PRODUCTION OF NUCLEI FAR FROM THE BETA STABILITY LINE USING INTERMEDIATE-ENERGY HEAVY IONS

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PRODUCTION OF NUCLEI FAR FROM THE BETA STABILITY LINE USING INTERMEDIATE-ENERGY

HEAVY IONS

D. Guerreau

To cite this version:

D. Guerreau. PRODUCTION OF NUCLEI FAR FROM THE BETA STABILITY LINE USING

INTERMEDIATE-ENERGY HEAVY IONS. Journal de Physique Colloques, 1986, 47 (C4), pp.C4-

207-C4-222. �10.1051/jphyscol:1986425�. �jpa-00225791�

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JOURNAL DE PHYSIQUE

Colloque C4, supplément au n' 8, Tome 47, août 1986 Cf-207

PRODUCTION OF NUCLEI FAR FROM THE BETA STABILITY LINE USING INTERMEDIATE-ENERGY HEAVY IONS

D. GUERREAU

GRNIL, BP 5027, F-14021 Caen Cedex, France

Résumé - La production de noyaux exotiques à l'aide des ions lourds aux énergies intermédiaires est passée en revue. Les différents mécanismes de production, essentiellement la fragmentation du projectile et les réactions de transfert, sont présentées ainsi que les premiers résultats.

Summary - The production of far unstable nuclei using heavy ion accelerators in the intermediate energy domain is reviewed. The various mechanisms responsible for the production of exotic species, mainly the projectile fragmentation and transfer reactions, are discussed, and the first experimental results presented.

INTRODUCTION

The production and study of nuclei far from stability has become one of the main goals of nuclear physics over the last decade /IS/. If the existence of about 2200 nuclides has been confirmed, a total of 7000 isotopes are predicted to be bound in their ground-states. But the ultimate goal is probably not to produce them all.

The ambition of nuclear physics is to get finally a good description of nuclear matter through a unique elementary interaction, by using self consistent theories.

Looking at nuclei far from stability may help to go in that direction. The same motivation which leads us to study hot nuclear matter near some critical temperature, should lead us also to study cold nuclear matter under extreme conditions with respect to the N/Z ratio. The most extreme N/Z ratios of nuclear ground states, accessible to experiments, are those at the border of nuclear stability, that is near the proton and neutron drip lines. These nuclides are those labelled "exotic" in this paper. Up to now, these limits have been reached for only the very light elements, the difficulty arising from the very sharp decrease of both production cross sections and lifetimes when moving away from stability.

Besides this first aim, maping the drip lines, there are strong motivations to get a broad systematic study of the main properties of nuclei far from stability by

varying the neutron and proton number over a very large scale. This provides challenging tests of semi-empirical models constructed near the stability and more self-consistent nuclear models. A good example is the discovery of a new region of deformation at N = 20, looking at the whole series of Na isotopes /6/. Such

measurements may concern half-lives, masses, energy of the 2+ state, particle decay modes as p delayed neutron or proton emission or even more exotic decay modes as proton radioactivity. For these very exotic species, the quite large energy release for p-decay may allow a more precise study of the weak force. Finally, it is worth noticing the interplay between astrophysics and nuclear physics, connected for example to an improved understanding of the nucleosynthesis process / 7 / .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986425

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C4-208 JOURNAL DE PHYSIQUE

Up t o now, v a r i o u s p r o j e c t i l e s have been used t o produce n u c l e i f a r from s t a b i l i t y : neutrons, h i g h and i n t e r m e d i a t e energy protons, low energy heavy i o n s /3,5,7-9/,

r e l a t i v i s t i c heavy i o n s /lo-13/. One o f t h e most powerful method i s based on spa1 l a t i o n , fragmentation and f i s s i o n r e a c t i o n s of heavy t a r g e t s induced by protons as i t is done f o r example a t the ISOLDE f a c i l i t y /14/. I n t h i s paper, I s h a l l discuss some o f t h e advantages and l i m i t a t i o n s o f these methods. My prime i n t e r e s t , however, w i l l be t o demonstrate t h a t i n t e r m e d i a t e energy heavy-ions can be a powerful and i n some r e g i o n s a unique t o o l f o r s t c ~ d y i n g c o l d m a t t e r w i t h very unusual N/Z r a t i o s . I s h a l l r e s t r i c t t h e d i s c u s s i o n t o t h e p r o d u c t i o n o f new

i s o t o p e s i n the l i g h t r e g i o n ( Z c 40) as i t appears t o be the most e a s i l y accessible r e g i o n f o r the e x i s t i n g f a c i l i t i e s . I n t h i s f i r s t section, t h e v a r i o u s r e a c t i o n mechanisms s u s c e p t i b l e t o produce e x o t i c species by means of intermediate-energy heavy i o n beans w i l l be reviewed. A simple model w i l l be presented which c o u l d be u s e f u l t o make p r e d i c t i o n s f o r t h e p r o d u c t i o n o f e x o t i c isotopes as w e l l as secondary beams /15/. The r e l a t e d t o o l s a r e b r i e f l y discussed i n s e c t i o n 11. I n s e c t i o n 111, measured and e x t r a p o l a t e d p r o d u c t i o n r a t e s o f i n t e r m e d i a t e energy heavy i o n r e a c t i o n s a r e compared t o those obtained a t i o n source based o n - l i n e separators.

F i n a l l y , r e s u l t s o b t a i n e d a t GANIL f o r e x o t i c n u c l e i w i l l be discussed i n s e c t i o n I V .

I

-

COMPETING REACTION MECHANISMS

I n t h e low energy region, and as f a r as heavy ions a r e concerned, a t l e a s t t h r e e processes have been used e x t e n s i v e l y f o r producing e x o t i c n u c l e i : f u s i o n f o l l o w e d by evaporation f o r c e n t r a l c o l l is i o n s , deep i n e l a s t i c and two-body t r a n s f e r r e a c t i o n s f o r p e r i p h e r a l c o l l i s i o n s /3/. The s i t u a t i o n i s q u i t e d i f f e r e n t a t i n t e r m e d i a t e energies where t h e competing mechanisms (which are o f i n t e r e s t i n t h i s paper) a r e mainly those a r i s i n g from p e r i p h e r a l c o l l i s i o n s , i.e. t r a n s f e r r e a c t i o n s i n t o continuum s t a t e s /16-17/ and t h e s o - c a l l e d fragmentation process /18-20/. The l a t t e r i s by f a r t h e dominant process and i n many r e s p e c t s resembles very much t h e one observed w i t h r e l a t i v i s t i c heavy-ions which has been proved t o be a l s o a good t o o l t o produce very neutron r i c h species /lo-13/. As several c o n t r i b u t i o n s t o t h i s conference are focused on t h i s mechanism /20,21/, 1 s h a l l j u s t r e c a l l t h e main c h a r a c t e r i s t i c s which a r e o f d i r e c t i n t e r e s t f o r the p r o d u c t i o n o f e x o t i c isotopes and f o r d e f i n i n g t h e experimental equipments :

i ) The p r o j e c t i l e - l i k e fragments (PLF) a r e e m i t t e d w i t h a v e l o c i t y very c l o s e t o t h e bean v e l o c i t y . (Only a small s h i f t i s observed r e s u l t i n g from the f r i c t i o n f o r c e due t o the b i n d i n g energy of t h e abraded nucleons). A t f i r s t order t h e monentum

d i s t r i b u t i o n s can be reproduced u s i n g t h e Goldhaber approach, t h e w i d t h o f t h e d i s t r i b u t i o n being expressed by t h e r e l a t i o n :

where Ap and AF stand r e s p e c t i v e l y f o r the fragment and p r o j e c t i l e mass, a0 i s c o n s t a n t over t h e whole mass range and c l o s e t o 90 MeV/c.

