Levels in 115In populated in the decay of 115mCd

Nuclear Physics A202 (1973) 385--395; ( ~ North-Holland Publishin9 Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

LEVELS IN l lSln POPULATED

IN THE DECAY OF llsmcd

V. SERGEEV +, J. BECKER, L. ERIKSSON, L. GIDEFELDT and L. HOLMBERG Institute of Physics, University of Stockholm, Vanadisviigen 9, S-11346 Stockholm, Sweden Received 6 November 1972 Abstract: The decay of a 15mCd to 115in is studied. Gamma intensities are measured with a Ge(Li) detector. Two new transitions are observed and placed in the level scheme. The following ~'-), directional correlations are determined (energies in keV): 484.35-933.6, 158.1-1132.5 and 316.1-1132.5. The M 1 strengths are derived for two transitions connecting members of the vibrational multiplet: B(MI, 1290.5 ~ + ~ 1132.5 ~+) = 0.5±0.1 /~g and B(M1, 1448.7 ~+ 1132.5 4} +) ~ 0.54-0.3/t2o . The half-life of the 933.6 ~+ state as measured by the delayedcoincidence technique is T~ = 57:t=5 ps, The partial half-life of the 933.6 ~}+ --+ 828.4 ~+ E2 transition is 55 times less than the Weisskopf estimate. The static quadrupole moment of the ~+ and ~,+ states is evaluated on the assumption that these levels are members of the proposed rotational band: Qo = 2.7±0.3 b. This is in agreement with the corresponding value for the 864.0 ½+-+ 828.4 ~+ transition, and the result of a recent direct measurement of the static quadrupole moment of the 828.4 ~+ state. The suggestion that the 1418.0 ~+ level is also deformed is supported by the fact that the decay occurs preferentially to the 933.6 ~+ state, whereas no transition connecting the 1448.7 -~+ state with the 933.6 z+ 2 state is observed. I E

RADIOACTIVITY 115mCd [from 11*Cd(n,7)]; measured E~,Ir, ~'y(0), V;e-delay. ~JSln deduced levels, J, T~, mixing ratios, B(E2), B(MI); evaluated Qo.

I. Introduction T h e study o f o d d i n d i u m isotopes is o f p a r t i c u l a r interest. T h e y have o n e hole in the m a j o r p r o t o n shell closing at Z = 50. T h e s t r u c t u r e o f these nuclei is s i m p l e e n o u g h to m a k e s h e l l - m o d e l c a l c u l a t i o n s feasible. T h e level s c h e m e o f a t 5 i n has b e e n s t u d i e d by a variety o f m e t h o d s , for e x a m p l e t h r o u g h the decay o f t l s C d a n d l l s m C d [refs. 1-8)], a n d in C o u l o m b excitation [refs. 9-12)], in inelastic s c a t t e r i n g a n d t r a n s f e r r e a c t i o n s t 3 - 1 8 ) , a n d in p h o t o e x c i t a t i o n a n d r e s o n a n c e scattering o f 7-rays 19-24). T h e 9+ g r o u n d state a n d the 1 - i s o m e r i c first excited state are m o s t readily interp r e t e d as s i n g l e - p r o t o n holes, g,~ 1 a n d p ~ 1. H o w e v e r , as p r o t o n p i c k - u p reactions o n 116Sn show, this d e s c r i p t i o n is o n l y a p p r o x i m a t e l y t r u e x 5). T h e s e c o n d excited state, w i t h I = ~ - , has even less single-hole (p~ ~) c h a r a c t e r t 5.16). T h e fact t h a t this state is excited in inelastic d e u t e r o n s c a t t e r i n g suggests the presence o f collective c o m p o n e n t s ~3). N e i g h b o u r i n g even C d a n d S n nuclei exhibit v i b r a t i o n a l excitations. S u c h excitations * Present address: Physics Institute, Leningrad University, Leningrad V-164, USSR. 385

