PHYSICAL REVIEW C VOLUME 49, NUMBER 1 JANUARY 1994 Neutron-proton bremsstrahlung from low-energy heavy-ion reactions N. Gan, K. -T. Brinkmann, A. L. Caraley, B. J. Fineman, W. J. Kernan, * and R. L. McGrath Physics Department, State University of New York at Stony Brook, Stony Brook, New York 117gg P. Danielewicz National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan $882$ (Received 6 May 1993) High-energy photon yields from the reactions ' C + ' ' Sn at 10 MeV/nucleon have been measured. The ratio of cross sections (E~ & 30 MeV) o (' Sn)/o (" Sn) is 1. 6 6 0.2, which is larger than expected within the equal-participant model. The elementary n-p-p cross section is evaluated within the neutral scalar 0 meson exchange model, and implemented into a BUU code. The results of the BUU calculation suggest that the high-energy p-ray yield differences for the two Sn isotopes arise from the differences in nucleon phase-space distributions. The sensitivity of the high-energy photon production to the initial phase-space conditions is explored. PACS number(s): 25.70. — z I. INTRODUCTION In the last few years, high-energy photon produc- tion (E~ & 30 MeV) in the intermediate energy region (E/A & 20 MeV/nucleon ) has been measured systemati- cally [1, 2]. Experimental and theoretical studies are con- sistent in attributing these hard photons to incoherent bremsstrahlung radiated from individual neutron-proton collisions during the early stage of nuclear reactions. Be- cause high-energy photons are only coupled with nucle- ons by the electromagnetic interaction and have no 6nal state interactions, they can serve as a possible "clean" probe for the collision chronology. Even at energies as low as 10 MeV/nucleon, previous studies [3, 4] found sim- ilar phenomena. Here the high-energy photon yield ex- ceeds substantially the standard statistical contribution from giant dipole resonance (GDR) decay. The observed forward-backward anisotropy in the laboratory frame, which is expected from a source moving at the half beam- velocity nucleon-nucleon kame, supports the view that high-energy photons from low-energy reactions have the same origin as at higher energies. However, the high- energy photon yield varies with projectile/target combi- nations more drastically than expected from the scaling relation [5,6] used at higher incident energies. Based on a simple geometrical overlap, equal-participant model, the relation predicts that the high-energy photon yield for different projectile/target combinations should be pro- portional to the first-chance proton-neutron collisions. The relation does not take into account details of nucleon momentum distributions. Since Pauli blocking constrains high-energy photon production much more severely at *Present address: EGSG Energy Measurements, Washing- ton Aerial Measurements Department, Suitland, MD 20746. lower energies, high-energy photons produced in low- energy reactions may provide a sensitive probe of the nucleon-phase space distributions inside the nucleus. In this paper, we present the results of a new measurement of the inclusive photons in C + ' Sn reactions at 10 MeV/nucleon, and the results of an analysis based on the BUU nuclear transport equation. II. EXPERIMENT The experiment was performed at the Stony Brook LINAC. Enriched 3. 0 mg/cm self-supporting foil targets of 96. 7% Sn and 98. 9% Sn were irradiated with a 120 MeV 2C beam. Photons were detected with a com- pact BaF2 array. The whole array was located at 90' with respect to the beam axis, and 25 cm kom the tar- get with a 10-cm-diameter lead collimator in front. The array consists of seven 5. 6 x 14-cm hexagonal BaF2 crys- tals surrounded by a 3-cm-thick NE102 plastic cosmic-ray veto shield. The detector performance was reported in a previous paper [7]. The beam was monitored by record- ing the ratio of elastic-scattering events detected by two surface-barrier detectors positioned at +15 with respect to the beam. These elastic events were later used to nor- malize the data. Two NaI arrays were also positioned in the scattering chamber to detect light charge particles and particle-p-ray coincidences. The coincidence results will be reported elsewhere. Good discrimination of high-energy photons from neu- trons is a crucial requirement for the high-energy p-ray detector. Neutrons were rejected by means of pulse-shape discrimination (PSD) deduced from the two components of light output of the BaF2 scintillator, and by measur- ing the time-of-flight (TOF) relative to the LINAC RF signal. The overall time resolution was about 900 ps. In Fig. 1, we show the TOF spectra with equivalent pho- ton energy exceeding 30 MeV for the two Sn targets. The 0556-2813/94/49(1)/298(6)/$06. 00 298 1994 The American Physical Society