Christopher S. Martin

Straw detector for high rate, high resolution neutron imaging

Instrument requirements in soon to be commissioned and planned neutron scattering facilities call for large area detectors that can sustain rates up to 108 cps and millimeter-level spatial resolution. Although 3He pressurized area detectors can provide good spatial resolution, sensitivity and gamma ray discrimination, this technology cannot achieve the required rates without further development. Moreover, achieving large detection areas with pressurized 3He technology is expensive because of the complexity of the pressure containing vessels required. We propose a detector technology, based on thin-walled straws, lined with a 1 mum thick sputter coating of enriched boron carbide (10B4 C). Neutrons converted in 10B generate charged particles that subsequently ionize the gas contained within each straw. Because the 10B4C coating is very thin, efficient escape of the reaction products can be achieved. A panel detector consisting of several thousand close-packed individual straws, which are read out independently, can easily support high event rates. We present performance testing of two 50-straw prototypes (non-enriched B4 C), including detection efficiency, spatial resolution, and two-dimensional imaging. Each straw has a diameter of 4 mm and a length of 1 m. Additional tests of a single 50 cm long straw, 2 mm in diameter are also presented

Novel neutron detector for high rate imaging applications

Next generation neutron science facilities, such as the Spallation Neutron Source, require improved thermal neutron detectors, with high counting rate capability, high spatial resolution, low cost per unit area, and adaptability to unique geometries. We propose a neutron detector technology, based on arrays of boron-lined plastic straws, that satisfies the above requirements. The elemental component of this detector is a B/sub 4/C-lined mylar straw, 4 mm in diameter and 1 m long, and operated as a proportional counter with an Ar/ethane gas mixture. Thermal neutrons captured in /sup 10/B are converted into secondary particles, which ionize the gas contained within the straw. Using a deep stack of such straws, with 20-25 layers, neutron detection efficiencies up to 80% can be achieved in the 1-10 /spl Aring/ neutron wavelength range. In such a highly segmented array, exceedingly high count rates can be achieved, on the order of 200/spl times/10/sup 6/ cps/m/sup 2/. Gamma ray discrimination was on the order of 10/sup 6/ for an energy threshold of 50 keV; position resolution of a 1-m long detector was measured to be 5.5 mm FWHM, and a highly linear readout was achieved using resistive charge division.

Performance of 1 Meter Straw Detector for High Rate Neutron Imaging

Instrument requirements at the newly commissioned Spallation Neutron Source call for large area detectors that can sustain rates up to 107 neutrons/s and millimeter-level spatial resolution. Although 3He pressurized area detectors can provide good spatial resolution, sensitivity and gamma ray discrimination, this technology cannot support the required rates without further development. We propose a detector technology based on thin-walled straws, lined with enriched boron carbide (10B4C. A panel detector consisting of several thousand close-packed individual straws, which are read out independently, can easily support high event rates. We present test results for a sub-scale 100times4 cm2 prototype detector, consisting of three 50-straw array modules. Each straw in the array has a diameter of 4 mm and a length of 1 m. Tests conducted in a thermal neutron beam focused on the count rate capability of the detector and the effect of high neutron rates and a high gamma ray flux on spatial resolution. Results showed that the detector can maintain a 7times4 mm2 resolution up to an integral rate of 40,000 cps, and a gamma ray flux of 1000 mrem/hr.

