Decay scheme of 25 min 131gTe

1.E.l: 3.A

I/

Nuclear Physics A161 (1971) 471-480; Not to be reproduced

by photoprint

DECAY

@

North-Holland

Publishing

Co., Amsterdam

or microfilm without written permission from the publisher

SCHEME

OF 25 min 131eTe

E. S. MACIAS t and W. B. WALTERS tt Department

of Chemistry,

Massachusetts

Institute of Technology,

Received

26 October

Cambridge,

Massachusetts

02139 ttt

1970

Abstract:

The nuclear level structure of i3*I has been investigated by studying the y-rays emitted in the decay of 25 mitt 13’=Te. They-rays were detected with Ge(Li) detectors. A total of 42y-rays were attributed to ‘aleTe, of which 21 were previously unknown. All but oney-ray was placed in the proposed decay scheme. The half-life of the 149.8 keV level was determined to be 0.76&0.05 ns. The levels are compared to those previously reported from reaction studies.

E

RADIOACTIVITY r3irTe [from r3’Te(n,y)] measured E,,, iv, W-coin, w-delay; deduced log ft. i3r1 deduced levels, J, _z, T+. Enriched target. Ge(Li) detectors.

1. Introduction

The structure of the low-lying excited states of 13rI seen in the decay of 25 min r31gTe was first studied with Ge(Li) detectors by Walters, Bemis and Gordon ‘) (hereinafter designated WBG). Similar previous studies were carried out with lowresolution NaI(TI) detectors and provided inconsistent data ‘-“). More recently Aublc, Ball and Fulmer “) have studied the levels in 1311using the 130Te(3He, d)1311 reaction. In that work several levels were seen which were reached by momentum transfer (I) for proton stripping equal to 0 or 2 indicating levels with spins and parities of .1.+ 2 9 3+, or 3’ which were not seen in the WBG work. As the spin and parity of 13rgTe is _z+, one would expect these levels to be populated in the /?-decay of r3rgTe. The results of the WBG study are for most part in agreement with the reaction work but the state of detection equipment at the time of the WBG work put severe limitations on the amount of fme structure which they could observe. Using improved counting equipment, we have reinvestigated the decay of 25 min r31sTe to clarify the discrepancies between the reaction and B-decay results. 2. Experimental procedure 2.1. SOURCE PREPARATION

The sources of r31ETe were produced by irradiating 2 mg of tellurium metal * enriched to 99.5 % 13’Te in the MIT reactor. Samples were irradiated for 90 set in a t Present address: Department of Chemistry, Washington University, St. Louis, Missouri 63130. t+ Present address: Department of Chemistry, University of Maryland, College Park, Maryland 20742. tit Work supported in part by the US Atomic Energy Commission under Contract AT(30-l)-905. t Obtained from Isotopes Division, Oak Ridge National Lab., Term. 471

472

E. S. MACIAS AND W. B. WALTERS

thermal-neutron flux of 2 x 10” n * cmw2 - set-’ and then were transported from the reactor to a radiochemical receiving laboratory in 7 set via a pneumatic-tube system. No chemical separation was performed before counting. Counting began 2 min after irradiation and continued for 75 min without appreciable interference from 1.2 d 13’Te. 2.2. COUNTING

