Revista Mexicana de Física 38, Suplemento 2 (1992) 53-65

Higher Spin States in Neutron Rich Nuclei

S. ZHU, X. ZHAO, J. H. HAMILTON, A. V. RAMAYYA, Q. LuW.-C. MA, 1. K. PEKER, J. KORMICKI, 11. XIE, W. B. GAO, J. K. DENG

Vanderbilt University, Physics Dcpartment, Nashville, TN 37235 (USA)

1. Y. LEE, N. R. JOlINSON, F. K. MCGOWAN, C. E. BEMIS

Oak Ridge National Laboratory, Oak Ridge, TN 37831 (USA)

J. D. COLE, R. ARYAEINEJAO

/daho National Engineering Lab" Jdaho Falls, ID 83415 (USA)

G. TER-AKOPIAN ANO Yu. OGANESSIANJoint Jnstitute lor NuclcU7' Resear'eh, Dubna, RUSSJA

ABSTRAeT. Recent studies of the products produced in spontaneous fission (SF) have shown aDunexpected richness of in[armalion aboul previollsly inaccessible higher spin studies in neutronrich nuclei. Recent evidence for octupole deformation in 144,146Ba, 164Yb and 226Ra have comefrom SF and other techniques. New insights into the structure of nuclei in the A = 100-120 and136-152 regions have beco obtained fraIn two different sr i - ¡ coincidence studies of 252Cf in aClase Paeked Ball with 20 Complan suppressed Ge detedors, where ane experiment also includeda fission fragment detector inside the ball at the 1I0lifield lIeavy Ion Research Facility, as well as asimilar sludy at Argonne. New data resol ve problems related to shape coexistence in lOOSr, 102Zrand provide new insights into the super deformation in this region. New high spin states up to16+, a record in su eh neutron rich nuclei, were found in 138,140Xe, 14°Ba, 146-150Ce aud 150,152Ndin our IIJlIRF work and levels identified for the first time in !36Te and !49Ce. The J! momentsof inertia for 148,150Ge and 150,152Nd exhibit an unexpected crossing pattern at high spins. Thesedata al50 provide new insight into the fission process itself.

RESUMEN. Estudios recientes de los productos de la fisión espontánea (SF) han mostrado unariqueza inesperada de información sobre estados previamente inaccesibles de espines altos ennúcleos ricos en neutrones. Evidencia" recientes de la deformación octupolar en 144,146Ba, 164Yb y226Ra se han obtenido de SF y otras técnicas. Nuevos puntos de vista en la estructura de nlJcleos delas regiones A = 100-120 Y 136-152, se han obtenido de dos estudios diferentes sobre coincidencias'Y- 'Yen la SF de 252Cf en un "Clase Paeked Ball" COIl 20 detectores de Ce con supresión Compton;en donde uno de los experimentos también incluyó un detector de fragmentos de fisión dentro dela bola en el laboratorio de investigaciones de iones pesados de 1I0lifield (lllllRF), así como unestudio similar en Argonne. Nuevos datos resuelven problema" relacionados a la coexistencia deformas en IOOSr, 102Zr y proporcionan visiones nuevas de la super deformación en esta región. Seencontraron en trabajos en el HHIRF nuevos estados de espines altos hasta 16+, un record, entales núcleos ricos en neutrones como 138,140Xe, 140Ba, 146-150Ce y 150,152Nd Y se identificaron,por primera vez, niveles en 136Te y 149Ce. Los momentos de inercia JI para 148,150Ge y 150,152Ndexhiben un patrón de cruzamiento inesperado en espines altos. Esos datos: también dan lugar anuevas consideraciones sobre el proceso mismo de fisión.

PAes: 21.10.-k; 21.90+£

54 S. ZUU ET AL.

INTRODUCTION

Nuelei on lhe neutron rieh side of oela slahi!ity have long oeen of interest for nuelearstrueture stndies oeeause they prooe ditrerenl regions of the single partiele speetrtlJII andditrerent shell gap eomhinalions for hoth spherica! aud deformed shapes. I1owever, su ehnuelei have heen a diffieult ehallenge experinlentally. ~Iueh iuformalion has heen gainer:Iahout the properlies of neutron rieh nuelei at 10\Vspin from the study of radioaelive iso-topes produccd in neutran in<1l1ced, and more rcccutly, protoll induced fission of uranium[1]. Sueh studies have been made possiole hy lhe use of isolope separalors on !ine toreaetors and more reeent!y to 10\Venergy lHotou aeeelerators. More reeently, new regionsof radioaetive neutron rieh nuelei have oeen opened up for sludy through the use ofmultinueleon lrausfer reaelions for examp!es, in lhe mass 50-80 and aetinide regions [2,3].

