Experimental Studies of Quasi-fission Reactions. B.B.Back, B.G.Glagola, D.Henderson, S.B.Kaufman, J.G.Keller, S.J.Sanders, F.Videbaek, and B.D.

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1 Expermental Studes of Quas-fsson Reactons The submtted manuscrpt hs been authored by a contractor o the U. S. Government under contract No. W ENG-3B. Accordmglv. the U. S- Government retans a nonexclusve, royally free lcense to publsh or reproduce the publshed lorm o ths contrbuton, or allow olhe's to do so. for U S. Government purposes. B.B.Back, B.G.Glagola, D.Henderson, S.B.Kaufman, J.G.Keller, S.J.Sanders, F.Vdebaek, and B.D.Wlkns Argonne Natonal Laboratory, Argonne, IL 6439 CONP DE Paper to be presented at Wnter Workshop on Nuclear Dynamcs V Sun Valley, Idaho February 22-26, 1988 DISCLAIMER Ths report was prepared as an account of work sponsored by an agency of the Unted State* Government. Nether the Unted States Government nor any agency thereof, nor any of ther employees, makes any warranty, express or mpled, or assumes any legal lablty or responsblty for the accuracy, completeness, or usefulneu of any nformaton, apparatus, product, or process dsclosed, or represents that ts use would not nfrnge prvately owned rghts. Reference heren to any specfc commercal product, process, or servce by trade name, trademark, manufacturer, or otherwse does not necessarly consttute or mply ts endorsement, recommendaton, or favorng by the Unted States Government or any agency thereof. The vews and opnons of authors expressed heren do not necessarly state or reflect those of the Unted States Government or any agency thereof.

2 Expermental Studes of Quas-fsson Reactons B.B.Back, B.G.Glagola, D.Henderson, S.B.Kaufman, J.G.Keller, S.J.Sanders, F.Vdebaek, and B.D.Wlkns Argonne Natonal Laboratory, Argonne, IL 6439 In recent years, a number of expermental studes have shown that a substantal fracton of the total reacton cross secton s found n fsson-lke processes, whch do not result from the fsson decay of a completely fused system. Smlar to the quas-fsson process predcted by Swateck these reacton products have total knetc energes typcal of those found n the fsson decay of compound nucle but may be dstngushed from compound fsson products on the bass of ther angular and mass dstrbutons. The quas-fsson products also exhbt substantally more net mass transfer than seen n deep nelastc scatterng. Thus, the quas-fsson processes can be vewed as brdgng the gap between complete fuson and deep nelastc scatterng n heavy systems. A large body of expermental data on these fsson-lke processes have become avalable n recent years usng a number of dfferent target-projectle combnatons. None of these expermental studes are specfcally addressng the queston of entrance channel effects. These effects are expected to play an mportant role f the dynamcs n the approach phase of the reacton s responsble for the occurrence of quas-fsson. An alternate nterpretaton n terms of the dsappearance of the fsson barrer at large partal waves, whch s entrely a property of the completely fused system ndependent of the formaton process, has also been proposed. In order to address these questons we have studed the system Th va three dfferent entrance channels, namely 32 S+ 182 W, 48 TH- 166 Er and 6 N+ 154 Sm. The analyss of the T+ Er data s, as yet, ncomplete and we shall therefore report only on the results of the 32 S+ 182 W and 6 N+ 154 Sm reactons n ths paper. The experments were carred out at the ATLAS faclty at Argonne usng beams of 136, 177, 222, and 26 MeV 32 S and 339, 39, and 421 MeV 6 N. Sngles reacton products were measured n an array of eght S surface barrer detectors over the angular ranges of, K=1-17 and 6, t=l - 9 for the nn f*n led ISLO S and N beams, respectvely. Fragment masses were deduced from the measured energy and flght-tmes after applyng sutable correctons for the pulse heght defect and plasma tme delay assocated wth the detecton of heavy ons n S-detectors.

3 In the Fsson decay of a compound nucleus t s expected that the mass dstrbutons are symmetrc around half the total mass of the system rrespectve of the center-of-mass angle at whch t s measured. Any devaton from ths symmetry therefore may be taken as a clear ndcaton that noncompound reactons contrbute to the measured mass dstrbutons. In Fg. 1 we show the angular dependence of the mass centrods for fsson-lke products 9 on from the S+ W reacton at four dfferent bombardng energes. At the two lowest bombardng energes, Ej ab =166, 177 MeV, the mass dstrbutons are clearly centered at half the total mass at A=17 at all angles, as expected for compound nucleus decay. At the two hgher energes, E Jab =222, 26 MeV, there appears, however, to be a weak preference for heaver fragments to be emtted at forward angles (and lghter fragments at backward angles). Ths trend s, however, very weak and may well be of nstrumental orgn,.e. nadequate correctons for the energy and tme response of S detectors to heavy ons. A devaton from the expected behavor for compound fsson would, however, be consstent wth the fact that partal waves for whch the fsson barrer has vanshed (t> r^e_. Q =65 h) contrbute to the fsson-lke cross secton at these beam energes. In the N+ Sm reacton we observe a strong component of non-compound fsson-lke reacton products. In Fg. 2 we show the angular dstrbutons for fragments belongng to dfferent mass bns for the beam energy of Ej 1 = :1O S + 26 MeV H- llr MeV T77MeV F 8-1: Angular dependence of the centrods MeV. The mass bn A = of the fragment mass dstrbutons are correspondng to symmetrc mass splts s 15 - L L 166 MeV 3 6 c. m (deg) shown for the "s+ lm W reacton - strctly forward-backward symmetrc as requred for bnary ext channels. Gong to lghter fragment masses we fnd a progressvely stronger preference for emsson at forward angles. Ths behavor s nconsstent wth the bnary decay of a compound system and t shows that the reacton tme s shorter or

