Multi-quasiparticle yrast states in 179Ta

Nuclear Physics A379 (1982) 20220 © North-Holland Publishing Company

MULTI-QUASIPARTICLE YRAST STATFS IN'~9Ta D. BARNÉOUD, S. ANDRÉ and C. FOIN Institut des Sciences Nucléaires (I N2 P3-USMG) 38026 Grenoble Cedex, France Received 25 August 1981 (Revised 30 November 1981) Ab®tract : Three 3-quasiparticle isomers with spins, parities and half-lives : ~-, 350 ns at 1253 .1 keV; ~*, 9 ms at 1317 .8 keV ; ~-, 1 .6 its at 1328 .4 keV and their associated rotational bands have been identified in "'Ta . The (g,c - gR) values deduced from the branching ratios are in good agreement with the proposed configurations . Two other isomers : ~*, 52 ms at 2640 .6 keV and -'~-, 22 ns at 2794 .1 keV, are interpreted as S~uasiparticle states. E

NUCLEAR REACTIONS "6Yb('Li, 4n), E = 355 MeV ; measured E;, l,, a(E,~,, E,, 0,), yy(t~'Li-y(t)" "9Ta deduced levels, J, n, T f , (qi - ga), ICC. Enriched targets:

1. Introduction During the last few years long-lived high-spin isomers have been observed in the heavy Hf, Ta and W isotopes. In this region isomerism arises from high-K multiquasiparticle states . These states correspond to rotation about the symmetry axis (this situation has some analogy with thôse observed in the N = 82 region where the spin of the yrast states are generated by aligning the spin of a few nucleons along the oblate symmetry axis). On the contrary the associated rotational bands correspond to rotation at right angles to the symmetry axis of the prolate core. "9T'a is a promising nucleus to investigate, for the further study of such long-lived yrast states . This nucleus was early studied by Manfrass et al. t). However, only p- and dinduced reactions were employed which do not excite states of very high spin. An investigation of the yrast level scheme of t '9Ta using'Li ions is reported in this paper. 2. Experimental procedure and results High-spin states in "9Ta were populated using the t '6Yb('Li, 4n)t'9Ta reao tion with ion beams from the Grenoble variable-energy cyclotron. Only some ex205

206

D. Barnéoud e~ at. / "~ Ta

perimental details different from our previous works 2 .3) will be discussed here . For the y-measurements the targets were prepared by depositing 8 mg/cm2 of ytterbium oxide enriched up to 96 ~ in t' 6Yb on 350 ~eg/cmZ mylar foils. A centrifugal method was used 4) . A "y-X Ortec" coaxial detector (resolution : 1 .9 keV at 1 .33 MeV, efficiency 15 /) and a high-resolution planar detector (resolution 0 .650 keV at 122 keV) were used to measure the singles y-ray spectra. Excitation functions measured between 35 and 45 MeV determine an optimum energy of 38 MeV for the t'6Yb('Li, 4n)t' 9 Ta reaction. Four-dimensional yy coincidence experiments (Eyt, Ey Z, tyty 2, tyt teF) were performed using two large germanium coaxial detectors (efficiency 15 ~). Particular

Fig. 1 . Some examples of prompt coincidence spectra (for the drawing the channels have beets addod in pairs) .

D. Barnéoud et al . / "'Ta

207

attention has been focussed on timing information. Three different time ranges have been investigated (i) The ns range between the beam bursts (82 ns intervals), by direct y-RF timing experiments. (ü) The ~s range using NaI x Ge or Ge x Ge delayed coincidences . (iü) The ms range by means of a two-parameter multi-scale technique using a slow pulsation of the beam (by acting on the cyclotron radiofrequency) . As most of the y-rays belonging to t '9Ta are delayed few results are expected from y-ray angular distribution data and thus we have performed these measurements at only two angles (32° and 90° with respect to the beam). Low-energy electron conversion lines were measured using the iron-free on-line Bray "orange" spectrometer s ).