386

V. SERGEEV et al.

may also be expected in alSln. Coupling a quadrupole phonon to a g~ proton hole should give rise to a quintet of states. The states originating from hole-vibration coupling are suitably excited in Coulomb excitation. During the course of this work an extensive investigation of 115in by Coulomb excitation and inelastic deuteron scattering was reported by Dietrich et al. [ref. 12)]. These authors also studied the properties of levels in 115In theoretically by the use of a model involving strong coupling of the g~ proton hole to vibrational states in ~16Sn" This model was expected to be especially useful in this case as there are no low-lying excited single-hole states of the same parity as the ground state capable of interfering with the hole-vibration multiplet. As the levels of 115in are populated quite differently in the decay of the Cd isomers than in, for instance, Coulomb excitation, decay studies are complementary and of importance for the elucidation of the structure of the nucleus 115in" Graeffe et al. 5) and Moret 6) studied the decay of 115reCd. Biicklin et al. 7) discovered a highly enhanced E2 transition between the 864 and 828 keV levels in 115in" This was interpreted as a rotational structure effect and was the first indication of strongly deformed states in this nucleus. Very recently the suggestion of large deformations gained independent support from a measurement of the static quadrupole moment of the 828 keV state by Haas and Shirley 25). Thus, the description of the low-lying excited states in 115in seems to be more intricate than anticipated for this "almost magic" nucleus. More detailed experimental investigations are obviously called for. Particular interest is attached to the search for higher members of the possible rotational band, having the 828 and 864 keV levels as the lowest members. This paper describes the determination of intensity ratios and transition probabilities in the decay of 115mCd" The spin of this isomer (I ~ = ~ - - ) permits fairly high spins to be excited in the decay. Three kinds of measurements were performed: (i) The y-ray spectrum was measured with a Ge(Li) detector. (ii) The y-y directional correlations of three cascades were determined by the use of a multichannel goniometer. (iii) The lifetime of the 933.6 keV state was measured.

2. Experiments 2.1. SOURCES The 115mCd source was obtained from the Radiochemical Centre, Amersham. The initial activity was about 1 mCi. The directional correlations were measured with the activity in HCI solution. A high-resolution measurement of the y-spectrum showed the presence of small amounts of 1°9Cd, 113Cd' 65Zn and 6°C0. 2.2. GAMMA-RAY SPECTRUM A Ge(Li) spectrometer of the Research Institute for Physics, Stockholm, was used for measurements of the y-ray spectrum. The active volume of the detector was 43

"Sin LEVELS

387

TABLE 1 Energies a n d intensities o f y-rays in the decay o f " 5reEd Present w o r k E (keV) intensity

105.14~0.07 158.055:0.07 231.355:0.10 260.8 =kO.1 316.1 5:0.2 336.2 5:0.2 386.0 5:0.5 477.0 ± 0 . 5 484.355:0.15 492.2 5:0.2 507.6 5:0.4 933.6 5:0.1 941.2 5:0.5 1132.5 ±0.1 1290.5 ±0.1 1418.1 4-0.2 1448.7 5:0.2 1462.5 5:0.5 1485.8 5:0.3

0.24 ± 0 . 0 4 1.0 ± 0 . 1 0.05 5:0.01 0.05 5:0.01 0.15 ± 0 . 0 2 0.31 5:0.03 0.010~0.005 0.010=L0.005 15 5:1 0.47 ~ 0 . 0 5 0.02 ±0.01 100 0.03 5:0.01 4.1 5:0.3 45 5:3 0.10 :::t0.01 0.83 ± 0 . 0 7 0.05 5:0.03 0.0255:0.003

Graeffe et al. 5) E (keV) intensity

E (keV)

