Electromagnetic transitions and structure in the Z = N nucleus 46 V

Electromagnetic transitions and structure in the ZÄNnucleus 46V F. Brandolini,1N. H. Medina,2R. V. Ribas,2S. M. Lenzi,1A. Gadea,3C. A. Ur,1,4 D. Bazzacco,1R. Menegazzo,1 P. Pavan,1C. Rossi-Alvarez,1A. Algora-Pineda,3G. de Angelis,3M. De Poli,3E. Farnea,3N. Ma ˘rginean,3,4 T. Martinez,3 D. R. Napoli,3M. Ionescu-Bujor,4A. Iordachescu,4J. A. Cameron,5S. Kasemann,6I. Schneider,6J. M. Espino,7 and J. Sanchez-Solano8 1Dipartimento di Fisica and INFN, Padova, Italy 2Instituto de Fı ´sica, Universidade de Sa ˜ o Paulo, Sa ˜ o Paulo, Brazil 3INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy 4National Institute for Physics and Nuclear Engineering, Bucharest, Romania 5McMaster University, Ontario, Canada 6Institut fu ¨r Kernphysik der Universita ¨tzuKo ¨ln, Ko ¨ln, Germany 7Departamento de Fisica, Universidad de Sevilla, Sevilla, Spain 8Departamento de Fı ´sica Teorica, Universidad Auto ´noma, Cantoblanco, Madrid, Spain 共Received 4 May 2001; published 24 September 2001兲 The nucleus 46V has been studied in the reaction 24Mg(28Si, ␣ pn)46V at 115 MeV beam energy, using both Au and Pb backed targets. Lifetimes were obtained for 14 levels with DSAM analysis. Experimental B(E2) reduced transition probabilities are well reproduced by the large scale shell model. The observed levels could be organizied in bands with a rather good Kvalue. DOI: 10.1103/PhysRevC.64.044307 PACS number共s兲: 21.10.Tg, 23.20.Lv, 27.40.⫹z I. INTRODUCTION In recent years an extensive experimental study of the structure of nuclei in the middle of the 1f7/2 shell has been made at LNL, in particular for 48Cr, 49Cr, 50Cr, and 47V 关1–4兴. Rotorlike bands have been found. Since experimental B(E2) values of good quality are essential to substantiate the rotational collectivity, the lifetimes of many levels 关2–4兴 have been determined with careful doppler shift attenuation method 共DSAM兲analysis. Large scale shell model 共LSSM兲 calculations in the full fp configuration space 关5兴reproduce very well the excitation energies of the observed natural parity levels and transition probabilities, while the unnatural parity bands are reasonably reproduced by extending the configuration space to include a d3/2 hole 关6兴. In this context new results are presented here for the Z ⫽Nnucleus 46V, which has recently aroused great interest owing to its special peculiarities 关7–11兴. As shown in the level scheme of Fig. 1, coexistence of natural parity T⫽1 states with T⫽0 and unnatural parity states occurs at low excitation energy, so isospin selection rules and isospin mixing can be tested. This nucleus has previously been studied at the spectrometer GASP of LNL with the reaction 24Mg on 28Si at 100 MeV bombarding energy, using a thin selfsupporting target 关7兴. A detailed level scheme was obtained and compared with LSSM predictions. Two other experimental works in 46V also appeared recently: a less extensive level scheme was obtained using the PEX array 关9兴in the same reaction as in Ref. 