DELAYED caNCIDENCEs GATED (5~ur) ON THE 9/2 BAPDr-RAYS _r N~ 0.1 Ns
N V1

DELAYED caNCIDENffs GATED(sroP)ON THE SY2 BAND~RAYs N0.1 Na
480

P q

DELAYED COINCIDENCß GATED (stogy ON THE 23/2 BAND g-RAYS 12 ns
Fig . 2, Some examples of delayed coincidence spectra (for the drawing the channels have been added in pairs) .

D. Barnéoud et al. / "~Ta

208

1~.0

180

220

260

300

I(A)

Fig. 3. Delayed (I-5 ms) conversion electron spectrum .

For the electron measurements the targets were self-supporting isotopically enriched metallic foils of 600 ~g/cmZ thickness. The out-of-beam electron spectra were obtained by coincidences (e- , RF), while to obtain the delayed spectra in the ms range the slow pulsation of the beam was synchronized with the automatic current-scanning of the spectrometer (fig. 3). In the same range the half-lives of e - lines were obtained using the multi-scale technique (fig. 6). 3. Level scheme Table 1 lists the energies, intensities, half-lives and angular distribution coefficients of the transitions assigned to "9Ta. Many new levels have been added to the previously known rotational bands (fig . 4). However our interest has been focussed on the new high-spin isomeric states and their associated rotational bands (fig. 5). Five isomers are observed, all connected to the ~- [514] band (see fig. 5 and table 3). The first one, namely the lowest lying at 1253 .1 keV was already given by Manfrass et al. 1 ) with spin ~ or ~ and half-life 8 .6 ms. In the present experiments the two following facts are observed (i) The main lines of the i_ band (150.2, 175.4, 222.2 keV . . .) observed during the slow pulsation of the beam actually show a time component T} = 9 ms but also a second one, T~ = 52 ms.

20 9

D. Barnéoud et al. / "'Ta

The delayed yy coincidences (stop gates on the same lines) show that the level at 1253 keV is an isomeric state fed by many y-rays (table 2). The half-life of this state is measured as Tt = 325 ± 25 ns. A spin and parity ~- is obtained as in the neighbouring t"Ta nucleus. The cascade of transitions (290 .1, 306 .2, 313.6 keV) placed above this state could constitute the associated rotational band. (ü)

TABLE I

Energies, intensities and angular distributions for y-rays assigned to "¢I'a in the ("6 Yb+'Li) reaction at 38 MeV E,

lr '

A Z/A o

(125°)

75 .34 loo .s2 lOS .32 lo8 .ßz 133 .17

2 .4 iz .2 10 .4 6 .1 7 .9

133 .73 135 .92 145 .9 1 150 .19 153 .18

17 .3 4.5 7 .1 165 .0 12.6

-0 .0810 .04 0 .2610 .08

159 .26 161 .01 168 .26 175 .42

16 .9 22 .6 6 .l 126 .7

+0 .17±0 .07 -0 .1110 .06 -0 .1210.19 -0.03±0.05

184 .14 186 .50 199 .az 204.77 207 .64 210.59 222.16 227.37 232 .60 234 .46 238 .58 242 .52 246 .0 252 .49 261 .58

23 .3 9 .0 1lo .a 5 .4 8 .7 4.3

0.2510.12

loo

24.7 74.1 6.3 57.0 69.6 4.8 2.3 13 .1

262 .2') 263 .81

15 .3

270 .00

3 .9

Assignment /;K; ~ /~K~

~

(keV)

-0 .1210 .09 -0 .12±0 .10 -0 .11 to .19 0 .0510 .08 -0 .8410 .23

-o .oa±o .oa -0 .0810 .07 0 .0810 .23 -0 .061o .oa -0.1910 .06 -0 .0710 .OS 0 .0910 .18 -0 .0910 .05 -0 .03±0 .04 -0.03±0.08 0.2110.07

