g-factors of high-spin isomeric states in 216Ra

Nuclear Physics A442 (1985) 361-368 ©North-Holland Pubhshmg Company

g-FACTORS OF HIGH-SPIN ISOMERIC STATES IN 2i6Ra M ADACHI, T KOHNO, A MAKISHIMA, S FUKUDA, M FUKUDA, M TAYA and H TAKETANI

Department of Apphed Phystcs, Tokyo Institute of Technology, Oh-Okayama, Meguro, Tokyo 152, Japan Received 15 April 1985 Almtraet: The g-factors of two lsomenc states at Ex = 3763 and 5170 keV m 2t6Ra have been measured to be 0 51 + 0 03 and 0 63 + 0 06, respectwely, with a TDPAD method Spin and panty assignments of 19- for the 3763 keV state and 25- or 24 + for the 5170 keV state are consxstent with the measured g-factors Proposed configurations for the 19- and 25- assignments are of the same type as those pre&cted by a deformed mdependent-partmle model for 214Rn, wluch is an lsotone of 216Ra NUCLEAR REACTION 2°spb(12C,4ny), E = 80 MeV, measured Lr(O,H, t) 216Ra deduced levels, isomer g, 1"1/2,J, or, configuratmns Pulsed beam, enrtched target, Ge(L0 detector

1. Introduction R e c e n t l y there has been a great deal of interest m n u c l e a r structure at high spins m nuclei n e a r Z = 82 a n d N = 126. H o r n et al. 1) have f o u n d m a n y h i g h - s p i n l s o m e n c states m the N = 126 lsotones 212R.86.~.126,213~87rr126 a n d 2141~-88..a126,a n d assigned the c o n f i g u r a t i o n s of the states from m e a s u r e d g-factors. The a p p e a r a n c e o f such i s o m e r i c s t a t e s in t r a n s l e a d nuclei are e x p l a i n e d wlttun the f r a m e w o r k of a spherical shell m o d e l w i t h residual lnteracUons 2'3) These isomeric states are also e x p l a i n e d w i t h a d e f o r m e d i n d e p e n d e n t - p a r t i c l e m o d e l 4 ) . The latter m o d e l r e p r o d u c e s the e x c i t a t i o n energtes of k n o w n i s o m e n c states in Po, A t a n d R n i s o t o p e s a n d p r e d i c t e d m a n y i s o m e r i c states in these isotopes. V e r y recently, lugh-spin states m 2t6Ra were intensively studied with m - b e a m y - r a y s p e c t r o s c o p y b y two groups, L o n n r o t h et al. 5) a n d I t o h et al.6). A l t h o u g h b o t h g r o u p s have e a r n e d out their e x p e r i m e n t s m a i n l y wtth the same reaction, 2°8pb(13C,5n-t)216Ra, the p r o p o s e d v - r a y d e c a y schemes i n c l u d i n g excitation energms, spins a n d parities of the m e a s u r e d isomeric states are quite different f r o m e a c h other. I n the p r e s e n t study, g-factors o f two l u g h - s p m isomeric states m 216Ra have b e e n m e a s u r e d w i t h the T D P A D (time differential p e r t u r b e d a n g u l a r distribution) m e t h o d a n d the e x p e r i m e n t a l g-factors o b t a i n e d are c o m p a r e d with those calculated for the p r e v i o u s l y p r o p o s e d configurations 5,6) 361