关7兴, but at a bombarding energy of 87 MeV. Low-lying levels have been studied in detail using the reaction 46Ti(p,n)46Vat15MeV关10兴. Preliminary data of a plunger lifetime measurement in the reaction 32S(16O,pn)46V have been reported 关11兴. Finally, the T⫽1 band has been extended up to I⫽10⫹at Gammasphere/FMA 关12兴. The present work, performed at LNL, complements the previous one 关7兴and has the main aim to extend the study of the electromagnetic properties in order to understand better the structure of this nucleus. No lifetime values in the DSAM range have so far been reported. II. EXPERIMENTAL PROCEDURE AND RESULTS The nucleus 46V was populated in the reaction 28Si on 24Mg at 115 MeV bombarding energy, using 0.8mg/cm2tar- gets of 24Mg backed with 15mg/cm2either of Au or of Pb. The incident energy in the center-of-mass reference was the same as in the previous experiment, so that experimental intensities can be directly compared. Gamma rays were detected with the GASP array, comprising 40 Comptonsuppressed HPGe detectors and an 80-element BGO ball which acts as a gamma-ray multiplicity filter. The level scheme of 46V of Fig. 1 is organized in rotational bands of definite K values, as suggested by the appearence of rotational bands in this nuclear region. At high spin it is a simplified version of that of Ref. 关7兴, since mainly levels observed in the present work are shown. However, by virtue of the better energy resolution for Doppler unbroadened peaks, several new transitions have been observed. The levels 6⫹at 3365 keV and 7⫹at 3642 keV, which were not observed in the present experiment, are reported for completeness. The arrow widths are proportional to the line intensities. It may be objected that a Kclassification for the positive parity levels is not justified, owing to the band irregularity, but the following discussion of the ␥ transitions will show that it may be appropriate as a first approximation. A summary of experimental data, which includes those of Ref. 关7兴, is reported in Table I. According to the Gallagher-Moszkowski 共GM兲rule 关8兴, the presence of K⫽3⫹and K⫽0⫹bands is expected at low energy, corresponding to a parallel and antiparallel coupling of the Nilsson orbitals 关321兴3/2⫺occupied by the odd nucle- PHYSICAL REVIEW C, VOLUME 64, 044307 0556-2813/2001/64共4兲/044307共7兲/$20.00 ©2001 The American Physical Society64 044307-1 FIG. 1. Level scheme of 46V. F. BRANDOLINI et al. PHYSICAL REVIEW C 64 044307 044307-2 TABLE I. Experimental and theoretical results in 46V. Errors in ␥ -ray energies are 0.4 keV rising to 1 keV above 2 MeV. Gamma lines in brackets were not observed. Theoretical B(E2) values and branches less than 0.1 are set to 0. Transition E ␥ E ␥ ␥ ⫺BR ␥ ⫺BR ␶␶ B(E2) B(E2) B(M1) B(M1) exp. SM exp. th. exp. th. exp. th. exp. th. keV keV % % ps ps e2fm4e2fm4 ␮ N 2 ␮ N 2 K⫽3⫹ 5⫹→3⫹423.3 496 100 100 903共107兲a67共14兲65 7⫹→5⫹379.3 447 100 100 1080共170兲b97共15兲61 9⫹→7⫹1490.1 1518 100 100 0.70共15兲c159共34兲145 11⫹→9⫹1471.3 1558 100 100 0.85共16兲c139共26兲130 13⫹→11⫹2537.4 2535 100 100 0.10共2兲c78共16兲89 15⫹→13⫹1382.8 1800 100 100 2.7共5兲60共12兲67 4⫹→3⫹377.9 507 100 100 514共119兲a208共50兲296 5⫻10⫺5 6⫹→4⫹360.0 294 45共5兲45 783 79 6⫹→5⫹315.1 306 55共5兲55 187 9⫻10⫺5 8⫹→6⫹1775.0 2081 70共8兲78 0.32 113 8⫹→7⫹1710.0 1939 30共8兲22 36 4⫻10⫺5 10⫹→8⫹1603.0 1569 60共9兲64 0.47 96 10⫹→9⫹1823.0 1988 40共9兲36 31 3⫻10⫺5 K⫽0⫹ 2⫹→0⫹915.0 993 98.0共4兲95 9.0共23兲a138共35兲142 2⫹→3⫹113.5 128 2.0共4兲5 2.0 0.21 1⫹→0⫹993.2 949 100 100 0.06 1.07 32 ⫹ →1⫹共383兲443 ⬍1 0.1 0.54 189 32 ⫹ →2⫹461.4 398 100 99.9 0.29共10兲a0.1 2.0共7兲1.07 32 ⫹ →3⫹共574兲526 ⬍5 0 1.3 ⬍10⫺5 42 ⫹ →2⫹1140.1 1398 8共1兲6 0.13 187 42 ⫹ →3⫹1253.1 1026 14共2兲34 0 0.08 42 ⫹ →4⫹共875兲520 ⬍3 1 0.4 0.027 42 ⫹ →32 ⫹678.4 499 65共3兲42 0 0.63 42 ⫹ →5⫹830.1 530 7共1兲4 0.2 0.032 42 ⫹ →52 ⫹328.9 242 6共1兲12 0.2 1.55 52 ⫹ →3⫹共923兲784 ⬍10 34 51 7.3 52 ⫹ →32 ⫹349.1 257 74共4兲44 160 52 ⫹ →4⫹547.1 277 8共2兲10 10.2 1⫻10⫺5 52 ⫹ →5⫹501.4 288 18共2兲13 17 7⫻10⫺5 52 ⫹ →6⫹共186兲8⬍5 0 2.6 7⫻10⫺5 K⫽0⫺ 2⫺→(0⫺) 130.5 44 0.9共2兲1424共600兲a160共75兲200 4⫺→2⫺590.1 684 85.0共9兲99 50 89 5⫺→3⫺721.6 871 91.2共8兲100 17 183 6⫺→4⫺967.2 1232 85.3共9兲100 3.3共7兲c, d 248共55兲203 7⫺→5⫺1134.0 1360 97.3共9兲100 1.5共3兲c, d 282共57兲270 8⫺→6⫺1259.8 1396 94.5共7兲100 0.90共23兲c, d 271共70兲242 9⫺→7⫺1410.0 1547 98.8共4兲100 0.61共13兲c, d 237共52兲234 10⫺→8⫺1098.4 1350 96.0共6兲100 4.5共9兲c, d 109共24兲92 11⫺→9⫺1323.6 1357 100 100 1.1共2兲c175共34兲140 12⫺→10⫺1882.6 2075 100 100 0.23共4兲150共28兲122 13⫺→11⫺1940.6 2032 100 100 0.24共5兲124共27兲130 14⫺→12⫺2644.2 2338 100 100 0.050共15兲126共38兲95 15⫺→13⫺2651.6 2835 100 100 0.10共2兲62共12兲88 16⫺→14⫺1969.1 2673 100 100 0.55共15兲50共14兲62 17⫺→15⫺2780.0 2725 100 100 ⬍0.2 ⬎25 68 aReference 关11兴.cAlso with a NGTB analysis. bReference 关20兴.dOnly from data using lead backing. ELECTROMAGNETIC TRANSITIONS AND STRUCTURE... PHYSICAL REVIEW C 64 044307 044307-3 ons. The antiparallel coupling gives rise to the K⫽0⫹ ground state band, shown on the left of Fig. 1, where the even spin levels have T⫽1. The intense, irregular, positive parity band, based on the yrast 3⫹level and terminating at the 15⫹level, can be approximately described as the K ⫽3⫹sequence with favored signature. Some triaxiality has been suggested for positive parity states of this nuclide 关9兴, which may give rise to the large irregularity in signature splitting. A third band, indicated in Fig. 1 with K⫽0⫺,is described as due to the excitation a d3/2 hole, with equal neutron and proton contributions. The GM rule predicts in this nucleus the presence of K⫽0⫺and K⫽3⫺bands, due to the parallel and antiparallel coupling of the orbitals 关202兴3/2⫹and 关321兴3/2⫺. A candidate for the 0⫺head of the former band has been observed, but none for the 1⫺ level. For the the predicted K⫽3⫺band, from the parallel coupling, candidates for its head at 1254 keV, and for the next two levels, have already been observed in Refs. 关7,10兴. These are weakly populated and not connected to other bands. A band, based on a level at 4225 keV, was already observed in the previous experiment 关7兴and is now classified as K⫽(7⫺).AK⫽7⫺band is indeed predicted in a Nilsson scheme, as due to the excitation of a nucleon from the 关202兴3/2⫹orbital to the empty 关321兴5/2⫺one, with parallel coupling of the four unpaired nucleons. A K⫽8⫺band has been observed in 48V, which arises in a similar way 关19兴. The assignment I⫽(6⫺) was made 关7兴for the level at 3406 keV, which seems not to belong to, or decay to the K⫽3⫺ sequence. Finally, several weak E1 branches have been observed connecting the K⫽0⫺band to positive parity levels 共shown in Table II兲. In Table I the experimental branches from those levels take into account the observed E1 decay. The present experimental data for the K⫽0⫹band are generally in agreement with those of Ref. 