in-beam γ-ray spectroscopy of 103cd

N U C L E A R PHYSICS A

ELSEVIER Nuclear Physics A 624 (1997) 210-224

In-beam y-ray spectroscopy of 1°3Cd M. Palacz a,b, j. Cederk~ill b, M. Lipoglav~ek c,e, j. Persson o, A. Ataq d, J. Blomqvist b, H. Grawe f,g, C. Fahlander c, J. Iwanicki h, A. Johnson b,

A. Kerek b, J. Kownacki h, A. Likar e, L.-O. Norlin b, J. Nyberg c, R. Schubart g, D. Seweryniak c'i, Z. Sujkowski a, R. Wyss b,

G. de AngelisJ, P. BednarczykJ, Zs. Dombr~idi k, D. Foltescu e, D. Jerrestam m, S. Juutinen n, E. M~ikel~i n, G. Perez k, M. de Poli j,

H.A. Roth e, T. Shizuma °, (). Skeppstedt e, G. Sletten o, S. T6rm~inen n, T. g a s s k

a Sottan Institute for Nuclear Studies, Swierk, Poland b Physics Department Frescati, Royal Institute of Technology, Stockholm, Sweden

c The Svedberg Laboratory, Uppsala University, Uppsala, Sweden d Department of Radiation Sciences, Uppsala University, Uppsala, Sweden

e j. Stefan Institute, Ljubljana, Slovenia f Hahn-Meitner Institute, Berlin, Germany

g Gesellschaftfiir Schwerionenforschung, Darmstadt, Germany h Heavy Ion Laboratory University of Warsaw, Warsaw, Poland

i Institute of Experimental Physics, University of Warsaw, Warsaw, Poland i INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy

k Institute of Nuclear Research, Debrecen, Hungary Chalmers University of Technology, G6teborg University, GOteborg, Sweden

m Department of Neutron Research, Uppsala University, NykOping, Sweden n Department of Physics, University ofJyvaskyla, Jyvaskyla, Finland

o The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

Received 9 April 1997; revised 28 May 1997

Abstract

Excited states of l°3Cd were studied in the reaction 58Ni(S°Cr,4pn)J°3Cd. The NORDBALL array with neutron and charged particle detectors was employed for the detection of y rays and light evaporated particles. The level scheme of l°3Cd was significantly extended. A strong cascade of E2 transitions connecting negative parity states was found. Shell model calculations were performed and positive parity excited states were interpreted in terms of neutron-particle and proton-hole excitations with respect to the doubly magic N = Z = 50 core. The negative parity band was well reproduced by the total routhian surface calculations. @ 1997 Elsevier Science B.V.

0375-9474/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved. PH S0375-9474(96 ) 00325 -4

M. Palacz et al./Nuclear Physics A 624 (1997) 210-224 211

Keywords: NUCLEAR REACTIONS 58Ni(5°Cr,4pny), E = 216-261 MeV; measured yy-coin, E~,, I r. 1°3Cd deduced high-spin levels, J, ~, band structure.

1. Introduction

Nuclei in the region of the doubly magic self-conjugate nucleus l°°Sn have recently been in focus of many experimental and theoretical studies. With the discovery of the first excited states in 98Cd [1] and 99Cd [2] the N = 50 shell closure has been reached in in-beam investigations just two proton holes below the Z = 50 proton magic number. This paper completes our study of the most neutron-deficient odd cadmium isotopes [2,3] providing new spectroscopic information on 1°3Cd. The 1°3Cd nucleus has previously been studied by Meyer et al. [4] and five low excited states were known prior to the present work.

2. Experiment

An experiment with the aim to populate and observe excited states of nuclei in the vicinity of the doubly magic nucleus J°°Sn was performed with the NORDBALL detector array [5,6] placed at the Tandem Accelerator Laboratory of the Niels Bohr Institute, Rise, Denmark. The experiment has already been described in Refs. [2,3,7-10] and the experimental setup is briefly summarized here. The reaction 58Ni+5°Cr leading to the compound nucleus l°STe was used at a beam energy of 261 MeV. Two 5°Cr targets enriched to 96.8% were used with thicknesses of 4.8 and 3.l mg/cm 2 deposited on gold backings of 11 and 19.5 mg/cm 2, respectively. NORDBALL was equipped with fifteen

Ge Compton suppressed spectrometers, a Neutron Wall [ 11 ] consisting of eleven liquid scintillator neutron detectors, a Silicon Ball [ 12] consisting of 21 silicon A E detectors for the identification of charged particles and 30 BaF2 scintillators for y-ray multiplicity and sum energy filtering.