362

M A d a c h t et al / g-factors

2. Experimental procedure and results The excited states In 216Ra were populated via the 2°8pb(12C, 4ny)216Ra reaction using a 12C b e a m of 80 MeV from the INS-Tokyo SF cyclotron. The tluckness of the enriched (99.5%) 2°spb target was 10 m g / c m 2. G a m m a rays were detected with a 90 cm 3 Ge(L1) detector with a resolution of 2.0 keV for 1 33 MeV "t-rays. The g-factors of the xsomenc states were measured with the T D P A D method with an external magnetic field applied perpendicular to the y-detection plane Using cyclotron b e a m burst (8 MHz), time distributions of the y-rays deexcmng the isomeric states were measured at 0y = 57 ° and 147 ° relatwe to the actual beam direction in 2-parameter (Ev-t v-r t ) list mode A part of the singles y-ray spectrum is shown in fig. 1 together with a partial T-ray decay scheme of 216Ra taken from ref. 6) In the present experiment, time distnbutlons of the 557 and 613 keV y-rays have been used to deduce g-factors of the lsomenc states at E x = 5170 and 3763 keV, respectively, since among a series of v-rays deexcltmg the isomeric state, the highest energy T-ray shows the best timing resolution The time distributions are shown in figs. 2 and 3 The external magnetic field, Hext = 2 23 + 0 01 T, was measured with a calibrated Hall probe. The b e a m bending correction to obtain the actual T-detection angle was evaluated to be - 4 ° by a precise mapping of the magnetic field The time distributions are analyzed as distributions composed of a p r o m p t c o m p o n e n t and a perturbed delay component convoluted with a response function derived f r o m a pure prompt distribution. For the 557 keV y-rays, two time dlstnbutlons at 0v = 57 ° and 147 ° were simultaneously fitted by the following expression. r(t,) =[°~P(t,-t)f(t)dt, "l-oo

where

P(t,)= /(t)=

1

e x p [ - : (1t , / o )

2

]

[b p + b2Pcos(20v)] + ( 1 / , ) e x p ( - - t / , ) [ b d o + bdcos2(Ov--tOat)]

In tlus expression, the response function is assumed to be of gausslan shape and and w L are the mean hfe and Larmor precession frequency for the lsomenc state, respectively. The intensities b[ and b~ are for prompt and delay components, respectively, and are related to the common angular distnbutlon formula, W ( O ) = a o + a2Pz(cosO ), by

b o = a o + ¼a 2 ,

b2 = 3a 2

The best-fit parameters are a 2 = - 0 258(48), T = 9.5(4) ns and w L = 6.66(68)× 107 s -1 with reduced chl-square X 2 = 1 03 F o r the 613 keV y-rays, a two-level formula 7) has been used for f ( t ) since the preceding lsomenc state at 5170 keV has nearly the same lifetime as that of the isomeric state at 3763 keV which is being analyzed The obtained mean hfe ~- and

o

8

"0

~z

Y

"I3NNVH:D H3d SINnOD

363

M Adacht et al / g-factors

364 I

I

557 keV

by=57*

¢~,

~

I

I

~

557 keY

~

~y-147*

I

..-i ILl Z

Z

"lOf_ LU 0.. 03 I.-Z 0 ¢...)

101

50

100

150 CHANNEL

50 NUMBER

100

150

Fag 2 Tame dastnbutaons for the 557 keV 3,-raysunder Hext = 2 23 T Sohd lines are theoretical fats, see text

Larmor precession frequency COL for the higher-lying isomeric state are used m the analysis of the 613 keV ,/-ray time distribution in order to mlnlrmze the free parameters in the expression, f ( t ) In fact the fitted values have proved that COL(613 keV) is rather insensitive to the variation of COL(557 keV) and ~(557 keV); a _ 50% variation of COL(557 keV) changs coL(613 keV) by -T-5% and a 10% variation of z(557 keV) changs COL(613 keV) by less than 1% This is probably because only one-tenth of the delay component of the 613 keV ,/-ray is fed through the higher-lying isomeric state The best-fit parameters a 2, • and COL are --0.178(15), 7.6(4) ns and 5.13(34) x 107 s - i , respectively, with X~ = 1.34. The Larmor precession frequency for the 613 keV ,/-rays is also obtained from the ratio

g(t)

N(Ov=sv°'t)-N(Ov=x47°'t) N(Ov=svo, t)+N(Ov=147o, t ) '

as is shown In fig. 4 The sohd line is a single slnusoldal fit, and a 2 and coL turned out to be - 0 157(11) and 5.70(38)× 107 s -i, respectively The effective magnetic

t'

M Adachl et al / g-factors

613 keY Oy=57"

~ /

\

365

613 keY Oy=147"