M o r e t 6) intensity

105.6~0.8 158.15:0.04

0.455:0.15 0.9 2:0.2

106.0i0.8 158.0~0.5

0.6 5:0.1 0.6 5:0.1

336.35:0.4

0.25±0.10

317 5=1 336.22:0.2

0.125:0.03 0.355:t:0.05

485.05:0.6 492.6~=0.3

13.6 ~:1.0 0.45:t_0.10

484.8~0.5 492.2±0.2

18.0 5:0.5 0.6 ±0.1

934.45:0.6

100

1133.0i0.6 1291.25:0.6 1419.45:1.0 1450.15:1.0

4.2 £ 0 . 3 46 ±2 0.11±0.02 0.85±0.10

934.7±0.5 1133.05:0.5 1291.0±0.5 1419.55:1.0 1450 ± 1

100 4.2 5:0.8 41.0 5:1.0 0.155:0.05 0.905:0.15

TABLE 2 U p p e r limits o f ~ - r a y intensities, expected in the decay o f 115mCd Position in the level scheme 115mCd ~ 933.6 -+ 941.2 -* 1078.0~ 1132.5 -+ 1290.5 ~ 1290.5 ~ 1418.1 ~ 1418.1 ~ 1448.7-+ 1448.7--*1462.5--~ 1462.5 ~

115gCd 597.0 597.0 0 933.6 941.2 933.6 1290.5 1132.5 1290.5 933.6 941.2 933.6

") Ref. 35) b) Units as in table 1.

Energy (keV) 173 a) 336.6 344.8 1078 198.9 349.3 359.6 128.6 285.6 158.2 515.1 521.3 528.9 1490

Multipolarity

E5 M2 El E2 E2 E4 M3 E2 M I , E2 E2 MI,E2 MI,E2 M1,E2

U p p e r limit b)

~ 0.1 × 10 -2 u n d e r 336.2 keV line u n d e r b a c k g r o u n d line -- 4 x 1 0 . 2 _< 1 . 0 × 1 0 . 2 :- 0.5 × 10 -2 ~0.5x10 -z ~ 0.2 x 10 . 2 ~ 0.3 x 10 . 2 u n d e r 158.0 keV line ~ 1.0xl0 -2 <0.7×10 -z < 0.7X10 -2 ~< 0.1 × 10 -2

388

V. SERGEEV et al.

c m 3 and the resolution at 1.33 MeV was 2.7 keV. A 4096-channel pulse-height analyser with a 60 M H z converter was used. The drift of the system was found to be less than 0.5 keV in 10 h. Organic and aluminium absorbers were used in order to reduce the intensity of bremsstrahlung and of the fl-background. Energy calibration of the spectrometer was carried out with a set of standard isotopes, including 22Na, 54Mn, 57C0, 6°C0, 8Sy, 137Cs and laZTa. The intensity response of the detector was calibrated with the t S E T a s o u r c e . Several runs were made with a combination of 115mCd, 182Ta, 137Cs and 22Na sources. The background activities of 4°K and 6°Co were also used for energy calibration. The counting time was about 100 h (7 runs) for the 115mCd spectrum and about 50 h for the background measurements. Several runs were made after two months in order to use the decay constant for identification of the lines. Energies and intensities of the 1t S~Cd ~-rays are given in table 1. There are several lines which have not been observed in this decay before: 231.35, 260.8, 386.0, 477.0, 507.6, 941.2, 1462.5 and 1485.8 keV. Of these transitions the first two also occur in the decay of 115Cd. The 7-rays of 260.8, 386.0, 941.2, 1462.5 and 1485.8 keV have been observed in Coulomb excitation experiments 12). In table 1, data from the present work are compared with those obtained in earlier investigations 5.6). The lines at 130, 200 and 285 keV reported by previous authors were not found in this work. The upper limits of intensity of these lines are presented in table 2, together with upper limits of unobserved transitions which could proceed between levels in t15In. The estimate found for the 173 keV E5 transition between the isomeric and ground states of 1~ 5Cd corresponds to a partial half-life for the 7-radiation of T, > 2 x 1013 S. The hindrance factor for the E5 transition, relative to the Weisskopf estimate t, is greater than two. The analogous E5 transition in 1x 3 C d is hindered 20 times 27). Three new, weak transitions of 477.0, 507.6 and 941.2 keV are suggested to be connected with the 941.2 keV state (fig. 1). The existence of this state was established in Coulomb excitation experiments t 0-12).

