Potential energy effects and diffusion in the relaxed components of the reaction 197Au + 40Ar at 288 and 340 MeV bombarding energies

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Nuclear Physics A259 (1976) 173--188; ( ~ North-HollandPubhshmg Co, Amsterdam

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Not to be reproduced by photoprmt or mtcrofilm without written permtssion from the pubhsher

POTENTIAL ENERGY EFFECTS AND DIFFUSION IN THE RELAXED COMPONENTS

O F T H E R E A C T I O N 197Au+4°Ar

AT 288 AND 340 MeV B O M B A R D I N G E N E R G I E S L G MORETTO t, j GALIN t t R. BABINETttt Z FRAENKEL ~, R. SCHMITT, R. JARED and S. G. THOMPSON

Department of Chemistry and Lawrence Berkeley Laboratory, University of Califorma, Berkeley, Califorma 94720 Received 3 October 1975 (Revised 17 November 1975)

Abstract: The fragments emRted m the reaction between

197Au and 4°Ar at 288 and 340 MeV bombarding energies have been stud~ed. The fragments have been identified in atomic number up to Z = 32 by means of an E-AE telescope. The kinetic energy dlstnbutlons, the cross sections and the angular distributions have been measured for each Z. The kinetic energy distributions show the typical quaslelastlc and relaxed components; the Z-distributions show a smooth increase m the cross sectmn with increasing Z, interrupted at relatively forward angles by a fmrly sharp peak close to Z = 18. The angular distributions are forward peaked in excess of 1/sin 0 for atomic numbers as large as Z ~ 30, as far as twelve atomic number units above the projectile; flus is at varmnce with other reactions hke Agq-z ONe, where the angular distributions become 1/sin 0 four or five atomic number umts above the projectile. This Is interpreted m terms of an enhanced diffusion towards symmetry, possibly promoted by the potential energy in the intermediate complex corresponding to two fragments m contact.

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REACTIONS X97Au(4°Ar, X), E = 288, 340 MeV; measured a(E(fragrnent Z), 0), Z ~_ 32.

1. Introduction R e c e n t studies o f h e a v y - i o n reactions have s h o w n the presence o f a c o m p o n e n t m the cross section c h a r a c t e r i z e d by t h e r m a h z e d , or relaxed, kinetic energy distributions 1 - t s). T h e m o r e or less p r o n o u n c e d lack o f e q m h b r a t i o n , visible in the mass or charge d~stribution, and especmlly, the lack o f s y m m e t r y a b o u t 90 ° in the c.m. a ng u l ar distributions, rules o u t a c o m p l e t e associatton o f these r e a c t i o n p r o d u c t s with c o m p o u n d nucleus decay. I n the reactions i n d u c e d by 14N, 2 ° N e a n d 4 ° A r o n Ag, a s t r o n g d e p e n d e n c e o f the f o r w a r d p e a k i n g in the a n g u l a r d i s t r i b u t i o n u p o n the atormc n u m b e r o f the e m i t t e d f r a g m e n t has been sh o w n 1 - s). T h e f o r w a r d p e a k i n g in excess o f the 1/sin0 c h s t n b u t io n a p p e a r s to fade a w a y as the Z o f the o b s e r v e d fragt Sloan Fellow 1974--1976. ,t On leave from Institut de Physique Nuclealre, Orsay, France. *it On leave from DphN/MF-CEN, Saclay, France. On leave from the Weizmann Instttute, Israel. 173