LLI Z Z < "10

C~

Z 0 rj

101

50

100

150

CHANNEL

50

NUMBER

100

150

Fig 3 T]me dlstnbuttons for the 613 keV y-rays under Hext = 2 23 T Sohd hnes are theoretical fits, see text

field which really acts on the nucleus differs from the external field due to the Kmght sfift K and the diamagnetic sluft o" Heff=Hext(1 + K ) ( 1 - o ) . These correcUon factors for Ra m Pb were obtained from the work of Hausser et al 8): K = (1.05 + 0.15) × 10 -2 and o = 2.04 × 10 -2. ~L(613 keV) Is obtained as a weighted mean of 0~L'S deduced from the tune dlstribuuon at a single detecUon angle and from the ratio R ( t ) . In table 1, the g-factors of two isomeric states obtained in the present experiment are compared with the calculated g-factors for proposed configurauons 5,6) In this table, prevlously-obtmned mean hves and spins and pantms are also shown. The mean hfe of the higher-lying state measured by Lonnroth et al 5) is about 1.5 times longer than both the present value and the one measured by Itoh et al 6). The present mean life for the 19- state zs slightly shorter than that obtained by Itoh et al. Tlus is probably due to the fact that Itoh et al. have deduced the mean hfe by neglecting the effect of the higher-lying isomenc state. In fact, applying a singledelay-component analysis of the summed (Ov = 57 ° and 147 °) Ume distribution for

M Adacht et al / g-factors

366

~t

613key

R(t) 0.3 0.2 0.1 0

IJ'pllIt

-0.1

-0.2 -0.3

50

!

25

TIME(ns)

Fag 4 Ratio R(t) for the 613 keV y-rays under Hext = 2 23 T The sohd lxne corresponds to o~L = 5 70(38) × 107 s l TABLE 1 g-factors and m e a n hves of two isomeric states in 216Ra compared with those for proposed configurations E x [keV]

J'~

z [ns]

Proposed configuration

gealc a)

9 5(4)

gexpb)

Ref

0 63(6)

PW c )

5170

25-

[214Ra(17-), ~r(hSg/2113/a)]v(g9/2)~*

0 59

6)

5868

24-

14(4)

';'r(J02+ h49/2 )12 + v (g9/2 J15/2 )12

0 28

5)

5868

24 +

14(4)

3 f7/2 )14 + v(g9/2111/2 )10 + '/r(Jo2 +h9/2

0 57

5)

9 7(9)

7 6(4) 3763

19-

8 7(6)

0 51(3) [214Ra(11-), ~(h59/2113/2)]v(g9/2)2+

0 49

PW c) 6)

a) g-factors of single partacles are taken from ref 11) b) K m g h t shaft K = (1 05 __ 0 15) × 10 - 2 and diamagnetic shift a = 2 04 × 10 2 are taken from ref 8) c) Present work

the 613 keV y-rays, the mean hfe of the 1 9 - state becomes 8 6 + 0.3 ns instead of 76+04ns. 3. Discussion T h e hagher-lymg isomeric state is tentatwely assigned by two groups J ~ = 2 5 a n d E x = 5170 keV by Itoh et al. 6), and J = 24 ( p a n t y undetermined) and 5868 keV b y L o n n r o t h et al.S). In both decay schemes 5"6) the 557 keV 3,-ray is one of the t r a n s m o n s between the two isomeric states Itoh et al. have interpreted this isomeric state as arising f r o m the weak c o u p h n g of the two-neutron state v(g9/2)2+ to the 1 7 asomerlc state (7"1/2 = 230 ns and g = 1.028 _+ 0 007) m 214Ra [ref 1)]. L o n n r o t h et al p r o p o s e d the following configurations q r ( j 02+ h 94/ 2 ) 1 2 + v ( g q / E J 1 5 / 2 ) 1 2 - f o r the 2 4 -