~, ~ ~ ~i, ~i ~, }- -~ ~, }+

~ ~+ ~ }, }+ ~, ~+ ~ ~. ~+ ~, ~+ -+ }, ~+

~ ~+ ~ }. }+

2930 .0 keV ~ ~, ~~ }+ ~ ~ }+

~, $- -~ ~. ~ -+ ~ }+ ~ } }+ ~, } -1 ~, ~+ ~

~+ ~

~

~ }+ ~ ~ ~, }+ -+ }

~  Lu

}+ }+

~, $- -~ ~. ~, } ~ ~. }-

~, }+ ~ ~}, }++b

~, ~+ ~ ~" }+

") ~, }+ -+ ~, } + ~ }+ ~ ~

~, }+ -~ ~, }' + "BTa

~- ~ ~,

~}, 3164 .5 keV -. 2930 .0 keV } }+ ~ ~ }+ ~, n ) ~, }+ ~ ;i, }+

~, ~ y ~, ~+ ~, }- -~ ~, }i, }+ --' ~, }+ ~, }+ ~ ~, 3+

D . Barnéoud et al . / "~Ta

210

TABLE 1 (continued)

E.,

1,'

A Z /A a '

273 .8s

39 .9

274.47 276 .12 280.8 ') 281 .68 283 .90 289 .06

18 .7 8 .3

J -0 .08 ±0 .10

14 .3 9 .4 16 .9

0 .02±0 .08 0 .01 t0.10

290 .10

19.9

O .l s ±0,04

0 .07±0 .05

291 .6 293 .8

4 .s 20 .9

294 .E 297 .4

7 .4 6 .6

306 .2

9 .4

0.02 f 0 .09

313 .6

11 .3

0.20 f 0 .08

319 .7

6 .1

32s .6

ss .9

332 .E 343 .7 347 .E 3s0.7 369 .s 370.s

12 .1 4 .3 14 .1 4s .4 9 .9

376 .s 388.0 ") 389.3 397.4 401 .E 410.8 417.8 421 .7 429.2 43s .1 443 .E 464.8 466 .8 a7s .1

~,

n

)

}, }+ ~ }, }++ b? }, } -~ }, }+ ~, }+ ~ } }+

~, ~. ~ ~ ~ .

}+

1,~2 ~++ -+ ~, ~++

0.21 t0 .07 -0.19±0 .11

~, } + --~ }, } + ~, ~+ --~ ~, ~+

~, ~. ~- --,

~r. ~-

~, ~+ -+ ~, ~+

~,~ -+~,~~, ~- -" ~,

"8 Ta

0.20 f 0 .16 0 .21 f0.07 0.2st0.08 -0 .2s t 0.13

13 .7

0 .10 f 0.07

11 .9 s .3 6 .7 2 .7 33 .1

0 .20 f 0.07

2.2 11 .0 33.0 s .3 60.1

~, ~+ ~ ~, ~+

~, }+ --. },

s .6

s.l

Assignment

0 .02 t 0 .06 0 .14f0 .08 0 .07 f 0 .06 O .IO f 0 .13 -0.01 to .os

-" "BHf

~, ~+ -+ ~, ~+ ~, }+ -+ ~, ~}+ ,~ }+ -+ i }+ partly b)

~, }--+~, }" b)

}, }

°) ")

-+ }, }+

~ }+ ~ ~ }+ ~, }+ -" ~}, }+ ')

b ) ~, ~ -~ ~,

°) ~z }+ ~ ~., }+ ~ }+ ~ ~ }+

~, }- ~ ~, } ~, ~- ~ ~, g> >eTa

D. Barnéoud et al. / "~Ta TABLE 1

E, (keV)

I, *

(12s°)

48s .s

11 .2

s04.3

10 .3

(continued)

A Z /A o "

0.19±0.09

7 .0

0 .30t0 .12

s37 .8

4 .4

0 .33±0 .15

s53 .s

6 .3

0 .18t0 .1s

s66 .2

6 .1 0 .21 ±0 .09

sn .l

13 .1 s .o

o .2s±o .13

s73 .8

8 .0

0 .23 t0 .11

s83 .2

1 .3

ss7 .s s9s " )

a .o

Assignment I,K, -~ I~K~

~, }+ ~ ~, ~+ bl ~,

s22 .1

s67 .8

21 1

o .osto .l7

s98 " ) 603 " ) 607 .E

12 .7

0 .19 to .08

617 .E 621 " )