2.3. GAMMA-GAMMA DIRECTIONAL CORRELATION EXPERIMENTS The instrument used for directional correlation measurements was a multichannel goniometer ( M C G ) [ref. 28)]. Eight NaI(Tl) scintillation detectors are used, and all coincidence combinations between these detectors were recorded with a fast-slow technique. The 56 coincidence channels are time analysed in one time-to-pulse-height converter (TPHC), and the information is stored in a 4096-channel analyser, each time spectrum being displayed over 64 channels. Due to the high efficiency of the M C G , this instrument is suitable for the study of weak cascades. Three directional correlations were measured: 484.35-933.6 keV, 158.1-1132.5 keV and 316.1-1132.5 keV. The values obtained for the correlation coefficients A 2 and A 4 are presented in table 3, together with the results of Van der Kooi et al. 3) and Pandharipande et al. 4). The 316.1-1132.5 keV correlation has not been measured before. t The Weisskopf units are defined in ref. 2~).

J'51n LEVELS

389

TABLE 3 Angular correlation coefficients Cascade

158.1 -1132.5 316.1 -1132.5 484.35- 933.6

Van der Kooi et aL 3) A2 A4 A2 A4 A2 A4

Pandharipande et al. '~) --0.107±0.011 0.021 ~0.017

--0.022 ±0.006 0.029 ±0.01 I

--0.023 ~0.005 0.013 ±0.012

Present work

--0.062~0.007 0.007±0.008 --0.103i0.022 --0.016±0.024 0.013 ±0.001 0.001 ±0.001

2.4. LIFETIME MEASUREMENTS

The lifetime of the 933.6 keV state was measured by registering the 484.35-933.6 keV 7-7 coincidences. Plastic NE 102A scintillators coupled to RCA 8575 phototubes were used together with an Ortec 437A TPHC. Organic and aluminium absorbers shielded the detectors from the fl-particles. The prompt time distribution was obtained with a 6°Co source. The setting of the energy-selecting single-channel analysers, and the singles counting rates in the two detectors, were the same in the registering of the p r o m p t and delayed curves. The slopes of the prompt curve were characterised by T~ = 60 ps and T~ = 50 ps, at the low- and high-energy sides, respectively. The time calibration was performed by use of a 50 f2 matched air line of 30 cm. The half-life of the 933.6 keV state was evaluated by a least-squares fitting procedure by the use of the full time-distribution curve 29). The result is T~ = 5 7 + 5 ps. This agrees with the result 7", < 130 ps of McDonald et al. 10), but disagrees with the value T~_ = 0.14+0.04 ns derived from a recent photoactivation measurement 24).

3. Level scheme of l~Sln The low-lying levels of 1~ Sin are shown in fig. 1. This scheme was established as the result of many investigations encompassing a variety of experimental methods. The lowest levels, 336.2, 597.0, 828.4 and 864.0 keV, are excited in the decay of 2.3 d 115Cd" Properties of levels higher than 864.0 keV are discussed in the following. The 933.6 k e V level. The energy of this level is determined by the sum (105.14+ 0.07)+(828.39__+0.08) = 933.53+0.11 keV and by the ground state transition of 933.6+0.1 keV. The characteristics are ~+. The 105.14 keV transition between the 933.6 keV and 828.4 keV @+) levels is of E2 character. The branching ratio Iy(105.1)/ 1~(933.6) = ( 2 . 4 + 0 . 4 ) x 10 -3 together with the half-life of 5 7 + 5 ps yields a partial half-life for the 105.1 keV transition of 27 ns, which is 55 times less than the Weissk o p f estimate. This enhancement is an argument for including the 933.6 level in the proposed K = ½ rotational band. The 941.2 and 1078.0 k e V levels. The characteristics of these levels are ~+. The B(E2) value for the excitation of the 941.2 keV level is almost equal to the Weisskopf

=,

•336.26 1

~- 336.2

(205

0 37

,,~

~.~"

~

~, ~o Iv

I

~--

93~. 6 9z.t-l..2 1078 I132.5

II

100 (3. Z~l

I ,~

I290.5

o

-~

l~l'l,~.I

t'tl

158. 0 5 1, 0

0-5" ~7~TO

I

0.I0

I C.

