Acceleration induced neutron emission in heavy nuclei
Résumé
The effect of the acceleration of a nucleus on the neutron states is studied in the frame of the independent-particle nuclear shell model. For this we solve numerically the time-dependent Schrodinger equation, with a moving mean-field of Woods-Saxon type. The time evolution of a neutron states at the Fermi level is calculated for U236 and acceleration parameter A=0.5 (in 10 44 [fm/sec 2 ]). It is the acceleration during the Coulomb repulsion of two U236 nuclei when they are 20 fm apart. We keep this acceleration constant for 10−21 sec before we switch it off (A=0) and follow the wave packet for another 10−21 sec. During the acceleration, the wave function oscillates with increasing amplitude until it escapes, mainly in the direction opposite to the motion of the nucleus. The mean value of its energy (in the nuclear system) increases from −4.80 MeV to −3.15 MeV and 12% of the wave packet leaves the nucleus. During the uniform motion, the wave packet continues to oscillate and to escape at a lower rate: an extra 2%. We repeated the calculations for two neighboring states and found the emission rate to depend strongly on the position of the neutron state with respect to the Fermi energy. Finally, the effect of the nuclear deformation on the acceleration induced neutron emission is studied. In this case the period of oscillation is larger and the amplitude smaller. The angular distribution with respect to the direction of motion is more complex: it has, in the nuclear system, an intense component almost perpendicular to the deformation axis.
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