Isomeric M3 transitions in 183Pt and 181Os

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A 643 (1998) 331-346

Isomeric M3 transitions in 183pt and

181Os

B. Roussibre a, E Kilcher a, A. Wojtasiewicz a,l , J. Genevey b, A. Gizon b, F. Ibrahim a,2, A. Knipper c, E Le Blanc a, G. Marguier d, J. Obert a, J. Oms a, J.C. Putaux a, M. Ramdhane e, J. Sauvage a and the ISOLDE collaboration a lnstitut de Physique Nucliaire, CNRS-1N2P3/Universiti Paris XI, F-91406 Orsay Cedex, France b Institut des Sciences Nuclgaires, CNRS-lN2P3/Universit~ Joseph Fourier, F-38026 Grenoble Cedex, France c lnstitut de Recherches Subatomique, CNRS-lN2P3/Universiti Louis Pasteur, F-67037 Strasbourg Cedex 2, France d lnstitut de Physique Nucl~aire, CNRS-1N2P3/Universit~ Lyon L F-69622 Villeurbanne Cedex, France e PPE Division, CERN, CH-12ll Geneva 23, Switzerland

Received 5 February 1997; revised 9 October 1998; accepted 12 October 1998

Abstract The M3 transitions linking the v71514] and u51 [521] states in 183n, 78rq05 and 18l~ 76us105 have been searched by high-resolution conversion-electron measurements. The reduced M3 transition probabilities have been estimated and compared to the values obtained in neighbouring isotones 179uz 184~ ,, ( 74,,1o5 and 79/-kUl05). All the known M3 transitions are presented and their properties discussed, especially as a function of deformation. The present work supports the rrh9®v½1521] and r r h 9 ® v 7 [514] particle configuration assignments proposed for the ~84Au isomeric and ground states, respectively. @ 1998 Elsevier Science B.V. PACS: 27.70.÷q; 23.20.-g; 29.30.Dn Keywords: RADIOACTIVITY 183'JS1Hg (/3+, EC) [from Pb(p, 3pxn)Hg, on-line mass separation],

measured Ee , l(ce). 183pt, 181Os deduced transitions, y-multipolarity, reduced M3 transition probabilities. Magnetic spectrograph

I Permanent address: Warsaw University, Warsaw, Poland. 2 Present address: lnstitut des Sciences Nuclgaires, F-38026 Grenoble, France. 0375-9474/98/$ - see front matter (~) 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 5 - 9 4 7 4 ( 9 7 ) 0057 1-5

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B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

1. Introduction 1 For a long time, the only case where the M3 pSI521 ] ~ v 7 [514] transition occurs was the N = 105 odd nucleus 179W [1]. This transition has been observed with a hindrance factor over the Weisskopf value equal to 1770 [2]. More recently, an M3 transition linking the isomeric and ground states was discovered in the doubly odd nucleus 184Au [3,4]. Nuclear spectroscopy studies performed in IS4Au [5,6] lead to assignment of the spin and parity values I ~ = 2 + for 184mAu and I ~ = 5 + for 184gAu, with the 7rh9¢~v½ [521] and 7rh9®p 7 [514] configurations, respectively. Thus in 184Au, the M3 transition appears to be the neutron transition linking the v ½[521 ] and v 7~ [ 514] states, but it occurs with a hindrance factor at least thirty times lower than in 179W. Fig. 1 summarizes the properties of the v 7 [514] and ~,½ [521] states known as isomeric and ground states in the N = 105 isotones. It is worth noting that no evidence for isomeric transitions was obtained in the previous studies of the 183Pt and J81Os low spin states [7,8], especially in 181Os where the M3 transition had been searched [ 10] even before the energy of the isomeric state was known. In order to determine to what extent this hindrance-factor difference between 184Au and 179W is significant, we have undertaken the search for the M3 vT[514]-+v½1521] transitions in 183pt and lSlOs, odd isotones of 179W and I84AH. In this paper, we first present the search for the M3 transitions in 183pt and lSlOs. Due to their low energy ( E < 50 keV) and high multipolarity (M3), low-energy internal-conversion-electron measurements with high energy resolution are needed to separate the L-lines and even the M-lines and to determine unambiguously the transition multipolarities from the L j / L 2 / L 3 / . . . intensity ratios. We then display the experimental results obtained in lS3pt and ~SlOs. Lastly, after having presented the systematics of the M3 transitions known in the literature, we discuss those observed in the four N = 105 isotones: 179W,181Os, JS3pt and 184Au.

221.9 keV, 6.4 m

i/z[521] ,

IN = 105 isobne~ I

\ A / / 48.9 keV,2.7 m /, ~ :34.5keV.43 s

/ 7/21514]

/

/t

68.6 keV,45 or 27 s 2", ~,I/21521]@~h9/2

,,/

~

, , ' "

~810s

/°apt

,,'"" ' " 1

IF.(M~)=57or~l

..........

