High-spin isomer in 93Mo

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  • DOI 10.1140/epja/i2004-10145-2

    Eur. Phys. J. A 24, 249257 (2005) THE EUROPEANPHYSICAL JOURNAL A

    High-spin isomer in 93Mo

    T. Fukuchi1,a, Y. Gono2, A. Odahara3, S. Tanaka4, M. Inoue4, Y. Wakabayashi4, T. Sasaki4, M. Kibe4, N. Hokoiwa4,T. Shinozuka5, M. Fujita5, A. Yamazaki5, T. Sonoda5, C.S. Lee6, Y.K. Kwon6, J.Y. Moon6, and J.H. Lee6

    1 Center for Nuclear Study, University of Tokyo, Hirosawa 2-1, Wako, Saitama 351-0198, Japan2 Cyclotron Center, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan3 Nishinippon Institute of Technology, Kanda, Fukuoka 800-0361, Japan4 Department of Physics, Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan5 Cyclotron and Radioisotope Center, Tohoku University, Sendai, 980-8578, Japan6 Department of Physics, Chung-Ang University, Seoul 156-756, Korea

    Received: 6 May 2004 / Revised version: 1 March 2005 /Published online: 12 April 2005 c Societa` Italiana di Fisica / Springer-Verlag 2005Communicated by C. Signorini

    Abstract. The high-spin states of 93Mo have been studied by a 82Se(16O, 5n)93Mo reaction at a beamenergy of 100 MeV using techniques of in-beam -ray spectroscopy. Measurements of -t, --t coinci-dences, -ray angular distributions and -ray linear polarizations were performed. The high-spin isomerwas found as a (39/2) state at about 9.7 MeV. The near-yrast states in 93Mo were interpreted using theweak-coupling picture of a d5/2 neutron to a neutron magic core nucleus

    92Mo.

    PACS. 27.60.+j 90 A 149

    1 Introduction

    High-spin isomers were reported in N = 83 isotones, sys-tematically, 143Nd [1], 144Pm [2], 145Sm [3], 146Eu [4],147Gd [5], 148Tb [6], 149Dy [7], 150Ho [8] and 151Er [9].Their lifetimes range from 10 ns to s. The excitation en-ergies of high-spin isomers in N = 83 isotones are closeto each other in a range of 8.59.0 MeV and their spinsand parities are 49/2+ and 27+ for odd and odd-odd nu-clei, respectively, except for 150Ho and 151Er. The decayscheme and the characteristics of the high-spin isomer in147Gd were studied in detail experimentally [5] and the-oretically [10]. This high-spin isomer has been known tobe the 49/2+ state at 8.6 MeV and has a 510 ns half-life [5]. The deformation parameter of this state was de-termined to be 0.19 based on the measurements of thestatic quadrupole moment [11]. The configuration of thisisomer was taken to be [(f7/2h9/2i13/2) pi(h

    211/2)]49/2+

    using the g-factor obtained experimentally [12]. Accordingto the deformed independent particle model (DIPM) cal-culations [10] the nuclear shapes suddenly change fromnear spherical into oblate shapes at high-spin isomericstates. In a high-spin region of these nuclei, the angularmomenta of individual valence particles align to the sym-metry axis so that the overlaps of nucleon wave functions

    a e-mail: fukuchi@cns.s.u-tokyo.ac.jp

    2d3/2

    2d5/2

    1g9/22p1/2

    1g9/2

    2d5/21g7/2

    2d3/23s1/2

    3s1/21g7/2

    proton neutron

    2p3/2

    2d3/23s1/2

    1g7/22d5/2

    1h11/2

    2d3/2

    2f7/21h9/21i13/2

    proton neutron

    1h11/2

    0

    2

    4

    6

    8

    -23s1/21h11/2

    Ener

    gy (M

    eV)

    Zr9040 50Gd14664 82

    10

    408264

    5050

    Fig. 1. Single-particle levels in the 146Gd region (left side) andin the 90Zr region(right side). The single-particle energies aregiven with respect to an arbitrary zero point.

    become maximum. This sudden shape change causes thehigh-spin isomer.

    In order to study the existence of the same kind of iso-mers in other mass regions, spherical single-particle orbitsnear Fermi levels were compared between those in mass-150 region and in mass-90 region. In the case of N = 83isotones, the neutron number is the magic number 82 plusone and proton numbers are near 64 sub-shell closure. InN = 51 isotones, the neutron number is the magic num-ber 50 plus one and proton numbers are near 40 sub-shell

  • 250 The European Physical Journal A

    Y9039 Zr

    9140 Nb

    9241 Mo

    9342 Tc

    9443 Ru

    9544 Rh

    9645 Pd

    9746

    0

    0.682

    3.2 h

    2-

    7+

    0 0 0 0 0 0 0

    21/2+

    15/2-

    5/2+

    3.167

    2.288

    29.0 ns

    4.35 s

    0.136

    0.2262-

    2+

    (7)+

    11-

    10.15 d

    5.9 ms

    2.203

    167 ns2.245

    6.85 h

    21/2+

    5/2+

    7+

    (2)+

    0.075

    52.0 m

    17/2+

    2.347

    4.5 ns

    (13+ )

    21/2(+)

    17/2(+)

