1.E.I: Nuclear PhysicsA121 (1968) 612--624;~)North-HollandPublishiny Co.,Amsterdam

3.A Not to be reproduced by photoprint or microfilm without written permission from the publisher

N U C L E A R S T R U C T U R E IN 133,13SXe AND 133,135Cs

PETER ALEXANDER and JOHN P. LAU Isotopes-A Teledyne Co., 50 Van Buren Avenue, kVestwood, New Jersey 07675 t

Received 28 August 1968

Abstract: The relative gamma-ray intensities of transitions depopulating the z3a, z85Xe ground and isomeric states as well as the 133Ba ground state have been measured using a semiconductor detector. The half-lives of these xenon states have been measured and are found to differ slightly from the reported values. The K internal conversion coefficient of the 183Xe isomeric transition was determined to have the value c( K = 7.68-t-0.25. The 13°Te(g, n)~3~,z33mXe cross sections were measured in a stacked-foil experiment. More than nine new transitions have been observed from unreported levels in 135Cs. New levels are proposed at 408.2, 981.7 and 1062.6 keV. The above results are discussed in terms of the level schemes.

E

RADIOACTIVITY 133,133mXe [from ~3sU(n, f), 18°Te(~, n)]; measured T~, E r, Ir ; cc. X33Ba; measured Er, It. 133Cs deduced levels. 1~5,18smXe [from 285U(n, f), 134Xe(n, y)];

measured Tt_, Er, I~, ?~-coin. lzsCs deduced levels, log ft. Enriched targets; Ge(Li), Si(Li) detectors.

NUCLEAR REACTION ZS°Te(~, n), E = 6-24 MeV; measured a(E), isomeric ratio. Enriched target.

1. Introduction

The existence of two regions of strong nuclear deformat ion where 150 < A < 190

and A > 210 have been known for some time. Recently a region of lesser nuclear

deformat ion has been observed 1, 2) for which 50 < N and Z < 82. The exact bounds

of this region have not been accurately determined. Evidence for weak nuclear

deformat ion in even-mass nuclei in this area arises f rom excited state systematics 2)

and B(E2) values 3). Person and Rasmussen 4) have shown that the energy levels

and t ransi t ion ampli tudes in the odd-mass nucleus 131Cs can be approximately fitted

by a model consisting of a strongly deformed non-axial ly symmetric core coupled

to the odd particle. Var tapetyan et al. 3) indicate that the nuclear coupling model

should also take into account ro ta t ion-vibra t ion odd-particle interact ion terms. Observat ion of a spin ~ - three-quasi-particle excitation 5,6) in 135Cs represents

another manifesta t ion of collective behavior in this region. High-lying collective

excitat ion data in the odd-mass nuclei in the weak deformat ion region give informa-

t ion about the importance of the rotat ion-vibrat ion-part ic le interaction. Addi t ional

in format ion concerning energy level locations, t ransi t ion probabil i t ies and internal

conversion coefficients will help to clarify the nuclear systematics in this area.

t Work supported by the Advanced Research Projects Agency under contract DA-49-146-XZ-517 with the Defense Atomic Support Agency.

612

Xe A N D Cs N U C L E A R S T R U C T U R E 613

In this work, additional information coL:erning the nuclear structure properties of 133,135Xe and 133,135 Cs is presented. Sew'ral problems are examined, Can the three- particle excitation found in 135Cs also be observed in laaCs ? The nuclear structure effects in the two cases are somewhat similar in that the odd-proton Nilsson states for both nuclei are probably identical. Another problem is that the internal conversion coefficient for the isomeric transition in 1 a aXe as measured by Bergstrom 7, 8) differs by 60 % from the theoretical estimates of Rose 9). There exist wide variations be- tween available values for the 13 aXe and ~ 3SXe half-lives. The most precisely measured value for the 133Xe half-life 1 o) is known to be incorrect 8). It appears that there may be some contradiction between the ~ 33Cs gamma-ray intensities measured by Bilger and Sherman 11) and the gamma intensity values which can be computed from the internal conversion data of refs. 12, x a). The energy level location of three of the four members of the l aSCs three-quasi-particle quartet are unknown. The high- spin member of this quartet is believed 6) to lie at 1621 keV. Experimental location of the remaining states would yield valuable information concerning the particle-par- ticle and particle-core interactions.