The trigger system required that either: a) at least two Compton suppressed y rays were detected in the Ge detectors and one y-ray in the BaF2 array, or b) at least one Compton suppressed T-ray was detected in the Ge detectors together with one neutron in the neutron detectors and one y-ray in the BaF2 array. About 1.1 x 109 events of type a) and 1.4 x 109 events of type b) were collected during 25 days of effective beam time. For the trigger generation, the neutrons and y rays in the neutron detectors were distinguished by an on-line technique based on the zero-cross-over principle. This discrimination was further improved in the off-line analysis.

In addition to the long run with the beam energy of 261 MeV, five short runs with beam energies of 201, 216, 231, 246, and 261 MeV were performed in order to study excitation functions. A thin, backed target of 0.25 mg/cm 2 was used in this part of the experiment. The simplest possible trigger condition was used and only one y-ray registered in the Ge detectors and one y-ray registered in the BaF2 detectors

212 M. Palacz et al./Nuclear Physics A 624 (1997) 210-224

were required. Selected results from this part of the experiment, related to the relative excitation functions of l°3Cd, are included in this paper.

3. Data analysis

Thirty-one residual nuclei were identified in the experiment with yields ranging from

about 0.001 to 23% of the total yield. Reaction channels were selected by gating on numbers of protons, a particles, and neutrons registered in the charged particle and

neutron detectors. The average efficiency for the detection and identification of protons,

a particles, and neutrons was about 60, 40, and 24%, respectively. The 1°3Cd nucleus

was produced with the emission from the compound nucleus of four protons and one

neutron and the relative yield for this reaction channel was about 15%, a number that

is not corrected for possible variations in the 3,-ray multiplicity of different residual nuclei. One-third of the registered l°3Cd events contained at least two y-ray signals and

were suitable for a 3"3,-coincidence analysis. Note that a lower value of the relative yield of l°3Cd was given previously in Ref. [7] and this was due to the fact that a new

strong, ground state feeding cascade found in the present work was not included in the determination of the yield presented before.

Two 3'3,-coincidence matrices were used to study l°3Cd. The first matrix was created with the requirement that one neutron and four protons were detected. Three protons

and one neutron were required for the second matrix. The two matrices contained about

4.1 and 23 million counts, respectively. The 4pln gated matrix was almost free from

contamination from other nuclei, whereas in the 3pln gated matrix only about 70%

of counts belonged to l°3Cd. The projected spectrum gated with four protons and one

neutron is presented in Fig. la. The Ge signals coming within a wide, two-dimensional,

banana shaped time gate were accepted. The width of the gate was about 75 ns at 200 keV and about 30 ns at 1.5 MeV. Gamma ray spectra and 3,3,-coincidence matrices were analyzed with the aid of the RadWare software package [ 13].

The most probable multipolarity of 3,-ray transitions was determined by calculating the ratio R of the intensity of 3/ lines observed at the two non-equivalent angles with respect to the beam axis (143 ° vs 79 ° and 101°). This was done in spectra gated by

a coincident 3,-ray observed at any direction. A stretched quadrupole transition should have an R value of about 1.5, whereas the value expected for a stretched pure dipole

transition is about 0.8. Other multipolarities may produce R values in the range 0.3

to 1.8, depending on the spin difference between the states and the mixing ratio. A variation of the R values of the order of 0.1 is possible, depending on the gating transition. The above mentioned theoretical values correspond to the situation when the involved states are fully rotationally aligned. Due to the dumping of the rotational motion of residues, the measured values of R may be distributed between the theoretical values and the value of 1, which corresponds to non-aligned states. It was assumed

that in the heavy-ion reaction used here, high spin states are preferably populated. A maximum possible spin which was not in conflict with the R values and with the


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Fig. 1. Projection of the yT-coincidence matrix gated with 4 protons and 1 neutron (a) and spectra gated with with the 188 (b), 740 (c), and 813 keV (d) y rays. The matrix contains 4.1 million counts.

coincidence relationship was assigned for states studied in this work. Only magnetic dipole and electric dipole and quadrupole transitions were considered. Positive parity was assumed for states which decay to lower, positive parity states by transitions for

which quadrupole character was deduced. A sequence of states mutually connected via transitions of a quadrupole character, decaying to positive parity states with high energy

transitions of a dipole character, was assigned the negative parity.


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Fig. 2. Excited states of m3Cd (a) observed in the reaction 58Ni(5°Cr,4pn) m3Cd at a beam energy of 261 MeV

and (b) calculated in the framework of the shell model. Transitions which are placed tentatively are marked with dashed lines. The width of the arrows is proportional to the intensities of the transitions as seen in the reaction studied here. The white parts of the arrows indicate internal conversion.

