Volume 173, number 4 PHYSICS LETTERS B 19 June 1986 OBSERVATION OF HIGH ENERGY ELECTROMAGNETIC DIPOLE RADIATION IN 14S + Ni REACTIONS AT Elab/A ----35 MeV N. ALAMANOS 1, p. BRAUN-MUNZINGER, R.F. FREIFELDER 2, p. PAUL, J. STACHEL Department of Physics, SUNY at Stony Brook, Stony Brook, N Y 11794, USA T.C. AWES, R.L. FERGUSON, F.E. OBENSHAIN, F. PLASIL and G.R. YOUNG Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 1 April 1986 High energy photons (20 < Ev < 150 MeV) produced in inclusive 14N + Ni --* • + X reactions were unambiguously observed in a Pb-glass detector array. The measured angular distributions exhibit a predominant dipole pattern. This rules out statistical and/or nucleon-nucleon production mechanisms. The data indicate instead a more coherent production mechanism reflecting the direction of relative motion of target and projectile. In this communication we report a direct measure- ment of high energy single photons produced with large cross section in reactions induced by 14N on Ni at Elab/A = 35 MeV. Photons with energies up to E7 150 MeV were unambiguously identified in this re- action. While the photon energy spectra are in gen- eral agreement with recent indirect measurements [1 ] and measurements at higher energies [2], we establish, for the first time, very anisotropic angular distribu- tions which indicate a predominant dipole character for the observed radiation in contrast to the expect- ed quadrupole pattern [3] for coherent nucleus- nucleus bremsstrahlung. Data were taken using the 35 MeV/nucleon 14N beam of the MSU K = 500 superconducting cyclotron with an average intensity of 1.4 pnA to bombard na- tural Ni targets with areal density of 19 mg/cm 2. The experimental set-up is depicted in fig. 1. The photons were detected in 4 telescopes each comprising 5 Pb- glass ~erenkov detectors and, in front of each central Pb-glass block, a i~erenkov plastic veto paddle ,1. Each I Permanent address: DPh-N/BE, Saclay, F-91191 Gif-sur- Yvette, France 2 Present address: GSI, D-61 Darmstadt, Fed. Rep. Germany. ,1 Cerenkov Plastic SCINTIPLEX-I, manufactured by Na- tional Diagnostics, Somerville, NJ, USA. 392 plastic ~erenkov paddle plastic .... ~ a rgel/30~"If- ~°~°~ Fig. 1. Experimental set-up. For details see text and ref. [5 ]. telescope was placed at a distance of 50 cm from the target and at scattering angles 0~ r = 30 °, 60 °, 90 ° and 120 °, respectively, with an angular aperture of A0 = -+8 °. The large distance was chosen to allow neu- tron-photon separation by time-of-flight and to avoid summing of low energy statistical 7-rays. Tu suppress cosmic ray background, and identify photons, a hit in the central detector of each stack was required to- gether with: (i) Prompt coincidence (within 9 ns) with the beam pulse. (ii) Prompt coincidence (within 3 ns) 0370-2693/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)