174

L G. MORETTOet al

ment is farther removed from the Z of the projectile. Furthermore, an asymmetry in the effect has been observed, namely the fragments with atomic numbers smaller than that of the projectile are more forward peaked than the fragments with atomic numbex s larger than that of the projectde The above observations have been interpreted in terms of a diffusion mechanism along the mass asymmetry degree of freedom of an otherwise completely thermahzed "intermediate complex" whose shape corresponds approximately to that of two touching fragments x- 3, s, 6) The diffusion along the asymmetry coordinate generates a progressive tnne delay in the population of fragments with Z-values farther removed from that of the projectile This time delay is reflected in the average decay times of the varmus fragments. As the average decay time increases and becomes comparable with or greater than the mean rotational period of the "intermediate complex", the angular distributions tend to become symmetric about 90 ° and to assume the 1/sin0 shape. The stronger forward peaking in the angular distribution for fragments with Zvalues lower than that of the projectile has been interpreted m terms of the potentia! energy of the intermediate complex along the mass asymmetry coordinate. The entrance channel mass asymmetries of the three above cases (t*N, 2°Ne and 4°Ar on Ag), correspond to a region where the potential energy, expressed in terms of the mass asymmetry degree of freedom, falls rapidly towards extreme asymmetries. Thus the diffusion process drives the system towards large asymmetry, resulting in the rapid formation of fragments smaller than the projectile fragments. Heavier fragments can be produced more slowly by "uphill" diffusion. It appears that a further test of the effect of the potential energy upon the rate of diffusion is desirable, especially If the drift in the diffusion along the mass asymmetry coordinate can be reversed by a statable choice of the target-projectile system. In the present experiment, this has been tried. The 4°Ar +a 97Au system generates an intermediate complex with an Injection asymmetry (asymmetry at the beginning of diffusion) such that the potential energy (as determined from the liquid drop model) decreases substantially m the directton o f symmetry (see fig. 5). In this case, diffusion should drive the system towards a fairly rapid formation of fragments larger than the projectile. Thus, a substantial forward peaking in the angular distributions should still appear for fragments much heavier than the projectile. In a sense this should be an "experimentum crucls" to prove the effect of the potentim energy of the intermediate complex upon the time evolution of the system. Thts kind of evidence, coupled with the relaxed kinetic energy distributions should provide great support to the assumption of a diffusive mechanism responsible for the mass and charge transfer in heavy-ion reactions.

2. Experimental techniques The Ar beams at energies corresponding to 7 20 MeV/A (288 MeV) and to 8 5 MeV/A (340 MeV) were provided by the Berkeley Super-HILAC. The beam was

197Au-l-4°Ar

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collimated to a spot about 3 m m in diameter on the target and was collected by a Faraday cup. Typical intensities of the stripped b e a m were in the neighborhood of 100 nA electrical The target thickness ranged between 400 and 600 pg/cm2; such a thickness does not appreciably degrade the energies of the beam or of the emitted particles. Two AE-E telescopes mounted on movable arms were used to detect and identify the fragments The AE counters were gas lomzation chambers of a type described elsewhere 2, 3, 17) The gas used was pure C H 4 at a pressure of approximately 8 cm Hg. The entrance window of the AE counter (Formvar) was approximately 50 pg/cm 2 thick The E-detector was a SI solid state counter, 300 p m thick The pulses originating in the two telescopes were fed to standard linear and logic circuitry and were routed to a single A D C system through an analogue multiplexer. The digitized data were fed on line to a PDP-15 computer through a C A M A C system The data were finally recorded on magnetic tape for off-line analysis. Monitoring of the experiment was performed on line by means of an x, y storage scope and off-hne by printing E-AE maps A more detailed presentation of the data acquisition system has been published elsewhere 2, 3).

3. Data reduction The data reduction has been performed off-hne on a PDP-9 computer The E-AE contour maps generated from the data at each angle show well defined ridges corresponding to the various atomic numbers. The Z-resolution is quite good up to and above Z = 32 under optimal conditions. Because of the great range of energies assoclated with the various fragments, the optimal resolution has not been attained at all angles. N o attempt to analyze the data has been made in the regmns where the atomic numbers could not be resolved. The valleys separating two adjacent elements were identified by means of the coordinates of two or three points m the E-AEmaps. Then these coordinates were fed to the computer to obtain cross sections and kinetic energy distributions for each Z The energy calibration for the E-detector was obtained by means of a pulserchopper system which was checked with the elastically scattered beam. The energy calibrataon for the AE detector was obtained from the energy loss of the elastically scattered beam as computed from the range-energy tables. Corrections for the mean energy loss inside the target and for the energy loss within the plastic window of the AE counter were performed for each atomic number. The corrected differential cross sections (02a/Of2OE)l,b were transformed to the c.m. assuming for each fragment a Z/A ratio equal to that of the combined system. Because of the spread m kinetic energy associated with each Z, a distribution in the c.m. angle results As a consequence, the quoted c.m kinetic energy distnbuhons correspond to a small distribution of c m. angles For each Z the following quantities were calcu-