11 .7

0.33 t0 .08

62s .0

4 .0

637 .9 646 .3

3 .1 12 .9

661 .7

12 .s

669 .8 728 .6

2 .9 12 .E

0.39t0 .21 0 .07 t 0 .08 0.2st0 .21

~},

~+ ~ ~, ~+

~, ~- ~ ~, ~, }+ -. ~, ~,

~

}+

-~ ~},

~, ~+ ~ ~, ~+ ~, ~+ ~ ~, ~+ ~,

b ) ~, ~- -. ~, ~, ~- -~ ~, ~+ b ) ~, i ~ ~, b) ~, ~+ ~ ~, ~+ b) 7,~2

~+

~ ~, t~+

b ) ~, }- ~ ~, } ~+ ~ ~. }+ ~, ~- ~ ~, ~, ~+ -.

~,

~+

b) + i37~ ~, ~- ~ ~, 1partly ~, }-

The errors in energies range from ±0 .10 keV for the strong lines to 10 .30 keV for weak or partially resolved lines. In the same way, the errors in intensities range from 8 % to 30 % . * The y-ray intensities (B = 12s°) and the A z/A o coefficients have been measured in two separate experiments . In the angular distribution experiment the assumption that the A 4 term could be neglected has been made . ") San in coincidence measurements only. b) Transition unplaced in level scheme, but preceding the 12s3 .1 keV isomer .

A more careful analysis of the four-parameter coincidences shows that there is another isomeric level above this 12s3.1 keV state with a half-life T.~ = 1.6 f 0 .4 Ees (fig. 2). The delayed coincidence results suggest also that these two isomeric levels

21 2

D . Barnéoud et al. / "~Ta

TABLE 2

y-rays feeding the ~-- 1253 .1 keV isomeric level observed in the delayed-coincidence (dT = 0 .1-1 .8 fis) 75 .30 15) ; 135 .9(10) ; 150 .0(6) ; 153 ') ; 184 .1(8) ; 204 .8(13) ; 225 .5(13) ; 234 .5(12) ; 246.0(13) ; 274 .5(100) ; 282 'x10) ; 290 .1(114) 297.4(29) ; 306 .2(50) ; 313 .6(29) ; 319 .7(29) ; 333 .0(8) ; 350.5 6 x29) 376 .5(56) ; 410.8(16) ; 418(8) ; 429 .2(16) ; 485(17) ; 573 .2 6x64) 583 ') ; 598(21) ; 603(15) ; 607 .6(7) ; 617 .6(53) ; 621(11) ; 662(17) The underlined y-rays feed the ~- level through the ~- 1328 .4 keV level . ') Weak, and then doubtful . b) Broad, may be double .

are connected by a 75.3 keV transition which, according to the electron measurements, has a M1 (or M1 + E2) nature (table 5). This second isomer is lying at 1328 .4 keV with spin and parity ~- . The identification of the three first levels of the rotational band associated with this 1328 keV isomeric level is straightforward. The delayed yy coincidences (stop gates on the lines of the K = z band) (fig. 2) still reveal the existence of a new isomeric level with a half-life Tt = 22 ± 5 ns. This level feeds the ~ band via a 573.8 keV l = 2 transition . This third isomer is lying at 2794.1 keV with spin and parity ~-. Several prompt transitions feed this isomeric level (table 4). Ofcourse two other isomeric states are responsible for the millisecond components (9 and 52 ms) observed on the time distribution of the main lines of the i- band. No further y-lines, other than those placed below the 1253 keV isomeric level, were seen with the 9 ms component : neither the delayed yy coincidences nor the ms y-timing measurements give evidence for the transition which depopulates the 9 ms isomer. Fortunately, the delayed electron spectrum (fig. 3) shows two lines, the energies of which are consistent with the L- en M-energies of a 64.7 keV transition. In order to establish that this is really the isomeric transition the decays of the L 64.7 and K 175 .4 keV electron lines have been measured . Fig. 6 shows that the two decay curves exhibit an identical time distribution . The nature of the 64 .7 keV transition is difficult to determine : the L/M ratio is TABLE 3