2. Experimental procedure

The nuclear structure investigations performed in this work centered on the nuclei 133,133raXe ' 133,135mXe ' 133Cs and 135Cs. Level structure in 133Cs and laSCs was

investigated through study of the energies and relative intensities of gamma rays produced in the decay of 133Xe and la3Ba (to 13aCs) and 135Xe (to 13SCs). Where

necessary ?-? coincidence techniques were employed to aid in placing new transitions in the level scheme. The half-lives of 133mXe, 133Xe ' 135mXe and 135Xe were measured, and the beta-decay population ratios 13amxe/la3Xe and 135mXe/135Xe as fed from 1331 and 1351 were determined. A re-measurement of the internal conversion coeffi- cient of the highly converted 133Xe isomeric transition was performed. While producing the source for the internal conversion coefficient measurement, it was also convenient to measure the t 30Te(ct, n)l 33m, 1 a aXe cross section. The level scheme for 133Xe and la3Cs is given in fig. 1 and the level scheme for 135Xe and IaSCs in

fig. 2. These schemes incorporate the results of this experiment as well as the results of other authors.

The gamma-ray data in this experiment were recorded using an 8 cm a Ge(Li) detector having a system resolution (FWHM) of 2.0 keV for 137Cs" The experimental- ly deduced relative efficiency correction curve for the germanium detector is estimated to be accurate to ___ 3-5 ~ . Internal conversion electron data were recorded with a cooled 1 cm 2 × 1 mm Si(Li) detector yielding a system resolution of 2.7 keV. The semiconductor detectors were connected through low noise electronics to a 3200- channel pulse-height analyser which could be operated in single-parameter, two- parameter or multiplex modes. Additional coincidence circuitry made possible the recording of Ge-NaI coincidence events with a resolving time of ,~ 40 ns.

614 P. ALEXANDER AND J. P. LAU

"~2- ; 288.7

~;+~° "437.3 2 1338o

~-~~. N i" F ~

~ 3 3 x e ~ 1 ,f384.,

f , ~ , ~+\ ~I

~, ~-1 81.0

-2" STABLE 133Cs

Fig. 1. Level scheme for the decay of xssXe and 13SBa ground and isomeric states. This scheme in- corporates the results of this experiment as well as the results of other authors.

3+ 9.1h g ^ X~, L - - ) ~ .q v . v . 7. 2 "'Xe ~ '~e~ e -/0"06%

2.6 ~.-J~\

2 135Cs

1062. 6 , 981.7

l I 608.6

U~ 408.2 i 249.8

0 2.0 x 106y

Fig. 2. Proposed decay scheme of ls6Xe. New levels in lssCs are proposed at 408.2, 981.7 and 1062.6 keV. The ~s6Cs ~t and ~ levels known to lie at 781 keV and 1621 keV are not populated in this decay.

Estimated log f t values are underlined.

3. Source preparation

Sources of 1331 and 1351 were produced through thermal neut ron fission o f 235U with subsequent milking o f the ingrown I. Sources o f 133Xe and 1 a 5Xe were prepared

Xe AND Cs NUCLEAR STRUCTURE 615

by pumping through a liquid nitrogen trap on the gas released in the decay of crystal- ized 133,135i. During this operation, the iodine was held at dry ice temperature.

A ~ 33Xe source in which the isomeric state at 233 keV was highly populated was produced through the reaction 13°Te(~, n)~33mXe, 133Xe using 30 MeV alpha particles from the MIT cyclotron. This reaction enhances the ~ 3 aXe isomer to ground state population ratio almost 20 times over the maximum value observed in the 1331 decay. The 99.5 ~o enriched ~ 3°Te target material was dissolved in HCI after bombardment, and the liberated xenon gas plus carrier was passed through a gas chromatograph to extract the xenon.

Electromagnetically separated x33Xe and 135Xe source strips were prepared by separation of fission produced xenon gas. This operation was carried out at the Argonne National Laboratory isotope separator.