L.G. MORETTO et aL

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l a t e d m the c.m.: (t) T h e d o u b l e differential cross section (d2a/~DdE)¢ m" (il) The i n t e g r a t e d cross sectton (da/dO)¢ m" (m) The m e a n kmetlc energy

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M o r e d e t a d e d m f o r m a u o n r e g a r d i n g the d a t a r e d u c t i o n p r o c e d u r e has been p u b h s h e d elsewhere 2. 3). S o m e k m e m a t , c features o f the react,ons are g,ven in table 1. TABLe 1 Characteristics of the reaction 1997Au+~°Ar at two bombarding energies 4°Ar lab energy (MeV) 288 340 Ec.m. (MeV) E*c ,. (MeV) T©.. (MeV) L=,x (B,) err (b) 0sr,zl.g, lab (deg) BC.ulomb (MeV)

239 120 2.0 156 1.99 48 154

283 164 2.4 191 2 54 37 154

An ro of 1.225 fin was used to calculate nuclear radu. In addition, 2 0 fln was added to the sum of the nuclear rachl m the calculation of the reaction cross section a,, Coulomb barrier Bc.uJomb, maxamum angular momentum/-~ax, and grazing angle 0sra,,,s. The temperatures were calculated from Tc ,. = ~/-E-dc.o./a assuming a = }A and 1 = 0 h

4. The kinetic energy distributions I n this reaction, as m o t h e r p r e v i o u s l y studied h e a v y - i o n reactions, the kinetic energy spectra are characterized b y t w o c o m p o n e n t s : a high e n e r g y c o m p o n e n t which we call " q u a s i e l a s t l c " , a n d a low energy c o m p o n e n t which we call " r e l a x e d " . T h e relaxed c o m p o n e n t is c o m m o n t o all f r a g m e n t s a n d t o all angles, w h d e the quasielastic c o m p o n e n t is restricted t o particles with Z close to t h a t o f the projectile a n d to angles close t o the g r a z i n g angle. T h e d e c o m p o s m o n is n o t always so obvious for particles o f Z < 18 where b r o a d kinetic energy d i s t r i b u t i o n s , whose widths d e p e n d s t r o n g l y u p o n angle, are observed. E x a m p l e s o f c m. kinetic energy d i s t r i b u t i o n s are s h o w n m fig. 1. I t can be seen that, at least at b a c k w a r d angles, the relaxed k m e U c energy d i s t r i b u t i o n s are quite c o n s t a n t a n d i n d e p e n d e n t o f angle. The m o s t p r o b a b l e

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klneUc energms for each Z as a f u n c t i o n o f c.m. angle are s h o w n m fig 2 One can observe that, at sufficmntly b a c k w a r d angles, the average kinetic energtes b e c o m e i n d e p e n d e n t o f angle A t the f o r w a r d angles close t o the grazing angle, the m e a n kmeUc energies tend to b e c o m e larger. Th~s is p a r t i c u l a r l y true close to Z = 18, b u t it Is also q m t e evident for o t h e r parUcles with Z < 18 I n fig. 2 the widths ( F W H M ) are also shown for v a r i o u s Z-values. A g a i n , the widths t e n d to b e c o m e c o n s t a n t with increasing angle. A c o n s t a n t w i d t h is a t t a i n e d at relatively small angles for Z > 18, at s o m e w h a t larger angles for Z < 18. The last figure gwes a g o o d idea o f the a n g u l a r range over which the relaxed c o m p o n e n t o f the cross section is essentially free f r o m quaslelastm contributions. A p l o t o f the m e a n c.m. k m e t m energms as a function o f Z (fig. 3) shows t h a t these

