Journal of Modern Physics, 2013, 4, 1504-1507 Published Online November 2013 (http://www.scirp.org/journal/jmp) http://dx.doi.org/10.4236/jmp.2013.411182 Open Access JMP From Sequential Processes to Multifragmentation in Proton Reactions with Gold Sergej P. Avdeyev 1* , Victor A. Karnaukhov 1 , Helmut Oeschler 2 , Waldemar Karcz 3 , Vahan V. Kirakosyan 1 , Pavel A. Rukoyatkin 1 , Edwin Norbeck 4 , Alexander S. Botvina 5 1 Joint Institute for Nuclear Research, Dubna, Russia 2 Institute für Kernphysik, Darmstadt University of Technology, Darmstadt, Germany 3 H. Niewodniczanski Institute of Nuclear Physics, Cracow, Poland 4 University of Iowa, Iowa City, USA 5 Institute for Nuclear Research, Moscow, Russia Email: * avdeyev@aol.com Received September 3, 2013; revised October 2, 2013; accepted October 28, 2013 Copyright © 2013 Sergej P. Avdeyev et al. This is an open access article distributed under the Creative Commons Attribution Li- cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT The distribution of relative angles between the intermediate mass fragments has been measured and analyzed for ther- mal multifragmentation in p + Au collisions at 2.1, 3.6 and 8.1 GeV. The analysis has been done on an event by event basis. The multibody Coulomb trajectory calculations of all charged particles have been performed starting with the initial break-up conditions given by the combined model with the revised intranuclear cascade (INC) followed by the statistical multifragmentation model. The measured correlation function was compared with the calculated one to find the actual time scale of the intermediate mass fragment (IMF) emission. It found transition from sequential evapora- tion for p(2.1 GeV) + Au to simultaneous multibody decay of a hot and expanded nuclear system in case of p(8.1 GeV) + Au. Keywords: Multifragmentation; Correlations; Emission 1. Introduction The main decay mode of very excited nuclei (E * ≥ 4 MeV/nucleon) is copious emission of intermediate mass fragments (IMF), which are heavier than -particles but lighter than fission fragments. An effective way to pro- duce hot nuclei is the reaction induced by heavy ions with energies up to hundreds of MeV per nucleon. But in this case, the heating of the nuclei may be accompanied by compression, rotation and shape distortion, which can essentially influence the decay properties of hot nuclei. The picture becomes clearer when light relativistic pro- jectiles (protons, antiprotons, pions) are used. In this case, fragments are emitted by only one source—the slow moving target spectator. Its excitation energy is almost entirely thermal. Light relativistic projectiles provide therefore a unique possibility for investigating thermal multifragmentation. The decay properties of hot nuclei are well described by statistical models of multifragmentation [1] and this can be considered as an indication that the system is in thermal equilibrium or at least close to that. The time scale of fragment emission is a key parame- ter for understanding the decay mechanism of highly excited nuclei. Is it sequential and independent evapora- tion of IMF’s or is it a multibody decay mode with al- most simultaneous emission of fragments governed by the total accessible phase space? As was suggested by D.H.E. Gross in ref. [2], “simultaneous” means that fragments are liberated during a time interval which is smaller than the Coulomb acceleration time τ c , when the kinetic energy of fragments amounts to ~90% of the asymptotical value. According to [2], τ c is estimated to be (400 - 500) fm·c −1 . Fragments emitted within this time interval are considered being not independent as they interact via the Coulomb force while being accelerated in the electric field of the source. As a result, the yield of events with small relative velocities of the fragments (or small relative angles between them) is suppressed. The magnitude of this effect drastically depends on the emis- sion time since the longer the time separation of the * Corresponding author.