Evidence for the inuence of reaction dynamics on the population of compound nuclei

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1 1 Submitted to the Proceedings of the First Latin-American Workshop on: On and O Line Beam Gamma Spectroscopy for the Study of Heavy Ion Reactions and Pre-Equilibrium Processes, September 4-8, 1995, Universidad Simon Bolivar, Caracas, Venezuela. Dynamical Eects in the Population of Compound Nuclei M. Thoennessen a a National Superconducting Cyclotron Laboratory and Department ofphysics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA Evidence for the inuence of reaction dynamics on the population of compound nuclei will be presented. Large dissipation in certain heavy-ion reactions leads to long compound nucleus formation times which can change the initial angular momentum and excitation energy distribution of the compound nucleus. This eect can be observed by measuring high energy -rays from the giant dipole resonance. The overall neutron multiplicity, however, is not a sensitive observable. The long formation times might even inuence the nal spin distribution of evaporation residues, which could possibly explain the entrance channel dependent population of superdeformed bands. 1. INTRODUCTION Evidence for reaction dynamical eects in the formation and decay of compound nuclei is still quite controversial [1,2]. The problem was rst observed in neutron multiplicity measurements and was discussed as \entrance channel eects" [3,4]. Bohr's compound nucleus hypothesis stating that the decay of a compound nucleus is independent of its formation was questioned. Neutron multiplicities in the reaction 64 Ni + 92 Zr could not be reproduced with standard statistical model calculations. Recently, the measurements of the -ray decay of the giant dipole resonance (GDR) conrmed this observation by comparing the reactions 64 Ni Mo and 16 O+ 148 Sm forming the same compound nucleus 164 Yb [5]. While the -ray spectrum following the latter reaction could be described, the former showed signicant deviations from the statistical model calculations. In contrast to the neutron data, the GDR spectra could be qualitatively described with a simple model which does not question the independence hypothesis. The model is based on the dissipative dynamical model which predicts long compound nucleus formation times for certain heavy-ion reactions [6,7]. The dierences in the GDR spectra arise from these formation times during which particle and -ray can occur which changes the population of the initial compound nuclei. This dierence in the initial population could also result in dierences in the nal spin distribution of the evaporation residues which might be important for the reaction dependent feeding of superdeformed bands which have recently been reported [8{11].

2 2 2. Formation Times The eects of the long formation times for the GDR spectra was rst observed in the reaction 64 Ni Mo forming 164 Yb at 49 MeV [5]. In the following it was demonstrated that several other systems did not show any eect in the -ray spectra [1,12]. Even in a seemingly very similar reaction to 64 Ni Mo, 64 Ni + 92 Zr forming 156 Er no deviations from the statistical model were observed [2]. These apparent contradictions can be explained within the dissipative dynamical model [6,7] coupled with the possibility of particle and -ray emission during the formation of the compound nucleus. From the semiclassical code HICOL [13] which is based on Swiatecki's model it is possible to extract the time evolution of the internal excitation energy as a function of time in addition to the total compound nucleus formation time. If the formation times become comparable to the neutron evaporation times the contribution of evaporation during the formation stage can become important for the subsequent decayof the compound nucleus. For the reaction 64 Ni Mo, HICOL predicted formation times of 10,20 s which are comparable to neutron evaporation times (1:6 10,20 s) for 164 Yb at 49 MeV excitation energy, whereas for more asymmetrically formed and/or lighter systems the formation times are much shorter than the evaporation times. In a rst approach to incorporate the evaporation during the formation times into the statistical model, the formation stage was treated not time dependent, but average values for the excitation energy and shape were used [1,5]. Particle and -ray emission was then calculated for a nite time (formation time). The -ray decay of the GDR originates predominantly from the rst decay steps of the compound nucleus decay and the shape of the -ray spectrum depends on the deformation of the system [14,15]. Thus the GDR is particularly sensitive to any inuence of the formation time which is expected to occur early on and involves extreme deformations. The total calculated -ray spectrum is then a sum of the contribution from the formation stage and the compound decay. The eect of the formation times on the -ray spectrum is due to two eects. The smaller one is the emission of -rays during the formation which has a dierent shape because of the large deformations involved. However, more important is the reduction of excitation energy available to the compound nucleus decay, due to the neutron emission during the formation. This lower average excitation energy of the compound nucleus reduces the emission probability of GDR -rays and thus changes the overall shape of the spectrum. This is demonstrated in the following for the reactions 18 O+ 92 Mo and 50 Ti + 60 Ni. Although both have fairly short formation times ( 2 10,21 s and 4 10,21 s, respectively) the dierence between the two reactions is sucient large to show the eect. Figure 1 shows the initial population of the compound nuclei after evaporation during the formation stage for the reactions 18 O+ 92 Mo and 50 Ti + 60 Ni. For these two reactions only 73% and 58%, respectively, populate the original compound nucleus 110 Sn. The difference is due to the longer formation time for the reaction 50 Ti + 60 Ni. The remaining cross section populates lighter nuclei due to evaporation of particles. The dominant nucleus is 109 Sn which has a relative initial population of 15% and 23% for 18 O+ 92 Mo and 50 Ti + 60 Ni, respectively, following neutron evaporation. This not only changes the initial compound nucleus but it also reduces the average initial excitation energy from 56 MeV

