In-beam study of the level structure of 122Te

Nuclear Physics A528 (1991) North-Holland

IN-BEAM

381-394

STUDY

OF THE LEVEL

STRUCTURE

OF ‘=Te

C.S. LEE, J.A. CIZEWSKI, D. BARKER’, R. TANCZYN’, G. KUMBARTZKI, J. SZCZEPANSKI, J.W. GAN, H. DORSETT3, R.%. HENRY and L.P. FARRIS Department

of Physics and Astronomy,

Rutgers University,

New Brunswick,

NJO8903,

USA

H. LI A. W. Wright Nuclear Structure Laboratory,

Yale University,

Received 5 November (Revised 27 December

New Haven, CT0651 1, USA

1990 1990)

Abstract: The level structure of ‘*‘Te has been studied using the “‘Sn( LY,2n) reaction at beam energies of 18 and 24 MeV. Standard in-beam y-ray spectroscopy measurements of excitation functions, yy-coincidences, and y-ray angular distributions were performed. A level scheme up to 3995 keV in excitation was constructed and spin-parity values up to 12+ were assigned to almost al1 of the states. The identification of several new levels and the first in-beam observation of the excited Of state at 1357.4 keV are discussed. The low-lying excitations are examined in the context of coexisting structures of collective, single-particle, and particle-hole intruder excitations.

E

NUCLEAR REACTION ‘*‘Sn(a, 2n), E = 14-22 MeV, measured .E._,,i,(E), I,(@), “‘Te deduced leveIs, J, GT,6(E2/Ml), B(A) ratios. Model calculations. yy-coinc. NU~LEARSTRUCTURE ‘*2Te; calculated, B(h) ratios, levels; deduced collective, intruder state features.

1. Introduction Transitional nuclei have been extensively studied in recent the complicated coexistence of single-particle and collective near shell closures.

The tellurium

nuclei,

with two protons

years to understand properties in nuclei

outside

the 2 = 50 shell

closure, are good candidates for study of this interplay. The systematics of the low-lying states in N a 66 Te nuclei ‘) are presented in fig. 1. The yrast 2+ and 4’ states have a similar parabolic behavior as a function of neutron number, reaching a minimum in excitation energy near midshell, whereas the 6’ states are rather constant in energy as a function of neutron number for N > 66. The first excited 2” states are believed to be mainly collective vibrational excitations. However, the * Present 2 Present 3 Present

address: address: address:

0375-9474/91/$03.50

LOGICA, Cobham, Surrey KTll3LX, UK. Lafayette College, Easton, PA 18042, USA. Department of Chemistry, University of Maryland, @ 1991 - Elsevier

Science

Publishers

College

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382

C.S. Lee et al. / “‘Te

constant energy dependence as a function of N of the yrast 6.+states for the heavier isotopes is characteristic of a proton excitation. The yrast 4.’ states are probably, therefore, a mixture of collective vibrational and two-proton configurations. A particular example is iz2Te: the energy ratio E(4+)/E(2’) = 2.09 resembles that of a vibrational nucleus, while the relative B(E2) ratio “) B(E2; 4+-+2’),‘B(E2;

2++0+) = 1.48

is closer to the rotational prediction of p than the vibrational expectation of 2. Furthermore, the systematics of electric quadrupole moments “) of 2+ states in Te nuclei have led to the suggestion that even the ground state may be weakly deformed, with maximal deformation near the middle of the shell. In addition to collective excitations and two quasiproton components in the first 6+ states, there are two quasineutron excitations involving an h11,z neutron that give rise to negative-parity states. There are also a 3- state at 2.2 MeV and 5- and 6- states, which may be of collective or two-quasiparticle character. A wide range of approaches have been used to study theoretically the complicated structure of the 2 = 52 tellurium nuclei, including two-proton core coupling calculations 4,5), the dynamic deformation model (DDM) (j), and the interacting boson approximation (IBA) model 7-1o). One of the main interests in the study of Te nuclei has been to study the systematics of shell-model intruder states ii). In the neighboring even Cd(Z = 48) and Sn(Z = 50) nuclei, rotational bands built upon the first excited O+ states have been identified. A definite spin-parity assignment of O+ to the 1357 keV state in 12’Te was made

6

Tellurium Fig. 1. Systematics

of energy levels of yrast states in Te nuclei as a function of neutron taken from present work, ref. ‘) and references therein.

number.

