NUCLIDES FAR OFF THE STABILITY LINE AND SUPER-HEAVY NUCLEI IN HEAVY-ION NUCLEAR REACTIONS

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NUCLIDES FAR OFF THE STABILITY LINE AND SUPER-HEAVY NUCLEI IN HEAVY-ION NUCLEAR

REACTIONS

Marc Lefort

To cite this version:

Marc Lefort. NUCLIDES FAR OFF THE STABILITY LINE AND SUPER-HEAVY NUCLEI IN

HEAVY-ION NUCLEAR REACTIONS. Journal de Physique Colloques, 1972, 33 (C5), pp.C5-73-C5-

102. �10.1051/jphyscol:1972507�. �jpa-00215109�

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JOURNAL RE PHYSIQUE C o L I o q u e C 5 , s u p p l e m e n t au no

8-9,

Tome 33, A o i i t - S e p t e m b r e 1972, p a g e

C5-73

NUCLIDES FAR OFF THE STABILITY LINE AND SUPER-HEAVY NUCLEI I N HEAW-ION NUCLEAR REACTIONS

b y Marc L e f o r t

Chimie N u c l e a i r e - I n s t i t u t de Physique Nucleaire-ORSAY, France

A b s t r a c t

A review i s given on t h e new species which have been produced i n t h e r e c e n t y e a r s by heavy i o n r e a c t i o n s , m a i n l y 12C, 160, ''0, 2 2 ~ e and 2 0 ~ e ions.

The f i r s t s e c t i o n i s devoted t o t h e f o r m a t i o n o f neutron r i c h e x o t i c l i g h t n u c l e i and t o t h e mecha- nism o f m u l t i n u c l e a r t r a n s f e r r e a c t i o n s r e s p o n s i b l e f o r t h i s formation. I n t h e second s e c t i o n d e a l i n g on medium atomic numbers a b r i e f account i s made on t h e d i s c o v e r y o f p r o t o n r a d i o a c t i v i t y and some com- ments a r e made on t h e d i f f i c u l t i e s encountered f o r producing v e r y neutron d e f i c i e n t isotopes 1 i ke ' ^!:~n.

The study o f t h e r e g i o n o f neutron d e f i c i e n c y f o r r a r e e a r t h i s one o f t h e most f r u i t f u l f i e l d f o r heavy i o n r e a c t i o n s . More than f i f t y new i s o - topes have been found. A d i s c u s s i o n i s made on t h e p o s s i b i l i t y f o r going f u r t h e r i n t o t h e neutron d e f i c i e n c y open by t h e use o f Ar, Kr and h e a v i e r ions. I t i s shown t h a t t h e r e i s no advantage t o bombard t a r g e t s w i t h i o n s h e a v i e r than "'Ca o r '%Ni,

Section 4 i s devoted t o heavy n u c l e i i n t h e r e g i o n o f t h e neutron s h e l l N = 126. F i s s i o n compe- t i t i o n i s discussed as w e l l as t h e l i m i t a t i o n o f t h e compound nucleus cross s e c t i o n due t o h i g h angular momentum e f f e c t s . Results a r e presented on t h e r o t a - t i n g l i q u i d drop and t h e v e r y s t r o n g d i m i n u t i o n o f f i s s i o n b a r r i e r s due t o r o t a t i o n a l energy. A b r i e f r e v i e w i s made o f t h e new decay f a m i l i e s discovered by Hyde e t a ? . t631 f o r l i g h t thorium and p r o t a c t i - nium isotopes, These r e s u l t s a r e one o f t h e g r e a t success o f heavy-ion induced r e a c t i o n s .

Section 5 reminds t h a t a l l isotopes o f e l e - ments beyond Z = 101 have been produced b y heavy- i o n r e a c t i o n s , i n s p i t e of t h e g r e a t f i s s i o n compe- t i t i o n .

The 1 a s t s e c t i o n describes t h e v a r i o u s

attempts a l r e a d y made f o r t h e s y n t h e s i s o f super- heavy elements. A d i s c u s s i o n i s presented on t h e f o l l o w i n g problems : r e a c t i o n thresholds and coulomb b a r r i e r s f o r h e a v i l y charged p r o j e c t i l e s , complete f u s i o n cross s e c t i o n as compared t o t h e t o t a l c r o s s section, main decay channels f o r e x c i - t e d compound n u c l e i i n t h e r e g i o n Z = 118,

N = 184-190, f u s i o n - f i s s i o n r e a c t i o n s and t h e p o s s i b i l i t y o f p r o d u c t i o n o f S.H. elements as f i s s i o n fragments, g r a z i n g r e a c t i o n s and exchange o f b i g

apsregates

.

Resume

Noyaux l o i n de l a S t a b i l i t e e t Noyaux Super-lourds dans 1es Reactions Nucltiaires p a r Ions tourds.

Cet expose c o n s i s t e en une revue s u r 1es nomt.reux nouveaux noyaux p r o d u i t s depuis quelques annees p a r l e s r e a c t i o n s par ions lourds, e t en une d i s c u s s i o n des p o s s i b i l i t e s ouvertes par l e s faisceaux d ' i o n s p l u s l o u r d s 'OAr, "Ca, 84Kr, '36Xe e t c . . . Les t e n t a t i v e s de synthese de noyaux superlourds sont d e c r i t e s brievement.

Dans l e premier paragraphe, on i n s i s t e sur- t o u t s u r l e s noyaux r i c h e s en neutrons depuis l e bore jusqu'a l ' a r g o n , p r o d u i t s g r t c e ii des t r a n s - f e r t s d ' u n grand nombre de neutrons depuis des c i b l e s lourdes vers l e p r o j e c t i l e . I1 e s t p o s s i b l e d'approcher l a l i m i t e d ' e x i s t e n c e des noyaux oil l ' e n e r g i e de l i a i s o n du neutron d e v i e n t n u l l e . Le second paragraphe t r a i t e des noyaux moyens pour l e s - quels l e s seuls e s s a i s ayant donne quelques r e s u l - t a t s o n t pennis de t r o u v e r que l e s t e l l u r e s t r e s l e g e r s (107, 108) e t a i e n t emetteurs alpha e t de d @ c o u v r i r l a r a d i o a c t i v i t e p a r p r o t o n s u r l e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972507

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c o b a l t 53. Les p o s s i b i l i t G s de f o r m a t i o n de noyaux t r e s d e f i c i e n t s en neutrons t e l s '!:~n ou t:Zr s o n t etudiees.

Des c a l c u l s s o n t presentes q u i montrent que meme s i des noyaux compos@s r e l a t i v e m e n t l e g e r s peuvent & r e p r o d u i t s , l e u r desexci t a t i o n a 1 ie u e s s e n t i e l l e m e n t p a r emission de p a r t i c u l e s chargees e t q u ' i l e s t t r e s improbable d ' a t t e i n d r e l a zone N = Z.

La t r o i s i e m e p a r t i e du memoire S t u d i e l e domaine des t e r r e s r a r e s dans l e q u e l l e s r e a c t i o n s par i o n s l o u r d s o n t f a i t d e c o u v r i r p l u s de 50 nou- veaux isotopes emetteurs a p a r t i e l l e m e n t pour l a

~ l u ~ a r t . La couche N = 82 e s t a i n s i a ~ ~ r o c h e e e t

L ' e x i s t e n c e d'une sous couche 1 164 neutrons l a i s s e quelques e s p o i r s de p r o d u i r e des isotopes r i c h e s en neutrons 1 durges de v i e encore acceptablespour Z = 105 e t Z = 106.

E n f i n , l a d e r n i e r e p a r t i e e s t consacree aux noyaux Superlourds. Apres quelques mots s u r l a s i - t u a t i o n t r e s o p t i m i s t e des p r e v i s i o n s theoriques concernant l a s t a b i l i t e des noyaux 1 l ' e t a t fonda- mental autour de Z = 114 e t N = 184, l e s problemes poses par l e s chances d ' a t t e i n d r e ces noyaux p a r l e s r e a c t i o n s n u c l e a i r e s connues sont d i s c u t e s .

1 " ) La b a r r i e r e de p o t e n t i e l pour l ' a p p r o c h e d ' u n p r o j e c t i l e l o u r d a e t @ mesuree e t se r e v e l e p l z s elevee que pour l e s i o n s p l u s 16gers f r o = 1,32 au

. .

, ,

on peut raisonnablement e s ~ e r e r au moven des i o n s 1 ie u de ro = 1,45-1,50).