~

5 ÷, tZT/~21514]@Trh9/2

~e4Au

Fig. 1. Experimental systematics of the ~1 [521] and 7 [514] states in the N = 105 odd isotones [2,7-91, known prior to the present measurements. Comparison with the properties of the isomeric and ground states in the odd-odd N = 105 184Aunucleus [3-5].

B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

333

2. Experimental procedure 2.1. Experimental setup States in 183pt and 1810S are populated through the decay of J83Hg and 181Hg massseparated sources. Mercury atoms are produced at ISOLDE (CERN) by bombarding a molten lead target [ 11 ] by the 1 GeV PS-Booster proton beam. The radioactive Hg ions extracted from the ion source are mass-separated and switched through a beam line onto the experimental setup devoted to the detection of low-energy conversion electrons with high energy resolution [ 12]. This apparatus consists of a semi-circular magnetic spectrograph associated with a tape transporter, a decelerating lens for the incoming ions and a preacceleration system for the electrons. Installed at first at the on-line isotope separator ISOCELE (acceleration high voltage 44 kV), it has been modified to work on line with the ISOLDE separator (acceleration high voltage 60 kV). Aluminium deposits 15 mm long and 9 mm wide have been evaporated each 7.5 cm onto the insulating tape used for the ion transport. The radioactive Hg ions are slowed down from 60 kV to 700 V before being collected on the A1 deposits. In this way, the 59.3 kV high voltage, opposite to the acceleration high voltage, is applied only at the collection point. The deceleration of the ions before collection prevents them from being too deeply implanted and so preserves high energy-resolution measurements for the very low-energy electrons. The radioactive source is then moved into the spectrograph where a preacceleration high voltage is applied to accelerate the conversion electrons emitted by the source in order to overcome the energy cutoff due to the minimum radius of the spectrograph and then to allow the very low-energy electrons to reach the photographic film (Kodak, DEF392) used as a detector. The magnetic induction (B) and the preacceleration high voltage (HV) applied define the energy range of the conversion electrons detected: with B = 54 x 10 -4 T and HV = - 1 0 kV, a conversion-electron energy range 1-90 keV is covered.

2.2. Measurements achieved Three measurements were carried out during the experiment. The first one was performed using 187Hg mass-separated sources. Two exposures corresponding to different experimental conditions, i.e. without and with preacceleration high voltage, were achieved on the same photographic film. The following counting cycles (teollectlon = 480 S, twaiting = 120 s and tmeasurement --- 600 s) were used to favour the 18YAH (TI/2 = 8.4 m) decay among the radioactive chain issued from lSVHg (Ti/~ = 2.4 m). As energies and intensities of the electron lines belonging to the J87Au --+ lS7pt decay are well known [ 13], precise values for both the magnetic induction and preacceleration high voltage as well as the response of the detection system (spectrograph + film) as a function of the impact energy of the electrons have been obtained. The second measurement was performed using 183Hg (TI/2 = 8.8 s) mass-separated sources. The collection-measurement cycle was chosen to optimize the counting rate

334

B. Roussi&e et al./Nuclear Physics A 643 (1998) 331-346

corresponding to the decay of 183raPt (T1/2 = 43 s): the lS3Hg ions were collected during 160 s and the source activity counted for 160 s immediately after each collection. The third measurement was carried out with lSlHg (T1/2 = 3.6 s) mass-separated sources using the following counting cycle: tcollection = tmeasurement = 9 0 0 S, in order to favour the 181mOs (T1/2 = 2.7 m) decay. No preacceleration was applied to the conversion electrons emitted since the main purpose of the experiment was to search for the isomeric M3 transition the energy of which, known from the location of the isomeric state in ISlOs (48.9 keV) [8,9], is not very low. In this case, the conversion-electron energy range from 11 keV to 100 keV was covered. After development, the photographic films were analysed with a microdensitometer at the CDSI Institut d'Optique (Orsay). They have been scanned with a 400 p~m high and 50 /xm wide slit: the blackening density was digitized and stored into a bidimensional X x Y matrix, where X represents, on the film, the position of the scanner in length and Y in width. Electron spectra were then extracted from this matrix and analysed using programs working on the VAX 6000-510 computer at Orsay.

