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 b 117 97, 121, 157, 358, 609 190 117, 152,358

    97, 121, 157, 609

    117, 152,333, 340, 672 190 190 258, 340 258, 333 117, 152 190

    117, 121, 221, 252, 339

    80, 138,217, 252

    Intensity from the singles spectrum. Not placed in the level scheme. Seen only in the yy spectrum. Intensity deduced from the y-spectrum.

  • 420

    L. 70 Te.0 a5 90

    s-i 1

    0 50 100 150 200 250 300 3

    ENERGY [kcVj

    i-

    . 0

    Fig. 1. The low energy part of the beta coincident gamma-ray spectrum at the mass number A = 113. Planar Ge detector spectrum is shown because of its superior resolution over coaxial Ge detectors and because most of the transitions are below 400 keV in energy. The inset shows the appearance of the isomeric 51.3 keV transition in the single gamma-ray spectrum. 97Y and *Sr decays are due to

    monoxide (YO* 1 and hydroxide (SrOH+ 1 impurities.

    resolved from the singles conversion-electron spectrum, Fortunately, the conver- sion electrons due to the f isomer disappeared in the /? coincident-electron spectrum, as did also the canversion electrons due to the 81.3 keV M2 transition de-exciting the f- isomeric state in 113Pd For the determination of the ICC for . the $1.3 keV transition, the calculated intensity af the K-conversion electrons due to the 79.7 keV Ml transition was subtracted from the intensity of the K-79.7/K- 81.3 doublet in the singles conversion-electron spectrum. The multipolarity of the 79.7 keV transition results from the ICC deduced from the /3-gated spectra. The deduced K/L ratio for the 81.3 keV transition is 4.14 1.2, which implies L = 2 for this transition. In the case of the 34.9 keV transition, it was possible to observe only L conversion electrons.

    3.2. bomeric transition ia 113+11s,117Pd

    Fig. 2 shows the conversion-&c&on spectra measured in coincidence with the K X-rays of Pd at the mass settings of A = 113, A = 115 and A = I1 7. Because of the gate in the K X-rays, only the K-electron lines are seen in the spectra. The labeled peaks were identified as the isomeric transitions, since hey could not be seen in coincidence with p pa&&. The &served isomeric states in 113,1*fPd are directly fed in fission, because their measured half-lives are much shorter than the

  • H. Pmttilii et al. / P-decay 421

    Table 2 Experimental internal conversion coefficients and deduced multipolarities for transitions in Pd. Theoretical conversion coefficients are taken from ref. [23]. D indicates simultaneous measurement of conversion electrons and gamma rays. DS means that intensities of conversion electron lines and gamma transitions are deduced from singles spectra taken in separated runs. B means the same as DS, hut the intensities of conversion electron lines are taken from the beta gated electron spectrum.

    Transition energy CkeV)

    u,(exp) Method cu,(theor) Multipolarity

    34.9 (Y,. = 29 (7)

    79.7 0.56 (15)

    x1.3

    x4.9

    116.8

    120.7

    135.1

    137.5

    151.8

    189.7

    211.0

    252.1

    348.9

    409.5

    5.4 (9)

    0.12 (3)

    0.31 (3)

    0.52 (11) 0.57 (12) 0.15 (5)

    0.16 (3)

    0.08 (2)

    0.063 (4)

    0.05 (3)

    0.04 (3)

    0.0144(20)

    0.020 (6)

    D Ml (YI_ = 1.5 E2 CYL=40

    B I El 0.254 Ml 0.645 E2 2.235

    Dh Ml 0.610 E2 2.096 M2 7.043

    B . El 0.212 Ml 0.539

    D Ml 0.220 E2 0.623

    D Ml 0.201 DS E2 0.557 D Ml 0.147

    E2 0.377 D Ml 0.140

    E2 0.355 D El 0.0403

    Ml 0.101 D El 0.0215

    Ml 0.0586 E2 0.117

    DS Ml 0.0411 E2 0.0145

    DS Ml 0.0278 E2 0.0445

    D Ml 0.0121 E2 0.0152

    DS Ml 0.00818 E2 0.00932 M2 0.0293

    E2

    Ml

    M2

    El

    Ml/E2

    E2

    Ml

    Ml

    Ml

    Ml

    Ml/E2

    E2/Ml

    Ml/E2

    E2

    Normalized to the 189.7 keV Ml transition in Pd. h The K-79.7 keV (Ml in Pd) intensity is calculated and subtracted from the electron intensity.

    /? half-lives of the parent Rh isotopes. For the isomeric state in lsPd evidence of feeding also through the P-decay of rsRh was observed [S].

    The half-life of (0.3 _t 0.1) s for 3mPd was deduced from the decay of the isomeric 81.3 keV gamma transition in the singles spectrum (see the insert in Fig. 1

    and Fig. 3). No other transitions could be identified to de-excite this isomer. The

    multipolarity of M2 was assigned to this transition using the experimental internal conversion coefficient of crK = 5.4 + 0.9 and the K/L ratio of 4.1 k 1.2. Based on

  • 422

    J

    H. Penttilii et al. / p-decay

    100

    50

    0

    150

    Lo 100

    -z 2 50 0

    O-

    200

    150

    100

    50

    O-

    A=113

    I

    I I I

    Electron energy (keV)

    Fig. 2. The conversion-electron spectra sated by the characteristic X-rays of Pd at mass numbers A = 113, A = 115 and A = 117. The labeled transitions are connected with the decay of isomeric states

    in Pd nuclei.

    300

    200

    E

    5

    8

    100

    I I I I

    \ i

    l1; I I I

    100 200 300 400 500 TIME [ms]

    Fig. 3. Decay curve of the 81.3 keV y-ray de-exciting the isomeric state in Pd. The single component fit results in a half-life of 0.3 f 0.1 s.

  • H. Penttilii et (11. / p-deco) 423

    the M2 multipolarity of the isomeric transition and the 1 assignment for the

    ground state of Pd 161 1 = p - was obtained for the isomeric state.

    The isomeric state in lsPd has also been observed by Fogelberg er al. [6,16]. In

    our work it was not possible to deduce the internal conversion coefficient due to

    background in singles gamma ray spectrum. Only a lower limit (Ye 2 11 could be

    deduced, implying an E3 multipolarity in agreement with ref. [161. M2 multipolari- ties were assigned to both transitions labeled in the A = 117 spectrum. No

    evidence for the isomeric transition was observed for Pd. Instead, two P-decay-

    ing states with opposite parity were proposed for this isotope [17].

    4. Level scheme of 13Pd

    The level scheme of jPd was constructed mainly on the basis of the yy

    coincidence relations given in T...