The yrast structure of the neutron-rich nucleus 97Zr has been studied using fission of target-like products in the reaction of a 48Ca beam on a thick 238U target. The level scheme known from the previous studies up to an energy and spin of approx. 4619 keV and 23/2-, respectively, has been extended by about 3 MeV and a few units of angular momentum. The located structure can be discussed in terms of shell model configurations

Broda, R.

Carpenter, M.P.

Fornal, B.

Janssens, R.V.F.

Królas, W.

Matejska-Minda, M.

Mazurek, K.

Pawłat, T.

Wrzesiński, J.

Zhu, S.

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Vol. 40 (2009) ACTA PHYSICA POLONICA B No 3YRAST STRUCTURE OF 97Zr∗M. Matejska-Minda, B. Fornal, R. Broda, W. KrólasK. Mazurek, T. Pawłat, J. WrzesińskiInstitute of Nuclear Physics, Polish Academy of Sciences, Kraków, PolandM.P. Carpenter, R.V.F. Janssens, S. ZhuPhysics Division, Argonne National Laboratory, Argonne, IL, USA(Received December 9, 2008)The yrast structure of the neutron-rich nucleus 97Zr has been studiedusing fission of target-like products in the reaction of a 48Ca beam ona thick 238U target. The level scheme known from the previous studies upto an energy and spin of approx. 4619 keV and 23/2−, respectively, hasbeen extended by about 3 MeV and a few units of angular momentum. Thelocated structure can be discussed in terms of shell model configurations.PACS numbers: 23.20.Lv, 27.60.+j, 25.85.–w.1. IntroductionThe mass region A ≈ 100 has attracted a lot of attention becauseneutron-rich nuclei above 96Zr demonstrate a clear shape transition withincreasing neutron number. Whereas in both 96Zr and 98Zr the lowest exci-tations can be described in terms of simple shell model configurations [1,2],102Zr already exhibits features of a strongly deformed nucleus [3]. In the97−100Zr isotopes, coexistence of spherical and deformed structures is ex-pected. Indeed, such features have been observed in 98,99,100Zr, but notin 97Zr. In this nucleus information on higher-lying yrast states is largelymissing.In earlier works performed by some members of the present collabo-ration, deep-inelastic processes occurring during collisions of a 48Ca beamwith a thick 238U target, at an energy roughly 20% above the Coulomb bar-rier, were successfully used to study the yrast structure of hard-to-reach,neutron-rich nuclei from the neighborhood of 48Ca. In the 48Ca+238U reac-tion, neutron-rich nuclei may also be produced in fission of the target-like∗ Presented at the Zakopane Conference on Nuclear Physics, September 1–7, 2008,Zakopane, Poland.(633)634 M. Matejska-Minda et al.fragments resulting from multi-nucleon transfer processes. Indeed, inspec-tion of the data revealed a sizeable population of nuclei with masses between80 and 140. In particular, Zr isotopes, with A = 90–104, were present withcross sections sufficient to examine in detail the coincidence relationshipsbetween gamma rays emitted from states with relatively high spins.2. Experimental setupThe experiment was performed at the Argonne National Laboratory,USA, using the ATLAS accelerator. Gamma rays emitted during reactionsinduced by a 48Ca beam (330 MeV) on a thick 238U target (50 mg/cm2),were measured with GAMMASPHERE which consisted of 101 Comptonsuppressed HPGe detectors [4]. Gamma-ray coincidence data were collectedwith a trigger requiring three gamma rays to be present in prompt coin-cidence. Energy and timing information of all Ge detectors firing within800 ns was stored. The beam was pulsed with a 412 ns repetition rate whichallowed for a clean separation between prompt and isomeric decays. In theanalysis, various versions of prompt and delayed gamma coincidence matri-ces and cubes were sorted. The data showed that the process of fission oftarget-like nuclei populates relatively high-spin states in nuclei near 96Zr.The nucleus of interest, 97Zr, was one of the intense products.3. Experimental resultsPrior to this work, the low-lying yrast excitations in 97Zr were knownmostly from the β−-decay study of the 9/2+ state in 97Y [5, 6] and from(d, p) and (α,3He) reaction investigations [7]. These were: the 3/2+ state at1103 keV, an isomeric state at 1264 keV with 7/2+ spin-parity assignmentand half-life of 103 ns, an excitation at 2264 keV with the proposed quantumnumbers 11/2− and a state at 2625 keV for which spin-parity 13/2− was sug-gested. Also, the location of higher yrast states at 3081, 3780 and 4620 keVwith spin-parity assignments of 15/2−, 19/2− and 23/2−, respectively, wastentatively proposed following the beta-decay study of the 27/2− high-spinisomer in 97Y [8].As a first step in the analysis, we inspected the spectra of gamma raysthat were coincident with the beam pulse, but also associated with subse-quent (in the range of 1 microsecond) detection of transitions at 161 and1103 keV deexciting the 7/2+ isomer — the spectrum for the 1103 keV gateis shown in Fig. 1(a). We confronted them also with the spectra doubly gatedon 457, 407, 1400 keV γ rays depopulating the 11/2− state at 2264 keV (theexample is shown in Fig. 1(b)). In this way, we displayed the 361, 457, 698,818 and 840 keV gamma rays that were reported in Ref. [8] as deexcitinghigher yrast states. In addition, at the energies of 729, 882, 949 and 986 keVYrast Structure of 97Zr 635Double gate: 882-1400 keV(d)729814457407361 4074575316987298188409499869991103102611571400161167cont.