2.3. Energy and efficiency calibrations The film case of our spectrograph is equipped with notches which are used as geometric references. Their locations to the source are known with a precision of i 0 . 0 2 ram, which allowed us to establish the relation between the channel number in the spectrum and the distance on the film which is directly related to the curvature radius. From the film obtained with the A = 187 sources, the 187Au ~ lsvpt electron lines corresponding to the exposure without preacceleration were used to determine the precise value of the magnetic field (B = 54.43 4- 0.03 x 10 - 4 T ) ; those corresponding to the exposure with preacceleration allowed us to define the value of the preacceleration high voltage with a 0.2% accuracy. We have to note that in the space region where the magnetic and preacceleration electric fields are applied, the electron trajectories are no more circular arcs but elongated cycloids. We have established that the trajectory from the source to the film can be approximated to a circular arc provided that a linear dependence of the preacceleration high voltage upon the electron impact position on the film is introduced, which considerably simplifies the energy determination. We have checked that the error relative to this process is less than 0.05 keV over the energy range whereas the total estimated error is 0.1 keV. For intensity estimations, one of the most important parameters is the response of the detection system as a function of the electron energy. This response depends strongly on the nature of the photographic emulsion used and important fluctuations can be observed due to variations in the film processing. Moreover the dependence of the film response on the electron intensity is linear only in a specific blackening-density range, but beyond there is a film-saturation effect. To avoid this effect, we set a longitudinal shutter system inside the film case which gives three different exposure times for each measurement. In this way, depending on its intensity, the electron line is analysed using one of these film parts. Here, we restricted ourselves to determining globally the efficiency of the

B. RoussiOre et a l . / N u c l e a r Physics A 643 (1998) 331-346

335

6000

-t

~I

09 4000

3000

-

~7

:

2O00

1O00

00

20

40

60 80 lo0 120 Electron impact energy (keV)

Fig. 2. Relative efficiency calibration of the spectrograph+film system measured by using 187Au and 187pt sources. spectrograph + film system using the electron rays belonging to the X87Au --~ 187pt decay, the intensities of which are well known in the 10-100 keV energy range [ 13]. Another efficiency determination has been made later using 187pt sources and the 187Ir electron intensities extracted from Ref. [ 14]. All the results are presented in Fig. 2, where it is noticeable that the different data sets can be fitted by the same efficiency curve.

3. Experimental results Fig. 3 shows a partial electron spectrum obtained with 183Hg sources. The high quality of the data is illustrated by the electron lines of the 16.0 keV transition in 183Ir: the L2 line, at 3.2 keV energy, is clearly observed and the doublet M2-M3, with a 0.36 keV energy difference, is well resolved. This is the first direct observation of the 16.0 keV transition in J83Ir; its multipolarity, obviously E2, confirms the interpretation proposed for the first excited level and the ground state linked by this 16.0 keV E2 transition [ 15]. This spectrum allowed us to determine the multipolarity of about twenty transitions in 183pt, 183Ir, 179Ir and 1 7 9 0 S , the A = 179 mass chain being fed by the o~-decay of lS3Hg. In the following, the results concern the isomeric transition in 183Pt. Electron lines arising from internal conversion in the L1 and L3 Pt electronic subshells have allowed us to pinpoint a 35.0 keV M3 transition. One can see in inset of Fig. 3 that the L3 electron line has been clearly observed and that the Li electron line has been detected, though it has a very weak intensity and mixed with another peak. The Ll, L2 and L3 intensities are compared in Table 1 with the theoretical conversion coefficients for

336

B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

N × 10-1 25 L1

20 L2

La

I

I

16.0keVE2 m3,r Auger LMM

_

I

_

M2M,

NO

II

TI

~

35.0 keV M3 183Pt L2 L5

I I ,,0

1

,5 O K95]keV s 4,

i

Kgg>.oVO%l

o]/t /

,

45.1 keV qgOs L 1L2

£

~

I

X Z K 115.3 keV 183PI

I 47.6 keV 183Pt LIL2 L3

II

I

I

0

500

1500

1000

Channel Fig. 3. Partial electron spectrum obtained with 183Hg sources. Table 1 Electron intensities, experimental L3/LI and L3/L2 ratios for the 35.0 keV in 183pt and theoretical conversion coefficients or ratios for various multipolarities [161