3 O + Mo 50 Ti + 60 Ni 15% n 7% p 4% 1% α 2n,1n1p... 10% p 7% α 2% 2n,1n1p... 73% Sn % 58% 110 n Sn Figure 1. Calculated particle evaporation during the compound nucleus formation for the reactions 18 O+ 92 Mo (left) and 50 Ti + 60 Ni (right). Only 73% (58%) of the cross section do not emit particles during the formation stage in the reaction 18 O+ 92 Mo ( 50 Ti + 60 Ni). to 51.6 MeV ( 18 O+ 92 Mo) and 48.9 MeV ( 50 Ti + 60 Ni). Although several systems could be explained with the formation time model, so far only 64 Ni Mo showed signicant contribution from the formation stage. The recent result of an experiment of 156 Er where not only the neutron multiplicities but also detailed - ray spectra were measured showed no evidence for deviations from the compound nucleus decay [2]. The -ray spectra from the reaction 64 Ni + 92 Zr and 12 C+ 144 Sm were identical when gated by isomer transition in the residues and the same compound nucleus angular momentum. 64 Ni + 92 Zr is very similar to 64 Ni Mo and thus these discrepancies seem to be surprising. However, a closer look at the formation time of the two reactions 64 Ni Mo and 64 Ni + 92 Zr exhibits signicant dierences [16]. Figure 2 shows the fusion times for the two reactions as a function of angular momentum. Although the overall predicted formation time for the reaction 64 Ni + 92 Zr is smaller than the 64 Ni Mo times this small dierence cannot account for the dierences in the observations. However, these formation times depend critically on the populated angular momenta. The 64 Ni Mo reaction covered angular momenta between 10 ~ and 30 ~ with contributions up to 50 ~ [5]. The formation times needed to t the -ray spectra were ,22 s, a factor of two longer than the predicted average time. However the strong increase around 25 ~ might indicate much longer times. HICOL is a semiclassical code that does not predict fusion to occur above angular momenta of 29 ~. However, experimentally, much larger angular momenta lead to fusion [17]. The -ray spectrum following the reaction 64 Ni + 92 Zr was gated by lower angular momenta between 14 and 28 ~ and an average of 25 ~ [2]. In addition, the strong increase of the fusion times is predicted to occur at larger values > 35~. This dierence might explain the fact that the inuence of formation times was observed in 164 Yb and not in 156 Er. Another dierence that might lead to an enhancement of the inuence of formation times is the excitation energy. The \rst-chance" neutron evaporation times are a factor of two larger in 156 Er at 47 MeV compared to 164 Yb at 49 MeV. This dierence reduces the contribution of the formation stage relative to the compound nucleus decay in 156 Er.