"Ca ou ' ^{O A r} p r o d u i r e des i s o t o p e s encore p l u s 2") La f u s i o n complete conduisant 1 un noyau compo- l e g e r s dans l a r e g i o n de noyaux t r e s d&formGs se p l u s l o u r d que l ' u r a n i u m semble t r e s d i f f i c i f e Z = 58-70,N = 70-82. L ' u t i l i s a t i o n d ' i o n s p l u s sinon impossible pour des p r o j e c t i l e s de Z s u p e r i e u r l o u r d s que ::~i ne p a r a f t p r e s e n t e r aucun evantage. ¹ 20. Ceci apparaTt a t r a v e r s c e r t a i n s r e s u l t a t s

o r e l i m i n a i r e s obtenus avec l e s i o n s Kr e t se t r o u v e Les noyaux l o u r d s de N p l u s ou moins e l o i g n e justifie par llanalyse de ltinfluence de

de N = 126 sont e n s u i t e e t u d i e s . Dans c e t t e r e g i o n , de rotation lEnerqie N Y r a s t ~ ) et des barrieres de l a c o m p e t i t i o n de l a f i s s i o n d e v i e n t importante e t

l i m i t e l e s s e c t i o n s e f f i c a c e s . De plus, l ' e n e r g i e de r o t a t i o n elevee i n t r o d u i t e par l e s grands mo- ments a n g u l a i r e s p r o j e c t i l e s l o u r d s c o n d u i t a

abaisser l a b a r r i e r e de f i s s i o n e t 1 rendre impos- s i b l e l a f o r m a t i o n de noyau compose pour l e s ondes p a r t i e l l e s de l ' h eleve. Oes v a l e u r s de aCF/aR ,

r a p p o r t de l a s e c t i o n e f f i c a c e de f u s i o n complete 1 l a s e c t i o n t o t a l e de r e a c t i o n s o n t c a l c u l e e s e t comparees 1 des r e s u l t a t s experimentaux obtenus avec l e s i o n s Ar e t Kr. E n f i n , dans c e t t e r e g i o n des elements Rn a Pa, l e p l u s grand succes des r e a c t i o n s par i o n s l o u r d s C , N, 0, Ne a e t @ l a decouverte de nombreux isotopes l e g e r s . De nouvel- l e s f a m i l l e s a d ~ c r o i s s a n c e s en chaine o n t e t e mises en evidence p a r Hyde e t ses c o l l a b o r a t e u r s

1631, 1 p a r t i r d ' i s o t o p e s l e g e r s de thorium e t de p r o t a c t i n i u m .

Dans l a 5e p a r t i e , on r a p p e l l e que tous l e s isotopes des transmendeleviens Z > 101 o n t ete mis en evidence grace aux r e a c t i o n s i n d u i t e s s u r des c i b l e s de 238U, 2 3 9 P ~ 9 2 4 Z P ~ , **248Cms * " C f** par des i o n s de bore, carbone, oxygPne on neon, e t c e c i malgre de f a i b l e s s e c t i o n s e f f i c a c e s i n f e r i e u r e s 1 1 microbarn en r a i s o n de l ' i m p o r t a n c e de l a f i s s i o n .

-

f i s s i o n pour une g o u t t e l i q u i d e en r o t a t i o n . 3O) La d e s e x c i t a t i o n d'eventuels noyaux compos@s d e v r a i t a v o i r l i e u p a r emission alpha e t permettre d ' a t t e i n d r e l e s noyaux superlourds 1 v i e longue.

Les remarquables donnees de ~ i x ~ l ~ l ) s u r c e t t e q u e s t i o n s o n t cornentees e t quelques r e a c t i o n s favorables indiquees. On mentionne l e s r e s u l t a t s obtenus j u s q u ' i c i pour l e s t e n t a t i v e s e f f e c t u e e s 1 Orsay ( * ' ~ r + ~ ~ ~ T h ) e t (84Kr+208Pb), r e s u l t a t s nega t i f s malgre l a mise en oeuvre de moyens de detec- t i o n elabores pour l e s masses autour de A = 300.

Le choix de p r o j e c t i l e s 82Se, 76Ge ou ?'Ga s e r a f t peut-&re m e i l l e u r .

Enfin, d ' a u t r e s p o s s i b i l i t e s que l e passage p a r noyau compose s o n t indiquees. Les experiences avec l e f a i s c e a u de xenon f a i t e s 5 Dubna s o n t men- tionnees c o m e approche de l a methode d i t e " f u s i o n - f i s s i o n " q u i c o n s i s t e 1 p r o d u i r e un enorme noyau de f u s i o n A = 400 e t esperer que parmi l e s f r a g - ments de f i s s i o n de ce noyau, des elements super-

l o u r d s s o i e n t crees. Les t r a n s f e r t s de gros agrP-

g a t s s o n t une a u t r e p o s s i b i l i t @ pour l a q u e l l e l e

manque d ' i n f o r m a t i o n p r e c i s e empPche de f a i r e des

p r e v i s i o n s .

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SUPER-HEAVY

NUCLEI . . .

C5-75

INTRODUCTION

The f i r s t n u c l e a r r e a c t i o n s induced by heavy i o n s were observed more than twenty years ago. I n 1949, carbon i o n s were a c c e l e r a t e d i n t h e 152cm Berkeley c y c l o t r o n and a low i n t e n s i t y beam was used b y Hamil ton e t a1 [I). These authors showed t h a t 3 4 ~ 1 was produced from aluminium t a r - gets and 2 0 5 ~ t from gold. Some years l a t e r , heavy i o n i n v e s t i g a t i o n s have extended a t a g r e a t r a t e and new isotopes were discovered although t h e reac- t i o n mechanisms were n o t e n t i r e l y explained. For example, Hollander (2,3) was able t o produce a new bromine isotope, 7 4 ~ r , by bombarding copper w i t h

12c

ions, and a new p a i r of i s o b a r s 1 2 6 ~ a and

'"CS

by bombarding indium w i t h "N. A t Saclay f 42 t h e bombardment o f copper by oxygen succeeded i n p r o - ducing 7 9 ~ b , and a t Birmingham "'TI and l g 3 ~ 1 were discovered c 5 ) i n the i r r a d i a t i o n products o f t h e r e a c t i o n "N on tungsten.

A g r e a t deal of work has been done b e t - ween 1950 and 1960, p a r t i c u l a r l y i n t h e s y n t h e s i s o f new t r a n s u r a n i c elements. As e a r l y as 1951,

1 2 c

induced r e a c t i o n s on

2 3 8 ~

were c a r r i e d o u t (6) t o produce 2 4 6 ~ f . Because f i s s i o n i s b y f a r t h e most prominent mode of decay o f compound n u c l e i , cross s e c t i o n s f o r t h e products o f (HI,xn) r e a c t i o n s on uranium o r on heavy n u c l e i are very low, o f the o r d e r o f tens o f microbarns. Therefore t h e y cannot compete w i t h neutron captures (nu) and w i t h cumula- t i v e neutron captures ( s o c a l l e d f a s t process) f o r t h e synthesis of elements up t o Z

=

98 o r 99.

However t h e possi b i 1 i t y o f jumping many u n i t s o f atomic number i n t h e r e a c t i o n was under- stood as an a t t r a c t i v e f e a t u r e f o r t h e p r e p a r a t i o n o f isotopes a t t h e upper f i m i t o f t h e transuranium elements, p a r t i c u l a r l y above 2 = 100. Moreover, heavy i o n bombardment i s the unique way a t the present t i m e f o r t h e s y n t h e s i s o f transmendelevium elements, s i n c e t h e h i g h e s t Z number obtained i n underground thennonucl e a r explosions i s Z, = 100.

appears t o be a l i m i t t o the process o f s y n t h e s i s by neutron f l u x e s o f very h i g h d e n s i t y .

Then, t h e o n l y p o s s i b i l i t y i s t o take t h e h e a v i e s t a v a i l a b l e t a r g e t s , c u r i urn, berkelium, c a l i f o r n i u m and e i n s t e i n i um, and bombard them w i t h heavy ions. Pionners i n t h i s challenge were t h e Nobel I n s t i t u t e i n Stockholm where A t t e r l i n g e t a1 ( 9 1 made t h e synthesis o f 250Fm w i t h 160, t h e Kurchatov I n s t i t u t e i n Moscow and l a t e r t h e labo- r a t o r y f o r n u c l e a r r e a c t i o n s headed by G.N. F l e r o v i n Dubna (101, and t h e Lawrence R a d i a t i o n Labora- t o r y i n Berkeley, where Mendelevium, element 1 0 1 was f i r s t produced (11) by bombarding l o 9 atoms o f 253Es w i t h p a r t i c l e s , and elements 102 (Nobelium) and 103 (Lawrencium) were made s y n t h e t i c a l l y (123 by bombarding 3 ug o f c a l i f o r n i urn w i t h ' ^'B ^and

^'OB.