Data

383

C.S. Lee et al. / lz2Te

from recent

conversion

electron

measurements

12,13); this state has been proposed

as the band head of a 4p2h intruder excitation of two gg12 protons across the shell gap. Detailed IBA-2 calculations, which include mixing with a deformed intruder configuration

lo), indicate

mixed with intruder momentum

increases.

B(E2)‘s, and electric between the proposed any Te isotope 14). To provide the even-even

that quasibands

configurations, Although

in the normal

and the intruder this calculation

quadrupole “intruder”

configuration

component

reproduces

are strongly

grows as the angular well the level energies,

moments in Te nuclei, intraband states have not been experimentally

transitions observed in

further spectroscopic details on the yrast and non-yrast excitations in Te nuclei with 66 < N < 74, we have begun a systematic study via

((.u, xn) reactions on Sn targets. Here we report our results for lz2Te. With the relatively non-selective (a, xn) reaction we have identified new levels of moderate angular momenta, and have made definite, or more restrictive, spin-parity assignments to yrast and non-yrast excitations. We have previously reported 15) the first in-beam observation of the first excited O+ state at 1357.4 keV. In the present work we also discuss the properties of the proposed intruder band.

2. Experimental

procedure and measurements

The nucleus 12’Te was populated with the fusion evaporation 12’Sn( a, 2n) reaction using an a beam from the Rutgers FN tandem Van de Graaff accelerator. An isotopically enriched 12’Sn target (98.39% purity) of thickness =l mg/cm’ was evaporated directly onto a 0.5-mm thick slab of Pb. The experiments included y-ra.y excitation function, y-ray angular distribution, and yy-coincidence measurements. The y-rays in the coincidence measurements were detected with three germanium detectors: two n-type high-purity germanium coaxial detectors with an efficiency of 25% and a Ge(Li) detector with an efficiency of 20%) relative to a 7.5 x 7.5 cm2 NaI detector. Typical energy resolutions were 2.2 keV at 1332 keV for these detectors. Gamma-ray excitation functions were measured for beam energies of 14-22 MeV in 1 MeV steps using a single germanium detector. The relative yields of each transition as a function of beam energy were extracted, normalized to the integrated beam current. The results are shown in fig. 2. The reaction yield of the 2n channel (122Te) starts to saturate at 20 MeV, whereas that of the In channel (123Te) peaks at 17 MeV. The 18 and 24 MeV beam energies were chosen for the subsequetrt yy-coincidence measurements. The lower energy was used to populate low-lying non-yrast states and the In channel lz3Te, while the higher energy favors higher angular momentum states in 122Te. The angular distribution measurements were made at E, =23 MeV. The y-ray yields were collected with two detectors; one germanium detector was kept at -90” to the beam direction and was used for normalization. The other germanium detector, shielded with a BGO-Compton suppressor in an asymmetric configuration 16), was

C.S. Lee

384

et al. / '"Te

1000 6 m ;j c 51 :

--o--

159(3/2-l/2)

-_t

331(7/2-3/2)

e

505(5/2-l/2)

u

564(2-O)

*

569(6-4)

*

692(2-2)

100

10 15

Fig. 2. Gamma-ray

17

1.9

E(alpha)

MeV

21

:!3

intensities as a function of beam energy for the “‘%n(Lu, xn) reaction. represent the states of 12*Te, closed ones the states of ‘23Te.

Open symbols

swept between the angles 0” and 90” in 15” steps. A 500 ns wide veto signal was used to reject Compton-scattered photons. The data at different angles were normalized using the yield of the 564.1 keV 2 ++ O+ transition in the fixed detector. Efficiencies of the system as a function of angle and energy were obtained by placing 133Ba and 15’Eu sources at the target position. The peak areas as a function of angle were fitted with a fourth-order truncated Legendre polynomial: W( 6) = I,[ 1 + A&(cos

0) + A,P,(cos

0)] .

