Formation and decay of hot nuclei

Volume 195, number 1

PHYSICS LETTERS B

27 August 1987

F O R M A T I O N AND DECAY O F H O T N U C L E I ~ C. V O L A N T , M. C O N J E A U D , S. H A R A R , M. MOSTEFAI, E.C. POLLACCO, Y. CASSAGNOU, R. DAYRAS, R. L E G R A I N Service de Physique Nucldaire - Basse Energie, CEN Saclay, F-91191 Gif-sur- Yvette Cedex, France

G. K L O T Z - E N G M A N N and H. O E S C H L E R lnstitut j~r Kernphysik, Technische Hochschule, D-6100 Darmstadt, Fed. Rep. Germany

Received 16 March 1987

Formation and decay properties of composite-like nuclei produced by 58Niprojectiles bombarding 232Thtarget nuclei are investigated at different energies by means of fission fragment angular correlation measurements. Experimental results are compared to the exciton model and excitation energies in the fissioning nuclei are deduced. By comparing the present data with the results on other systems, the influences of entrance channel conditions and composite-like nuclei temperatures are discussed to explain the evolution of the high linear momentum transfer region. The coexistence of both massive transfer mechanism and head-on collisions followed by preequilibrium is suggested.

The fusion-fission process was studied recently for the 4°Ar+232Th [ 1,2] and 238U [3] systems by means of fission fragment angular correlation measurements at incident energies around the Fermi energy. It was shown that the central collisions ( C C ) peak located around a linear m o m e n t u m transfer ( L M T ) Pi ~ 7 GeV/c has a decreasing yield with the incident energy and vanishes at 44 MeV/u leaving a continuous spectrum. It has been suggested that a limiting temperature in the quasi-compound nuclei is responsible for the disappearance o f this b u m p [ 4 ]. Nevertheless, at the highest incident energy, the fission events located around 7 GeV/c are shown to correspond to very high excitation energy deposit [ 1 ]. In order to check if the entrance channel could influence the formation and decay of hot nuclei, we have studied the SSNi + 232Th system which eventually can deposit higher m o m e n t u m and excitation energy than for the 4°Ar induced collisions for the same projectile velocity. The experiment was performed at the G A N I L facility using S8Ni beams at 20, 25 and 30 MeV/u with intensities ranging from 50 to 100 nA. The experiExperiment performed at the GANIL National Laboratory. 22

mental set-up was similar to the one described in ref. [ 2]; in the present measurement two time-of-flight telescopes were located at 30 ° and 70 ° in coincidence with eight silicon detectors separated by 10 ° on the opposite side o f the beam, the most forward being placed at 40 °. Angular correlations between fission fragments (FF) have been measured and both their masses and velocities after particle evaporation determined event-by-event. This method allows the measurement of the recoil velocity (VR) and the direction (0R) of the fissioning nucleus as well as the relative FF velocity (VFF). Results at 30 MeV/u bombarding energy are presented in fig. 1 for two different regions of folding angles (0VF). At low m o m e n t u m transfer (Ovv= 170 °), the two-dimensional plot o f both FF masses (fig. la left) shows a well-defined region and their relative velocities (fig. lb left) are located around 2.34 cm/ns, in good agreement with the Viola's systematics [ 5 ] of FF total kinetic energies; the detected sum mass is around that o f the target mass. The recoil velocity matrix (perpendicular versus parallel components) shows two peaks: the peak labelled (A) in fig. lc left centered at ( Vii, Vl ) ~ (0.05-0.2)cm/ns and the other labelled (B) at ~ (0.14, 0.2)cm/ns are compatible, respec0370-2693/87/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Volume 195, number 1

