are based on the angulardistribution and conversion coefficient analysis of the relevant transitions.Excitations identified at energies up to approx. 2.7 MeV and spin 14− canbe described in terms of valence proton–neutron couplings. Higher-lyingyrast states, including two isomers, one at 3470 keV with Jπ = 15+ andT1/2 = 16(1) ns, and a second (which could not be firmly located) lying210 N. Cieplicka et al.at excitation energy higher than 6 MeV, must involve neutron or protonexcitations across the N = 126 and Z = 82 shell gaps, respectively. Thesenew findings will serve as a testing ground for shell-model calculations whichconsider 208Pb core excitations.This work was supported by the National Science Centre under ContractNo. DEC-2011/01/N/ST2/04612 and by the U.S. Department of Energy,Office of Nuclear Physics, under contract No. DE-AC02-06CH11357.REFERENCES[1] J.R. Erskine et al., Phys Rev. 128, 720 (1962).[2] J.S. Tsai, et al., Phys. Rev. C27, 2397 (1983).[3] B. Fornal, Habilitation thesis, Report No. 1939/PL, 2004.[4] N. Cieplicka, et al., Phys. Rev. C86, 054322 (2012).[5] T. Yamazaki, Nucl. Data A3, 1 (1967).[6] E. der Mateosian, A.W. Sunyar, At. Data Nucl. Data Tables 13, 391 (1974).

Angular distributions of rays from 210bi produced in 208 pb+208pb deep-inelastic reactions

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Angular distributions of rays from 210bi produced in 208 pb+208pb deep-inelastic reactions

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