V. E. Viola

Nuclear systematics of the heavy elements—II Lifetimes for alpha, beta and spontaneous fission decay☆

Abstract The experimental half-lives for alpha, beta and spontaneous fission decay are analysed for the heavy elements (A ≥ 140). From the systematic behaviour of these data, semi-empirical relationships are derived for use in the prediction of lifetimes for undiscovered nuclides. α-decay predictions are based on a square well nuclear model; β-decay is approximately treated in terms of average log (tft) values, and spontaneous fission is considered from the point of view of the liquid drop model. Particular emphasis is given to nucleosynthesis problems involving the trans-ferium nuclei. On the basis of this study, it is estimated that there exist a number of relatively stable nuclei with mass number A∼270 to 290. For nuclei having 184 neutrons, or slightly less, special stability is expected because of the closed neutron shell at N = 184. Nuclei with N > 184 should have extremely short alpha and spontaneous fission decay lifetimes. The implications of these predictions for various problems relating to chemistry and cosmology are also briefly discussed.

The liquid to vapor phase transition in excited nuclei

The thermal component of the 8 GeV/c pi+ Au data of the ISiS Collaboration is shown to follow the scaling predicted by Fishers model when Coulomb energy is taken into account. Critical exponents tau and sigma, the critical point (p(c),rho(c),T(c)), surface energy coefficient c(0), enthalpy of evaporation DeltaH, and critical compressibility factor C(F)(c) are determined. For the first time, the experimental phase diagrams, (p,T) and (T,rho), describing the liquid vapor coexistence of finite neutral nuclear matter have been constructed.

Mechanisms of very heavy-ion collisions: The 209Bi + 136Xe reaction at ELab = 1130 MeV

Abstract The mechanisms of kinetic-energy dissipation and nucleon exchange operating in damped heavy-ion collisions, and the corresponding time scales are investigated. As an example of a very heavy system, the reaction 209 Bi + 136 Xe at E Lab = 1130 MeV is choosen for a comparison with various reaction models. The experimental data include atomic number, energy and angular distributions of the projectile-like fragments and the correlations between these experimental observables. The integrated fragment Z distribution is found to be broad, but centered at the charge of the projectile. The kinetic energy distribution extends from quasi-elastic down to Coulomb energies of highly deformed fragments. The angular distribution of the reaction products is focussed into a narrow angular range a few degrees forward of the quarter-point angle. Examining the correlations of experimental observables with fragment charge and energy loss in detail, it is concluded that the kinetic-energy loss is a fundamental parameter indicating the stage of evolution of the reaction, i.e., the interaction time. The energy loss is inferred to decrease linearly with angular momentum, and this relation is used to construct an average experimental deflection function. The latter is compared to the predictions of classical dynamical models employing various potentials and frictional forces. These calculations do not provide a consistent description of the experimental results. Employing a simple phenomenological reaction model, l -dependent interaction times are deduced which are used to evaluate proton-number diffusion coefficients. A rather model-independent comparison of the time dependence of the mechanisms of nucleon exchange and kinetic energy dissipation is achieved by using the microscopic time scale provided by the nucleon-exchange mechanism. Energy dissipation is found to be of the one-body type, nucleon exchange contributing about 30% to the total energy loss. A consistent description of the observed reaction phenomena has been achieved in terms of a statistical mechanism of damped reactions evolving in time as nucleons are exchanged and energy is dissipated. It connects in a continuous fashion the domains of quasi-elastic scattering and strongly damped collisions.

Nuclear systematics of the heavy elements—I energetics and masses☆

Abstract From examination of nuclear decay energy systematics, the mass-energy surface of the heavy elements has been extended to experimentally unknown isotopes. The mass predictions are made by empirical extrapolation of α- and β-decay energies which fit together to form closed cycles. Experimental masses and energies are based upon the most recent work of Wapstra et al.(7) The following quantities are tabulated for nuclei with 54≤Z≤108 and 139≤A≤277: nuclidic mass, α- and β-decay energies, and neutron and proton binding energies. In addition the systematic behaviour of the kinetic energy release in fission is examined.

