Low-energy states in the odd-odd 126I nucleus produced by the (p, n) reaction

Nuclear Physics A402 (1983) 205-219 @ North-Holland Publishing Company

LOW-ENERGY STATES IN THE ODD-ODD -1 NUCLEUS PRODUCED BY THE (p,n) REACTION? J. BURDE, Rncah Institute

V. RICHTER

and I. LABATON

of Physics, The Hebrew University, Jerusalem, Israel Received 6 Janugry 1983

Gamma spectra produced in the “‘Te(p, n)? reaction were taken as a function of proton bombarding energy E, = 2.9 MeV to 6.0 MeV. A level scheme composed of 52 excited states was constructed on the basis of singles y-spectra and extensive y-y coincidence measurements. The coincidence measurements were carried out utilizing a three-parameter Ge(Li)-Ge(Li) timing system. Angular distribution measurements combined with the results from the coincidence counting rates enable some spin and parity assignments to be made. The mean lifetime of the first three excited states 56.45, 110.84 and 122.2 keV were determined, respectively, as 23(2), 81(4) and 19.5(15) ns. The structure of the states was discussed on the basis of a de-excitation mode comparison with the corresponding levels in the neighbouring odd-odd **? and lz41 nuclei.

Abstract:

E

NUCLEAR

REACTIONS ‘*%e(p, n), E = 2.9-6 MeV; measured E,, I,., yy-coin, y-y(t), I(@). ‘*? deduced levels, 1, 7r, y-multipolarity. Enriched target.

1. Introduction

Recently the level structure of the ‘*‘I and 1241excited states produced by the (p, n) reaction have been investigated ***).The mode of the decay of some positiveparity states in ‘*‘I could be accounted for by describing them in terms of admixed two-quasiparticle multiplets. Also a striking resemblance has been attained between 13 positive-parity levels in 1241and corresponding ones in lz2Sb, on the basis of the mode of de-excitation. The latter has be&n described mainly by two-quasiparticle multiplets. It should be interesting to elucidate the level structure of the neighbouring odd-odd iodine isotopes lz61 and 13’1,whose level schemes are completely unknown and to see to what extent they can be compared with the 1241 and 1281 nuclei following the description of two-quasiparticle multiplets. In the present paper we report on the results obtained via the 126Te(p, ny)‘261 reaction. Starting the excitation function below the threshold of the (p, n) reaction (Q. = 2.938 MeV), we were able to identify the y-transitions produced in the lz6Te(p, ny)‘? reaction and to locate roughly their position in the level scheme of ‘*?. However, only with the aid of elaborate coincidence measurements and ’ Work supported in part by the Israel Academy of Sciences and Humanities. 205

206

.f” Bide

et al. / Low-energy

states

utilizing two high-resolution (in energy) detectors could we disentangle the rather complex decay scheme. In the present investigation, the information of the timing order between two coinciding y-transitions, had to be used sometimes in order to decide between two alternative positions of the lines in the decay shceme. Spin and parity were assigned to some levels with the aid of angular distribution of y-rays with respect to the incident beam measurements and coincidence counting rate results. 2. Experiments ~~~gernent The rz6Te target enriched to 98.7% and 0.8 rng~crn~ thick was evaporated onto a 40 vg/cm2 carbon foil. The proton beam from a tandem Van de Graaff accelerator was collimated onto the target by two tantalum apertures 3 mm in diameter, placed 180 cm and 20 cm from the target and finally dumped into a tantalum Faraday cup 2.5 m away from the target. Singles y-spectra produced at various proton bombarding energies were taken by a 36 cm3 Ge(Li) detector which had a resolution of 2.5 keV FWHM for the 1332 keV y-ray of 6oCo and an efficiency of 6% in this region and was placed 2.5 cm away from the center of the target at 90” to the incident beam. For y-y coincidence measurements a second Ge(Li) detector with a resolution of 2.7 keV and similar efficiency was pfaced at 270” at the same distance from the target. The three-parameter Ge(Li)-Ge(Li) timing system block diagram and the method of analysis of the coincidence data were described elsewhere “). The singles y-spectrum produced by 6 MeV protons was taken also at 90” and 10 cm away from the target using a 5 cm3 Ge(Li) detector which had a better resolution particularly in the low-energy region (0.85 keV for the 122 keV y-line of “Co). For angular distribution measurements, one detector with 6% efficiency at 270” served as a monitor and the second was placed on a turntable at angles of t? = 27.5”, 35”, 40”, 45”, 60”, ‘75” and 90” with respect to the incident beam. Both detectors were placed 15 cm away from the target.

