Unusual backward emission of multifragmentation products in ultrarelativistic nucleus-nucleus collisions

Abstract

Light fragments such as Na resulting from the multifragmentation of Au by 232-GeV ' 0 are emitted preferentially backwards in the laboratory system (F/B =0.81+0.05). This effect is not seen for similar products from V, Cu, or Ag targets, or for fission products from Au.Recent developments in particle accelerator technology have opened a new area of research involving ultrarelativistic nucleus-nucleus collisions at energies above 10AGeV. Of special interest to us was the opportunity to study target fragmentation, i.e. , the production of large, low velocity, target-related fragments in such collisions. In our work, we looked for any substantial deviations from previously observed behavior for reactions induced by ultrarelativistic protons and relativistic heavy ions ( 5 2A GeV). This Brief Report reports evidence for such deviations in the form of an unusual backward emission in the laboratory system of multifragmentation products from the interaction of 232-GeV ' 0 with Au. Standard thick-target-thick-catcher techniques' were used to determine the average kinematic properties of a number of target fragmentation products formed in the reaction of 232-GeV ' 0 with V, Cu, Ag, and Au targets. Stacks of metal foils with forward and backward catcher foils were irradiated in an external beam of the Brookhaven alternating-gradient synchrotron (AGS) with 232-GeV ' 0 ions for periods up to 54.5 hours (total particle fluence of 6.3X 10' ions). The descriptions of each foi1 stack are summarized in Table I. Assay of the radioactivity in the central target metal foil and the Mylar catchers was begun within a day after the end of the irradiation. Standard techniques which have been described elsewhere' were used to identify the radionuclides present and to determine the activity of each nuclide in the forward and backward catchers and in the target.

Experimental Indications for the Response of the Spectators to the Participant Blast

“…Using a thick-target thickcatcher technique, target-like fragments around A = 60, produced in the bombardment of gold by 12 C at 25 GeV [9], were found to be produced with mean velocity close to zero in the laboratory frame. An experiment of Loveland et al [10], based on the same technique, revealed a backward emission of 24 Na, produced by multi-fragmentation of gold by 16 O at 232 GeV. Results from a time-of-flight measurement [11] in a comprehensive projectilefragmentation experiment with a 197 Au beam of 600 A MeV and targets from carbon to lead showed a levelling off of the velocities of the fragments with increasing mass loss.…”