Vinita J. Ghosh

Calibration and testing of a large-area fast-neutron directional detector

We have developed a new directional fast-neutron detector based on double proton recoil in two separated planes of plastic scintillators with continuous position-sensitive readout in one of two dimensions. This method allows the energy spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. The planes are made up of 100-cm long, 10-cm high paddles with photomultipliers at both ends, so that the location of an event along the paddle can be estimated from the time delay between the optical pulses detected at the two ends. The direction of the scattered neutron can be estimated from the locations of two time-correlated events in the two planes, and the energy lost in the first scattering event can be estimated from the pulse amplitude in the first plane. The direction of the incident neutron can then be determined to lie on a cone whose angle is determined by the kinematic equations. The superposition of many such cones generates an image that indicates the presence of a localized source. Setting upper and lower limits on time of flight and energy allows discrimination between gamma rays, muons and neutrons. Monte Carlo simulations were performed to determine factors affecting the expected angular resolution and efficiency. These models show that this design has a lower energy limit for useful directional events at about 250 keV, because lower energy neutrons are likely to scatter more than once in the first plane.

The momentum distribution of annihilating positron-electron pairs in aluminum

Experimental and theoretical results for the momentum distribution of the positron-electron pairs in defect-free bulk Al and Al containing vacancies are presented. The experimental data was obtained using a two-Ge-detector coincidence measurement of the Doppler broadening of the annihilation radiation. This setup enables an accurate measurement of the high-momentum part of the spectrum, thereby making it possible to compare experimental and theoretical results over a wider momentum range. Various approximations of the localized positron wave function (in the presence of an ideal monovacancy) are discussed.

The effect of the detector resolution on the Doppler broadening measurements of both valence and core electron–positron annihilation

Abstract The measured value of the Doppler profile of the electron–positron annihilation radiation depends on the resolution function of the detectors used in the experiment. This is illustrated by the results of calculations of the profile convoluted with resolution functions of different FWHM. These results are compared with Doppler broadening measurements for Al. The calculated results agree qualitatively with the experimental data. The positions of the peaks in the curves which give the ratio of a Doppler profile to a reference profile are found to shift towards higher momenta when the width of the resolution for the reference material was increased. Since the peaks in the ratio curves are used for identifying the chemical species, it is important that the peak positions be unambiguously characterized. Hence, the detector resolution should be carefully measured and quoted when reporting and comparing results of Doppler profiles.

Directional detection of fission-spectrum neutrons

Conventional neutron detectors consisting of 3 He tubes surrounded by a plastic moderator can be quite efficient in detecting fission spectrum neutrons, but do not indicate the direction of the incident radiation. We have developed a new directional detector based on double proton recoil in two separated planes of plastic scintillators. This method allows the spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. It also allows the determination of the angle of scattering in the first plane. If the planes are position-sensitive detectors, then the direction of the scattered neutron is known, and the direction of the incident neutron can be determined to lie on a cone of a fixed angle. The superposition of many such cones generates an image that indicates the presence of a localized source. Typical background neutron fluences from the interaction of cosmic rays with the atmosphere are low and fairly uniformly distributed in angle. Directional detection helps to locate a manmade source in the presence of natural background. Monte Carlo simulations are compared with experimental results.

Stand-off detection of special nuclear materials using neutron imaging methods

Neutrons can travel considerable distances through the air, and can be used for stand-off detection of special nuclear materials. Plutonium emits neutrons by spontaneous fission, while uranium can be induced to fission by active interrogation with either energetic photons or neutrons. Traditional neutron detectors consisting of 3He tubes embedded in polyethylene moderator do not record the directions of incident neutrons. Their efficiency is usually less than 10% for fission spectrum neutrons. We have developed two neutron imaging methods, one for fast neutrons and one for thermal neutrons. The fast neutron directional detector is based on double proton recoil in two layers of plastic scintillators consisting of arrays of paddles with photomultipliers at both ends. This arrangement provides relatively smooth spatial uniformity and allows neutrons to be distinguished from background gammas and muons using time of flight. The thermal neutron imager is a coded aperture camera based on a 3He wire chamber. Neutrons that are thermalized by materials close to the source have a mean free path in air of about 20 meters, and can be imaged at distances up to 60 meters. In this case, no moderation takes place at the detector, and the quantum efficiency of the 3He wire chamber is about 60%. Background neutrons generated by cosmic rays arrive from all directions at a low rate, whereas a manmade source is likely to produce a bright spot in an image. Various methods of image enhancement can be applied, such as auto filtering, but the practical limitation stems from the requirement that the real source must stand out against artifacts in the image.

