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TEMPERATURE AND LEVEL DENSITY
PARAMETER OF EVAPORATION RESIDUES
PRODUCED IN THE REACTION 165Ho + 600 MeV
20Ne
D. Hilscher, H. Rossner, A. Gamp, U. Jahnke, A. Giorni, C. Morand, A.
Dauchy, P. Stassi, B. Cheynis, B. Chambon, et al.
To cite this version:
TEMPERATURE AND LEVEL DENSITY PARAMETER OF EVAPORATION RESIDUES PRODUCED IN THE REACTION 1 6 5 ~ o
+
600 MeV 2 0 ~ eD. HILSCHER, H. ROSSNER, A. GAMP, U. JAHNKE, A. GIORNI*
,
C.
MORAND',A. DAUCHY', P.
STASSI*,B.
CHEYNIS",B.
CHAMBON*',
D. DRAIN",
C.
PASTOR**and
G.PETITT"'
Hahn-Meitner-Institut fiir Kernforschung, 0-1000 Berlin 39,
F.R.G.
'~nstitut
des Sciences NuclBaires, 53, Avenue des Martyrs.
F-38026 Grenoble. France
"~nstitut
de Physique NuclBaire, Universite Claude Bernard
Lyon I, 43, Boulevard du 11 novembre 1918, F-69622 Villeurbanne
Cedex, France
"'~eorgia State University, Atlanta, GA 30303, U.S.A.
Resume'
-
Les neutrons de pre'equilibre e t l e s neutrons Caporesa
p a r t i r des m s d'e'vaporation dans l a r d a c t i o n Ne (609 MeV) + Ho ont i t 6 u t i l i s e spour deduire l ' b n e r g i e d ' e x c i t a t i o n thermique E* e t l a temperature T des
re-
sidus. Une valeur du paramGtre de densite' de niveaux est alors de'duite de ces quantites, pour une temperature de 4.1 MeV.Abstract
-
Evaporative and preequilibr.ium neutrons emitted from evaporation residues i n the r e a c t i o nHo +
600 MeV neon are e x p l o i t e d t o deduce the t h e r - mal e x c i t a t i o n energy E* and temperature T o f t h e residues. From these quan- t i t i e s the l e v e l d e n s i t y parameter i s deduced a t a temperature o f 4.1 MeV.I
-
INTRODUCTIONSince the observation o f t h e disappearance
/l/
o f binary f u s i o n - f i s s i o n events i n hot n u c l e i the properties/2/
o f hot nuclear matter have obtained a renewed i n t e - ,rest. I n t h i s c o n t r i b u t i o n we t r y t o determine the l e v e l density parameter a a t highe x c i t a t i o n energy. I n order t o do t h i s we measure d i r e c t l y the temperature T o f an e q y i l i b r a t e d fused system and compare t h i s temperature employing t h e r e l a t i o n E* = aT
+
Bn w i t h the thermal e x c i t a t i o n energy E* deduced from other observables. Nuc- l e i can be heated i n heavy-ion c o l l i s i o n s t o high temperatures, however, w i t h i n - creasing bombarding energy an increasing number o f p r e e q u i l i b r i m (PE) l i g h t - p a r t i c - l e s are emitfed p r i o r t o the attainment o f thermal equilibrium. Thus i n order t o study the s t a t i s t i c a l properties o f hot nuclei, it i s necessary t o e x p l o i t observab- l e s t h a t measure ( i ) the transfer o f l i n e a r momentum as a measure o f dissipated k i - n e t i c energy, ( i i ) the e x c i t a t i o n energy and l i n e a r momentum c a r r i e d away by PE- l i g h t - p a r t i c l e s , and ( i i i ) the e q u i l i b r i u m temperature o f a nuclear system. I 1-
EXPERIMENTAL METHOD AND RESULTtrons were me ured i n coincidence w i t h evaporation residues (ER) i n the r e a c t i o n "'H0
+
600 RV ' i e . The ER were detected w i t h two AE-AE s o l i d s t a t e detector te- lescopes positioned a t k 5.3O 27 cm from the target. The ER were separated from other heavy fragments by means o f the measured t i m e - o f - f l i g h t and energy. Neutrons were detected by 9 NE213 s c i n t i l l a t o r s w i t h dimensions 5 cm X 10 cm (thickness X dia-meter) which were positioned outside o f a 2
rnn
t h i c k s c a t t e r i n g chamber 175-125 cmJOURNAL DE PHYSIQUE
from the t a r g e t a t angles between l @ and 1 5 9 . I n f r o n t o f each n-detector a 2 mm t h i n NE102 s c i n t i l l a t o r was positioned i n order t o detect high energy charged par- t i c l e s . The time r e s o l u t i o n o f a l l detectors was mainly given by t h e beam s t r u c t u r e o f the SARA-cyclotron a t Grenoble and was t y p i c a l l y 2.5 ns.