EQUIPMENT

Direct y-ray spectra were observed on a cooled 26 cm3 Ge(Li) detector with an operating energy resolution of 2.4 keV FWHM for the 1332 keV y-ray of 6oCo. These spectra were recorded on a 4096-channel pulse-height analyzer with a 4906-channel analog-to-digital converter in conjunction with a fast magnetic-tape readout system. The yy coincidence measurements were carried out using the 26 cm3 Ge(Li) detector in conjunction with a 45 cm3 Ge(Li) detector with energy resolution of 3.0 keV (FWHM) for 1332 keV y-rays. The addresses of each coincidence event were stored on magnetic tape as separate events simulating a 4096 x 4096 channel configuration using the MIT multi-parameter buffer-memory - magnetic tape system described in detail elsewhere “). With this system the y-ray spectrum detected in the 26 cm3 crystal in coincidence with each region of interest in the y-ray spectrum measured in the 45 cm3 crystal was examined after the data was collected. The lifetime of the 149.8 keV level was measured with a delayed-coincidence system described in detail elsewhere ‘). The detectors used were two cylindrical Naton plastic scintillators, covered with 8 mm of aluminum to stop p- particles emitted from the 13i*Te source, coupled to Amperex XP1021 photomultiplier (PM) tubes. Anode pulses from the PM tubes were passed through high-speed discriminators to a time-to-amplitude converter (TAC). Simultaneously a slow-coincidence system accepted the PM dynode pulses from radiation below 200 keV from one detector and pulses from radiation above 300 keV from the other scintillator. The output of the TAC was displayed on a 400-channel analyzer which stored only those TAC pulses which were accompanied by a pulse from the slow-coincidence system. The FWHM and slope of the time spectrum from prompt 6oCo y-rays were 0.3 ns and 0.1 ns respectively. 3. Results We show in fig. 1 the y-ray singles spectrum obtained by counting a number of samples. The counting time for each sample was limited to M 75 nun in order to avoid interference from 1.2 d 131mTc.In table 1, we list 42 y-rays attributed to the decay of 131*Te, 21 of which were not observed by WBG. The half-lives of these */-rays were determined by counting each of a number of samples for three 25 min periods and summing all of the first, all of the second, and all of the third spectra. The y-rays at 402, 496, 575, 605.4 and 825 keV were not present in sufficient intensity to permit a reliable half-life determination, The y-ray energies were determined from a sixth-

473

131*TeDECAY

I

I

i

1

I

c

474

E. S. MACIAS AND W. B. WALTERS TABLE 1

Energies and relative intensities ofy-rays emitted in the decay of 25 min 131gTe Energy (keV) “)

this work 109.8 149.8 221.4 278.3 298.3 342.9 353&l 384.2 402&l “) 421.0 452.4 492.8 49651’) 545.1 550.6 567.4 575&l “) 602.2 605;4 654.0 696.0 727.0 82551”) 841.9 855.6 898.6 934.6 948.5 951.8 997.4 1008.1 1098.7 1147.4 1278.2 1294.8 1309&l 1351.6 1427.5 1501.0 1527.9 1579fl 165211

WBG ‘)

150 279 343 384

453 493 544

603 654 695 727 842 898 933 948 952 997

1007 1098 1147 1295

Relative intensity b, (y-ray at 149.8 keV used as a reference) this work 0.14 I100 0.07 0.18 0.26 1.1 0.07 1.4 0.01 0.05 26.6 7.6 0.04 0.56 0.03 0.13 0.05 7.1 0.06 2.2 0.25 0.75 0.08 0.28 0.08 0.22 1.4 3.3 0.51 5.3 1.3 0.31 8.4 0.18 0.95 0.01 0.09 0.16 0.18 0.08 0.01 0.01

WBG I)

100 &3 l.OAO.2 l.lf0.2 1.1 f0.2

24.Ort1.5 7.010.4 0.4fO.l

5.9f0.3

1.810.2 0.5iO.l 0.4kO.06 O.ZhO.08 0.3&0.08 1.23Io.2 3.010.4 0.6kO.2 5.1 hO.4 1.5*0.2 0.8f0.2 9.410.6 1.4f0.2

‘) Unless otherwise stated the uncertainties in energy are 50.2 keV. b, Uncertainties in relative intensity are & 10 % or 0.03 relative intensity units, whichever is larger. ‘) These y-rays were not present in sticient intensity to permit an accurate half-life determination.