Half-life and decay of multi-quasiparticule states in'~~I'a E

K`

Half-life

Decay mode

1317 .8 1328 .4 2640.E z79a.1

~* ~~* ~-

9 .Of0 .2ms 1 .6 t 0 .4 ~s 52 t 3 ms 2z ts ns

~,~ ~}, ~, ~* ~, ~- (~ so ~), ~, ~* (~ zo ~)

D. Barnéoud et al . / "'Ta

X + O~

213

m

0

~o

N

O~~O nn ~ I~d

u1~ 1~1~

O~

n

9' l04

s~L9z I 19'l6Z

u ô X

i

I

05Z9

B'99V

S'LSS

PN

N N ~O

ON

Ô

~O

~O O

C

n

n

O N n

V N

N

009

SL9

m N N

6'LL9

m

n d

~ ~n V1 ~

9'6LS

S'LSS OLZ

n~ V

n~ 1~

~. O

53 r

~N

S'SBf L6 l'ZZS 9r99 S'ZSZ 19'Z

I

N N N

A

C

O ~ aD

n

Z'99S £' 90S S LBS B'LLS 9'Lli I Z'90L I Y'L62 I l'O6Z I L'9LZ 19'l9Z~S'ZVZJZ' N N N N N N T n n N N N N N

~

~.

0 d

C C Cs n n^ O~a n

A

L09

n

d'

Ç

O; ~O

ri n

âo1~ Çu,n ~ oQ.r~,oÇ ^ â ^ fV n

Na d

B~

P

9'l9Zl ZL92~1 t'L

NO M 0529

N

S'69L I B' L9Z

Y'66LIY'SLL N N N \ \ \ N \ ,n n O~

u Û

O'

y

_

E n

ir s~ N

OG W

21 4

D . Barnéoad et al. / "'Ta

Fig . 5 . Multi-quasiparticle level scheme of " 9Ta . The marks "c" indicate transitions observed in coincidence only . The width of the arrow is proportional to the y-intensity .

not very sensitive and the y-ray energy is close to those of the X-rays Ka.Z (Hf) (65 .0 keV) and K~, t (Ta) (65.2 keV) . However using the theoretical X-ray intensity ratios and the intensity balance one obtains a lower limit for the total conversion coefficient a,o, z 18.6. This high value is consistent with an E2, E3 or M2 nature of the 64.7 keV transition (higher multipolarity does not agree with the observed half-life) . For an E2 or E3 transition the I,Q and LW lines would be predominant in the Lgroup. Neither the experimental shape of the L-group nor the gap between the Land M-groups agree with this hypothesis . The M2 nature of the 64.7 keV transition is therefore established. This fourth 9 ms isomer is lying at 1317.8 keV with spin and parity ~+ .

~ 02

i

02~~~

i

~ 1 .0~

,ecu ~20 -

Fig. 6 . Dewv curves of the L 64.7 keV and K 175 .2 keV electron lines .

D . Bornéoud et al. / "'Ta TABLE

21 5

4

y-rays feeding the ~- 2794 .1 keV level 135 .9(4.5) 413 .3(1 .5)

234 .5(6 .3) 451 .0(1 .7)

280.8(1 .8) 516.2(1 .9)

370 .5(2 .0) 539 .4(2 .3)

388 .0(2 .6)

393 .2(2 .4)