I~8.7

083

E --386.,

i

aOl-

I ~ 6 2 . 5 0.05 I~8~ 8

I

I

i 15in LEVELS

391

estimate. It has been argued that this level might belong to the rotational band. The B(E2) value for the excitation of the 1078.0 keV level equals 10 Weisskopf units [refs. lO-11)]. This level is considered to be a member of the vibrational multiplet. The 1132.5 k e V level. This level is regarded as the 12~+ member of the vibrational multiplet 12). The M1-E2 mixing ratio of the ground state transition is of importance for the analysis of the 158.1-1132.5 keV and 316.1-1132.5 keV directional correlations. The mixing ratio is calculated from the M1 + E2 width obtained by Alston 22), and a mean value for the E2 strength from the works of Dietrich et al. 12), Bernstein et al. [ref. 11)] and McDonald et al. 10). The result is 6 = +0.54+0.08 or 6 = - 0 . 4 5 + 0.08. The E2 component of the 1132.5 keV transition is enhanced by a factor of 20, whereas the MI component is retarded by a factor of about 10. The 1290.5 keVlevel. Two transitions are known to de-excite this level, the ground state transition and the 158.1 keV transition to the 1132.5 keV level. The 1290.5 keV level seems to be the only candidate for the -~+ member of the vibrational multiplet, and this spin value is suggested by Dietrich et al. 12). The result for the directional correlation of the 158.1-1132.5 keV cascade is analysed assuming the spin sequence 1_23 ½1 9. The values obtained for the mixing ratio of the 158.1 keV transition are 6 = +0.04+0.02 for 6(1132.5)= +0.54+0.08 and 6 = +0.157+0.006 for the alternative 6(1132.5) = - 0 . 4 7 + 0 . 0 8 * Both values for the mixing ratio indicate that the 158.1 keV transition is predominantly of M1 character, the relative E2 strength being less than 3 ~o. The branching ratio 1~(158.1)/I~(1290.5)= 0.022+0.004 and the mean value for B(E2, 1290.5) from refs. 10-12) yield the partial half-lives of the M1 and E2 components of the 158.1 keV transition. The E2 component is enhanced by a factor of 10 for 6(158.1) = 0.04, and by a factor of 200 for 6(158.1) = 0.157. The M1 component is retarded by a factor of 3, the transition probability being B(M1, _13__~ 3~_) = 0.5+0.1 ~t~. In comparison, the result arrived at in the hole-vibration coupling calculation of Dietrich et al. 12) is B(M1) = 0.34/toz. No ~,-transition feeding the 1290.5 keV level was found, either in the Ge(Li) singles, or in the NaI(TI) coincidence spectra. The number of counts in coincidence with the 1290.5 keV 7-line in an interval appropriate for a line of 127.5 keV is less than 0.1 ~o of the number of 158.1-1132.5 keV coincidences. The 1418.0 k e V level. This level has not been observed in either Coulomb excitation or 7-resonance experiments. It is populated in the/3-decay of 115mCd with log f t = 8.2,which limits the possible spin values to .}, 3~_ or L3. The presence of transitions from this level to ~+, -~+ and ~+ levels is consistent only with the choice I = ~. The energy of this level is determined by the sum ( 4 8 4 . 3 5 + 0 . 1 5 ) + ( 9 3 3 . 6 + 0 . 1 ) = 1417.95+0.2 keV, and by the ground state transition 1418.1 +0.2 keV. The measured correlation for the 484.35-933.6 keV 7-ray cascade does not contradict a spin assignment o f / = ~ for the 1418.0 keV level. However, the mixing ratio of the 484.35 keV transition is still indefinite. t Emission matrix elements 30.3~) are understood, which results in a sign-convention for 6 opposite to that of Rose and Brink 32).

392

V. S E R G E E V

et al.