Between 1958 and 1971, many experiments were car- r i e d o u t i n Dubna and Berkeley, and new elements 102 t o 105 were discovered w i t h various combina- t i o n s o f p r o j e c t i l e s and t a r g e t s [13,14,15).

Nowadays a l a r g e number o f new isotopes f a r from t h e b e t a s t a b i l i t y l i n e are b e i n g synthe- t i z e d o v e r a l l t h e c h a r t o f isotopes, mainly by two methods o f p r o d u c t i o n : s p a l l a t i o n r e a c t i o n s indu- ced by e n e r g e t i c protons o r alpha p a r t i c l e s , and heavy i o n induced r e a c t i o n s . I n several cases, b o t h t e c h n i c s have been used t o produce neutron d e f i c i e n t n u c l e i , i n t h e r a r e e a r t h r e g i o n f o r instance.

However t h e r e a r e s p e c i f i c cases where (HI ,xn) r e a c t i o n s o r mu1 t i n u c l e o n t r a n s f e r r e a c t i o n s a r e a t t h e present t i m e t h e o n l y p o s s i b i l i t i e s f o r t h e synthesis o f very l i g h t o r very heavy isotopes. On t h e c h a r t o f isotopes, around two hundred new spe- c i e s produced i n t h e l a s t decade by heavy i o n reac- t i o n s are shown on f i g u r e s 1, 6, 7, 11, 12 and 19.

For each p a r t i c u l a r area we s h a l l comnent more spe- c i f i c a l l y which problems a r i s e i n t h e s y n t h e s i s and which are t h e p o s s i b i l i t i e s f o r l o o k i n g f o r unknown, bound n u c l e i and f o r t h e hope o f f i n d i n g e x o t i c n u c l e i .

The amount o f 2 5 7 ~ m r e c o v e w d i n 1969 from "Hutch"

l o o I. NEUTRON RICH EXOTlC LIGHT NUCLEI

experiment (71 was l a r g e enough t o serve as a t a r -

g e t m a t e r i a1 f o r i r r a d i a t i o n . However a1 though t h e Some years ago, i t was b e l i e v e d t h a t

y i e l d o f v e r y heavy n u c l e i was very much improved o n l y neutron d e f i c i e n t n u c l e i c o u l d be formed a f t e r

compared t o t h e "Cyclamen" e x p e r i m n t (83 of 1967, heavy i o n r e a c t i o n s . I t has been one o f t h e g r e a t

no i s o t o p e h e a v i e r than 2 5 7 ~ ~ Was discovered. There i n t e w s t o f t h e work made by V01

~ O V

and h i s c01-

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C5-76

M. LEFORT

l a b o r a t o r s 1 1 8 1 t o show t h a t t h i s was n o t t r u e . Mg, A l , S i , P, S and C1. The p r o d u c t i o n o f neutron M u l t i n u c l e o n t r a n s f e r r e a c t i o n s may produce neutron

r i c h l i g h t n u c l e i by t h r e e processes, i) p i c k up o f a number o f neutron from t h e t a r g e t , ii) s t r i p p i n g o f protons t o t h e t a r g e t , i i i ) complex exchange o f nucleons l e a d i n g t o a ' l i g h t p r o d u c t r i c h e r i n neu- t r o n s than t h e p r o j e c t i l e ( p r o j e c t i l e - x protons

+ y neutrons).

It was shown t h a t t h e y i e l d o f a g i v e n t y p e of t r a n s f e r i s governed e s s e n t i a l l y by t h e Q value o f t h e r e a c t i o n . Several authors (16,171 have observed t h i s e f f e c t and d e r i v e d an e m p i r i c a l r e l a t i o n s h i p between t h e cross s e c t i o n and t h e energy balance EC

^t

AM)^^, where aEc i s t h e change i n coulomb energy and 4Y t h e mass balance between t h e i n i t i a l s t a t e and t h e f i n a l s t a t e . When c o n s i d e r i n g t h e p i c k up process o f a l a r g e number o f neutrons, f a v o r a b l e c o n d i t i o n s occur when t h e h e a v i e s t elements are used as t a r g e t s , s i n c e t h e b i n d i n g energy o f neutrons becomes l a r g e r i n neutron r i c h l i g h t elements than i n t h e heavy t a r g e t nucleus.

Using 2 3 2 ~ h as a t a r g e t , Volkov (18') and h i s c o l l a b o r a t o r s have bombarded w i t h ''0, 2 2 ~ e ,

1 5 ~

and ''6 i o n s . They were able t o i d e n t i f y and t o measure t h e p r o d u c t i o n y i e l d f o r a number o f a l r e a d y known e x o t i c n u c l e i : ' ~ e ,

ll~i,

12Be, 156,

17c, 19N, 2100

2 2 ~

and 2 4 ~ e . They measured much l a r g e r y i e l d s than by t h e fragmentation process through which these n u c l e i had been formed pre- v i o u s l y by bombardment o f heavy n u c l e i w i t h GeV protons. Cross s e c t i o n s were estimated between 0 , l and 100 mb. I n a d d i t i o n , new species were found, much c l o s e r from t h e boundary f o r t h e p a r t i c l e s t a - b i l i t y w i t h l a r g e neutron excess. ''c,

^19c, ^{2 0 ~ ,}

2 1 ~ ,

220, 230, 240,

2 3 ~ , 2 4 ~ , 2 5 ~ ,

2 5 ~ e , 2 6 ~ e were i d e n t i f i e d . It was a l s o very c l e a r l y demonstrated t h a t 1 ° ~ e , 13Be and 146e a r e p a r t i c l e unstable.

Very r e c e n t experiments made 1191 w i t h 290 MeV argon i o n s g i 2 ~ h a r e even more promising.

Because o f t h e l a r g e r b i n d i n g energies o f neutrons i n t h e v i c i n i t y o f 4 0 ~ r , cross s e c t i o n s f o r neu- t r o n p i c k up a r e r a t h e r l a r g e . I n one s e t o f expe- riments, 17 new e x o t i c n u c l i d e s have been i d e n t i - f i e d by Volkov e t a1 [19], i n t h e Z r e g i o n between 12 and 17. The r e p r o d u c t i o n o f t h e c h a r t o f i s o - topes i n t h i s area ( f i g . 1) shows t h e new species o f

r i c h n u c l i d e s f a r away from t h e s t a b i l i t y opens t h e p o s s i b i 1 i t y f o r a v e r i f i c a t i o n o f t h e t h e o r e t i c a i boundary f o r t h e p a r t i c l e s t a b i 1 i t y and more gene- r a l l y o f the n u c l e a r models and mass formula. For a

F i g u r e

1

: C h a r t o f i s o t o p e s f o r l i g h t n u c l e i . - B o u n d a r i e s f o r s t a b i l i t y a r e drawn a f t e r

Garvey ( 2 0 ) .

- S t a b l e i s o t o p e s a r e shown i n shaded a r e a . - N e u t r o n r i c h i s o t o p e s p r o d u c e d w i t h heavy i o n r e a c t i o n s a r e i n d i c a t e d by u n d e r l i n e d s q u a r e s .

nucleus w i t h l a r g e neutron excess, more s i n g l e psr- t i c l e l e v e l s o f h i g h energy must be occupied than f o r t h e same A nucleus w i t h N - Z

=

0. Whether these h i g h l y i n g l e v e i s a r e bound o r unbound i s an open question.

The boundary r e g i o n c a l c u l a t e d by Garvey 1201 has been reached i n t h e r e g i o n Z

=

2 t o Z

=

5 and perhaps a t Z

=

11. ( F i g . 1 ) . F o r l a r g e r Z, t h e r e i s s t i l l a gap o f several neutrons between t h e p r e - s e n t l y known n u c l i d e s and t h e boundary. F o r example 3 6 ~ i i s a t 10 neutrons from t h e l i m i t . Cross sec- t i o n s f o r 240 o r

2 1 ~

a r e a l r e a d y r a t h e r low (some m i c r ~ b a r n s ) and i t i s d i f f i c u l t t o p r e d i c t t h a t h e a v i e r i s o t o p e s up t o 280 m i g h t be formed w i t h a s u f f i c i e n t y i e l d f o r t h e a c t u a l d e t e c t i o n e f f i c i e n c y . However i t i s known t h a t m u l t i n u c l e o n t r a n s f e r reac- t i o n s e x h i b i t cross s e c t i o n s which increase w i t h t h e energy, s i n c e a compound nucleus can o n l y be formed w i t h a l i m i t e d angular momentum (21,22] . ^A ^{1 arge}

number o f p a r t i a l waves c o n t r i b u t e t o d i r e c t pro-

(6)

SUPER-HEAVY NUCLEI . . .