i i ) The angular d i s t r i b u t i o n i s s t r o n g l y forward peaked and may be r o u g h l y

c a l c u l a t e d assuming an i s o t r o p i c d i s t r i b u t i o n i n t h e p r o j e c t i l e r e s t frame. I n such a f a s t abrasion process, one expects the i s o t o p i c d i s t r i b u t i o n o f t h e o u t g o i n g fragments t o be dominated by t h e i s o s p i n asymmetry o f t h e p r o j e c t i l e ; more n e u t r o n - r i c h w i l l be the p r o j e c t i l e , more n e u t r o n - r i c h w i l l be t h e r e a c t i o n

products. [ t h i s has been checked already a t r e l a t i v i s t i c energies comparing 40Ar and k8Ca p r o j e c t i l e s /11/]. A c h a r a c t e r i s t i c feature i n t h i s energy range i s t h e

s u b s t a n t i a l r o l e o f t h e neutron excess o f t h e t a r g e t f o r t h e neutron excess o f t h e PLF i n general and of the very neutron r i c h fragments i n p a r t i c u l a r /22,23/. The most p l a u s i b l e e x p l a n a t i o n f o r t h i s e f f e c t i s the occurence o f nucleon-nucleon c o l l i s i o n s i n t h e o v e r l a p zone : combining t h e existence o f a neutron s k i n i n heavy t a r g e t s w i t h t h e h i g h s c a t t e r i n g cross s e c t i o n s between 20 and 100 MeV/u tends t o increase the N/Z r a t i o o f t h e PLF over t h a t o f t h e p r o j e c t i l e /24/.

During t h i s fragmentation process, i t appears the PLF remain n o t much e x c i t e d , as shown by r e c e n t e x c l u s i v e experiments /25/. One may t h e r e f o r e reasonably estimate

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t h e e x c i t a t i o n energy o f t h e primary PLF by the excess s u r f a c e energy o f t h e abraded fragments. Typical values r a n m e e n 30 MeV f o r the r e a c t i o n Ca + Ta and 50 MeV f o r K r

+

Ta.

A simple a b r a s i o n - a b l a t i o n model has been used f o r comparison w i t h t h e measured data. I t i s a geometrical model where the mass d i s t r i b u t i o n i s c a l c u l a t e d under p u r e l y geometrical c o n s i d e r a t i o n s on t h e degree o f o v e r l a p between t h e two i n t e r a c t i n g n u c l e i . Then the charge d i s t r i b u t i o n i s deduced assuming z e r o - p o i n t motion o f t h e g i a n t d i p o l e resonance i n t h e p r o j e c t i l e /26/. S t a r t i n g from t h e above mentioned estimate f o r t h e e x c i t a t i o n energy o f t h e PLF, t h e d e e x c i t a t i o n stage, which p l a y s a major r o l e f o r determining t h e eventual p r o d u c t i o n o t very u n s t a b l e n u c l e i , i s c a l c u l a t e d u s i n g t h e code LILITA /27/ and t h e p r e d i c t e d masses o f Uno and Yamada /28/. This d e e x c i t a t i o n i s c a l c u l a t e d f o r a l l primary products (i.e. around 280 f o r an Ar induced r e a c t i o n ) and t h e f i n a l mass and charge d i s t r i b u t i o n s deduced.

Results o f such a c a l c u l a t i o n a r e shown i n f i g . 1 f o r t h e system 44 A.MeV Ar + Ta and compared w i t h experimental r e s u l t s obtained a t LISE on t h i s system /29/ ( c l o s e d c i r c l e s ) and those from a very s i m i l a r system, 44 A.FleV Ar

+

Au /22/ (open c i r c l e s ) .

Fig. 1

-

C a l c u l a t e d f i n a l i s o t o p i c d i s t r i b u t i o n s f o r t h e r e a c t i o n 44 A.MeV 40Ar

+

Ta ( s o l i d l i n e s ) . Comparison i s made w i t h r e s u l t s o f r e f . 22 (open c i r c l e s ) and those o f r e f . 29 ( s o l i d c i r c l e s ) . For more d e t a i l s , see t e x t .

The agreement i s q u i t e s a t i s f a c t o r y f o r t h e peaks o f t h e d i s t r i b u t i o n s ( p r o d u c t i o n c r o s s s e c t i o n s 1 t o 100 mb) and i s s u r p r i s i n g l y good f o r the most neutron r i c h isotopes observed ! For t h e l a t t e r , t h e cross s e c t i o n s have been deduced from measurements performed a t zero degree w i t h a somewhat small e f f i c i e n c y ( c and a r e t h e r e f o r e s u b j e c t t o l a r g e e r r o r bars ( f o r more d e t a i l s , see s e c t i o n 111).

The i n f l u e n c e o f t h e N/Z r a t i o o f t h e p r o j e c t i l e on t h e f i n a l mass d i s t r i b u t i o n s o f t h e PLF i s e x a m p l i f i e d i n f i g . 2 f o r 40Ca, 40Ar and 98Ca induced r e a c t i o n s and Z = 14 products. [It should be noted t h a t t h e cross s e c t i o n f o r t h e most probable i s o t o p e o f a given Z i s about 5 times h i g h e r than t h e one measured w i t h h i g h energy protons

1.

Besides t h i s f r a g n e n t a t i o n channel, t h e t r a n s f e r r e a c t i o n s a r e s u r e l y a promising way i n some very s p e c i f i c cases, e s s e n t i a l l y f o r producing p r o t o n - r i c h n u c l e i . However one should keep i n mind t h a t i t i s l i m i t e d t o a few nucleon t r a n s f e r ( s t r i p p i n g o r pick-up). The g r e a t i n t e r e s t l i e s i n t h e f a c t t h a t t h e momentum d i s p e r s i o n i s much smaller than f o r t h e fragmentation products /23,30/ and t h e a n g u l a r d i s t r i b u t i o n i s a l s o peaked a t very forward angles. Appreciable c r o s s s e c t i o n s measured, even a t the Fermi energies, l e a d t o t h i n k t h a t i t c o u l d a l s o be a very good t o o l f o r p r e p a r i n g secondary beams w i t h h i g h p r o d u c t i o n rates. T h i s has been e f f e c t i v e l y shown by Bimbot e t a1 /31-32/ i n Ar (and 0) induced r e a c t i o n s where 39Ar, 38Ar and 41K have been obtained w i t h p r o d u c t i o n r a t e s as h i g h as 6 ( w i t h r e s p e c t t o t h e beam i n t e n s i t y ) .

