β-decay of 113Rh and the observation of 113mPd : Isomer systematics in odd-A palladium isotopes

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  • Nuclear Physics A561 (1993) 416-430 North-Holland

    NUCLEAR PHYSICS A

    P-decay of 13Rh and the observation of 113mPd *: Isomer systematics in odd-A palladium isotopes

    H. Penttila, T. Enqvist, P.P. Jauho, A. Jokinen, M. Leino, J.M. Parmonen, J. Aysto

    Department of Physics, UniLwsity of .lyrv?skylli, SF-40351 Jy~C&ylii, Finland

    K. Eskola

    Department of Physics, University of Helsinki, SF-001 70 Helsinki, Finland

    Received 22 March 1993

    Abstract Decay of 13Rh to the levels of i3Pd was studied at the IGISOL-facility by means of p-, y-

    and conversion-electron spectroscopy. The level scheme of 13Pd was constructed using 33 gamma transitions on the basis of observed yy-coincidence relations and half-life analysis. A P-decay half-life of (2.80+_0.12) s was measured for 13Rh. A new s- isomeric state with (0.3 + 0.1) s half-life and excitation energy 81.3 keV was discovered in 13Pd, This state and the other recently observed low-lying 4m or y- isomeric states in *5,7Pd isotopes are directly populated in proton-induced fission. The decay of these isomers is unusually strongly hindered compared with Weisskopf estimates. Our observation of two strongly hindered M2 transitions in 3,*17Pd with hindrance factors of 7600 and 6800, respectively, imply coexistence of nuclear shapes in odd-A Pd nuclei.

    Key words: RADIOACTIVITY i13Rh, *3mPd mass separated [from 23xU(p, f), E = 20 MeV]; measured T,,,(P-l, E,, L,,,,, Pr-, yy-, Xy-, p(ce)-, Xfcel-coin, 13Pd deduced levels, J, rr, T *,21 log ft.

    1. Introduction

    Nuclear shapes and their coexistence represent a challenge for both experimen-

    tal and theoretical studies of structure of transitional nuclei with A > 100. Coexis-

    tence of prolate and oblate shapes at low excitation for odd-A Pd isotopes has been suggested in studies of the isomeric decays and P-decays of very neutron-rich

    Pd isotopes up to Pd [l-3]. Previous studies have identified the $- isomeric states in 10s~07~109~11Pd isotopes [4] and the ;- or y- isomers in lsPd [5,6] and

    Pd [1,2].

    l Supported by the Academy of Finland.

    0375-9474/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

  • H. Penttilii et al. / p-decuy 417

    The heaviest nucleus that can be studied via a transfer reaction is Pd.

    However, most of the data of Pd and Pd are from decay studies [7-91. The

    levels of 3*5,7Pd can be studied via P-decay of their 3~s~7Rh precursors

    produced in fission of heavy neutron-rich element. The P-decays of 3~s*7Rh

    were discovered at IGISOL and reported in refs. [1,2,10]. In addition, fission

    populates directly nuclear states over a large range of energy and spin values.

    These states include isomeric states that are not populated in P-decay. For

    example, an isomer with I = 27 _ z has been observed in Y [ll]. In this work new

    experimental data is presented on the discovery of the negative-parity isomer in

    Pd and on the P-decay of 13Rh to the levels of 13Pd. A detailed study of the

    P-decay of jRh was necessary for the search and identification of the isomer in

    Pd.

    2. Experimental techniques

    Because of the bulk of other fission products, studies of short-lived neutron-rich

    species produced with relatively low cross sections can only be performed using

    on-line separation. The physical and chemical properties make it hard to produce

    ion beams of Rh for mass separation. On the other hand, chemical separation

    without proper mass assignment may result in error, as was the case with the

    previously reported 13Rh decay [12].

    The Ion Guide Isotope Separator On-Line, IGISOL [131, can produce mass-sep-

    arated ion beams of any element in the millisecond time scale. Furthermore,

    because all the mass-separated ions are primary ions from the reaction, the

    situation is much better compared with conventional ion sources, in which the

    long-lived species accumulate in the target and are mass separated with a much

    higher efficiency than the short-lived species in the same mass.

    As a relatively fast device, IGISOL provides an effective way to study rapid p

    decays but also isomeric decays of mass-separated samples with half-lives as short

    as 0.1 ms. However, one remaining difficulty is the identification of Z, especially, if

    an isomeric state decays via a single transition directly to the ground state.

    Fortunately, such transitions are often strongly converted, and the most effective

    method for the Z assignment is a coincidence measurement between the charac-

    teristic X-rays and the conversion electrons. At IGISOL the mass-separated ion beam can be injected directly into the source position of the electron transport

    spectrometer ELLI [14]. Thus, no mechanical transportation of the produced

    activity is required and the conversion electron spectroscopy can be performed as

    rapidly as the mass separation, i.e., in the millisecond time scale. This has made it

    possible to search for the isomeric states in odd-A Pd nuclei over a large range of half-lives.

