The Angular Distribution of Photoneutrons From O16

The relative importance of competing reaction channels in the photo-excitation of the giant resonance in O16 has been studied by measurement of the angular distribution of the emitted photo-neutrons through the corresponding energy range using a multi-angle neutron time of flight spectrometer installed at the Kelvin Laboratory of the University of Glasgow. Design factors for the spectrometer are discussed with regard to optimisation of energy resolution and counting rates under the constraints of the source, and the obtainable accuracy of normalisation between neutron detectors at different angles has been measured, A reliable method of calibration of the neutron energy scale at each angle is described and corrections to the neutron spectra for absorption by materials in the flight path have been made. The energy dependent efficiency of the neutron detectors is calculated in an appendix by a Monte Carlo technique using a new measurement of the scintillation response curve for recoil protons. Angular distributions of photoneutrons from the D2 (gamma,n)p reaction with 26.9 MeV bremsstrahlung have been measured and are found to agree with previously published results and theoretical predictions to within the expected accuracy. This is taken as confirmation of good angular normalisation in the system. Measurements by the author of the angular distributions of photo-neutrons from 26.9 MeV bremsstrahlung on O16 are presented and absolute cross sections have been obtained with respect to that known for deuterium. Photoproton data from other authors is analysed in a similar way for comparison with the photoneutron results. In the E1 approximation, the relative intensity of s/d wave admixture is found to follow the resonance structure for both protons and neutrons and the corresponding phase difference is found to vary about the value expected from optical model predictions. The E2 cross section has been extracted from the data and is found to rest mainly in two broad states centred at about 23 MeV and 24.7 MeV, while a Barker-Mann analysis shows the isospin impurity in the O16 giant resonance to be small and to be mainly associated with the regions of E2 strength. The pure 1p1h dipole approximation fails to describe these features and the observed splitting of the giant resonance in O16 but the presence of more complicated excitations is shown to be more consistent with experiment

Phonon drag thermopower and weak localization

Previous experimental work on a two-dimensional (2D) electron gas in a Si-on-sapphire device led to the conclusion that both conductivity and phonon drag thermopower $S^g$ are affected to the same relative extent by weak localization. The present paper presents further experimental and theoretical results on these transport coefficients for two very low mobility 2D electron gases in $\delta-$doped GaAs/Ga$_x$Al$_{1-x}$As quantum wells. The experiments were carried out in the temperature range 3-7K where phonon drag dominates the thermopower and, contrary to the previous work, the changes observed in the thermopower due to weak localization were found to be an order of magnitude less than those in the conductivity. A theoretical framework for phonon drag thermopower in 2D and 3D semiconductors is presented which accounts for this insensitivity of $S^g$ to weak localization. It also provides transparent physical explanations of many previous experimental and theoretical results.Comment: 19 page Revtex file, 3 Postscript figur

Blood pressure and blood lead concentration in bus drivers.

San Francisco bus drivers have an increased prevalence of hypertension. This study examined relationships between blood lead concentration and blood pressure in 342 drivers. The analysis reported in this study was limited to subjects not on treatment for hypertension (n = 288). Systolic and diastolic pressures varied from 102 to 173 mm Hg and from 61 to 105 mm Hg, respectively. The blood lead concentration varied from 2 to 15 micrograms/dL. The relationship between blood pressure and the logarithm of blood lead concentration was examined using multiple regression analysis. Covariates included age, body mass index, sex, race, and caffeine intake. The largest regression coefficient relating systolic blood pressure and blood lead concentration was 1.8 mm Hg/ln (micrograms/dL) [90% C. I., -1.6, 5.3]. The coefficient for diastolic blood pressure was 2.5 mm Hg/ln (micrograms/dL) [90% C. I., 0.1, 4.9]. These findings suggest effects of lead exposure at lower blood lead concentrations than those concentrations that have previously been linked with increases in blood pressure

Compilation of extended recursion in call-by-value functional languages

This paper formalizes and proves correct a compilation scheme for mutually-recursive definitions in call-by-value functional languages. This scheme supports a wider range of recursive definitions than previous methods. We formalize our technique as a translation scheme to a lambda-calculus featuring in-place update of memory blocks, and prove the translation to be correct.Comment: 62 pages, uses pi

Ranking ligand affinity for the DNA minor groove by experiment and simulation

The structural and thermodynamic basis for the strength and selectivity of the interactions of minor-groove binders (MGBs) with DNA is not fully understood. In 2003 we reported the first example of a thiazole containing MGB that bound in a phase shifted pattern that spanned 6 base-pairs rather than the usual 4 (for tricyclic distamycin-like compounds). Since then, using DNA footprinting, nuclear magnetic resonance spectroscopy, isothermal titration calorimetry and molecular dynamics, we have established that the flanking bases around the central 4 being read by the ligand have subtle effects on recognition. We have investigated the effect of these flanking sequences on binding and the reasons for the differences and established a computational method to rank ligand affinity against varying DNA sequences

Control over phase separation and nucleation using a laser-tweezing potential

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a poorly understood practice. Although there are now numerous examples of control using laser-induced nucleation, the absence of physical understanding is preventing progress. Here we show that the proximity of a liquid–liquid critical point or the corresponding binodal line can be used by a laser-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in a liquid mixture using a low-power laser diode confirm the effect. Phase separation and nucleation using a laser-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter

Biogem: an effective tool-based approach for scaling up open source software development in bioinformatics

Summary: Biogem provides a software development environment for the Ruby programming language, which encourages community-based software development for bioinformatics while lowering the barrier to entry and encouraging best practices