Gas gain on single wire chambers filled with pure isobutane at low pressure

The gas gain of single-wire chambers filled with isobutane, with cell cross-section 12x12 mm and wire diameters of 15, 25, 50 and 100 $\mu$m, has been measured at pressures ranging 12-92 Torr. Contrary to the experience at atmospheric pressure, at very low pressures the gas gain on thick wires is higher than that on thin wires at the same applied high voltage as was recently shown. Bigger wire diameters should be used in wire chambers operating at very low pressure if multiple scattering on wires is not an issue.Comment: 9 pages, 6 figure

Benchmarking calculations of excitonic couplings between bacteriochlorophylls

Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. We also caution against computationally optimizing a crystal structure before calculating couplings, as it can lead to large, uncontrollable errors. Understanding the unavoidable uncertainties can guard against striving for unrealistic precision; at the same time, detailed benchmarks can allow important qualitative questions--which do not depend on the precise values of the simulation parameters--to be addressed with greater confidence about the conclusions

Anomalous tunneling of bound pairs in crystal lattices

A novel method of solving scattering problems for bound pairs on a lattice is developed. Two different break ups of the hamiltonian are employed to calculate the full Green operator and the wave function of the scattered pair. The calculation converges exponentially in the number of basis states used to represent the non-translation invariant part of the Green operator. The method is general and applicable to a variety of scattering and tunneling problems. As the first application, the problem of pair tunneling through a weak link on a one-dimensional lattice is solved. It is found that at momenta close to \pi the pair tunnels much easier than one particle, with the transmission coefficient approaching unity. This anomalously high transmission is a consequence of the existence of a two-body resonant state localized at the weak link.Comment: REVTeX, 5 pages, 4 eps figure

Stability of C20 fullerene chains

The stability of (C20)N chains with N = 3 - 7 is analyzed by numerical simulation using a tight-binding potential and molecular dynamics. Various channels of losing the cluster-chain structure of the (C20)N complexes are observed, including the decay of C20 clusters, their coalescence, and the separation of one C20 fullerene from the chain.Comment: To appear in JETP Letter

Precision planar drift chambers and cradle for the TWIST muon decay spectrometer

To measure the muon decay parameters with high accuracy, we require an array of precision drift detector layers whose relative position is known with very high accuracy. This article describes the design, construction and performance of these detectors in the TWIST (TRIUMF Weak Interaction Symmetry Test) spectrometer.Comment: 44 pages, 16 Postscript figures, LaTeX2e, uses Elsevier class elsart.cls, package graphicx, submitted to Nuclear Instruments & Methods in Physics Researc