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C4-210 JOURNAL DE PHYSIQUE

It appears more d i f f i c u l t t o answer t h e q u e s t i o n o f t h e e f f e c t i v e n e s s o f c e n t r a l c o l l i s i o n s a t these energies. L e t us mention t h e use o f i n v e r t e d kinematics i n f u s i o n r e a c t i o n s t h a t vould h e l p t o g e t a s t r o n g f o c u s i n g o f t h e r e c o i l i n g products and t h e e x i s t e n c e o f a multifragmen- t a t i o n process where t h e t a r g e t breaks i n t o many pieces m i g h t w e l l be another way t o produce l i g h t neutron r i c h fragments.

F i n a l l y , as we do n o t know p r e c i s e l y what's o c c u r i n g f o r h e a v i e r p r o j e c t i l e s as Kr o r Xe, we should remain open f o r surprises, especial l y those o f f e r i n g new p r o d u c t i o n ways f o r e x o t i c n u c l e i . ( F i r s t r e s u l t s obtained w i t h a Kr bean w i l l be presented l a t e r on i n t h i s paper).

I 1

-

THE TOOLS

The main ~ r o ~ e r t i e s of these PLF. i .e. A forward peaked angular d i s t r i b u t i o n s and a r a t h e r broad momentum w i d t h peaked near t h e bean v e l o c i t y , a r e e v i d e n t l y very favourable f o r a p p l y i n g t h e technique o f r e c o i l separation. L e t us mention the 0"

LISE magnetic analyser /33/, t h e SPEG spectrometer a t GANIL /34/, t h e RPEIS /35/

( r e a c t i o n product mass separator) a t MSU.

As an example, t h e 0" LISE spectrometer i s shown i n f i g . 3. I t has an angular

acceptance o f 1 iasr and e s s e n t i a l l y c o n s i s t s o f two i d e n t i c a l d i p o l e s ; t h e f i r s t one i s used as a d i s p e r s i v e element whereas t h e second one gives t h e necessary compensation t o the d i s p e r s i o n i n the f i r s t element. With such a system, one g e t s a t the achromatic f o c a l plane a double achromatism i n angle and i n p o s i t i o n . The maximum r i g i d i t y acceptance i s t 2.5 %.

Moreover, t h e o p t i c a l p r o p e r t i e s o f LISE are conserved even i n t h e presence o f a t h i c k energy degrader between t h e two dipoles. I n t h a t case, t h e f i r s t d i p o l e provides t h e A/Z a n a l y s i s whereas the Z dependance o f t h e energy l o s s i n t h e degrader, a l l o w s a s e l e c t i o n o f a g i v e n i s o t o p e a t the f o c a l p o i n t . The l a t t e r method i s discussed i n F. Hubert' paper /36/ and a l s o by R. Bimbot e t a1 /32/. 1 want t o emphasize t h a t t h i s powerful method i s e s s e n t i a l f o r p r o d u c t i o n o f secondary beam w i t h a h i g h p u r i t y , and a l s o f o r p r o v i d i n g c o n d i t i o n s f o r spectroscopic decay s t u d i e s o f PLF products.

An on l i n e mass separator has been t e s t e d a t GANIL /37/ and t a r g e t as w e l l as pro- j e c t i l e fragmentation have been 1 ooked f o r . The observed p r o d u c t i o n r a t e s were however

Fig. 2

-

C a l c u l a t e d f i n a l d i s t r i - b u t i o n s f o r Z = 14 i s o t o p e s produced w i t h 3 d i f f e r e n t p r o j e c t i l e s . The i n f l u e n c e o f t h e N/Z r a t i o o f t h e p r o j e c t i l e appears very c l e a r - l y [Einc = 44 MeV/u, Ta target].

Fig. 3

-

Lay-out o f t h e t r i p l e focusing LISE r e c o i l spectrometer.

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too limited t o allow spectroscopic studies.

Finally, other tools such a s an He j e t system should be useful f o r example t o look f o r incomplete fusion reactions.

I11 - PRODUCTION RATES

In section I, i t has been shown t h a t q u i t e high cross sections can be reached

w i t h

intermediate energy heavy ions. However the only really meaningful parameter i s the production r a t e ( i n atoms per second) a t which a given isotope can be e f f e c t i v e l y measured. This r a t e determines the kind of study one i s able t o undertake

:

roughly speaking,

1

atom per hour may be enough t o prove the existence of a new isotope, 1 t o 10 per minute i s necessary t o g e t the h a l f - l i f e and more than 1 per second t o deduce the f i r s t spectroscopic infornation.

The production r a t e 1, i s expressed by the following r e l a t i o n

:

where

a

i s the production cross section, I the beam i n t e n s i t y (atoms/s), N the t a r g e t thickness (atornslcm2) and

R

the efficiency of the experimental device. Typical cross sections a t the Fermi energies have been given i n f i g . 1 and

2.

The beam i n t e n s i t y available a t GANIL i s ranging from 4.1011s-1 f o r an Ar beam down t o 5.1010s-1 f o r an heavier beam as 8 6 ~ r . Target thicknesses range between 0.1 and 0.5 g/cm*. The efficiency

R

is d i r e c t l y related t o t h e spectrometer.

A

typical example i s shown i n f i g . 4 f o r the reactions

58EIi

+

S8Ni

and '+OCa + S8Ni a t 55 tleV/u (with a t a r g e t thickness of 0.1 g/cm2). I t takes i n t o account the angular d i s t r i b u t i o n of the products (assuming an i s o t r o p i c d i s t r i b u t i o n i n the r e s t frame of t h e p r o j e c t i l e ) , the angular acceptance of LISE, the energy spread due t o the reaction mechanism (expression I ) , the energy loss in the t a r g e t /38/ and the r i d i g i t y acceptance of the spectrometer. This figure shows c l e a r l y t h a t the efficiency i s maximum f o r the products which mass i s close t o the p r o j e c t i l e mass and therefore demonstrates than one given p r o j e c t i l e defines one optimum region of study.

Fig. 4 - The efficiency

R

of the LISE spectrometer i s strongly depending on both the PLF' mass and the nature of the p r o j e c t i l e . For both p r o j e c t i l e s , a 100 mg/cm2 Ni t a r g e t i s considered in the calculations. For more d e t a i l s on t h i s calculation, see t e x t .

Typical expected production r a t e s , a t the focal point of LISE, a r e plotted in f i g . 5-6 f o r the reactions

77

A.HeV A r + Ta and 55 A.!leV

5 8 N i + 5 8 N i .