  • 418 H. Penttilii et al. / p-decay

    The activity studied was produced using 20 MeV proton induced fission of 238U

    and mass separated using the ion-guide technique, as described in ref. [13]. The

    P-decay of 13Rh was investigated with Pry, PrX, p(ce) and X(ce) coincidence

    set-ups. High-purity Ge detectors with 23% and 25% relative efficiencies were

    used to detect gamma rays up to 2 MeV, and 7 mm and 10 mm thick planar Ge

    detectors with active areas of 200 mm* and 1000 mm2, respectively, were used to

    detect X rays and low-energy gamma rays up to 400 keV. In the j!Irr and /3yX

    set-ups, the p particles were detected with a 1.0 mm thick NE102 type plastic

    scintillator AE, detector. The coincidences between X rays and conversion elec-

    trons, as well as the singles conversion electron and low-energy gamma-ray spectra

    were recorded. The P(ce> coincidence measurement was performed using a sur-

    face-barrier silicon detector as a AE, detector. The lack of beta coincidences

    indicates isomeric transition.

    The cyclotron and separator beam was pulsed for the half-life measurements.

    The p-decay half-lives were deduced from the decay of beta-gated gamma rays

    during the cyclotron beam-off period. The half-life of the isomeric transition was

    deduced from the decay of gamma rays in the singles spectrum. More details of the

    experimental set-ups can be found in refs. [3,5,14,15].

    3. Experimental results

    3.1. P-Decay of 13Rh

    Gamma transitions were assigned to the P-decay of 13Rh via observed coinci-

    dences between characteristic K X-rays of Pd and gamma rays, via yy- coinci-

    dences or via their observed half-life. One gamma transition (348.9 keV) was assigned via the observation of its K-conversion electrons in coincidence with the

    characteristic K X-rays of Pd. Altogether 42 gamma transitions assigned to the decay are listed in Table 1. Coincidences with P-particles confirmed the assigned

    gamma ray to follow the P-decay of 13Rh Fig. 1 shows a part of the beta-coinci- .

    dent gamma spectrum recorded at A = 113. The P-decay half-life for 13Rh was

    deduced from the decay of the 84.9, 117.0, 137.5, 189.7 and 348.9 keV gamma rays observed in coincidence with P-particles during the beam-off period of the

    cyclotron. The half-life value of (2.80 + 0.12) s is the weighted average of the measured values. The value agrees well with our previous result [lo], but the

    accuracy is somewhat improved. The conversion-electron measurements resulted

    in internal K-conversion coefficients for 13 transitions and an L-conversion

    coefficient for one transition (34.9 keV) in 13Pd. These are given in Table 2. At

    low energy, the copiously produced 43.2 keV G isomer in 13Ag tended to disturb

    conversion electron measurements. Also, because of the resolution of the Si(Li) detector used, the K-79.7 and K-81.3 conversion-electron lines could not be

  • H. Penttilii et al. / p-decay 419

    Table 1 The gamma transitions following the P-decay of Rh and the observed yy-coincidence relations. Note that 81.3 keV transition does not follow the P-decay of t3Rh but the isomeric decay of 13mPd, but its intensity is given because of completeness. Intensities are gamma transition intensities normalized to the 348.9 keV transition and not corrected for internal conversion.

    Transition Relative energy (keV) intensity

    Coincident gamma rays (keV)

    34.9 (3) a 1.2 (2) 79.7 (3) 2.7 (3) 81.3 (3) a 6.9 (4) 84.9 (2) b 8.2 (5) 96.8 (3) 1.8 (3)

    100.4 (3) 0.7 (1) 116.8 (2) 9.7 (5) 119.4 (3) h 0.5 (1) 120.8 (3) 2.2 (3) 135.0 (2) h 2.8 (3) 137.5 (2) 7.8 (3) 151.8 (3) 7.4 (4) 157.1 (3) 5.7 (4) 159.9 (3) 4.8 (5) 189.7 (2) 45.0 (8) 197.0 (4) 0.9 (3) 217.0 (2) 9.1 (4) 219.6 (3) 10.3 (6) 221.0 (3) 4.3 (5) 236.7 (4) 0.9 (3) 252.1 (3) 6.8 (5) 254.8 (5) 1.2 (4) 257.5 (4) 2.7 (4) 265.0 (3) 2.8 (4) 310.8 (4) 1.2 (3) 332.7 (3) h 2.0 (3) 339.1 (4) c weak 348.5 (6) 2.2 (5) 348.9 (5) d 2.1 (5) 348.9 (3) 100.0 (9) 357.6 (3) 4.5 (3) 373.1 (4) 1.8 (4) 409.3 (3) 42.2 (8) 454.7 (4) 2.8 (4) 500.3 (3) 5.5 (4) 538.8 (4) 7.0 (5) 543.0 (4) 3.8 (4) 609.0 (3) 6.8 (5) 671.1 (4) 2.3 (5) 749.1 (4) 1.7 (4) 932.7 (4) 3.8 (5) 980.0 (5) 2.0 (4)

    1053.0 (5) 1.9 (4)

    97, 121, 138, 157, 609

    119, 135,225,980 , 1053 , 1124 b 217,252

    100, 197,221, 258,349

    138, 217, 252,358 85, 119 80, 157, 237, 609 100, 197, 221, 258, 349, 358, 747.5 80, 138,217, 252 190 160, 220, 265,311. 349,542, 933 b, 1226