The rotational band associated with the ~+ isomeric state is built, without any difficulty, up to spin ~+ . All the y-rays of this band show a 52 ms time component. Two more transitions with energies of 108.8 keV and 261.6 keV are seen in coincidence with the y-rays of the band. The decay of the 108.8 keV y-ray only shows the 52.ms half-life (fig. 7). This isomeric transition has an E2 nature according to the intensity balance and electron measurements . This 52 ms isomer is lying at 2640 .6 keV with spin and parity ~+. The 262.2 keV y-ray (though weak) appears on the delayed spectra start-gated by the 135.9 and 234.5 keV transitions. This line connects the 2794 .1 and 2531 .8 keV levels. These last two isomers (9 and 52 ms) are yrast isomeric states . 4. Discussion Three-quasiparticle states. The three isomeric states observed at 1253 .1, 1328.4,

1317.8 keV have spin too high (~-, ~-, ~+ respectively) to be interpreted as singlè quasiparticle states. Such excitation energy and spin suggest a three~uasiparticle structure. The tentative configuration assignments obtained by coupling a proton state with the 2-quasiparticle states of the t '8Hf core') are summarized in table 6. The ~-- state is probably, as in the lighter tantalum isotopes 3 ' 8 .9, to)~ a mixing of configurations (see table 6). It decays by the 232.6 keV M1 and 475.1 keV E2 transitions which have hindrance factors with respect to the Weisskopf estimates TABLB S

Multipolarities of some transitions in "¢I'a Energy

a exp

64 .7 75 .3 108 .8

total > 18 .6 L 2 .0 f 0 .8 tota12 .6 t 0 .6 K/L 0 .75 f 0 .3 total 0 .36 f 0 .10

232 .E

El

MI

a theor s) E2

M2

E3

0 .011 0 .28 6 .0 .04 0

2 .75 1 .4 3 .45 6 .4 0 .41

23 .5 0 .85 2 .7 0 .52 0 .16

81 30 .5 29 .3 3 .23 1 .9

1030

Multipolarity M2 (see text) M 1 (or M 1 + E2) M 1 (or M 1 + E2)

D . Barnéoud et a1 . / "'Ta

21 6

=i U W O 2

103 5 .102

2 2 .10 102

Fig. 7 . Decay of 108 .8 keV y-ray compared to two y-rays of the ~+ band.

x 105 and FW = 2.5 x 103. These values correspond to hindrance factors for each degree of K-forbiddenness fof 13 and 7.1 respectively. Such lowf-values can be explained by the possible K-admixtures in the ~- state levels and also in the ~-- state. These K-admixtures in the ~- state may also explain why the ~- state is the lowest observed three-quasiparticle state (3 q .p .s .) . As a matter of fact it is surprising to find the ~- level lower than the ~+ and ~- states which are well explained by the coupling of the 8 - [~ + n; ~ - n] state (the lowest highspin two-neutron state in the t'BHf core) with the lowest-lying single-quasiparticle Fw = 4 .3

TABLE 6

Proposed configurations of observed multi-quasiparticle states Band head energy

K`

Components' " °)

1253 .1 1317 .8 1328 .4 2640 .6 2794.1

~p , (~P +

' ip .

ZP )

y n , 27 0 ) 37P , (i içP,(37n,iyn)7 (3n , in , 3P , ZP )

iv+(~tô .~n,iP,~v)

Possible admixture' "b) 2SD + (Qin , Za ) 3y p + 3y0 , ~'n iP , (in . 3n )

7

i

i y P .~y v , ~' n_ , i7n , i1n ÏD + in , Sn , Tn , 3s

') Singleßuasiparticle orbits are }p , ~+ (404) ; ~o , ~ - (Sl4) ; }v , } + (402) ; ~v , } - (541) ; ~~ , ~-(514) ; ié , i + (624) ; }~ , } (512) ; }~ , } (521) ; ~ ; , ~+(633) . °) The corresponding 2 or 4 q .p.s . observed') in the "°Hf core are grouped in parcntheses.

