Charles L. Fink

Evaluation of neutron techniques for illicit substance detection

We are studying inspection systems based on the use of fast neutrons for detecting illicit substances such as explosives and drugs in luggage and cargo containers. Fast-neutron techniques can determine the quantities of light elements such as carbon, nitrogen, and oxygen in a volume element. Illicit substances containing these elements are characterized by distinctive elemental densities or density ratios. We discuss modeling and tomographic reconstruction studies for fast-neutron transmission spectroscopy.

A neutron detector to monitor the intensity of transmitted neutrons for small-angle neutron scattering instruments

A semiconductor-based neutron detector was developed at Argonne National Laboratory (ANL) for use as a neutron beam monitor for small-angle neutron scattering instruments. The detector is constructed using a coating of 10 Bo n a gallium–arsenide semiconductor detector and is mounted directly within a cylindrical (2.2 cm dia. and 4.4 cm long) enriched 10 B4C beam stop in the time-of-flight Small Angle Neutron Diffractometer (SAND) instrument at the Intense Pulsed Neutron Source (IPNS) facility at ANL. The neutron beam viewed by the SAND is from a pulsed spallation source moderated by a solid methane moderator that produces useful neutrons in the wavelength range of 0.5–14 ( A. The SAND instrument uses all detected neutrons in the above wavelength range sorted by time-of-flight into 68 constant DT=T ¼ 0:05 channels. This new detector continuously monitors the transmitted neutron beam through the sample during scattering measurements and takes data concurrently with the other detectors in the instrument. The 10 B coating on the GaAs detector allows the detection ofthe cold neutron spectrum with reasonable efficiency. This paper describes the details of the detector fabrication, the beam stop monitor design, and includes a discussion of results from preliminary tests using the detector during several run cycles at the IPNS. r 2003 Elsevier Science B.V. All rights reserved.

Development of semiconductor detectors for fast neutron radiography.

A high-energy neutron detector has been developed using a semiconductor diode fabricated from bulk gallium arsenide wafers with a polyethylene neutron converter layer. Typical thickness of the diode layer is 250 to 300 μm with bias voltages of 30 to 150 volts. Converter thicknesses up to 2030 μm have been tested. GaAs neutron detectors offer many advantages over existing detectors including positional information, directional dependence, gamma discrimination, radiation hardness, and spectral tailoring. Polyethylene-coated detectors have been shown to detect 14 MeV neutrons directly from a D-T neutron generator without interference from gamma rays or scattered neutrons. An array of small diode detectors can be assembled to perform fast neutron radiography with direct digital readout and real-time display of the image produced. In addition, because the detectors are insensitive to gamma rays and low energy neutrons, highly radioactive samples (such as spent nuclear fuel or transuranic waste drums) could be ...

Accelerator requirements for fast-neutron interrogation of luggage and cargo

Several different fast-neutron based techniques are being studied for the detection of contraband substances in luggage and cargo containers. The present work discusses the accelerator requirements for fast-neutron transmission spectroscopy (FNTS), pulsed fast-neutron analysis (PFNA), and 14 MeV neutron interrogation. These requirements are based on the results of Monte-Carlo simulations of neutron or gamma detection rates. Accelerator requirements are driven by count-rate considerations, spatial resolution and acceptable uncertainties in elemental compositions. We have limited our analyses to luggage inspection with FNTS and to cargo inspection with PFNA or 14-MeV neutron interrogation.

Key research issues in the pulsed fast-neutron analysis technique for cargo inspection

Non-invasive inspection systems based on the use of fast neutrons are being studied for the inspection of large cargo containers. A key advantage of fast neutrons is their sensitivity to low-Z elements such as carbon, nitrogen, and oxygen, which are the primary constituents of explosives and narcotics. The high energy allows penetration of relatively large containers. The pulsed fast-neutron analysis (PFNA) technique is currently the baseline system. A workshop on the PFNA technique involving industrial, government, and university participants was held at Argonne National Laboratory in January 1994. The purpose of this workshop was to review the status of research on the key technical issues involved in PFNA, and to develop a list of those areas where additional modeling and/or experimentation were needed. The workshop also focused on development of a near-term experimental assessment program using existing prototypes and on development of a long-term test program at the Tacoma Testbed, where a PFNA prototype will be installed in 1995. A summary of conclusions reached at this workshop is presented. Results from analytic and Monte Carlo modeling of simplified PFNA systems are also presented.