4 Ni + Ni + 92 Zr Mo Figure 2. Extracted fusion times for the reactions 64 Ni Mo (solid) and 64 Ni + 92 Zr (dashed) as a function of spin. The other observable measured by Heller et al. [2] was the neutron multiplicity. This had been a controversial probe before [18,19] and they also did not observe any eect due to long formation times. The simple formation time model which includes evaporation actually does not predict any dierences in the neutron multiplicities even for long formation times. Again, the system 110 Sn is used to demonstrate the eect. Figure 3 shows the total neutron, proton and multiplicities for the two reactions, as well as the individual contributions from decay during formation (bottom) and the following compound nuclear decay (top). The formation-stage contribution is < 10% and < 15% for the 18 O and 60 Ti induced reactions, respectively. The larger values for 50 Ti + 60 Ni are due to the longer formation time. The sum of both contributions is equal to the multiplicities calculated with the standard statistical model. For example, the standard statistical model predicts a neutron multiplicity of 1.92 and 1.82 for the reactions 18 O+ 92 Mo and 50 Ti + 60 Ni, respectively, which corresponds exactly to the sum of the formation stage and the subsequent compound nucleus decay as shown in Figure 3. The smaller overall particle multiplicity in the 50 Ti + 60 Ni reaction is due to the slightly wider accepted angular momentum distribution in this reaction, which reduces the eective mean excitation energy because of the larger rotational energy. This shows that the neutron multiplicities are extremely sensitive to the applied gate on the angular momenta, whereas this gate is not as crucial for the -ray spectra. Even in the reaction 64 Ni Mo where signicant dierences in the -ray spectra due to decay during the formation period were observed, the total particle multiplicities from the formation and the compound decay are equal to the total multiplicities calculated

5 O + Mo Ti + Ni 5 Particle Multiplicity Neutron Proton Alpha Figure 3. Calculated particle multiplicity for the reaction 18 O+ 92 Mo (left panels) and 50 Ti + 60 Ni (right panels). The sum of the contributions from the formation stage (dark, top) and the subsequent compound nucleus decay (light, top) is equal to the multiplicities calculated with the standard statistical model. with the standard model. This eect is currently being investigated in the 164 Yb system in detail [20]. Another observable which should be similarly sensitive to long formation times is a measurement of the ratio of deuterons to protons [21]. During the initial stage, predominantly protons and neutrons are emitted and the emission of deuterons is suppressed because of the relatively low excitation energy. Deuterons are then again suppressed during the compound nucleus decay because the eective excitation energy is reduced by the emission of protons and neutrons during the formation. The inuence of the formation should thus result in a reduction of deuteron emission. This eect was indeed observed in the reaction 64 Ni Mo forming 164 Yb where the deuteron to proton ratio was signicantly reduced compared to the reaction 16 O+ 148 Sm where no contributions from the formation are expected [21]. Finally another GDR -ray experiment which shows possible indication of formation times should be mentioned. Zelazny et al. [22] measured two compound systems diering by one neutron and by 12 MeV excitation energy ( 162 Yb at E =50:8 MeV and 161 Yb at E =38:8 MeV) The dierence between these -ray spectra should then correspond to emission during the rst stage of the compound nucleus decay. In the reactions with 17 O and 18 O projectiles on 144 Sm the dierence spectrum agreed nicely with statistical model predictions. However, the -ray spectra following the reactions 48 Ti Cd and 48 Ti Cd at 225 MeV and 210 MeV, respectively, were identical. This eect has not yet been explained. One possible explanation would be a stronger inuence of long formation times at the higher energy, thus reducing the excitation energy and yielding similar -ray spectra as in the lower energy reaction. A quantitative analysis of this hypothesis has not yet been performed.

6 res_diss res_stat σ /σ Mo( Ni,4 n) 160 Yb Angular Momentum ( ) Figure 4. Ratio of the spin dependent4nevaporation residue cross section of the standard statistical model over the model including evaporation during formation. 3. Evaporation Residues The large feeding of superdeformed bands following heavy-ion fusion evaporation reactions has been studied extensively [8{11,23{25]. It was pointed out that the inuence of dissipation on the ssion process could possibly change the entry population leading to evaporation residues and thus possibly enhanced the population of superdeformed bands [26]. In addition to the surprisingly large feeding, several experiments suggested that the population of superdeformed bands depended on the asymmetry of the reaction forming the compound nucleus [8,9]. It was speculated that these dierences might be due to the long formation times, however no calculations were performed [26]. In order to search for possible eects of the formation time on the population of superdeformed bands calculations for the decay of the compound nucleus 164 Yb formed with the reaction 64 Ni Mo were performed. As an example, the population of the Yrast line of the 4n channel 160 Yb was calculated with the standard statistical model and with the modied model which included decay during the formation stage. As mentioned before, the overall particle multiplicity for both cases was exactly the same and thus the cross section for populating 160 Yb was identical. However, the population distribution of the Yrast line was slightly shifted towards larger spin for the latter calculation. Figure 4 shows the ratio of the cross section for the standard calculation and the formation time calculation as a function of spin. It is apparent that the incorporation of the formation times reduces the population at low spins and enhances the population at larger spins, whichwould thus favor the population of superdeformed bands. This eect is fairly small, because it only eects the tails of the distributions. In addition, these initial calculations have to be taken with care and have tobeveried for each specic case. It is certainly