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energies are strongly correlated with the energies expected to arise from Coulomb repulsion. The energies obtained from the Coulomb repulsion o f two touching spheres, and those obtained from the Coulomb repulsion of two touching spheroids allowed to attain their equdlbnum deformation, are also shown in the figure for comparison. While some doubts may remain as to which configuration is responsible for the final kinetic energy, the association of the experimentally observed kinetic energies with those of the calculated to arise from Coulomb repulsion is unmistakable. These features are m complete agreement with the results obtained from other heavy-ran reactions. The conclusions that can be drawn from the study of the kinetic energy distributions are uncertain. The only relatively clear conclusion refers to the relaxed component. Its fisslon-hke appearance suggests a nearly complete statistical eqmhbratmn of the kinetic energy with the Internal degrees of freedom 17). One can presumably expect a good deal of ordinary fission to be revolved in these reactions, and its mass/ charge distribution could well extend down below Z = 30. And yet lack of symmetry about 90 ° m the angular &stnbutlons, which we shall conslder later, Indicates the presence of a faster process. Such a process we have associated with the formation of an intermediate complex, retaining the approximate shape of two touching fragments, and chffusmg along the mass asymmetry degree of freedom 1-3, 6. 7). It is not clear at the moment how to separate two such components, namely the fissmn and the diffusion components. The quasielastlc component at times appears as a well defined peak; at others it appears to mix with the relaxed component, thus generating broad distributions. In fact in these cases it is not at all clear whether ~t is legitimate to assume the existence of two distract processes or whether one ~s dealing with a continuum of intermediate stages of relaxation

5. The charge distribution The lab cross sections at each angle are plotted as a function of atomic number m fig. 4 for both energies. The c.m. cross sections integrated over a fixed angular range are given in table 2. The general appearance of these cross sections is quite slmdar at both bombarding energies. At forward angles a very sharp peak is visible close to Z = 18. This peak is quite asymmetric. T o the right, for Z > 18, the cross section drops very rapidly, whtle to the left, for Z < 18, it decreases rather slowly. F o r sufficiently large lab angles, for instance 01ab = 70 ° at 288 MeV bombarding energy and 01ab = 60 ° at 340 MeV bombarding energy, the peak is absent and the cross sectmns appear to vary smoothly with Z. The asymmetry of the peak is closely associated with the kinetic energy spectra. Above Z = 18, the kinetzc energy spectra are relaxed and their w~dth Is essentially constant throughout the entire angular range. Below Z = 18, the kinetic energy spectra cannot be easily separated into the relaxed and quasielastlc components. The cross sectmns reported here are obtained by integrating the kinetic energy spectrum irrespective of its width Therefore they may

191Au+ 4OAr

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TABLE 2

Integrated c.m. cross sections for the observed charges E = 288 MeV 01 (deg)

25 24 86 85 85 85 85 84 85 85 85 85 86 86 26 26 26 26 26 26

E = 340 MeV

/~o2 d a /~130° do" O, 2~/ - - smOdO 2 z r / - - smOdO Oz (deg) Jo, d'Q .J3o ° d-Q (deg) (mb) (mb) 105 141 163 163 163 163 163 163 163 163 163 164 164 164 158 143 143 136 119 88

1.56 3 53 0 57 0 62 0.73 0.84 1 10 1 22 2 25 2 71 3.80 4 73 5.56 649 20 06 21 52 22.34 23.89 21.67 17.26

1.59 2 67 3.31 4.01 7.12 3.98 3 15 5 08 6.18 7 48 5 11 11.25 12 60 13.21 16 53 18 99 19 66 21 85 22 92 22 81