PHYSICS LETTERS B

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Fig. 1. (a) Two-dimensional plot of the two fission-fragment masses (MA versus Ma) for peripheral collision events (OFF = 170 °, left) and for the CC events (sum of 0 FF= 90 o, 100 ¢ and I l0 °, right) for the SSNi+232Th system at 30 MeV/u incident energy. (b) Same as in (a) for the total mass MToT=MA+MB versus the relative FF velocity for events above the solid lines in (a). (c) Same as in (b) for the parallel ( Vii) versus perpendicular ( V±) components on the beam axis of the fissioning nucleus recoil velocity VR. Because of the rapid variation of V~,with relative angle only data for OFF=90 ° are displayed in the right side. Lines are discussed in the text. tively, with one- a n d four-nucleon transfers to the target assuming the ejectile is e m i t t e d at the grazing angle ( ~ 9 ° lab). F o r these processes corresponding to L M T Pm~ 1.25 a n d 2.25 GeV/c, respectively, only one o f the two possible solutions (heavy recoil in both side o f the b e a m ) is detected for the given OFF and the second solution lies at a different correlation angle. Thus for large 0~F, the present o b s e r v a t i o n shows, as expected, that we are dealing with quasielastic induced fission a n d illustrates at the same time

27 August 1987

the precision o f the e x p e r i m e n t a l technique. At high m o m e n t u m transfer ( 0 r F ~ 100 ° ) the recoil angle is smaller a n d one is able, in the massive transfer picture, to assign an average L M T to a m e a s u r e d OFF assuming symmetric mass splitting (Pip ~ 9.5 GeV/c). The F F mass d i s t r i b u t i o n (fig. l a right) is significantly b r o a d e r than the one observed for peripheral collisions but nevertheless well defined within a 5 0 - 1 5 0 a m u range. Because o f high excitation energies reached in the fissioning nuclei, the m e a n total detected mass is about 200 a m u (fig. l b right). The most probable VFF value for s y m m e t r i c mass division is located at ~ 2.5 crn/ns which is consistent with the Viola's systematics for A ~ 2 8 0 composite-like nuclei. F u r t h e r m o r e the VFF distribution exhibits a large tail reaching up to 4 cm/ns; an analysis o f these unusual high kinetic energy events indicates that they arise from b i n a r y a s y m m e t r i c mass splitting (50 ~
Volume 195, number 1

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PHYSICS LETTERS B

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As already noted for lighter projectiles [6] one observes again a remarkable scaling of momenta imparted to target nuclei with the projectile mass: P~ = (180_+20 MeV/c)XAproj although the present 58Ni result ( ~ 165 MeV/c) is at the lower boundary. This scaling is important since it strongly suggests that the whole projectile interacts with the target nucleus in head-on collisions. To explain the shape of the measured correlations and their evolution with the bombarding energy we invoke the abrasion-ablation model, extended to include kinetic effects due to separation energy [ 7 ] and preequilibrium emission. For a given incident energy the abrasion-ablation model predicts a decreasing cross section with LMT, as seen in the data and a concentration of cross section at full LMT, when the projectile is stopped in the target. Thus in this picture the fall in cross section with Ovv corresponds to the decreasing probability of massive transfer [ 1 ] and the CC peak arises from the interaction of the entire projectile with the target. Further, the calculations as a function of incident energy shows that the cross section for the low LMT peak 24

