High-spin isomer in 93Mo

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<ul><li><p>DOI 10.1140/epja/i2004-10145-2</p><p>Eur. Phys. J. A 24, 249257 (2005) THE EUROPEANPHYSICAL JOURNAL A</p><p>High-spin isomer in 93Mo</p><p>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</p><p>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</p><p>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</p><p>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</p><p>92Mo.</p><p>PACS. 27.60.+j 90 A 149</p><p>1 Introduction</p><p>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</p><p>211/2)]49/2+</p><p>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</p><p>a e-mail: fukuchi@cns.s.u-tokyo.ac.jp</p><p>2d3/2</p><p>2d5/2</p><p>1g9/22p1/2</p><p>1g9/2</p><p>2d5/21g7/2</p><p>2d3/23s1/2</p><p>3s1/21g7/2</p><p>proton neutron</p><p>2p3/2</p><p>2d3/23s1/2</p><p>1g7/22d5/2</p><p>1h11/2</p><p>2d3/2</p><p>2f7/21h9/21i13/2</p><p>proton neutron</p><p>1h11/2</p><p>0</p><p>2</p><p>4</p><p>6</p><p>8</p><p>-23s1/21h11/2</p><p>Ener</p><p>gy (M</p><p>eV)</p><p>Zr9040 50Gd14664 82</p><p>10</p><p>408264</p><p>5050</p><p>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.</p><p>become maximum. This sudden shape change causes thehigh-spin isomer.</p><p>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</p></li><li><p>250 The European Physical Journal A</p><p>Y9039 Zr</p><p>9140 Nb</p><p>9241 Mo</p><p>9342 Tc</p><p>9443 Ru</p><p>9544 Rh</p><p>9645 Pd</p><p>9746</p><p>0</p><p>0.682</p><p>3.2 h</p><p>2-</p><p>7+</p><p>0 0 0 0 0 0 0</p><p> 21/2+</p><p> 15/2-</p><p>5/2+</p><p>3.167</p><p>2.288</p><p>29.0 ns </p><p>4.35 s</p><p>0.136</p><p>0.2262-</p><p>2+</p><p>(7)+</p><p>11-</p><p>10.15 d</p><p>5.9 ms</p><p>2.203</p><p>167 ns2.245</p><p>6.85 h</p><p>21/2+</p><p>5/2+</p><p>7+</p><p>(2)+</p><p>0.075</p><p>52.0 m</p><p>17/2+</p><p>2.347</p><p>4.5 ns</p><p>(13+ )</p><p>21/2(+)</p><p>17/2(+)</p><p>2.285</p><p>3.05 ns</p><p>2.540</p><p>10.05 ns</p><p>5/2+</p><p>5/2+</p><p>2.244</p><p>0.052</p><p>1.51 m</p><p>6+</p><p>3+</p><p>2.3 ns</p><p>2.67d</p><p>stable</p><p>3.47x107 y</p><p>4.0x103 y</p><p>4.88 h</p><p>1.643 h</p><p>9.90 m3.10 m</p><p>0</p><p>1</p><p>Exci</p><p>tati</p><p>on E</p><p>ner</p><p>gy (</p><p>MeV</p><p>)</p><p>2</p><p>3</p><p>4</p><p>Fig. 2. Experimental systematics of isomers in N = 51 isotones.</p><p>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</p><p>known for odd nuclei. A configuration of these isomers was[(f7/2) pi(h</p><p>211/2)]27/2 . On the other hand, in N = 51</p><p>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</p><p>29/2)]21/2+</p><p>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</p><p>29/2)]39/2 for odd nuclei,</p><p>and [(d5/2g7/2h11/2)pi(g29/2p</p><p>11/2)]20+ for odd-odd nuclei.</p><p>In this work, a member of the N = 51 isotones 93Moas well as its neighboring nuclei were studied using -rayspectroscopic methods.</p><p>2 Experimental procedure and results</p><p>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</p><p>0</p><p>1</p><p>2</p><p>3</p><p>4</p><p>5</p><p>6</p><p>7</p><p>8</p><p>9</p><p>10</p><p>262*573*</p><p>476*x</p><p> 1.1 s</p><p>0.8 ns</p><p>(31/2,33/2)</p><p>(33/2,35/2)</p><p>(35/2,37/2)(39/2--)</p><p>9669</p><p>9001</p><p>8353</p><p>1067</p><p>669*</p><p>648*</p><p>1516*</p><p>2399*</p><p>260*</p><p>1442</p><p>242</p><p>1147686</p><p>24741734</p><p>263</p><p>685</p><p>1477</p><p>114</p><p>1363</p><p>6.85 h</p><p>278</p><p>Ex(MeV)</p><p>1552*</p><p>13631477</p><p>0</p><p>1571*</p><p>6837</p><p>21622425</p><p>41594437</p><p>7097</p><p>5585</p><p>4899</p><p>70267268</p><p>83358597</p><p>9170</p><p>9646</p><p>8820</p><p>23/2--27/2--</p><p>33/2--</p><p>29/2+</p><p>25/2+</p><p>21/2+</p><p>13/2+</p><p> 9/2+</p><p> 5/2+</p><p>35/229/2</p><p>93Mo</p><p>(35/2,37/2)(37/2)</p><p>(37/2)(39/2)</p><p>(41/2)</p><p>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.