4. Results

4.1. Level scheme

The level scheme of 1°3Cd constructed in this study is presented in Fig. 2a and the information about the observed y-ray transitions is summarized in Table 1. Examples of coincidence spectra are shown in Fig. lb-d. The intensity values listed in Table 1 are determined using the ESCL8R program [ 13]. Note that uncertainties of these values are subject to assumptions on shapes of the peaks and on the background subtraction procedure (5% uncertainty assumed). The intensity error bars of weak transitions may be underestimated, in some cases. The efficiency calibration uncertainty was estimated to be about 5%.

The states at an excitation energy 188, 908, 1830, 2452, and 2571 keV are known from a previous study of ~°3Cd [4]. The transitions 1195, 1025, 170, and 259 keV,


Table 1 Gamma-ray transitions assigned to l°3Cd. The following quantities are listed: the ~,-ray energy (E;,), the transition intensity (1), the excitation energy of the initial state (Ex), the angular distribution ratio (R) and the spin and parity assignments of the initial (IF) and the final (17) state. Only statistical uncertain±ties are included in the errors of the R values

Ey 1 Es R I F 17 (keV) (relative) (keV)

118.7-4-0.1 39-4-2 2570 .8±0 .3 0 .93±0 .01 21/2 + 19/2 + 168.0 i 0.2 0.7 i 0.1 908.0 .4- 0.2 11/2 + 9 /2 + 170.1 .4- O. 1 6.6 .4- 0.3 3766.1 ± 0.3 0.79 -4- 0.02 25/2 + 23/2 + 187 .5±0 .2 1.5.4,0.1 2798.4.4-0.3 19/2 + 187.9 .4- 0. I 100.0 .4- 3.6 187.9 4- 0.2 0.87 4- 0.01 7 /2 + 5/2 + 2 0 8 . 4 ± 0 . 1 3.4.4-0.2 2779.1 .4-0.4 0.75.4-0.02 21/2 + 250.2 -4- 0.1 0.6 .4- 0.1 5289.5 .4- 0.7 258.9 4- 0.1 24.7 .4- 1.3 4025.0 4- 0.4 0.81 4- 0.01 27/2 + 25/2 + 261.1.4,0.2 0 . 4 i 0 . 1 5039 .3+0 .6 288.2.4-0.2 0.3 4- 0.1 6777.04-0.5 317 .9±0 .1 1 .4±0 .1 1829.74-0.2 0 . 7 0 ± 0 . 1 5 15/2 + 13/2 + 330.94-0.1 3.7.4-0.2 6045.1.4,0.5 3 1 / 2 - 354.8 .4- 0.1 2.7 ± 0.2 2184.5 .4- 0.3 0.83 ± 0.05 17/2 + 15/2 + 425.9.4,0.1 1.7.4,0.1 3205.1 .4-0.4 443.5 .4- 0.1 1.9 -4- 0.1 6488.6 .4- 0.5 552.2 -4- 0.1 7.3 -4- 0.5 740.0 + 0.2 0.76 ± 0.09 9 /2 + 7/2 + 604.7.4,0.1 5 .04-0.3 1512 .5±0 .2 0.73.4,0.11 13/2 + 11/2 + 622.6 ± 0.1 62 ± 3 2452.1 ± 0.3 1.45 ± 0.02 19/2 + 15/2 + 641.3 4- 0.1 2.9 ± 0.2 3252.6 ± 0.4 19/2 + 643.1 .4.0.1 22.1 .4- 1.2 2314.0 4- 0.2 1.42.4,0.10 1 5 / 2 - 1 1 / 2 - 671.9.4,0.1 11.8.4,0.7 2184.5.4,0.3 1.5 ± 0.3 17/2 + 13/2 + 689.0 .4- 0.2 0.9 -4- 0.1 3766.1 .4- 0.3 25/2 + 21/2 + 720.1.4.0.1 94.4-5 908.0.4-0.2 1.434,0.01 11/2 + 7 /2 + 740.0 4- 0.1 54 .4- 4 740.0 4- 0.2 1.37 + 0.02 9 /2 + 5 /2 + 772.64.0.1 304 -2 1512.5.4.0.2 1.5.4,0.3 13/2 + 9 /2 + 781.84,0.1 9 . 4 ± 0 . 6 2611.34,0.3 1.43.4,0.05 19/2 + 15/2 + 801.44,0.1 17.8.4. 1.0 2314.0.4,0.2 0.894-0.09 15 /2 - 13/2 + 810.64.0.1 4.1 4-0.2 4835.5.4-0.5 0.70 ± 0.13 29/2 27/2 + 813.1 .4,0.1 3 6 ± 2 4 8 1 3 . 5 ± 0 . 4 1 .38±0 .09 2 7 / 2 - 2 3 / 2 - 8 1 8 . 9 ± 0.1 38.4-2 3132.94-0.3 1.45.4,0.02 19 /2 - 15 /2 - 859.6 .4- 0.1 4.3 ± 0.3 3657.9 ± 0.3 867.5 ± 0.1 33 -4- 2 4000.4 ± 0.3 1.39 ± 0.09 2 3 / 2 - 1 9 / 2 - 887.1 .4,0.1 3 .94-0 .2 4545.0.4-0.4 892.5 -4- 0.1 7.0 ± 0.4 3077.0 .4- 0.3 21/2 + 17/2 + 900.64.0.1 26.6.4- 1.4 5714.1 ± 0 . 4 1.36.4,0.10 3 1 / 2 - 2 7 / 2 - 921.5.4,0. l 83.4-4 1829.74.0.2 1.344,0.04 15/2 + 11/2 + 923.3 ± 0.1 2.9 ± 0.2 4000.4 .4- 0.3 2 3 / 2 - 21/2 + 930.9.4,0.1 24.5.4- 1.4 1670.94,0.2 0.774,0.01 1 1 / 2 - 9 /2 + 948.4.4,0.1 3.2.4-0.2 3132.94,0.3 1 9 / 2 - 17/2 + 962.9 .4- 0.1 2.2 ± 0.2 4729.0 .4- 0.4 25/2 + 963.14.0.1 3.3.4-0.3 4 0 9 6 . 0 ± 0 . 4 19 /2 - 968.5.4,0.1 4 . 6 ± 0 . 3 2798.4.4-0.3 15/2 +