3.1. SINGLES SPECTRA

The energies of the excited states or any de-exciting transitions in 12? were completely unknown. Hence, singles y-spectra were taken as a function of the proton bombarding energy, starting below the threshold for the (p, n) reaction at 2.9 MeV in steps of 50 keV up to 3.3 MeV and continuing in 100 keV intervals up to 6 MeV, in order to be able to identify and to locate to some extent the positions of the y-transitions in the decay scheme. Fig. 1 shows the low-energy region of the y-singles spectrum produced by 6 MeV protons as taken by the higher resolution

_T.Burt& et al. f Low-mergy

stales

207

TABLE 1 Energies and intensities of the transitions in I’&, produced at the proton incident energy of 6 MeV Energy of

transition (keV) 7.5 “) 54.65 (15) 56.45 (8) 56.75 (15) b, 62.56 (18) 81.00 (10) 83.50 (IO) 86.34 (17) 88.70 (9) 93.45 (6) 107.oa (7) 109.60 (20) “f 1 IO.85 (6) 111.80 (6) h, 115.14 (8) 117.05 (7) 120.66 (10) 122.20 (IO) 122.70 (20) ‘) 125.78 (9) 132.20 (10) h, 133.20 (12) 140.70 i2tlf 142.90 (7) 146.00 (6) 147.70 (lot hf 149.110 (11) 166.00 1171 166.20 (5) 171.42 (7) 178.95 (7) 187.15 (8) 189.50 is) 194.45 (5) 196.60 (10) b, 201.3u (7) 206.60 (IO) 209.12 (61 211.65 (10) 216.35 (8) 222.60 (6) 226.80 (13) b, 227.70 (10) 233.30 (8) b, 234.20 (6) 237.47 (81 247.40 (10) 254.65 (11) ‘) 254.75 (11) b, 262.80 (12)

Intensity at 90”

120 (20) 951(25) 49 (25) “1 21.8 (26) 5.0 (12) 9.7 (14) 30.0 (IS) 4.4 (13) 39.3 (20) 25.3 (17) 7.7 (22) “) 538.6 (2.5) 31.5 (20) “f 38.0 (13) 34.9 (14) 6.7 (10) 210.0 (34) 108.0 (34) 7.8 (14) 5.4 (12) 5.2 (12) 10.8 (141 25.5 (14) 44.1 (16) 2.7 (14) “1 l&2 (14) “) 15.5 (26) 156.0 (26) 131.6 (22) 8.7 (17) 4.3 (16) 5.2 (17) 8.3 (19) 2.9 (15j”) 57.7 (22) 6.3 (18) 13.0 (IS) a.5 (22) 27.7 (20) 64.0 (24) 3.7 (20) “) 6.3 (20) 9.4 (22) “) 14.4 (22) 10.7 (21) 19.2 (25) 15.0 (64) “1 43.0 (64) “) 3.9 (231

Angular distribution

4

0.0302 ziz0.047

-0.0389*0.01

-o.oKm

rt 0.006

-~*0018~~.008~ -0.085 It 0.011 0.0298It 0.28

-0.0038* 0.06

-0.0049 * 0.032

-0.221 *to,112 -0.0052 * 0.004

Initial state

Final state

ikeV

WV)