lexp a

LI

L2

L3

L3/LI

L3/L2

36 ± 7

< 36

179 4- 38

5.0 4- 1.4

> 5

2.00 x 4.81 x 1.04 x 5.42 × 4.04 x 2.68 x 1.94 x 6.07 x

0,0107 0,481 4.81 15,7 1,09 60,5 54,2 64,5

0.11 7.11 68.9 195.0 1.29 1.11 1.06 1.07

e-

MI M2 M3 M4 El E2 E3 E4

1.87 x 1.00 × 2.16 x 3.45 x 3.69 x 4.43 3.58 x 9.41 x

101 103 104 105 10- t 102 103

1.81 6.76 1.51 2.78 3.13 2.42 1.83 5.66

× x x x x x x

101 103 104 10-I 102 104 105

10- I 102 105 106 10-1 102 104 105

a /cxp obtained with tcollcction = tmeasurcmcnt = 160 s and normalized to the 115.3 keV 183Au----~I83pttotal e intensity (ltot(115.3 keV) = 1000). v a r i o u s m u l t i p o l a r i t i e s . T h i s c o m p a r i s o n s u p p o r t s with n o d o u b t the M 3 m u l t i p o l a r i t y for this 35.0 k e V t r a n s i t i o n in p l a t i n u m . T h e L2 intensity is very difficult to e x t r a c t b e c a u s e the L2 line is m i x e d with the s t r o n g K - l i n e o f the 95.4 k e V t r a n s i t i o n in 1790s. H o w e v e r , o n e c a n e s t i m a t e that it is w e a k e r t h a n the LI intensity, s i n c e n o d i s t o r t i o n is o b s e r v e d o n the r i g h t - h a n d side o f the p e a k l a b e l l e d K 95.4 k e V in Fig. 3, W e c o n c l u d e that the 3 5 . 0 k e V t r a n s i t i o n is m a i n l y M 3 a n d c o n t a i n s less than 3 % E4. T h e e n e r g y o f the i s o m e r i c state in 183pt g i v e n in our p r e v i o u s w o r k was 34.3 k e V [ 7 ] , T h i s value h a s b e e n r e e v a l u a t e d to 3 4 . 5 0 ( 8 ) k e V b y F i r e s t o n e [ 1 7 ] , u s i n g the data o f Ref.

[ 7 ] . T h u s , o n e m a y w o n d e r w h e t h e r the o b s e r v e d 35.0 k e V M 3 t r a n s i t i o n is

B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

337

actually the isomeric transition in 183pt. We have to note that, in the 0-160 keV range, the transition energies obtained for 183pt from the electron spectrum are systematically 0.2 or 0.3 keV higher than the values given in Ref. [7], whereas the energy determination for the transitions belonging to the other nuclei present in the electron spectrum is in quite good agreement with what is known in the literature. This seems to indicate that the transition energies obtained in the present work are more accurate than those given in Ref. [7]. Therefore we have determined the energy of the isomeric state taking as energies for the transitions in the 0-160 keV range the values obtained from the electron lines observed in this work. In this way, the first two levels built on the ground state lie at 84.8 and 96.3 keV (instead of 84.6 and 96.0 keV in Ref. [7] ). These levels are both used to determine the energy of many other states and more especially the energy of the isomeric state which is found under these conditions to be 34.8 ± 0.2 keV. Moreover, we can note that, taking the energy of the isomeric state equal to 34.8 or 35.0 keV, the energy precision obtained for the levels, particularly for those located above 1.5 MeV, is in both cases better than 0.5 keV. We conclude that the 35.0 keV M3 transition observed in the electron spectrum is the isomeric transition in 183pt. The percentage branching of the 183"Pt decay by isomeric transition (btw) has been determined from the intensity of the M3 isomeric transition. Indeed the ratio between the 183mpt --* 183gPt and 183Au --+ 183m+gPt disintegration numbers can be evaluated using: (i) the relative intensities obtained in the electron spectrum from the internal-conversion electron lines corresponding to the M3 transition and to other transitions in Pt, and (ii) the 183Au --~ 183pt level scheme. This ratio can also be expressed as a function of the timing conditions chosen to record the electron spectrum, the percentage branching of the J83"'pt decay by isomeric transition and the half-lives of 183Hg (Ti/2 = 8.8 s [18] ), J83Au (T1/2 = 42 s [ 18] ) and 183'"Pt (Tl/2 = 43 s [ 19] ). The experimental data lead to biT = 3.1 ± 0.8%. With regard to the search for the M3 transition in lSlOs, one can see in Fig. 4 that no peaks appear in the spectrum at the positions expected for the L-electron lines corresponding to a 48.9 keV transition in Os, even though the energy of the transition would differ by 1.0 keV. In this case, only upper intensity limits can be estimated for the L1, L2 and L3 electron lines. On the other hand, the K-electron lines of the 117.9 and 144.5 keV transitions belonging to the I81Os --~ ISlRe decay appear strongly in the electron spectrum. Contrary to the 1 17.9 keV transition observed in both lSJgOs ~ JsJ Re and lsl"'Os --~ ~81Re decays, the 144.5 keV transition proceeds only from the 181"Os --~ 18iRe decay and its multipolarity (El) as well as its absolute intensity (I v = 87%) are known [20]. Therefore the comparison of the 144.5 keV line intensity extracted from the electron spectrum and of the upper intensity limit for the 48.9 keV L3-ray (the strongest line expected for a multipolarity M3) leads to a direct estimate of the percentage branching of the 181"'Os decay by isomeric transition: biT ~< 3%. Table 2 presents the experimental M3 reduced transition probabilities calculated using the bit values obtained for 183rapt and JalmOs. We have also reported the results related to the isomeric M3 transition in 179Wand 184Au, both isotones of 183pt and 181Os since 1 ,v715141 one-particle the isomeric M3 transition consists mainly in the v~[521],

B. RoussiOre et al./Nuclear Physics A 643 (1998) 331-346

338

N x 10-3

18-

K 107.6keV ImOs K 111.9keV ISllr

2-

I L 48.9keV ISlOs

16-

14-

1-

12-

0

i

Y

× lOZ 8-

|

i

i

I

i

i

50 100 150 200 250 300 350

6-

4-

2-

0

500

1000

1500

EO00 Channel

2500

~

3500

4000

Fig. 4. Partial electron spectrum obtained with 181Hg sources.