The solid-angle correction was negligible due to the small BGO shield and the 26 cm target-to-detector distance.

entrance Angular

angle of the distribution

coefficients and the extracted y-ray intensities are summarized in table 1. To assign multipolarities to all transitions, the alignment attenuation coefficients 17), (Ye, were extracted using the method described in refs. 18,‘9). The measured angular distribution coefficients for known stretched E2, El and Ml transitions were compared to theoretical predictions to extract the gaussian widths of the population. For our present measurements, the extracted widths ranged from 0.2-0.3 per unit of angular momentum. For essentially all transitions a!,> 0.6, indicating that considerable alignment is retained as the nucleus deexcites. The fact that the alignment is essentially preserved aids in assigning the multipolarities of stretched transitions and determining the E2/Ml ratios for mixed transitions. For the yy-coincidence measurements three germanium detectors were placed at SO”, 102”, and -105” with respect to the beam direction. The event trigger was the coincidence of one time-to-amplitude (TAC) signal and 2 Ge signals; all TACs and Ge analog-to-digital convertors (ADCS) were read in each event and the data were recorded on magnetic tapes. The time resolutions of all three TAC spectra were

C.S. Lee et al. / “‘Te

385

TABLET Results

E, (keV)“1 273.1 297.3 351.0 532.8 564.1 569.X 617.2 621.1 692.6 694.3 704.6 712.0 72X.3 769.7 793.3 “) 860.7 909.9 91X.6 1049.4 1102.6 1139.4 1162.2 1220.8 1226.6 1256.7 1345.5 13X6.9 1477.9

from angular

I,“) 15 (3) 144 (3) 14 67 1000 559 839

(1) (2) (14) (11) (17)

33 (1) 74 (2) 42 (1) 22 (1) 20 (1) 21(l) 12 (1) 25 (1) 15 (1) 22 (1) 204 (4) 165 (3) 45 (1) 32 (1) 43 (1) 20 (1) 65 (1) 11(l) 1X (1) 10 (1) 13 (1)

distribution

measurements

A, “1

A, “1 0.45 0.33 -0.84 0.29 0.33 0.39 0.37 0.43 0.16 0.18 0.45 -0.18 -0.03 0.15 0 0.33 0.30 0.36 -0.11 0.37 -0.88 0.28 -0.34 -0.20 0.15 0.15 0.14 0.16

(38) (9) (15) (3) (2) (2) (2) (4) (4) (3) (5) (4) (5) (10) (8) (5) (2) (2) (3) (4) (3) (33) (2) (7) (4) (8) (7)

of -y-ray transitions Multipolarity

in “*Te S(EZ/Ml)

0 0

0.19 0.05 -0.05 -0.07 -0.09 -0.15 -0.14 -0.09 -0.20 0 -0.19 0 0 -0.25 -0.11 -0.16 0 -0.28 0.04 -0.20 0 0 -0.05 -0.02 0.04 -0.03

(13) (4) (4) (4) (4) (5) (5) (4) (7) (7)

(10) (6) (3) (4) (4) (4)

(9) (5) (10) (8)

E2 E2fMl E2+Ml E2 E2 E2 E2 E2+Ml”) E2+Ml”) E2 El or Ml E2+Ml E2 isotropic E2+Ml E2 E2 El E2 E2+Ml E2 El or Ml El E2 E2 E2+Ml E2

-1.5+1.ra 0.75 -0.2:;:;:

-0.57

(5)

+1.3?3 0.4

-0 2XlO.O’

.

0.04

-0.3 < s < 0.0

“) Energies are rounded to the first decimal point. See table 2 for more precise values with errors. b, All intensities are normalized to the intensity of the 564.1 keV y-ray. Errors on the last digits are only statistical uncertainties coming from the peak-area fitting. ‘) Large uncertainties are due to weak intensities for Z, 6 20. d, The intensity was extracted from the total-projected spectrum. ‘) No mixing ratios can be determined because the components of the 692.6 + 694.3 keV doublet cannot be resolved.

about 20 ns. The data were first analyzed by sorting 13.5 million real, prompt events into a single 2048 x 2048 matrix. Fig. 3 shows a total-projection spectrum. Examples of several background-subtracted gated spectra are shown in fig. 4.

3. Results and level scheme Combining the results of the yy-coincidence and angular distribution measurements, a level scheme up to 3995.4 keV in excitation was constructed, as shown in fig. 5. The present scheme was built upon the compiled data in ref. ‘O). Three new

C.S. Lee et

386

0

200

400

600

800

af. /

1000 CHANNEL

“‘Te

1200

1400

1600

1800

2OCIO

NUMBER

Fig. 3. Total-projected spectrum of the coincidence measurements in the ““Sn(cu, nn) reaction at E, = 24 MeV. The transitions in PO come from reactions with the Pb backing.