An attempt was made to populate isomeric states in 13~Cs through the 33Cs(a, a') 13 a*Cs reaction using spectroscopically pure natural cesium. Lanthanum

and barium activities are also produced in this bombardment. After irradiation the target was dissolved in 15N HNO3, and lanthanum and barium carriers were added. The cesium from the target was then extracted onto AMP.

Gas sources for gamma-ray measurements were contained in small thin-walled glass vials. Internal conversion electron gas sources were frozen onto an aluminium disc in a system which will be described later. Care was exercised to minimize electron energy degradation effects due to source absorption. Barium and cesium sources were placed in thin-walled plastic vials for gamma-ray counting.

4. Results

4.1. GAMMA-RAY ENERGIES AND RELATIVE INTENSITIES IN THE XSaBa AND 13aXe DECAYS

The measured relative 133Cs gamma-ray intensities from the decay of 133Ba are presented in table 1. The intensity errors shown include both statistical errors and the error in the efficiency calibration of the germanium detector. Also presented for comparison are the intensity values of Gurfinkel and Notea 14), Stewart et al. ~5), Bilger and Sherman :1) and Thun et aL 12). The relative gamma-ray intensities resulting from this measurement compare well with the recent data of Gurfinkel and Notea. The relative gamma-ray intensities of the 79.60 and 80.997 keV ~ 33Cs transi- tions were measured in this experiment using a small area semiconductor detector giving a F W H M of 1.0 keV (at 80 keV). It was found that the measured gamma-ray energies agreed well with the data of Thun et al. 12). The two new lines recently reported ~6) at 35 and 391 keV in the decay of 133Ba could not be observed in this experiment and may possibly result from summing or other spurious effects.

The relative gamma-ray intensities observed in the decay of x 33Xe are presented in table 2. Also given for comparison are the intensity values Erman et al. ~7) and Jha et al. ~s). The relative intensity value given in table 2 for the 79.60 keV transition

616 P. ALEXANDER AND J. P. LAU

TABLE 1

Relative gamma-ray intensities in the decay of a88Ba

Relative gamma-ray intensity

Gamma-ray a) Gurfinkel Bilger energy this b) and Stewart and Thun) (keV) measurement Notea 14) et al. 1~) Sherman ix) et a l : O

53.174-0.04 3.3 ~0 .5 3.7 4-0.09 3 79.604-0.05 . , _ f l l 4-4 9 80.9974-0.006 65.9 ±~'w155 64.7 4-4.2 55

160.66 4-0.06 1.20:t:0.06 1.2 4-0.05 2 223.43 4-0.26 0.744-0.06 0.804-0.42 ~ 0 . 3 276.43 4-0.26 12.0 4-0.4 11.6 4-0.17 8 303.09 4-0.21 30.6 4-0.9 29.7 4-0.29 27 356.26 4-0.15 100 e) 100 100 384.09 4-0.20 14.2 4-0.5 14.1 4-0.26 10

9 4.7} a) 55 e) 55

11.0 25.8

100 14.7

a) From the results o f T h u n et aL 1~). b) The error values quoted include statistical errors and the error in the efficiency calibration of

the germanium detector. e) Normalized. a) These results have been deduced from the relative K internal conversion line intensities and the

measured ~K values. These results are separately normalized.

TABLE 2

Relative gamma-ray intensities in the decay of 5.3 d ~3aXe

Gamma-ray Relative gamma-ray intensity energy (keV) Erman and Jha

this measurement a) Sujkowski 1~) et al. is)

79.60 )/982 :[: 59 1000 1000 80.997 1000 e" f 16-4-7 b) 8 3.7

160.66 1.74 4-0.09 1.09 14 223.43 0.00647 4-0.00613 303.09 0.135 4-0.004 0.123 0.84 384.09 0.618 4-0.019 0.062 0.43

a) The errors shown include statistical errors and the error in the efficiency calibration of the germanium detector.

b) Deduced from 160, 79, 81 keV intensity ratios measured in decay ofl83Ba. e) Normalized.

was d e d u c e d f r o m t he 7 9 / 1 6 0 k e V b r a n c h i n g r a t i o as m e a s u r e d in t h e 13 3Ba decay .

T h e t r a n s i t i o n s 19) r e p o r t e d a t 312 a n d 356 k e V in t he d e c a y o f 133mXe a p p e a r to

b e t h e r e s u l t o f c o n t a m i n a n t s in t he source .

