Jan Toke

Particle identification via pulse shape analysis for large-area silicon detectors of the CHIMERA array

Mass and atomic-number identification (ID) of reaction products is a fundamental requirement of any nuclear reaction study. An effective particle-ID method is demonstrated, based on pulse shape analysis/discrimination (PSD) applied to large-area, single-element silicon detectors. This technique uses commercial electronic modules and achieves atomic number resolution rivaling that typically obtained with multi-element (/spl Delta/E-E) detector telescopes. The method is applied to the CHIMERA detector system without compromising its time-of-flight (TOF) resolution. In-beam tests of the PSD method have been performed with large-area, 300-/spl mu/m thick CHIMERA silicon detectors, measuring particles from the /sup 19/F+/sup 12/C reaction at Tandem energies. Performance of a simple PSD set up is discussed, for front and rear particle injection.

Shapes of α-particle spectra from energetic heavy-ion fusion reactions

Abstract The shapes of α-particle spectra from energetic heavy-ion fusion reactions are analyzed within the framework of a statistical model. Contrary to claims made in a number of earlier papers, good agreement is obtained between calculated and experimental spectra without the need to lower the evaporation barriers relative to the respective absorption barriers.

Dynamical production of intermediate-mass fragments in peripheral 209Bi+136Xe collisions at ElabA=28 MeV

Abstract Intermediate-mass fragments (IMF) from the reaction 209 Bi+ 136 Xe at E lab A =28 MeV have been measured in coincidence with other charged reaction products, as well as with neutrons. The IMF emission patterns are seen to exhibit signatures of two mechanisms — statistical emission from the fully accelerated massive reaction partners and a fast dynamical emission from a single effective source. The latter mechanism, possibly involving a multiple neck rupture, becomes dominating for least dissipative collisions characterized by low associated light particle multiplicities.

Caloric curve for mononuclear configurations.

The caloric curve for mononuclear configurations is studied with a schematic model. We investigate the dependence of the entropy on the density and effective-mass profiles. In finite nuclei, a plateau in the caloric curve is a consequence of decreasing density and the destruction of correlations rather than an indication of phase coexistence. The mononuclear regime is metastable with respect to binary fission at low excitation energy and with respect to multifragmentation at high excitation. The statistical framework presented here is suitable to treat scenarios where experimental conditions are set to favor a population of highly excited mononuclei.

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.

Damped reaction dynamics in 197Au + 208Pb collisions at 29 MeV/nucleon

Abstract Velocity and angular distributions of neutrons from the reaction 197Au + 208Pb at E A =29 MeV were measured in coincidence with charged reaction products. The neutron emission pattern is found to erolve smoothly with atomic number and energy of the coincident fragments, up to the highest degrees of energy dissipation observed. The velocity distribution of neutrons in coincidence with any type of fragment is essentially bimodal and can be understood in terms of sequential emission from fully accelerated primary projectile-like and target-like fragments. A relatively weak nonequilibrium neutron component is also observed. Neutron multiplicities and spectral slope parameters, as well as the deduced emitter velocities indicate consistently that, even for highly dissipative collisions associated with intermediate-mass fragments, the average damping of kinetic energy available in the entrance channel is incomplete.

Fission of spin-aligned projectile-like nuclei in the interactions of 29 MeV/nucleon 208Pb with 197Au

Abstract Binary fission of projectile-like nuclei has been investigated in the interaction of 29 MeV/nucleon Pb on Au, together with the associated neutron multiplicity. Fission is only observed in rather peripheral collisions and represents approximately 20% of the total reaction cross-section. The fission process occurs after collisions in which up to 550 MeV have been dissipated. The angular and energy distribution of the fragments can be accounted for by assuming a noticeable spin alignment of the fissioning nuclei.

A simple pulse processing concept for a low-cost pulse-shape-based particle identification

Abstract A simple method of pulse processing is described, which allows one to achieve at a very low cost a high-quality pulse-shape discrimination, using two commonly gated current-integrating ADCs. Results of application tests for a liquid NE213 scintillator detector (neutron-γ discrimination), as well as for a CsI(Tl) detector (p, d, t, α discrimination) are presented.

Retardation of particle evaporation from excited nuclear systems due to thermal expansion

Particle evaporation rates from excited nuclear systems at equilibrium matter density are studied within the Harmonic-Interaction Fermi Gas Model (HIFGM) combined with Weisskopf’s detailed balance approach. It is found that thermal expansion of a hot nucleus, as described quantitatively by HIFGM, leads to a significant retardation of particle emission, greatly extending the validity of Weisskopf’s approach. The decay of such highly excited nuclei is strongly influenced by surface instabilities.

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.