Measurement of the lunar neutron density profile

An in situ measurement of the lunar neutron density from 20 to 400 g/sq cm depth between the lunar surface was made by the Apollo 17 Lunar Neutron Probe Experiment using particle tracks produced by the B10(n, alpha)Li7 reaction. Both the absolute magnitude and depth profile of the neutron density are in good agreement with past theoretical calculations. The effect of cadmium absorption on the neutron density and in the relative Sm149 to Gd157 capture rates obtained experimentally implies that the true lunar Gd157 capture rate is about one half of that calculated theoretically

Digital Signal Processing Education: Technology and Tradition

In this paper we discuss a DSP course presented to both University students and to participants on industrial short courses. The "traditional" DSP course will typically run over one to two semesters and usually cover the fundamental mathematics of z-, Laplace and Fourier transforms, followed by the algorithm and application detail. In the course we will discuss, the use of advanced DSP software and integrated support software allow the presentation time to be greatly shortened and more focussed algorithm and application learning to be introduced. By combining the traditional lecture with the use of advanced DSP software, all harnessed by the web, we report on the objectives, syllabus, and mode of teaching

Potential landscapes and induced charges near metallic islands in three dimensions

We calculate electrostatic potential landscapes for an external probe charge in the presence of a set of metallic islands. Our numerical calculation in three dimensions (3D)uses an efficient grid relaxation technique. The well-known relaxation algorithm for solving the Poisson equation in two dimensions is generalized to 3D. In addition,all charges on the system, free as well as induced charges,are determined accurately and self-consistently to satisfy the desired boundary conditions. This allows the straightforward calculation of the potential on the outer boundary using the free space electrostatic Green's function,as well as the calculation of the entire capacitance matrix of the system. Physically interesting examples of nanoscale systems are presented and analyzed.Comment: 6 pages, 6 figures, submitted to PR

Weak Localization and Transport Gap in Graphene Antidot Lattices

We fabricated and measured antidot lattices in single layer graphene with lattice periods down to 90 nm. In large-period lattices, a well-defined quantum Hall effect is observed. Going to smaller antidot spacings the quantum Hall effect gradually disappears, following a geometric size effect. Lattices with narrow constrictions between the antidots behave as networks of nanoribbons, showing a high-resistance state and a transport gap of a few mV around the Dirac point. We observe pronounced weak localization in the magnetoresistance, indicating strong intervalley scattering at the antidot edges. The area of phase-coherent paths is bounded by the unit cell size at low temperatures, so each unit cell of the lattice acts as a ballistic cavity.Comment: some revisions, to appear in New Journal of Physics, Special Issue Graphen

Voltage-flux-characteristics of asymmetric dc SQUIDs

We present a detailed analysis of voltage-flux V(Phi)-characteristics for asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry alpha_I in the critical currents of the two Josephson junctions, the minima in the V(Phi)-characteristics for bias currents of opposite polarity are shifted along the flux axis by Delta_Phi = (alpha_I)*(beta_L) relative to each other; beta_L is the screening parameter. This simple relation allows the determination of alpha_I in our experiments on YBa_2Cu_3O_(7-x} dc SQUIDs and comparison with theory. Extensive numerical simulations within a wide range of beta_L and noise parameter Gamma reveal a systematic dependence of the transfer function V_Phi on alpha_I and alpha_R (junction resistance asymmetry). As for the symmetric dc SQUID, V_Phi factorizes into g(Gamma*beta_L)*f(alpha_I,beta_L), where now f also depends on alpha_I. For \beta_L below five we find mostly a decrease of V_Phi with increasing alpha_I, which however can only partially account for the frequently observed discrepancy in V_Phi between theory and experiment for high-T_c dc SQUIDs.Comment: 4 pages, 7 figures, Applied Superconductivity Conference 2000, to be published in IEEE Trans. Appl. Supercon

Evaluation of the micro-carburetor

A prototype sonic, variable-venturi automotive carburetor was evaluated for its effects on vehicle performance, fuel economy, and exhaust emissions. A 350 CID Chevrolet Impala vehicle was tested on a chassis dynamometer over the 1975 Federal Test Procedure, urban driving cycle. The Micro-carburetor was tested and compared with stock and modified-stock engine configurations. Subsequently, the test vehicle's performance characteristics were examined with the stock carburetor and again with the Micro-carburetor in a series of on-road driveability tests. The test engine was then removed from the vehicle and installed on an engine dynamometer. Engine tests were conducted to compare the fuel economy, thermal efficiency, and cylinder-to-cylinder mixture distribution of the Micro-carburetor to that of the stock configuration. Test results show increases in thermal efficiency and improvements in fuel economy at all test conditions. Improve fuel/air mixture preparation is implied from the information presented. Further improvements in fuel economy and exhaust emissions are possible through a detailed recalibration of the Micro-carburetor

Ratchet effect in dc SQUIDs

We analyzed voltage rectification for dc SQUIDs biased with ac current with zero mean value. We demonstrate that the reflection symmetry in the 2-dimensional SQUID potential is broken by an applied flux and with appropriate asymmetries in the dc SQUID. Depending on the type of asymmetry, we obtain a rocking or a simultaneously rocking and flashing ratchet, the latter showing multiple sign reversals in the mean voltage with increasing amplitude of the ac current. Our experimental results are in agreement with numerical solutions of the Langevin equations for the asymmetric dc SQUID.Comment: 10 pages including 5 Postscript figure