PhysicsLenersB 317 (1993) 505-509 PHYSICS LETTERS B North-Holland Neutron-proton interferometry in 30A MeV 4°Ar + 197Au collisions CHIC Collaboration M. Cronqvist a, ~. Skeppstedt a, M. Berg b, L. Carl6n b, R. Elmer b, R. Ghetti b, j. Helgesson b, B. Jakobsson b, B. Nor~n b, A. Oskarsson b, F. Merchez c, D. Rebreyend c, L. Westerberg d, V. Avdeichikov e A. Bogdanov e, O. Lozhkin e, Yu. Murin ~, K. Nybe f, E. Olberg f and T.-F. Thorsteinsen f " Department of Physics, Chalmers Institute of Technology, S-412 96 G~teborg, Sweden b Department of Physics and Lund Institute of Technology, Lund University, S6lvegatan 14, S-223 62 Lund, Sweden c lnstitut des Sciences Nucl~aires, F-38026 Grenoble, France d Svedberg Laboratory, Uppsala University, S- 751 21 Uppsala, Sweden e V.G. Khlopin Radium Institute, Roentgen Str. 1, 197 022 St. Petersburg, Russia f Department of Physics, Bergen University, .411~gaten 55, N-5000 Bergen, Norway Received7 May 1993;revisedmanuscriptreceived 13 September1993 Editor: R.H. Siemssen Neutron-proton correlationfunctionsin 30.4MeV 4°Ar+ 197Au, 12C, CH2 reactionshave been measured. In contrast to the nn and pp correlations,which exhibit the shape expectedfrom quantum symmetrization effects and nuclear + Coulomb (pp) f'mal- state interaction,the np functionshowsmuch less correlation strength.This maybe due to Coulombrepulsionon the proton and/ or to a largereffective spin than expected.Selecting highenergy nucleonsstrengthensboth the np and pp correlations. The initial phase of a 30A MeV heavy ion reaction is normally binary. An asymmetric reaction, like "OAr+ tg?Au, leads to incomplete fusion [ 1,2] fol- lowed by (fission+) evaporation [3]. In addition pre-equilibrium emission occurs from a statistical process [4] in hot regions and/or from quasi-elastic scattering. In this paper we present small angle nn, np and pp correlations and compare them to predictions from fermion interferometry calculations. If the emission time is long a dominating anti-correlation is expected for small relative pp or nn momenta (q= ½ IPl -P21 ) due to the anti-symmetrization of the two-fermion wave function. This should be characteristic for re- actions where evaporation from a compound nucleqs is the only source of particle emission [ 5 ]. Pre-equi- librium processes should on the other hand probe fi- nal state effects, i.e. the short range attractive nuclear interaction and the long range Coulomb interaction. In order to isolate the nuclear part, np correlations ought to be favourable since nn pairs are affected by Pauli exclusion and pp pairs also by mutual Coulomb scattering. A review over fermion interferometry in general is found in ref. [ 6 ]. The experimental arrangement has been presented in ref. [7]. Currents of 1-10 nA of a 30,4 MeV 4°At beam from the SARA two-cyclotron system at ISN, Grenoble, bombarded 3 mg/cm 2 CH2, C and Au tar- gets. Every second beam burst was suppressed, giving 166 ns between the bursts which corresponds to the time-of-flight of a 2.3 MeV neutron. The burst width was 1.5 ns which gives an energy resolution in the TOF measurements, AE/E< 5% for neutrons. In this paper we report on np correlations from Ar+Au, C, H (extracted from C and CH2) collisions and com- pare them to simultaneously measured pp and nn correlations. Six cylindrical and five hexagonal liquid scintilla- tors (Bicron BC-501 ) with a thickness of 15.6 cm and diameters of 30.5 and 18.3 cm (diagonal across the corners) were used to detect neutrons. Plastic scintil- lators and lead absorbers in front of the detectors were 0370-2693/93/$ 06.00 © 1993 ElsevierSciencePublishersB.V. All rights reserved. 505