[This calculation i s using the model described in section I which has been shown t o reproduce with a r a t h e r good accuracy the observed cross sections f o r the most probable isotopes and t o some extend a l s o f o r very neutron rich ones in Ar induced reactions]. From these r e s u l t s , two conclusions can readily be drawn

:

i ) the most probable isotopes f o r a given Z a r e produced with q u i t e high r a t e s ,

specially with Ar p r o j e c t i l e s

:

10%-1 30A1, 3.106s-

1 35P.

Even higher r a t e s a r e predicted f o r products c l o s e r t o the beam.

Table I gives the r e s u l t s of the calculation f o r masses 35 and

36

compared t o the measu- rements of Binbot e t a1 /31/ in the reac- tion 44 A.FleV Ar + Be. The agreement, within a factor

3

or 4, i s quite s a t i s f a c t o r y . This calculation, based on a fragmentation model f a i l s i n predicting the high r a t e s f o r frag- nents very close t o the p r o j e c t i l e origina- t i n g mainly from t r a n s f e r reactions ( s e e r e s u l t s in table I f o r nuclides a s 39Ar o r

'+1K

resulting from 1 neutron and 1 proton

pick-up).

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JOURNAL DE PHYSIQUE

Fig. 5

-

Typical p r o d u c t i o n r a t e s Fig. 6

-

Same as f i g . 5 f o r a 5 8 ~ i ( i n atoms s - l ) expected a t t h e focal p r o j e c t i l e [The r i g i d i t y a p e r t u r e

p o i n t o f t h e LISE spectrometer w i t h i s i: 0.5

%1.

an '+OAr p r o j e c t i l e a p e r t u r e i s

+

^{2.5 %}

i i ) Rather h i g h p r o d u c t i o n r a t e s a r e expected f o r several unknown e x o t i c n u c l i d e s o r o t h e r s f o r which we have almost no spectroscopic i n f o r m a t i o n . Floreover, these examples show t h a t the neutron d r i p l i n e i s w i t h i n the reach o f t h e e x p e r i m e n t a l i s t f o r t h e l i g h t elements

(Z <

81, as t h e p r o t o n d r i p l i n e should be reached up t o Z = 28 u s i n g b°Ca and 58Ni p r o j e c t i l e s .

Table I

-

The i n t e n s i t i e s ( w i t h respect t o t h e primary beam i n t e n s i t y I o ) o f secondary beams produced i n t h e r e a c t i o n 44 A.MeV Ar + Be ( r e f . 31) are compared t o t h e fragmentation c a l c u l a t i o n s .

How c o u l d these r a t e s compete w i t h o t h e r e x i s t i n g methods. L e t us f i r s t discuss n e u t r o n - r i c h i s o t o p e s and t a k e as an example t h e r e s u l t s obtained w i t h low-energy heavy i o n s a t GSI. An example o f t h e y i e l d s measured a t t h e GSI on l i n e mass separator i s shown i n f i g . 7 f o r t h e r e a c t i o n 7.5 A.MeV 82Se + 186W 139,401. This may be compared t o p r e l i m i n a r y r e s u l t s o f a r e c e n t experiment performed a t LISE u s i n g t h e r e a c t i o n 43 A.MeV 86Kr + 197Au 1411. During these measurements, t h e magnetic r i g i d i t y o f t h e LISE spectrometer was swept over a l a r g e range i n order t o p r o p e r l y measure complete i s o t o p i c d i s t r i b u t i o n s o f PLF a t 0". Fig. 8 shows f o r Z = 26, 28 the measured p r o d u c t i o n r a t e s 1, f o r t h e optimal Bp s e t t i n g o f each i s o t o p e [i.e. t h e one corresponding t o the most probable momentum o f t h e e j e c t i l e ] . As t h i s experiment was performed w i t h a t h i n t a r g e t ( 2 mg/cm2) and a very small r i g i d i t y a p e r t u r e ( + 0.25 %), one may then reasonably expect t o increase the r a t e s by a f a c t o r 200 i n f u t u r e experiments w i t h t h i c k e r t a r g e t s and l a r g e r momentum acceptance (see r i g h t scale i n f i g . 8). A s u r p r i s i n g r e s u l t i s t h e broadening, observed f o r t h e i s o t o p i c d i s t r i b u t i o n s as compared t o those o b t a i n e d w i t h 1 ig h t e r

Secondary beam

(111,) Experiment (111,) C a l c u l a t i o n

-3 "S

2.8 3.1 10-6

35S

3.10-6 1.2 10-5

j6S

6.10-6 2.2 10-5

3 6 C l

2.10-6 3.10-6

37C1

8.10-6 3.5 10-5

39C1

3.10-5

-

39Ar

5.10-5

-

(8)

p r o j e c t i l e s as Ar o r Ca. A broad mass ( o r i s o s p i n p r o j e c t i o n range T i s covered e.g between 53Ni (T, =

-

3/21 and 72Ni (TZ + 8). Comparing f i g . 7 an8 8 show t h a t i n t e r m e d i a t e energy heavy ions may compete favourably f o r the p r o d u c t i o n o f neutron r i c h isotopes. On t h e p r o t o n - r i c h side, t h e s i t u a t i o n does n o t seem t o be e q u a l l y favourable f o r i n t e r m e d i a t e energy p r o j e c t i l e s , since fusion-evaporation r e a c t i o n s appear t o o f f e r favourable p r o d u c t i o n c o n d i t i o n s except f o r very s h o r t l i v e d nuclides. [e.g 200 atoms/s f o r T, =

-

^1, 150 ms 48Mn from 12C ('+OCa, p3n) r e a c t i o n s /42/]. For l i g h t e r elements ( Z

<

10) the very neutron r i c h isotopes cannot be formed e a s i l y by any low-energy mechanism and fragmentation r e a c t i o n s a r e s u r e l y by f a r superior.

56 58 60 62 6L 66 68 70 72 7L 76 78

MASS NUMBER A

Fig. 7

-

Experimental y i e l d s of r e a c t i o n products observed w i t h t h e r e a c t i o n 7.5 A.MeV 82Se

+

W w i t h t h e GSI on-1 i n e mass separa- t o r ( r e f . 39,401.

Fig. 8

-

P r e l i m i n a r y y i e l d s 1, measured f o r Fe and Ni isotopes i n t h e LISE spectrometer through t h e r e a c t i o n 43 A.MeV 86Kr + Au. R e s u l t s were o b t a i - ned w i t h a 2 mg/cm2 t h i c k t a r g e t and a r i g i d i t y a p e r t u r e o f t 0.25 %. I n addi- t i o n , estimated i n t e n s i t i e s I,a r e given, which one m i g h t reach u s i n g a 50 mg/cm2

\ Au t a r g e t and a r i g i d i t y a p e r t u r e of t 2.5 %. An example o f t h e measured charge s t a t e d i s t r i b u t i o n $ s g i v e n f o r 6 2 ~ i ( r e f . 41).