^C5-77

cesses, which are no longer only surface reactions.

Therefore, one might expect t h a t higher bombarding energies f o r 2 2 ~ e and

' O A ~

induce a great deal o f many complex t r a n s f e r reactions.

It seems a l s o possible t o produce neu- t r o n r i c h n u c l e i f o r elements heavier than Z = 18, now p r o j e c t i l e s l i k e * ' + ~ r are available. For exam-

3 6

p l e r e s u l t s have been obtained (233 a t energies between 450 and 500 MeV i n which 232Th t a r g e t nu- c l e i have l o s t 7 o r 8 neutrons. Cross sections are above lmb and i t i s reasonable t o believe t h a t se- veral c f these neutrons have been captured by t h e p r o j e c t i l e .

For e x o t i c n u c l e i heavier than mass 40, a d i f f i c u l t technical problem i s c e r t a i n l y the mass assignment, P a r t i c l e i d e n t i f i e r s cannot work e f f i - c i e n t l y w i t h a good r e s o l u t i o n f o r l a r g e masses, i f they are based on A E - E measurements. However progress made on the r e s o l u t i o n f o r time measure- ments pemSt t o b e l i e v e t h a t the time o f f l i g h t d i s c r i m i n a t i o n w i l l be a good i d e n t i f i c a t i o n technic.

On the other hand, accurate measurements o f the k i n e t i c energies and angular d i s t r i b u t i o n s o f t h e product open the p o s s i b i l i t i e s f o r determi- n i n g Q values and then masses f o r t h e new isotopes.

This has been done (243 only i n very few cases, l i k e f o r 210 and 220, and t h e w i s a l a r g e open f i e l d .

11. INTERMEDIATE MASSES. PROTON AN0 ALPHA EMITTERS.

LIMITATIONS ON

...

THE PRODUCTION OF VERY NEUTRON DEFICIENT NUCLEI.

I n the region o f medium atomic numbers, neutron r i c h nuclides can be produced as f i s s i o n fragments w i t h a much b e t t e r y i e l d than by any heavy i o n reaction. Therefore the only attempts t o use heavy i o n reactions f o r producing new species i n t h i s region were made i n order t o obtain neutron d e f i c i e n t nuclides and p a r t i c u l a r l y t o look f o r proton r a d i o a c t i v i t y , and beta delayed proton emi ssiot-i.

Although beta delayed proton emitters can a l s o formed as s p a l l a t i o n products, many o f them have been obtained e i t h e r i n heavy i o n trans- f e r reactions 1251 l i k e 17Ne, o r i n (HI ,xn) reac-

t i o n s 126') l i k e 9 6 R ~ ( 160,4n) lo8Te. Proton radio- a c t i v i t y has been discovered (27) f i r s t i n 1970 on an isomer o f 53Co, which was the r e s u l t o f the reac- t i o n '+OCa (160,2np) 5 3 ~ o m . I t was checked t h a t no cor n c i dence appeared w i t h any p o s i t r o n decay and therefore 5 3 ~ d n was assumed t o be a proton e m i t t e r .

A new region o f alpha r a d i o a c t i v i t y around masses 100 has been found c281 by MacFarlane, w i t h 9 6 R ~ (160,~n) 12-'~e reactions. f sotopes 107 and 108 o f t e l l u r i u m are alpha e m i t t e r s (energr'es o f 3.28 and 3.08 MeV).

An important problem t o be studied i n nuclear spectroscopy i s the i s o s p i n invariance o f nuclear forces. Medium nuclei w i t h N = Z which are f a r from t h e s t a b i l i t y l i n e are o f p a r t i c u l a r i n t e - rest, because i t i s desirable t o determine what are t h e important c o r r e l a t i o n s among the nucleons f o r N = Z and t h e p a i r i n g e f f e c t s . P a i r i n g correla- t i o n s could be observed d i r e c t l y by p a i r i n g vibra- t i o n a l o r r o t a t i o n a l spectra. I t would be i n t e r e s - t i n g t o have some experimental knowledge o f t h e isospin s t r u c t u r e o f N = Z odd-odd nuclei. Does T = 1 p e r s i s t as t h e ground s t a t e as i t i s f o r l i g h t n u c l e i ?

According t o the Garvey's mass formula, some o f the n u c l e i i n t h e region o f ':;~n should be s t a b l e against s i n g l e proton emission, but unstable against double proton emission. P a i r i n g e f f e c t t r a n - s i t i o n s may be observed i n double proton emission.

Also i n the region N = Z

=

40, there i s the open question t o know i f 40 remains a magic number f o r protons.

For the above motivations, several pro-

posals have been made t o use heavy ions f o r produ-

cing e x o t i c nuclei l i k e ::~r o r ';:~n. The fusion

o f a t a r g e t and p r o j e c t i l e nucleus tends always t o

produce a neutron d e f i c i e n t compound nucleus, due

t o the increasing neutron excess o f heavier nuclei'

along the v a l l e y o f beta s t a b i l i t y . These compound

systems sharing a l a r g e amount o f e x c i t a t i o n energy,

a number o f p a r t i c l e s should be evaporated. A t a

f i r s t glance, i t was thought t h a t the coulomb bar-

r i e r should i n h i b i t the emission o f charged p a r t i -

c l e s and t h a t i t would r e s u l t i n a strong prefe-

rence f o r evaporating neutrons. This tends t o pro-

duce s t i l l more neutron-deficient products. However,

(7)

C 5 - 7 8

M.

LEFORT

p a r t i c u l a r l y i n t h e r e g i o n Z

=

30, Z = 50, t h i s i s n o t t r u e .

I n h a y , we have shown (29) t h a t i n the ' 2 3 ~ e compound nucleus e x c i t e d a t 60 MeV and r e s u l - t:ig from t h e r e a c t i o n '!Pg

^t

'IfN, most o f t h e eva- porated p a r t i c l e s were protons and alpha p a r t i c l e s , i n good agreement w i t h t h e o r e t i c a l p r e d i c t i o n s . The same conclusions have been obtained very r e c e n t l y

[30] on t h e evaporated p a r t i c l e y i e l d s i n t h e reac- t i o n (77Se + ::Ar) f o r which t h e compound nucleus i s ' k z ~ e . F o r

3 4

a c e n t e r o f mass energy o f 140 MeV, t h e cross s e c t i o n f o r alpha p a r t i c l e s was found a t 840 mb and f o r protons 854 mb, t o be compared t o a t o t a l r e a c t i o n cross s e c t i o n c a l c u l a t e d w i t h t h e o p t i c a l model a t 1.7 barn.

More r e c e n t l y , Stephens e t a1 ( 3 9 , and Blann e t a1 (32.331 have discussed t h i s q u e s t i o n o f p r e d i c t e d l i m i t s f o r p r o d u c t i o n o f new p r o t o n r i c h n u c l e i . As t h e compound n u c l e i become more and more p r o t o n r i c h , t h e usual predominant decay by neutron evaporation gives way t o p r o t o n and alpha p a r t i c l e evaporation. This can be shown using t h e Weisskopf- Ewing evaporation formalism, where the p r o b a b i l i t y f o r e m i t t i n g a p a r t i c l e j w i t h t h e k i n e t i c energy c , from a compound nucleus w i t h e x c i t a t i o n energy ECN *

may be w r i t t e n n e g l e c t i n g t h e c o n s i d e r a t i o n o f angu- l a r momentum

I

where a j i s t h e l e v e l d e n s i t y parameter, S j t h e s e p a r a t i o n energy o f t h e p a r t i c l e ,

6 j

t h e p a i r i n g energy c o r r e c t i o n .

I n t e g r a t i n g W ( ~ ) d e between t h e k i n e t i c energy t h r e s h o l d f o r t h e p a r t i c l e j,

C j

and t h e maxi- mum energy given by R j = E C N - S j - 6 j - c j , y i e l d s t h e

X

w i d t h r j f o r e m i t t i n g a p a r t i c l e j. And t h e compari- son w i t h neutron emission i s given by :

r . ( 2 s j + l ) m j

f I

A = . ^{R j}

^*

q ^an exp 1 ^2(*-%)1

^{( I}⁾

rn (2Sn+l)mn

^/

where s u b s c r i p t n corresponds t o t h e neutron, and Rn = ECN-Sn-6,,

X

s i n c e c n = 0.