A conspicuous feature of the de-excitation of the 1418.0 keV level is the absence of the possible 285 keV M1 transition to the 1132.5 keV level, the ratio of reduced transition probabilities being B(MI, 285, ~+ ~ ~-~+) < 0.001. B(MI, 484.35, ~ 2 + ~ +) This fact, as well as the non-appearance of Coulomb excitation, bears witness to the marked difference between this state and the nearby $+ 1448.7 keV state. The intensity ratio between the 484.35 keV M1 + E2 and 477.0 keV E2 transitions, both de-exciting the 1418.0 keV level, is about 1500. If the B(E2) value is supposed to be of the same order of magnitude in both transitions, the following strong retardation of the ground state transition of 1418.0 keV is observed: B(M1, 1418.0, -~-+ -~ ~z+) < 0.0003. B(M1,484.35, ~z+ ~ ~+) The 1448.7 k e V level. The ground state transition and the 316.1 keV transition to the 1132.5 keV state de-excite this level. As argued by Dietrich et al. t2), there is evidence from pick-up reactions 16) for the spin and parity to be $+. This assignment is used for the analysis of the 7-~' directional correlation of the 316.1-I 132.5 keV cascade. Accepting 6 = + 0.54 + 0.08 or fi = - 0.47 + 0.08 for the 1132.5 keV transition, the values 6(316.1) = - 0 . 0 5 + 0 . 0 3 and 6(316.1) = - 0 . 2 1 7 + 0 . 0 1 5 result. Using the branching ratio 17(316.1)/17(1448.7 ) = 0.18+0.03 and the M I + E 2 width for the ground state transition 22), the M1 component of the 316.1 keV transition was found to be retarded by a factor of about 3. The E2 component is enhanced by a factor of 6 for 6(316.1) = - 0 . 0 5 and 100 for ~(316.1) = -0.216. The low E2 strength of the 316.1 keV transition, the relative strength being less than 5 %, is in qualitative accordance with what is expected for transitions connecting members of a vibrational multiplet. The result for B(M1) is 0.5+0.3/~2. This conforms with the result for the M1 transition connecting the 1290.5 keV and 1132.5 keV levels. The 1462.5 and 1485.8 k e V levels. These levels are weakly populated in the decay of 1~5mCd" The line in the Ge(Li) spectrum corresponding to the 1462.5 keV ground state transition is quite close to the 4°K background line. The transition of 386.0 keV from the 1462.5 keV to the ~2+ 1078.0 keV level appears very weakly in the spectrum. This line is observed in Coulomb excitation, and Dietrich et al. 12) conclude that the characteristics of the 1462.5 keV level are ~+, an assignment not contradicted by the large value of logft = 10. The 1485.8 keV level is appreciably excited in l = 4 pick-up reactions, which is interpreted as an indication of an admixture of the ground state configuration 16); the characteristics are then z9 + . 4. Concluding remarks Summarising the foregoing discussion we note that two different models have been found necessary to describe the levels of 115in in the 0.9-1.5 MeV region. Six levels

115In L E V E L S

393

appear as candidates for the multiplet arising from coupling of a quadrupole phonon to the ground state proton hole (energies in keV): 1078.0 ~+, 1132.5 ~1+, 1290.5 ~~- 3 ÷ , 1448.7 9_+, 1462.5 ~+ and 1485.8 ~-+. The Coulomb excitation probability of the 941.2 keV ~+ level indicates that this level should be ascribed some of the vibrational strength. The directional correlation measurements of this work provide new information for tests of the proton-hole plus vibration description of al 5in" For two transitions, of intra-multiplet nature, absolute M1 strengths are determined. In the case where the transition strength has been calculated from the model with strong coupling, the agreement is acceptable. However, for the mixing ratios there are at present no theoretical predictions. The levels that might be regarded as members of the proposed K = ½ rotational band are: 828.4 ~+, 864.0 1 ÷ , 933.6 -7÷ , 941.2 ~Z+ and 1418.0 ~+. The E2 strength of the 933.6 ½+ --* 828.4 3+ transition is determined from the present lifetime and intensity measurements. The value for the intrinsic quadrupole moment, corresponding to a rotational transition of this strength, is evaluated, neglecting, however, the off-diagonal terms pertaining to a K = ½ band: Qo = 2.7_+ 0.3 b. This result agrees with that obtained from the 35.6 keV 1 + ~ k+ transition, Qo = 2.6-t-0.3 b [according to B~icklin et al. 7); a lower value is obtained by Meyer and Struble 33)], and the recently determined static moment of the 828.4 keV k+ state, Qo = 3.0-t-0.4 b. The 941.2 keV ~+ level is weakly populated in the decay ofllsmCd. Dietrich et al. found the 344.2 keV ~+ ~ k - E1 transition to be retarded by a factor 1.8 × 104 relative to the Weisskopf estimate. The 1418.0 keV ~ + level is found to decay preferentially to the 933.5 keV state, the suggested I = ~ member of the rotational band. The M1 transitions to the vibrational 1132.5 keV and ground states exhibit high relative retardation. Thus, there seems to be evidence for the proposed members of the rotational band to form a set characterised by appreciable deformation. However, the energies of TABLE 4 Levels suggested to belong to " t h e rotational b a n d " I