4 comparable to the rotatonal perod of the nteracton complex. The sold curves are calculated on the bass of the standard formalsm for fsson angular dstrbutons. The angular asymmetry s acheved by multplyng the standard expresson wth an exponental functon such that the full expresson s Z 21*1 P I «* E A 21*1 d 1 \ 2 (2) where P(I) s the partal wave dstrbuton, f! s a parameter controllng the angular asymmetry, p{k) s the K-dstrbuton (K s the axal component of the total spn I) and 6Q [6) s the ^-dependent part of the symmetrc top wave functon. The partal wave dstrbuton P(I) s taken from model o calculatons whch reproduce the exctaton functon for fsson-lke processes 2 whereas the K value ( the varance of the gaussan K-dstrbuton, p{k) ) s K3 obtaned from a ft to the angular 9 dstrbuton for the A=46-5 mass bn. The sold curves for the other mass bns are obtaned by adjustng the slope parameter ft and the overall normalzaton. We observe that ths sngle choce for the K -value gves a good representaton of the data for all mass bns. Ths ndcates that, wthn the expermental senstvty, the varance of the K-dstrbuton s not strongly dependent on the mass splt n the process. These data are, however, not suffcently accurate n the extreme forward regon to rule out a weak mass 2 dependence of the K value as observed n the 5 T, 56 Fe + 9fJ8 Kratz et al. 4. Pb reactons by Another effect of the entrance channel mass asymmetry s evdent n the wdth of the mass dstrbutons of fsson-lke products as llustrated n Fg. 3. The standard devatons of the mass dstrbutons, all normalzed to the same scsson pont temperature of T = (degrees) on "^ ' Fg. 2: Angular dstrbutons fo? fragments fallng nto dfferent mass bns are shown for the N+ 154 Sm reacton at E,.= 339 MeV. The sold ponts are data for the mass bn ndcated n the fgure. Open crcles correspond to the complementary fragment plotted at the supplementary angle.

5 MeV, are here shown as a functon of the mean square angular momentum contrbutng to the fsson-lke processes. We observe that the angle-ntegrated mass dstrbutons for the N+ Sm products (sold squares) are substantally wder than the ones for the W reacton (sold crcles). The mass dstrbuton of products emtted only at =9 (open squares) s also c m oo substantally wder than seen for the S nduced reacton. Ths ncrease n the wdth of the mass dstrbuton for the more mass asymmetrc entrance channel agan leads to the concluson that these * 1 ' Th 1 c a Ht+ t *Sr a * N+ tl4 Srn@>9 5 KXXX) 15 2 Fg. 3: The standard devaton of the mass dstrbutons of fsson-uke fragments are products are the result of a reacton.. r shown as a functon of the mean square nvolvng a rapd net transfer of mass sp(l of the system from the heavy to the lght reacton partner durng a tme perod, whch s shorter or comparable to the rotatonal perod of the system. We fnd that the observed dfferences n angular and mass dstrbutons for the two reactons lead to the followng concluson. Whereas the S+ W products essentally dsplay the characterstcs expected for the fsson decay of a compound system (although small devatons are found for the two hghest energes, where partal waves above the vanshng pont for the fsson barrer are populated), these characterstcs are not found for the N+ Sm system. On the contrary, the angular dstrbutons for asymmetrc mass splts n the N+ Sm reactons show evdence of short reacton tmes and fast mass transfer rates, whch are characterstc of quas-fsson processes. Ths work was supported by the U.S. Department of Energy, Nuclear Physcs Dvson, under contract W Eng-38. Present address: Gesellschaft fur Schweronenforschung, Darmstadt, Germany Hv.Q.Shen et al., Phys. Rev. C36, 115 (1986) and references theren. 2 W.J.Swateck, Phys. Scr. 24, 113 (1981); Nucl. Phys. A376, 275 (1982) 3 B.B.Back et al., Phys. Rev. C32, 1 (1985) 4 J.Kratz et al., Nucl. Phys. A452, 351 (1986) -