Angular-momentum effects in pre-equilibrium processes

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Abstract
Cross sections and the first three moments of the γ-ray multiplicity distribution were measured for many transitions corresponding to xn and xnα products from bombardments of Er166 with He4, Nd150 with Ne20, and Te130 with Ar40. The He4 and Ne20 bombarding energies were chosen to produce Yb170 compound nuclei at similar excitation energies, approximately 63, 77, and 92 MeV. A Ge(Li)-NaI anti-Compton spectrometer was used in coincidence with nine 5.1-cm by 7.6-cm NaI(Tl) detectors. For the reactions (Ne20,xn) and (Ar40,xn), the average γ-ray multiplicity 〈M〉x increases linearly with decreasing x as expected from the statistical decay of compound nuclei. In the (He4,xn) reactions, 〈M〉x is similar for large x but for small x it decreases substantially with decreasing x. The latter corresponds to a shift in the population toward the low J values of levels in Yb170−x prior to γ decay. Also, in the few-neutron channels the average multiplicity associated with a given gating transition in the residual nucleus is much smaller for transitions between low-lying levels. These features, absent in the heavy-ion bombardments, indicate a strong contribution from pre-equilibrium neutron emission. The measured (He4,xn) cross sections are in reasonable agreement with those calculated from the geometry-dependent hybrid model of pre-equilibrium emission except for the smallest values of x; the latter results and the low 〈M〉x values for x≤4 suggest that more than one pre-equilibrium neutron is emitted in these channels.