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,11’7Pd

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 Table I. Altogether 33 gamma transitions were

placed in the level scheme shown in Fig. 4. The multipolarities of 14 transitions

were deduced from the internal conversion coefficients, as shown in Table 2.

These as well as the P-decay properties of “‘Rh were used for the spin and parity

assignments of the levels, as discussed below.

l712+l 2.805

I--

‘13Rh +=5.13 MeV

T 09% 65

27% 60

0.8% 6

I’ 6

:

3 7 % 6 0

36% 60

* ma 6

24.4% 5 2

22% 63

645% 50

12% 66

110.‘% r57

15% 65 0 61

log tt ‘13Pd E IkeVl

Fig. 4. The decay scheme of “‘Rh Internal conversion is included in the given intensities of the transitions. The isomeric state at 81.3 keV in ‘13Pd is, however, not populated in the P-decay but

directly in fission.

424 H. Penttilii et al. / P-decay

The ground state. The spin and parity $ + were uniquely assigned to the ground

state of ‘13Pd in ref. [6]. The assignment was based on the observed P-decay

properties of ‘13Pd. The 2’ ground state is also observed for the lighter odd-A

nuclei. Its origin for the light odd-A Pd nuclei clearly lies in the spherical shell

model d5,2 neutron orbital. However, it is difficult to understand, how the d5,2

neutron orbital could alone be responsible for the ground-state spin of heavier Pd

nuclei.

Even though the relative location of single-particle orbits changes remarkably

from N = 50 to N = 82, so that the g,,, orbital is well below the d5,2 orbital near

“‘Sri, the g,,, is still above the d,,, orbital in ““Pd, in which neutron should

already fill the g7,* orbital. In fact, it is known that the 3’ ground state of odd-A

Pd nuclei has a mixture of both the ds,2 and the g7,2 spherical shell-model

orbitals. The considerable mixing of the g7,2 spherical shell-model state in the

ground-state wave function results in fast P-decay to the f ’ state in the odd-A Ag

nuclei [ 181.

The 34.9 keV lecel. The observed y-y coincidence data can be explained only by

introducing a level at 34.9 keV, as seen in the level scheme given in Fig. 4. The

34.9 keV gamma transition did not appear in the yy spectra, but it was observed in

the low-energy gamma singles spectrum. More evidence of this transition was

obtained from the singles electron spectrum, where a line with an energy of 32.4

keV was observed, which corresponds to the L-34.9 transition in Pd. The K-con-

version electrons had too low energies to be observed in the present measure-

ments. The deduced L-conversion coefficient for the 34.9 keV transition implies

an E2 multipolarity. An $’ level at low energy is expected on the basis of the level

systematics of the lighter odd-A Pd nuclei. Spin and parity i’ were thus assigned

for the 34.9 keV level. Furthermore, these can also be deduced from the multipo-

larities of the 116.8, 137.5 and 217.0 keV transitions populating the 34.9 keV level.

Due to the E2 multipolarity a relatively long lifetime of the corresponding state is

expected. The non-observation of the 34.9 keV gamma transition in coincidence

with /3 particles implies that the level has a lifetime much longer than the width of

the 1 ps coincidence window used. The Weisskopf estimate for the half-life of a

34.9 keV E2 transition is 5 ps.

The 81.3 keV level corresponds to the the isomeric state found in ‘13Pd. The

multipolarity of the observed 81.3 keV transition was deduced to be M2. No

coincidences were observed for the 81.3 keV gamma transition. Thus, there are

two possibilities to place the transition: leading to the ground state, or to the 34.9

keV level, which also has a lifetime long enough to prevent coincidences. On the

basis of the experimental systematics of the odd-A Pd nuclei a high spin is

expected for the isomeric state, which supports the f + assignment for the 34.9 keV

level and the placement of the 81.3 keV transition leading to the %’ ground state.

If the 81.3 keV M2 transition is populating the i’ state, the possible spins of the

initial state would be i, s or $. A state with such a low spin could be expected to

H. Penttilii et al. / p&my 42s

decay in very competitive way also to the ground state. The highest possible spin

for the 81.3 keV level is 4 and this spin with negative parity was assigned for this

level.

The 151.8 keV letjel is connected to the ground state via an Ml and to the 34.9

keV level via an Ml/E2 transition. Spin and parity $’ are thus suggested.

The 172.4 keV lerlel. The placement of this level is fixed by the 236.7 keV gamma

transition and the intensity ratio between the 79.7 and 137.5 keV transitions.

