An experimental study of mass asymmetry in subbarrier photofission of 238U

A measurement of symmetric and asymmetric photofission yields of²³⁸U in the energy region from 4.5 MeV to 6.5 MeV has been performed. As γ-source the bremsstrahlung from a microtron has been used. The fission yields are obtained from β-counting of chemically separated isotopes ¹¹¹Ag, ¹¹⁵Cd and ¹¹⁷Cd for symmetric fission and ¹³⁹Ba for asymmetric fission. Some structures in the yield curves at about 5.3 MeV and a pronounced maximum at about 6.0 MeV in the valley-to-peak ratio in the fission fragment yield distribution are observed. It is suggested that these effects are connected with the double-hump fission barrier concept and indicate differences between fission barriers for symmetric and asymmetric fission.     

The accuracy of ab initio molecular geometries for systems containing second-row atoms

The performance of the standard hierarchy of ab initio models--that is, Hartree-Fock theory, second-order Moller-Plesset theory, coupled-cluster singles-and-doubles theory, and coupled-cluster singles-doubles-approximate-triples theory--in combination with correlation-consistent basis sets is investigated for equilibrium geometries of molecules containing second-row elements. From an analysis on a collection of 31 molecules (yielding statistical samples of 41 bond distances and 13 bond angles), the statistical errors (mean deviation, mean absolute deviation, standard deviation, and maximum absolute deviation) are established at each level of theory. The importance of core correlation is examined by comparing calculations in the frozen-core approximation with calculations where all electrons are correlated.     

Induced losses in strained loose-tube fiber cables

Measurements of induced losses on primary coated single-mode fibers strained against the inner wall of loose tube cable materials are reported. Fibers have been chosen from current types such as match and depressed clad with mode field diameters (MFD) and cut-off wavelengths spanning the expected range of production tolerances. The loss dependence on MFD and cut-off is characterized, and the combination high MFD/low cut-off turns out to be the most bend sensitive parameter combination while the opposite case with low MFD/high cut-off gives the best performance. Different tube materials provide different bend loss performance depending on the microstructure of the tube and the amount and type of filling jelly.     

Electronic circular dichroism of disulphide bridge: ab initio quantum-chemical calculations

Electronic circular dichroism (ECD) parameters of the disulphide chromophore have been calculated for dihydrogen disulphide, dimethyl disulphide, and cystine using density-functional theory, coupled-cluster theory, and multiconfigurational self-consistent field theory. The objective is twofold: first, to examine the performance of the Coulomb-attenuated CAM-B3LYP functional for the calculation of ECD spectra; second, to investigate the dependence of the ECD parameters on the conformation around the disulphide bridge. The CAM-B3LYP functional improves considerably on the B3LYP functional, giving results comparable to CCSD theory and to MCSCF theory in an extended active space. The conformational dependence of the ECD parameters does not change much upon substitution, which is promising for the application of ECD in structural investigations of proteins containing disulphide bridges.     

Linear-scaling symmetric square-root decomposition of the overlap matrix

We present a robust linear-scaling algorithm to compute the symmetric square-root or Lowdin decomposition of the atomic-orbital overlap matrix. The method is based on Newton-Schulz iterations with a new approach to starting matrices. Calculations on 12 chemically and structurally diverse molecules demonstrate the efficiency and reliability of the method. Furthermore, the calculations show that linear scaling is achieved.     

Structure of amyloid aggregates of lysozyme from small-angle X-ray scattering data

The structure of filament amyloid aggregates of hen egg white lysozyme in water has been investigated by the small-angle X-ray scattering method. The experimental data are described by different cylindrical models, among which the best agreement is reached with the long helix model. A comparison of the results with the small-angle neutron scattering data reveals the influence of the heavy component of the solvent (a H₂O/D₂O mixture) on the structure of the filaments.     

The effects of service disciplines on the performance of a nonreliable terminal system

In this paper, we consider a stochastic queueing model for the performance evaluaton of a real-life computer system consisting of n terminals connected with a CPU. A user at terminal i has thinking and processing time depending on the index i. Let us suppose that the operational system is subject to random breakdowns, which may be software or hardware ones, stopping the service both at the terminals and at the CPU. The failure-free operation times of the system and the restoration times are random variables. Busy terminals are also subject to random breakdowns not affecting the system operation. The failure-free operation times and the repair times of a busy terminal i are random variables with distribution function depending on index i. The breakdowns are serviced by a single repairman providing preemptive priority to the system's failure, while the restoration at the terminals are carried out according to the FIFO rule. We assume that each user generates only one job at a time, and he waits at the CPU before he starts thinking again, that is, the terminal is inactive while waiting at the CPU, and it cannot break down. All random variables involved in the model construction are assumed to be exponentially distributed and independent of each other. The aim of this paper is to investigate the effect of different service disciplines, such as FIFO, processor sharing, priority processor sharing, and polling, on the main performance measures, such as utilizations, response times, throughput, and mean queue length. Supported by the Hungarian National Foundation for Scientific Research (grant No. ORKA T014974/95). Proceedings of the Seminar on Stability Problems for Stochastic Models, Hajdúszoboszló. Hungary, 1997, Part II.