Two-quasi-particle states at 1095 keV and 1543 keV in 168Er

I

1.E.l: 3.A

I

Nuclear

Physics

86 (1966)

405416;

@

North-Holland

Publishing

Co., Amsterdam

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

TWO-QUASI-PARTICLE

STATES

AT 1095 keV AND 1543 keV IN “‘Er J. JURSiK Nuclear

Research

and

V. ZVOLSKA

Institute,

Received

Kei,

4 April

Czechoslovakia 1966

Abstract: The gamma-ray spectrum of lesTm has been studied by means of Ge(Li) detectors. The The spin and parity of the 1095 and multipolarities of transitions in laaEr were determined. 1543 keV levels were determined as 4- and 3-, respectively. By the delayed coincidences method, the half-life of the 1095 keV level was found to be T+ = (1.07*0.10) x lo-’ sec. The structure of the 1095 and 1543 keV two-quasi-particle states is discussed. RADIOACTIVITY

E

‘@Tm[from la*Er deduced

Ta(p, f)]; measured &,, Zy, yy-delay. aK, levels, J, z, T+.

1. Introduction In recent years several theoretical papers dealing with non-rotational excited states 16*Er was one of the investigated nuclei. in even deformed nuclei have appeared; In this study an attempt is made to obtain new or more exact experimental data about excited states of this nucleus. Besides the data related to two-quasi-particle states, some new facts about the gamma-ray spectrum of 16*Tm and the multipolarities of the transitions in 16*Er were obtained. 2. Experimental

Procedure

2.1. SOURCE

The source was obtained earth elements produced by the synchrocyclotron at the urements were started after the transitions belonging to 2.2.

GAMMA

by chromatographic separation from a mixture of rare bombardment of a Ta target with 660 MeV protons on Joint Institute of Nuclear Research, Dubna. The measthe short-lived Tm isotopes had decayed, so that only 168Tm were observable.

SPECTRUM

The gamma-ray spectrum of ‘@Tm (T+ = 85 d) was measured by lithium-drifted germanium detectors produced by Dr. Trousil at the Institute of Solid State Physics, Prague. The resolution of one of the detectors with an area of 1.7 cm2 and depletion depth 6 mm was 5 keV on the 137Cs 661 keV peak. The resolution of the other, coaxial detector, which had an active volume of 2.5 cm3, was 6 keV on the 137Cs 405

406

J. JURSiK AND V. ZVOLSKA

661 keV peak. The signals from the detectors we,e fed into a charge-sensitive preamplifier and a 200-channel pulse-height analyser Intertechnique.

(a) 1

7'

.-.1

O~ O~

<_. 50

100

150

200 channel

Fig. 1. G a m m a - r a y spectra of 168Tm obtained with a Ge(Li) detector, (a)-(f) the spectrum f r o m 40-210, 170-360, 360-700, 700-860, 860-1400 and 1400-2000 keV respvctively. The peaks at 1543 and 1591 keV are sum peaks.

TWO-QUASI-PARTICLE

407

STATES

All parts of the gamma spectrum were measured in at least two series. Some parts of the obtained gamma-spectrum are shown in figs. la-f. Relative photon intensities 2 Q

E

2

lOi6

(b)

!5XlO’

4

100

150

200 channel

are given in table 1. For comparison, we have also listed in table 1 the data of Reidy et al. ‘) obtained with a scintillation crystal. There is a good agreement between the sums of values of relative intensities of those lines which were not resolved by Reidy et al. ‘) and their corresponding values, with the following exceptions: (i) Our value for the intensity of the 1016 keV gamma ray is about four times lower than that given by Reidy et al. The part of the spectrum near the 1016 keV line was re-investigated carefully using different geometries, with the lead absorber and without it. We have found that the relatively intense sum peak 1015 keV (y198+y817) is present. Some of these measurements are shown in fig. 2. The area of the sum peak was subtracted from the line 1016 keV. (ii) The intensity excess of the 1166 keV line in ref. ‘) could be caused by the fact that the position of this line coincides with the

408

I.

position of the Compton to determine its intensity

JURSiK

AND

V.