Without the 236.7 keV transition the order of the 79.7 and 137.5 transitions could

be switched. This level is connected to the +’ level at 34.9 keV via an Ml

transition and to the 252.1 keV level also via an Ml transition. Since no decay to

the ground state was observed, a spin difference as large as possible between the

172.4 keV level and the ground state is assumed and a spin and parity t + are

suggested also for this level.

The 189.7 keVleLle1 is connected to the ground state via the second most intense

transition in the ““Rh decay. The Ml assignment of the 189.7 keV transition

implies a spin and parity of 5 +, : + or ; + for this level. The relatively large

P-decay branching to the 189.7 keV level implies a spin of 2, $ or +, if the ground

state of “‘Rh is assumed to have spin and parity of :‘. Since no decay to the 4 +

level at 34.9 keV is observed, : + is preferred over $ +.

The 252. I keV Ie~~el decays to the $ + ground state and the + + level at 34.9 keV

via Ml/E2 transitions. It is also connected via an Ml transition to the 172.4 keV

level, for which spin and parity of i + were deduced. The $’ assignment for the

525.1 keV level is suggested. If the 252.1 keV transition is E2 with no Ml mixing,

which is possible within the precision of the LYE measurement, a $’ assignment

also becomes possible.

The 348.9 keV leclel. The 348.9 keV gamma transition is the most intense in the

decay of . “‘Rh It was assigned to this decay via X(ce) coincidences and its

half-life.

The 348.9 keV level is the de-excited by several gamma cascades to the ground

state, as shown by the level scheme in Fig. 4. The intense 348.9 keV transition was

also placed to de-excite this level. However, some weak coincidences between the

349 keV gamma rays and the 189.7 keV, 151.9 keV and 116.8 keV transitions were

observed. These were far too weak to be coincidences between the mentioned

transitions and the very intense 348.9 keV ground-state transition. The 189.7 keV

transition is the second most intense transition in the p decay of ““Rh. Further-

more, the exact energy of the 349 keV gamma rays observed in coincidence with

the 116.8 keV and 151.9 keV gamma transitions was (348.5 _t 0.6) keV. The

observed coincidences and intensities are explained by the multiple placement of

349 keV transitions. The 348.5 keV transition in coincidence with the 116.8 and

151.9 keV transitions fits energetically to the transition from the 500.3 keV level to

the 151.9 keV level. Another 348.9 keV transition was placed above the 189.7 keV

426 H. Penttib et al. / p-decay

level, resulting in a level at 538.8 keV. This level is confirmed by a cross-over

transition to the ground state.

Most of the observed gamma transition intensity in the 349 keV line belongs to

the transition from the 348.9 keV level to the ground state. No coincidences

between this 348.9 keV gamma transition and other gamma transitions were

observed. From the yy coincidence spectra the intensities of the 348.5 keV

transition and the 348.9 keV transition from the 538.6 keV level to the 189.7 keV

level can be deduced to be 2.2 + 0.5 and 2.1 k 0.5, respectively, as compared with

100.0 + 0.9 for the 348.9 keV gamma transition to the ground state. The angular

correlation effects are not taken into account in these numbers, because these are

believed to be negligible due to the large solid-angle detector geometry used.

The two low-intensity transitions were neglected in deducing of the czK coeffi-

cient for the most intense 348.9 keV transition. Their effect is less than 5% and it

is taken into account in the error of the deduced (Ye. The Ml/E2 multipolarity for

the 348.9 keV transition suggests a i +, s + or $ + assignment for the 348.9 keV

level. A characteristic feature of the P-decay of odd-A Rh nuclei up to “‘Rh is the

strong population of a $+ state at about 300 keV excitation energy. Its energy

corresponds to the energy of the first 2+ state in even-even Pd nuclei, and its

origin may be the 2+ state coupled to the neutron in the d5,* or g,,, orbital. The

fast p decay from the 5’ ground state of odd-A Rh isotopes to these 1’ states

implies large g,,, component in the wave function. The strong P-feeding to the

348.9 keV level, which results in log ft of 5.0, fits very well in this picture. The

experimental systematics thus gives a reason to suggest spin and parity values of

$ + for this level.

The 373.1 keV level is connected via a 120.8 keV E2 transition to the + + (4 ‘1

level at 252.1 keV, implying positive parity for the 373.1 keV level. The multipolar-

ity of the 221.0 keV from the 373.1 keV level transition was unmeasurable due to

its low electron intensity. Since the 221.0 keV transition connects positive-parity

states, El is ruled out. The multipolarity of this transition has to be Ml or E2. If

Ml is assumed, the 3 + assignment of the 151.9 keV level gives the possible spin

and parity values of i +, 5 + and z +. Low beta feeding to this level supports an 4 +

ora 5’ assignment over $+.