244.9 110.84 56.45 179.0 373.7

237.4 56.45 *. x%:2 311.2

311.2 331.3 311.2 204.25 311.2 166.0 110.84 222.6 237.4 227.7 348.45 322.2 244%9 348.45 369.6 506.9 479.0 365.5 373.7 479.0. 393.8 166.0 222.6 227.7 179.0 535.7 393.8 422.2 570.3 566.8 434.2 331.3 434.2 338.4 222.6 570.3 227.7 800.1 479.0 237.4 369.6 365.5 311.2 373.7

227.7 244.9 222.6 110.84 204.25 56.45 1 l!&4 122.2 110.84 227.7 &?? 222.6 237.4 373.7 338.4 222.6 227.7 331.3 244.7 5?G5 56.45 3:Ck 204.25 227.7 373.7 365.5 227.6 122.2 222.6 122.2 3&5 56g$sg 244.9 1z 110.84 56.45 110.84

~._~~

._ ~~

Energy of transition WV)

Intensity at 90’

264.50 (IO) 275.35 (10) 287.87 is) 292.00 (Sf 292.00 (10) “) 306.60 (iOl 309.30 (12) 311.20 (5) 312.30 368.74 (6) 377.75 (3) 383.20 (10) 391.85 (Sf 397.60 (10) [email protected] (S) 414.60 110) “) 422.42 (14) 434.60 (IO) 459.50 (1.5) 469.32 ilO) 475.40 (fO) 479.25 (IOf 488.20 (ro) 496.50 (IO) 521.20 (IO) 535.70 (IO) 548.70 {XC!, 565.60 (20) 601.2O (20) 624.20 (IO) 641.20 (IO) 609.50 (30) by 507.00 (30) h, 6237.90irq 709.60 ilO! 7x70 (XC@ 744.20 (l@) 752.3 (12)

4.9 (23) 12.5 130) 122 @‘Pf 51.2 (80) 28.1 (sfft “) 26.3 (33) 16.8 (30) 49-3 (42) 29.2 [42) “t 1 r.7I(so) 15.6 &2) “) 24.4 (45) 19.6 (45) Q.l(30) 8.9 (30) 13.8 (38) ‘i34.3 (481 4.4 {39> 4.4 (40) “) 21.4 (38) 34.4 (50) 7.3 138) 21.9 IS2f 50.5 @2f 2o.u (45, 36.0 (56f 38.6 (56) 64.0 (65) 12.4 (52) 45.5 (54) 16.2 (53) 66.9 i62f t2,3 (55) 12.1 (54) 14.3 (61) 88,7 (68) 13.4 {&I) 20.1 @Zf 9.7 (68) 10.8 (68) 3.9 (68) 4.8 (70) “) 30.6 (90) “) 46.5 (89) 22.2 isa1 53.8 p4f 5.9 WI 19.4 (861

Anglllar

distribution AZ

-rs.f54;ii0&9 -0.0795 10.037

Initial state: (kev) 698.3 397.6 M02.5 348.45 458.0

617.8 513.5 676.7’ 479.0 544.6 688.0 544.6 4342 470.3

--IZOJCMG

-0.134*0.086 -0.010*tw.08 -0x-Y

f O&8

-r).ZOl40*11 O,QS9*&45

“) Deduced from the coincidence measurements. 7 Observed in coincidence measurements. ? Derived on the assumption that A&= 0.

Final state (keV 393.8 1222 714*7 56.45 166.0 311.2 2#4*25 36.v IS&Q 227.7 369.6 222.6 110.84 244.9

393.5 343.45 47Q,O S9l.B SQX.5 434.2 748.7 7032 397& 513.5 S80#6 479.0 491J.l 570,3 812.9 7413.t S3S.T 8lM 8QO.J lOC?(T‘l 7x47 714.7 676.7 944.7 868.9 8615.9 979.1

617.8 688,O 1102.2 714.7 tCn32.7 956.6

20425

210

J. Bwde

et al. / Low-energy states

detector at 90”. The energies of the transitions and the efhciencies of the detectors as a function of the y-energy line were calibrated with the aid of 24*Amt “Co, l=Eu, ‘*Na and “OCo sources. In the first two columns of table I are given the energies and the relative intensities which were obtained from the high-resolution detector measurements. Unresolved transitions in the singles spectrum which were observed or deduced from the coincidence measurements were also included. 3.2. COINCIDENCE