    2.285

    3.05 ns

    2.540

    10.05 ns

    5/2+

    5/2+

    2.244

    0.052

    1.51 m

    6+

    3+

    2.3 ns

    2.67d

    stable

    3.47x107 y

    4.0x103 y

    4.88 h

    1.643 h

    9.90 m3.10 m

    0

    1

    Exci

    tati

    on E

    ner

    gy (

    MeV

    )

    2

    3

    4

    Fig. 2. Experimental systematics of isomers in N = 51 isotones.

    closure. Figure 1 shows empirically deduced sphericalsingle-particle levels in the 146Gd [10] region and sphericalsingle-particle energies in the 90Zr region calculated usingthe Woods-Saxon potential.In N = 83 isotones, low-spin isomers of 27/2 were

    known for odd nuclei. A configuration of these isomers was[(f7/2) pi(h

    211/2)]27/2 . On the other hand, in N = 51

    isotones, 21/2+ isomers were reported. Experimental sys-tematics of isomers in 90Y [13], 91Zr [14], 92Nb [15],93Mo [16], 94Tc [17], 95Ru [18], 96Rh [19], and 97Pd [19] areshown in fig. 2. These isomers have [(d5/2)pi(g

    29/2)]21/2+

    configurations in odd nuclei. Both are of the stretch cou-pled configurations with proton core excitations. Theseisomers which have the same excitation mechanism inboth isotones suggest the existence of the high-spin iso-mer in N = 51 isotones. Based on the analogy of configu-rations of isomers in N = 83 and N = 51 isotones, the ex-pected configurations of high-spin isomers in N = 51 iso-tones are [(d5/2g7/2h11/2)pi(g

    29/2)]39/2 for odd nuclei,

    and [(d5/2g7/2h11/2)pi(g29/2p

    11/2)]20+ for odd-odd nuclei.

    In this work, a member of the N = 51 isotones 93Moas well as its neighboring nuclei were studied using -rayspectroscopic methods.

    2 Experimental procedure and results

    Experiments to search for the high-spin isomer in 93Mowere performed at Cyclotron and Radioisotope Center(CYRIC) at Tohoku University. States in 93Mo were pop-ulated using the reaction 82Se(16O, 5n)93Mo. The 82Setarget of 5.2 mg/cm2 was enriched to 90% and a targetfoil was backed by 500 g/cm2 of gold for a mechanicalsupport. The target was thick enough to stop the reactionproducts in it. The pulsed 16O beam of 100 MeV witha repetition time of 83 ns and pulse width of less than

    0

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    262*573*

    476*x

    1.1 s

    0.8 ns

    (31/2,33/2)

    (33/2,35/2)

    (35/2,37/2)(39/2--)

    9669

    9001

    8353

    1067

    669*

    648*

    1516*

    2399*

    260*

    1442

    242

    1147686

    24741734

    263

    685

    1477

    114

    1363

    6.85 h

    278

    Ex(MeV)

    1552*

    13631477

    0

    1571*

    6837

    21622425

    41594437

    7097

    5585

    4899

    70267268

    83358597

    9170

    9646

    8820

    23/2--27/2--

    33/2--

    29/2+

    25/2+

    21/2+

    13/2+

    9/2+

    5/2+

    35/229/2

    93Mo

    (35/2,37/2)(37/2)

    (37/2)(39/2)

    (41/2)

    Fig. 3. The proposed level scheme for 93Mo. The width of thearrows indicates -ray intensities, while the energies are givenin keV units. The transitions marked by an asterisk are newones.

  • T. Fukuchi et al.: High-spin isomer in 93Mo 251

    100

    200

    300

    260*

    278

    e+e- 64

    8*66

    9*

    1516

    *

    1734

    56Fe

    100

    200

    300

    400

    278

    476*

    573* 68

    656

    Fe

    1147

    1442

    1734 2474

    e+e-

    100

    200

    300

    400

    242

    e+e- 6

    86

    1442

    2474

    (a)

    0 500 1000 1500 2000 2500 3000Energy (keV)

    Coun

    ts/C

    hann

    el

    (b)

    (c)

    Fig. 4. Spectra obtained by gating on 1067 keV (a), 1571 keV (b) and 2399 keV (c) -rays.

    2 ns was provided by the AVF cyclotron at CYRIC. Theaverage of the O4+ beam intensity was about 2 enA.The calculated angular momentum l brought into a

    compound nucleus was 38~ according to the Bass modelcalculation [20]. The beam energy was selected to be100 MeV according to the results of the calculation madeby the statistical-model code CASCADE. The predictedfusion cross-section of about 300 mb is consistent with theexperimental result.For the -ray detection, 2 BGO anti-Compton shielded

    and one unshielded clover-type Ge detectors, 2 co-axialand 1 LEPS Ge detectors were used. The clover de-tectors were operated in the add-back mode for the- coincidence measurement and in direct mode forangular-distribution measurement. The relative efficien-cies of 2 co-axialtype Ge detectors were 40% and 50%.Two clover-type Ge detectors were located at an angle =90 with respect to the beam axis and were used for linear-polarization measurements. Two co-axial and LEPStype Ge detectors were placed at angles = 50, 158 and141 with respect to the beam axis. The distances betweenthe target and the surfaces of the Ge detectors were 13 cm.The total detection efficiency of the -ray was 0.73% at

    1.3 MeV in the singles mode. A total of 1.2 109 coinci-dence events were recorded in event-by-event mode for off-

    line analysis. In addition to the coincidence measurement,a beam-delayed two-dimensional measurement, -ray en-ergy versus time, was performed in order to