The s i t u a t i o n i s d i f f e r e n t w i t h h i g h energy protons where, i n p r i n c i p l e , s p a l l a t i o n , f i s s i o n and fragmentation r e a c t i o n s w i t h a heavy t a r g e t are a very powerful t o o l t o produce e x o t i c isotopes over a broad mass range. The use o f much t h i c k e r t a r g e t s ( t y p i c a l l y 100 gIcn2) and h i g h e r primary beam i n t e n s i t i e s ( t y p i c a l l y 1013s-1) r e s u l t i n the very h i g h y i e l d s a v a i l a b l e a t t h e ISOLDE f a c i l i t y re.g. a p r o d u c t i o n o f 500 atoms/s f o r T =

-

2, 98 ms 32Ar]. There a r e however, a l s o l i m i t a t i o n s t o t h e a p p l i c a t i o n o f ion-source based o n - l i n e separators, which do n o t e x i s t w i t h magnetic r e c o i l spectrometers a t t h e Fermi energy. They a r e due t o t h e r a t h e r l o n g and

Z

dependant delay times, connected w i t h the o p e r a t i o n o f an i o n source, which l i m i t measurements a t t h e edges of t h e s t a b i l i t y i n general an become v e r y s t r i n g e n t i n case o f very s h o r t periods (T1/2 < f e w ms). These e f f i c i e n c i e s a r e very h i g h f o r t h e l o n g - l i v e d a l k a l i isotopes

( R -

^11, b u t a r e s t r o n g l y reduced f o r elements as Fe, CO, Ni o r even more f o r elements as N

(R -

10-4). However, f o r many elements, i t remains t h e most powerful method, w i t h more experimental improvements expected t o occur i n the near f u t u r e /14,43/.

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C4-214 JOURNAL DE PHYSIQUE

I n conclusion, i t appears t h a t 0" magnetic spectrometers w i t h Fermi-energy heavy i o n beans show a l e a d over o t h e r methods f o r i n v e s t i g a t i n g extremely s h o r t - l i v e d , f a r u n s t a b l e n u c l e i w i t h A

<

70. The advantages are obviously the f a s t c o l l e c t i o n t i m e ( < 200 ns), the absence o f Z s e l e c t i v i t y a p r i o r i (i.e. w i t h o u t i n t e r m e d i a t e

degrader) and t h e p o s s i b i l i t y of i n t r o d u c i n g mass and charge s e l e c t i v i t y by degrader techniques /44/. The main l i m i t a t i o n s a r i s e from the use o f r a t h e r t h i n t a r g e t s ( < 0.3

-

0.5 g/cm2), t h e mass dependance o f t h e transmission, b o t h e f f e c t s

r e s t r i c t i n g t h e p r o d u c t i o n r a t e s , and from t h e appearance o f uncompletely s t r i p p e d PLF using very heavy beams (see t h e example o f a charge s t a t e d i s t r i b u t i o n f o r Z = 28 i n f i g . 8). However i t appears c l e a r l y t h a t t h e r e a r e s t r o n g needs f o r systematic s t u d i e s o f p r b d u c t i o n c o n d i t i o n s under 0" which are r e l e v a n t t o heavy i o n r e a c t i o n mechanism as w e l l as atomic physics. For i n s t a n c e the broad i s o t o p i c d i s t r i b u t i o n observed i n B6Kr induced r e a c t i o n cannot be reproduced u s i n g t h e fragmentation model described above ( f i g . 8) and might r a t h e r i n d i c a t e t h e occurence o f a break-up of an e x c i t e d K r - l i k e fragment. Several groups have a l r e a d y emphasized the appearance o f new behavl'ours w i t h such heavy beams /45,46/.

I V

-

EXPERIMENTAL RESULTS

I V . l

-

Towards t h e f r o n t i e r s o f s t a b i l i t y

I V . l . l

-

I d e n t i f i c a t i o n techniques

The LISE spectrometer, which has been already described i n s e c t i o n 11, i n p l i c i t e l y d e f i n e s how we can achieve t h e i d e n t i f i c a t i o n o f the products. The f l i g h t time o f the PLF i s measured between the t a r g e t and the f i n a l f o c a l p o i n t o f LISE ( 1 = 18 m).

A t t h i s p o i n t standard s o l i d s t a t e d e t e c t o r s ensure the Z i d e n t i f i c a t i o n by usual E-AE techniques. Combined i n f o r m a t i o n from t h e telescope, the time o f f l i g h t (TOF) and t h e magnetic r i g i d i t y s e t t i n g allows i n p r i n c i p l e an overdetermination o f the fragment mass and atomic number. Depending on the beam, t h e o v e r a l l t i m e r e s o l u t i o n (between t h e RF and telescope s i g n a l s ) f o r t h e 18 m f l i g h t path i s ranging between 0.2 and 1

X.

A Z r e s o l u t i o n o f AZ/Z

-

^{1 %} i s e a s i l y achieved.

The i d e n t i f i c a t i o n o f t h e products i s e v i d e n t when 1 ooking a t t h e bidimensional p l o t ( f i g . 9) r e p r e s e n t i n g t h e TOF ( p r o p o r t i o n a l t o A/Z) versus Z. It e x h i b i t s

c h a r a c t e r i s t i c curves, each o f them r e l a t e d t o a given T,. The l i n e o f constant TOF corresponds t o A/Z = 2, Tz = 0, N = Z s e l f - c o n j u g a t e n u c l e i

and t h e absence o f we1 1 known unbound _{. * a *}

isotopes as SBe, 9~ and 16F a l l o w s f o r _S, an unambiguous i d e n t i f i c a t i o n .

IV.1.2

-

Towards the neutron d r i p l i n e

:x I

Two d i f f e r e n t p r o j e c t i l e s have been

used up t o now t o emphasize the p r o d u c t i o n o f n e u t r o n - r i c h isotopes : 44 A.MeV 4 0 ~ r , 33 and 43 A.MeV 86Kr p r o j e c t i l e s . As mentioned before, t h e r a t e a t which t h e n e u t r o n - r i c h isotopes a r e produced i s much h i g h e r when u s i n g a n e u t r o n - r i c h t a r g e t . Comparing Ta and Be t a r g e t s shows a d i f f e r e n c e o f several orders o f magnitude f o r t h e p r o d u c t i o n of very n e u t r o n - r i c h species /19/.

. . . * a

1

t t

T,=+l T + Tz=0 Tz=-1/2Tz,.l

0

160 m no

400 c k l s

Tof

Fig. 9

-

A t y p i c a l two dimensional p l o t o f events i n t h e diagram Z versus time o f f l i g h t ( f o r more d e t a i l s , see t e x t ) .