216

Table 1 - -cont inued

M. Palacz et aL/Nuclear Physics A 624 (1997) 210-224

E~ 1 Ex R 1T 17 (keY) (relative) (keY)

1001.74-0.1 3 . 1 ± 0 . 2 5546 .8±0 .5 1001.9 ::[: 0.2 1.24-0.1 5098.04-0.5 1015.94-0.1 1.84-0.1 5040.94-0.5 27/2 + 1025.2 4- 0.1 10.0 4- 0.5 3596.0 4- 0.3 0.90 4- 0.03 23/2 + 21/2 + 1046.54-0.1 1.74-0.1 3657.9 i 0.3 19/2 + 1063.04-0.1 10.5 i 0.6 6777.04-0.5 3 1 / 2 - 1144.1 4-0.2 1.34-0.1 3596.04-0.3 23/2 + 19/2 + 1195.34-0.1 26.74- 1.4 3766.1 4-0.3 1.364-0.03 25/2 + 21/2 + 1233.84-0.1 2 .54-0.2 8010.84-0.5 1331.9 4- 0.3 0.5 4- 0.1 5098.0 4- 0.5 25/2 + 1429.9 4- 0.9 0.8 ± 0.1 4000.4 ::l:: 0.3 2 3 / 2 - 21/2 + 1573.24-0.6 0 . 7 t 0 . 1 4778.24-0.5 1.54-0.2

above the 2571 keV level, were also observed in the work of Ref. [4], but they were placed in the level scheme in a different order. The 1195 keV transition is assigned a stretched E2 character, whereas the 1025, 170, and 259 keV transitions are of a dipole type. Spins and parities 23/2 +, 25/2 +, and 27/2 + are assigned to the states at 3596, 3766, and 4025 keV, respectively.

A new strong cascade of transitions linking the states at 740, 1512, 2184, and 3077 keV is observed. Several transitions connecting the new cascade with the previously known part of the decay scheme strongly support the proposed level scheme. Positive parity and spins 9/2, 13/2, 17/2, and 21/2 are assigned to the levels at 740,

1512, 2184, and 3077 keV, respectively. This is corroborated by the angular distributions of the 740, 773, and 672 keV transitions, by the angular distributions of the linking transitions 552, 605, and 355 keV and by the observation of the 689 keV transition linking the 25/2 + state at 3766 keV with the level at 3077 keV.

Another strong cascade consisting of the 643, 819, 867, 813,901, 1063, and 1234 keV transitions was observed, as shown in the right-hand side of Fig. 2a. The stretched quadrupole character is deduced for the five lowest transitions of this cascade. The two strong transitions 931 and 801 keV, linking the cascade with the levels at 740 and 1512 keV, respectively, are of dipole character. Three other transitions 948, 1430, and 923 keV link the levels at 3133 and 4000 keV with the 2184 and 3077 keV levels. Negative parity and spins 11/2, 15/2, 19/2, 23/2, 27/2, and 31/2 are assigned to the states at 1671, 2314, 3133, 4000, 4813, and 5714 keV, respectively.

