VOLUME 68, NUMBER 14 PH YSICAL REV I EW LETTERS Multifragmentation of 40Ca+ 40ca 6 APRiL 1992 K. Hagel, ' M. Gonin, ' ' " R. Wada, ' J. B. Natowitz, ' B. H. Sa, ' Y. Lou, ' M. Gui, ' D. Utley, ' G. Nebbia, D. Fabris, G. Prete, 3. Ruiz, D. Drain, B. Chambon, B. Cheynis, D. Guinet, (3) X. C. Hu, (3) A. Demeyer, (3) C. Pastor, (3) A. Giorni, (4) A. Lleres, (4) P. Stassi, (4) J. B. Viano, (4) and P. Gonthier ' Cyclotron Institute, Texas Ac%M University, College Station, Texas 77843 -' Istituto Nazionale di Fisica Nucleare-Legnaro, 1-35020, Legnaro, Italy '"Institut de Physique Nucleaire-Lyon, 69622 Villeurbanne CEDEX, France Institut des Science Nucleaires-Grenoble, 38026 Grenoble CEDEX, France "'Hope College, Holland, Michigan 49423 (Received 12 November 1991) The multifragment emission of "completely characterized" events in the Ca+ Ca system at 35 MeV/nucleon has been compared to the predictions of several models. The observed multifragment emission is not in agreement with models based on conventional statistical binary decay, but is in agree- ment with both a simultaneous multifragmentation model and a sequential emission model in which ex- pansion is treated. PACS numbers: 25. 70.Pq, 24.60. Dr, 25.70.Gh The observation of multifragment emission following heavy ion collisions in the intermediate energy regime of 20 to IOO MeV/nucleon has been clearly established in numerous recent studies [1-5]. However, the dominant mechanism of multifragment emission has not yet been unambiguously determined. Standard equilibrium mod- els which treat fragment emission as sequential binary decay from an equilibrated system predict an increase in the probability of sequential fragment emission with in- creasing excitation energy [6], as do formalisms which as- sume the simultaneous breakup of the system into mul- tifragment final states [7, 8]. We have studied the mul- tifragment events in the Ca+ Ca system at 35 MeV/nucleon in a nearly 4z configuration and have com- pared the experimental results to the predictions of vari- ous models. These comparisons indicate that the ob- served multifragment events result from hot nuclei which have undergone significant expansion. In the experiment 35 MeV/nucleon Ca ions were in- cident on a Ca target in the AMPHORA [9] detector at the SARA Accelerator Facility . This detector con- sists of 120 Csl phoswich detectors that together cover 80% of 4x, 92 of which at angles less than 45' are covered by fast plastic detectors. To concentrate on cen- tral collisions, we recorded events only if a minimum of fourteen detectors fired. Hydrogen and helium isotopes were identified using pulse-shape discrimination. Heavier elements were identified with approximately unit Z reso- lution up to Z=20 using the ultrafast light component from the plastic and the slow component of the CsI. The energy calibration for protons was determined from the sharp cutoff in the energy spectrum which reflects the punchthrough energy. Energy calibrations for intermedi- ate mass fragments (IMF's) were determined using the approximate beam velocity peaks in forward-angle spec- tra observed during a singles calibration run and deter- mining quenching factors relative to the proton energies. The quenching factors deduced from the forward-angle spectra were used for all of the detectors. Uncertainties in the fragment energies are estimated to be + 10%. We have analyzed only events in which at least 90% of the total Z of the system was detected. These analyzed events are virtually complete, missing only a few particles (most likely very forward preequilibrium protons). The total energy of an event was calculated by summing the kinetic energies of all detected particles with the Q value and appropriate corrections for neutrons. We estimated the neutron contribution by assuming the same number of neutrons as protons and that their energies differ from the protons only by the Coulomb energy. Since our "corn- plete" events are only about 1% of the total number of recorded events, accidental events where two reactions occur within the same beam burst could be emphasized. For these analyzed events, we estimate using a reaction simulation an accidental rate of 10% after restricting the total detected energy of an event to be less than 1500 MeV as well as demanding that particles be in the true time peaks in the time spectra. Further tests show that contamination by random events does not affect the re- sults sufficiently to change the conclusions of this paper. In the following we compare some of the experimental observables to the predictions of several statistical mod- els. To make such comparisons, the mass (A) and excita- tion energy (E*) of the deexciting system must be specified. At 35 MeV/nucleon, significant A and E* may be removed by preequilibrium emission, even for very central collisions. For this symmetric system the experi- mental separation of preequilibrium and equilibrium emission from the particle spectra is difficult. We have therefore relied on several dynamic models to estimate the starting conditions of A, E*, and, where applicable, angular momentum (J„;i) for the deexcitation ealeula- tion. Landau-Vlasov [10,11] and Boltzmann-Uehling- Uhlenbeck (BUU) [12, 13] calculations indicate that at 35 MeV/nucleon the collision of Ca+ Ca results in an initial compression followed by expansion. This takes 1992 The American Physical Society 2141