32 32 32 39 62 62 62 62 63 50 51 51 52 52 52 52

/~o2 da 02 2~rJ smOdO (deg) ol d-~ (mb) 77 117 163 163 163 163 163 163 164 164 151 152 144 137 119 120

2 02 3 64 5.87 5.25 2 18 2 42 2.87 3.67 5.09 8 10 7.95 9.12 9.56 9 73 8.64 9 18

( 130° da 2~z - - smOdO J 3 0 o d.Q (mb) 10 42 3 94 6 07 7.29 4 62 4 81 5 99 7 35 8 81 11.38 ti.06 11.86 13.23 14.14 13 54 14 07

?he hmlts of integration 01 and 02 define the angular range of experimental data where the relaxed component nlnated. The cross sections for th~s range are given m the next column. The last column gives cross sections obled from polynomial fits to the data from 30 ° to 130 °. Because of the experlmental errors and errors mtro:ed m the interpolation and extrapolation procedure, the quoted values may be in error by as much as 20 ~o.

incorporate a substantml amount of quaslelastic cross section which, not surprisingly, ~s c o n c e n t r a t e d a b o u t t h e a t o m i c n u m b e r o f t h e p r o j e c t i l e .

Still, t h e p h y s i c a l r e a s o n

f o r t h e a s y m m e t r y is n o t c l e a r ; m o t h e r w o r d s , it is n o t u n d e r s t o o d w h y t h e q u a s i elastic component

is p r e v a l h n g f o r Z < 18 T h e o b s e r v a t i o n t h a t t h i s e x c e s s c r o s s

section may be due to the break-up of the projectile could be of significance, but, at t h i s s t a g e , t h i s is j u s t a n o b s e r v a t i o n a n d n o t a n e x p l a n a t i o n The distributions observed at large angles, and thus assocmted with the relaxed component

of the kinetic energy spectrum are, m a sense, now qmte familiar. There

is a s o m e w h a t l a r g e r c r o s s s e c t i o n f o r t h e v e r y l i g h t Z - v a l u e s , r a p i d l y d e c r e a s i n g t o a m i n i m u m a r o u n d Z = 9, f o l l o w e d b y a d r a m a t i c i n c r e a s e m c r o s s s e c t i o n w i t h i n c r e a s i n g Z , c o v e r i n g a r a n g e o f a l m o s t t w o o r d e r s o f m a g n i t u d e f r o m Z = 9 t o Z = 28, 29 These features quahtatlvely resemble those observed in the reaction Ag+Ar [ r e f s . 2, 4, 5)], t h o u g h i n t h a t r e a c t i o n t h e rise i n c r o s s s e c t i o n w i t h Z ~s n o t q m t e as dramatic

Again, the correlation between the general trend of these cross sections

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Fig. 5. Potential energy of the intermediate complex relative to the corresponding Hlskes shape as a function of the atomic number of one of the fragments for various/-values The calculatmn has been performed for a configuration of two spherical llqmd drops in contact with Z and the potential of the lnterme&ate complex as a function of Z (fig 5) is qmte compelling and easdy observable in fig. 6. The potential shows the BusmaroGallone mountains well &splaced toward small Z-values and the potential sloping quite dramatically on b o t h sides towards both small and large Z-values Thus, ff one expects that the yield be approximately proportional to the Boltzmann factors

Y(Z) = k(Z, T) exp ( - Vzlr), one obtains a good qualitative explanation of the observed distributions On the other hand, as observed elsewhere, the quahtatlve agreement between the experiment and thin statistical prediction only indicates that the mechamsm responsible for the observed cross section is sensitive to the ratio Vz/T. It does not imply automatically that the dlstnbutmn is purely statistical in nature. Further evidence o f a Vz/T effect can be seen in the cross sectlons when plotted as a functlon of angle for each Z (fig 7). At small Z the cross sectlons for the 288 MeV bombarding energy are lower than the corresponding cross sections for the 340 MeV bombarding energy. At large Z the