=ge

47.5

DE!CAY

order polynomial calibration curve constructed by ascxxtaining the centroids of a number of well-known y-rays prior to and following counting 131gTe y-rays. The relative y-ray intensities were obtained by correcting the photopeak areas with an empirical efficiency curve for the Ge(Li) detector “). In order to utilize the full capabilities of Ge(Li)-Ge(Li) coincidence system coupIed to a buffer-tape analyzer, only small regions (x 10 keV) of the spectrum detected

I

I

1319

I

I

I

I

I

I

1

A

TC3 r-Ray Spectrum Coincident With Region Containing 492.8+ 4?6-keV ‘d Ray

Coincident 452.4-keV

Coincident 149.8-keV

j

200

400

ii

I

With YRay

B 1

With XRoy

_

S(xt

600

CHANNEL

NUMBER

Fig. 2. The yy coincidence spectra of 13QTe. A spectrum of y-rays coincident with an adjacent Compton region has been subtracted. The regions of the spectrum used as gates are listed below adjacent gate main gate (principle y-rays) 471-485 keV (492.8 -t-496 IreV) a) 485-499 keV 461-475 keV (452.4 keV) b) 446-460 keV 158-172 keV c) 143-157 keV (149.8 keV)

476

E. S. MACIAS AND W. B. WALTERS I

300-

I

I 1319

I

Te

A

/

149 8

X-Roy Region

ZOO-

Spectrum including

Coincident Il47.4-keV

With Y Ray

”

.“.

149.8

6

(x2

Coincident

With

602.2+605.4-keV I

Rays

LJ Coincident 384.2-keV

550.6

CHANNEL

NUMBER

With Cy Ray

5674

.A.... ----A--

Fig. 3. The 3n/ coincidence spectra of rslSTe. A spectrum of y-rays coincident with an adjacent Compton region has been subtracted. The regions of the spectrum used as gates are listed below: adjacent gate main gate (principle y-rays) 1125-1139 keV a) 1140-1154 keV (1147.4 keV) 610- 624 keV b) 595- 609 keV (602.2+605.4 keV) 368- 379 keV c) 379- 389 keV (384.2 keV)

in the 45 cm3 detector were used as coincidence gates. Furthermore, an adjacent gate was also analyzed in order to determine the amount of real coincident events due to Compton-scattered radiation detected in the 45 cm3 detector. When counting statistics were large enough, an adjacent Compton gate was subtracted from the gate containing the peak of interest. Since all coincident events were stored on tape, there was no limitation on the number of different gates set or the number of different modes of analysis used. In all 56 gates including adjacent Compton gates were analyzed but only those which reveal new features of the decay scheme are shown here. Figs.

3/i’

‘3~-L

_

wQ.017.3 "0.017.5 "0.05 6.9 1.3 6.0 1.4 6.1 1.4 6.1 ~0.05 7.7 0.2 7.2 Il.75.7 2.5 6.4

