Estimates of the Nuclear Time Delay in Dissipative U + U and U + Cm Collisions Derived from the Shape of Positron and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>δ</mml:mi></mml:math>-Ray Spectra

Abstract

Positron and $\ensuremath{\delta}$-ray spectra have been measured in coincidence with quasielastic scattered particles and fission fragments from the bombardment of Pd, U, and Cm targets with U beams of energies between 5.9 and 8.4 MeV/u. For collisions leading to a fission reaction, the atomic positron and $\ensuremath{\delta}$-ray spectra fall off more steeply at high energies than expected from calculations based on pure Rutherford trajectories. A quantitative analysis of this effect is in accord with a nuclear contact time of about ${10}^{\ensuremath{-}21}$ s.