Table 2 Experimental reduced M3 transition probabilities in 179W, 18lOs, 183pt and |84Au. In column 6 is indicated the Weisskopf hindrance factors for the v 1 I521]---+v 7 [514] transition Initial state

Final state

Nucleus

Ei (keV), I T 12v[N, n z , A ] TI/2, biT (%)

E f (keV), 1f~ ~lv[N, nz,A]

Transition E z, (keV) O/tot(M3)

179w74-[21

221.9, I u½]5211 6.4 m, 99.72

0, 7 v715141

181c~-76v~

48.9, 7 v~15141 2.7 m, ~<3

lS3Pt78 -~

1784Au [41

BeXp(M3) (#2 fm4)

Fw(M3) a

221.9 1.04× 10 l

0.94

1770

0, ½vi[5211

48.9 1.82 × 104

~<1.7

~>250

35.0, 7 v71514] 43 s, 3.1

0, 1 v½1521]

35.0 1.80 × 105

6.8 -t- 1.8

64+] 2

68.6, 2 + v½[521]®vrh~_

0, 5 + v715141®orh9

68.6 3.32 × 103

30 or 51

57 or 34

45 or 2 7 s , ~ 3 0 a Fw(M3) = B w ( M 3 ) / B ( M 3 , u½15211 ~ v715141) with Bw(M3) = 1.6501 X A 4/3 /A2 f i n 4 and B(M3,v½15211---*v 7 [514] ) the experimental reduced transition probability in the case of 179W and 184Au and the experimental reduced transition probability for the reverse transition ( B exp(M3) × (21i + 1 ) / (21f + 1) ) in the case of 18lOs and 183pt.

B. Roussi~re et al./Nuclear Physics A 643 (1998) 331-346

339

transition for these four N = 105 isotones (see Fig. 1). Indeed, the neutron configurations v~[51417 and v ½[521 ] correspond respectively to the isomeric and ground states in 183Pt and JSIOs and to the ground and isomeric states in lV9W. Concerning the 184Au oddodd nucleus, these two neutron configurations coupled to the same proton (~'h 9) have been proposed to describe the ground and isomeric states [5]. In order to compare the M3 reduced transition probabilities in these four N = 105 isotones, we have reported in column 6 of Table 2 the Weisskopf hindrance factors for the v½1521]-+vV [514] transition.

4.

Discussion

In the region of the 150 < A < 190 deformed nuclei, it has been shown that the reduced transition probability between intrinsic states in odd nuclei provides a sensitive test in the determination of nuclear level configurations [21,22]. In doubly odd nuclei, specific selection rules connected with the two-quasiparticle character of the intrinsic states have to be considered. However, in the special case of transitions between states differing only through the neutron or the proton configuration, which in the odd-odd nucleus implies a one-particle transition, the hindrance factor is usually similar to that obtained in neighbouring odd nuclei for the corresponding one-particle transition [23]. Up to now these points have been extensively studied for the lowmultipolarity transitions, but for the M3 transitions, systematic studies suffer from lack of data. The present work offers the opportunity to update the available experimental data [ 2125] in order to (i) analyse the properties of the M3 transitions as a function of the particle configurations involved and (ii) compare the M3 transitions in odd and odd-odd nuclei. Fig. 5 shows the Weisskopf hindrance factors against the number of nucleons for all the known M3 transitions. Looking at this global picture we can underline several points: although the Fw values are rather scattered, average Fw increases slightly as a function of the mass number. This trend has already been observed for the lowmultipolarity magnetic transitions [25] ; - the Weisskopf estimate is supposed to reproduce results in spherical nuclei, however, the Fw obtained for the quasi-spherical nuclei are not lower than those found in the deformed nuclei; - very few M3 transitions exist in odd nuclei: none in the A < 100 nuclei, only two in the 100 < A < 150 region and twelve in the A > 150 nuclei. This is not unexpected since an M3 transition can exist only if no decay mode of lower multipole order competes and since few single-particle states with the same parity and AI = 3 appear close in energy in Nilsson diagrams; in odd-odd nuclei, most of the M3 transitions link states having the same v ® ~" configurations but with the proton and neutron spins coupled parallel for one state

340

B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

10 a

107

-

106

-

o d d - o d d n u c l e u s ( s a m e c o n f i g u r a t i o n for t h e g r o u n d o d d - o d d n u c l e u s ( o t h e r cases) + odd n u c l e u s

and isomeric states)

-"

I74Lu

j"

10 5

_

10 4

-

.jjjT"

-''"

I~IOs

18lOs 18g~

-'"

l,,w+T + us

. ~ iO

3

-

-'" ."

lO z

_ •-

101

-

10 0

-

."