V a l u e s f o r t he r e l a t i ve b e t a d e c a y a n d e l e c t r o n c a p t u r e f e e d i n g o f levels in 133Cs

a re p r e s e n t e d in t a b l e 3. T h e s e v a l u e s were d e d u c e d f r o m t h e g a m m a - r a y i n t e n s i t y

XeAND CsNUCLEARSTRUCTURE 617

data of tables 1 and 2 and the internal conversion coefficients of ref. 12). The principal reason for the difference between this set of electron capture values and that of ref. z o) appears to lie in a difference between internal conversion coefficient values used for the two compilations.

TABLE 3

Relative feeding of levels in 133Cs in

Level Beta decay Electron capture energy (keV) this measurement data cards this measurement data cards

437.3 88.8 76 384.1 0.030 ~0.015 6.5 11 160.7 1.7 ~ 1 4.7 13 81.0 98.3 ~ 9 9 ~ 0

TABLE 4

Gamma-ray energies and relative intensities in the decay of la~Xe

Energy Relative intensity (keV)

this measurement b) Thulin zl)

158.54-0.4 a) 0.262 199.94-0.7 a) ~0.02 e) 249.8 -4-0.4 100 358.64-0.4 0.239 373.1 4-1.0 a) 0.012 e) 408.24-0.4 a) 0.339 454.0 <0.006 a) 573.34-0.9 a) 0.006 c) 608.64-0.4 2.63 654.64-0.4 a) 0.035 731.94-0.4 a) 0.050 812.64-0.4 a) 0.055 981.7 <0.003 a)

1063.0~_0.9 a) ~0.003 e)

100 0.09

a) These transitions have not previously been reported. b) Except where noted relative intensity errors are estimated to be 7-12 ~ . e) Intensity error exceeds 13 ~ . a) Not observed.

4.2. TRANSITIONS IN THE lZSXe DECAY

The energies and relative intensities of the 12 transitions observed in the decay of 135Xe are presented in table 4. The gamma intensities of the transitions listed in table 4 were observed to decay with a half-life consistent with 9.1 h. With the exception of the two weakest observed lines, these transitions were also found on the mass 135

618 P. ALEXANDER AND J. P. LAU

source strip produced by electromagnetically separating the xenon source gas. Gamma-gamma coincidences (Ge-NaI) produced the following results. When gating on the 249.8 keV transition, the 158.5 and 358.6 keV lines appeared in coincidence. A coincidence gate set on all gamma rays between 640 keV and 1100 keV revealed coincidences with the 158.5, 249.8 and 408.2 keV transitions. A gate set on the combined 608.6 and 654.6 keV peaks resulted in coincidences with the 158.5, 373.1 and possibly the 408.2 keV lines. A level scheme tentatively placing the transitions of table 4 is given in fig. 2.

4.3. THE HALF-LIVES OF 188Xe AND lsamxo

The half-life of * 33Xe was measured with a mass spectrometer as 5.270 d (126.48 h) by Macnamara et al. * o) who were unaware of the existence of the 233 keV isomeric transition. Decays from this isomeric level into the 13~3Xe ground state can signi- ficantly distort the 133Xe half-life. The 13 aXe half-life has been re-measured in this experiment using an electromagnetically separated 133Xe source. The mass-133 strip activity was allowed to decay for one month until the ~ 33Xe isomeric transition intensity became negligible. The peak area of the 81 keV 133Cs transition was then monitored on a multi-channel analyser for 14 d with the source fixed on a Ge(Li) detector. The 133Xe half-life data points were then computer analysed using the FRANTIC least-squares analysis program 22). The resulting half-life value was

z~r ( laaXe) = 127.5___0.6 h.

A 4 ~ discrepancy exists between the 56.40 h 133raXe half-life value determined by Bergstrom s) and the 54.24 h value of Erman and Sujkowski 17). This half-life value has been re-measured in this experiment using a Ge(Li) detector. Background subtraction statistics become a significant part of the error in estimating the area of the 233 keV 133mXe peak as observed in a fission produced 133Xe source. For this measurement, a 133raXe source was prepared through the 13°Te(ot, n ) 133ra' 133Xe

reaction in which the 133mXe level is populated twice as heavily as the x 3'3Xe ground state. The change in intensity of the 233 keV gamma ray was followed for 15 d with the source fixed. The data were then fitted by the least-squares method by a computer. The half-life value for 133mXe obtained in this experiment was

~r(133mXe) -- 52.58+__0.70 h.