The ordering of the three lowest transitions in this negative parity cascade is supported by the linking transitions, whereas the ordering of the higher lying transitions is deduced from their relative intensity. The highest lying transitions of the cascade, 1063 and 1234 keV, are strongly Doppler broadened, indicating that these transitions are faster than the stopping time of the residues in the target and backing material. The negative parity cascade is more populated at higher effective beam energy than the positive parity states, as shown by the data from the excitation function run (see below). A side


branch of the negative parity cascade is observed, consisting of the 963 and (possibly)

1002 keV transitions extending up to the level at 5098 keV. This level decays into the

positive parity state at 3766 keV via the 1332 keV transition. Two other sequences of transitions feeding the positive parity states 15/2 + and 21/2 +

at 1830 and 2571 keV, respectively, were also observed. The former one is relatively

strong and consists of the 782, 1047, 641, 188, 969, 860, 887, and 1002 keV transitions.

The spin and parity 19/2 + was deduced for the 2611 keV level lying at the bottom

of this cascade. The latter cascade, feeding the 21/2 + state, is only tentatively placed

in the level scheme. The level scheme is completed with a few other single transitions

feeding different levels. Some of these transitions are only tentatively placed. Several other transitions were observed in coincidence with the transitions belonging

to l°3Cd, but could not be convincingly placed in the level scheme. This includes the

following y rays: 69, 208 (a doublet with the 208 keV transition depopulating the level

at 2779 keV), 275, 288, 319, 381, 416, 457, 587, and 1155 keV. One of these lines, namely 457 keV, was reported earlier [4] as a transition feeding directly the 11/2 + level

at 908 keV and being in coincidence with a 819 keV line, but our data do not support such a placement. There is also evidence for a highly fragmented, thus not established,

link between the state at 6777 keV and various positive parity states. This link includes the 443 and 288 keV y rays depopulating the levels at 6489 and 6777 keV, respectively.

4.2. The 19/2 + isomeric state

The 19/2 + level at 2452 keV was found to be isomeric with a half-life of 1.3+0.2 ns.

The half-life was determined by analyzing time distributions of y-ray lines registered in the Ge detectors. A decay curve G(t ) convoluted with a prompt shape P ( t ) was

fitted to the time distributions. A direct prompt feeding of a depopulated level was also allowed. The fitted function F ( t ) had the following general form:

F( t) = a P ( t) + B( P ® GT,~/2T~/2) ( t ) ~- C . (1)

The parameters A and B describe the relative contributions of the delayed and prompt

components, the C parameter is added in order to take into account a flat background. The decay curve G is a sum of up to two exponents with the half-lifes Tll/2 and T~/2. An experimental, non-Gaussian prompt shape was used. The prompt shape was deduced

from the time distributions of prompt y-ray lines of a similar energy as the analyzed one. A small variation of the prompt width could be allowed in the fits, the results were,

however, not sensitive to releasing or fixing the width parameter. The transitions below the 19/2 + level consistently show a delayed component, whereas

the transitions above this level do not. The results of the fits to the time distributions of the 720, 921, and 623 keV are presented in Table 2 and the time distribution of the

720 keV transition is shown in Fig. 3. The statistical error of the measured half-life is smaller than the value quoted above. The error bar was increased in order to take into

account systematic uncertainties. A non-zero prompt contribution was obtained for the 623 keV line, which is not supported by the presented level scheme. A second delayed


Table 2 Results of the fits to the time distributions of the "), rays which depopulate the 19/2 + level. The half-life value is determined as the weighted average of the values from fits 1, 2, and 3. The error bar was increased in order to account for systematic uncertainties (see fits 4 and 5) and the final value Ti/2 = 1.3 4- 0.2 ns was adopted

Fit Gate energy Tl/2 Prompt component Remarks (keV) (ns) (%)

1 720 1.18 4- 0.05 46 4- 4 2 921 1.21 4-0.03 354-3

3 623 1.42 ~- 0.03 19 ± 2

Mean value 1.29 4- 0.02

4 623 1.22 ± 0.01 0 5 921 1.33 ± 0.02 40 4- l

Second delayed component 3% contribution TI/2 = 7.9 4- 0.7 ns Non-zero prompt

No prompt Only one delayed component

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Fig. 3. Time distribution of the 720 keV transition. The dashed line shows the fitted curve which is a sum of the delayed and a prompt component. The delayed component is responsible for 54% of the intensity of the 720 keV line. The dotted line shows a curve which is obtained from the fit if the transition is assumed to be prompt. The time calibration on the abscissa axis is 0.53 ns/channel.

c o m p o n e n t ( 3 % cont r ibu t ion) wi th a half- l i fe o f about 8 ns was seen in the d is t r ibut ion

o f the 921 keV line. The results o f the fits wi thout p rompt feeding for the 623 keV l ine

and wi th only one de layed c o m p o n e n t cont r ibut ing to the shape o f the 921 keV line,

are also shown in Table 2.