No measurable lifetime was connected to this level. The Weisskopf estimate for

the partial half-life of a 120.8 keV E2 transition is 385 ns. This should result in a

lifetime of about 100 ns for the level when the other gamma branchings are taken

into account. The observed fast E2 transition thus implies collectivity of the 373.1

keV state.

The 408.3 keV level is placed in the level scheme on the basis of the yy

coincidence data. The 408.3 keV cross-over transition to the ground state was

impossible to observe because of the strong 409.3 keV transition and the 407.0 keV

gamma transition from the “Y decay. The 408.3 keV cross-over transition is thus

not placed in the level scheme.

H. Penttilii et al. / p-decay 427

The 409.3 keVleLle1 decays to the ground state via an E2 transition. The deduced

(Ye coefficient given in Table 2 is high for an E2 transition even within the quoted

errors, but the intensity supports the E2 assignment. Strong p feeding to this level

implies spin values similar to those of the other strongly populated levels at 189.7

and 348.9 keV, namely s or :. A p spin assignment is not very probable for any of

these levels. If Ml multipolarity is assumed for the 157.1, 219.6, and 236.7 keV

transitions, spin and parity i + result for the 409.3 keV level. The Ml choice is

reasonable considering of the conversion electron yields of these transitions, and it

is also consistent with the previous spin and parity assignments for the levels at

172.4, 189.7 and 252.1 keV.

The 500.3 keV lecef is another level resulting from the multiply placed 349 keV

transition. A transition energy of 348.5 keV was adopted from the yy spectrum.

The decay properties of the 500.3 keV level to the levels below support a low spin

assignment for this level. This fact combined with the relatively high p feeding

results in a 2’ assignment for the level.

The 538.8 keV lellel. The third 349 keV gamma transition was placed to de-excite

this level. No spin or parity are suggested.

The 730.6 keV level results from the placement of the 357.6 keV transition. A

730.9 keV gamma transition was seen in the P-gated gamma spectrum, but there

was no other evidence for this cross-over transition. Thus, the 730.9 keV cross-over

transition was not placed in the level scheme.

The 741.0 keV level is based on the observation of the 332.7 keV and the 339.1

keV gamma transition in coincidence with each other and also with the 257.9 keV

gamma transition. The order of the 332.7 keV and 339.1 keV gamma transitions is

based on the higher intensity of the 332.7 keV transition. No cross-over transitions

from this level were observed.

The 861.2 keV feuel results from the placement of the 609.0 keV gamma

transition. A 861.2 keV gamma transition was observed also in the /3 coincident

spectrum, but its intensity was too low for a half-life analysis and it can be assigned

to the level scheme by the energy only. Therefore, the 861.2 keV cross-over

transition was not placed in the level scheme.

The 2080.1 keVleue1 is fixed by the 671.1 keV gamma transition. The intensity of

the 339.1 keV gamma transition could not be determined from the P-coincident

spectrum, but it was placed in the level scheme on the basis of the yy coinci-

dences.

5. Isomer systematics of odd-,4 palladium isotopes

The level systematics of the odd neutron rich Pd nuclei has been remarkably

extended, see Fig. 5. Prior to our present studies, only the (50 + 3) s ($ -, t ->

428 H. Penttilii et al. / p-decay

L26. L33

/,’ ._ 412

7,2*393, ---~l5/2,7/2*1

3L9c5/2*1 327

,*- I'

5,2*302~,- '\ /' '\

296

7/2. 2L5 252

g/2*--

':;;., 253 f --1712.9/2*1 .,

1112‘215

230~

'\

'\\ IQ,

,I -l312'1

\ 151; ,,2*116 \ 128 132

-_ 113 i ?,,:)-(712.

‘1 \ ‘\ . 72, ‘,a1 9J2~~';9/2,1112-,

'.

.912-

512'1

300

5’ f?

200 !f? .

z

100

“‘L35,,2+) ~(712.512')

512'0 ___A ___o___o___o '3'2* ~15/2,3/2'1

‘07Pd ‘13Pd 5/2+)

0

logPd “‘Pd “‘Pd ‘17Pd

Fig. 5. The systematics of the low-lying levels in odd Pd isotopes from A = 107 to A = 117.

isomeric state at 89 keV in “‘Pd [6,16] was known about the excited levels of

The spin and parity of these isomeric states were previously deduced to be q -

up to “‘Pd [4]. In the present study the isomeric state in ‘13Pd was deduced to

have spin and parity of 4 -. For “‘Pd the ground-state branching in the P-decay of

“‘Rh supports $ + assignment for the ground state of “‘Pd and thus spin and

parity of i- for the isomeric state [51. This result is not in disagreement with ref.

