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Angular distributions of various Mg, Sc, Cu, Ag, and Ba nuclides produced in the interaction of 238 U with 400-GeV protons have been measured. While all the angular distributions peak at ~ 90°t those obtained for certain products are characterized by greater backward than forward emission in the laboratory system. This category includes both neutron-deficient and neutron-excess Sc fragments as well as neutron-deficient Cu and Ag nuclides.Measurement of the angular distributions of light fragments and deep spallation products formed in the interaction of heavy elements with high-energy protons has been of value in the elucidation of the reaction mechanisms. While forward peaking in the laboratory system has been observed up to 3 GeV, 1 " 4 the results obtained at 11.5 and 29 GeV indicate the occurrence of sideward peaking. 4 " 6 By contrast, the angular distributions of fission products are sideward peaked over the entire GeV regime. 2 ' 6 The change from forward-to sideward-peaked angular distributions may be the result of a change in the nature of hadron-nucleus interactions at high energies and has been interpreted as the effect of a nuclear shock wave, 5 ' 7 or the result of a coherent interaction with a part of the nucleus. 8 ' 9 However, a definitive explanation has not as yet been given.It is of interest to determine whether additional changes in the angular distributions occur at yet higher energies. We report here the first results of angular distribution measurements per formed on light fragments, deep spallation products, and fission fragments emitted in the interaction of 238 U with 400-GeV protons. While some of these products have distributions that are very similar to those obtained at lower energies, others reveal a novel and highly unusual feature, namely, a larger number of fragments emitted at backward than at forward angles in the laboratory system.Thin (200-300 /ig/cm 2 ) UF 4 targets evaporated onto pure Al backings were irradiated in an evacuated chamber with a 400-GeV proton beam in the Neutrino Hall at Fermilab. The targets, which were inclined at 45° to the beam, faced a cylindrical holder at a radial distance of 9 cm on which were mounted pure Al or Mylar recoil catcher foils. The assembly was irradiated with approximately (3-5)xlO 13 protons/min for about a oneweek period. Autoradiographs and assays of the segmented target indicated that the beam had a full width of ~ 3 mm and did not shift during the irradiations. The catcher foils intercepted relatively large solid angles (0.06-0.35 sr) in order to ensure that adequate counting rates would be obtainable. The catchers were therefore cut along constant-0 lab lines 10 and the solid angles intercepted by each foil were determined with a code which, in addition to the target-catcher geometry, 11 took into account the beam profile at the target location. 12 In a given experiment, the angular distribution was determined between 5° and 105° (or 75°-175°). The foils were cut in 15°-wide segments except for the 5°-15° and 165°-175° intervals. T...

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