L. G MORETTO et ol

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197Au+ 4OAr

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opposite is true, namely the cross section for 288 MeV ~s actually largerthan that for 340 MeV. The pivoting Z appears to be somewhere between 18 and 21. This can be explained as a Vz/T effect, whereby, at the larger energy (or temperature), the regions o f high and low potential are more nearly equalized m the cross section than at the lower energy (or temperature). In this respect, it is quite possible that the unknown change in the l-window assocmted with the change m bombarding energy may be partially responsible for such an effect However, the explanation in terms of temperature change seems more plausible. As a general comment, the similarity of the Z-distribution of the relaxed component with fission is quite striking. One can hardly avoid the association of the observed distrlbution with the left-hand side tail of a fission distribution. And yet, at least up to Z = 30, asymmetric angular distributions m&cate that there ~s at least part of the cross section which cannot be called just fission. And even for the rest of the cross section, higher m Z, one must be dealing with a very peculiar kind of fission, with a compound nucleus that has hundreds of MeV of excitatlon energy and, even at the lowest/-wave, has hardly any fission barrier left.

6. The angular distributions The c m. angular distributions for the fragments of various atomic numbers and for b,~th 288 and 340 MeV bombarding energies are shown in fig. 7. In these graphs, any quas~elastlc component of the cross section identified as a separate peak has been ehmmated. For the case m which no decomposition of the kinetic energy spectrum appeared to be feasible, the point has not been plotted. This procedure has been adopted m the attempt to obtain angular &str~butlons for the relaxed components only. Unfortunately, as noted before, for atomic numbers below Z = 18 the kinetic energy spectra m the forward direction appear to be anomalously broad. In other words, for these fragments either the relaxation of the kinetic energy is not complete or the quasielastlc component severely overlaps with the relaxed component, so that no correct decomposition ~s possible. Consequently, st is not clear how to Interpret these angular &stnbut~ons, because the kinetic energy spectra change qmte substantially in width when one moves from forward to backward directions The very strong forward peaking of these angular distributions appears to be associated more with the presence of a non-relaxed component than otherwise. It is qmte possible that for higher bombarding energies, the overlapping of quaslelastlc and relaxed components is not so severe On the other hand, the fragments with Z > 18 do not suffer from such a problem. The kinetic energy spectra become relaxed at reasonably forward angles and their widths remain constant for more backward angles. Consequently, it appears possible to study the angular distributions of the relaxed component of the kinetic energy for fragments w~th Z > 18. Inspection of these angular distributions shows that the excess forward peaking is retained for fragments with Z well above 18. At the lower