M~ASUREME~S

The y-y coincidence set-up, utiltizing the three parameter ~e~Li~-~e~~i~-tirni~~ system and the method of deducing the coincidence spectra, were described elsewhere “>. Fig. 2 shows examples of such genuine coincidence spectra with some chosen prominent transitions. The results deduced from the coincident spectra shown in fig. 2 and other similar ones are given in table 2. The energy of the selected y-line window is indicated to the left and the energies of the tra~sitio~s which have been found to be in coincidence with it are given to the right in the same row. 3.3. LlEETiME MEASUREMENTS

3.3.2 Lifet&~e ~~~er~~~~~ti~~of &hs 210.84 keV state, Fig. 3 shows the delayed time spectrum obtained by setting both energy windows, of spectrum 1 and spectrum 2 that starts and stops the TAC respectively, on the sum of the 110.84 and 111.80 keV lines. The 111.80 keV transition populates the 110.84 keV fevel and the latter is de-excited to the ground state by the 110.84 keV line. The slope towards higher channel numbers corresponds to the events that the 111.80 keV line appears in spectrum 1 and the 110.84 transition in spectrum 2, and towards the lower numbers when the order of the lines is interchanged. Two time spectra were also obtained by setting windows in spectrum 1 on the 93.45 keV and then on the 254.65 keV lines that populate the 110.80 keV level and accepting the 110.80 keV line in spectrum 2. Two additional time-inversed spectra were obtained when the windows in both energy spectra were interchanged. The centraids of these four time spectra were compared with the corresponding prompt time spectra, The latter were obtained by switching off the proton beam and replacing the target by a **Na source and using the same energy windows 4>SThe Iifetimes deduced from the shifts of the centroids and the slopes as observed in fig. 3 agreed, within the experimental errors, with each other. The combined result for the mean lifetime of the 110.84 keV state thus obtained was 8 l(4) ns. 3.3.2. L~f~~i~~ ~e~~r~~e~~ of the 56.45keV &XX%Fig. 4A shows the time spectrum obtained by setting a window on both feeding lines, 166.2 and 171.4 keV from spectrum 1, and channeling the 56.45 keV de-exciting transition from spectrum 2. In fig. 4B the time-inversed spectrum is obtained by inter~han8ing the

CC3lJNTS

201(via 2% and!42)

COLJN‘I-S

212

.T. Surde et al. / Low-energy

states

TABLES Coincidence of transitions in I’?, Selected r-fine ikeV1 56.45

-I-

56.7

62.56 86.34 93.45 109.6+110.85+111.8

122.2+ 122.7

132.2+ 133.2 142.9 146.0 149.0 166.0+166.2 171.42 178.95 189.5 194.45 201.3 209.12 216.35 222.6 234.2 247.40 254.65 e 254.75 275.35 287.87 292.0+292.0 306.6 343.45 397.6 422.42 521.2

obtained at the proton incident energy of 6 MeV Enegy af transitions in coincidence. (keV)