关10兴. There are two relevant differences: the 383 keV 32 ⫹ →1⫹transition, reported there to have an intensity of 1.6(4)%, was not found, whereas a 1253 keV 42 ⫹ →3⫹transition was observed. LSSM calculations were performed with the code ANTOINE, using the KB3 residual interactions 关5兴. Such calculations have already been reported for both positive and negative parity in Ref. 关7兴, where the calculated energies are compared with experimental ones. The only change is that the Kurath term aT(T⫹1), which regulates the relative position of T⫽0 and T⫽1 levels, has been adjusted as in Ref. 关10兴. In Table I the calculations for transition energies, M1 and E2 reduced rates, branching ratios and lifetimes are reported. The branching ratios, in this case, do not account for possible E1 branches. For the lifetime measurements, data were sorted into seven ␥ ⫺ ␥ matrices having on the first axis the detectors in rings at 34°, 60°, 72°, 90°, 108°, 120°, 146° and on the second axis any of the other 39 detectors. More details about the analysis procedure are reported in Ref. 关2兴. The program LINESHAPE 关13兴modified and used in order to allow the narrow gate on transition below 共NGTB兲procedure 关14兴, which is free from systematic errors related with side-feeding uncertainties. As an example, the NGTB line shape analysis of two transitions in the K⫽3⫹band is shown in Fig. 2. With an upper coincident transition as a probe, the comparison of its full line shape 共upper part of the figure兲with the partly suppressed line shape 共lower part兲, obtained with a narrow gate on the lower transition to be studied, provides the lifetime associated to the gating transition. The Northcliffe- Schilling stopping power 关15兴, corrected for atomic shell effects 关16兴, was used. The use of lead as backing material was important in order to extend the lifetime measurement to low members of the K⫽0⫺band. The accuracy of stopping powers is discussed elsewhere 关17兴. Lifetime values have been obtained for 14 transitions and are reported in Table I. Different analyses were performed. Whenever it was possible the NGTB analysis was used. For completeness, LSSM lifetimes values are reported for levels when no experimental value could be obtained. Two examples of line shape analysis for transitions in the K⫽0⫺band are displayed in Fig. 3. Lifetimes for 11 levels of the K⫽0⫺band were determined, and led to B(E2) values well reproduced by LSSM calculations, as Fig. 4 shows. TABLE II. E1 transitions in 46 V. Errors in ␥ -ray energies are 0.4 keV rising to 1 keV above 2 MeV. ␥ lines in brackets were not observed. Transition E ␥ BR ␶ W.u. keV % ps 10⫺6 0⫺→1⫹242.4 2⫺→1⫹373.0 57共6兲1424共600兲a5.9 2⫺→2⫹451.1 21共4兲1.2 ⌬T⫽1 2⫺→3⫹共564兲⬍0.3 3⫺→2⫹750.7 91共3兲⌬T⫽1 3⫺→3⫹864.2 0.8共2兲 3⫺→4⫹共486兲⬍0.4 4⫺→3⫹1154.6 1.0共2兲42b0.1 4⫺→4⫹772.2 2.7共5兲1.1 4⫺→5⫹732.0 0.5共2兲0.2 4⫺→32 ⫹580.1 9.5共8兲9.0 5⫺→4⫹共1585兲⬍0.4共1兲16b 5⫺→5⫹1162.8 1.7共4兲0.5 5⫺→6⫹848.3 1.8共4兲1.4 5⫺→42 ⫹333.0 5.3共6兲69 ⌬T⫽1 5⫺→52 ⫹662.0 1.4共4兲2.3 6⫺→5⫹1699.2 2.6共5兲3.3共7兲1.3 6⫺→6⫹1384.5 3.6共8兲3.5 6⫺→7⫹1320.0 3.6共7兲4.0 6⫺→52 ⫹1198.2 4.9共6兲7.4 7⫺→6⫹共1983兲⬍0.4 1.5共3兲 7⫺→7⫹1918.0 2.7共6兲2.0 7⫺→62 ⫹共156兲⬍0.4 8⫺→7⫹2579.7 5.5共13兲0.90共23兲2.7 8⫺→8⫹共870兲⬍0.5 8⫺→9⫹共1090兲⬍0.5 9⫺→8⫹共1618兲⬍0.5 0.61共13兲 9⫺→9⫹1837.6 1.2共4兲2.5 10⫺→9⫹2188.3 4共1兲4.5共9兲0.7 aReference 