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bombarding energy, the angular d~strlbutlons are more sharply forward peaked than at the higher bombarding energy and the minimum is more displaced towards the backward hemisphere In both cases the excess forward peaking slowly decreases with increasing Z. However, the hmltmg 1/sin0 form of the angular &stnbutlon seems to be attained only around Z = 29, 30. Th~s IS to be contrasted with the results obtained for different systems, hke N +Ag, Ne +Ag, or Ar + A g [ref. 1- 6)], where the 1/sin0 limit ~s attained only four to five Z-units above the projectde. In the present case, the system must &ffuse more than ten Z-umts above the projectde before the I/sin0 limit is attained. Th~s evidence appears to be m good support of the &ffuslon hypothesis. As was mentioned in the introduction, the injection points for N + A g and Ne + A g are located to the left or close to the top of the Busmaro-Gallone mountain in the potentml energy versus asymmetry curve of the mtermedmte complex, a region where the potential energy slopes down rapidly towards the extreme mass asymmetries In the case of Ag + A t the rejection point is somewhat to the right of the Busmaro-Gallone summit However, the potentml energy trough at symmetry IS not well developed for most of the/-waves, so that, for Z > 18, the potentml energy is rather flat In the present case of Au +Ar, the rejection point is well to the right of the Busmaro-Gallone mountain, m a region where the potential energy IS steeply falling towards symmetry. Consequently, in the latter case, d~ffusion is driven by the potentml energy towards a large Z, which m turn, perimts their early decay and results in forward peaking In the former cases the potentml energy either hampers or does not help diffusion to populate large Z-values, thus resulting in a rapidly fading forward peaking A possible temperature effect is also seen in the angular &strxbutlons at the two energies At the lower energy the angular distributions appear to be sharper than at the higher energy, perhaps because the d~ffuslon process depends upon Vz/T and not upon Vz alone Thus the higher temperature appears to reduce the drift velocity assocmted with the potentml energy. A quantitative comparison of the present results with the theory presented in ref. 7) would be highly desirable. In fact, sueh a comparison has been performed successfully for the reaction Ag +288 MeV Ar [ref. 7)]. The most critical assumption made in the theoretical calculation for the latter reaction was the total absence of compound nucleus fission. In other words, it was assumed that all of the observed relaxed cross section was assocmted with the intermediate complex and not w~th compound nucleus fission Th~s very tentative assumption can be perhaps justified for relatively light combined systems on the following basis: (1) The low /-waves form a compound nucleus with a very high fission barrier. All of the compound nucleus cross section is to be found in the evaporation residue cross section. (ii) At h~gher/-waves the fission barrier goes down to zero, and with it the driving forced leading the system into an approximately spherical shape typical of the compound nucleus. An intermediate complex is then formed which ts responsible for the relaxed component. 0ii) At the h~ghest /-waves, quas~elastlc processes take place. Under

197Au"[-'~ °Ar

187

these assumptions, the l-window associated wtth the relaxed component can be tentatwely defined. The lower bound 1, is set by the evaporation residue cross section

=

Z (2l+ 1)T,. 1=0

The upper bound ~s set by the relaxed cross section

In the present case of Au + A r it 1s not clear how to determine the/-window. In fact, at all/-waves the fission barrier is rather small, if not absent Therefore, one may expect true fission to overlap with the relaxed component anslng from the decay of the Intermediate complex. Consequently one does not have available for the present reaction a c o m p o u n d nucleus cross sectmn to set a lower bound m the l-window as was the case m the prewously d~scussed reaction. Only a dynamical calculation, not avadable as yet, would be able to prowde this kind of reformation. In the mean time, one must rely on the quahtat~ve features of the reaction, keeping in mind that an excess forward peaking in the angular dlstnbut~on is observed despite the possible presence of a compound nucleus component behawng hke 1/sin0 7. Conclusion

In the reaction studied m the present work, the very same patterns wslble in prewously stud~ed systems have been observed The kinetic energies present b o t h the "quasielast~c" and the "relaxed" components, and the latter is s~mllar to what is to be expected from fission. The Z-distribution, once the quaslelast~c component is excluded, as very smooth and seems to reflect the qualitative pattern to be expected from the potential energy studies. The angular d~strlbutlons of the relaxed component are forward peaked and, d~fferent from previously d~scussed systems, retain their forward peaking up to very large Z = 29, 30 or eleven atomic numbers above that of the projectile. This has been interpreted m terms of the diffusion model proposed by Moretto and Sventek 7) The potentml energy of the lntermedlate complex, which ms rapMly decreasing towards symmetry, generates a rapid drift towards large atomic numbers, thus keeping the time delay between injection and decay fairly short for each atomzc number. In turn th~s seems to reflect ~tself on the forward peaking of the angular distnbutlons. A serious argument, still open, is whether th~s decay process associated wlth chffus~on is mixed with ordinary fission, and if so, how to separate the two Unfortunately, in th~s range of angular m o m e n t a and excitation energies, the ordinary compound nucleus theories are expected to be unreliable, because of vamshmgly small fiss,~on burners and because of predicted decay t~mes of the order of, or shorter than, the predicted collectwe periods of the compound nucleus

188

L. G MORETTO et al References

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