-

-"

90~-~O " i-

• 38

.'0

Cl

.o ~Br

-II 34C'~

t~ q

I| 179... l

ll2In --

t~'" ~

gSTc

121o 0 ~ • k,UQ.~ + ~SnII3~JrSISgHO + h ' , a H o

• Z4AI

4e V

seC9u

--~

--

24

_

10 - 2

, 0

- -. - - - ~ -

I

30

~

,

142° Pr

94Nb 0 ~ h

#OCo •

i

10 -1

82Br

•lg°Ir 188~

HI 0 i-~e 1 q 'v~77Ybn83~" eo't 113~ Yb 7 1 ~ 184 ~L !0'0 ~n tZZ,La -To t72 7 [ I ~ AU + • 13o^ t44pr L u l + + | lO41~h -}- US l@6r Ol 0

158T b I79Lu ta laSRe + .......

.....

- .......... ....

--

~ -. - - -

I

60

I

I

I

I

I

I

I

I

90

120

MASS

NUMBER

I

150

I

180

210

Fig. 5. Weisskopf hindrance factors for the known M3 transitions. Data are taken from Table 2 and Ref. 126 ]. The nuclei with quasi-spherical shape ( 1/32[ < 0.1 from Ref. 127] ) are underlined. The two dashed lines have been drawn to show the minimum and maximum values of Fw versus A.

and anti-parallel for the other. In this case, provided the spin of the single neutron 3 the spin difference of the two states in the o d d - o d d nucleus or proton is equal to 3, is obviously 3, which allows the existence of an M3 transition. Table 3 shows the Weisskopf hindrance factors for the one-particle transitions known in the 150 < A < 190 deformed odd and o d d - o d d nuclei. Concerning the odd nuclei, it is worth noting the strong dependence of the Fw ( M 3 ) values on the particle configurations involved: F w ( M 3 ) are > 1000, ~ 20 and < 8 for the ~,~[512] --~ u91505], u½1510] --~ v71514] and 7r½1411] -+ v71404] transitions, respectively. Moreover, for these particle transitions, the change in F w ( M 3 ) seems to be connected with a small change in deformation (see Fig. 6). Concerning the o d d - o d d nuclei of this mass region, the only one-particle M3 transition observed is that in 166Lu and its hindrance factor is in keeping with the general pattern of the systematics of the F w ( M 3 ) values obtained for the same proton transition in odd nuclei with neighbouring Z number, namely Z - 4 , Z, Z 4- 2 (see Table 3 and Fig. 6). Therefore, in this mass region, in spite of the scarcity of experimental data on M3 transitions, it seems that the analysis of the M3 hindrance factors may provide information on the structure of the initial and final states.

341

B. Roussi&e et a l . / N u c l e a r Physics A 643 (1998) 331-346

Table 3 M3 transition probabilities and Weisskopf hindrance factors for one-particle transitions in odd and odd-odd nuclei Nucleus

Er

1[r ---+ I rr

( keV )

configurations

19JOs

74.4

189Os

30.8

179Hf

160.7

177yb

227.0

175yb

514.8

ISlTa

615.2

179LU

592.1

159Ho

40.0

166Lu

34.4

~ ---+ ~ v~ [ 5121--~v-9215051 9~ - ---+ g3 - 3 v 9 [ 505 ]---+v~ [5121 $I - - ~ 7~v½1510]--+v71514] 71-- ---* ~7 - v l 15101-+v~ [5141 I721 ---+ ~ 7 v~[5101~v~[5141 1+ ~ 7 rr} Dlll--+cr 7 [4041 $1 4-I ---+ ~7 + ~-14111--+Tr 71404]

Bexp(M3) [ ~2 fin4 ] 0.14 0.63 62 68 129 405 1400

l~ ~7 + 7 5 " 14111--+¢r514041 ¢r~

193

3- ~ 6rr½1411]--+7r 7 [4041 b

108

B(M3) [~2 fm4] configurations

Fw(M3) a

0.14 v~ [512 ]---+v9 15051 1.6 v 3 [512]---+v 9 15051 62 v½1510[---+v71514] 68 v½1510]--+v 7 [5141 129 11510]___+v71514] v~7 405 7r½ [ 4 1 1 1 ~ r 7 [404] 1400 rr½ [4111--+7"r7 [404]

13000

193 ~r½1411 I ~ r r 7 [4041 108 ~r½ [4111---+~r714041

1120 27 24 12.5 4.2 1.2 7.4 13.9

a Fw(M3) = B w ( M 3 ) / B ( M 3 ) with Bw(M3) = 1.6501 x A 4/3 /z2fm4. b The v configuration is v~ [523] for both initial and final states.