关11兴. bCalculated rotor values with ␤ ⫽0.27 were adopted. F. BRANDOLINI et al. PHYSICAL REVIEW C 64 044307 044307-4 The lifetimes of the 4⫺and 5⫺levels could not be measured because they are too long, in the range of a plunger measurement. The regular pattern of the low part of the K⫽0⫺band, before the backbending, suggests a rotor-like behavior down to I⫽2⫺. The experimental reduced rate for the 2⫺→(0⫺) transition, reported in Table I, agrees with this description. The deduced transition quadrupole moment Qtof approximately 90efm2, corresponds to ␤ ⯝0.27, which is sligthly smaller than that of the unnatural parity band in 47V关4兴. The band turns out to be somewhat more deformed than predicted. The strong back bending of the ␣ ⫽0 signature band at I⫽10⫺is accompanied by a sharp decrease of the B(E2) value. Both of them are well reproduced by LSSM, as shown in Table I. The ⌬I⫽1 branches were not observed in the negative-parity band, except for the 4⫺→3⫺transition, which was found with only 1.0(3)% intensity. According to the isospin selection rule 关18兴,⌬T⫽0M1 transitions are predicted to be strongly hindered, so that this branch is likely mainly E2. In the K⫽3⫹band, only the B(E2) values for transitions between levels of the favored signature could be determined and were found to be in good agreement with LSSM calculations, as shown in Fig. 4. A deformation parameter ␤ ⯝0.22 is deduced from the reduced rates of 11⫹→9⫹and the 9⫹→7⫹transitions. The agreement between theory and experiment is good also for levels of the K⫽0⫹band. For the 2⫹→0⫹transition in the T⫽1 sequence, we note that the B(E2) value in 46Ti is known to be 191(15) e2fm4, while the LSSM prediction is 114 e2fm4. This is interpreted as arising from mixing of two-hole and four-hole configurations, which bring additional collectivity. This has been also noticed in g-factor measurements 关21兴.AB(E2) value of 137共35兲e2fm4is reported in Ref. 关11兴for the isobaric analogous transition in 46V, which is much smaller than the experimental value in 46Ti, while it should reasonably be bigger, due to the presence of a valence proton. A more precise lifetime measurement has to be performed for a definitive conclusion. In the case where no lifetimes are available, one can compare calculated and experimental branching ratios. The well-predicted branching ratios for the 42 ⫹level suggest agreeement also for absolute value of reduced rates. It turns out that strong E2 transitions only connect levels inside a band, confirming the validity of the Kquantum number. This is to some extent surprising, owing to the irregular behavior of positive parity bands. Detailed data were previously reported for the lower spin levels from the (p,n) reaction 关10兴, where experimental branches are compared with LSSM calculations using both the KB3 and the FPD6 residual interaction. The only significant difference for positive parity levels between those KB3 FIG. 2. NGTB lifetime analysis for the 1471 keV 11⫹→9⫹ transition 共a兲and the 1490 keV 9⫹→7⫹transition 共b兲, respectively. A gold backing was employed. FIG. 3. 