(  )c400 400CountsDouble gate: 407-1400 keV0400800120016000 0200 200600 600800 8001000 10001200 12001400 1400361457698729 818840882cont.cont.cont.Energy [keV] Energy [keV]CountsGate: delayed1103 keV05001000150020002500300035000 200 400 600 800 1000 1200 14003614575436987298188408829499499709869991244cont.cont.cont.cont.cont.161 167cont.cont.cont.( )a( )b4000 200 600 800 1000 1200 1400CountsCounts0040080040240012008020001600160Sum of double gates: 698-818 &818-999 & 698-999 & 407-1400 &698-1400 & 818-1400 keV120cont.Fig. 1. The γ-ray spectra from the 48Ca+238U reaction obtained by setting: (a)a single gate on the delayed 1103 keV, (b) a double gate on the 407 and 1400 keVlines, (c) sum of double gates on selected 97Zr transitions used to find the highestspin transitions, (d) a double gate on the 882 and 1400 keV γ rays.other candidates for transitions preceding the 11/2− state appeared. Sub-sequently, the sum of double coincidence gates placed on the members ofthe 999–818–698–840 keV cascade in the prompt γγγ cube as well as on thepairs of lines belonging to the cascade 457–407–1400 keV that deexcites the11/2− level at 2264 keV, showed distinct 949 and 986 keV gamma rays andalso weaker transitions at 1026, 990, 1157, 531 and 167 keV clearly belong-ing to 97Zr (Fig. 1(c)). On the basis of these findings and the coincidencerelationships between the new lines, the following states were placed in the97Zr level scheme: a state at 5569 keV decaying by a 949 keV transition,a 5606 keV state connected to the 4620 keV level by a 986 keV transitionand states at 6595, 6763, 7127, 7294 keV. The cascade consisting of threegamma rays 729, 814 and 882 keV forms a branch parallel to the main branchfeeding the 2264 keV state — this feature is illustrated in Fig. 1(d). Theresulting level scheme is presented in Fig. 2.As mentioned above, the states at 3082, 3780 and 4620 keV were iden-tified in the beta-decay study of the 27/2− high spin isomer in 97Y [8] andtentative spin-parity assignments of 15/2−, 19/2− and 23/2−, respectively,were proposed. The excitations located in the present work at 5569, 5606,6595, 6763, 7127, and 7294 keV are very likely higher-lying yrast states. Thetwo levels placed at 5569 and 5606 keV are very good candidates for excita-tions with 25/2− and 27/2− that were searched for in Ref. [8] as states fed636 M. Matejska-Minda et al.1103097Zr1/2+3/2+12647/2+14001807226422341619995431400970407110345788231464689387572981830822625457361378069846209499905569(54) (100) (13)814(5)(13)(22)(82)(35)(22)(26)(44)(24)(10) (9)(4)4056066763712711573641026(6)65955317294(5/2 )+(7/2 )(7/2 )+(11/2 )-986-297(13/2 )------(27/2 )(25/2 )(23/2 )(19/2 )(15/2 )840167103nsFig. 2. The level scheme of 97Zr obtained from the 48Ca+238U reaction. Relativetransition intensities are shown in parentheses.in beta decay of the 27/2− isomer in 97Y. The gamma decay pattern of thestates located at higher energies indicates that their spin values are in therange of 29/2 − 33/2.4. Summary and conclusionsWe have studied the yrast structure of the neutron-rich nucleus 97Zr pro-duced in fission of target-like fragments following the 48Ca(330 MeV)+238Ureaction. By using prompt and delayed γ–γ coincidence spectroscopy, wehave extended the 97Zr level scheme up to an excitation energy of ∼ 7.3 MeVand a spin of approximately 33/2. The new findings point out that 97Zr,with one neutron outside the 96Zr core, retains the character of a sphericalnucleus up to quite high excitation energy and spin.This work is supported by the U.S. Department of Energy, Office ofNuclear Physics, under contract DE-AC02-06CH11357 and by the PolishMinistry of Science, Grant No 1P03B05929.Yrast Structure of 97Zr 637REFERENCES[1] D. Pantelica et al., Phys. Rev. C72, 024304 (2005).[2] W. Urban et al., Z. Phys. A358, 145 (1997).[3] J.L. Durell et al., Phys. Rev. C52, R2306 (1995).[4] I.Y. Lee, Nucl. Phys. A520, 641c (1990).[5] E. Monnand et al., Proc. Int. Conf. Nuclei Far from Stability, Cargese, Corsica,1976, p. 477; CERN-76-13 (1976).[6] G. Lhersonneau et al., Z. Phys. A323, 59 (1986).[7] C.R. Bingham, G.T. Fabian, Phys. Rev. C7, 1509 (1973).[8] G. Lhersonneau et al., Phys. Rev. C54, 1117 (1996).View publication stats

Yrast structure of 97Zr

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Yrast structure of 97Zr

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