EVOLUTION OF THE FUSION LIKE PROCESS AROUND THE FERMI ENERGY

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EVOLUTION OF THE FUSION LIKE PROCESS

AROUND THE FERMI ENERGY

E. Pollacco, J. Alarja, Y. Cassagnou, B. Chambon, B. Cheynis, M. Conjeaud,

R. Dayras, A. Douchy, D. Drain, A. Giorni, et al.

To cite this version:

E. Pollacco, J. Alarja, Y. Cassagnou, B. Chambon, B. Cheynis, et al.. EVOLUTION OF THE

FUSION LIKE PROCESS AROUND THE FERMI ENERGY. International Conference on Heavy

Ion Nuclear Collisions in the Fermi Energy Domain, 1986, Caen, France.

pp.C4-377-C4-380,

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MORAND' , J .B. VIANO' C. VOLANT'

+Service de Physique NuclBaire, Basse Energie, CEN-Saclay, F-91191 Cif-sur-Yvette Cedex, France

' Institut des Sciences NuclBaires, F-38026 Grenoble, France " ~ n s t i t u t de Physique Nucl@aire, UniversitrS Claude Bernard, Lyon I, F-69622 Villeurbanne Cedex

"'oak Ridge National Laboratory. Oak Ridge, TN 37831, U.S.A. Risum6

-

L86tude des r i s i d u s d86vaporation du systzme Ne + Ag montre un chan-

qual i t a t i f dans l e dcanisme de r i a c t i o n aux alentours de 1 ' C e r g i e de Fermi. A 20 MeVIu l e p r o j e c t i l e e s t essentiellement absorb5 par l a c i b l e tandis qu8S 30-37 MeV/u on observe un t r a n s f e r t de masse continu accompagne d'un grand moment transverse.

Abstact

-

The study of evaporation residue from the Ne

+

Ag system shows t h a t t h e r e s q u a l i t a t i v e change i n the reaction mechanism i n the Fermi energy domain. A t 20 MeVIu the p r o j e c t i l e i s mostly absobered by the target, while a t 30-37 MeV/u a continious range o f mass t r a n s f e r w i t h a l a r g e transverse momentum i s observed.

I n v e s t i g a t i o n o f the evaporation residue a f t e r col l i s i ons i n v o l v i n g l a r g e momentum t r a n s f e r s plays an important r o l e i n t h e determining the change i n the reaction mechanism i n the Fermi energy d m a i n.

The reaction Z0Ne + Ag a t 20, 30 and 37 MeVIu was studied using the SARA f a c i l i t y . The evaporation residues were detected using a time o f f l i g h t consisting o f a

MCP

system and a t r i p l e detector telescope t o gfve a f l i g h t path o f 42 cm. Recoil mass versus v e l o c i t y spectra were measured f o r angles between 4' and 35', r e l a t i v e t o the beam axis. The contour p l o t f o r the v e l o c i t y as a f u n c t i o n o f mass i s shown i n f i g . I. As seen, the m a j o r i t y o f the events i n the choosen v e l o c i t y region consists main-

l y o f residus which are characterised by t h e i r mass ( % 100 amu). These events are e a s i l y discriminated against f i s s i o n and low energy p r o j e c t i l e l i k e fragments. O f

i n t e r e s t i s t h a t a t 30 MeV/u, a l a r g e f r a c t i o n o f the events l i e below the compound nucleus v e l o c i t y VCN.

The mass p r o j e c t i o n a t 20, 30 and 37 MeV/u are e s s e n t i a l l y Gaussian i n shape and have mean values o f 106, 100 and 95 amu respectively. This suggests t h a t the o v e r a l l e x c i t a t i o n energy, E' increases w i t h bombarding energy. I n f a c t using mean values o f the i n v a r i a n t v e l o c i t y spectra (V > 0.4 cm/ns) and the massive t r a n s f e r description gives an increase o f approximatly 100 MeV i n E* between 20 and 37 MeV/u.