!lo\Vever, to test many of the theoretical predietions of nuelear models one needsinformation ahout the higher spiu states in nueld in addition to their low spin statespopulated in radioaetive deeays. I1igher spin states in neutron rieh nuelei have been aneven more diffieult ehallenge thau the lower spin states aeeessihle through deeay studies.One eannot reaeh the higher spiu states in these nuelei hy heavy ion fusion evaporationreaetions as earried out extensive!y for prolon rich nuelei. Until reeeut!y, untapped soureesof information on higher spin states, as \Vell as 10\Vspiu states, of neutron rich nuelei haveheen the study of the prompt gamma rays from tl,e fragmeuts produeed in spontaneous(51') and indueed (IF) tission of heavy elemeuts.

Many years ago prompt SI' studies were used to suggest for the tirst time that 98Srand IOOZrhad unusually Jarge grouud state deformations [4]. The avai!ahility of higherefficieney multi-deteetor arrays of Comptou suppressed Ge detectors has hrought on arene\Ved interest in studies of the prompt gamma rays of the fragments from spontaneousand induced tission. Groups at Argonne, Dareshury, and a Vanderoilt-Oak Ridge-Idaho-Duona collahoratiou have carried out several sueh studies from spontaneous [.5-9] andheavy-ion indueed [10] tission \Vhieh have revea!ed ne\V insights into our knowledge ofneutron rich nuelei [.5-10]. This paper is primarily a revie\V of these studies, ineludingreeent, unpuolished results [6,9].Mueh of the work presented here comes from t\Vo experiments earried out at the !lo-

litield !leavy Ion Researeh Faei!ity (!l!lIRF) in the 20 Compton suppressed Ge detectorClose-Paeked Hall array [6,9] with 252Cf SI' sourees. In the tirst experiment a 252Cf soureewith 3,900 tission eventsjsee \Vas eoated on the iuside surfaee of a gas-tilled ionizationehamber \Vhich was plaeed inside the Ge hall to allo\V tission-fragment-')'-')'-coineideneesto be reeorded [6J. A total of 1.5 X 107 fragment-')'-')'-coineidenees were recorded. New,higher spin studies up to 14+ in some cases, were observed in 19 nuelei, and the levels of1361'e wcrc cslahlished [01' the first lime. Thesc data allow ane lo study lhe changC's fromvibrational lo dcformed strllrtufC's and lo scc the influence of rotation aligned structuresat higher spins from only a short runo In order to improve the statistics and to see higherspin states, a stronger 252Cf souree \Vas run in the same Close-Paeked Ball and 1.9 X 109')'-')' events \Vere recorded [9]. In these data states to 16+, a record for neutron rieh nuelei,were observed in some of the level sehemes. A uumher of new iusights are ootained fromthese data.

IIIGI!ER SPIN STATES IN NEUTRON RICI! NUCLEI 55

18-

12+

10+

6+

FIGURE 1.

OCTUPOLE DEFORMATIONS

2381

3891992

3681624

345~ 1279

320~ 959

290669

253416.6

~ 205.1 211.5•• 143.8 67.

Stable ground state octupole deformations I,ave beeu oue of the exciting recent newiusights iuto uuclear structure with the discovery of parity doublets in the regions of At,Ra and Th nuclei (see review of Nazarewicz, Ref. [11]). Nucll'i with permanent octupoledeformation do not have re"ection symmetry (see the c1assic paper of Leauder et al.,Ref. [12]). Reflection asymmetry gives rise to a doubling of states with opposite parity inodd.A nnclei, as first observed in the actinide elements, for example 223Th (Ref. [13]).

TAIILE I.Reduccd lransition probabililies and chargc deforlnalion paramcters iu 226Ra (ReL (14]).

A

234

O(EA; O ~ A)[e2b'j

.5.15(14)1.1O( 11 )1.08( 15)

.B,(exp)

0.165(2)0)0.104(5)0)0.123(8)

.B, (Ref. [12])

+0.164.0.112

+0.096

4)Sign of deforrnation paralllPt.er not determined experimenlally.