One meter square high rate neutron imaging panel based on boron straws

In order to fully realize the enhanced potential of the powerful neutron scattering technique provided by high intensity facilities like SNS, large area high rate detectors must be developed that do not depend upon the dwindling stock of 3He. We have developed a neutron detector that offers a large sensitive area (1 m2), 3D spatial resolution, high sensitivity and high count rate capability, and it is economical and practical to produce. The detector technology is based on 10B thin film conversion of neutrons in long straw-like gas detectors, as a novel and superior replacement of 3He detectors. Twenty-two detector modules, each consisting of 50 straws (1 m long, 4 mm in diameter, natural boron carbide coated), have been constructed and mounted into a robust housing to form a square meter panel detector. Readout electronics including preamplifiers, signal conditioning modules and digitization modules have been designed and tested to meet the required count rate in a 1 m2 detector. We have successfully tested the panel detector, by imaging objects of different materials and different shapes. Additionally, one enriched detector module has been fabricated. The enriched module was integrated into the panel and produced a factor of four enhanced sensitivity in agreement with modeling predictions based on five fold higher 10B content. The neutron imaging detector has achieved the required performance for SNS applications and substantially increases performance compared with 3He while utilizing a cheap and inexhaustible conversion medium.

Long range neutron-gamma point source detection and imaging using unique rotating detector

We propose a new radiation detection technology that offers high sensitivity to both gamma rays and neutrons, and can be applied, cost effectively, to survey monitoring. The detector consists of a close-packed array of many small thin- walled copper straws, each 1 m in length, and lined with a very thin (1 mum) coating of enriched boron carbide (10B4C). Gammas are converted in Cu, while thermal neutrons are converted in 10B. The detector design draws upon low-cost technology developed by the high energy physics community for large particle detectors such as ATLAS, currently being commissioned at CERN. The feasibility of the detection technology has been demonstrated previously [1], [2]. The current work presents a unique application of the straw detectors, whereby a rotating panel is used to significantly improve performance. The proposed concept is based on the characteristic signature of a point source, that differentiates it from background noise. We present and evaluate an algorithm for detecting the presence of a source and estimating its net count, direction, and the background rate. Simulation results show that the proposed technique can detect in about 20 minutes time and in an area 100 m in diameter, a 1 mCi gamma ray source, with 90% sensitivity and false alarm probability of 0.1/hour. At 30 min time, 270 g of neutron-emitting 240Pu can be detected within a circular area 100 m in diameter, assuming 20% of neutrons thermalize near the source.

Boron coated straw detectors as a replacement for 3 He

The disappearing inventory and minute natural abundance of 3He gas necessitate the adoption of new technologies for the detection of neutrons. The exclusive source of 3He on Earth is derived from the tritium stockpile, which decays to 3He at a rate of 5.5% per year. Despite the low 3He supply, and uncertain production rate in the future, this medium remains by far the most attractive for many applications. The DHS and DOD plan to equip major ports of entry with large area monitors, in an effort to intercept the smuggling of nuclear materials. The desired world deployment of such monitors alone could consume the entire 3He supply, limiting the prospects of nuclear science and other applications that rely very heavily on 3He-based detectors as well. Clearly, alternate neutron detection technologies must be developed. We propose a technology based on close-packed arrays of long aluminum or copper tubes (straws), 4 mm in diameter, coated on the inside with a thin layer of 10B-enriched boron carbide (10B4C). A close-packed array of straw detectors offers a stopping power for neutrons equivalent to that of 2.68 atm of 3He gas. In addition to the high abundance of boron on Earth and low cost of 10B enrichment, the boron-coated straw (BCS) detector offers distinct advantages over conventional 3He-based detectors, including faster signals, short recovery time (ion drift), low weight, safety for portable use (no pressurization), and low production cost. The above are all critical for large detector deployments, as in portal monitoring, and for active interrogation applications, where fast signals can significantly improve performance. Furthermore, in imaging applications, the BCS high level of segmentation supports high count rates and parallax-free position encoding, both difficult to achieve in conventional 3He pressure vessels. We review the use of the BCS detector in a variety of applications, pointing out its distinct advantages over conventional 3He tubes.