(10)

Results o f t h e Ar experiments /29,47/ are shown i n fi 10. They can be summarized as f o l l o w s : t h e e x i s t e n c e of four new isotopes 22C, jiN, 2 9 ~ e , 3ONe, t h e absence of 21C, 18B and t h e c o n f i r m a t i o n of the e x i s t e n c e o f 198 already seen by Flusser and Stevenson /13/ a t t h e Bevalac. Moreover t h e p a r t i c l e u n s t a b i l i t y o f 250 i s very 1 ik e l y . These r e s u l t s c a l l f o r two remarks :

i ) t h e neutron-drip l i n e has been reached up t o Z = 7 according t o a l l e x i s t i n g mass formula.

i i ) The somewhat s u r p r i s i n g i d e n t i f i c a t i o n o f 29Ne, which was p r e d i c t e d unbound by almost a l l mass p r e d i c t i o n s . On the o t h e r hand Uno and Yamada /28/ p r e d i c t an i n c r e a s i n g p a r t i c l e s t a b i l i t y i n the Ne r e g i o n (even Ne isotopes a r e p r e d i c t e d t o be bound up t o 3 8 ~ e ) . The o b s e r v a t i o n o f 2 9 ~ e i l l u s t r a t e s c l e a r l y t h a t a simple

d e t e r m i n a t i o n o f t h e bound c h a r a c t e r o f an i s o t o p e can p r o v i d e already a t e s t of t h e mass formulas. The p r o d u c t i o n o f such n e u t r o n - r i c h isotopes m i g h t be very

i n t e r e s t i n g f o r f u t u r e spectroscopic s t u d i e s as HF c a l c u l a t i o n s /48-49/ i n d i c a t e t h e onset o f a new deformation region, most apparent f o r 32Ne. It seems however very d i f f i c u l t t o go f u r t h e r o f f t h e s t a b i l i t y l i n e w i t h an Ar p r o j e c t i l e ; n u c l i d e s such as 260 o r 32Ne might, however, be w i t h i n t h e reach o f experiments w i t h nery

n e u t r o n - r i c h p r o j e c t i l e s such as 48Ca (see s e c t i o n

I ) .

i i i ) The observed cross s e c t i o n s f o r these very n - r i c h products a r e i n r a t h e r good agreement w i t h t h e c a l c u l a t i o n s presented i n s e c t i o n I (black c i r c l e s i n f i g . 1 a r e t h e estimated experimental t o t a l cross s e c t i o n s assuming an i s o t r o p i c angular d i s t r i b u t i o n i n t h e p r o j e c t i l e r e s t frame). It shows t h e good s e n s i t i v i t y o f t h e measurement as cross sections as low as 50 pb can be measured.

Fig. 10

-

Mass histograms of neutron r i c h isotopes f o r t h e l i g h t elements o b t a i n e d i n t h e r e a c t i o n 44 A.lleV Ar + Ta w i t h t h e LISE spectrometer (As t h e r i g i d i t y was s e t t o o p t i m i z e t h e p r o d u c t i o n of very e x o t i c isotopes, t h e d i s t r i b u t i o n s do n o t r e f l e c t t h e e n t i r e i s o t o p i c d i s t r i b u t i o n f o r a g i v e n

Z)

( r e f . 29,471.

Extending e a r l i e r i n v e s t i g a t i o n s based on deep i n e l a s t i c '9,501 and n 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 /7,40/, 14 new n u c l i d e s i n t h e r e g i o n 18

<

Z ( 30 have been observed /51/ f o r the f i r s t time i n the r e a c t i o n 33 A.MeV 8 6 ~ r

+

T i and p r e l i m i n a r y r e s u l t s w i t h t h e r e a c t i o n 43 A.MeV 86Kr + AU i n d i c a t e t h e bound c h a r a c t e r o f 71,72Ni (see f i g . 8) /41/. Obviously, these new isotopes are ranging f a r away t h e neutron d r i p - l i n e . However, t h e p r o d u c t i o n r a t e s o f n e u t r o n - r i c h species a r e expected t o increase considerably i f t h e e x t r a p o l a t i o n o f the r e c e n t Kr

+

Au r e s u l t s , d i s p l a y e d i n f i g . 8, h o l d t r u e ; t h i s i s very encouraging f o r f u t u r e spectroscopic s t u d i e s as 8-delayed neutron emission i n t h i s mass r e g i o n which i s i n

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C4-216 JOURNAL D E PHYSIQUE

the neighbourhood o f t h e

M

= 40 s h e l l and moreover o f a g r e a t i n t e r e s t f o r a s t r o p h y s i c s /7/.

On t h e o t h e r hand, t h e i s o t o p e 67Fe, which was observed i n LISE experiments f o r t h e f i r s t time, c o u l d be produced w i t h the r a t e o f 0.5 s-1 i n f u t u r e experiments (see f i g . 8). This i s o t o p e c o u l d be a good candidate f o r t h e observation o f neutron r a d i o a c t i v i t y as shown by c a l c u l a t i o n s o f Peker e t a l . /52/ which p r e d i c t t h e neutron emission frorn a h i g h s p i n isomeric state. (However no experimental r e s u l t i s p r e s e n t l y a v a i l a b l e concerning t h e angular momentum t r a n s f e r i n these fragmentation r e a c t i o n s ) . Obviously medium-heavy p r o j e c t i l e s as 76Ge, 82Se and 86Kr a r e very promising f o r f u t u r e spectroscopic s t u d i e s o f n e u t r o n - r i c h isotopes w i t h 18

<

Z c 30 ; above Z = 30, f u f t h e r experiments w i t h heavier beams such as Xe a r e needed t o decide whether o r not, conventional neutron-induced f i s s i o n i s becoming a more performing p r o d u c t i o n process.

IV.1.3

-

Towards t h e p r o t o n d r i p l i n e

I n t h e same way n e u t r o n - r i c h p r o j e c t i l e s have been used f o r producing n e u t r o n - r i c h nuclides, the p r o t o n d r i p l i n e has been i n v e s t i g a t e d u s i n g p r o t o n - r i c h p r o j e c t i l e s /53/. Two p r o j e c t i l e s have been used so f a r : 77 A.FleV 40Ca and very r e c e n t l y 55 A.MeV ! j 8 W i . I n b o t h cases a q u i t e neutron d e f i c i e n t t a r g e t was used ( 5 8 N i ) i n o r d e r t o enhance the p r o d u c t i o n r a t e o f p r o t o n - r i c h n u c l e i .

P r e l i m i n a r y r e s u l t s concerning the use o f t h i s 58Ni beam i n d i c a t e the bound

c h a r a c t e r ( l i f e t i m e 200 ns) o f 43V, 46-47Mn, 48Fe, 5°-52C0, S2Ni and 55-56C~. (The l a t t e r were obtained w i t h an A1 t a r g e t ) . The observation o f these new Cu isotopes would demonstrate c l e a r l y t h e e f f e c t i v e n e s s o f t r a n s f e r r e a c t i o n s even a t these The most o u t s t a n d i n g r e s u l t i s t h e c l e a r evidence f o r t h e bound c h a r a c t e r o f t h e T =

-

5/2 s e r i e s : 23Si, 27S, 31Ar and 35Ca as shown i n f i g . 11. [35Ca was f i r s t i a e n t i f i e d by i t s 8 delayed 2p a c t i v i t y

/54/]. Owing t o t h e steepness o f t h e

v a l l e y o f 8 - s t a b i l i t y on t h e proton-

z

r i c h s i d e and sometimes some remaining

background.events one must be c a u t i o u s ^20- b e f o r e making d e f i n i t e statement on

whether o r n o t t h e d r i p l i n e has been reached up t o Z = 20 /55/. Fig. 12 shows f o r the s e r i e s o f n u c l i d e s w i t h

TZ =

-

^3,

-

^5/2,

-

^2,

-

3/2 t h e p r e d i c - 15- t e d b i n d i n g energies

S

c a l c u l a t e d u s i n g the charge symme!;ys?g rmula o f Garvey and Kelson /56/ as done by Janecke using t h e most r e c e n t e x p e r i -

mental masses. From these p r e d i c t i o n s 10- /58/, one may then reasonably consider

t h e p r o t o n d r i p l i n e t o be mapped e x p e r i m e n t a l l y up t o Z = 20. The o n l y p o s s i b l e exception c o u l d be *2Si

p r e d i c t e d t o be bound a g a i n s t 2p emis- ^5-

._.