7 7 possible that in other cases the formation time could enhance the lower spins or would not eect the distribution at all. 4. Conclusion Some aspects of the eect of dissipation on the formation and decay of compound nuclei have been described. The experimental observations are still controversial and the calculations including long formation times into statistical evaporation models are still fairly crude. Clearly, detailed measurements of GDR -rays in coincidence with evaporation residues are desirable. In the calculations the detailed shape and excitation energy evolution during the formation stage has to be included. REFERENCES 1. M. Thoennessen, E. Ramakrishnan, J. R. Beene, F. E. Bertrand, M. L. Halbert, D. J. Horen, P. E. Mueller, and R. L. Varner, Phys. Rev. C 51, 3148 (1995). 2. F. Heller, Ph.D. Thesis, Heidelberg, MPI-H-U13 (1995). 3. W. Kuhn, P. Chowdhury, R. V. F. Janssens, T. L. Khoo, F. Haas, J. Kasagi, and R. M. Ronningen, Phys. Rev. Lett. 51, 1858 (1983). 4. R. V. F. Janssens, R. Holzmann, W. Henning, T. L. Khoo, K. T. Lesko, G. S. F. Stephans, D. C. Radford, A. M. van den Berg, W. Kuhn, and R. M. Ronningen, Phys. Lett. 181B, 16 (1986). 5. M. Thoennessen, J. R. Beene, F. E. Bertrand, C. Baktash, M. L.Halbert, D. J. Horen, D. G. Sarantites, W. Spang, and D. W. Stracener, Phys. Rev. Lett. 70, 4055 (1993). 6. J. Blocki, J. Randrup, W. J. Swiatecki, and C. F. Tsang, Ann. Phys. (N.Y.) 105, (1977). 7. W. J. Swiatecki, Physica Scripta 24, 113 (1981). 8. G. Smith et al., Phys. Rev. Lett. 68, 158 (1992). 9. S. Flibotte et al., Phys. Rev. C 45, R889 (1992). 10. S. M. Mullins, J. Nyberg, A. Maj, M. S. Metcalfe, P. J. Nolan, P. H. Regan, R. Wadsworth, adn R. A. Wyss, Phys. Lett. B312, 272 (1993). 11. S. M. Mullins et al., Phys. Rev. C 52, 99 (1995). 12. R. K. Choudhury, et al., Nucl. Phys. A569, 93c (1994). 13. H. Feldmeier, Rep. Prog. Phys. 50, 915 (1987). 14. K. A. Snover, Ann. Rev. Nucl. Part. Sci. 36, 545 (1986). 15. J. J. Gaardhje, Ann. Rev. Nucl. Part. Sci. 42 (1992). 16. M. Thoennessen, \Proc. of the Groningen Conf. on Giant Resonances", 28. June - 1. July 1995, to be publ. in Nucl. Phys. A. 17. M. L. Halbert, J. R. Beene, D. C. Hensley, K. Honkanen, T. M. Semkow, V. Abenante, D. G. Sarantites, and Z. Li, Phys. Rev C (1989). 18. B. Fornal et al., Phys. Rev. C42, 1472 (1990). 19. J. L. Barreto et al., Phys. Rev. C48, 2881 (1993). 20. J. R. Beene, M. L. Halbert, and M. Thoennessen, to be published. 21. M. Korolija et al., to be published. 22. Z. Zelazny et al., Nucl. Phys. A569, 1c (1994). 23. K. Schier and B. Herskind, Nucl. Phys. A520, 521c, (1990).

8 24. F. Soramel et al., Phys. Lett. B350, 173 (1995). 25. G. Viesti et al., \First Latinamerican Workshop on: On and O Beam Gamma Spectroscopy for the Study of Heavy Ion Reaction and Pre-equilibrium Processes", this proceedings. 26. M. Thoennessen and J. R. Beene, Phys. Rev. C 45, 873 (1992). 8

Submitted to the Proceedings of the Third International Conference on Dynamical Aspects of Nuclear Fission

Submitted to the Proceedings of the Third International Conference on Dynamical Aspects of Nuclear Fission August 30 - September 4, 1996, Casta-Papiernicka, Slovak Republic Dynamical Fission Timescales