54.65; 93.4; 111.8; 142.9; 146.0; 166.2; 171.4; 254.7; 292.0; 122.2; 337.7; 422.4; 434.6; 479.2; 535.7 83.5; 88.7; 107.0; 133.2; 254.7 115.2; 122; 147.7; 237.4; 488.2 54.6; 107.0; 110.8; 189.5; 264.5; 309.3; 496.5; 752.3 56.4; 88.7; 93.4; 107.0; 111.8; 117.0; 142.9; 146.0; 189.5; 201.3 254.6; 262.8; 292.0; 311.2; 312.9; 323.4; 368.7; 402.6; 41d.6; 211.6 459.5; 507.0; 565.6 56.7; 86.3; 115.2; 122.7; 140.7; 147.7; 209.1; 216.3; 247.4; 275.4; 356.9; 488.2; 535.7; 521.2; 132.2 149.0; 234.2; 325.6; 624.2 115.2; 122.2; 318.4; 609.5 146.0; 171.4; 262.8; 62.5 56.4; 110.8; 201,3; 166.2; 222.2; 311.2; 383.2 56.4; 117.0; 171.4; 227.7; 110.8; 133.2; 196.6 115.2; 122; 264.5; 496.5 292.0; 312.9; 414.6 56.4; 88.7; 125.8; 62.5; 142.9; 201.3; 211.6; 311.2; 322.0; 368.7 56.4; 83.5; 120.7; 146.0; 187.2; 194.5; 206.6; 317.0; 363.5: 475.4; 641.2 535.7 93.4; 410.8; 264.5; 496.5 171.4; 117.0 56.4; ‘111.8; 142.9; 166.2; 222.6; 233.3 122.2; 147.7; 488.2 122.2: 140.7; 521.2; 744.2 88.7; 125.8; 142.9; 201.3; 211.6; 322.0; 368.7 115.2; 122; 521.2 122.2; 318.4; 609.5 56.4; 110.8; 201.3; 311.2; 383.2; 62.5; 306.6; 391.9 122.2; 704.6 535.7; 714.7 56.45; 109.6; 166.0; 187.2 254.7; 107.0 226.8; 469.4; 601.2 704.6 56.4; 521.2 140.7; 122; 147.7; 234.2; 312.9; 422.4

energy windows in both spectra. The lifetime of the 56.45 keV state was also deduced from the centraid shifts of the time spectra obtained separately between the 166;2, 171.4 and 254.75 keV feeding transitions and the de-exciting 56.45 keV line and of the corresponding prompt spectra. The centroid shifts for the time-inversed spectra were also obtained. The combined result from these 6 centraid shifts and the two slopes of fig. 4 yielded 23(2) ns for the mean lifetime of the 56.45 keV state.

J. Burde et al. / Low-energy states

213

cn

2 3

E 10

1

300

v--?----+

500

1100 CHANNELS

1300

1700

1900

2100

Fig. 3. Delayed time spectrum of the 110.84 keV state de-excitation, obtained by setting both energy windows on the sum of the 110.84 and the Ill.80 keV lines.

53.3. Lifetime deter~~n~t~o~ of the1222 kevstate. The lifetime of the 122.2 keV state was determined, in a similar way, by comparing separately the centroids of the time-delayed coincidences, between the 56.75, 209.12 and 216,35 keV feeding lines and the 1122.2keV de-exciting line, with the corresponding prompt time spectra. The mean lifetime obtained from these three centroid shifts and from the three inverse time spectra, was lQS(15) ns.

Fig. 4. Delayed time spectra of the 56.45 keV state de-excitation. (A) Obtained by setting windows on the 166.2 and 171.4 keV tines of spectrum 1 and the 56.45 keV transition of spectrum 2. (Bf Obtained by setting the windows of the papnlating lines of spectrum 2 and the 56.45 keV transition of spectrum 1.

214 3.4. THE

.I. Burde et al. j Law-energy LEVEL

states

SCHEME

Fig. 5 shows the level scheme of 12? produced in the (p, n) reaction at a proton energy of 6 MeV. The level structure was constructed by utilizing the excitation function and the sum rules for the energies of the y-lines subject to proper balancing the intensities of the populating transitions with the de-exciting lines. The final acceptance of the positions of the transitions in the level scheme, however, was based on the extensive coincidence measurements. Filled circles at the end of the arrows indicate that the position of the transition that terminates in a definite state is verified by a coincidence result between this line and at least one line that is

Fig. 5. Level scheme of 12? produced in the (p, n) reaction at a proton energy of 6 MeV. The energies of the levels and the transitions are given in keV.