levels were found in the present work and placed at E, =33&1.7, 3579.6, and 3995.4 keV, based on the observation in the 24 MeV data of 712.0,909.9 and 704.6 keV y-rays, respectively. The angular dist~bution results show that both the 704.6 and 909.9 keV y-rays are stretched E2 transitions deexciting to 10” and 8’. states, respectively. A tentative assignment of (12*) to the previously established 3995.4 keV state is based on the measured A4 = -0.20 (7). The 3381.7 keV state is assigned I = 9, based on the stretched dipole character of the 712.0 keV transition; no parity assignment can be made from our work. In our 18 MeV data we also observed the first excited 0’ state at 1357.4 keV via the detection of the 793.3 keV transition deexciting to the 2’ state at 564.1 keV. The isotropic angular distribution of this y-ray supports the previously established ‘2B’3)Oi assignment. Additional spin-parity assignments were made for the previously known I’) states at 2758.9,2890.5 and 2971.9 keV. The 351.0 and 1139.4 keV y-rays are mixed E2 + Ml transitions to the S and 6’ states, respectively. Thus, the 2758.9 and 2890.5 keV states are assigned as 6.. and 7+, respectively. The stretched dipote character of the 1220.8 keV y-ray supports the i = 7 assignment to the 2971.9 keV state. Definite spin-parity of the non-yrast state at 1951.0 keV is determined as 2+, and the 4+ assignment *‘) of the 2042.0 keV state is confirmed. The spin-parity of the 3073.6 keV state is not clear because of the large uncertainty in the A2 coefficient of the 273.1 keV transition; however, I = 7,8 or 9 is likely. Several previously measured low-lying states were not observed in the present work. The second excited 2+ state at 1753 keV found in p-decay 20) and Coulomb

387

C.S. Lee ef al. / ‘“Te

(4 9 18.6 300

c

fEx

LeV

= 2669.7)

200

cl U N T 5 100

a

4

I

600

800 CHANNEL

f000

1200

1

1200

1

NUIIBER

I 400

600

800 CHANNEL

Fig. 4. background-sub~ra~~d

1000 NUMBER

spectra gated on the (a) 918.6, (b) 704.6, and (c) 712.0 keV transitions.

C.S. Lee et al. / ‘?e

388

7 12.0 (Ex

400

600

1000

800 CHANNEL

keV

= 338

1.7)

1200

1400

NUMBER

Fig. 4-continued

excitation 21) has been considered to be an important collective state near the three-phonon excitation. This state has also been proposed *-lo) as an intruder excitation built on the first excited Ot state at 1357.4 keV. Although we searched for this state, we did not find any sign of its population in the present measurements. The second excited O+ state at 1940 keV is now well established through conversion 12,*3). While another excited O+ state at 1747 keV has been electron measurements suggested,

its existence

previously study “).

seen at 2200 keV in Coulomb

is still ambiguous

13). We also did not populate excitation

the 3- state

21) and at 2190 keV in a (p, t)

4. Discussion In the simplest configuration of valence particles in 122Te, two protons occupy the rdg,2 and rgTj2 orbitals, while twenty neutrons fill the lower udg,2 and vg,,, orbitals and half fill the vhll,z orbital. For the yrast members in the ground-state band (g.s.b.), the 2+ state is the lowest-lying collective excitation. The constant energy systematics (fig. 1) as a function of neutron number suggest that the 6+ state has significant two-proton components, namely n(g7,2)2 or rr(d5,2 x g,,J. Also, the low-lying collective 4+ state probably has an admixture of two-proton structure. The higher spin states I Z=8+ in the g.s.b. probably involve quasineutron excitations with a major contribution from v(h11,2)2 components.

389

l **Te Fig. 5. Level scheme of “‘Te . The widths of the arrows are approximately proportiona of the y-rays. Dotted levels were not observed in the present work, but are taken

to the intensities from refs. 12,‘3S20).

The negative-parity states in lighter Te nuclei have been identified as two-neutron excitations 2”). Applying similar arguments to 122Te,the 5- and 6- states are mainly v(~s,,~ x lh,,J, and the 7- state v(2dSj2 x lhlr,J. Higher-lying 1= 8,9 states can be constructed from the neutron configurations y(lg,,2x 1h11,2)8,9.The Y(lh1rJ2 configuration could give rise to a lO+ state. The first excited O+ state at 1357.4 keV has been proposed lo) as a 4p2h proton excitation across the 2 = 50 shell closure. To test further the role of intruders in the low-spin states, we have examined the collective nature of several non-yrast states by comparing relative reduced transition probabilities. In lig. 6 and summarized in table 3, the extracted relative E2 branching ratios are presented and compared with the predicted values from various models. The 2f state resembles a normal collective state of either a vibrational or y-unstable nucleus, indicated by its forbidden 2; + 0: transition in each dynamical symmetry “). The 2,’ state is also a normal collective state in view of the near-zero strength of the 2; + 2: transition. The finite value of the 2,” + 2; transition supports a vibrational description, although its strength could not be accurately determined in the present work. The significant strength to the 2:

C.S. Lee et al. / “*Te

390

1357.4 1256.7 1181.3

s1.00 T=

0.008(l)

I

564.1-

O-

2:

.