</p></li><li><p>T. Fukuchi et al.: High-spin isomer in 93Mo 251</p><p>100</p><p>200</p><p>300</p><p>260*</p><p>278</p><p>e+e- 64</p><p>8*66</p><p>9*</p><p>1516</p><p>*</p><p>1734</p><p>56Fe</p><p>100</p><p>200</p><p>300</p><p>400</p><p>278</p><p>476*</p><p>573* 68</p><p>656</p><p>Fe</p><p>1147</p><p>1442</p><p>1734 2474</p><p>e+e-</p><p>100</p><p>200</p><p>300</p><p>400</p><p>242</p><p>e+e- 6</p><p>86</p><p>1442</p><p>2474</p><p>(a)</p><p>0 500 1000 1500 2000 2500 3000Energy (keV)</p><p>Coun</p><p>ts/C</p><p>hann</p><p>el</p><p>(b)</p><p>(c)</p><p>Fig. 4. Spectra obtained by gating on 1067 keV (a), 1571 keV (b) and 2399 keV (c) -rays.</p><p>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</p><p>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</p><p>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</p><p>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-</p><p>line analysis. In addition to the coincidence measurement,a beam-delayed two-dimensional measurement, -ray en-ergy versus time, was performed in order to search forthe short-lived isomer. This measurement was performedin parallel with coincidence measurements using anotherindependent data acquisition system.</p><p>2.1 Prompt - coincidences</p><p>A level scheme of 93Mo was established in the past up tothe 35/2 states at 7.3 MeV [16]. Figure 3 shows the pro-posed level scheme of 93Mo based on the - coincidencedata. The gate width of prompt - timing was set to200 ns. The coincidence relations of known -rays agreewith those in the previous report except for the 1067 keVtransition. A known -ray of 1067 keV which was previ-ously reported [16] as a transition populating the 29/2+</p><p>state at 5585 keV was placed to populate the 35/2 at 7268keV state based on the present coincidence result.The ordering of each level was determined considering</p><p>the intensity balances and the coincidence relations. Therewere coincidence relations between the 648 keV -ray and-rays below the 8353 keV state as well as those below the29/2+ state at 5585 keV. However, no transition linking</p></li><li><p>252 The European Physical Journal A</p><p>Table 1. Measured -ray energies, intensities, angular-distribution coefficients and linear-polarization values in 93Mo. Relativeintensities are normalized to the 1734 keV -ray intensity. Uncertainties are given in parentheses.</p><p>E (keV) I A2/A0 Polarization Ei Ef Ji Jf</p><p>241.6 43(7) 0.19(4) 7268 7026 35/2 33/2</p><p>260.2 doublet(b) 6837 7097</p><p>262.4 doublet(b) 8597 8335263.0 2425 2162 21/2+ 13/2+</p><p>278.4 60(5) 0.10(5) 0.15(4) 4437 4159 27/2 23/2</p><p>476.0 9(3)(a) 9646 9170</p><p>573.4 14(5)(a) 9170 8597</p><p>647.6 2(1)(a) 9001 8353</p><p>668.6 2(1)(a) 9669 9001684.5 2162 1477 13/2+ 9/2+</p><p>685.9 58(4)(a) 0.072(6) 0.06(3) 5585 4899 29/2+ 25/2+</p><p>1066.6 10(3)(a) 8335 72681147.4 18(2) 0.37(13) 0.11(6) 5585 4437 29/2+ 27/2</p><p>1441.5 29(2) 0.12(9) 0.44(14) 7026 5585 33/2 29/2+</p><p>1477.1 1477 0 9/2+ 5/2+</p><p>1516.3 2(1)(a) 8353 6837</p><p>1552.4 6(2)(a) 8820 72681570.7 5(1) 8597 70261734.4 100(6) 0.39(7) 0.023(6) 4159 2425 23/2 21/2+</p><p>2399.4 8(1) 6837 44372474.1 57(3) 0.17(8) 0.019(9) 4899 2425 25/2+ 21/2+</p><p>(a) Estimated from coincidence data.(b) Unresolved doublet.</p><p>the 8353 keV and the 5585 keV levels was observed in thiswork. The same occurs for a transition between the 9001keV and the 33/2 states at 7026 keV.Figure 4 shows background-subtracted -ray spectra.</p><p>Figure 4(a) is the spectrum gated by the 1067 keV -ray.Figures 4(b), (c) are the spectra gated by newly found -rays and new -rays can be seen. The transitions markedby an asterisk in figs. 3 and 4 are new ones. Intensitiesand placements in the level scheme of all -rays observedin this work are given in table 1. The 276 keV transitionwas not listed because the coincidence relation could notbe determined. Gamma-ray intensities given with (a) wereobtained from singles data. Those with (b) were deducedusing gated spectra.