PREEQUILIBRIUM EMISSION OF NUCLEONS IN THE SYSTEM 40Ar + 40Ca AT 20 MeV/u

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PREEQUILIBRIUM EMISSION OF NUCLEONS IN THE SYSTEM 40Ar + 40Ca AT 20 MeV/u

W. Rösch, A. Richter, G. Schrieder, R. Genter, K. Keller, L. Lassen, W.

Lücking, R. Schreck, W. Cassing

To cite this version:

W. Rösch, A. Richter, G. Schrieder, R. Genter, K. Keller, et al.. PREEQUILIBRIUM EMISSION OF

NUCLEONS IN THE SYSTEM 40Ar + 40Ca AT 20 MeV/u. Journal de Physique Colloques, 1986,

47 (C4), pp.C4-389-C4-391. �10.1051/jphyscol:1986446�. �jpa-00225812�

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JOURNAL DE PHYSIQUE

C o l l a q u e C4, supplbment au n o 8, Tome 47, aoOt 1986

PREEQUILIBRIUM EMISSION OF NUCLEONS IN THE SYSTEM 4 0 ~ r + ^{' O C ~}AT 20 M@V/U( l )

W. R ~ S C H , A. RICHTER, G. SCHRIEDER, R. GENTER', K. KELLER*, L. LASSEN', W. L ~ C K I N G * , R. SCHRECK* and W. CASSING"

Institut fur Kernphysik, Technische Hochschule Damstadt, D-6100 Darmstadt, F.R.G.

'~hysikalisches Institut, Universitzt Heidelberg.

D-6900 Heidelberg, F.R.G.

. .

Institut fiir Theoretische Physik, Universitat Giessen, D-6300 Giessen, F.R.G.

R&sm6

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Pour l e systsme s p 6 t r i q u e 4 0 ~ r + 4 0 ~ a 2 20 MeV/u nous avons mesur6

P.

les neutrons e t l e s particules lbg&res chargees en coincidence avec l e s r6si- dus d'evaporation. Une composante de preequilibre e s t presente tant dans l e spectre d'energies que dans l a distribution angulaire. Nous avons aussi me- sure l e s multiplicit6s. Les donn6es ont 6t6 comparbs avec des calculs avec un nodsle qui t i e n t cmpte de l'espace de phase.

Abstract

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In the mass symmetric system 4 0 ~ r + 40 Ca a t 20 MeV/u neutrons and light charged particles were measured i n coincidence with evaporation residues. The shape of the neutron energy spectra and t h e i r angular distribution show a preequilibrium component. Multiplicities were deternined. The data are compared to phase-space calculations.

I

-

INTRODUCTION

The equilibration process of f i n i t e systems of fermions and the related problem of the competition of mean f i e l d dynamics and successive nucleon-nucleoncollisions are

~f great i n t e r e s t i n heavy ion physics. A tool t o study these questions are the emitted l i g h t particles. Especially neutrons are suited for the investigation of the equilibrium process, since they do not underly Coulomb interactions. Experiments de- signed to measure preequilibrium emission have been done up t o now only f o r a s p e - t r i c systems / l / . The interpretation i s complicated due to the different velocities of the particle emission sources. In symetric systems the source velocities are a l l the same and preequilibrium components can be extracted from angular distributions and the shape of the neutron energy spectra,

I1

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EXPERIMENTAL METHODS

In f i g . l the experimental set-up is shown. The pulsed 4 0 ~ r beam (300 ps) from the UNILAC of GSI with an energy of 20 MeV/u has h i t a 4 0 ~ a target (800 pg/un2) and was dumped i n a shielded Faraday cup. Fast neutrons, protons and deuterons were measured i n coincidence with evaporation residues (trigger f o r central collisions). The evaporation residues were detected by surface barrier detectors i n the angular range of -3' t o -1 1

'

and t h e i r masses were determined by the time of f l i g h t method. The neutrons were measured i n two position sensitive neutron detectors (PSNCI

,

PSNCZ) and three standard neutron detectors (NI, N2, N3). Position sensitive p l a s t i c scin-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986446

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C4-390 JOURNAL DE PHYSIQUE

t i l l a t o r s placed i n front of the neutron counters served a s detectors f o r the highly energetic protons and deuterons. This set-up allowed measurement of a complete angular distribution.

P u l s e d 4 0 ~ r - earn

0 20 40

E

n-cm

(MeV)

Fig. 1 Experimental set-up Fig. 2 Neutron energy spectra

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^RESULTS

In f i g . 2 the yield of neutrons measured i n coincidence with evaporation residues i n the angular range Om = 28' t o 50° is shown a s a function of the neutron energy i n the center of mass system

.

Emission from an equilibrated nucleus a t a temperature of 6 MeV (dashed l i n e ) , which was obtained universally over the whole angular range, can account f o r the low energy p a r t of the spectrum. A t higher energies we find an additional conponent i n the neutron spectrum. The experimental data can be reprodu- ced Ey adding a preequilibriun component (dotted l i n e ) calculated according t o a phase-space model t o the thermal emission yields. The f u l l y drawn l i n e shows the sum of the two components.

In f i g . 3 the angular d i s t r i b u t i o n of neutrons i n coincidence with the evaporation residues is shown together with calculations (solid l i n e ) from a phase-space mo- d e l /2/. The dashed l i n e s a r e the limits of the model predictions.

The angular d i s t r i b u t i o n of the neutrons of energy 2 0 MeV i s nearly isotropic i n the center of mass system. A t 30 MeV and 40 MeV there is a c l e a r forward and backward peaking. This r e s u l t can be interpreted by a superposition of an isotropic equilibrium cm.ponent, and a p r e e q u i l i b r i m yield, which i s s i g n i f i c a n t l y enhanced a t higher energies 'and oriented along the beam axis.

The cross section f o r the production of evaporation residues i s about a f a c t o r 2 higher than expected from t r a j e c t o r y model calculations /3/ extrapolated t o higher energies. This enhancement can be explained by p a r t i c l e emission i n the e a r l y stage of the reaction.

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The total neutron multiplicity is _12.Taking the relative yield of the phase-space model calculations into account, a multiplicity of 2 for preequilibrium neutrons is derived.

4 0 ~ r + 4 0 ~ a E,,, = 2 O M e V l u

2 0 0 E,.,,= 2 0 M e V

Fig. 3 Angular distributions of neutrons at various neutron center of m s s energies

In summary, the double differential neutron yield in coincidence with evaporation residues indicates that neutrons might well serve as a probe for nonequilibrium nuc- lear dynamics. Energy spectra show very energetic neutrons which cannot be descri- bed by classical Fermi-jet models /4/ and require a quantum-mechanical treatment of the emission process /2/. Furthermore, the angular distributions demonstrate that up to two successive nucleon-nucleon collisions must be considered in the nonequilibrium component, indicating a rather fast equilibration process in the reaction zone of finite nuclei.

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