^_.

"

<=*

^:*,-e=-

s i o n by o n l y 16 KeV ! On t h e o t h e r 180 170 160 if0 of ns

hand, t h e observation o f 3 1 ~ r , p r e d i c -

t e d t o be unbound by 180 KeV a g a i n s t F i g . 11

-

Two dimensional p l o t o f d i r e c t 2p emission i s p a r t i c u l a r l y events ( s i m i l a r t o f i g . 9) which i n t e r e s t i n g as i t may be a'candidate e x h i b i t s c l e a r l y the p r o d u c t i o n o f f o r l o o k i n g f o r t h i s novel type o f ra- f o u r TZ =

-

5/2 isotopes i n t h e d i o a c t i v i t y . Such a decay has been r e a c t i o n 77 A.MeV 40Ca

+

Ni ( r e f . 53).

discussed 1 ong time ago by Go1 dansky /59/ and l a t e r on by Janecke /60/. A

b e t t e r candidate ti.e., h i g h e r Q value, l e s s 8-decay c o m p e t i t i o n ) f o r t h e 2p r a d i o a c t i v i t y c o u l d be 39Ti, pre$8cted t o be unhound a g a i n s t 2p emission by 600 KeV.

Unfortunately, i t was n o t observed even w i t h a 58Ni p r o j e c t i l e .

(12)

energies f o r producing e x o t i c isotopes. These p r e l i m i n a r y r e s u l t s would a1 so i n d i c a t e t h a t the p r o t o n d r i p 1 in e has been reached f o r odd elements Z = 23, 25, 27, 29.

IV.2

-

Study o f n u c l e a r ground-state

10 20 30

Besides the experimental p r o o f o f the e x i s t e n c e o f an e x o t i c isotope, even a t t h e d r i p - l i n e s , one needs o b v i o u s l y t o go f u r t h e r i n our knowledge o f t h i s e x o t i c n u c l e a r matter. Several groups a r e p r e s e n t l y working i n t h i s f i e l d using t h e

a c c e l e r a t o r s a v a i l a b l e i n the i n t e r m e d i a t e energy domain. F. Hubert e t a1 /61/ a r e studying 8-y c o r r e l a t i o n s i n n - r i c h isotopes. They have demonstrated the e f f i c i e n c y o f t h e LISE spectrometer associated w i t h a degrader i n t h e i n t e r m e d i a t e f o c a l plane. Several nucl i d e s have been s t u d i e d (8 6 Z

<

16) and t h e i r h a l f - l i v e s deduced. These r e s u l t s as w e l l as the method a r e discussed i n F. Hubert's paper /36/.

An i n t e r e s t i n g t o o l f o r p-n c o r r e l a t i o n s has been used i n an experiment w i t h t h e 180 beam d e l i v e r e d by t h e CERN synchrocyclotron /62/. It c o n s i s t s o f a 4% neutron b a l l , a very h i g h e f f i c i e n c y neutron d e t e c t o r which provides a p r e c i s e measurement o f t h e n m u l t i p l i c i t y . The p delayed neutron e m i t t e r 15B has been i n v e s t i g a t e d t h i s way. T h i s

technique should a l l o w a p r e c i s e measurement o f t h e branching r a t i o s Pons Pln, P2,,.

10 Other $ decay measurements have been performed a t

t h e NSCL l a b o r a t o r y a t MSU u s i n g t h e r e a c t i o n r o d u c t mass separator. The l i f e t i m e s o f 14Be and e7C have been deduced along w i t h 5 o t h e r i s o t o p e s /63/. They a l s o measured t h e pf branching r a t i o s

o f 9C t o t h e s t a t e s i n t h e unbound nucleus 9B Fig. 12

-

P r e d i c t e d s e p a r a t i o n

/64/ energies S and SEp f o r t h e

s e r i e s o f h ~ c l i d e s w i t h i sos- Another fundamental p r o p e r t y o f the nucleus i s p i n p r o j e c t i o n Tz =

-

3/2, t h e b i n d i n g energy s t r o n g l y r e l a t e d t o

-

^2,

-

^5/2,

-

³( f o r more de- s i n g l e - p a r t i c l e a n d c o l l e c t i v e e f f e c t s . E x t e n d e d t a i l s s e e t e x t .

systematic measurements over a l a r g e nunber o f isotopes may reveal new i n t e r e s t i n g trends.

Such a program devoted t o mass measurements has been r e c e n t l y s t a r t e d by M i t t i g e t a l . /65/ a t GANIL. A very o r i g i n a l method i s p r e s e n t l y used i n which t h e SPEG spectrometer i s coupled t o a t i m e o f f l i g h t measurement over a very l o n g f l i g h t path.

The mass i s deduced from t h e w e l l known r e l a t i o n :

The t a r g e t i s l o c a t e d a t the e x i t o f t h e l a s t c y c l o t r o n o f t h e a c c e l e r a t o r and t h e standard beam l i n e i s now used t o t r a n s p o r t t h e secondary beam up t o t h e

spectrometer. This provides a f l i g h t path o f 116 m which, w i t h a time r e s o l u t i o n o f 0.5 ns ensures a r e s o l u t i o n A V / V

-

^5.10-4. With a r i g i d i t y r e s o l u t i o n o f 10-4, t h e mass excess o f several l i g h t neutron r i c h n u c l i d e s (7

<

Z

<

9) has been measured w i t h an e r r o r c l o s e t o 500 KeV. R e s u l t i n g separation energies Sgn a r e p l o t t e d i n f i g . 13 f o r t h e s e r i e s o f N, 0 and F i s o t o p e s along w i t h a l l e x i s t i n g experimental r e s u 7 t s and compared w i t h the c a l c u l a t i o n o f Uno and Yamada /28/ and M i i l l e r and N i x 1661. An i n t e r e s t i n g r e s u l t i s t h a t t h e i n v e s t i g a t e d n e u t r o n - r i c h isotopes o f oxygen

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As f a r as the near f u t u r e i s concerning, we may conclude t h a t t h e measured y i e l d s f o r n u c l e i f a r from s t a b i l i t y a r e l a r g e enough t o s t a r t e x t e n s i v e decay studies. I n a d d i t i o n t o 6-y and mass measurements, systematic s t u d i e s o f 6 delayed neutron and p r o t o n emission can now be performed over a l a r g e mass and charge range. Moreover, somewhat more e x o t i c behaviour should be looked for, as f o r i n s t a n c e 2p and neutron r a d i o a c t i v i t i e s . However, t h e r e i s s t i l l a s t r o n g need f o r measuring p r o d u c t i o n c r o s s s e c t i o n s a t 0' and f u r t h e r i n v e s t i g a t e t h e r e a c t i o n mechanisms, e s p e c i a l l y f o r heavy beans. ( I t has been shown t h a t I(r r e s u l t s already d i s p l a y some new beha- v i o u r s ) . On the o t h e r hand, t h e r e i s a l a c k o f i n f o r m a t i o n on the charge s t a t e d i s - t r i b u t i o n s f o r t h i c k t a r g e t s o f various atomic numbers when u s i n g beams w i t h Z

>

30.