</p><p>2.2 Angular distributions</p><p>In order to assign spin values of the states, angular-distribution measurements were performed. The spinchange, I, of each transition was deduced based on theseresults. Gamma rays were detected in singles mode byclover-type Ge detectors located at angles 38, 50, 76,90 and 158. The singles -ray spectra from each Ge crys-tal in clover detectors were treated independently. Theangular-distribution data were fitted to the Legendre poly-nomial expansion</p><p>W() =</p><p>AP(cos ), (1)</p><p>where s are even numbers. The extracted A2/A0 coeffi-cients are listed in table 1.</p><p>2.3 Linear polarizations</p><p>Linear-polarization measurements are useful in determin-ing the electric or magnetic character of -radiation as wellas spins of initial and final states. By combining the re-sults of linear-polarization and angular-distribution mea-surements, information on the multipolarity of -radiationas well as spins of the initial and final states and the spinand parity of the concerned nuclear state can be assigned.These measurements were performed by using a spec-</p><p>trum taken by the clover detectors located at an angle = 90 with respect to the beam axis. The clover de-tectors act as a polarimeter when the Compton-scatteredevents give signals in adjacent crystals. The coincidenceevents were separated according to the condition whetherthe coincidence events from the adjacent crystals are ei-ther from parallel or perpendicular ones to the reactionplane. The linear polarization P is defined by the follow-ing equation:</p><p>Pexp = A/Q, (2)</p><p>whereA = (W W)/(W +W), (3)</p><p>W and W denote the perpendicular and the parallelCompton scattering amplitude, respectively. Q represents</p></li><li><p>T. Fukuchi et al.: High-spin isomer in 93Mo 253</p><p>28</p><p>30</p><p>32</p><p>34</p><p>36</p><p>38</p><p>40</p><p>0 500 1000 1500 2000 2500 3000</p><p>Tim</p><p>e (</p><p>ns)</p><p>Energy (keV)</p><p>Prompt</p><p> 92</p><p>Mo Isomer</p><p>93Mo</p><p>278</p><p>1147</p><p>1734</p><p>2474</p><p>0</p><p>200</p><p>400</p><p>600</p><p>800</p><p>0</p><p>100</p><p>200</p><p>300</p><p>0</p><p>200</p><p>400</p><p>600</p><p>800</p><p>0</p><p>200</p><p>400</p><p>600</p><p>800</p><p>30 40 50 60</p><p>Time (ns)</p><p>(a)</p><p>(c)</p><p>(b)</p><p>(d)</p><p>278 keV</p><p>Cent. 36.11</p><p>1734 keV</p><p>Cent. 30.98</p><p>1147 keV</p><p>Cent. 30.53</p><p>2474 keV</p><p>Cent. 29.20</p><p>93Mo </p><p>93Mo</p><p>93Mo </p><p>93Mo</p><p>Counts</p><p>/Channel</p><p>30 40 50 60</p><p>Fig. 5. The examples of the time distribution spectra of-ray transitions 278 keV (a), 1147 keV (b), 1734 keV (c) and2474 keV (d) (top). Time versus -ray energy plot of extractedcentroids (bottom). The prompt -rays are represented by thefilled circles. The -ray from isomeric states in 92Mo and fourtransitions in 93Mo are denoted by a box and crosses, respec-tively.</p><p>the polarization sensitivity of the polarimeter and is afunction of the incident -ray energy.Table 1 shows measured linear-polarization values and</p><p>transition assignments were made assuming pure multipo-larities for -rays. The transition multipolarity assignmentwas based on angular-distribution measurements. Basedon these results, spins and parities of 5 states in 93Mowere determined.</p><p>2.4 Off-beam -rays</p><p>To search for the short-lived isomer, a beam delayed two-dimensional measurement, E versus time, was performedin parallel with the coincidence measurement. In this mea-surement, the -ray energy spectra were recorded withtime information between -rays and beam pulses of thecyclotron. These pulses, which were produced by the RFsignal, had a spacing of 83 ns. The time distribution mea-surements allow one to find delayed transitions.An isomer search using the centroid-shift method was</p><p>performed. In the lifetime measurement, the usual proce-dure is to measure a decay curve. However, for a short</p><p>Table 2. Observed -rays in 93Mo and their intensities ob-tained in a delayed spectrum. Relative intensities are obtainedby normalizing to those of a prompt spectrum.</p><p>-ray energy Area Relative(keV) (counts) intensity</p><p>242 doublet with 92Mo 278 7701 0.48(3)</p><p>6...</p></li></ul>