I n fig. 2 the I n v a r i a n t v e l o c i t y spectra a t t h e three energies are superimposed. A t 20 MeVIu t h e spectrum shows a peak corresponding t o 70 % o f VCN, which i n a massive

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JOURNAL DE PHYSIQUE

40 so 120

M A S S a m u

Fig. 1

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Mass vs v e l o c i t y spectrum a t 10". 2 0 0 0 0 L l l O D 16000 2 0 M eV/u 8 0 0 0 0 .,.Q '000 t r a n s f e r p i c t u r e t r a n s l a t e s i n t o 14 amu being embedded i n t o the target. What i s s i n g u l a r l y i n t e r e s t i n g i s t h a t increasing the energy t o 30

MeVIu leaves only a minor evidence _.4

o f t h i s peak. A t 37 MeV/u the peak 1. 1.6

disappears completely

.

The same

_V

cm/ns

shown when plotting _Fig.

₂

-

_{I n v a r i a n t v e l o c i t y spectra a t} 10" for the i n v a r i a n t cross section i n a

p a r a l l e l versus perpendicular vel o- 20Ne + Ag a t 20, 30, 37 MeVIu). c i t y plane ( f i g . 3). Again a strong N e + A g 2 0 MeV/u source i s present a t 20 MeVIu,

while a t 30 MeVIu the events are spread over a l a r g e range o f sour- ces.

Before deal i ng w i t h the angular d i s t r i b u t i o n i t w i l l be p e r t i n e n t t o remark t h a t comparing the 20 MeV/u w i t h other systems, namely 160 + Al, Ca, Ni a t d i f f e r e n t energies /l/ shows, i n a V i o l a p l o t t h a t the Ne data i s approximatly

12 % below the trend. O.=Som/n-

The angular d i s t r i b u t i o n s were obtained by i n - t e g r a t i n g the evaporation residue events f o r v e l o c i t i e s l a r g e r then 0.4 cmlns and are pre- sented i n f i g . 4. Again there e x i s t s a strong d i f f e r e n c e between 20 and 30 MeVIu and s t r i c k - i n g l y enough l i t t l e change between 30 and 37 MeVIu. Thus a t 20 MeVIu the events are concen- t r e t e d i n t h e forward angles as expected from a q u a s i - f u l l momentum t r a n s f e r process. A t 30 and 37 MeV/u t h e data i s s h i f t e d t o l a r g e angles

--

and implies t h a t a strong transverse momentum Fig.

-

_Invariant_velocity_contour plots.

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t o b 1 4 . The calculated d i s t r i b u t i o n of evaporated masses give a reasonable descrip-

t i o n t o t h e evaporative component measured by Y. Cassagnou e t al./3/ f o r the same

system and energy. The calculation a r e given i n f i g . 6 and show t h a t the data (0.8 0.9 cm/ns) have a considerably larger width. Thus f a s t p a r t i c l e s and evaporation do not reproduce the width and thus l a r g e transverse momentum, such a s two-body proces- s e s i s required t o expl ane t h e data.

To summarize the data a t 20 MeV/u shows t h a t the p r o j e c t i l e i s mostly absorbed by

t h e target. A t 30-37 MeV/u there

i s

a continuous range of mass t r a n s f e r s and large

transverse momentum. Hence a change i n reaction mechanisms i n the formation of

residues

is

present between 20 and 30 MeV/u.

REFERENCES

/ l / Chan, Y. e t a1

.,

Phys. Rev. C27 (1983) 447.

/ 2 / Blann, M., Phys. Rev. C31 (1V85) 1245.

/3/ Cassagnou, Y

.

e t a1

.

,

m e pub1 i shed.

Fig. 4

-

Residue angular d i s t r i b u t i o n s f o r 20Ne

+ Ag a t 20, 30 and 37 MeV/u. A-

..

'. J \

'.

-

A E0 M.VIU

-

QS

'.

'A

-

N e

+

A g \-

-

_{tor V s 0 . 4 s m h .}

-

~ r 6 0 1

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30URNAL DE PHYSIQUE

Fig. 5

-

Residue angular d i s t r i b u t i o n gated by

the vel oci ty.

S... Id..,