It cannot be exc luded that the observed delayed c o m p o n e n t should be associa ted wi th

the 2 1 / 2 + level at 2571 keV, instead o f the 19/2 + level. This uncer ta inty is due to

M. Palacz et aL/Nuclear Physics A 624 (1997) 210-224 219

the fact that the time distribution of the 119 keV transition is very broad so it is not

possible to verify if this transition is delayed on a 1 ns scale. The analogy to the l°lCd

nucleus [3] suggests, however, that the isomeric level has a spin of 19/2 +. Note also that the prompt contribution to the distribution of the 623 keV line should be larger

than the measured 19%, if the half-life were associated with the 21/2 + state. A prompt component of 38% is expected in such a case, as deduced from the intensities of the

623 and 119 keV lines (see Table 1). An indication of a half-life of the 19/2 + level in l°3Cd was also observed in the

unpublished work of Ref. [ 14]. In that experiment prompt transitions were emitted in

flight and thus Doppler shifted or broadened. The 119 keV transitions was Doppler broadened, whereas transitions below 19/2 were not and this additionally supports the

assignment of the isomer to the 19/2 + level.

4.3. Relative excitation functions

Relative excitation functions were deduced for the states populated by the strongest y-ray transitions in 1°3Cd. The y-ray intensities were compared at four different beam

energies; 216, 231, 246, and 261 MeV. For each beam energy, the y-ray intensities

were normalized to the intensity of the 188 keV transition. The yield of 1°3Cd was unmeasurably small for the lowest beam energy of 201 MeV used in the experiment, so

only data for beam energies of 216, 231,246 and 261 MeV are presented.

The transitions from the main positive parity cascade, namely 720, 921, 623, 119,

1195, and 259 keV (Fig. 4a) show a moderate increase of the intensity as the beam

energy increases. This is in contrast to the transitions depopulating the negative parity

states, which are more than twice as strong at a beam energy of 261 MeV as at 231 MeV (Figs. 4c,d). The intensity of these transitions at a beam energy of 216 MeV

was unmeasurably small. The 931 keV line shows an increase of the intensity at the beam energy of 216 MeV. This exceptional behaviour of the 931 keV line is probably

due to a contribution from another transition of a similar energy, perhaps from a different

nucleus. A weak dependence of the transition intensity on the beam energy is seen for the

transitions 672 and 892 keV, whereas the transitions 773 and 740 keV show a similar

behaviour as the negative parity cascade (Fig. 4b). Note that the 773 and 740 keV transitions are strongly fed by the negative parity cascade, whereas such feeding is

much weaker for the transitions 672 and 892 keV. The population of the positive parity cascade consisting of the 782, 969, 860, 887, and

1002 keV transitions hardly depends on the beam energy (Fig. 4e). A steep increase of

the intensity of the 860 keV line is due to the increasing contribution of the 861 keV transition from 1°2Ag (9 - ---+ 8 +) [ 15].

The difference in the excitation functions observed for various lines indicate a different

microscopical structure of the corresponding parts of the level scheme. The observed consistency in the behaviour of groups of lines gives additional support for the presented placements in the level scheme.


~O 720 a) 100 I ~

80

II © 4 ~ c 60 co > o "1-259 © ~ 1 ()25 Q2 4()

20 • '~

0

b) O 74-0 [ ] 775

A 672 0 892

c) O801 Z951 & 6~3 0819

CO I~ ~D ~ C O CO - - ~ 3 ~

Cq C'q ~ OqCq f'q cq £'qCN ['~ ("q f'qCN ¢q

Beam Energy (MeV)

O 867 []813 A901 <> 1063 © 1234

e) O 782 [] 969 A 860 0 887 C= 1 OO2

i I ~ I I

~:3 ~ t.D CO

CN C ' q f N c q f ' q

Fig. 4. Excitation functions of the 25 strongest transitions in m3Cd. The energies of the transitions are marked in the plots. The intensities are normalized to the intensities of the 188 keV transition. The transitions are grouped in the plots according to their placements in the level scheme: plots (a) and (b) show transitions from the two main positive parity cascades, plots (c) and (d) show the transitions depopulating the negative parity states and plot (e) shows the positive parity cascade on the left-hand side of the level scheme in Fig. 2, feeding the 1830 keV level.

5. Discuss ion

The excited states in 1°3Cd can be interpreted in terms of neutron-particle and proton- hole excitations with respect to the doubly magic l°°Sn core. The l°3Cd nucleus has

two proton holes and five neutron particles outside the core. Numerical shell model calculations were performed using the code RITSSCHIL [16] . The 88Sr nucleus was

used as a core in the calculations. Proton excitations to the Pl/2 and g9/2 orbitals and neutron excitations to the d5/2, g7/2, Sl/2, d3/2, and h l l / 2 orbitals were allowed. There was no artificial restrictions on the numbers of particles in the included orbitals.