The se'luences of rotational states in 223Th can be understood as the coupling of anoctupole deformed core with an llnpaired neutron. In even-cvCIl Iluclei alle ohserves even-and odd-parity rotational bands, as shown in Fig. 1. The eS! Coulomb excitation grouphave Coulomb excited the radioactive target 22GRa(t¡/2 = 1600 yr) with a 208Pb beam[14]. They extracted dectric 'luadrupole, hexadecapole, aud, for the first time, octupole

56 S. ZHU ET AL.

~13~.1

o -B.o

"<r.>ee-_

(!:\~<9.2

~.~(21.~

~, .7

'3120~(3 1"

---.J!:.l¡~e

"

O'

161.0

'<163.1

961.4 :"'" ~8>a'.•~..• ."/ __ r".."" /

/

"". //",. "" (o>oJ I

~, ..., ~/

.' ...,• O

"'80 .'80

~" _.60U (6.$j '" fui

.,..," fu]

¡"7O.1

~ ()6.7J

FIGURE2. Levels in142-I46Ba from SF (Ref. [.')]).

transitiolls moments up .lo spin 18. Their extracted f32, {33 and f34 deformations for 226Ra,in Table 1, are in excellent agreement with theoretical calculations [12J for a stable octupoledeformation,

The Iirst evidence for ground state octupole deformation outside the At-Ru-Th regionwas found in H4,H6Da from studies of the prompt 'i"-ray frolll SF of 252Cf. Positive- andnegative-parity bands with enhanced El crossing transitions were observed as shown inFig. 2 (Ref. [5]). For colllpleteness, while theoretical and experimental investigations ofoctupole deforlllations have focused on the Ra-Th (Z '" 88, N '" 134) and Da-Sm (Z '"56, N '" 88) nudei, there exists some evidence that indicates an instability with respectto the octupole degree of freedom for N '" 94 nudci. In }62Er and 164Yb, the lowestnegative-parity band is seen to be systcmatical!y depressed about 200 keV rclative tothe next lowest negative-parity band, and thcre is a lack of signature splitting in thelowest negative-parity decay sequence in the neighboring odd-N isotopes, To darify thepossibility of strong interaction betwecn the octupole and aligned quasineutron contig-urations observed in the N = 94 erbium and ytterbium, D(EI)/D(E2) branching ratios

HIGl!ERSPIN STATESIN NEUTRONRICl! NUCLEI 57

were exlracted from our 164Yb lifelime dala [15]. As a resull of lhe slrong oclupole bandcharacler, one mighl expecl enhanced El lransilions from lhe (-, I)-band lo lhe yraslbando The El/E2 branching ralios were exlracled for 162,166Yb and 164Yb and, indeed,lhe branching ralios in 164Yb are enhanced by more lhan a faclor of 4 wilh respecl lolhe neighboring isolopes. Furlhermore, lhe absolule B(El) values of 164Yb are of lheorder of 10-3 W.U. comparable lo lhose observed in lhe Ba-Sm nueleL The enhancedoclupole slrenglh for 164Yb probably is lhe result of lhe proximily of lhe second pair ofoclupole-driving orbilS; i.e. lhe n = 3/2 componenl. of lhe Vi13/2 and v 17/2 slales nearlhe Fermi level for N = 94. 1I0wever, lhe octupole inslabilily in 164Yb is less pronouncedlhan for Ba-Sm nuelei [rr(hll/20 d5/2) and V(iI3/20 f7/2)] and Ra-Th nuelei [rr(iI3/20f7/2) and 11(j15/20 g9/2)], where al leasl one sel of lhe oclupole driving orbilals bolh haven = 1/2. These syslemalics indicale lhal lhe larger spacial localizalion of lhe 11= 1/2,high-j conliguralions probably is imporlanl in slabilizing lhe oclupole shape. So, 164Ybprovides an example of enhanced oclupole deformalion arising from lhe less pronouncedpair of oclupole-driving orbilals, i.c. /I(iI3/20 f7/2) wilh 11= 3/2.