For the v11521] -~ v71514] transition in the J83pt, 18lOs and 179W isotones, the Fw(M3) dependence upon/92 is much less obvious than for the,other transitions shown in Fig. 6. Indeed the /32 deformation for 183pt is predicted between the values for 181Os and 179W whereas the hindrance factor is smaller in 183pt than in 181Os and 179W. However, it is well known that the Os, Pt and Au nuclei present potential energy surfaces soft relatively to the deformation, which can give rise to quick changes in the nuclear deformation. In this case, it would be better to draw the evolution of Fw(M3) versus the experimental deformation instead of the predicted one. The/32 values extracted from 2\1/21i = 0 . 2 4 ( 1 ) [28]) and IS4Au ( 1 ( /32 2 ) 1/21 __ isotope shift measurements for 183pt (]\//3 2/ 0.25(1) [29]) differ from the predicted ground state values [27] (/32 = 0.256 for 183pt and /32 = - 0 . 1 5 6 for 184Au) but unfortunately, no experimental value is available for 181Os. Nevertheless, as the deformation parameters found experimentally for X83pt and 184Au are very close, the similarity of the Fw(M3) values obtained in both nuclei (see Table 2) allows us to conclude that the same neutron states are involved in the M3 transitions of ~83pt and 184Au. Thus the v configurations proposed for the isomeric and ground states in 184Au are confirmed. In order to check if the deformation parameter is really the cause of the fluctuating hindrance facti~rs in the N = 105 isotones and of the smooth change in Fw for the other cases, we have calculated, for the odd nuclei, the transition probabilities and the

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B. Roussi~re et al./Nuclear Physics A 643 (1998) 331-346

105

104

~"

103

iImOs 1?gWe

10a

18aPL• le0

10~

10 -1

~l"mLu

[] "n1/2 [411]~'rr?/2 [40 4] ® r'i/21510]~P7/215t 4] • ~,I/215m]~,7/21514]

0.15

77

~ wYb

"~a~Ho N-~ITa

o ~3/2[~12p~9/2Esos] 10°

179Hf

I

I

I

I

I

I

p

0.17

0.19

0.E1

0.23

0.25

0.27

0.29

0.31

t~2 Fig. 6. Weisskopf hindrance factors as a function of ground-state quadrupole deformation. /~2 are taken from Ref. [ 27].

hindrance factors in the frame of the Nilsson model [30] that describes the odd deformed nuclei. The obtained values are reported in Table 4. For all the transitions, the variation of the theoretical transition probabilities is much less sharp than the experimental one. The low values of FN(M3) corresponding to the ~1 [411 ] ~ ~-77 [404] transition have to be considered with caution because, although the theoretical Nilsson M3 transition probabilities are very similar for 181Ta, 179Luand 159Ho,they exhibit not only important changes as a function of deformation but also a cancellation for/~2 ,-v 0.22. In the other cases, the Nilsson hindrance factors obtained are systematically lower than the Weisskopf hindrance factors, which shows the improvement obtained when the deformation of the nucleus is taken into account. In particular, the hindered u½ [510] ---+u 7 [514] transitions are quite well described by the Nilsson model since the Nilsson hindrance factors, which are about eight times lower than the Fw values, are rather close to one. With regard to the u ½[ 521 ] --* u 7 [ 514 ] transitions, the Nilsson hindrance factor in 183Pt is quite reasonable but the FN(M3) value in 179Wremains abnormally high. This is quite surprising since the main difference between 179Wand 183pt refers to nuclear deformation: the former is rather rigid and the latter exhibits indications of softness. Then the Nilsson model is expected to describe the experimental results better in 179Wthan in t83pt, which is not the case for the M3 transition probability between the u ½[ 521 ] and u 7 [ 514 ] states discussed here. Furthermore high FN(M3) values are also obtained in 191'1890S. It is worth noting that, in 179W and t91'1890S, the high FN(M3) values are obtained for transitions, namely u½1521] ---+ u71514] and p-321512] ---, u91505], which are unhindered ( A n = 3, AN = 0, An z = --1 and AA = 3) according to the selection rules for the electromagnetic transition [ 31 ]. Therefore, to understand better the behaviour of these hindrance factors, it would be necessary to use a theoretical approach quite sophisticated taking into

B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

343

Table 4 Nilsson M3 transition probability and hindrance factor for one-particle transition in various odd nuclei Nucleus

/32 a

Transition type d

B(M3) b (/22 fm4)

BN(M3) (/./2 fm4)

FN(M3) c

Fw(M3)

179W 74- - 11)5

0.268

v½ [5211--+v71514l

0.94

401

427

1770

~<6.8

354

/>52

/>250

27.2

380

14+5 -3

64+22 -

0.14

511

3650

13000

1.6

627

392

1120

62

223

3.6

27

68

221

3.3

24

129

211

1.6

12.5

405

20

0.049

4.2

1400

20

0.014

1.2

193

18

0.093

7.4

1818o76_~105 0.248 183p 78_tm5

0.256

191076vS115 0.164 189°76vS113 0.183 17914f 72--1117

0.278

177Vh 70_v107

0.279

175Vh 70--~ 1(15

0.287

181T"73--"108 0.269 179/,,

0.279

1591"*O92 467

0.263

71~u1(}8

unhindered V½[5211---+V715141 unhindered 7 vg, [5211---~v515141 unhindered 3 ___+V515051 9 v~[5121 unhindered v 3 [5121 ---+v91505] unhindered v½ [5101---~v7[5141 hindered AA=4 v½15101--,v715141 hindered AA=4 u½ [510]-~v715141 hindered 2xA--4 ~-½[411 ]--+Tr7 [404] unhindered 1 7 14041 ~71411 ]---,Trg unhindered ~½ [411 ]--~Tr~ [4041 unhindered

