Romualdo deSouza

Confronting measured near- and sub-barrier fusion cross sections for

Recently measured fusion cross sections for the neutron-rich system 20 O + 12 C are compared to dynamic, microscopic calculations using time-dependent density functional theory. The calculations are carried out on a three-dimensional lattice and performed both with and without a constraint on the density. The method has no adjustable parameters, and its only input is the Skyrme effective nucleon-nucleon interaction. While the microscopic density constrained time-dependent Hartree-Fock calculations lie closer to the experimental data than standard fusion systematics, they underpredict the experimental data significantly.

^{20}O+^{12}C

We demonstrate a novel concept for a position sensitive microchannel plate detector. This detector provides sub-millimeter spatial resolution by examining the signal induced on a wire harp by the electron cloud from a microchannel plate detector. Wires in the harp are efficiently read out by coupling them to a delay line.

with a microscopic method

A method for achieving good position resolution of low-intensity electron signals using a microchannel plate resistive anode detector is demonstrated. Electron events at a rate of 7 counts s(-1) are detected using a Z-stack microchannel plate. The dependence of position resolution on both the distance and the potential difference between the microchannel plate and resistive anode is investigated. Using standard commercial electronics, a measured position resolution of 170 μm (FWHM) is obtained, which corresponds to an intrinsic resolution of 157 μm (FWHM).

Using induced signals to sense position from a microchannel plate detector

Digital signal processing techniques were employed to investigate the joint use of charge division and risetime analyses for the resistive anode (RA) coupled to a microchannel plate detector (MCP). In contrast to the typical approach of using the relative charge at each corner of the RA, this joint approach results in a significantly improved position resolution. A conventional charge division analysis utilizing analog signal processing provides a measured position resolution of 170 μ m (FWHM). By using the correlation between risetime and position we were able to obtain a measured resolution of 92 μ m (FWHM), corresponding to an intrinsic resolution of 64 μ m (FWHM) for a single Z-stack MCP detector.

Optimizing the position resolution of a Z-stack microchannel plate resistive anode detector for low intensity signals

Equilibration of N/Z in binary breakup of an excited and transiently deformed projectile-like fragment (PLF*), produced in peripheral collisions of 64Zn + 27Al, 64Zn, 209Bi at E/A = 45 MeV, is examined. The composition of emitted light fragments (3<=Z<=6) changes with the decay angle of the PLF*. The most neutron-rich fragments observed are associated with a small rotation angle. A clear target dependence is observed with the largest initial N/Z correlated with the heavy, neutron-rich target. Using the rotation angle as a clock, we deduce that N/Z equilibration persists for times as long as 3-4 zs (1zs = 1 x 10^-21 s = 300 fm/c). The rate of N/Z equilibration is found to depend on the initial neutron gradient within the PLF*.

Using pulse shape analysis to improve the position resolution of a resistive anode microchannel plate detector

Isospin transport occurring within dinuclear projectile-like fragments (PLFs) produced in heavy- ion collisions is explored as a probe of the nuclear symmetry energy. Within the framework of the Constrained Molecular Dynamics model (CoMD), the existence of the long-lived dinuclear PLFs, for up to 800 fm/c, is observed. It is demonstrated that changes in the of the two fragments resulting from the breakup of the dinuclear PLF is due to isospin transport. The rate of the transport between the two fragments is shown to be dependent on the slope of the symmetry energy at saturation density. Comparison of the CoMD calculations with experimental data establish that the evolution of could be used to constrain the density dependence of the symmetry energy.

Timescale for equilibration of N/Z gradients in dinuclear systems

We report the first measurement of the fusion excitation functions for K + Si at nearbarrier energies. Evaporation residues resulting from the fusion process were identified by direct measurement of their energy and time-of-flight with high geometric efficiency. At the lowest incident energy, the cross-section measured for the neutron-rich K induced reaction is ∼6 times larger than that of the β-stable system. The experimental data are compared with both a dynamical deformation model and coupled channels calculations (CCFULL).

Symmetry energy dependence of long-timescale isospin transport

Microchannel plates (MCPs) are routinely used in the detection of photons, electrons, and ions. Due to their high amplification and excellent timing characteristics they are particularly useful in the detection of a single photon, electron, or ion. In addition to sub-nanosecond time resolution, MCP detectors can also provide position resolution making them useful in imaging applications. In the described approach, the position of the incident particle is determined by sensing the electron cloud as it emanates from a MCP stack. The bipolar nature of the induced signals provides a particularly unique signal. By coupling a sense wire plane to a delay line a position resolution of 466 pm has been achieved. A setup to characterize the dependence of the induced signals on the position and magnitude of the electron cloud, through an independent measurement of its position, is presented. A second generation prototype detector which employs a differential approach is also described.