共a兲NGTB lifetime analysis for the 1410 keV (9⫺→7⫺) transition, using a gold backing 共b兲Standard line shape analysis for the 1098 keV (10⫺→8⫺) transition, using a lead backing. FIG. 4. Experimental B(E2) reduced transition rates in 46V, compared with LSSM predictions 共empty squares兲. Data are taken from the present work 共full squares兲, except for those indicated as full triangles 共see text兲. ELECTROMAGNETIC TRANSITIONS AND STRUCTURE... PHYSICAL REVIEW C 64 044307 044307-5 calculations and the present one, using ANTOINE, is the large value predicted there for the branch of the 501 keV 52 ⫹ →5⫹transition. The 1.6(4)% branch of the 383 keV 32 ⫹ →1⫹transition in the K⫽0⫹band, quoted in Ref. 关10兴,is too large to be explained theoretically. In fact, using that branching ratio and the large B(M1) observed 关11兴for the 461 keV 32 ⫹ →2⫹transition, a very large B(E2) value of 55(15)⫻102e2fm4would be estimated for the 383 keV transition. One notes that in 46Ti, strongly populated by inelastic scattering in that experiment, the transition 4⫺→3⫺ is also 383 keV and is therefore a likely contamination. Experimental information has also been obtained for the negative parity levels belonging to the K⫽(3⫺) and (7⫺) bands, but the agreement with calculations is worse in this case. LSSM calculations predict the 3⫺,4 ⫺, and 5⫺levels of a K⫽3⫺band to be close to the corresponding levels of the K⫽0⫺band. Accordingly, the levels based on the 1254 keV level are assigned to a K⫽3⫺band. The negative parity assignment is favored by the fact that no other low-lying positive parity levels are predicted by LSSM. Indeed the spacings, 541 and 634 keV, agree well with the calculated values 571 and 645 keV from LSSM. Although the members of the K⫽3⫺band are calculated to be connected by E2 transitions to the K⫽0⫺band, this is not observed. Such interaction between the two bands results in irregularity of the calculated B(E2) values, while the experimental ones seem to be more regular. In the spectroscopic electric quadrupole moments calculated by LSSM, there is evidence that both the reduction of B(E2) and the backbending at I⫽10⫺are due to a band crossing of the K⫽0⫺band with a K⫽7⫺band. A similar situation occurs for the K⫽8⫺band in 48V关22兴. The two high-Kbands are interpreted as due to the excitation of a nucleon from the 关202兴3/2⫹orbital to 关321兴5/2⫺with a parallel coupling of the normal parity bands with K⫽3⫹and the K⫽4⫹, respectively. It is tempting to relate the (6⫺) level at 3406 keV to a 3⫺octupole vibration core coupled to a neutron-proton pair with I⫽3⫹, the head term of the favored signature sequence. A similar fact seems to occur for the (8⫺) level observed at 4257 keV in the cross-conjugate Z ⫽Nnucleus 50Mn 关22兴, where the coupling would be with the yrast 5⫹state. In support of this suggestion it is noted that the 3⫺level at 3737 keV in 40Ca decays with a collective B(E3) strength of about 27 W.u. Levels with I⫽3⫺of presumable vibrational character are observed in Z⫽Nnuclei 44Ti at 3943 keV, 48Cr at 4067 keV, and 52Fe at 4396 keV. Such 3⫺states are also present in N⫽Z⫹2 nuclei 42Ca at 3446 keV, 46Ti at 3570 keV, and 50Cr at 4052 keV. Unfortunately no B(E3) strengths have so far been determined, so that the suggestion remains unsubstantiated. The properties of 21 E1 transitions, depopulating the levels of the K⫽0⫺band, are summarized in Table II. For eight of them experimental B(E1) values were