Exp. (keV)

Calc. (keV)

-~ ~-

864.0 828.4 941.2 933.5 1418.0

(864.0) (828.4) 1050.3 967.2 1366.6 1236.2

A least-squares fit o f the five p r o p o s e d rotational levels to the f o r m u l a E~(1) -- E ½° + h 2 / 2 J [ l ( l + l ) + a ( - - 1 ) I + ~ (I÷½)].

T h e energies o f the ½ a n d ~ states were a s s u m e d to be unperturbed. T h e resulting values for the rotational p a r a m e t e r s are h 2 / 2 J ~ 16.3 keV a n d a ~ --1.7.

394

V. SERGEEV et

al.

these levels do not obey the formula for a K = ½ rotational band. A least-squares fit was performed, where the ½+ and g2+ energies were assumed unperturbed (ef. table 4). As seen, the 941.2 keV ~_-+ level is a particularly bad fit. However, as mentioned in the foregoing the excitation probability in Coulomb excitation shows that this state can not be purely rotational. In addition, the ½+ and -~-+ states may be expected to be perturbed by vibrational states in the same energy region. In view of this possible mixing, the difference in the decay modes of the 1418.0 and 1448.7 keV 9+ states seems remarkable. The rotational parameters obtained in the fit of the energy values may be used to predict the energy of the 1@+ rotational state, E~ = 1236 keV. The 1418.0 keV ~+ level is affluently populated in the decay of the 1@- cadmium isomer, and the 1@+ state may be expected to be seen in this study. However the only -~-+ candidate observed is the 1132.5 keV level, which seems to be well accounted for by the vibrational model. The "rotational" levels were populated by proton-transfer reactions in the recent investigation of Thuri6re iv). The 933.6 keV ½+ level was strongly excited in the 114Cd(3He, d) 115In reaction. This is in agreement with the interpretation of this level as a lp-2h state, with the odd proton occupying the ½+ [431] orbital, the predominant component of which is g~_. The occupation probabilities found for the various states are in fairly good agreement with that predicted for the deformation 6 ~ 0.1. However, this value for the deformation parameter is smaller than that obtained from the quadrupole moment Qo = 2.6, and from the decoupling parameter a = - 1 . 7 : 6 ~ 0.2. The origin of deformed states in 115in is envisaged by Thuri~re as a result of the lp-2h excitation. The core of 114Cd may be thought of as "soft", and the odd g~ proton being able to stabilise a deformed shape. The coexistence of two sets of levels in ~~5In with markedly different properties seems well established. The description of "the deformed set" in terms of a rotational band is tempting. As recently demonstrated 34) the occurrence of a quadrupole deformation in this Z = 49 nucleus can be understood on the basis of a microscopic theory.

We express our gratitude to Prof. T. R. Gerholm for excellent research facilities and for his continuous interest. We are indebted to Drs. H. Ryde and Z. Sawa for putting their 3,-spectrometer at our disposal. We thank Prof. B.-G. Pettersson for stimulating discussions and Drs. A. Biicklin and J. McDonald for valuable suggestions. Chr. Bargholtz generously gave us access to his program for the analysis of lifetimes. One of us (V.S.) is grateful to Prof. T. R. Gerholm for the hospitality during his stay at the Institute of Physics. This work was supported by Statens Rgtd f6r Atomforskning.

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395

References 1) 2) 3) 4) 5) 6) 7) 8) 9)

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