NEUTRON RICl! NUCLEI IN Tl!E A = 100-120 REGION

New levels were observed in IOO,I02Zr(independenlly reporled in Ref. [8]), I04,I06Mo,108,1I0,1I2Ru, 112,1I4,1I6Pd and 120Cd in our lirsl experimenl [6]. Some of lhe level schemesare given in Table 11. As reporled in Refs. [7,16], Peker and lIamilton [17] analyzed lhedala [6] for 9851', 100,I02Zr, 104-106Mo and 106-1I0Ru in lerms of ~Ery = Ery2-Eryl VS. spin.This funclion is very sensilive lo shape changes [181 and is a smoolhly decreasing functionof spin for a single band wilh breaks when lhere is a band crossing 01' inleraclion. Therehad be en a puzzle for 9851' and IOOZrin lhal, while lheir 2i - 0+ energies all(l lifelimesindicaled lhey were slrongly deformed, lheir E4/E2 ralios were 2.99 and 2.67, respeclively,considerably lower lhan lhe 3.3 predicled in apure rolalional mode!. These lwo nueleihave lhe lowesl Oi exciled slales known in any nueleus, 215 and 331 keV, respeclively.These exciled oi sIal es are near spherical in nalure. Wilh lhe new 8+ and 10+ levels[6], lhe ~Ery values as shown in Fig. 3 for I02Zr, I04,lOGMo, 106-1IORu and lhe recenllydiscovered IO'Zr (Ref. [8]) are aH smoolhly decreasing funclions of spin (wilh some smaHf1uclualions for 104Mo) lo indicale a single rolalional band characlerislic of defonnedrolors up lo lhe highesl spin observed. 1I0wever, bolh 9851' and IOOZrhave quile differenl~Ery patterns. Their ~Ery's exhibil a break al lhe lowesl spin. This break was inlerpreled[17], jusI as for 74,76Kr (Ref. [19]) as arising from lhe inleraction of near- spherical exciledand superdeformed ground slales lo shifl lhe O:, down all(l Oi up. By assuming lhal ~Eryfor IOOZrand 9851' vary in lhe same way al higher spins and exlrapolaling lhal varialionback lo low spins, we found lhe superdeformed ground slale lo be pushed down 20 and40 keV in 9851' and IOOZr, respeclively [171. Afler correcling for lhese shifls, lhe E4+/E2+Tatías are 3.3 and 3.1, which are compatible with thc rigid rotor valuc. Following OUTwork, Mach el al. [20] analyzcd aH lhe lifelime dala for lhe 2i, oi and 4i levels in lhesenueleL They also found lhal lhe superdeformed ground slales only weakly mix wilh lheexciled near-spherical oi slales. Their exlrac.tcd shifls of 23.3 and 46.9 kcV, for 9851' and

58 S. ZIIU ET AL.

----- 1G.t Z•.-16"~V

120--{)-- 102 "b-- lOOJ'.r

--o- lO-! 'IQ

uJ<l

160

200

~ ,oaRo'80 ~- 112 Ru

~ 110 Ru.101<.eV

'6<l .t.E.!keV

"O

"O

"lO

80

spin6<l

O 2 , 6 8 10

200

xoO 2 ~ Ó S

FIGURE 3. ~E, = E,2 - E,I •., function of spin [6,8,9J

IOOSr, respectively, are in excellent agreement with our values [7,16) determined earHerfrom 6E~.This c10se agreement demonstrates the power of 6E~p!ots to yield detailedand quantitative information on energ)' level patterns, changes in structures, and shapecoexistence. Similar analysis indicate perhaps onl)' 5-10 keV shifts in IOOSr and 102Zr(see !ter. [16] for additional details and discussion of lIlass 70 nuelei). There remains theimportant question of why the excited near-spherical and superdeformed ground stateswhíeh are so e10se in energ)' in 9SSr and IOOZrinteract so w{'ak!y (20-50 keV shifts) whenthey interact so strongly (200 keV shlfts) in 74,7GKr where these same two shapes are muchfarther apart [7,16). It is dimcu!t to understand these diff{'rences in interaction strengthssince the origín of the sup{'rdeformation in both cases r{'lates to tJI<' r{'inforc{'m{'nt of theN (38,60) and Z (38) sll<'ll gaps at {3~ 0.4 and possihly t}¡p salIl{' gol> and g7/2 intruderorbitals [7J.

