W. Udo Schroder

Surface entropy in statistical emission of massive fragments from equilibrated nuclear systems

Statistical fragment emission from excited nuclear systems is studied within the framework of a schematic Fermi-gas model combined with Weisskopf’s detailed balance approach. The formalism considers thermal expansion of finite nuclear systems and pays special attention to the role of the diffuse surface region in the decay of hot equilibrated systems. It is found that with increasing excitation energy, effects of surface entropy lead to a systematic and significant reduction of effective emission barriers for fragments and, eventually, to the vanishing of these barriers. The formalism provides a natural explanation for the occurrence of negative nuclear heat capacities reported in the literature. It also accounts for the observed linearity of pseudo-Arrhenius plots of the logarithm of the fragment emission probability versus the inverse square-root of the excitation energy, but does not predict true Arrhenius behavior of these emission probabilities.

Liquid-gas coexistence and critical behavior in boxed neutral, isosymmetric pseudo-Fermi matter

A schematic model is presented that allows one to study the behavior of interacting pseudo-Fermi matter locked in a thermostatic box. As a function of the box volume and temperature, the matter is seen to show all of the familiar charactersitics of a Van der Waals gas, which include the coexistence of two phases under certain circumstances and the presence of a critical point.

A simple method for rise-time discrimination of slow pulses from charge-sensitive preamplifiers

Performance of a simple method of particle identification via pulse rise-time discrimination is demonstrated for slow pulses from charge-sensitive preamplifiers with rise times ranging from 10 to 500 ns. The method is based on a comparison of the amplitudes of two pulses, derived from each raw preamplifier pulse with two amplifiers with largely differing shaping times, using a fast peak-sensing ADC. For the injected charges corresponding to energy deposits in silicon detectors of a few tens of MeV, a rise-time resolution of the order of 1 ns can be achieved. The identification method is applicable in particle experiments involving large-area silicon detectors, but is easily adaptable to other detectors with a response corresponding to significantly different pulse rise times for different particle species.

Cluster Emission in Complex Nuclear Reactions

Possible mechanisms of the emission of nuclear clusters in heavy-ion induced nuclear reactions are discussed in the context of the overall dissipative reaction environment. Essentials of model approaches to the reaction dynamics are briefly reviewed. Analysis of the overall reaction dynamics provides an estimate of time scales for cluster emission and the buildup or preexistence of nucleonic correlations. Experimental results demonstrate the presence of at least two different mechanisms of cluster emission. Energetic nuclear clusters are likely emitted during early stages of a nuclear reaction, containing memory of entrance-channel conditions. In addition, sequential, statistical cluster emission is observed from the hot remnants of projectile and target nuclei. The difficulty encountered previously by statistical models to describe statistical emission of massive clusters from excited nuclei is resolved in the framework of a simple interacting Fermi-gas model of expanded nuclear matter. It is suggested that sequential cluster emission is entropy-driven and can be understood in terms of a rearrangement of the surface of hot nuclei produced in energetic nuclear reactions.