J. Burde et al. / Low-energy states

215

de-exciting this level. The existence of 48 excited states out of 52 proposed levels has been supported in each case by more than one coincidence result. The 458.0, 580.6, 800.1 and 1082.7 keV levels were constructed solely on the basis of one coincidence measurement in each case, and hence were indicated by dashed lines. A number of 96 transitions out of the 98 identified lines were placed in the level scheme. The construction of the present level scheme became more complicated due to a number of unresolved lines corresponding to different transitions and to the presence of many levels which could lead sometimes to other alternative insertion of the lines in the level structure. Thus, for instance, the coincidence result between the 56.7 keV line and the 122.2 keV transition could be accounted for, within the energy resolution of the system, by placing the 122.2 keV line above the 56.45 keV level. However, the timing information from the present coincidence measurement indicated that the emission of the 56.7 keV -y-line preceded the 122.2 keV line, in contradiction to this latter alternative proposition. Likewise, a comparison between the prompt and delayed coincident spectra which are gated by the 111.80 + 110.85 + 109.60 keV transitions (presented, respectively in fig. 2B and 2C) shows that the 292 keV line is absent in the delayed spectrum. This fact indicates that the 292 keV transition should be placed above the 166.0 keV level which is de-excited also by the 109.60 keV line and not via the long-lived 110.80 keV level. The initial and final state for each transition is given also in table 1.

4. Spins and parities 4.1.

MULTIPOLARITIES

OF SOME

TRANSITIONS

An experimental and theoretical fractional yielded the mutlipolarites of some transitions.

AND

PARITIES

OF STATES

y-decay coefficient comparison ‘**) The coefficients can be defined by

the expression

where the 1; and It are respectively the intensities of a given y-line and the competing y-line that de-excite the same level. The corresponding total internal conversion coefficients are denoted by (Y’ and ok. The quantity J&. which is very sensitive to the multipolarity of low-energy transitions, is proportional to the coincidence counting rate between a feeding y-line i and the de-exciting transition j, divided by the intensity of the transition i. The angular correlation effects were minimized due to the very large solid angles subtended by both detectors in the coincidence measurements. The experimental xi values and the corresponding theoretical values for various multipolarities are given in table 3 for five low-energy transitions. In deducing the

J. Burde et al. / Low-energy states

216

TABLE 3 Experimental

and theoretical y-decay coefficients xl obtained by dividing the intensity by the sum intensities of all the transitions that de-excite the same state

Energy of de-excited state state (keV)

Energy of transition

56.45 110.84 122.2 221.7 331.3 343.45

56.45 110.84 122.2 171.42 86.34 343.45

(keV)

“) The 54.65 keV transition ‘) The 54.65 keV transition

of the y-line

i

XV

ew 0.42 (7) 0.44 (7) 0.46 (6) 0.57 (10) 0.40 (5) (0.974) is assumed is assumed

El

Ml

E2

0.51 0.425 “) 0.893 0.909 0.577 0.993

0.19 0.500 b, 0.700 0.807 0.394 0.974

0.07 0.79 b, 0.526 0.734 0.236 0.973

Adopted multipolarity

El El, Ml E2 (Ml) E2 (Ml) Ml

to be Ml. to be El.

value for the 110.84 keV transition the branching due to the 54.65 keV line was taken into account. The experimental xl values were normalized by taking the experimental quantity of the 343.45 keV transition to be equal to the theoretical Ml ,& value. (At this energy the difference between the ,Y$ values for El and Ml is only 2% .) As is evident from table 3, the 56.45, 122.2, 171.42 and 86.34 keV transitions have most probably El, E2(Ml), E2(Ml) and Ml multipolarities respectively. The multipolarity of the 110.84 keV line could not be deduced from this comparison. The parity of the ground state is negative 5), Hence the levels 56.45 and 122.2 keV which are de-excited to the ground state have respectively positive and negative parities and the 227.7 keV level that decays to the 56.45 keV state has a positive assignment.

4.2.

SPIN

ASSIGNMENTS

Angular distribution measurements were taken at a bombarding energy of 6 MeV and were compared with the theoretical predictions calculated with AMANDA 6), an extended version of the computer code MANDY ‘). The protons and neutrons best fit potentials of Becchetti and Greenlees “) were used for these calculations. Usually the angular distribution is given by the expression W(O) = ao[l +AzP,(cos

B)+A$,(cos

e)] .