IIowever, the 101,103Zr levels (8) provide evidence for the im portance of the hl1 /2 in truderorbital in the superdeformation in this region, as ear1í{'r propos{'d in !ler. (16). IIotchkisct al. [8J propose that the side hand in IOIZr and the ground hand in I03Zr have 5/2-band heads associated with the 5/2-[532) intruder configuration (/lh!!I»' These results

IIIGI!ERSPIN STATESIN NEUTRONRICI! NUCLEI 59

are consistent with the predic.tions of mean-fie1d calculations [21-23) and correspond withpredicted minima in the potential energy surfaces [22,2:11at {32::::0.4. The near degeneracyof the 5/2-[532] orhital with the ground state in IOIZr, they propose, suggests substantialoccupaucy in lOoZr of the íl = 1/2 and 3/2 Nilsson orhitals of the hll/2 intruder. So, in102-104Zr they snggest there should be significant orcupanry of the íl = 1/2-, 3/2- and5/2- orhitals to support predirtions of deformed mean ficld calculations that the hll/2orbit near the Fermi surfare provides the driving forre toward large deformation. This isin contrast to shell model ralculations which attribute the deformation to the g7/2 orbitwith the only small orcupanry of the hll/2 orbi\. Earlier, it was pointed out [24] that it isthe strong down-sloping, I/hll/2 orbit and to a lesser extent the I/g7/2 orbit whirh shouldbe important in driving the deformation, and that it is not clear what this .loes to thep-n coupling scheme interpretation (25) of the strong deformation in Zr nuclei.

NEUTRON Rlcl! NUCLEI IN TI!E A = 136-],50 REGION

TA liLE 11. Vra,l gamma-ray ellergios of nelltron-rich 10<Mo lo 1<20aw¡lh A - (100-1!i0) ill prompleven.cveIl products of 252Cf spont.all(,olls fission. The new 142Xe dala are Ref. {2G]and c:onfirmedin our work [9]' an<ilhe olhers Ref. [6,9J. New lrallsiliolls are indicaled by '. The ¡42.IHlJa dalaare Ref. [5] (and confirm(>d !Jere). Some of •.he higher spin assignments are tentativc.

I'~ Ir En (keY)1 ,

104~.lo ~~Mo 108Ru IIORu 112Ru 112rd 114pd 116rd42 44 44 4.' 46 46 462+ 0+ 192.8 172.1 242.3 241.8 2:17.!i 349.0 333.3 341.04+ 2+ 369.0 3!i1.1 423.3 423.2 409.2 !i:14.6 .520.2 538.26+ 4+ 519.7 !i11.6 !i76.1' !i76.2 5'1 !j,4 * 667.9" 649.1 682.3"8+ 6+ 642.7' 65!i.1' 702.6' 706.0' 6!i 1.1 7:33.0" 715.9' 785.3'10+ 8+ 734.1" 785.1' 815.8' 722.9' 643.8'

l20Cd 13GTe 138Xe 140Xe 142Xe 140lla 1421la I44Ha48 52 54 54 54 56 56 562+ 0+ .506.3 1134.5" 589.5 377.0 287.2" 60:1.2 360.1 199.94+ 2+ 697.0 288.8" 484 458.2 403.5" 529.0 47.5.7 331.46+ 4+ 830.4 " .573.6" 482" 583.1" 490.4 • 809.1' 63!.9 43!.98+ 6+ 925.6" 1697.0" 730.4" ,567.3" 551.8* 694.2 .510.110+ 8+ 688.6" 608.1" 610.4 " ,574.012+ 10+ !i99.8" 679" 671.7" 622.614+ 12+ 848.8"

New, higher spin states have been found in 138,140Xe,14olla, 146,148.150Ce,150,I52Nd andl58Sm in our III1lRF work [6,9J and the leve]s in 136Te (Rer. [6)) and H2Xe (Rer. [26))eslablished for the tirsl lime, as shown in Table II and Fig. 6 shown later. Transilions inl49Ce also have be en identified for the first time [91. One can see from Fig. 4 the influenceof the N = 82 spherical shell gap alld the increase ill dcformation as a functioll of N andas one Z goes away from the Z = .50 shell gap fol' a given N. There are some anomalies,

60 S. ZUU ET AL.

1.5 --"-S2Te

-----o- 54Xe

--S6Ba

1.0-Q--SSCe

>~~.-NW

05

N

00!lO 70 80

FIGURE 4. The 2{" energies in Te, Xe, Ba and Ce nuclei.

