Thomas J. Yule

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.

An on-Line Monitor for Alpha-Emitting Aerosols

An extremely sensitive on-line monitor for alpha-emitting aerosols has been developed. The monitor incorporates discrimination against background radon and thoron daughter product activity which is based on two factors. Firstly, the background activity is associated with small-sized dust particles, while the aerosol to be monitored has most of its activity associated with large-sized particles. Secondly, the alpha-particle energies from the two sources are different. The monitor utilizes virtual impactors to concentrate the largesized particles into a reduced flow. This size-selected sample is then collected on a filter which is viewed by a detector. The monitor is capable of detecting less than one maximum permissible airborne concentration, MPCa, of these hazardous aerosols in less than one-half hour.

Response functions for proton-recoil proportional counter spectrometer

Abstract The accurate determination of neutron spectra with proton-recoil proportional counters requires that corrections be made for non-ideal electric-field effects and for wall-and-end effects. The use of field tubes to define a sensitive volume leads to a distortion of the internal field lines near the ends of the sensitive region and produces a counter whose multiplication is a function of position of the event along the anode. Wall-and-end effects are present when some proton-recoil tracks deposit only part of their energy in the sensitive region of the counter. Correction schemes have been obtained for electric-field distortion based on the solution of the electric field everywhere in the counter together with measured multiplication versus voltage data and for wall-and-end distortion based on analytically determined track length probability functions and measured stopping-power and range-energy data. The investigation of electric-field effects indicated that by using an unconventional counter design in which the diameter of the cathode over the field tube is reduced, it is possible to reduce the influence of electric-field distortion and produce a counter better suited for spectroscopy and absolute counting applications. Calculated pulse-height distributions have been compared with experimentally determined distributions. The extent of corrections for these effects has been assessed for a representative fast-reactor spectrum.

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.

The Argonne ACWL, a potential accelerator-based neutron source for BNCT

The CWDD (Continuous Wave Deuterium Demonstrator) accelerator was designed to accelerate 80 mA cw of D{sup {minus}} to 7.5 MeV. Most of the hardware for the first 2 MeV was installed at Argonne and major subsystems had been commissioned when program funding from the Ballistic Missile Defense Organization ended in October 1993. Renamed the Argonne Continuous Wave Linac (ACWL), we are proposing to complete it to accelerate either deuterons to 2 MeV or protons to 33.5 MeV. Equipped with a beryllium or other light-element target, it would make a potent source of neutrons (on the order of 10{sup 13} n/s) for BNCT and/or neutron radiography. Project status and proposals for turning ACWL into a neutron source are reviewed, including the results of a computational study that was carried out to design a target/moderator to produce an epithermal neutron beam for BNCT.

Development of an On-Line Tritium Monitor with Gamma Ray Rejection and Energy Discrimination

With the prospect of large fusion facilities coming on-line in the not-too-distant future, it is becoming increasingly important that an on-line tritium-monitoring system be developed which is capable of detecting small amounts of released tritium. Since tritium oxide is some 400 times as hazardous as elemental tritium, it is necessary to distinguish between the two in order to properly evaluate the hazard. Presently available on-line instrumentation has marginal sensitivity, is unable to distinguish between the two forms of tritium, and has poor discrimination against background gamma radiation and air activation products. The objective of our program is to develop a monitoring system with the capability of distinguishing between the two forms of tritium, detecting tritium with a sensitivity of a fraction of an MPC/sub a/ (1 MPC/sub a/ = 5. x 10/sup -6/ Ci/M/sup 3/) for the oxide, and discriminating against gamma activity and airborne activity other than tritium.

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.

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>>

Characterization of aerosols containing fissionable elements using solid-state track recorders

Abstract An aerosol of U 3 O 8 highly enriched in 235 U was generated with a nebulizer from a suspension of U 3 O 8 powder in distilled water. The aerosol was collected on a membrane filter. Polycarbonate plastic, placed in good contact with the filter, was used to record fission tracks when the package was exposed to known fluences of slow neutrons. Fission-track stars associated with individual particles of U 3 O 8 were observed in the plastic. The fission-track distributions were converted to a particle-size distribution for the aerosol. For a log normal distribution the geometric mean and standard deviation can be determined with better than 5% accuracy. This method can be applied to plutonium and other transuranic aerosols.