ZV0LSK.i

edge for the 1280 keV gamma ray. This makes it difficult by the scintillation method. Besides the gamma rays listed

Cc)

50

100

60

200 ctwnnel

in table 1, Reidy et al. ‘) give gamma rays with energies M 1085 and = 1525 keV and relative intensities 3 and 5, respectively (in the units of table 1). If such gamma rays exist, according to our results, their relative intensities should be considerably lower, i.e. < 1.2 for the 1085 keV gamma ray and < 0.8 for the 1525 keV gamma ray (in the same units). 2.3. TRANSITION

MULTIPOLARITIES

Our relative gamma-ray intensities together with those of conversion electrons given by Jacob et al. “) were used for the determination of conversion coefficients rK. For the transition 184 keV (4+ + 2+, E2), the theoretical value CI~ = 2.0 x 10-r

409

TWO-QUASI-PARTICLE STATES

was adopted. Experimental conversion coefficients ~K obtained in this way together with theoretical values amd conclusions about multipolarities of transitions are listed in table 2. eo

(d)

6 r,,,i

x--

1

5b

t50

channel

It should be noted that for the 80 keV (2 + --+ 0 +) and 917 keV (4 + ~ 2 +) transitions, which should be pure E2, the difference between the experimental and theoretical conversion coefficients is less than 10%. Experimental ~K values for the 817, 743, 732 and 633 keV transitions, which take place between levels with A I = 0 or 1, belonging to two rotational bands (K = 0 and K = 2) are in good agreement with theoretical values (see table 2) for E2 with a possible small admixture of M I .

410

J.

JURSiK

AND

V.

ZVOLsKA

2.4. log ft VALUES

The relative gamma-ray intensities together with relative intensities of conversion electrons made possible the determination of total transition intensities (which are

cc :

2

(e)

fO’x:

‘

-A

50

75

100

125

150 channel

shown in fig. 3). On the basis of these data the electron capture branches to the levels in ‘(j8Er and corresponding logft values were calculated. The results are shown on the decay scheme of 168Tm (fig. 3). The value of 1720f 38 keV was assumed for the 168Tm-‘68Er mass difference 3). 2.5. HALF-LIFE

OF THE 1095 keV LEVEL

The half-life of the 1095 keV level was determined from delayed y-y coincidences of the 198-448 keV cascade. Scintillation detectors with 3.8 cm x 3.8 cm NaI(T1) crystals were used. The energy resolution was z 8% on the 137Cs 661 keV peak.

TWO-QUASI-PARTICLE

411

STATES

the resolving time of the coincidence circuit was r = 6.2 x IO-* sec. The delay time was changed within the region 0- 1.85 psec by steps of 0.05 psec. The half-life of the 1095 keV was found to be (1.07f0.10) x low7 set in good agreement with the results of ref. 4).

(f)

c

,

150

50

200 channel

3. Discussion

The data concerning multipolarities of transitions (see table 2) make possible the unambiguous determination of the spin and parity for the 1095 and 1543 keV levels. 3.1. THE

1543 keV LEVEL

The czx of the 721 keV transition to El multipolarity (with possible

leading to the 2+ level (see fig. 3) corresponds small M2 admixture) and is inconsistent with

412

J. JURSiK AND V. ZVOLSKA TABLE 1 y-ray intensities Relative (keE;.) 80

99

This

work

155 53

*30 ill

174 184 198 273

7 +5 280 &) 790 +I0 <4

349 422 448 547 558 633 646 721 732 743

6 11 604 70 <12 170 6 201 89 217

822 817 831 917 1016 1166 1280 1355 1464

gamma-ray

intensities Reidy

176*30 54+27 I 1190*120 I t30

1t2 $4 i150 .c30 *60 *3 *4-O +20 +I4

228 lt42 “) 1000 140 1180 +26 54 f10 1.310.5 1.710.9 44 -19 2.511.3 8 +I.6

a) The value I,,,, = 280 taken as reference intensity. b) Decomposition of the 817 and 822 keV lines was performed keV transition is E2.

et al. ‘)

420 75f30 ’ 2301~30 I 630-k90

137O:k120 1 5 ‘1.5 m7 40 + 6 m4 5+2

on the assumption

that

the 822

other multipolarities. It follows that the parity of the 1543 keV level is odd and the spin has one of the values 1, 2, 3. The 547, 646 and 1280 keV transitions which lead to the 4+, 3+ and 4+ levels, respectively are all El. This enables us to restrict the spin of the 1543 keV level to the value Z = 3. 3.2. THE