90

however. Note for N = 82 the first exeited 2i energy for the N = 82 isotones inereaseas one goes away from the Z=50 shelJ e1osure, 1280(Z = 52), 1:114(54), 1436(56) and1596(58). The 2i energy in the N = 84 isotones shows a similar inerease a~ Z inereasesfrom 54 to 58. For N = 86 the 2i energy oseilJates but is highest for Z = 58. For N =88 the 2i euergy is again higher in 146Ce(Z = 58) than in H111a(56). Then the 2i trendreverses for N = 90 for H6Ba and 148Ce. If one looks al the moment of inertia, the ÓE~plots or the -y ray energies as a funelion of spin, one sees some other interesting features.Note (Table Il) for N = 84 tl,ere is a baekbend in the moment of inertia as a funelionof hw (hreak in ÓE, Fig. 5) at the .1+ level fm 138Xe and HOBa to indieate some type ofbland erossing at low spin. Then in 138Xe there is a second baekbend at 10+. For N = 86there is no haekhend in "IOXe and 142Ba, hut I10Xe has a haekbeud at 8+, but 142Ba doesnot! One also sees that while I10Xe starts out with slightly smaller deformation at the 2+level than 142Ba (a higher 2i energy), the !evels and theír depopulating transitions at 4+and above are lesser in euergy (presumably larger deformation) thau those in the groundbland of 14211a.By N = 88, 11-llla already looks like a good soft rotor. As already stressed,reccntly [1), the sharp jUIIlJl in clf'formation hetw('en N = 88 amI no 1I0t("<I many )'f'ars agofor Sm, Gd and Dy isotopes ís a shell gap reinforeement effeet where the Z = 64 spheriealsubshelJ gap at Gd keeps these nuelei spherical further from the N = 82 spherieal shelJgap ulltil there is a sudden ehange hetween N = 88 and 90 for tl,ose three nuelei, justas the N = 56 spherieal shell gap is reinforeed by the Z = 40 suhshelJ gap to keep theSr and Zr nuelei spherieal furtherl from N = 50 until there is a sud den switch (onset ofdeformation) hetweeu N = .58 and 60 in the Sr and Zr nuclei [1]. This difference also is

IIIGI!EIl SPIN STATES IN NEUTIlON RICI! NUCLEI 61

300 -D--&-

200-O-

~1006w" O

-'00

-200.4 .2 O 2 4 6

Spin8 10 12 14

180

'60

'40

'2()'>• 100"-w

80u

60-D- 1428a(86)-&- 1448a(88)

40 -O- 1468a(90)

20-2 O 2 4 6 8 , O '2

Spin

40 40

30 30

-, 20 -, 20

-D- 138Xe(84) '42Ba(86)'0 -&- 140Xe(86) '0 144Ba(88)

-O- 142Xe(88) 146Ba(90)

O O- 2 3 8 , 3 -2 O 2 4 6 8 'O '2

Spin Spin

'00 .120

O 100-D- 1428a(86)

-&- 144Ba(88)-'00 80 -O- 146Ba(90)

'"-200 60-, '"-300 -,

40138Xe(B4)

-<00 140Xe(86) 20-500 142Xe(86)

O-600

-2 3 8 '3 -20Spin -2 O 2 4 . 6 8 , O '2

Sptn

FIGURE 5. Mornents ofinertia JI ami Jz and LlE.., for 138-142Xe <\JI<!142-146Ba.

62 S. Znu ET AL.

,,.1652

10+ J5311'19

".\90

"09

.' 3'2 .....,

"2'-

2392'

,...75

O' (b\{)

I~Nd92,.J,l6S

,.- 57.Z8Il9

,,' 5M)2329

lO'537

1792

,, 5011291

• '5' =,--~S4S

2' '95 1S9O' O"Il'V),.-

5JC Ce 911

12:17 '.7'10 1211

, '"'"3¡" ",O. ...,.••. 24H 2J.I,' __ 162 72O" (keY)

I 5-1 ""do(j ¡ .•• <M

,/ ,w.12' 5'7 1919

10' '96•• 23

. "", "3

• 37•""7,

"" 31\62:__ ~~O 150 U•.eV)

SIC Cen

F'GURE 6. New levels for '''.150Ce and 152.154Nd(ReL [9]).

seen in laser spectroscopy IIwasurpments of the mean square charge radii wlwrp there isno sudden jump in deformation bptween N = 88 and 90 in th" Ha.Ce region like there isin Sm.Dy nudei (see ReL [1]).