27 August 1987

and its high LMT tail is essentially constant while the yield for CC events decreases, as observed experimentally. Assuming preequilibrium emission the recoil velocity for CC events is lowered. In fact, calculations using the exciton model of Blann [ 8 ] give values for the position of the peak which reproduces the data as a function of bombarding energy and projectile mass as shown in table 1 where results for the ~4N projectiles at 60 MeV/u on the same target are also added [ 9 ]. Thus the exciton model is also capable of reproducing the momentum scaling. Through the exciton model we can also comment on the excitation energies (E*) reached. To simulate the central collisions, the number of excitons was set equal to the projectile mass number and the initial excitation energy was chosen to that of the compound nucleus with corrections for the Q-value. The nucleon binding energies for the exotic nuclei formed were derived from the liquid-drop model. Variation of the exciton number by a few percent does not effect the predicted LMT and only weakly the excitation energies. Table 1 also gives the detected mean total mass of both fragments which diminishes with incident energy, indicating an increase in E*. The predicted E* are in good agreement with the total detected masses when assuming that each nucleon evaporated from the thermalized system carries away 12 MeV. It has been suggested [4] that the temperature of the fissioning nuclei is responsible for the disappearance of the CC events. In fig. 3, the cross section for the high LMT events, as extracted by Conjeaud et al. [ 1 ], are plotted as a function of e given in table 1. In fig. 3 is also shown a value for the 14N+232Th system obtained from an extraction of the CC events [ 9 ]. It is encouraging that the N, Ar and Ni data fall close to a line giving a limiting value of e ~ 4 MeV, which is in turn consistent with values for the limiting temperature predicted by theories [ 10]. It is, on the other hand, important to note that this agreement does not exclude other possible quantities as key parameters. Thus, for example, the data plotted as function of EcM--VcB, where EcM is the CM energy and VCB is the Coulomb energy at the barrier, show the same clustering along a straight line (fig. 3b). In other words, the fall in cross section can also be related to the initial available energy and the dynamics in which it is dissipated. To be able to dis-

Volume 195, number 1

PHYSICS LETTERS B

27 August 1987

Table 1 Comparison between the experimental data for central collisions and the results of the exciton model for the SSNi, 4°Ar and laN q-232Th systems. system

Ei a)

Plib)

MCLN c)

E~LN d)

Ee)

Pliexpf)

mexpg >

Mcalc h)

(MeV/u)

(GeV/c)

(amu)

(MeV)

(MeV/u)

(GeV/c)

(amu)

(amu)

58Ni+232Th

20 25 30

9.8 10.2 10.4

283 279 275

553 670 772

1.95 2.4 2.8

9.5 9.5 9.5

233 222 203

237 223 210

4OAr+ 232Th

31 35 39 44

7.4 7.5 7.5 7.6

262 260 258 256

640 700 755 818

2.45 2.70 2.93 3.2

7 7 7 -

210 203 196 193

209 202 195 188

14N+232Th

60

2.5

237

419

1.77

2.4

205

202

") Incident energy per nucleon. b~ Calculated linear momentum imparted to the composite-like nuclei after the pre-equilibrium stage. c~ Calculated residual masses in the composite-like nuclei after the preequilibration time evaluated to be ~ (5-6) × 10 22 s. d) Calculated remaining excitation energies in the composite-like nuclei after pre-equilibrium emission. e~ Excitation energies per nucleon of the composite-like nuclei: e = E*LN/McL N. n Average experimental momentum transfer at the CC bump. g~ Sum of the two detected fragment masses for the central collisions. 4°At and 14N results are from ref. [ 1 ] and ref. [9], respectively. The absolute scale for the present 58Ni data is accurate within + 7%. h) Calculated residual masses assuming that the evaporation from the composite-like-nuclei (McLN, E~L N) removes 12 MeV per nucleon. To be compared to the experimental values Mexp.

i

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1500

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500

05 (MeV/A)

1.5 EC.M.-VcB (6eV)

Fig. 3. Absolute cross sections of coincident FF for the high LMT events versus the excitation energy per nucleon ~ taken from table 1 (a) and versus the center of mass energy above the Coulomb barrier (b) for the 58Ni, 4°Ar and 14N+232Th systems. A I/sin OCMangular distribution has been assumed for the fission fragments. All data have been recorded with the same experimental set-up, the relative errors are given for a few points. The errors on the absolute cross sections are estimated around + 20%. The lines are to guide the eye.