(2)

the expected A4 In the present case, similar to the “‘1 and lz41 investigations, values I**) should mostly be as low as 0.001 which was beyond experimental detection. Hence the experimental angular distributions were expressed in terms of the A2 values which were given with their specified errors in the third column of table 1.

.i. Burde et al. / Low-energy staates

217

The Az values were calculated for all posible initial and final spin combinations for transitions with dipole-quadrupole admixture and were compared with the experimental values. In the following we discuss briefly the main justification for the spin assignment of some levels. The ground-state spin and parity has been determined 5, as 2-. The 56.45 keV state is de-excited to the ground level by an El transition (table 3). Using the presently measured lifetime value for this level and assuming that the M2 transitions probability rate would not exceed the single-particle estimate, we get an upper limit of 0.01% for an M2 admixture. A theoretical comparison with the experimental Az value, under the rather high El purity condition, yielded uniquely the spin and the parity of the 56.45 keV state as l+. The 227.7,348.45and 479.0 keVs#ates are de-excited respectively by the 171.4, 292.0 and 422.4 keV transitions to the 56.45 keV level. The relatively high negative A2 values for these three de-exciting lines determine uniquely the spins of these three states as 2. Furthermore the parity of the 227.7 keV level is positive (table 3). The 688.0 and 397.6 kevlevefs are de-excited to the ground state. The relatively high negative Aa values support an assignment of spin 3 to the states. However, a spin 2 can be also fitted within the experimental errors. 5. Discussion In the present work, similarly to the ***I, rz41 and the r2’Sb investigations lW3), most of the low-lying states are expected to decay to levels with the same parity. For El transitions between opposite-parity states, the latter must have admixed configurations from one higher major shell. This admixture apparently is very low for the low-energy levels under present investigation. Although the B(E1) reduced transition probability of the 56.45 keV y-line in 1241is about two times larger than the corresponding l*+ 2- 121.5 keV transition in r**Sb and about three times faster than the 133.47 keV line in *“I, it is still only about 5 X 10e5 in units of the s.p. estimate. Thus, for instance, the 178.95, 237.4, 338.4, 343.45, 369.6 and the 397.6 keV levels are de-excited only either to one of the two well established negative-parity states (the ground state and the 122.2 keV level) or to both. Likewise, the 244.9, 331.3,688.0,714.7,812.9,819.5,944.7,979.1,1002.5,1082.7andthe 1102.2 keV states decay to or via the latter. Hence, these 16 states, in addition to the ground state and the 122.2 keV level, have most probably negative parities. Following the same reasoning, the probably best candidates for the positive-parity states can be selected. The 222.6 keV level is fed by the 311.2, 348.5 and the 434.2 keV levels which each one of them populates also the two well-established positive-parity states (the 56.45 and 227.7 keV levels) and by the 544.6 and 591.3 keV levels which are de-excited also to the 227.7 keV state. These five