This value is 3 ~o below the 54.24+_0.40 h value given by Erman and Sujkowski. Due to this difference in half-life values, another half-life measurement was performed yielding a value in agreement with the 52.58 h 133mXe half-life determination.

4.4. THE HALF-LIVES OF xa6xe A N D 18amXe

The half-lives of x35Xe and x3SmXe were measured using a Ge(Li) detector and a xenon source prepared through the 134Xe(n ' y)135m, 13SXe reaction on isotopically

Xe AND Cs N U C L E A R S T R U C T U R E 619

enriched 134Xe. This reaction mechanism produces a particularly pure 135Xe source. The x 35Xe half-lives as measured in this experiment are

z½(135Xe) = 9 . 1 4 ± 0 . 1 0 h,

z½(135mXe ) = 15.20 + 0.70 min.

These half-life values appear to be in good agreement with the results tabulated in the literature z o).

4.5. INTERNAL CONVERSION COEFFICIENT OF THE 233 keV lssmXe TRANSITION

A 13 3raXe ' 13 3Xe source with a preferentially populated isomeric state was prepared

through the ' 3OTe(~ ' n) ' 33~aXe ' , s 3Xe reaction. The purified xenon gas sample

extracted f rom this source was admitted into an evacuated chamber and frozen onto a thin a luminium disc which was precooled to 77 ° K. Cooled Ge(Li) and Si(Li)

g~ o ;

,===

13iXe /~64

I I 150 200

t3,~Xe 233

203Hg

~ 279

250 300 ENERGY (keV)

A~I64K _ 3 I I ~,~L ~2S3K

I - ~ Q: z ~ 3 3 L

~ 2 233 M,N

Z m O <

279K

z

8 r~ o ,& ,~o "" 2'0~ 2so' - " "

ENERGY (keV)

Fig. 3. Top - The Oe detector gamma-ray spectrum. Shown is a portion of the gamma-ray spectrum containing the 164.9 keV 13XXe and 233.4 keV 183mXe transitions. The 203I-Ig transition at 279 keV was used to make a comparative internal conversion coefficient measurement. Bottom - The Si detector electron spectrum. Shown is a portion of the spectrum containing the conversion electron lines from the 164.9 keV 18xXe and 233.4 keV 13smXe transitions.

detectors were posit ioned so as to observe source radiations. This system is described

in another publicat ion zz). The electron and gamma spectra were stored s imultaneous-

ly in the multipl ied 3200-channel analyser.

620 P. ALEXANDER AND J. P. LAIr

Measurement of the 233 keV isomeric transition internal conversion coefficient was accomplished by employing a mixed source technique using 2 o 3Hg" The 279 keV transition in 2o 3Hg has an accurately known conversion coefficient and the K-con-

version line falls at about the same energy as the 233 keV K-conversion line from 33mXe" This removes the need for a large relative efficiency correction in the electron

detector energy spectrum. The electron and gamma spectra from the Z°3Hg source

were recorded prior to the introduction of the xenon and after its removal from the system establishing the relationship between the 279 keV gamma intensity and the

TABLE 5 Comparison of experimental and theoretical M4 internal conversion coefficients for the 233 keV

~33Xe isomeric transition

~K K

L+M

This measurement 7.68 ±0.25 2.04 ±0.12 Bergstrom 7,g) 4.4 ~1.4 2.23±0.17 Rose 9) 6.9 3.15 a) Sliv ~5) 6.6 2.98 a) Hager and Seltzer 26) 6.1 2.79

a) These represent K/L ratios. The L/M ratio calculated by Hager and Seltzer is 10.8 : 1.