f.3 7.0

408 481 532 709 766 782 862 912 1013 Ilff

1333

25.0 min QP =2’210keV

76

Fig. 4. Decay scheme of 29 min 1318Te+

53 I

3

478

E. S. MACIAS

AND

W. B. WALTERS

2 and 3 show the y-ray spectra detected with the 26 cm3 crystal in coincidence with regions of the spectrum containing y-rays at 149.8, 452.4 and 492.8 keV; and 384.2, 602.2-605.6 and 1147.4 keV respectively, detected with the 45 cm3 crystal. In each case the y-ray spectrum in coincidence with an adjacent Compton gate has been subtracted from the spectrum of interest. It should be noted that although this technique removes Compton y-rays in the gate area, it does not correct for random events. Time spectra were analyzed on an IBM 360/65 computer using a revised FRANTIC program ‘). The lifetime measurements yielded a half-life for 149.8 keV level of 0.76kO.05 ns. This result is in fair agreement with the less precise measurements by Devare et al. of 0.95+0.10 ns [ref. lo)]. 4. Construction of the decay scheme We show in fig. 4 the decay scheme of 13’sTe as well as the normalized spectroscopic factors obtained by Auble et al. “). The B-ray intensities were computed from differences in intensity of y-rays into and out of each level and utilized an C(kLMof 0.24 for the 149.8 keV transition from the work of Beycr and Kelly ‘I). The coincidence data shown in figs. 2 and 3 confirm the presence of levels at 149.8, 492.8, 602.2, 877.0, 1098.7, 1147.4, 1427.4, 1444.4 and 1501.0 keV proposed earlier by WBG. These data also establish p-decay to levels at 1298.2, 1348.5, 1677.7, 1729.1 and 1801.3 keV which were observed in the reaction studies. In the WBG study, a level was erroneously placed at 1188 keV on the basis of coincidences thought to exist between the 696 keV y-ray and the 493-keV y-ray. As is seen in fig. 2, the 696 keV y-ray is in coincidence with the 452 and 602 keV y-rays, establishing a level at 1298.2 keV. The 1348.5 keV level is established by the coincidence between the 492.8 and 855.6 keV y-rays. The latter is not to be confused with the strong 852 keV transition in the decay of r31mTe. 5. Assignments of spin and parities The log ft values for the fi- transitions in the decay of i31gTe were calculated with the use of Moszkowski’s nomogram 12). The percentage decay to each level depends on a measurement of Devare etaL4) indicating nodetectable P-decay to the 1311ground state. The levels at 149.8,602.2,877.0, 1098.7, 1147.4, 1427.5, 1444.4, 1501.0 and 1677.7 keV have log ft values < 7.0 which is indicative of allowed /?-decay (LIJ = 0, 1; An = no). The other levels at 492.8, 1298.2, 1348.4, 1729 and 1802 keV have slightly higher log ft values (5 7.7) which may indicate first-forbidden p-transitions (AJ = 0, 1; An = yes). However, allowed b-transitions with logft values as high as 9.7 have been found in the decay of neighboring odd-A tellurium nuclei i3). Therefore ptransitions to any of these levels may be allowed. Furthermore, the ‘30Te(3He, d)13’I reaction data of Auble et al. “) listed in table 2 assign I = 0 or 2 momentum transfer values from the O+ 13’Te ground state to these levels. These even I-values indicate positive parity for the levels. Thus the spins of all levels fed in the decay of 131sTe

479

x31rTeDECAY

are $,$, or 5 with positive parity and all p-transitions are allowed. The spin and parity of the ground state and 149.8 keV level are known to have J” of 3’ and 3’ respectively for reasons discussed in the WBG paper. The 492.8 and 602.2 keV levels are assigned J” of 3’ and 3’ respectively because of the similarity of the logft and y-decay properties with states of those J” values in lz71 and “‘1 [ref. ‘“)I. The (3He, d) TABLE2 Results of i30Te(3He, d)1311 reaction study of Auble, Ball and Fulmer 6, for states below 2 MeV Level IlO.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Energy

I

J=

c2s.J

WV)

0.64 0.53 0.07 0.47 0.21

0

1491 5 491 601 874 1094 1145 1296 1345 1425 1499 1638&10 1672 1718 1797

2 2 0 2 (:)

(0.14, 0.26) (O-25,0.13) 0.03 (0.03,0.02) (0.10,0.05) 0.70

0 2

0.31 (0.05,0.03)

The 1is the momentum transfer to the level, Jn is the spin and parity of the-level;parentheses indicate tentative assignments and CzSJ is the spectroscopic factor measured for the level. The two values given in parenthesesin this column correspond to two J-values possible for the level.

reaction indicated I = 0 values for levels at 877.0, 1348.4 and 1729 keV which necessitates 3’ assignments for these levels. The remaining levels at 1098.7, 1147.4, 1298.2, 1427.5, 1444.4, 1501.0, 1677.7 and 1802 keV have logft values indicative of allowed decay. All of these levels except the state at 1444.4 keV were seen in the (3He, d) reaction and were assigned 1 = 2 values. However, there is not sufficient evidence to distinguish between $’ and 3’ assignments for any of these levels. 6. Discussion The results of this study have shown that all the levels of ’ 3‘1 below 2 MeV seen in the (3He, d) study “) with I-values of 0 or 2 are fed in the decay of 25 min 131sTe (3’). However, as noted by Auble et al. “) the configuration of the states in 1311are not simply explained. The ground state and first excited state at 149.8 keV might be expected to be formed by placing the odd proton in the lg,, and 2d% orbitals. The spectroscopic factors for these states, 0.64 and 0.53 respectively, indicate that other configurations are contributing to the wave functions for these states. In fact the spectroscopic factor for :he 602.2 keV level, 0.47, indicates that the 2d+ single-particle strength is almost equally divided between the 602.2 and 149.8 keV levels. The