tinguish between these two aspects more data are n e e d e d . N e v e r t h e l e s s p l o t t i n g t h e c r o s s s e c t i o n as a function of the relative velocity of the ions at contact s h o w s it n o t t o b e a n a p p r o p r i a t e p a r a m e t e r . T h i s a g r e e s w i t h t h e fact t h a t a n g u l a r c o r r e l a t i o n s m e a s u r e d w i t h l i g h t e r p r o j e c t i l e s still s h o w p r e s e n c e o f C C e v e n t s e v e n at v e l o c i t i e s u p t o 6 0 M e V / u [ 9,1 1 ]. An alternative, but somewhat different interpretation of the data can be drawn by comparing the a n g u l a r c o r r e l a t i o n s as a f u n c t i o n o f i n c i d e n t e n e r g y . T h e c o r r e l a t i o n s suggest t h a t t h e C C p e a k lies on the massive transfer continuum which extends b e y o n d t h e b u m p . A s r e m a r k e d earlier, t h i s tail is independent of incident energy and thus the CC e v e n t s lie o n a c o n s t a n t b a c k g r o u n d . T h i s o b s e r v a t i o n is p a r t i c u l a r l y e m p h a s i z e d b y t h e 4°At ( 4 4 M e V / u ) + 232Th c o r r e l a t i o n w h e r e t h e C C p e a k ceases to exist, l e a v i n g b a c k g r o u n d e v e n t s w h i c h e v e n beyond 7 GeV/c have characteristics consistent with t h e r e s t o f t h e d a t a . T h u s o n e c a n suggest t h a t t h e C C and background events arise from primary interact i o n s w h i c h a r e i n t r i n s i c a l l y d i f f e r e n t , yet l e a d i n g t o the same exit channel. Since the background and CC e v e n t s d i f f e r b y t h e i r i m p a c t p a r a m e t e r , it is t h e r e 25

Volume 195, number 1

PHYSICS LETTERS B

fore likely that the fall in CC cross section is also related to the dynamic effects in the entrance channel. In summary, it is shown that using 58Ni projectiles it is possible to attain linear m o m e n t u m transfer values up to 9.5 GeV/c with significant cross section. To our knowledge this is the highest transfer to a heavy target ever reported. Using the a b r a s i o n - a b l a t i o n model followed by preequilibrium emission in the angular correlations are semi-quantitatively understood and the derived excitation energies are consistent with the mass measurements. The present results show that the fall in CC cross section is consistent with predictions for limiting temperature, however, it is stressed that other descriptions, such as geometrical and d y n a m i c effects in the entrance channel can also play a n i m p o r t a n t role. We thank M. Blann for m a k i n g available to us his exciton model code a n d the G A N I L accelerator staff for the good quality of the Ni beams.

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References [ 1] M. Conjeaud et al., Phys. Lett. B 159 (1985) 244. [2] E.C. Pollacco et al., Phys. Lett. B 146 (1984) 29. [3] D. Jacquet et al., Phys. Rev. Lett. 53 (1984) 2226; S. Leray et al., Nucl. Phys. A 425 (1984) 345; Z. Phys. A 320 (1985) 533. [4] S. Leray, J. Phys. (Paris) 47, C4 (1986) 275, and references therein. [5] V.E. Viola et al., Phys. Rev. C 31 (1985) 1550. [6] V.E. Viola et al., Phys. Rev. C 26 (1982) 178. [71 R. Dayras et al., Nucl. Phys. A 460 (1986) 299. [8] M. Blann, Phys. Rev. C 31 (1985) 1245. [9] C. Volant, Proc. XXIII Intern. Winter Meeting on Nuclear physics (Bormio, 1985) p. 370. [ 10] S. Levit and P. Bonche, Nucl. Phys. A 437 (1985) 426; D.H.E. Gross et al., Phys. Rev. Lett. 56 (1986) 1544. [ 11 ] J. Galin et al., Phys. Rev. Lett. 48 (1982) 1787; M. Fatyga et al., Phys. Rev. Lett. 55 (1985) 1376.