0,’ '**TCt

Fig. 6. Relative

B(E2) branching

ratios for low-lying states of ‘*‘Te. The 2, + 2, transition to be a pure E2 transition.

was assumed

state indicates that the 4: state is probably not a simple collective excitation. However, neither does it exhibit the signature of any anomalous collective structure, such as a 4p2h intruder state. Using the Alaga rules 24) in the rotational limit for the 4+ member of a K" = 0' band, the square of the ratio of Clebsch-Gordan coefficients (4020(20) to (4020(40) yields 1.1, which is six times larger than the measured branching ratio to the 2+ and 4+ members of the g.s.b. Therefore, a sizeable component of the wavefunction of the 4; state is probably of single-particle structure, rather than a shape-coexisting structure. The deexcitation of the quasiband on the right-hand side of the g.s.b. in fig. 5 is characterized by rather strong interband transitions to the g.s.b. and the absence of intraband transitions. The quasiband was previously suggested 14) as a deformed intruder band arising from the 4p2h proton excitation across the 2 = 50 shell closure. The data do not support a simple rotational band interpretation for this quasiband: the experimental energy spacing does not follow the I(1 + 1) rule and intraband transitions have not been observed. However, invoking strong mixing between

C.S. Lee et al. / “‘Te

391

TABLE 2 Placement of y-rays in “*Te from (q 2n) measurements Placement IT-If”

E, (keV) “)

273.06 (7) 297.26 (7) 351.0 (1) 532.84 (11) 564.1 (1) 569.83 (12) 617.19 (12) 621.05 (13) 692.6 (3) 694.3 (5) 704.59 (14) 712.0 (2) 728.32 (15) 769.7 (2) 793.30 (16) 860.7 (3) 909.93 (19) 918.6 (2) 1049.4 (2) 1102.6 (2) 1139.43 (17) 1162.19 (18) 1220.76 (20) 1226.6 (2) 1256.78 (23) 1345.5 (2) 1386.9 (2) 1477.9 (2)

Ei (kev)

Ef (keV)

3073.6 3210.6 2758.9 2283.9 564.1 1751.1 1181.3 3290.8 1256.7 1951.0 3995.4 3381.7 1909.6 1951.0 1357.4 2042.0 3579.6 2669.7 2800.5 2283.9 2890.5 2913.3 2971.9 2407.9 1256.7 1909.6 1951.0 2042.0

2800.5 2913.3 2407.9 1751.1 0 1181.3 564.1 2669.7 564.1 1256.7 3290.8 2669.7 1181.3 1181.3 564.1 1181.3 2669.7 1751.1 1751.1 1181.3 1751.1 1751.1 1751.1 1181.3 0 564.1 564.1 564.1

?-710+-s+ 6--56+-6+ 2+-o+ 6+-4+ 4+-2+ lo+-8+ 2+-2+ 2+-2+ (12+)-10’ 9-8+ 4+-4+ 2+-4+ o+-2+ 4+-4+ lo+-8+ 8+-6+ 7--6+ 6+-4+ 7+-6+ 8+-6+ 7 -6+ 5--4+ 2+-o+ 4+-2+ 2+-2+ 4+-2+

“) Errors on the last digits are a combination of uncertainties coming from peak fitting and energy calibration.

intruder

and normal

configurations

could

explain

the data. In particular,

previous

arguments that support the intruder, more deformed character of this quasiband include: (i) the non-yrast states 1~6 are strongly mixed; thus the yrast 8+ state at 2669.7 keV could be of intruder character 14), (ii) the IBA-2 calculation with mixing of an intruder configuration predicts an increasing intruder component as the spin increases, reaching almost 90% of the wavefunction of the 8+ state lo). To study the collectivity of this quasiband, we have extracted upper limits for the intensity of the unobserved intraband ST(2669.7) + 6:(2283.9) transition. The upper limit for the intensity of a ~386 keV transition was extracted from a sum of