480

E. S. MACIAS AND W. B. WALTERS

higher energy levels are even more complex and make simple interpretations impossible. For the levels below 1 MeV there is an inverse relationship between the spectroscopic factor measured in the c3He, d) reaction and the logft value for the allowed B-decay to these levels. The $-’ levels at 149.8 and 602.2 keV are both fed with logfl values equal to 6.1 and have spectroscopic factors of 0.53 and 0.47 respectively. The 4’ level at 492.8 keV and +’ level at 877.9 keV are /?-fed with log ft values of 7.5 and 7.0 respectively and have spectroscopic factors of 0.07 and 0.21 respectively. As mentioned previously, a log ft value of 7.5 is high for an allowed /?-decay 14). It was suggested in the WBG paper that such /?-decay hindrances may result from large phonon mixing in the wave function of the daughter state when the wave function of the parent state has a large single-particle component. The ground state wave function of 131gTe is expected to have a large 2d, single-particle component ‘). The states with the low spectroscopic factors, which indicates large phonon mixtures, are fed in p-decay with the most hindered allowed logft values. Thus, at least qualitatively, the WBG hypothesis is confirmed. However, Apt et al. 13) have pointed out that for lz7Te and lz9Te decay, the hindrances may also depend on other factors such as the detailed characterization of the wave function of the initial state. Preliminary directional correlation measurements were performed in order to determine the spins of several of the upper levels, particularly the 1147.4 keV level. The correlation measurements are dificult because of the short half-life of the 131gT’eparent state. Even more troublesome is the 0.76 ns half-life of the 149.8 keV level in 1311. In order to interpret the correlation of these cascades which include the 149.8 keV y-ray it will be necessary first to investigate the possibility of extra-nuclear perturbations of the angular correlation functions. The authors wish to express thanks to Dr. M. B. Perkal for his help with the lifetime measurements. tion for support

One of us (E.S.M.)

during

wishes to thank the National

Science Founda-

part of this work.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

W. B. Walters, C. E. Bemis and G. E. Gordon, Phys. Rev. 140 (1965) B268 C. A. Mallman, A. H. W. Aten, Jr., D. R. Bess and C. M. de McMillao, Phys. Rev. 99 (1955) 7 J. M. Fergusson and F. M. Tomnovec, Nucl. Phys. 26 (1961) 457 S. H. Devarc, P. N. Tandon and H. G. Devare, Phys. Rev. 131 (1963) 1750 N. Ahmed, Nuovo Cim. 48 (1967) 2567 R. L. Auble, J. B. Ball and C. B. Fulmer, Phys. Rev. 169 (1968) 95.5 L. S. Kisslinger and R. A. Sorensen, Rev. Mod. Phys. 35 (1963) 853 E. S. Ma&s, Ph.D. Thesis, Dept. of Chemistry, Massachusetts Institute of Technology (1970) unpublished M. B. Perkal, Ph.D. Thesis, Dept. of Chemistry, Massachusetts Institute of Technology (1969) unpublished S. H. Devare, R. M. Singru and H. G. Devare, Phys. Rev. 140 (1965) B536 L. M. Bcyer and W. H. Kelly, Nucl. Phys. A104 (1967) 274 S. A. Moszkowski, Phys. Rev. 82 (1951) 35 K. E. Apt, W. B. Walters and G. E. Gordon, Nucl. Phys. Al53 (1970) 344 C. E. Gleit, C.-W. Tang and C. D. Coryell, in Nuclear Data Sheets (National Research Council National Academy of Science, Washington, D.C. 1963) 5-5-109 and 5-S-145