279 keV K-conversion line intensity in this geometry. The joint 2°3Hg and 133mXe electron and gamma spectra (fig. 3) were recorded. The observed intensity of the 233 keV K-conversion line was corrected for the presence of the 203Hg K-conversion peak. The K-conversion coefficient and K/L ratio for the 233 keV transition were then calculated by comparison to ~i~ of 2°aHg as measured by Taylor 24). The experi-

mental 233 keV transition internal conversion coefficient values as determined in this measurement and in the earlier measurement of Bergstrom et al. 7, 8) are given in table 5. Also presented for comparison are the theoretical values of Rose 9), Sliv 25) and Hager and Seltzer 26).

The 233 keV transition conversion coefficient was also computed by comparison to c~: of the 164 keV transition 2 7 ) from 13~mXe which was present in the sample. The 131mXe was probably formed from the reaction tZSTe(c~, n) ~ 31mXe. This mea-

surement yielded a value ei~ (233) = 7.83+__0.8.

4.6. CROSS SECTIONS FOR THE l~°Te(ct, n) 18a, laaraXe REACTIONS

The magnitudes of the 1 aOTe(e ' n)133, 1 a 3mXe cross sections were determined as a function of alpha energy using a stacked Te foil package containing 13OTe enriched

to 99.5 ~ evaporated onto aluminium foil. This package was bombarded with 8 pAh of alpha particles at the MIT cyclotron. The activated Te coated foils were analysed shortly after irradiation. The intensities of the 81 keV 1 a3Cs and 233 keY

Xe A N D Cs N U C L E A R S T R U C T U R E 621

1 3 3 X e gamma transitions from each foil were determined using a Ge(Li) detector. Separate experiments established that < 2 % of the gaseous xenon escaped from the evaporated foils after irradiation. The ~ 3 3 X e cross-section curves resulting from these measurements are presented in fig. 4. The cross section curves of fig. 4 have been corrected for the 130Te(~ ' p)f331 cross section and for small contributions to the 81 keV transition intensity from the 80 keV transition in 1 3 t X e . This transition appears due to a-reactions on the 128Te present in the target. The energy values shown in fig. 4 were determined at low energy from the alpha-energy-loss data of Comfort

I0 K ....

G E

b.I co

o

o

~ o -

OOI --

~ ~ ~ 1 3 3 rn Xe

1-33X e

~ ~ . ~ RATIO

j 130].e (COn) t35, 133mXe

I t I I I I 0 I0 20 3 0

ALPHA ENERGY (MeV)

0

o o3

Fig. 4. Variation o f the l~°Te(ct, n)lasmXe, ~aaXe cross sections with alpha energy. The ratio of isomeric to ground state cross section is given by the solid circles.

et al. 28). At high alpha energy, the degradation was calculated from the range energy relations of Bichsel et al. 29). The error bars shown on the isomer ratio curve in fig. 4 represent the statistical errors in counting.

4.7. POPULATION RATIO OF 133mXe/1aaXe AND x35mXe/1Z~Xe FR OM THE DECAY OFlaZI A N D IaBI

The population ratio 133rnXe / 1 a 3Xe in the 133][ decay was

1 3 3mXe - 0.029+__0.001.

13 a x e

622 P. A L E X A N D E R A N D J. P . L A U

This measurement was made after the decay of all short-lived ~ 33Xe states. Xenon gas from the decay of an ~ 331 source was allowed an ingrowth period of 50 min and was then swept into a vacuum-tight vessel. The intensity ratio of the 81 and 233 keV peaks was determined and corrected to the time of collection. The xenon half-lives and internal conversion coefficients as measured in this experiment were employed to obtain the corrected population ratio. Eichler et al. 30) have measured a value of 2 ~o for the 133Xe population ratio. If they were to employ the eK(233) measured in this experiment, their ratio would become ~ 3 ~ .

The population ratio 135mXe/135Xe in the decay of ~ 35I was

135mXe -- 0.095+__0.015.

135Xe

The xenon ingrowth period in this case was 11 min. The 250 keV]528 keV intensity ratio was recorded immediately after collection of the xenon.

5. Discussion

5.1. THE NUCLEI lasXe AND 188Cs

The internal conversion coefficient of the 233 keV isomeric transition in 1 ~ aXe had been measured previously by Bergstrom et al. 7, 8). Their value differed significantly from the theoretical values of Rose 9) and Sliv 24) for a pure M4 transition. As can be seen from table 4, measurements carried out during this experiment appear to be somewhat more consistent with these calculated values.