When t h e compound nucleus i s a l r e a d y n e u t r o n - d e f i c i e n t , Rp o r R, becomes l a r g e r than Rn,

d e s p i t e t h e e f f e c t o f cp o r c, because Sp o r S, are much s m a l l e r than Sn. Then, t h e exponential becomes p o s i t i v e and rp/rn o r r,/rn l a r g e r than 1. Such an e f f e c t was u n d e r l i n e d by Bodansky [34) and i l l u s t r a - t e d on f i g u r e 2.

But i n a d d i t i o n o f

wv, t h i s s e p a r a t i o n energy

dependence, t h e use o f heavy i o n s b r i n g s an angular momentum e f -

2

^0'

_-'{. .

^o ^w

0 E R ~

f e c t which has been

0 0 0

discussed w i t h g r e a t

d e t a i l s by Thomas e t

O J

I - M 5 A lor <-d MINI

a1. (351 . Because neu-

t r o n s and protons a r e unable t o t a k e away

F i g u r e 2 : from t h e compound sys-

r n / r a s

a

f u n c t i o n tem l a r g e f r a c t i o n s o f o f N ~ Z ( a f t e r Bodansky

( 3 4 ) ) . .

. .

a n g u l z r momentum, t h e i r emission y i e l d s a r e lowered a t t h e b e n e f i t o f h e a v i e r p a r t i c l e s , and p a r t i c u l a r l y alpha p a r t i c l e s . I t has been shown (363 t h a t the r a t i o (r,/r ) can be r e l a t e d t o

( r a / r n ) J, O c a l c u l a t e d by expression (1)

J~~

where

T

i s t h e n u c l e a r temperature,

J

t h e moment o f i n e r t i a and R t h e r a d i u s o f t h e compound nucleus.

r,/rn r a t i o s have been c a l c u l a t e d as a f u n c t i o n o f JCN and E;~, assuming f o r t h e momentum o f i n e r t i a a r i g i d s p h e r i c a l value. As an example, r,/rn f o r t h e compound system ( 6 3 C ~ + 1 6 0 ) increases from 0.5 a t J

=

o up t o 5 a t J = 40h a t an e x c i t a t i o n energy o f 60 MeV. F i g u r e 3 shows t h a t Tn drops down t o very

1 2 3

low y i e l d s f o r Xe compound n u c l e i when J l i e s between 20 and 30h a t an e x c i t a t i o n energy o f 20Mev.

A r a t h e r crude way t o p l a c e an upper

1 im i t on t h e i n f l u e n c e o f angular momentum

OR

t h e

compound nucleus decay i s t o reduce f o r each p a r t i a l

wave t h e e x c i t a t i o n energy by the c l a s s i c a l r i g i d -

body r o t a t i o n a l energy Erot = J(J+1)/2 . Very i n t e -

r e s t i n g r e s u l t s have been r e c e n t l y o b t a i n e d by

(8)

SUPER-HEA

W

NUCLEI . . . ^C5-79

d e - e x c i t a t i o n . I t i m p l i e s t h a t " i t w i l l be e x t r e m l y u n l i k e l y t h a t l o O S n c o u l d be made w i t h reasonable

n

cross s e c t i o n by any evaporation mechanism". Very heavy p r o j e c t i l e s l i k e T i o r 40Ca do n o t h e l p t h e

a

p r o d u c t i o n p r o b a b i l i t y n e i t h e r f o r

loo

~n n o r f o r

* O

Zr.

J

-

3 0 h

The s i t u a t i o n i s b e t t e r above mass 130 and t h e cross s e c t i o n s f o r a l a r g e number o f e x o t i c n u c l i d e s are p r e d i c t e d t o be f e a s i b l e f o r spectros- copy i n t h e r e g i o n o f r a r e e a r t h t h a t we are going t o consider now.

10 20 30 40 50

E ( M e V )

111. NEUTRON DEFICIENT NUCLIDES I N THE RARE EARTH Figure 3

:

Percentage of p,n,a and

y

emission in REGION. ALPHA EMITTERS AN0 DEFORMED NUCLEI.

the de-excitation of the compound nucleus

'23xe.

The r a r e e a r t h r e g i o n c o u l d be

- - - .

a) As a function of J%at E*

=

20 MeV.

b) As a function of E at J

=

30h.

Figure 4

:

Contour diagram for maximum yields following decay o f compound nuclei.

The numbers are log for the maximum yields in units o f mb (after M. Blann et a1 (33))

Marshall Blann (33) i n several regions o f t h e c h a r t o f isotopes. A few cases are presented here w i t h h i s gracious permission. They show f o r t h e case o f medium n u c l e i ( f i g . 4) t h a t xp emission i s t h e dominant evaporation made i n t h e e a r l y stages o f

d e f i n e d as t h e p r e d i l e c t i o n area f o r (H1,xn) reac- t i o n s , because t h e compound nucleus deexcites predominantly by neutron evaporation. The f i s s i o n process w i d t h i s s t i l l very low and the Coulomb b a r r i e r i s l a r g e enough t o i n h i b i t s t r o n g l y charged p a r t i c l e evaporation.

Reactions have been induced by

l 2 ~ , " + ~ ,

15N, l G O , 180, 19F, 2 0 ~ e , 2 7 . ~ e on many t a r g e t s b e t - ween Z = 50 and Z = 72. Cross bombardments have been used i n o r d e r t o check t h e i d e n t i f i c a t i o n o f r e s i - dual n u c l e i which were produced by several t a r g e t - p r o j e c t i l e combinations. A r a t h e r good knowledge o f e x c i t a t i o n f u n c t i o n s f o r (HI ,xn) r e a c t i o n s w i t h x between C and 8 o r 10 has been acquired. F o r example Toth and Hahn $373 have bombarded w i t h

1 4 ~

1 4 " ~ d and they assumed t h a t i n (14N,6n) and (14N,7n) isotopes

S2Ho and 151Ho were produced. Furthermore, they observed e x c i t a t i o n f u n c t i o n s o f t h e same alpha rays by bombarding 14*Nd a t lower energies where ( 1 4 ~ , 4 n ) and ( 1 4 ~ , 5 n ) are t h e m s t probable r e a c t i o n s . This knowledge o f t h e p o s i t i o n o f e x c i t a t i o n f u n c t i o n s was a g r e a t h e l p t o d i s c o v e r new h i g h s p i n isomers i n l53H0 and "Ho.

A t t h e maximum o f t h e e x c i t a t i o n f u n c t i o n

cross s e c t i o n s l i e around several hundreds o f mb f o r

x l o w e r than 6. L i g h t e r isotopes a r e produced w i t h

cross s e c t i o n s l o w e r and the decrease depends on how

f a r t h e n u c l i d e s are from the b e t a s t a b i l i t y . As an

example, f i g u r e 5 shows e x c i t a t i o n f u n c t i o n s (38)

drawn from x = 3n up t o x

=

12n f o r t h e r e a c t i o n

1 3 0 ~ e ( 1 2 ~ , x n ) ' 4 2 - x ~ e . I3Oce, which i s a t 10 masses

(9)

C 5 - 8 0 M. LEFORT

u n i t s from t h e s t a b i l i t y , i s produced w i t h a cross s e c t i o n o f t h e o r d e r o f 20 mb.

1 ' " " " " ~

F i g u r e 5

:

E x c i t a t i o n f u n c t i o n s f o r t h e r e a c t i o n s 1 3 c ~ e ( ' 2 ~ , x n ) 4 2 - x ~ e . ( A f t e r Oganecian e t al.)

Since t h e compound system i s always l i g h t e r than t h e b e t a s t a b i l i t y mass o f 2 t o 6 u n i t s , when various t a r g e t s are bombarded w i t h '%,

14N, 160, "F o r 2 0 ~ e , i t has been p o s s i b l e t o reach neutron d e f i c i e n t r a r e e a r t h n u c l e i s l i g h t l y h e a v i e r than t h e 82 neutron closed s h e l l , i n o r d e r t o observe t h e expected increase o f alpha decay ener- gies. This was very n i c e l y done i n 1964 by R.D.