[6], where the ground state of “‘Pd was assigned to have spin and parity G + or : +.

The $’ assignment for the ground state was favoured on the basis of the

experimental level systematics and the y- assignment for the isomeric level was

expected according to lighter Pd nuclei. Since the isomeric state in ‘13Pd is now

assigned to I” = 4 ~ this argument does not hold any more. The ground state of

‘17Pd was deduced to have I” = %’ on the basis of P-decay of l17Rh, for which the

G’ ground state was assumed from the systematics [2]. More detailed data on the

P-decay of the ground state of ‘17Pd are still needed to confirm the spin and parity

assignments. The isomeric state in “‘Pd has spin and parity of either y- or t-

depending on the nature of ground-state properties.

The breaking of the systematics of the y- states is probably due to the onset of

deformation in the middle of the neutron shell. If the structure of the odd

4-proton-hole Pd nuclei is compared to the odd 4-proton-particle Xe nuclei, a

similar change in the spin of the lowest negative-parity state is observed when

moving towards the mid-shell. The ‘29-‘35Xe nuclei near the magic neutron number N = 82 have q _ . isomeric states, whereas the ‘25,‘27Xe nuclei have 4 -.

H. Pmttiki et al. / /!-decay 429

Table 3

The hindrance of E3/M2 transitions in odd-A Pd nuclei compared with the Weisskopf estimates.

Isotope Ref.

lmpd 4

““Pd 4 “‘“Pd 4

“‘Pd 4

“‘pd

‘liPd h

“‘Pd 1,

-

Transition Branch

CkeV) o/c

182.8 100

214.9 100

190.0 100

172.2 73

P-decay 27

81.3 100

89.3 8

P-decay 92

71.5 5

168.6 95

MA Weisskopf

TI,, (s)

M2 6.94 x lo- h E3 81.7 x 10m3

E3 0.187

E3 0.35x

M2 0.38 x lo-”

E3 33.1

M2 0.73 x lo-’

M2 9.67 x lomh

Observed Partial y Hindrance

T,,: (5) T,,, (s)

36.1 x lo-” 52.5 x lo-’ 7.6 21.3 31 379

281 495 2 654 lY.8 x 10” 59.1 x loh 165 200

0.3

50

19.1 x lo-J

2.88 7590 16.4 x 10” 497

4.95 h780 32.9 x 10 ’ 3400

Lighter odd-,4 Xe isotopes tend to be well deformed and have a 5 - state as the

lowest negative-parity state [19]. A comparison of the even-even Xe and Pd

isotones shows a striking similarity of their structure [20].

The structure of the negative-parity states in odd-A Xe isotopes has been

explained in the framework of the particle-core model by coupling of the h,,,*

neutron to a triaxial core [21]. However, the hindrance in the decay of the

negative-parity isomeric states in odd-A Pd nuclei is much larger than that in the

odd-A Xe nuclei, reflecting the importance of the large neutron excess. The

hindrance of the 172.2 keV E3 transition in “‘Pd is the largest known for E3

transitions in this mass region [22]. The hindrances of the isomeric transitions

given in Table 3 are all among the largest known hindrances for E3 and M2

transitions in this mass region [21].

6. Conclusions

The hindrance of the studied isomeric transitions is suggested to be a conse-

quence of pairing, which causes the cancellation of the transition rate between the

states above and below the Fermi level [161. If the ground state and the isomeric

state have pure single-quasiparticle character, the expected pairing reduction

factors are (uiuj - L~~C,)~ for electric transitions and (u,uj + L:~L;>~ for magnetic

transitions. Thus, the hindrance factor of about 7000 for an M2 transition cannot

be only due to pairing effects. It can be argued that the 4 - state does not have a

single-particle character and there are other reasons to the observed hindrance of

M2 transitions. However, it would be natural if the same common reason had

caused the exceptional hindrances in all odd-A Pd nuclei.

The hindrance may reflect very different nuclear shapes of the isomeric states

compared with the ground states. This is predicted also by the macroscopic-micro-

430 H. Penttilii et al. / P-decay

scopic model with axially deformed Woods-Saxon potential, which suggests a

coexistence of oblate and prolate shapes [15]. The Hartree-Fock type calculations

performed using Skyrme-type interaction also predict coexistence of prolate and

oblate shapes [24]. More detailed measurements of these states, for example, by

laser spectroscopic methods would solve the question of nuclear shapes and

provide other useful information.

The authors would like to thank Dr. Tom Liinnroth for fruitful discussions.

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