An improved knowledge of t h e experimental c o n d i t i o n s m i g h t be u s e f u l t o make p o s s i b l e t h e use o f o t h e r t o o l s such as He j e t systems o r o n - l i n e mass separators.

and f l uor are considerably

(1

t o 4 MeV1

Upgraded f a c i l i t i e s a t GANIL and MSU w i l l p r o v i d e us i n a few years w i t h even h e a v i e r beans a t s u f f i c i e n t l y h i g h energies. (For example 45 A.MeV Xe and 35 A.MeV Ta beams w i l l be a v a i l a b l e a t GANIL). New p e r s p e c t i v e s may be opened a l s o a t h i g h e r energies w i t h t h e appearance o f new f a c i l i t i e s as the SIS p r o j e c t a t GSI and Mimas a t SATURNE.

more s t r o n g l y bound than p r e d i c t e d ; t h i s corresponds t o a weaker decrease o f t h e b i n d i n g energies as compared t o t h e c a l c u l a t i o n s ( i n t h i s r e g i o n the I d 5/2 s h e l l becomes f i l l e d by t h e e x t r a neutrons w i t h r e s p e c t t o the 160 core).

This f i r s t experiment appears t o be v e r y promising f o r f u t u r e systematic study o f t h e b i n d i n g energies ; neon and magnesium i s o t o p e s would be o f p a r t i c u l a r i n t e r e s t i n vie^ o f t h e deformation e f f e c t s observed f o r heavy sodium isotopes. (There a r e c l e a r l y some i n d i c a t i o n s f o r Mg /67/ which r e q u i r e a d d i t i o n a l

measure~nents)

.

V. CONCLUDlNG REMARKS

Extension o f t h e f i r s t BEVALAC experiments a t GANIL and tlSU w i t h heavy i o n s a t i n t e r m e d i a t e energies,

way f o r producing has been and shown stud.ying t o be a n u c l e i very promising f a r from t h e p - s t a b i l i t y l i n e . The l a r g e i n t e r e s t o f

s 1 2 n ) IMCVI

N o F

X )

-\\(

¹² ¹⁶ ²⁰ ¹⁸ ² ^D ^2> ³⁶^+f

using r e c o i l spectrolieters i s now c l e a r l y established. A t these energies, b o t h

fragmentation and t r a n s f e r r e a c t i o n s c o n t r i b u t e Fig. 13

-

Experimental separa- t o t h e p r o d u c t i o n o f these e x o t i c species. t i o n energies Sen f o r N, 0 and Using t h e d i v e r s i t y o f the a v a i l a b l e beams a t F as a f u n c t i o n of t h e mass GANIL, neutron d e f i c i e n t ones such as 40Ca, ( b l a c k c i r c l e s : r e f . 58, open S8Ni o r neutron r i c h ones such as '+OAr, 86Kr, c i r c l e s : r e f . 65). S o l i d and has made p o s s i b l e t o map t h e neutron d r i p l i n e dashed curves correspond t o the up t o Z = 7, t h e p r o t o n d r i p l i n e up t o Z = 23. p r e d i c t i o n s o f ref. 56 and 66

r e s p e c t i v e l y . Several new o t h e r isotopes c o u l d be produced

u s i n g medium heavy-ion beams w i t h Z

<

36. L e t us mention 39Ti u s i n g 56Fe p r o j e c t i l e , 260,

3 2 ~ e w i t h 48Ca and n e u t r o n - r i c h species i n t h e r e g i o n 18 ⁶Z

<

30 w i t h 82Se o r 86Kr beams. The l a t e r should a1 low t o produce Ni isotopes may be up t o the doubly magic nucleus 78Mi. These new r e s u l t s , together w i t h those concerning mass and h a l f - l i f e measurements a r e summarized i n f i g . 14. These experiments have a l s o demonstrated t h e e f f e c t i v e n e s s o f t h i s energy range i n producing secondary beams w i t h a q u i t e h i g h p r o d u c t i o n r a t e .

(14)

&.@yE

I ]

a

^Newtso

+

M a - r r

Fig. 14

-

Summary o f experimental measurements performed w i t h i n t e r m e d i a t e energy heavy i o n p r o j e c t i l e s [GANIL, MSU, CERN-SC1 ( f o r n o r e d e t a i l s , see t e x t ) .

The c a l c u l a t e d d r i p l i n e s a r e a l s o shown as p r e d i c t e d by Uno and Yanada /28/ and Gorvey-Kel son formulae /56-58/. The l a b e l s 1, 2, 3 f o r ha1 f - 1 i f e measurements correspond r e s p e c t i v e l y t o r e f . 61, 63, 62.

F o r each 2, t h e two nunbers r e f e r t o t h e l a s t known i s o t o p e on b o t h t h e neutron r i c h and t h e p r o t o n r i c h sides. T r i a n g l e s i n d i c a t e t h e bound c h a r a c t e r o f new isotopes observed i n t h e r e a c t i o n 55 A.MeV 58Ni

+

58Ni and 43 A.MeV 86Kr

+

AU ( p r e l i m i n a r y ) . F i n a l l y , l e t us t h i n k o f t h e p o s s i b i l i t y t o g e t very h i g h i n t e n s i t y beams (as h i g h as 1014 s-1) as p r e s e n t l y under d i s c u s s i o n f o r t h e GANIL accelerator. That may o f f e r a l s o completely new p o s s i b i l i t i e s f o r producing secondary beams as w e l l as f o r t h e decay study o f n u c l e i very f a r from t h e 6 s t a b i l i t y l i n e .

ACKNOWLEDGEMENTS

I an g r a t e f u l t o a l l my colleagues from GANIL and ORSAY f o r f r u i t f u l discussions.

Special thanks a r e due t o F. Pougheon and D. Bazin f o r a n a l y s i n g r a p i d l y very r e c e n t r e s u l t s obtained w i t h Ni and K r beams, and E. Roeckl f o r a c r i t i c a l reading o f t h e manuscript. It i s a pleasure t o thank S. Geswend f o r t y p i n g and assembling t h e manuscript.

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