I n f i g u r e
1
neutron energy spectra are shown i n coincidence w i t h ER having veloci- t i e s between 0.68 and 1.0 cmlns corresponding t o a l i n e a r momentum t r a n s f e r (LMT) o f 89 %. Two components can be seen, a low- and a high-energy p a r t corresponding t o evaporation and PE-emission o f neutrons. These spectra have been f i t t e d w i t h a l e a s t square f i t method assuming two sources I 3 1 moving w i t h d i f f e r e n t . v e 1 o c i t i e s t o@
and e m i t t i n g i n each res't frame i s o t r o p i c a l l y . The spectra? shape i s assumed t o be &exp (-EIT). For the low energy component the v e l o c i t y was set t o the measured ve- l o c i t y of ER corrected f o r energy l o s s i n the 500 pm/cm Ho-target. For the high en- ergy component the source v e l o c i t y was used as parameter. Temperature and m u l t i p l i - c i t y was determined i n both cases from the l e a s t square fit. The dotted and dashed l i n e i n f i g u r e 1 are the r e s u l t o f such a f i t f o r the evaporative and PE component, t h e s o l i d l i n e i s the sum of both contributions. For the PE component t h e neutron energy spectra at a l l angles between l@ and 1590 were used whereas f o r the evapo- r a t i v e component o n l y t h e angles between 101° and 1 5 9 were used since at these angles the c o n t r i b u t i o n from PE neutrons i s s u f f i c i e n t l y small so t h a t the deduced temperature o f t h e low energy component i s not effected by PE neutrons. The deduced parameters from the f i t are shown i n f i g u r e2
as a f u n c t i o n o f ER-velocity f o r the evaporative component. We associate w i t h the deduced temperature o f t h e low energy component the mean nuclear temperature averaged over the n-cascade a f t e r a thermal equi l i brium has been reached. The temperature and the associated n-mu1 t i p 1 ic i t y i s increasing w i t h LMT up t o <Tn>= 3.76+
0.13 MeV and <M>
= 16.8 k 1.0 neutrons. The temperature Tn of the f i r s t neutron can be calulcated by Tn= (12/11).
<Tn> = 4.1+
0.14 MeV.For the PE component the m u l t i p l i c i t y i s approximately constant 2.5 t o 3.2 neutrons whereas the v e l o c i t y o f the hot moving source i s decreasing from 0.66 t o 0.49 times t h e beam v e l o c i t y whereas the temperature parameter T p ~ i s increasing w i t h i n - creasing LMT. Using the deduced v e l o c i t y V ~ E and m u l t i p l i c i t y w p ~ we f i n d t h a t the l i n e a r momentum given by VpE MPE corresponds t o 17%, 31% and 65% o f the missing LMT t o the ER a t LMT o f 51%, .70%, and 89%, respectively. Since we f u r t h e r - more f i n d t h a t about h a l f as many PE protons as neutrons are emitted f o r the high- est LMT, t h e complete missing LMT has been found f o r the highest LMT i n PE nucleon emission. For smaller LMT other not measured processes f o r instance emission o f a-
p a r t i c l e s o r other heavy fragments are contributing. This can also be seen from f i g . 3 where the integrated y i e l d f o r low energetic neutrons between 5.5 and 17.5 MeV i s shown f o r a l l three LMT. Only f o r the highest LMT o f 89% i t i s possible t o describe the n - y i e l d also at forward angles by using t h e parameters obtained a t backward angles (101-1593). For the low LMT a t forward angles there i s additional n-yield which i s not consistent w i t h the assumption o f i s o t r o p i c emission i n a rest-frame moving w i t h the v e l o c i t y o f detected ER. I n summary, from these f i n d i n g s we conclude t h a t o n l y f o r the highest LMT o f 89%, the measured q u a n t i t i e s are com- p l e t e i n order t o explain the f u l l missing l i n e a r momentum by P E - l i g h t - p a r t i c l e - emission. Thus we assume t h a t we can deduced from the measured data also the mean e x c i t a t i o n energy <EpE>
=
I.5*MpE=(B+
1.5-TpE) c a r r i e d away by the PE l i g h t - p a r t i c l e s i n addition t o the mean l i n e a r momentum M ~ E-
VPE taken away by these p a r t i c l e s ; B i s the neutron o r proton binding energy plus Coulomb b a r r i e r . We can c a l c u l a t e t h e e x c i t a t i o n energy o f t h e ER a f t e r attainment o f thermal equilibrium:where m ~ , EP, Vp, Qgg, <Erot>, and c i s t h e mass o f the target, energy per
as the value a = 17-20.MeV-~ which i s given by D i l g e t al.
NI.
References
/l/ Conjeaud, M. e t al., Phys. Rev. L e t t . 159B (1985) 244. 121 Dean, D.R. and Mosel U., Z. Phys. A322 (1985) 647.
131 Holub, E., e t
a l . ,
Phys. Rev. C33 (1986) 143 and Phys. Rev. C28 (1983) 252. I 4 1 Dilg, W., e t al., Nucl. Phys. A217 (1973) 267E,-,,,
(MeV) V,, (cm/ns)JOURNAL