M Adacht et al / g-factors

367

state and ,n'(j2+h39/2fT/2)14+v(g9/2111/2)lO + for the 24 + state. The configuration of 2 3 rt(J0÷h9/2f7/2)14+ IS the same as that proposed for the 14 + isomeric state (1"1/2 = 275 ns and g = 1 022 + 0.009) in 214Ra [ref. i)]. The lower-13nng isomeric state is assigned by Itoh et al. 6) to be the 19- state at 3763 keV with the configuration of v(g9/2)2+ coupled to the 11- isomeric state (7"1/2 = 333 ns and g = 1.085 + 0.010) in 214Ra [ref. i)]. Lonnroth et al. 5), on the other hand, located the lower-lying Isomeric state at the 1708 keV 8 + state by simply quoting the early work of Nomura et al 9). The latter assignment, however, seems to be unhkely from the present observation that the time spectrum of the 557 keV -/-rays has more prompt component than that of the 613 keV y-rays as shown in figs. 2 and 3, since if there exists no long-lived isomeric state between the two v-rays as assigned by Lonnroth et al, the succeeding "t-rays of 613 keV should show more prompt component in the time spectrum. Chevalher et al. i0) have also pointed out the erroneous assignment of the lower-lying Isomeric state to the 1708 keV 8 ÷ state from their experiment and proposed the state to have a spin higher than 14 and excitation energy greater than 3.29 MeV. The comparison of the present experimental g-factors with those calculated for the proposed configurations 5,6) is also presented m table 1 to check the previous assignments. Here, the experimental g-factors of 17- and 11 - states in 214Ra [ref. 1)], whose configurations are gdven in parentheses, are used to calculate the g-factors for the proposed configurations In 2i6Ra. The spin and parity of 24- with the configuration proposed by Lonnroth et aL 5) could not reproduce the experimental g-factor for the higher-13qng isomeric state. Both the spins and parities of 24 + with the configuration proposed by Lonnroth et a! 5) and 25- with the configuration proposed by Itoh et a! 6) reproduced well the experimental g-factor. So it is not possible to give a unique assignment of spin and panty for the higher-lying isomeric state from the present experimental g-factor. The experimental g-factor for the lower-lying isomeric state is well reproduced by the assignment of 19- wtth the configuration proposed by Itoh et al 6). It is noted that the configurations arising from the weak coupling of two-neutron state v(g9/2)2+ to the 11 - and 17- states in 214Ra are of the same type as those predicted by the deformed independent-particle model 4) for 214Rn, wluch is an isotone of 216Ra The authors would like to thank Dr. Y. Gono for valuable discussions and kindly providing us the 2°spb target The authors are grateful to the INS cyclotron crew for their cooperation. Numerical calculations were carried out with FACOM R380 computer at the INS.

References 1) D Horn, O Hausser, B Haas, T K Alexander, T Faestermann, H R Andrews and D Ward, Nucl Phys A317 (1979) 520

368

M Adacht et al / g-factors

2) J Blomqvxst, I Bergstrom, C J Herrlander, C G Lmd6n and K Wdkstrom, Phys Rev Lett 38 (1977) 534 3) D Horn, O Hausser, T Faestermann, A B McDonald, T K Alexander, J R Beene and C G Herrlander, Phys Rev Lett 39 (1977) 389 4) K Matsuyanag~, T Dossmg and K Neergbxd, Nucl Phys A307 (1978) 253 5) T Lonnroth, D Horn, C Baktash, C J Laster and G R Young, Phys Rev C27 (1983) 180 6) Y Itoh, Y Gono, T Kubo, M Sugawara and T Nomura, Nucl Phys .4,410 (1983) 156 7) R Lutter, O Hausser, D J Donahue, R L Hershberger, F Raess, H Bohn, T Faestermann, F v Felhtzsch and E K G Lobner, Nucl Phys A229 (1974) 230 8) O Hausser, J R Beene, T Faestermann, T K Alexander, D Horn, A B McDonald and A J Ferguson, Hyp Int 4 (1978) 219 9) T Nomura, K Hlruta, M Yoshle, H Ikezoe, T Fukuda and O Hasbamoto, Phys Lett 58B (1975) 273 10) A Chevalher, J Chevalher, B Haas, S Khazroum and N Schulz, Z Phys A308 (1982) 277 11) R Bauer, J Speth, V Klemt, P Rang, E Werner and T Yamazakl, Nucl Phys A209 (1973) 535