Macfarlane ( 3 9 . 'Z:~rn (N

=

84),

1

" ~ m (N

=

85), '::~b (N = 84) were produced w i t h cross s e c t i o n s o f t h e o r d e r o f 50 mb by bombardments o f 141pr w i t h

2.0

Ne ('ONe,8n), o f 1 4 2 ~ d ("F, 7n), and o f 14'sm

F i g u r e 6 : C h a r t of i s o t o p e s i n t h e r e g i o n of l i g h t r a r e e a r t h . New Neutron d e f i c i e n t i s o t o p e s a r e shown on t h e l e f t s i d e .

w i t h oxygen. Twenty new alpha e m i t t e r s have been discovered ( F i g . 6) f o r Z between 65 and 72, j u s t above t h e 82 neutron she1 1 , by Macfarl ane e t a1 [40 1,

Ho (151,152,153) E r (152,153,154) Tm (153,154), Yb (154,155) Lu (155,156) H f (157,158), and by Hahn, Toth e t a1 137,443 , Ho (150), Tm (155,156), E r (151), Yb (156,157). I n some cases t h e (H1,xn) r e a c t i o n r e s u l t s were checked by ('He,xn) r e a c t i o n s as f o r example 1 6 2 ~ r ( 3 ~ e , 9 n ) 1 5 6 ~ b . The new species disco- vered have h a l f - l i v e s i n t h e range around 1 minute although t h e l e s s s t a b l e decay i n a f r a c t i o n o f secund.

A very convenient method f o r c o l l e c t i n g r e c o i l i n g n u c l e i and measuring

a

rays from t h e i r decay, i s t h e helium j e t technique which has been developed by Macfarlane e t a1 c42], Hyde e t a1 143) and r e c e n ~ l y i n Orsay by Le Beyec e t a1 (44). There are s t i l l a number o f new n u c l e i t o be discovered i n t h e empty r e g i o n between t h e two i s 1 ands o f known alpha e m i t t e r s , around 160Lu (See f i g u r e s 6 and 7 ) .

Attempts f o r t h e s y n t h e s i s o f very neutron d e f i c i e n t r a r e e a r t h ( N lower than 82).

The v i c i n i t y o f t h e 82 neutron s h e l l i s an i n t e r e s t i n g region. Moreover, a very i n t e r e s t i n g r e g i o n o f deformed n u c l e i i s expected f o r N < 82 and 60

<

Z

<

75. I t would be o f i n t e r e s t f o r example t o see i f n u c l e i i n t h i s r e g i o n d i s p l a y r o t a t i o n a l spec- t r a . I t was suggested t h a t o b l a t e deformations occur f o r l i g h t baryum isotopes (451.

Then t h e question a r i s e s whether i t i s p o s s i b l e o r n o t t o produce n u c l i d e s w i t h a neutron

F i g u r e 7 : C h a r t of i s o t o p e s i n t h e r e g i o n o f

heavy r a r e e a r t h .

(10)

SUPER-HEAVY

NUCLEI . . .

-7

d e f i c i e n c y o f 15 t o 20. Because t h e cross s e c t i o n f o r a d e e x c i t a t i o n channel corresponding t o more than 10 neutrons w i l l always be small, even a t t h e convenient energy, one should t r y t o produce a com- pound nucleus already as n e u t r o n - d e f i c i e n t as pos- s i b l e . A study o f t h e combinations t a r g e t + p r o j e c - t i l e which can be made i n o r d e r t o reach t h e r a r e e a r t h region, leads t o t h e conclusion t h a t t h e b e s t p r o j e c t i l e s a r e : i ~ r ,

' O C ~ ,

5 8 ~ i , i:~n, ::Ga and

2 0 2 8

:;Ge.

Experiments were made (46,47) w i t h 'OAr on l14Cd and l 1 6 c d which have shown t h a t t h e e x c i - t a t i o n f u n c t i o n f o r producing known isotopes 1 4 9 ~ b , lS0Dy and lS1Dy, corresponding t o (HI,6n) and (HI,7n), were very s i m i l a r t o t h e e q u i v a l e n t e x c i t a t i o n func- t i o n observed w i t h l i g h t e r i o n s . Cross s e c t i o n s a t t h e peak o f the e x c i t a t i o n f u n c t i o n are about 2.2 t o 2.7 t i m e s m a l l e r than w i t h C, N, 0 and Ne p r o j e c t i - l e s . We s h a l l discuss t h i s p o i n t l a t e r . However, t h e r e s u l t s o f Natowitz and Alexander (46) demonstrate t h a t more than 100 mb c o u l d be obtained f o r (Ar,6n) r e a c t i o n s , i n a r e g i o n o f neutron d e f i c i e n c y around 10. This m i g h t n o t be t r u e i f one goes down t o a neutron d e f i c i e n c y o f 20, because r, and r p become much l a r g e r than rn. Stephens and Diamond have c a l - c u l a t e d t h e curve where rp/rn = l w i t h a r a t h e r sim- p l e formula rp/rn = exp((Sn/.r) - (Sp+0,8B/.r)) where B i s t h e b a r r i e r energy f o r t h e charged p a r t i c l e . They deduced t h a t rp/rn = 1 a t N = 82 f o r Z

=

65 (Terbium), a t N

=

86 f o r Z = 72 (Lutetium) and N = 70 f o r Z = 60 (Neodymium).

Gauvin e t a1 (48) have r e c e n t l y d e t e r - mined t h e r a t i o a(Arxn)/(oAr,p(x-1)n) f o r Argon

1 2 1 - 1 2 3

induced r e a c t i o n s on l18Sn, Sb and

1 2 7 ~

t a r - gets. They were able t o show, by measuring t h e cross- s e c t i o n s a t t h e peak o f e x c i t a t i o n f u n c t i o n s f o r 58Er isotopes around N = 84, t h a t t h e r a t i o i s equal t o 1 f o r x

=

6 and as low as 0.04 f o r x

=

9. Furthermore o(Ar,xn)/o(Ar,a(x-4)n) i s 0.04 f o r x = 9. N.T. P o r i l e has given i n t h i s confe- rence more d e t a i l s on these r e s u l t s . They b r i e f l y show t h a t when compound n u c l e i are produced around N

=

84 i n t h e r a r e e a r t h region, l a r g e cross sec- t i o n s cannot be expected f o r t h e (xn) d e - e x c i t a t i o n channels f o r x > 6 ( F i g u r e 8). Going back t o calcu- l a t i o n s , we quote again t h e very i n t e r e s t i n g r e s u l t s

Figure 8 : Comparison of c r o s s s e c t i o n s f o r : (Ar,xn) , (Ar,p(x-1)n) and ( ~ r ,a(x-h)n) r e a c t i o n s i n r a r e e a r t h r e g i o n ( a f t e r Gauvin e t a1 ( 4 8 ) ) .

F i g u r e 9 : Contour diagram f o r maximum y i e l d s f o l l o w i n g decay of 1 3 2 ~ m and

l S 0 y

( s e e f i g u r e 4) .

o f M. Blann e t a1 on very l i g h t compound n u c l e i l i k e 1 3 2 ~ m ( 4 0 ~ a + 9 2 ~ o ) o r l S 0 y b ( 5 8 ~ i + 9 2 ~ ~ ) . ( F i g u r e 9). They c l e a r l y i n d i c a t e t h a t charged p a r t i c l e s are p r e f e r e n t i a l l y e m i t t e d and t h a t r e s i -

dual n u c l e i should have consequently a l a r g e r r a t i o

N/Z than t h e compound system.

(11)

Figure 10 : Main de-excitation channel for various neutron deficient compound nuclei :O residual nuclei

:

@

lightest known isotopes f3' decay : lightest known isotopes a decay _: a

Made on t h e same basis, I should l i k e t o present, on f i g u r e 10, t h e r e s u l t o f c a l c u l a t i o n s made i n o r d e r t o e s t i m a t e t h e dominant d e - e x c i t a t i o n process f o r l i g h t isotopes l i k e l;:Tb ('::Ag + :;Ar).

I n these estimations, e x c i t a t i o n energies were choo- sen a t values around 80 MeV corresponding t o bombar- d i n g energies o f t h e o r d e r o f 200 MeV f o r which i s a l r e a d y l a r g e r than 1 barn. Therefore we b e l i e v e t h a t p r o d u c t i o n o f l i g h t i s o t o p e s o f r a r e e a r t h (N < 82) i s s t i l l p o s s i b l e a t a l e v e l o f t h e o r d e r o f s e v e r a l mb.

A l a s t remark on t h i s problem. There i s no advantage t o i n c r e a s e very much t h e bombarding energy. F i r s t o f a l l , i t has a1 ready been shown t h a t l a r g e e x c i t a t i o n energies cannot c o n t r i b u t e t o t h e evaporation o f a l a r g e number o f neutrons. Secondly, i t would increase t h e average angular momentum, and many experimental r e s u l t s as w e l l as t h e o r e t i c a l arguments have shown t h a t complete f u s i o n cross s e c t i o n s are w e l l below t h e t o t a l r e a c t i o n cross s e c t i o n s a t h i g h p r o j e c t i l e energies. Such d a t a have been i n t e r p r e t e d i n terms o f a l i m i t i n g angular momentum above which complete f u s i o n may n o t occur

and they w i l l be discussed a b i t more l a t e r on

@9,50,51').