From onr second 2S2Cf SF experiment, I rays have been identified from states up to arecord 16+ for very neutron.rich nudei as shown in Fig. 6 where our new level schemesfor 148,ISOCeand IS2"s4Nd are given [9]. Plots of,h and .12moments of inertia are sbownin Fig. 7. Note the moment of inertia plots where JI starts out considerably smaller forH8Ce than ISOCe, but as 1 inrrpases ,h rearhes and passes .1, for 1S0Ce. In 1s2Nd, JIalso shows the same crossing pattern but at 10IVerspin and less sharp but more definiteerossing. The .12for H8Ce exhibits a striking iucrease at tl,e tlVOhighest spins. As anotherexample, the levels iu '''Xl' have been identified from the Argonne 248Cf SF data andhave been used to test the ¡""dietion of the NpN" seheme [261. Finally, as seen in Fig.8 significant illformation ahout the spontaneolls fission process itself can be ga.inrd fromthese studies. The faet that one sees states up to 16+ earries signifieant informationabout the neeks at fission. The nerk must be reasonably thiek iu order to impart surhlarge angular momentum. As sepu in Fig. 8, oup also gains iuformation about tllP relative

IlIGIIER SPIN STA TES IN NEUTRO N RICII NUCLEI 63

.100

25<l ----tJ- 14XCe

200 --o- 150Ce

'5<>N..,

'00

50

O Spin

••----tJ- I4SCe

50 -O- 150Ce

20Spin

10.2 O 2 oC 6 S 10 12 14 16 I S

70--tr- 152Nd--o- 154Nd

••

---l::s- 152Nd50 --o- I54Nd

Spin

35-2 O 1 .•

FIGURE 7. JI and J2 for 148,150Ce, 152,154Nd.• • 10 I 2

ncutron emission channcls. Notc in Fig. 8 onc sccs transitiolls associatcd with levels fromI03Zr, the In dlalmel, to D8Zr (1I0t ShOWII), the Gn channel.

These data dcmonstrate that studies of the prompt -y rays from thc fragmcnts producedin SF can provide considerahle important information to test our theoretical models,including the interplay of spherical and deformed shapcs and single partide orbitals as a

64 S. Znu ET AL.

1.6(+05

•• Tren~ition cf l02Zr -2n

'1 : Tren~ltion of 101Zr -3nL 3(.05 ... Tren~ition of lOOlr -4n

M: Trensition of 99Zr -5n ..127

9.6(+04 ~;.: Trensition of lO3Nb I127.159,219 Mixed ••froM 159 gateo 152

I

6.4[+04

••,159[ H8Ce 2-0

++ 219

2121

I

o'O 95 131 168 204 240 277 313 350

FI(;UH.E 8. Plol sho'\'ing lhe 9'Zr_1o..1Zr transitiolJ in ('oillcidellcc w¡th 148Ce (Ref. [9]).

fllnclion of spin and N and Z in addilion lo lhe fission proeess ilself. \Ve have jllsl begunlo look al lhese dala lh('orelieally in order lo idenlify lhe differenl slrllctllres involved.There also is considera.ble lIlore illfoflllatioll illlcutling; lIle idcntification of additional new¡solapes in tite prcscnt data sets. \Vc are contilllling our analysis. Also, heavy.ioll inducedfission shollld provide an I'VPn wider mass range of fission fragmenls for 5ludy. Our INELcollahoralors have prepared, for lhe firsl lime, a 2421'11 SF souree (essenlially lhe world'ssllpply) whieh will be rnn al 1111m F soon. This SOllree, wilh ils differenl mass dislribulion,shollld yield addilional valnable insighls.

ACI\NO\VLEDGEtvl ENTS

\York al Vanderhill Universily and ¡NEL are sllpporled by lhe U.S. Deparlmenl ofEnergy IInder granl No. DE-FGO!j-88ER40.107 and conlrael No. DE-AC07-7GIDOI570,respeelively. The work al ORNL is sllpporled by I>larlin Mariella Energy Syslems IInderconlrarl No. DE-ACO!j-8,10H21400 for U.S. Deparlml'nl of Energy. L. K. Pl'kl'r is in lheNllrll'ar Dala Corollp al llrookhaven Nalional Laboralory. Parl of lhe analysi, was donehy S, Zhll al Tsi"ghlla U"iversily.

IlIGIIER SI'IN STATES IN NEUTRON RICII NUCLEI 65

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