1095 keV LEVEL

The rK of the 99, 198 and 831 keV transitions leading from the 1095 keV level to the 4+, 3+ and 4+ levels, respectively, correspond to the multipolarity El (with a possible small M2 admixture) and are inconsistent with other multipolarities. This fact restricts the possible values of spin and parity of the 1095 keV level to 3- or 4-. This result is in agreement with the value of rzKof the 448 keV transition between the 1543 keV level (3-) and the 1095 keV level which indicates that this transition is of Ml (+E2) multipolarity. The 1095 keV level is connected with 2+ states by the 273 and 1016 keV transitions. Since the parity of the 1095 keV level is odd, both

TWO-QUASI-PARTICLE

TABLE

Transition

Er

99

3.7io.7 “) (0) 2.3 kO.5 (- 1) 2.0 b) 5.510.5

(-1) (-2)

273 349 422 448

>2.3 2.8kl.l 1.710.7 2.910.9

(-1) (-2) (-2) (-2)

547 633

m2.8 9.414.0

(-3) (-3)

646 721 732 743 817

w3.7 2.8kO.6 6.6k1.6 5.8kl.2 3.9hO.8

(-3) (-3) (-3) (-3) (-3)

822 831 917

4.3 “) 1.410.3 3.6kO.7

(-3) (-3) (-3)

ml.1 -8

in lasEr

Ctg(theOr)

184 198

1016 1280

2

multipolarities

=K(exP)

El 80

413

STATES

(-2) (-4)

7.8 &)(-2) 2.8 (-1) 5.5 (-2) 4.6 (-2) 2.0 (-2) 1.1 (-2) 6.9 (-3) 6.1 (-3) 3.9 (-3) 3.0 (-3) 2.8 (-3) 2.2 (-3) 2.1 (-3) 2.0 (-3) 1.7 (-3) 1.7 (-3) 1.6 (-3) 1.4 (-3) 1.1 (-3) 7.5 (-4)

E2 4.0 “) 1.1 2.0 1.6 6.5 3.5 2.0 1.8 1.1 7.8 7.4 5.8 5.6 5.4 4.4 4.3 4.2 3.4 2.7 1.7

Ml (0)

(0) (-1) (-1) (-2) (-2) (-2) (-2) (-2) (-3) (-3) (-3) (-3) (-3) (-3) (-3) (-3) (-3) (-3) (-3)

6.6 “)(1.4 4.2 3.5 1.5 7.9 6.6 4.1 2.5 1.7 1.6 1.2 1.2 1.1 8.6 8.4 8.2 6.5 5.0 2.8

1)

(0) (-1) (-1) (-1) (-2) (-2) (-2) (-2) (-2) (-2) (-2) (-2) (-2) (-3) (-3) (-3) (-3) (-3) (-3)

M2 1.5 *)(+l) 2.0 (+l) 2.4 (0) 1.9 (0) 5.7 (-1) 2.8 (-1) 1.5 (-1) 1.2 (-1) 7.0 (-2) 4.6 (-2) 4.3 (-2) 3.2 (-2) 3.0 (-2) 2.9 (-2) 2.2 (-2) 2.2 (-2) 2.1(-2) 1.6 (-2) 1.2 (-2) 6.6 (-3)

Multipolarity E2 El E2 El M2 E2, El + M2 E2, E2+Ml Ml +E2 El E2 El El E2 E2 E2 E2 El E2 M2 El

b, Theoretical value was accepted. “) See footnote b), table 1.

transitions should be El or M2. As may be seen in table 2, multipolarity El is excluded for both transitions. Apparently, both are of multipolarity M2. Therefore, it can be concluded, that the spin of the 1095 keV level is 4. Thus, our experimental data result in the conclusion that the quantum characteristics of the 1543 and 1095 keV levels are 3- and 4-, respectively. According to Gallagher and Soloviev 5), two two-quasi-particle states with 16*Tm; one with the configuration K” = 3- should be excited in the beta decay of nn [633f - 52111 at the energy E 1.1 MeV and the other with the configuration pp [523f-41111 at the energy M 1.3 MeV. Since, according to previous experimental work ‘), the quantum characteristics of the 1095 and 1543 keV levels were both assumed to be 3-, it was considered, that the configurations nn [633t--52111 and pp [523t -41111 were realised at energies 1095 and 1543 keV, respectively. However, Pyatov and Soloviev 6, pointed out that there were some contradictions in this interpretation. (i) The experimental logft value for the beta decay of 16’Trn to the 1095 keV level (which is 8.2 according to our results) differs considerably from the theoretical value of 6.2, which would correspond to the configuration nn [633t - 52111. (ii) On the basis of the Gallagher-Soloviev interpretation 5, of the 1095 and 1543 keV levels it is impossible to explain why there are no experimental indications for

.I.