Bogdanov, Karnaukhov e t a1 [52) have searched f o r p r o t o n r a d i o a c t i v i t y on very neutron d e f i c i e n t r a r e e a r t h by bombardment o f

9 6

w i t h

~ ~ 3 2 ~

and 3 5 ~ 1 i o n s . Complete f u s i o n n u c l e i should be ' * * ~ d (neutron d e f i c i e n c y = 14) and 13'pm (neutron d e f i c i e n c y = 15). A g a s - f i l l e d mass s e p a r a t o r was used t o ensure a r a p i d s e p a r a t i o n o f t h e r e a c t i o n products from t h e beam and they looked a t t h e p r o t o n delayed emission w i t h a AE.E telescope. From t h e r i g i d i t y value, i t was shown t h a t p r o t o n e m i t t e r s do e x i s t i n t h e r e g i o n o f mass 123. A h a l f - l i f e o f about 1 sec was observed f o r protons having energy o f 0.83 $ 0.05 MeV. Results were o b t a i n e d a l s o i n t h e case o f l 1 2 s n +

^{3 2 ~ .}

A1 though t h e cross s e c t i o n s were small, ( l e s s than 1 microbarn) these r e s u l t s a r e o f a b i g i n t e r e s t s i n c e t h e measurement o f rp/rB+ i s a good way f o r checking mass formula and p r o t o n b i n d i n g energies f a r away from t h e s t a b i 1 i t y l i n e . There w i l l c e r t a i n l y be more experiments i n t h i s f i e l d when h i g h i n t e n s i t y beams o f

3 2 ~ ,

4 0 ~ a and S 8 ~ i w i l l be a v a i l a b l e .

I V . HEAVY NUCLEI I N THE REGION TANTALUM TO URANIUM.

FISSION COMPETITION AN0 LIMITS DUE TO HIGH ANGULAR MOMENTA.

Many new alpha e m i t t e r s have been disco- vered by heavy i o n r e a c t i o n s i n t h i s region. There has been f i r s t a g r e a t deal o f research on t h e alpha systematics. Seaborg and Y i o l a 1532, Seeger 1541, S i i v o l a f55), have p r e d i c t e d energies f o r a1 pha rays o f unknown isotopes and i t has been a good guide f o r t h e i n v e s t i g a t i o n o f new e m i t t e r s . The general trend:

o f alpha emission has been reviewed a t t h e L y s e k i l

Conference "Nuclides f a r o f f t h e S t a b i l i t y L i n e " .

F i g u r e 11 shows t h a t below t h e 126 neutron close

s h e l l , alpha e m i t t e r s were found f o r i r i d i u m , p l a -

tinum, go1 d, mercury, l e a d and bismuth. Branching

r a t i o s f o r

a

t o E.C. decay a r e n o t w e l l known and

t h e r e are s t i l l u n c e r t a i n t i e s on h a l f - l i v e s and

isomers f o r a number o f isotopes. Very n e u t r o n

d e f i c i e n t n u c l i d e s have been i d e n t i f i e d since. F o r

example 17'1r o r

'"AU

are a t 20 neutrons from t h e

s t a b i l i t y number.

(12)

SZTPER-HEAVY NUCLEI

. . .

C5-83

Roughly t e n years ago, a l l t h e n u c l i d e s i n t h e Pb-Th r e g i o n w i t h h a l f l i v e s l o n g e r than one

"<-I minute had been i d e n t i f i e d ( F i g u r e 12). A f a s t e r technique f o r c o l l e c t i n g n u c l e i , t h e helium j e t t r a n s f e r method, and an e x t e n s i v e use o f heavy i o n r e a c t i o n s have open a new f i e l d o f i n v e s t i g a t i o n f o r nucl ides w i t h ha1 f - 1 i ves between a mi 11 isecond and a minute.

F i g u r e

1 1 :

C h a r t o f i s o t o p e s i n t h e r e g i o n Ta-Pb.

New n e u t r o n d e f i c i e n t i s o t o p e s a r e i n under- l i n e d s q u a r e s .

Since an extensive c o m p i l a t i o n was made by Eskola (567 i n 1967, we s h a l l r e s t r i c t o u r s e l f t o t h e more r e c e n t data. New l i g h t isotopes o f I r and

Ta, w i t h h a l f - l i v e s around several minutes were F i g u r e 12 : C h a r t of i s o t o p e s i n t h e r e g i o n found (572 by bombarding r e s p e c t i v e l y holmium and ^Pb - Pa. ( N e u t r o n d e f i c i e n t p a r t o f t h e

c h a r t ) . gadolinium t a r g e t , w i t h 22Ne and *ONe.

They decay by e l e c t r o n capture i n t o known os,nium and hafnium n u c l e i . Alpha decays o f new osmium i s o - topes (170 t o 174) were detected by Borggreen and Hyde (58) and by Toth and Hahn (59) . ^Branching

r a t i o s ~ / B + . E c a r e low 0.02% f o r t h e case o f 1740s.

However t h e s l i g h t p r o b a b i l i t y f o r alpha emission i s i m p o r t a n t f o r t e s t i n g t h e r e l i a b i l i t y o f mass p r e d i c - t i o n s . Myers and Swiatecki ' s (60) c a l c u l a t i o n s come c l o s e r from t h e experimental alpha r a y energies than Garveyls (61) e x t r a p o l a t i o n .

There has been some controversy on t h e alpha emission o f l i g h t l e a d and bismuth isotopes around N

=

106-110. I n very r e c e n t experiments (62) we have s t u d i e d t h i s r e g i o n by producing a l r e a d y

very neutron d e f i c i e n t compound n u c l e i 'ZZBi i n t h e r e a c t i o n ( ' i z ~ b + : i ~ r ) and ' i : ~ b i n t h e r e a c t i o n ('2:~d + y : ~ r ) . The e x c i t a t i o n f u n c t i o n s o f (Ar,xn) r e a c t i o n s were compared t o t h e corresponding e x c i -

204-x

t a t i o n f u n c t i o n s o f Po induced on 1 6 4 ~ y by 4 0 ~ r where polonium isotopes are w e l l k n o ~ n . ' ~ ~ ~ o , l g 0 ~ i and '06pb are t h e l i g h t e s t isotopes w i t h h a l f l i v e s

between 5 and 20 seconds and r a t h e r l a r g e alpha branching r a t i o s . Although i t w i l l be r a t h e r d i f - f i c u l t t o go much f u r t h e r i n t h e neutron d e f i c i e n c y , mainly because t h e r a t h e r low f i s s i o n b a r r i e r s , t h e r e i s some i n d i c a t i o n t h a t '06pb and l E 7 p b a r e produced.

Mow than 50 new n u c l i d e s were i d e n t i f i e d by bombar- d i n g t a r g e t elements Pt, Au, Hg, T I , Pb, B i w i t h p r o j e c t i l e s l i k e l l B ,

^{1 2 C ,}

14N, 160, 19F, 20Ne, 22Ne and more r e c e n t l y 40Ar. Nuclides j u s t t o t h e l e f t o f t h e b i g d i s c o n t i n u i t y a t N

=

128 i n decay energies

as d i s p l a y e d on f i g u r e 13 have been discovered, e i t h e r as d i r e c t p r o - ducts o f (H1,xn) w a c - t i o n s o r as daughters o f l i g h t isotopes o f Th and Pa. Most o f t h e work was done a t Berke- l e y by Hyde e t a1 (63) and by Macfarlane e t a1 (641. A t t h e Dubna I n t e r n a t i o n a l Confe-

Neulran numb.,

rence on Heavy I o n Reactions, E. Hyde (65) has made a review paper

F i g u r e 13 : on t h e alpha spectrosco-

S y s t e m a t i c s f o r a l p h a py of isotopes i n t h e d e c a y e n e r g i e s i n t h e

r e g i o n

N =

126. elements francium, tho- r i um, p r o t a c t i n i u m .

The l i g h t e s t iden-

t i f i e d isotopes o f Po

(193), and A t (196)

(13)

have been produced by 185'187Re (20Ne,xn) r e a c t i o n s (661, and isotopes o f emanation w i t h 126 o r fewer neutrons have been prepared 167) from 20,LRn t o 2:zRn.