Transport simulation and image reconstruction for fast-neutron detection of explosives and narcotics

Fast-neutron inspection techniques show considerable promise for explosive and narcotics detection. A key advantage of using fast neutron is their sensitivity to low-Z elements (carbon, nitrogen, and oxygen), which are the primary constituents of these materials. We are currently investigating two interrogation methods in detail: fast-neutron transmission spectroscopy (FNTS) and pulsed fast-neutron analysis (PFNA). FNTS is being studied for explosives and narcotics detection in luggage and small containers for which the transmission ration is greater than about 0.01. The Monte Carlo radiation transport code MCNP is being used to simulate neutron transmission through a series of phantoms for a few (3-5) projections angles and modest (2 cm) reolution. Areal densities along projection rays are unfolded from the transmission data. Elemental abundances are obtained for individual voxels by tomographic reconstruction, and the reconstructed elemental images are combined to provide indications of the presence or absence of explosives or narcotics. PFNA techniques are being investigated for detection of narcotics in cargo containers because of the good penetration of the fast neutrons and the low attenuation of the resulting high-energy gamma-ray signatures. Analytic models and Monte Carlo simulations are being used to explore the range of capabilities of PFNA techniques and to provide insight into systems engineering issues. Results of studies from both FNTS and PFNA technqiues are presented.

Beam characterization with video imaging systems at the ANL 50 MeV H/sup -/ beamline

Video imaging systems consisting of scintillators, charge-coupled device (CCD) cameras, and in-beam CCD detectors are being used to characterize beams at the Argonne National Laboratory (ANL) 50 MeV H/sup -/ beamline. The characterization technique consists of placing pinholes, slits, or wires in the beam and viewing the resulting images or shadows on a downstream scintillator or CCD. The images are digitally recorded using a frame grabber and stored in computer memory where they are analyzed to determine Twiss parameters and local beam divergence. Since many of the measurements involve low-intensity beams and require good position resolution, studies have been performed on scintillators to obtain sensitivity and resolution data. Various scintillators, including Rarex, CsI, and CaF/sub 2/, have been evaluated. An in-beam CCD imager has also been tested.<<ETX>>

NPBTS-overview and capabilities

The Neutral Particle Beam Test Stand (NPBTS) provides a versatile facility for scientific and engineering studies on large-diameter, low-divergence neutral and charged particle beams. It consists of a linac that accelerates H/sup -/ atoms to 50 MeV at 10-12 mA and two experimental areas. Typical pulse widths are 30-150 mu s at repetition rates of 0.5-30 Hz. A small RMS-emittance is achieved by using a series of collimators to shave the 1.6- pi -mm-mr emittance measured at the output of the linac. Typical current in the experimental areas is 500-600 mu A. Experimental area A has been used to study the physics of beam diagnostics and foil neutralization and to measure (p,n) reaction cross sections. Experimental area B has a series of quadrupole objectives built by Los Alamos National Laboratory to reduce beam divergence. Typical beam characteristics are RMS diameters of 10-20 cm and a full-angle divergence (RMS) of 12-24 mu r. The facility contains a wide variety of diagnostics including segmented Faraday cups, beam toroids, stripline beam-position monitors, and wire scanners. In addition, several new diagnostic systems for large-diameter beams have been developed by Argonne and Los Alamos National Laboratories.<<ETX>>

Angular Response Functions for Sodium Iodide Detectors

This paper presents analytical, Monte Carlo and experimental investigations on the angular response functions to gamma sources of typical cylindrical scintillation detectors. A general analytical approach capable of computing the total counting efficiency of geometrically-simple detectors to isotropic point sources is introduced. MCNP5 calculations are performed for several cylindrical NaI(Tl) detectors with different sizes in order to verify this analytical approach. Photopeak efficiency is also computed based on analytically determined total counting efficiency and MCNP5 calculated peak-to-total ratio. Sets of angular measurements are executed for cylindrical NaI detectors of different sizes using Cs-137 and Co-60 sources with appropriate radioactivity. The three means of determining the angular response functions for the total efficiency and photopeak efficiency give consistent results with less than 5% discrepancy over the measured range.

Use of a high-current accelerator (CWDD) for neutron radiography

The status of the physics and engineering databases in high-current accelerators, high-power target design, and neutron reaction rates is discussed. It is concluded that while accelerators are attractive, the necessary system tradeoff studies cannot be made reliably without additional research, particularly in target design and neutron production rates. Since the high-current continuous wave deuterium demonstrator (CWDD) being built at Argonne can play a key role in developing this database, a brief description of this accelerator is provided.<<ETX>>