a Theoretical /32 from Ref. [27]. b B(M3) is the experimental reduced transition probability modified by the factor (21i + l ) / ( 2 I f + 1) when necessary (t83pt and 181'189Os). c FN(M3) = BN(M3)/B(M3), where BN(M3) is the Nilsson transition probability defined in Ref. [301. Hindered or unhindered according to the selection rules for the electromagnetic transitions [31 ].

account possible admixtures in the wave functions describing the states linked by these M3 transitions and higher multipole order deformation parameters. However, it seems that this latter point should not have a strong effect since the variation in/34 is predicted to be less important [27] between 191Os (/34 = - 0 . 0 8 1 ) and 189Os (/34 = - 0 . 0 8 7 ) or between 179W (/34 = - 0 . 0 5 7 ) , 18lOs (/34 ---- - - 0 . 0 5 2 ) and 183pt (/34 = - 0 . 0 3 4 ) than b e t w e e n 166Lu (/34 = 0 . 0 0 8 ) , 159Ho (/34 = 0 . 0 3 5 ) , 181ya (/34 = - 0 . 0 9 ) and 179Lu (/34 = - 0 . 0 9 6 ) . On the contrary, the former point may considerably affect the results. For example, the high hindrance factors found for 191'1890S indicate that other c o m p o n e n t s than the v~ [512] and v 9 [505] Nilsson states are probably present in the wave functions describing the levels linked by the M3 transition in Os. Moreover, the very high F w ( M 3 ) value observed in 19lOs results in internal conversion anomalies in the M3 transition [ 3 2 , 3 3 ] , which could indicate that the M3 operator has to be improved. Indeed, abrupt changes in the hindrance of M1 transitions between states having the same Nilsson orbital assignments are also k n o w n for a long time. The case of M I transitions in 181ya and 175Lu, exhibiting anomalies in the M1 conversion, has

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B. Roussi~re et al./Nuclear Physics A 643 (1998) 331-346

been carefully studied [34]. The authors have shown that important Coriolis mixing exists between the states linked by these M1 retarded transitions, but is not sufficient to explain the large hindrance factor observed in lSlTa. In particular, they have established that a tensor component in the effective MI operator is crucial to understand the strong retardation of the M 1 transition. The 189.1910s nuclei with their retarded M3 transition, as well as the N = 105 isotones (I79W, 1810s and lS3pt) which exhibit important changes in the hindrance of the M3 transitions, could offer the possibility of improving the knowledge on the effective M3 operator.

5. Conclusion Low-energy high-resolution internal-conversion-electron measurements have allowed us to determine the percentage branching of the lS3mpt decay by isomeric transition. In the case of lSJmOs, an upper limit of blv has been obtained. The reduced M3 transition probabilities have been calculated from the experimental results and the deduced Weisskopf hindrance factors have been compared to the values known in the neighbouring odd (179W) and odd-odd (184Au) isotones. Whereas the Weisskopf hindrance factors obtained for the one-particle M3 transitions in the 150 < A < 190 region show an obvious dependence on the quadrupole deformation, this is not the case for the ~,½[521 ] --, p71514] M3 transition in the N = 105 isotones. Nevertheless, the similarity of the Fw(M3) values in IS3pt and 184Au, nuclei that have close deformations, tends to confirm the neutron configurations proposed to describe the isomeric and ground states in 184Au. The determination of a precise B(M3) value in ISIOs, instead of a limit, would be helpful to specify the hindrance-factor difference through the N = 105 isotones from 179W to IS3pt. However, the only way to deeply understand why high M3 hindrance factors are found for unhindered transitions would be to carry out a theoretical work devoted to the study of the following M3 transitions, u½ [521 ] ~ u 7 [514] in the N = 105 isotones and u 3 [512] -~ ~9 [505] in the Os isotopes. This could bring interesting information on the M3 effective operator.

Acknowledgements We would like to thank Dr N. Boos, R. Breuil, J.-E Clavelin, M. Ducourtieux, A. Ferro, Dr D. Lunney, J. Munsch, J.-C. Potier and D. Sznajderman for technical support and help during the preparation and/or the running of the experiment. We are indebted to Dr C.E Liang and G. Blain for the making of calibration sources. We also thank all the members of the ISOLDE group and especially J. Lettry for his work on the Pb target. Acknowledgements are due to S. Equilbey for his help with the microdensitometer operation. We are grateful to Dr R. Lombard for fruitful discussions and suggestions during the preparation of the paper.

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B. Roussikre et al./Nuclear Physics A 643 (1998) 331-346

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