Many i s o m e r i c s t a t e s have been observed below t h e 126 neutron s h e l l when t h e lowest l y i n g l e v e l s are expected t o be s t a t e s P ~ , ~ , fgI2 and i13/2. Since i s o m e r i c s t a t e s have been given a 13/2+ assignment i n t h e odd mass isotopes o f t h e even elements lead, mercury and p l atinuin, t h e o c c u r r e n c e o f t h e same t y p e o f isomerism was expected i n polonium and radon i s o - topes. Experiments have shown t h a t t h i s i s t r u e f o r A = 195, 197, 199 and 201 i n 8 4 P ~ and f o r A

=

201, 203 i n 86Rn.

Franci um isotopes o f mass number 204- 213 were produced by heavy i o n r e a c t i o n s on t h a l l i u m by G r i f f i o e n e t a1 (68) and a b i t l a t e r t h e same n u c l i d e s p l u s 203Fr were prepared by V a l l i e t a l . Then 22 isotopes a r e known f o r element ,,Fr ; w i t h ha1 f l i v e s ranging between 0.7.10-~s ( 2 1 6 F r ) and 19 min ( n 2 F r ) . The occurrence o f isomerism i n nu- c l i d e s o f odd elements above l e a d c o n t a i n i n g 127 neutrons appears c l e a r l y on t h e s e r i e s 2 k g A ~ , 2L;Fr, 2LgAt and 2k;Bi. I t can be e x p l a i n e d by the h i g h s p i n s o f t h e odd neutron and p r o t o n g9/2 and hg/2 r e s p e c t i v e l y , so t h a t t h e c l o s e - l y i n g l e v e l s o f 0 - and 9- a r e p o s s i b l e .

Improvements i n t h e c o l l e c t i n g method and i n t h e o n - l i n e techniques o f

a

spectroscopy were c a r r i e d o u t by Hyde e t a1 (63,701, l e a d i n g t o t h e discovery o f f i v e new t h o r i u m isotopes w i t h mass numbers ranging from 213 t o 217, f i v e new a c t i n i u m i s o t o p e s (209-213), 7 new r a d i um isotopes (206 t o 214) and 4 new p r o t a c t i n i u m isotopes (222-226). New decay f a m i l i e s d i s p l a y e d on f i g u r e 14 and f i g u r e 15 have been discovered s t a r t i n g w i t h 222Th, 221Th, Z25Pa, 224Pa, 223Pa and 222Pa. Up t o now t h e l i g h - t e s t p r o t a c t i n i u m i s o t o p e (216Pa) has been synthe- s i z e d [ 7 1 ] when bombarding 1890s and 1900s w i t h 31P and 197Au w i t h 24Mg. Although t h e cross s e c t i o n l i e s around 1 microbarn i t i s q u i t e i n t e r e s t i n g t o a t t a i n a neutron number below the 126 neutron s h e l l and s t i l l observe a p r o t a c t i n i u m nucleus w i t h a h a l f l i f e o f 0.2s.

As i t was p o i n t e d o u t by Hyde i n 1971 (&J, n o t many n u c l i d e s remain t o be discovered by

because f o r l i g h t e r n u c l e i , y i e l d s w i l l be bery low owing t o f i s s i o n c o m p e t i t i o n i n t h e d e - e x c i t a t i o n of t h e e x c i t e d compound nucleus, and a l s o owing t o t h e p r o t o n emission c o m p e t i t i o n a t such g r e a t neu- t r o n d e f i c i e n c y . I n o r d e r t o produce a complete f u s i o n nucleus a l r e a d y very neutron d e f i c i e n t , one i s tempted t o bombard l i g h t e r t a r g e t s w i t h heavier p r o j e c t i l e s . We have a l r e a d y seen t h a t ::P on

2 2 0

';:OS

gives a compound nucleus

^9,-.

Argon heavy i o n s are b e i n g used a t Orsay ( 6 2 1 f o r p r e p a r i n g compound n u c l e i ';:Bi (::~r + ';:Tb), ';:Pb ( 4 0 A r

+ 1 5 5 ~ d ) and ";~r ( " O A ~ + 'EZTm). I n p r i n c i p l e , even more neutron d e f i c i e n t compound systems are p o s s i b l e w i t h h e a v i e r p r o j e c t i l e s l i k e 5 8 ~ i , "zn, 7 0 ~ e , 7 6 ~ e o r 8 2 ~ r .

-

s h o r t 1 1 4 p s

.. -

-/--

. .- . .. . .

/

F i g u r e 14

:

New decay c h a i n f o r l i g h t t h o r i u m n u c l e i . ( A f t e r Hyde (70)).

F i g u r e 15 : New d e c a y c h a i n f o r l i g h t p r o t a c t i n i u m n u c l e i

a p p l i c a t i o n o f c u r r e n t techniques i n t h e r e g i o n Pb-U,

(14)

SUPER-HEAVY NUCLEI

. .. ^C5-85

However, i t w i l l be necessary t o devote some t i m e t o study t h e n a t u r e of t h e r e a c t i o n s indu- ced by such complex n u c l e a r p r o j e c t i l e s b e f o r e t o p r e d i c t w i t h confidence what f r a c t i o n o f t h e t o t a l r e a c t i o n w i l l go by compound nucleus type. For t h e moment one might speculate on t h r e e unresolved p r o - blems which occur when heavy p r o j e c t i l e s b r i n g l a r g e o r b i t a l angular momenta and e x c i t a t i o n energy i n t o heavy t a r g e t n u c l e i which are unstable a g a i n s t de- formation. i) which f r a c t i o n o f t h e t o t a l cross s e c t i o n w i l l go by a compound nucleus process ; ii) which f r a c t i o n o f t h e d e - e x c i t a t i o n channels w i l l go by f i s s i o n ; i i i ) which f r a c t i o n o f t h e non f i s s i o n i n g e x c i t e d nucleus w i l l go by neutron and gamma de-exci t a t i o n .

L i m i t a t i o n o f UCN due t o h i g h angular momentum e f f e c t s .

The f a c t t h a t compound-nucleus formation cannot t a k e p l a c e f o r h i g h p a r t i a l waves seems t o be r a t h e r we1 1 e s t a b l i s h e d b o t h by experimental r e s u l t s and t h e o r e t i c a l treatments. Measurements o f cross s e c t i o n s we1 1 below t h e t o t a l r e a c t i o n cross sec- t i o n s f o r complete f u s i o n o f heavy i o n p r o j e c t i l e s w i t h v a r i o u s t a r g e t n u c l e i ( l i g h t medium and heavy)

have been i n t e r p r e t e d i n terms o f a l i m i t i n g angular momentum above which complete f u s i o n may n o t occur.

B r i e f l y t h i s i s demonstrated by t h e f o l l o w i n g expe- r i m e n t a l observations ; i ) aCF/oR f o r a given t a r g e t and p r o j e c t i l e s goes through a maximum when t h e bom- b a r d i n g energy i s increased, i .e. when t h e o r b i t a l momentum increases; i i ) oCF/oR decreases when t h e p r o j e c t i l e becomes h e a v i e r ; iii) a corresponding i n c r e a s e o f t h e cross s e c t i o n s f o r d i r e c t r e a c t i o n s a t h i g h e r energies has been measured by several authors [21,22] . Also r e a c t i o n s a r e more and more complicated s i n c e a t h i g h e r energies a l a r g e r over- l a p between t a r g e t and p r o j e c t i l e w i l l be p o s s i b l e and i n t e r a c t i o n w i l l become s t r o n g e r .

T r y i n g t o f o r m u l a t e t h i s concept where compound nucleus formation i s i n c r e a s i n g l y hindered because o f t h e l a r g e angular momntum, t h e s i m p l e s t approach i s t h e sharp c u t o f f model proposed by M i l l e r e t a1 [49]and developed by N a t o w i t z [50).

The r e a c t i o n cross s e c t i o n can be given by

aR

is c a l cu'iated w i t h an o p t i c a l model program o r w i t h t h e sharp c u t o f f approximation f o r TR :

where B i s t h e Coulomb b a r r i e r and ? t h e k i n e t i c energy o f t h e p r o j e c t i l e i n t h e c e n t e r o f mass system :

c

oCI( =

T @

( Z a t l ) w i t h z C = minimum (rcr,amax) a =O

and

R,$

= maximum angular momentum f o r f o r m a t i o n o f t h e compound nucleus.

F o r a , , amax oCN = oR.

YR (amaX+l)i

Then =

CM (acr+1)2

The measurement o f c r ~ / o c ~ p e r m i t s a s t r a i g h t f o r w a r d d e t e r m i n a t i o n o f a , , , s i n c e amax = k.R -

k)l

=

G) ^{and R}

⁼

r 0 ( ~ ; j 3

i- <'3)