Effect of the local energy distribution of x-ray beams generated through inverse Compton scattering in dual-energy imaging applications.

X-ray sources based on the inverse Compton interaction between a laser and a relativistic electron beam are emerging as a promising compact alternative to synchrotron for the production of intense monochromatic and tunable radiation. The emission characteristics enable several innovative imaging techniques, including dual-energy K-edge subtraction (KES) imaging. The performance of these techniques is optimal in the case of perfectly monochromatic x-ray beams, and the implementation of KES was proven to be very effective with synchrotron radiation. Nonetheless, the features of inverse Compton scattering (ICS) sources make them good candidates for a more compact implementation of KES techniques. The energy and intensity distribution of the emitted radiation is related to the emission direction, which means different beam qualities in different spatial positions. In fact, as the polar angle increases, the average energy decreases, while the local energy bandwidth increases and the emission intensity decreases. The scope of this work is to describe the impact of the local energy distribution variations on KES imaging performance. By means of analytical simulations, the reconstructed signal, signal-to-noise ratio, and background contamination were evaluated as a function of the position of each detector pixel. The results show that KES imaging is possible with ICS x-ray beams, even if the image quality slightly degrades at the detector borders for a fixed collimation angle and, in general, as the beam divergence increases. Finally, an approach for the optimization of specific imaging tasks is proposed by considering the characteristics of a given source.

Meningococcal B vaccine and the vision of a meningitis free world.

A century of traditional vaccinology lost the fight against meningococcus serogroup B (MenB). However, thanks to an innovative genome-based approach, the first broadly effective MenB vaccine, Bexsero® (GSK Vaccines), was developed and has been licensed for use in various age groups by the European Commission and other regulatory authorities. Genes encoding for the main meningococcus B antigens were identified and screened in order to achieve a broadly protective vaccine, taking into account the fact that meningococcus B has many different subtypes whose membrane proteins may be different. Since the antigens selected for Bexsero® are also harbored by meningococci belonging to other serogroups there may be the potential for Bexsero® to offer a certain level of protection against non-B serogroups. Therefore preliminary studies were carried out to investigate the potential of the vaccine to also provide a degree of cross protection against non-B serogroups. Here we review the potential for Bexsero® to offer a certain level of protection against the diversity of meningococcus type B subtypes and its potential ability to offer some cross protection from non-B serogroups. Lastly, we describe the future perspectives in pentavalent meningococcal vaccine (ABCWY) development which hopefully will result in a vaccine able to help prevent Invasive Meningococcal Diseases (IMD) from the majority of currently circulating meningococcal strains.

Equation of state of superfluid neutron matter and the calculation of the 1S0 pairing gap.

We present a quantum Monte Carlo study of the zero-temperature equation of state of neutron matter and the computation of the 1S0 pairing gap in the low-density regime with rho < 0.04 fm(-3). The system is described by a nonrelativistic nuclear Hamiltonian including both two- and three-nucleon interactions of the Argonne and Urbana type. This model interaction provides very accurate results in the calculation of the binding energy of light nuclei. A suppression of the gap with respect to the pure BCS theory is found, but sensibly weaker than in other works that attempt to include polarization effects in an approximate way.

Meningococcal disease: a review on available vaccines and vaccines in development.

Meningococcal disease continue to have a major public health impact in many countries. Five major groups of Neisseria meningitidis (A, B, C, Y and W135) are responsible for most meningococcal diseases. Plain polysaccharides vaccines for Neisseria meningitidis groups A, C, Y and W-135 have been in use for approximately 20 years, both to prevent invasive disease in high-risk population and to control disease outbreaks. However, these conventional meningococcal vaccines induce a relatively short-lasting T-cell independent immune response, are not effective in children under two years of age and can induce hyporesponsiveness. New meningococcal group C conjugate vaccines have since been developed, which offer solid advantages over the currently licensed plain polysaccharide vaccines. Tetravalent serogroup A, C, Y and W135 meningococcal vaccines are under development and one has already been licensed. There is still no universal vaccine available against the serogroup B, which is a major cause of invasive disease. This report summarises the different approaches to the development of vaccines against the pathogenic meningococci.

Auxiliary field diffusion Monte Carlo calculation of nuclei with A < or = 40 with tensor interactions.

We calculate the ground-state energy of (4)He, (8)He, (16)O, and (40)Ca using the auxiliary field diffusion Monte Carlo method in the fixed-phase approximation and the Argonne v(6)' interaction which includes a tensor force. Comparison of our light nuclei results to those of Green's function Monte Carlo calculations shows the accuracy of our method for both open and closed-shell nuclei. We also apply it to (16)O and (40)Ca to show that quantum Monte Carlo methods are now applicable to larger nuclei.

The spectra of mixed 3He-4He droplets.

The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of 3He atoms bound to a cluster of 4He atoms by using a previously determined optimum filling of single-fermion orbits with well-defined orbital angular momentum L, spin S, and parity quantum numbers. The study concentrates on the energies and shapes of the three kinds of states for which the fermionic part of the wave function is a single Slater determinant: maximum L or maximum S states within a given orbit, and fully polarized clusters. The picture that emerges is that of systems with strong shell effects, whose binding and excitation energies are essentially determined by averages over configuration at fixed number of particles and spin, i.e., by the monopole properties of an effective Hamiltonian.

Quantum monte carlo algorithm based on two-body density functional theory for fermionic many-body systems: application to 3He.

We construct a quantum Monte Carlo algorithm for interacting fermions using the two-body density as the fundamental quantity. The central idea is mapping the interacting fermionic system onto an auxiliary system of interacting bosons. The correction term is approximated using correlated wave functions for the interacting system, resulting in an effective potential that represents the nodal surface. We calculate the properties of 3He and find good agreement with experiment and with other theoretical work. In particular, our results for the total energy agree well with other calculations where the same approximations were implemented but the standard quantum Monte Carlo algorithm was used.

Ambiguities in the implementation of the impulse approximation for the response of many-fermion systems.

Within the impulse approximation, the response of a many-body system at large momentum transfer can be directly related to ground state properties. Although the physics assumptions underlying impulse approximation are well defined, their implementation involves ambiguities that may cause significant differences in the calculated responses. We show that, while minimal use of the impulse approximation assumptions naturally leads to write the response in terms of the spectral function, the alternative definition in terms of the momentum distribution involves a more extended use of the same assumptions.

Equation of state of solid 3He

We present results of diffusion Monte Carlo calculations for the bcc and hcp phases of solid 3He, using a recent ab initio interatomic potential, including two- and three-body terms. This potential is found to yield an equation of state for condensed 4He in excellent agreement with experiment, in a wide density range. For 3He, we find a systematic discrepancy, worth 0.7 K, between our computed equation of state and a commonly accepted experimental one. We attribute such a discrepancy to an improper choice of reference energy in the determination of the experimental equation of state.

Josephson effects in dilute bose-einstein condensates

We propose an experiment that would demonstrate the dc and ac Josephson effects in two weakly linked Bose-Einstein condensates. We consider a time-dependent barrier, moving adiabatically across the trapping potential. The phase dynamics are governed by a "driven-pendulum" equation, as in current-driven superconducting Josephson junctions. At a critical velocity of the barrier (proportional to the critical tunneling current), there is a sharp transition between the dc and ac regimes. The signature is a sudden jump of a large fraction of the relative condensate population. Analytical results are compared with a numerical integration of the Gross-Pitaevskii equation, in an experimentally realistic situation.