Optimal discrimination and classification of neuronal actionpotential waveforms from multiunit, multichannel recordings usingsoftware-based linear filters

Describes advanced protocols for the discrimination and classification of neuronal spike waveforms within multichannel electrophysiological recordings. The programs are capable of detecting and classifying the spikes from multiple, simultaneously active neurons, even in situations where there is a high degree of spike waveform superposition on the recording channels. The protocols are based on the derivation of an optimal linear filter for each individual neuron. Each filter is tuned to selectively respond to the spike waveform generated by the corresponding neuron, and to attenuate noise and the spike waveforms from all other neurons. The protocol is essentially an extension of earlier work (S. Andreassen et al., 1979; W.M. Roberts and D.K. Hartline, 1975; R.B. Stein et al., 1979). However, the protocols extend the power and utility of the original implementations in two significant respects. First, a general single-pass automatic template estimation algorithm was derived and implemented. Second, the filters were implemented within a software environment providing a greatly enhanced functional organization and user interface. The utility of the analysis approach was demonstrated on samples of multiunit electrophysiological recordings from the cricket abdominal nerve cord     

Quasi-isoplanatic radiation transfer through randomly movingparticles

An iterative solution of an improved forward-scattering approximation for the radiative-transfer equation is derived. The narrow-angle approximation is useful for optical and acoustic beam propagation in the ocean and the atmosphere, where particle sizes are much larger than the wavelength. The improved narrow-angle approximation is useful for a cluster of anisotropic scatterers, and when the cluster is subjected to winds or currents. The solution of the derived equation is sought in terms of uniformly convergent iteration series via the two-scale embedding technique. The lowest iterative of the technique provides an approximate solution that differs from the commonly used successive substitution technique because it takes into account some of the multiple scattering terms. An example of wave propagation through randomly moving particles, useful for remote sensing, is considered     

A method to improve the estimation of conduction velocitydistributions over a short segment of nerve

Accurate, noninvasive determination of the distribution of conduction velocities (DCV) among fibers of a peripheral nerve has the potential to improve both clinical diagnoses of pathology and longitudinal studies of the progress of disease or the efficacy of treatments. Current techniques rely on long distances of propagation to increase the amount of temporal dispersion in the compound signals and reduce the relative effect of errors in the forward model. The method described in this paper attempts to reduce errors in DCV estimation through transfer function normalization and, thereby, eliminate the need for long segments of nerve. Compound action potential (CAP) signals are recorded from several, equally spaced electrodes in an array spanning only a 10-cm length of nerve. Relative nerve-to-electrode transfer functions (NETF's) between the nerve and each of the array electrodes are estimated by comparing discrete Fourier transforms of the array signals. NETF's are normalized along the array so that waveform differences can be attributed to the effects of temporal dispersion between recordings, and more accurate DCV estimates can be calculated from the short nerve segment. The method is tested using simulated and real CAP data. DCV estimates are improved for simulated signals. The normalization procedure results in DCV's that qualitatively match those from the literature when used on actual CAP recordings.     

Linac based photofission inspection system employing novel detection concepts

Rapiscan Systems is developing a LINAC based cargo inspection system for detection of special nuclear material (SNM) in cargo containers. The system, called Photofission Based Alarm Resolution (PBAR) is being developed under a DHD/DNDO Advanced Technology Demonstration (ATD) program. The PBAR system is based on the Rapiscan Eagle P9000 X-ray system, which is a portal system with a commercial 9 MeV LINAC X-ray source. For the purposes of the DNDO ATD program, a conveyor system was installed in the portal to allow scanning and precise positioning of 20 ft ISO cargo containers.The system uses a two step inspection process. In the first step, the basic scan, the container is quickly and completely inspected using two independent radiography arrays: the conventional primary array with high spatial resolution and a lower resolution spectroscopic array employing the novel Z-Spec method. The primary array uses cadmium tungstate (CdWO₄) detectors with conventional current mode readouts using photodiodes. The Z-Spec array uses small plastic scintillators capable of performing very fast (up to 10⁸ cps) gamma-ray spectroscopy. The two radiography arrays are used to locate high-Z objects in the image such as lead, tungsten, uranium, which could be potential shielding materials as well as SNM itself.In the current system, the Z-Spec works by measuring the energy spectrum of transmitted X-rays. For high-Z materials the higher end of the energy spectrum is more attenuated than for low-Z materials and thus has a lower mean energy and a narrower width than low- and medium-Z materials.The second step in the inspection process is the direct scan or alarm clearing scan. In this step, areas of the container image, which were identified as high Z, are re-inspected. This is done by precisely repositioning the container to the location of the high-Z object and performing a stationary irradiation of the area with X-ray beam. Since there are a large number of photons in the 9 MV Bremsstrahlung spectrum above the photofission “threshold” of about 6 MeV, the X-ray beam induces numerous fissions if nuclear material is present. The PBAR system looks for the two most prolific fission signatures to confirm the presence of special nuclear materials (SNM). These are prompt neutrons and delayed gamma rays. The PBAR system uses arrays of two types of fast and highly efficient gamma ray detectors: plastic and fluorocarbon scintillators. The latter serves as a detector of fission prompt neutrons using the novel threshold activation detector (TAD) concept as well as a very efficient delayed gamma ray detector. The major advantage of TAD for detecting the prompt neutrons is its insensitivity to the intense source related backgrounds.The current status of the system and experimental results will be shown and discussed.     