D . Barnéoud et al. / "'Ta

21 7

proton states ~ - [514] and ~ 1 [404] respectively . Another possible explanation is that the lowest-lying 8 - state in "aHf is mainly a two-proton state [but this hypothesis disagrees with the assumptions of ref. ")] . Then the coupling of this lowlying 8- state with the ~1 [402] proton state would give an energy near that obtained by wupling the higher-lying 8 - state with the ~1 [404] or ~ - [514] proton states . The 2s1 T state expected to be a pure 3 q.p.s. (~P , ~ô , ~~ ) is connected to the ~-state th~rOUgh the 64.7 keV M2 transition . The only one-quasiparticle transitions are those involving the {~rp , ~ n , ~~ } and {~v , ~ô , ~~ } components of the ~- state. The first one is equivalent to the ~ô -" ~~ transition which is observed in ' a 'W with FW = 1 .4 x 102 and the second one is a ~a -" ~P transition which is observed in 'a'Ta [ref. 'e)] with FW = 35 . The higher hindrance factor F,,a = 8 .5 x 102 of the 64.7 keV transition is in good agreement with the fact that the {~rp , urn , ~n } configuration and especially the {~P , ~ô , ~~ } configuration are not the main components of the ~-- state. A comparison with the M2 transitions in "aHf i.e. the 406.7 keV transition (6(6 1) -" 8(8 -)) and the 125.1 keV transition (14(14 - ) -. 16(161)) which have hindrance factors of46 and 73 respectively leads to the same conclusions. The ~- state (T~ = 1 .6 ~s) is as the 251 state expected to be a pure 3 q.p.s. It decays also to the ~-- state. Now the one-quasiparticle transitions involved are ~n

~ Zn

~.t ~ ~. t zP ZP

for the {~rv , ~ô , ~~} final-state component,

for the {gyp , ~ , ~~} final-state component.

The first one is observed in "9.'s'W and "9Hf [ref.' 6)] with no measured halflife and the second one has a half-life of 95 ns in " 9Ta (fig. 4) corresponding to a hindrance factor Fw = 4.6 x 103 . The high hindrance factor (Fv = 3.4 x lOs) of the 75.3 keV M1 transition indicates that the {~â , ~ , ~~ } component and especially the {gyp , ~ ô , ~~ } component are weak in the ~- state. The ~- state wind also decay to the ~1 state through a 10.6 keV E1 transition which is unobservable in our experiments. Using the half-life (T~ = 1 .4 ~s) of the ~(~-) level at 30.7 keV which decays by the same transition (gyp -" ~P ) to the ~(~ 1) level one finds a T} = 4.8 ~s for the partial half-life of the ~- level. This value does not rule out completely the possibility of such an E1 transition but it is compatible with the observed half-life of the ~ state. Five-quasiparticle states . The configuration of the ~ 1 state at 2640.6 keV is very probably the one given in table 6 obtained by coupling the ~1 proton state with the 161 4-quasiparticle state of the ocre {~ô , ~~ , ~ p , ~ p } because there are no other low-lying single-particle states available to give such a high Kvalue at such low energy . The four times K-forbidden isomeric E2 transition which depopulates this ~1 level has a hindrance factor f = 24 . This low value can be explained by assuming a K-admixture in the ~t state. This value can be compared to that of the similar transition .l4(14 -) -a 12(8-), f = 33 in l'aHf.

218

D. Barnéoud et al. / "~Ta TABLE

Iex-eRl (calc) ")

Band

o.si o. ~ a

lex-eRl (exP) o.ls t o.oa

0.14

0.09 ±0 .05

') The gx values are taken from refs . e.'a) ; the adopted configurations are those of table 6 without admixture.