The gamma-ray intensity ratio of the partially resolved 80.99 keV and 79.60 keV transitions produced in the decay of 133Ba was measured at high resolution (1.0 keV). This ratio (table 1) was observed to be 5 : 1. This result is in reasonable agreement with the intensity ratio of 6 : 1 of Bilger and Shermann 1~). On the other hand, employing the internal conversion line intensities and 0%t values of Thun et al. 12), the 80.99 keV/79.60 keV calculated gamma intensity ratio has a value of 11.7. The internal conversion results of Thun et al. seem to be supported by the data of Hennecke et al. ~ 3). Thus it appears possible that there is a conflict between the gamma-ray intensity ratio observed for the 80.99 keV and 79.60 keV transitions (which has admittedly large errors) and the internal conversion electron results for

these two transitions. The ~a3Cs(0q e')~a3*Cs reaction was used in an effort to excite a three-quasi-

particle state in ~ 33Cs" No de-excitations from this state were observed. These mea- surements were made "off-line". Thus it is only possible to say that this state does not appear to possess a long half-life or to have a large cross section for this reaction.

The appearance of the 13°Te(~, n)133mXe, 133Xe cross-section curves tend to favor the compound nucleus interaction. Excitation of the low-spin (½+) 133Xe ground state would be favored at low alpha energies in both the compound nucleus and direct interaction mechanisms. The large value for the isomer ratio (2.3) suggests

Xe AND Cs NUCLEAR STRUCTURE 623

that at 17 MeV 133, 133mXe formation proceeds primarily through the compound nucleus interaction.

5.2. THE NUCLEUS assCs

The level scheme for 135Cs proposed as a result of this work is presented in fig. 4. The spin ~- three-quasi-particle level at 1621 keV and the spin ~- state at 781 keV are not shown in fig. 4, as they are not fed in this decay. It is likely that the low spin 135Xe ground state decays primarily via allowed fl-transitions to levels in 135Cs. These levels in turn will probably decay through M1-E2 transitions to the Cs ground state. The underlined logft values shown in fig. 4 were calculated using the gamma- intensity data of table 4 and assuming "reasonable" M1-E2 internal conversion coefficient values for the transitions. The logft values for fl-transitions to the 249.8, 608.6, 981.7 and 1062.6 keV levels are consistent with those for allowed transitions and suggest ½÷, ~+ or ~+ spin assignments for these levels. Because of its log ft value, the transition to the level at 408.2 keV might be allowed or possibly first for- bidden, thus yielding possible spin values of ½+, ~2 ~, 52 or 5-- Beta-spectra mea- surements performed with a Si(Li) detector indicate a lower limit of > 8.8 on the logft value for the fl-transition to the 135Cs ground state. This would be consistent with a non-unique second-forbidden assignment for the//-transition feeding this level.

The various three-particle configurations suggested by Hailer and Jung 6) to account for the spin ~- level in x 35Cs would also result in the formation of states of low spin (½, ~, ~, ~ and 2). It would be of interest to determine if any of the new levels proposed in fig. 4 were low-spin three quasi-particle states. Although little experimental information is available on the splitting of the three-quasi-particle quartet, a first guess would place all components above 1 MeV. A low-spin three- quasi-particle level may sometimes be distinguished from a single-particle state if there exists a difference in the log ft value anticipated for the beta decay to the two different states. An accurate value for the beta disintegration energy to one of the levels in 135Cs is required in order to draw any meaningful conclusions concerning the nature of the new levels in this nucleus. Due to the strong beta decay feeding of the 249.8 keV 135Cs level, no clear information concerning the conversion coefficients of the new 135Xe transitions or the inner beta groups was obtained in this experiment at ~ 1 ~ electron energy resolution.

The authors are indebted to I. Bergstrom, P. Erman, J. Matuszek, B. Mottelson, J. Thun and E. Willis for helpful discussions. They are also indebted to J. Lerner of the Argonne Isotope Separator for making the isotopic separations and to C. Hansen, A. Sahagian and J. Melosh for help in data collection and analysis.

R e f e r e n c e s

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624 P. ALEXANDER AND J. P. LAU

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