derived from the present lifetime measurement and for two more levels from lifetimes elsewhere quoted 关11兴. Furthermore, order of magnitude estimates have been made for the B(E1) rates from the 4⫺and 5⫺levels, using the rigid rotor estimate with transition quadrupole moment Qt⫽90 efm2. This estimate is favored because the levels seem to be not mixed with those of the K⫽3⫺band. In a Z⫽Nnucleus ⌬T⫽0, E1 transitions are only permitted by the small isospin mixing caused by the Coulomb interaction 关18兴. The average retardation of ⌬T⫽0, E1 transitions with respect to the Weisskopf estimate is about 106, as shown in Table II. This is not much larger 共perhaps one order of magnitude兲than that of non-Z⫽Nnuclei. An explanation has recently been suggested 关23兴. In any case, this feature is different from what occurs in 48Cr, where the retardation is about 109. Finally one may ask why the 1⫺level of the K⫽0⫺band was not observed. No reliable predictions can be made for the lifetime of the 3⫺level, which decays mainly E1 via the 751 keV3⫺→2⫹transition. The decay of this level is predicted to have a 366 keV branch to the unobserved 1⫺level. Assuming for the 3⫺→2⫹transition a realistic B(E1) value of the order of 10⫺4W.u., a lifetime of few tens of picoseconds is estimated. In this case a 300–400 keV E2 branch to the unobserved 1⫺level would be smaller than one percent and could escape the observation. III. CONCLUSIONS Much new information has been obtained for the structure of 46V. In spite of the relatively small deformation the classification of the observed levels according to bands of definite Kvalue appears to be successful, at least in first approximation. The agreement with LSSM calculations for positive parity levels is very good. Calculations for negative parity states are satisfactory as they are able to reproduce both the back bending and the drop in the B(E2) value at 10⫺in the well-deformed K⫽0⫺band. The possibility of an octupole vibration has been suggested. Several E1 transitions have been observed and it has been found that the isospin forbidden ⌬T⫽0E1 transitions are in the average only one order of magnitude retarded with respect to the isospin allowed ⌬T⫽1E1 transitions. ACKNOWLEDGMENTS R.V.R. and N.H.M. would like to acknowledge financial support from the Brazilian agency CNPq 共Conselho Nacional de Desenvolvimento Cientı ´fico e Tecnolo ´gico兲and INFN 共Italy兲. J.S.S. was supported by DGES 共Spain兲Grant No. PB96-53. J.A.C. was supported by NSERC 共Canada兲. Fruitful discussions with Professor P. G. Bizzeti are acknowledged. 关1兴S.M. Lenzi et al., Z. Phys. A 354,117共1996兲. 关2兴F. Brandolini et al., Nucl. Phys. A642, 387 共1998兲. 关3兴F. Brandolini et al., Phys. Rev. C 60, 041305共R兲共1999兲. 关4兴F. Brandolini et al, Nucl. Phys. A 共to be published兲 关5兴E. Caurier et al., Phys. Rev. Lett. 75, 2466 共1995兲. 关6兴A. Poves and J. Sanchez-Solano, Phys. Rev. C 58, 179 共1998兲. F. BRANDOLINI et al. PHYSICAL REVIEW C 64 044307 044307-6 关7兴S.M. Lenzi et al., Phys. Rev. C 60, 021303共R兲共1999兲. 关8兴C.G. Gallagher and S.A. Moszkowski, Phys. Rev. 111, 1282 共1958兲. 关9兴C.D. O’Leary et al., Phys. Lett. B 459,73共1999兲. 关10兴C. Friessner et al., Phys. Rev. C 60, 011304共R兲共1999兲. 关11兴I. 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