Imposing such limits would lead to significantly changed results, if limits were lower than the number of valence particles in the nucleus. The single particle energies with


Table 3 Single particle energies (SPE) used in the shell model calculations. Energies relative to the 88Sr and I(~Sn cores are shown

Orbital SPE (MeV) SPE (MeV) 88Sr I~Sn

~g9/2 -6.248 -2.993 ~pl/2 -7.124 -3.379 Pg7/2 --3.667 - 10.95 pals~ 2 -6.397 - 11.15 vd3/2 -3.807 -9.254 Psi/2 -4.783 -9.353 Phjl/~ -3.825 --8.608

respect to the 88Sr core and the corresponding single particle energies with respect to

J°°Sn are listed in Table 3. Other details of the calculations are given in Ref. [ 17]. Note that the shell model parameters were not specifically adjusted for the l°3Cd nucleus and

the same set of parameters was used for calculations for other nuclei in the region. The calculated energies of excited states in 1°3Cd are presented in Fig. 2b. The particle and

hole excitations discussed below are expressed relative to the l°°Sn core.

The lowest excited states of l°3Cd can be qualitatively interpreted as seniority one

states with the odd neutron placed either in the d5/2 or in the g7/2 orbital. The calculations

show that although all the low lying states are very mixed, with a large contribution from

the higher lying neutron orbitals (about 50%), the placement of the unpaired neutron

is to a large extent in agreement with such a qualitative picture. In terms of squared

amplitudes, a 50-60% contribution of the d~/2 unpaired neutron is calculated for the yrast states 5 /2 +, 9 /2 +, 13/2 +, and 17/2 +, whereas the states 7 /2 +, 11/2 +, and 15/2 +

have a comparable contribution from the unpaired g7/2 neutron. The contribution from

the unfavoured odd-neutron configurations is below 10%, for both groups of states. The angular momentum for the states mentioned above is mainly generated by the neutron

pairs, with the two g9/2 proton holes coupled to low spin values, mainly spin 0.

Two 19/2 + states are calculated at almost the same excitation energy. One of the calculated 19/2 + states should be associated with the experimental 19/2 + state at

2452 keV. The wave functions of the two states calculated at 2745 and 2748 keV have

dominating components with the odd neutron in the d5/2 and g7/2 orbitals, respectively. Both these calculated states have the two proton holes coupled to the maximum spin

value of 8, while the four neutrons couple to spin 0. The transition depopulating the 19/2 + level is hindered, with an experimental B(E2) value of 0.18 ± 0.01 W.u. This is analogous to the situation in J°lCd [3,18] where a B(E2) value of 0.16 4-0.01 W.u. was measured for the transition depopulating the 19/2 + state. In both l°lCd and l°3Cd

isotopes, the hindrance of the transition depopulating the 19/2 + level is related to the

fact that this is the lowest level with the angular momentum generated by proton-holes. An isomeric effect of the same origin is observed for 8 + levels in the even cadmium isotopes l°°Cd and J°2Cd [19].


The shell model calculations reproduce relatively well the B(E2) value in l°lCd, but they fail in that respect in the case of l°3Cd. The B(E2) values calculated for transitions from the two 19/2 + states, 2745 and 2748 keV, to the 15/2 + state, are 1.7 and 3.5 W.u., respectively. These values were obtained assuming effective charges of 1.72 and 1.44 for protons and neutrons, respectively. The discrepancy between the calculations and the experiment may perhaps be explained by the contribution to the wave functions from the poorly known higher neutron lying orbitals d3/2, Sl/2, and h11/2. This contribution increases with the number of valence neutrons and is larger in l°3Cd than in 10JCd. Another possible explanation is the increasing significance of the particle-hole excitations over the N = 50 gap. Such excitations are not included in the calculations. A similar behaviour is observed for the 8 + --~ 6~- transitions in l°°'l°2Cd,

with the former being in a better agreement with the experiment than the latter.

The calculations reproduce well the layout of the positive parity yrast states, up to the spin of 29/2 + . The calculated 29/2 + state can probably be associated with the experimental level at 4836 keV.

A systematic tendency to overestimate the energies of the levels in the calculations is observed. Such an effect was also seen for 99Cd [2] and J°lCd [3] (see also Ref. [20] ).