C.S. Lee et al. I “‘Te

392

TABLE 3 B(E2)

(Ii +

Ij)/(li

+

Ik)

c7.*-+W(2*~21) (4,+2,)/(4,+4,)

branching

Present

work

ratios for low-lying W5)

“)

states of ‘*‘Te O(6) “)

IBA-2 “)

0.008 (1) “)

0

0

0.009

0.16 (3)

0

0

0.0007 f) 0.78 “) 0.014

(2s+2s)/(2s+4,)

< 5.86 (51) “)

0.56

(2s+2,)/(2s-t4r)

< 0.0036

0

Other 0.011 d) 0.056 ‘)

“) Theoretical values from the IBA-1 symmetries of ref. ‘“). The 2, state is assumed to be of o < o,,, character for the O(6) comparison. b, IBA-2 calculation of ref. lo) including an intruder configuration, unless otherwise noted. ‘) The ratio was obtained by using the mixing ratio 6 = -3.48 (4) for the 2,-2, transition, tabulated in ref. “). d, Ratio determined from E2 matrix elements from ref. 26): M.E.(2,-0,) = 0.925 e. b, M.E.(2,-2,) = 0.097 e. b. “) Theoretical value from the DDM calculation of ref. 6). ‘) IBA-2 calculation of ref. ‘). “) An upper limit is obtained by assuming a pure E2 2, + 2, transition. h, Taken from the standard IBA-2 calculation of ref. lo).

the gated spectra for the (6:+ 4:), (4:+ taking into account the Et dependence extracted

2:), and (2:+ 0:) g.s.b. transitions. By of E2 transition probabilities, we have

B(E2; 8: + 6;) B(E2; S:+ This number

is considerably

6:)

<4.

smaller than the values of 20-50 which usually

character-

ize the enhancement of intraband to interband transition rates. Rather, the deexcitation of the 8: state indicates that the states of interest have similar character. Possibly, strong mixing could explain the observed deexcitation of the 8: state. In a two-state

mixing

calculation,

the final energy

separation

A’ is given by

A’= [A*+ (2&)*]“*, where A is the initial energy separation and HI2 is a mixing matrix element. Thus, the mixing matrix element is at most one-half of the final energy separation. Applying this to the two lO+ states at 3210.6 and 3290.8 keV, the mixing matrix element is at most 40 keV at I = 10 in ‘**Te. More typically, mixing matrix elements of ~100 keV have been necessary to explain the mixing between intruder and normal configurations in tin and cadmium nuclei 29). Either the mixing strength between normal and intruder configurations in tellurium nuclei exhibits a strong dependence on spin, decreasing as the angular momentum increases (an opposite conclusion to the results of ref. 14), or intruders do not play a significant role in the near-yrast excitations. This conclusion is consistent with a recent study of electric monopole transitions

C.S. Lee et al. / ‘“Te

between

the O+ states. The observed

to the E2(0:+2:)

transition

ruling out an anomalous

12) relative

ratio of the EO(O: + 0:) transition

agrees well with the IBA-2 calculation

character

Here the 8: state would

of ref. ‘*), thus

for the 0: state. The same conclusion

for the 0: state at 1940 keV. An alternative explanation for the near-yrast picture.

393

be a 4-phonon

was obtained

states could be a normal state with allowed

vibrational

transitions

to

the 6: 3-phonon state, and hindered transitions to the 6:, 4-phonon state; this picture is consistent with the observed deexcitation of the 8: state. However, no single picture seems to be able to provide a consistent explanation of the yrast and near-yrast excitations in 122Te. As mentioned earlier, the 6: state probably has strong two-proton admixtures and many of the low-lying absolute and relative E2 transition probabilities are not in agreement with vibrational expectations. Also, two closely spaced lO+ states cannot be understood simply in a vibrational framework and the presence of a third one at 3579.6 keV could affect the validity of the two-state mixing discussion. 5. Conclusion The interplay between single-particle and normal collective structure governs the low-lying states in 122Te. In order to draw a better picture of collective properties one needs a more accurate measure of structure quantities such as mixing ratios, and absolute and relative E2 transition rates for many non-yrast states. The lack of observation of intraband transitions for the previously proposed intruder band sheds doubt on the role intruder configurations play in 12*Te. It is hoped that a systematic study of neighboring even Te nuclei, as well as the odd isotopes, will be valuable in enlightening the role of coexisting structures in these transitional nuclei. Such work is in progress. This work was supported

in part by the US National

Science

Foundation.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

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