Correlation of fission fragment kinetic energy data

Recent measurements of the most probable kinetic energy release in fission are compared in order to study the dependence on nuclear species. Appropriate corrections to data obtained with semi-conductor detectors, particularly with regard to calibration procedures, are discussed and applied. Corrected results are listed in Table I and plotted in Fig. 1. It is found that the most probable kinetic energy release in fission can be simply expressed by the function. E k > = 0.1071 Z 2 / A 1 / 3 + 22.2 MeV . The corrected values are compared with recent liquid drop calculations and good agreement is found for Z2/A less than 35, but not above this value. Effects of mass asymmetry are also briefly discussed. Analysis of the data indicates that the most probable kinetic energy release is essentially independent of excitation energy. Use was made only of results published since 1961. Prior data can be found in the review of Huizenga and Vandenbosch .

Signals for a transition from surface to bulk emission in thermal multifragmentation.

Excitation-energy-gated two-fragment correlation functions have been studied between E(*)/A = (2-9)A MeV for equilibriumlike sources formed in 8-10 GeV/c pi(-) and p+197Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E(*)/A = (2-5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.

Event-by-Event Analysis of Proton-Induced Nuclear Multifragmentation: Determination of the Phase Transition Universality Class in a System with Extreme Finite-Size Constraints

A percolation model of nuclear fragmentation is used to interpret 10.2 GeV/c p+197Au multifragmentation data. Emphasis is put on finding signatures of a continuous nuclear matter phase transition in finite nuclear systems. Based on model calculations, corrections accounting for physical constraints of the fragment detection and sequential decay processes are derived. Strong circumstantial evidence for a continuous phase transition is found, and the values of two critical exponents, sigma = 0.5+/-0.1 and tau = 2.35+/-0.05, are extracted from the data. A critical temperature of T(c) = 8.3+/-0.2 MeV is found.

Fission barriers and half-lives of the trans-radium elements

Abstract Spontaneous fission half-lives and fission barriers are calculated for the elements with 88 ≦ Z ≦ 108. The method is semi-empirical, based on the liquid drop model with modifications for the effects of nuclear structure. Spontaneous fission stability is predicted to decrease for nuclei just beyond the N = 152 neutron subshell, but should increase regularly for neutron numbers greater than 160. This effect should become quite strong as the N = 184 neutron subshell is approached, resulting in a number of experimentally observable isotopes in the mass region A ≈ 270−290. The results are discussed briefly with regard to problems of nucleo-synthesis as they pertain to the discovery of new isotopes and heavy element generation in stellar reactions.

The Indiana silicon sphere 4π charged-particle detector array

A low threshold charged particle detector array for the study of fragmentation processes in light-ion-induced reactions has been constructed and successfully implemented at the IUCF and Saturne II accelerators. The array consists of 162-triple-element detector telescopes mounted in a spherical geometry and covering 74% of 4π in solid angle. Telescope elements are composed of (1) an axial-field gas ionization chamber operated with C3F8 gas; (2) a 0.5 mm thick passivated silicon detector, and (3) a 2.8 cm thick CsI(Tl) scintillation crystal with photodiode readout. Discrete element identification is obtained for ejectiles up to Z ∼ 16 over the dynamic range 0.7 ≤ EA ≤ 95 MeV/nucleon. Isotopes are also distinguished for H, He, Li and Be ejectiles with 8 ≲ EA ≲ 95 MeV. Custom-designed electronics are employed for bias supplies and linear signal processing. Data are acquired via a CAMAC/VME/Ethernet system.

Intermediate-mass-fragment production in the reaction of 200 MeV 3He with Ag☆

Abstract Energy spectra of Z = 4–12 fragments emitted in the reactions of 198.6 MeV 3He with natAg were measured over the full angular range. Back-angle data are consistent with evaporation from the compound nucleus; the spectra evolve with Z from maxwellian to gaussian shapes, in agreement with earlier theoretical predictions. A two-component moving source model gives evidence for the existence of a strong non-equilibrium component, with a steep Z−5.3 dependence. The data are compared with the predictions of an accreting source model.