In the region of 2800 keV several configurations are possible (see table 6) ; then the ~- state observed at 2794 .1 keV is probably a mixing of configurations . However the weakness of the E1 transitions,to the ~+ band levels seems to indicate that the contribution of the {~P , ~ , ~~ , ~~ } component is not important in the ~state. The very low hindrance factor (f = 5.3) of the ~(~-) -" ~(~-) transition is probably due to a strong lower-K admixture in the ~- state. Three-quasiparticle rotational bands. From the rotational model the (gR - gR)/Qo values are given by lyK - gR)Z /QÔ

with

= 3.48 EZ(M1)/S Z(21+2x2I-2),

sign lflR - 9R)/QO = sign S

[ref. 9)],

where Qo is given in b, E(M1) is the energy of the cascade transition in MeV and S is the E2/M1 mixing amplitude. Unfortunately the cascade transitions are weak or partially resolved in the angular distribution spectra. So it is not possible to obtain S from the angular distribution experiments. Then the branching ratios were analyzed to extract the IgR - gR I values, using the Qo value of the t'sHf core Qo = 6.95 f0.02 b [ref. tt)] . In table 7 these experimental values are compared to those calculated, taking for gR the value of the even core and taking for

gR,

assuming a pure three-quasiparticle configuration, 3

gR

= K-t ~ Ki9Ri, ~1

where the gRi values, for the involved orbitals, are taken from ref. t a) . Although this calculation neglects possible K-admixtures the agreement obtained supports the proposed configurations for the ~- and ~- states. For the state the high IgR-eRl value given by the three-proton configuration (table 6) agrees well with the weakness of the crossover transitions in the band .

D . Barnéoud et al. / "'Ta

219

60

Fig. 8. Apparent moments of inertia of the 3 q .p . bands of "9Ta .

The moments of inertia were calculated from the transition energies using the formulae 'a) 2J/fi2 = d(RZ)/dE, (ttw)2 =

(dE/dR)Z .

In fig. 8 the moments of inertia of the ~- and ~+ 3-quasiparticle bands are compared with those of 0-, 1-, and-2~uasiparticle bands of neighbouring nuclei. It has been pointed out that, for the Hf nuclei, the average moment of inertia increases with the number of quasiparticles and that a negative slope as a function of it~w2 is only seen for the bands with an i~ neutron component [ref.' s)] . Although the situation is not as clear as in the Hf isotopes the trend is the same and supports a ~ +[624] n component in the configuration of the ~- and ~+ bands in t'~fa. References 1) P. Manfrass, W. Andrejtscheff, P. Kemnitz, E. Will and G. Winter, Nucl . Phys. A226 (1974) 157 2) D. Barnéoud, C. Foin, A. Baudry, A. Gizon and J. Valentin, Nucl . Phys . A154 (1970) 653 3) C. Foin, S. André and S. A. Hjorth, Nucl . Phys. A219 (1974) 347 4) J . P. Richaud, Nucl. Insu. 167 (1979) 97 5) S . André, J. Treherne and D. Barnéoud, J. de Phys. Lett. 18 (1977) 369 6) R. S. Hager and .E. C. Seltzer, Nucl. Data tables A4 (1968) 1 7) T . L. Khoo and G. L~vh~iden, Michigan State University cyclotron laboratory, Report 237 (1977) 8) C. Foin, Th . Lindblad, B. SkAnberg and H. Ryde, Nucl. Phys. A195 (1972) 465 9) B. Skt;nberg, S. A. l~jorth and H. Ryde, Nucl . Phys. A134 (1970) 641 10) S. André, D. Barnéoud, C. Foin, B. Ader and N. Perrin, Nucl. Phys . A279 (1977) 347

220

D . Barnéoud et al. / "'Ta

1 I ) F . W. N . De Bcer, P. F. A . Goudsmit, B. J . Meijer, J. C . Kapteyn and J. Konijn, Nucl. Phys. A263 (1976) 397 12) O . Prior, F. Bcehm and S . G . Nilsson, Nucl . Phys . A110 (1968) 257 13) I . L. Lamm, Nucl . Phys . A125 (1969) 504 14) W. Klamra, S . A . Hjorth, J . Boutet, S . André and D . Barnéoud, Nucl . Phys . A199 (1973) 81 15) G . D . Dracoulis and P. M . walker, Nucl . Phys. A342 (1980) 335, and references therein 16) Table of Isotopes, ed . C . M . Lederer and V . S. Shirley (Wiley, New York, 1978)