One of the reasons for this discrepancy can be related to the proton matrix elements. As recently shown by G6rska et al. [ 1 ], the level scheme of 98Cd is best reproduced using

proton matrix elements given by Blomqvist and Rydstr6m [21], whereas proton matrix elements from the work of Gloeckner and Serduke [22] were used here. On the other hand, the energies of the first excited states in the recently studied nuclei l°2Sn [23] and l°lIn,cede96 are also calculated too high, which suggest that the neutron-neutron and

neutron-proton matrix elements may contribute to the observed discrepancy. In addition, two orbitals contribute mainly to the ground state wave function, which may lead to the lowering of the calculated ground state energy and thus shifting all other calculated states up, if the two orbitals are too strongly bound. These are the proton~hole Pl/2 and neutron-particle h11/2 odd-parity orbitals. They contribute to the ground state wave function with two holes (or particles) coupled to spin 0, building up 20% and 9% of

the wave function, respectively. The properties of the ~Pl /2 and vhll/2 orbitals are in the open N > 50 shell poorly known. According to our calculations, such a relatively

-2 large contribution of the ~PI/2 and vh~l/2 configurations to the ground state is typical

for most of the nuclei in the region. In the case of 1°3Cd, a large contribution of the -2 ~Pl/2 configuration is also seen for the 7/2 + state (17%). Excitation of Pl/2 protons

has been found to yield too low lying states in a number of cases [24,25]. This is a deficiency of the residual interaction used in the present shell model calculation.

The 11/2- state is calculated about 500 keV higher than the level observed experimentally. The regular structure of the negative parity levels is not reproducedin the Shell Model calculations. To shed more light on the structure of the negative parity band, we have performed pairing- and deformation self-consistent total routhian surface calculations [26,27]. The experimental and calculated aligned angular momentum of the negative parity band are presented in Fig. 5. In order to determine the aligned angular


20 ~_ * exoeriment

16

34 _

8

4 ATRS neutrons

2 [] TRS protons / /

0.1 0.2 0.5 0.4 0.5 0.6 0.7 (MeV)

Fig. 5. The aligned angular momentum as a function of the rotational frequency for the negative parity band in 1°3Cd. The experimental values and the values calculated in the TRS calculations are marked. The theoretical values from the TRS calculations are shown both for neutrons and protons separately as well as their sum.

momentum we assume that the states at 6777 and 8011 keV have a spin and parity

3 5 / 2 - and 3 9 / 2 - , respectively.

The structure of the deformed bands in the Cd-isotopes is a question open for debate

and different interpretations have been given (see Refs. [ 27,28 ] and references therein).

Although l°3Cd is very close to doubly magic l°°Sn, the solution of the cranked Strutin-

sky calculations results in a deformed minimum at /32 ~ 0.14 with rather large positive

values o f /34 --~ 0.03 and positive y ~ 10 °. At this deformation the proton structure

is mainly determined by g9/2. The g9/2 proton-holes align their angular momenta at

hw ~ 0.5 MeV, together with g7/2 neutrons corresponding to the experimentally ob-

served backbend at hw = 0.45 MeV (cf. Fig. 5). The opposite driving force of proton

holes and neutron particles polarize the core towards spherical shape and the deformation

is calculated to decrease above the backbend. The 3 9 / 2 - state corresponds mainly to

a fully aligned five quasi-particle configuration (~'hll/2(pg2/2)6 , -2 ( 7"rg9/2 ) 8 ~ ) 39/2-. States

above will be non-colective and the band is calculated to terminate at 4 7 / 2 - . The

termination may occur in a smooth manner as observed in heavier Sb and Te nuclei.

6. Conclusions

The level scheme of l°3Cd was extended up to the excitation energy of about 8 MeV.

The 19/2 + level at 2452 keV was found to be isomeric with a half-life of 1.3 ns.

The posit ive parity yrast levels are well described in the shell model calculations. The

observation of the negative parity cascade is in agreement with the total routhian surface

calculations. The negative parity cascade exhibits a back-bending which is interpreted

as a simultaneous al ignment of the proton-hole g9/2 and neutron-particle g7/2 pairs.

224 M. Palacz et al,/Nuclear Physics A 624 (1997) 210-224

Acknowledgements

This w o r k was par t ia l ly suppor t ed by the the Po l i sh Scient i f ic Resea rch C o m m i t t e e

( g r a n t no. 1 8 1 / P 0 3 / 9 5 / 0 9 and 1 9 3 / P 0 3 / 9 5 / 0 8 ) , Swed i sh Natura l Sc ience Resea rch

Counc i l , the G 6 r a n G u s t a f s s o n Founda t ion , the A c a d e m y of F in land , the D a n i s h Na tu ra l

Sc ience R e s e a r c h C o u n c i l and the H u n g a r i a n Fund for Scient i f ic Research . The exce l l en t

c o o p e r a t i o n o f the s ta f f o f the T a n d e m Acce le ra to r Labora to ry o f the Niels B o h r Ins t i tu te

is apprec ia ted . Ch. L i n g k is a c k n o w l e d g e d for c o m m u n i c a t i n g u n p u b l i s h e d resul t s o f the

i somer ana lys is .

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in-beam γ-ray spectroscopy of 103cd

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