Defect profiling in elemental and multilayer systems: correlations of fitted defect concentrations with positron implantation profiles

Abstract The results of several positron annihilation (Doppler broadening) experiments have been analyzed using the BNL Monte Carlo implantation profiles and the program VEPFIT. The program VEPFIT has been modified so that scaled, parameterized multilayer profiles can also be used as the initial condition for the diffusion equation solution. We have looked at both elemental (e.g. amorphous silicon) and multilayer (e.g. Pd Si ) systems. Strong correlations between the input implantation profile parameters and the fitted values obtained for the diffusion lengths and overlayer thicknesses for the multilayer systems have been found. The effect of uncertainties in the mean depth on the value of the diffusion length and hence the defect concentrations will be discussed. The impact of reimplanting backscattered positrons on both the implantation profiles and the fitted diffusion lengths will also be presented.

Monte Carlo studies of positron implantation in elemental metallic and multilayer systems

We have used a Monte Carlo computer code developed at Brookhaven to study the implantation profiles of 1‐10 keV positrons incident on a wide range of semi‐infinite metals and multilayer systems. Our Monte Carlo program accounts for elastic scattering as well as inelastic scattering from core and valence electrons, and includes the excitation of plasmons. The implantation profiles of positrons in many metals as well as Pd/Al, and Al/Co/Si multilayers are presented. Scaling relations and closed‐form expressions representing the implantation profiles are also discussed.

POS-SPRITE — an extensible calculation of positron and electron implantation in metals

Abstract Accurately modeling the behavior of a beam of positrons incident upon a solid is of crucial importance for depth-profiling and data analysis in slow positron experiments. We describe a suite of Fortran programs to perform a Monte Carlo calculation of positron implantation in amorphous metals and small bandgap semiconductors. This provides statistical information about penetration (stopping profiles, thermalization time, etc.), energy loss and reemission (backscattering, thin film transmission, etc.). The calculation is implemented using a user-friendly Monte Carlo transport “engine”, which is capable of treating particle transport in infinite, semi-infinite, and multilayer systems. A contingent of elastic (up to 10 keV) and inelastic (down to 30 eV) scattering mechanisms is included, but different mechanisms can be added via external data files or code modification. As a consequence of the scattering theory involved, this calculation is amenable to electron implantation and scattering.

Data mining for ontology development.

A multi-laboratory ontology construction effort during the summer and fall of 2009 prototyped an ontology for counterfeit semiconductor manufacturing. This effort included an ontology development team and an ontology validation methods team. Here the third team of the Ontology Project, the Data Analysis (DA) team reports on their approaches, the tools they used, and results for mining literature for terminology pertinent to counterfeit semiconductor manufacturing. A discussion of the value of ontology-based analysis is presented, with insights drawn from other ontology-based methods regularly used in the analysis of genomic experiments. Finally, suggestions for future work are offered.

The effects of low-energy scattering on positron implantation

Existing Monte‐Carlo models are capable of simulating the behavior of positrons incident at keV energies, then following the energy loss process to arbitrary final kinetic energies of from 20 eV to 100 eV. In the present work we describe a Monte‐Carlo simulation of the final stages of positron thermalization in Al, from 25 eV to thermal energies, via the mechanisms of conduction‐electron and longitudinal acoustic phonon scattering. We show that the later stages of thermalization can have important effects on the stopping profiles and mean depth. We describe a novel way to obtain information about positron energy loss by considering the time‐evolution of a point‐concentration (delta‐function distribution) of positrons. We examine, for the first time in the context of a positron Monte‐Carlo calculation, the effects of a positive positron work function. Finally, we discuss some issues relating to the agreement of Monte‐Carlo calculations with experimental data.