Electron emission and positron production in deep inelastic heavy-ion reactions

Abstract

Atomic excitations are used to obtain information on the Course of a nuclear reaction. Employing a semiclassical picture we calculate the emission of S electrons and positrons in deep inelastic nuclear reactions for the example of U + U collisions incorporating nuclear trajectories resulting from two different nuclear friction models. The emission spectra exhibit characteristic deviations from those expected for elastic Coulomb scattering. The theoretical probabilities are compared with reCent experimental data by Backe et al. A simple model is used to estimate the influence of a threebody breakup of the compound system upon atomic excitations. I. THE DESCRIPTION OF ELECTRONIC EXCITATIONSIn collisions of very heavy ions superheavy quasiatomic Systems are created, where the electrons experience for a period of time T-10-=' s the combined nuclear charge Z = Z p + Z T of the projectile and the target nucleus. The resulting enormous binding energies and the highmomentum components of the wave functions necessitate a relativistic treatment of the dynamical behavior of the electrons. Under these conditions excitation Drocesses of electrons and positrons are appropriately described in a semiclassical picture based on the time-dependent twoCenter Dirac equation <fi=c= 1) where HTCD is the relativistic two-center Hamiltonian depending on the time-dependent classical intemuclear separation R(t). Equation (1) determines the motion of a single electron, initially in state i, in the external timevarying electromagnetic field. Since for nonrelativistic bombarding energies the inner-shell electrons move quasiadiabatically, the total electron wave function is expanded into the Set of adiabatic eigenstates 4j of the Hamiltonian:The expansion includes the bound states as well as an integration over continuum states of positive and negative frequencies. The phase factors Xi are chosen conveniently to eliminate the diagonal matrix elements of the Hamiltonian HTCD. Inserting the expansion (2) into ( 1) and projecting with stationary eigenfunctions we obtain a Set of coupled differential equations for the amplitudes aij(t), with the initial condition aij( -CO )=tiij.Since multistep excitation processes are crucial for a quantitative understanding of inner-shell vacancy formation in scattering processes involving very heavy ions, one has to integrate the coupled equations for the electron occupation amplitudes rigorously

Investigation of the dynamics of dissipative U+Au collisions with δ-electron spectroscopy

“…The coupled-channels approach has been developed by Reinhardt et al and uses an adiabatic basis for the quasi-atomic eigenstates calculated in monopole approximation [41,42]. The calculated spectra are weighted with the experimental impact parameter distribution corresponding to each collision class.…”

Mott scattering as a probe of long range QCD effects

“…[16]. The coupled channels equations for the transition amplitudes from state~i ) to state~j ) are given by with the initial condition that a; (t =oo) = $, and an initial configuration of occupied states.…”

Electromagnetic probes of nuclear collisions at intermediate energy

“…It is steepest for large values of ~ and decreases if the deceleration becomes more abrupt (shorter times ~) with a tendency towards a limiting slope for z .~ 10-23 S. In order to obtain absolute predictions which can be compared with measurements, a full coupled channel calculation, also including Pl/2 states, is required which could increase the probabilities by about an order of magnitude. Furthermore an ansatz for the outgoing trajectory [22] and its impact parameter dependence for close collisions is needed. Electron screening effects are negligible [23].…”

“…We suggest to utilize energetic photons and electrons as probes of the nuclear reaction process at intermediate energies, because their emission mechanisms are well understood and their final state interactions are negligible. While lepton-pairs and photons are recognized as experimental signals for ultra-relativistic nuclear reactions [7], electrons, positrons and photons are known to reflect nuclear sticking times in low-energy collisions [8,22]. More than 20 years ago a bremsstrahlung experiment was proposed [9] to measure the nuclear time delay in order to offer an unambiguous separation between compound nucleus reactions and direct reactions by utilizing the fact, that the compound nucleus reactions are delayed.…”