Differential die-away analysis system response modeling and detector design

Differential die-away-analysis (DDAA) is a sensitive technique to detect presence of fissile materials such as and . DDAA uses a high-energy (14 MeV) pulsed neutron generator to interrogate a shipping container. The signature is a fast neutron signal hundreds of microseconds after the cessation of the neutron pulse. This fast neutron signal has decay time identical to the thermal neutron diffusion decay time of the inspected cargo. The theoretical aspects of a cargo inspection system based on the differential die-away technique are explored. A detailed mathematical model of the system is developed, and experimental results validating this model are presented.     

Simulation methods for photoneutron-based active interrogation systems

Modeling methods have been developed to accelerate the simulation of a photoneutron-based active interrogation system of nuclear materials. The proposed technique segments the simulation of a full system into several physical steps, representing functional approximations. Each approximation is carried-out separately, resulting in a major reduction in computational time and a significant improvement in tally statistics. Although more human effort is required to separate each step, the net time required to produce results is drastically reduced. In addition, the results of previous steps can be used as inputs to proceeding steps without the need for re-simulation. We show that for a photoneutron interrogation system, the final results are in good agreement with the full, single-step simulation and also with experimental results.     

Nuclear-based techniques for explosive detection

The status of bulk explosive detection techniques based on nuclear interrogation methods is reviewed. The desirable characteristics of an operational system capable of meeting the requirements for civil aviation security are compared with what can be expected from nuclear-based techniques. The comparison is limited to those techniques that utilize penetrating neutron and photon probes on the target elements of explosives most likely to be encountered in the airport scenario. The physical properties of a relevant group of explosives are surveyed for unique characteristics that could provide detectable signatures to nuclear-based techniques. A survey of the accessible reactions and their relative detection sensitivities are tabulated for a selection of practical interrogating probes. Some results obtained with systems currently under development are reported.     

Optimization of S/B in the detection of nuclear fission signatures via different accelerator pulsing modes

In the search for concealed special nuclear materials (SNM) there are a number of fission specific signatures that can be measured. These include prompt and delayed neutron and gamma ray signatures. Here the focus will be on the delayed gamma signature with the assumption that a pulsed electron linac with a constant peak current will be used to generate bremsstrahlung radiation and induce photofission in ²³⁵U.In this case, the signal to background ratio (S/B) will depend on the choice of linac frequency, pulse mode, and “active” background due to linac activation products. The linac frequency is simply the rate at which it produces short bursts of radiation, typically 2-4 μs in duration. There are two pulse modes, micro-pulsing, and macro-pulsing. In the micro-pulsing mode, the linac runs continuously at its set frequency and data is collected between bursts. In the macro-pulsing mode, the linac is turned on for a given length of time, on the order of seconds, and then turned off for a period of time typically equal to the length of time it was turned on. Counting takes place during the time the linac is off and stops when the linac is turned on for another cycle.The time dependence of the delayed gamma population can be approximated by the use of 5 time groups with half-lives of 0.29, 1.7, 13, 100, and 940 s, respectively. Each group has its own relative population, which together with its half-life determines what time frame the group contributes most to the measured signal. For example, a group with a short half-life will contribute more signal to a short cycle macro pulsed measurement than it would to a macro pulse measurement with a very long cycle.An analytical expression can be derived that calculates the maximum obtainable signal (delayed gamma photons per fission gamma ray) in either a micro- or macro-pulsed measurement. Using this information along with the observed active background present in a given situation (which can constrain the micro-pulsing parameters), the preferred mode of operation can be chosen to maximize S/B and the detection sensitivity. The principles and experimental application of the optimization process will be shown.