414

JURSfK

AND

V.

ZV0LSK.i

the existence of two levels with K” = 4- (with configurations nn [633t + 52111 and pp [523? +41 l-11, which should be situated 6, at the energies zz 400 keV lower than the corresponding states with K” = 3-. (iii) In the work of Soloviev et al. ‘) it is

Js

s

,o 2

500

400

300

2oc

10G

100 Fig. 2. The upper

12.5 chorinei

and lower parts of the figure show the part of the spectrum near the 1016 keV line measured with and without a lead absorber, respectively.

shown that the r6*Erlevels with configurations nn [633f - 52111 and pp [523T -411-11 are practically pure two-quasi-particle states, since admixtures of other two-quasi-

TWO-QUASI-PARTICLE

STATES

particle states do not exceed 0.5 %. Hence, gamma transitions between be strongly forbidden, whereas the intensity of the gamma transition 1543 and 1095 keV levels is 33 % per decay.

415

them should between the

Fig. 3. The decay scheme of 16*Tm.

Pyatov and Soloviev 6, show that these contradictions are removed if the configurations pp [523t +41 lJ] and pp [523f -41111 are assigned to the levels 1095 keV and 1543 keV, respectively, i.e., the levels are assumed to be components of the doublet of the same two-quasi-particle state. Their spins and parities should be, in such a case, 3- for the 1543 keV level and 4- for the 1095 keV level. This is confirmed by our experimental data. We suppose however, that the nn state doublet is excited rather than the pp state doublet in the decay of 168Tm. The following arguments are in favour of the suggestion that in the decay of 16*Tm the states nn [633f + 52111 are excited: (i) Gallagher et al. ‘) and Soloviev zt al. 7, calculated excitation energies of the levels with K” = 3in 168Er. The value for the neutron-neutron state was found to be lower. Due to large errors in the calculated energy values this argument cannot be, however,

J. JURSiK

416

AND

V.

2VOLSK.k

considered as a strong one. (ii) According to our results the experimental log ft value for the beta transition to the 1095 keV level is 8.2. On assumption that the 1095 keV level corresponds to the nn [633?+5214] configuration, this log ft value can be explained on the basis of a A-forbiddenness (dn = 2) for the beta transition from the 168Tm state [633?--41111 to the [633t+5211] state. Assuming the 1095 keV level as a pp [523t +4111] state, a considerably higher log ft value should be, apparently, expected. (iii) Assuming that at 1095 keV the neutron-neutron state is realised, it is easier to understand, why there is no experimental evidence of the proton-proton state, which according to the theoretical calculations should be at z 1.3 MeV. The component pp [523f-41111 lies at the energy 2 1.7 MeV and the component pp [523f +4111], which should lie z 400 keV lower 6), is not excited due to the strong forbiddenness of the corresponding beta transition. The authors of the source.

wish to thank

Dr. K. J. Gromov

and Dr. Zh. T. Zhelev for the loan

References 1) 2) 3) 4) 5) 6) 7)

J. J. Reidy, E. G. Funk and J. W. Mihelich, Phys. Rev. 133B (1964) 556 K. P. Jacob, J. W. Mihelich, B. Harmatz and T. H. Handley, Phys. Rev. 117 (1960) 1102 Nuclear Data Sheets (1965) J. W. Mihelich and B. Harmatz, Phys. Rev. 106 (1957) 1232 C. J. Gallagher and V. G. Soloviev, Mat. Fys. Skr. Dan. Vid. Selsk. 2, No. 2 (1962) N. I. Pyatov and V. G. Soloviev, Izv. Akad. Nauk SSSR (ser. fiz.) 28 (1964) 1617 V. G. Soloviev, P. Vogel and A. L. Korneychuk, Izv. Akad. Nauk SSSR (ser. fiz.) 28 (1964)

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