218

J. Surde et al. / Low-energy states

populating levels have most probably positive parities and their feeding the 226.6 keV level, which is predominantly de-excited to the 56.45 keV state, indicate also that the 222.6 keV level must have the same parity. The 110.84 keV level is populated via the 222.6, 227.7, 311.2 and 434.2 keV states and by the 373.7 and the 365.5 keV levels. The latter two levels, respectively are de-excited also to the 227.7 and 222.6 keV states. On the other hand the 110.84 level and all the feeding states are not populated by any one of the 16 most probable negative-parity states which have been discussed above. Hence these states have most probably positive parities. Selecting the other positive-parity states on the basis of de-excitation to and via the 11 proposed positive-parity states and not to any negative-parity levels, we can add also the 204.25,422.2,491.0, 506.9, 535.7,566.8,617.8,676.7 703.1, 748.7 and 956.6 keV states as having most probably positive parities. On the basis of the spin and parity of the low-lying states in the neighbouring 125 127 I, 1, r2’Te and ‘27Xe nuclei, the levels in 12?, similar to those le3) in 1281,i2*I and in iZ2Sb, are expected to have the following two-quasiparticle configurations: rg7/2vsr/2: rg7/2vd3/2; rd5/2vsr/2, vds/2vd3/2, .ng7/2Vhii/2 and nd5/zvhrr/2. The sequence of the 22 interconnected positive-parity states appears to be reasonably well separated from the 18 negative states. Some of these positive-parity states are apparently members of the first 4 two-quasiparticle configurations, whereas some states of the negative sequence can be probably related to the last two configurations. In ref. ‘) the mode of the-excitation of nine positive-parity excited states in “‘1 could be accounted for by assigning these levels and the ground state as admixed two-quasiparticle configurations. Hence, it becomes very interesting to see whether there exists a similar behaviour in 1261.Considering first the positive-parity sequences, a striking resemblance is attained between the 56.45, 110.84, 204.25, 222.6, 227.7, 311.2, 348.45, 365.5, 373.7, 422.2, 434.2, 491.0, 544.6, 566.8, 676.7, 703.1, 748.7 and 956.6 keV levels in ‘*‘? and the ground state, 27.4, 85.5, 220.9, 160.62, 344.69, 385.43, 151.56, 750.6, 866.42, 391.9, 374.3, 426.2, 794.56, 295.59, 678.8, 613.03 and 232.62 keV states in **sI, respectively, on the basis of the mode of de-excitation of the corresponding levels. The structure of the negative-parity states in 12?, on the other hand, appears to have a rather convincing correspondence with the sequence of states in 124I. In fact, a close observation reveals that there is a strong resemblance between the ground state, 122.2, 178.95, 237.4, 244.9, 331.3, 338.4, 343.45, 369.6, 397.6, 688.0, 714.7, 812.9, 819.5, 1002.5 and 1102.2 keV levels in 12? and the ground state, 123.05, 184.2,404.65, 275.55,493.15,291.15,446.85,297.0,361.9,496.7, 596.4, 765.1, 605.09, 985.4 and 909.8 keV negative-parity states in 1241,respectively. Here also the resemblance is deduced on the basis of the de-excitation mode of the corresponding levels. The correspondence between the *261and 1241for the negative-parity states and between the 126I and 128I for the positive-parity levels can be considered at present only tentative because the spins and parities of only some of the levels are assigned uniquely in all the three odd-odd isotopes of iodine.

L Burde et al. f Low-energy states

219

In the “*Sb investigation 3), the negative- and positive-parity low-lying states were assigned as members of the two-quasiparticle configurations. Here, however, similarly to the 124I investigation 2), the mode of the de-excitation of all the corresponding negative-parity states cannot be accounted for by assigning the levels merely as members of the two rrg7,2vh11,2 and rd5,2uh11,2 multiplets. Likewise, not all the 18 corresponding positive-parity states can be assigned only as members of the four rg7/2&/2, pg7/2vd3/2, rrdS,2vS1,2 and vd5J2vd3/2 multiplets. Apparently the transition from antimony to iodine isotopes by interchanging one proton outside the closed shell by a cluster of three protons produces additional multiplets of low-lying states. The effect of such a transition on the level structure has been discussed 9P10)for the odd iodine isotopes. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)

J. Burde, V. Richter, I. Labaton and A. Moalem, Nucl. Phys. A351 (1981) 238 .I. Burde, V, Richter, J. Tsaliah and I. Labaton, Nucl. Phys. A385 (1982) 29 V.L. Alexeev et al., Nuct. Phys. A297 (1978) 373 J. Burde, V. Richter, 1. Labaton, A. Moalem and D. Kalinsky, Nucl. In&r. lSl(l978) R.L. Auble, Nucl. Data Sheets 9 (19’73) 125 E. Friedman, an extended version of the computer code MANDY ‘), unpublished E. Sheldon and D.M. van Patter, Rev. Mod. Phys. 38 (1966) 143 F.D. Becchetti, Jr. and G.W. Greenlees, Phys. Rev. 182 (1969) 1190 R. Almar, 0. Civitarese and F. Krmpotic, Phys. Rev. CS (1973) 1518 V. Paar, Nucl. Phys. A211 (1973) 29

261