The Evolution of Neutron Straw Detector Applications in Homeland Security

The dwindling supply of 3He has necessitated the adoption of new technologies for detection of neutrons especially in Homeland Security (HS), where detectors require large volumes of the precious gas. The boron-coated straw (BCS) technology consists of very thin-walled metal tubes equipped with a highly robust coating of 10B4C on the inner wall. The coating, applied with Physical Vapor Deposition (PVD), is atomically bound to the metal substrate, and it is able to resist delamination over a temperature range from -200° C to 1000° C. Accelerated lifetime testing of sealed straw detectors over extreme temperature ranges (-70° C to +125° C) have recently demonstrated a 99% expected lifetime of 30 years. At the same time, the coating purity is high, with a 10B content approaching 77%. Straws stacked 5 deep in a dense array have demonstrated an efficiency of > 70% when evaluated in a cold neutron beam. A number of field worthy HS detectors have been developed over the past 2 years based on the BCS. Examples are presented, including neutron modules for Radiation Portal Monitors (RPM), a vehicle mounted modular detector with an intrinsic efficiency of 30% for unmoderated 252Cf neutrons, and handheld detectors. All of these systems have been government tested. The 10B4C coated straw offers a highly versatile, robust and low cost technology meeting multiple needs in homeland and international security.

Small Animal PET Camera Design Based on 2-mm Straw Detectors

Depth of interaction (DOI) errors in commonly employed crystal detectors cause severe degradation of off-axis resolution in current positron emission tomography (PET) scanners used for small animal or single organ imaging. We propose a PET camera design that uses low cost, easily fabricated, lead-walled straw (LWS) detectors. Photons converted in Pb generate photoelectrons that escape into the straw interior producing an avalanche whose position can be readily measured with high precision. Straws can be of considerable length (up to 50 cm), but small in diameter (2 mm), and can be packed to form scanners with high sensitivity and large fields of view (FOV). The rigorous three-dimensional position encoding eliminates DOI errors encountered in crystal detectors, and provides submillimeter resolution and sensitivity response that is uniform over the full extent of the FOV. We present preliminary results of the operation of a prototype 200-straw module. We then propose a PET scanner design that consists of two rotating trapezoidal detector panels, each panel containing fourteen 200-straw modules (a total of 5600 straws). The FOV has a diameter of 5 cm and extends axially to 15 cm. This unique geometry affords a long field of view, with near constant axial sensitivity of 4.7%, and uniform resolution of 0.9 times 0.9 times 1.0 mm3 maintained over the entire extent of the FOV, as determined in Monte Carlo simulations.

Cylindrical high pressure xenon spectrometer using scintillation light pulse correction

Cylindrical high-pressure xenon detectors utilizing Frisch grid electrodes have been employed for many years. These detectors are limited to no better than 2% energy resolution at 662 keV, a factor of 3-4 above the intrinsic spectroscopic limit of xenon. This likely results from imperfections in the structure of the Frisch grid combined with the high capacitive load which increases amplifier noise substantially. Also, high sensitivity to microphonic noise of the sensitive grid severely limits field use. We propose a highly stable cylindrical structure consisting of concentric anode and cathode electrodes and use of scintillation light collected through a transparent end window. Measurement of the time interval between the prompt scintillation light from the interaction vertex and stimulated light produced by primary electrons arriving at the anode electrode provides determination of the radial deposited charge distribution, including effects of electron range and xenon fluorescence. In a 5 cm diameter detector employing a MgF/sub 2/ end window we have shown that anode signal levels can be corrected using the time digitized light output signal to achieve resolution superior to the Frisch grid systems and approaching the intrinsic xenon Fano limit. We anticipate employing the technique in arrays of kilogram scale detectors for high sensitivity high resolution field compatible gamma ray spectroscopy.

Ultra-high resolution PET detector using lead walled straws

In the last few years a high emphasis has been placed on the design and use of PET cameras for small animal studies, for example, to aid in the development of human gene therapies by imaging transgenic animals such as mice. Although, such techniques have an extraordinary potential for both clinical and basic biomedical science applications, its full realization is severely hampered by the expense, complexity and physical limitation of crystal detectors widely used in todays cameras. In particular, crystals are costly and very difficult to segment to the desired 1 mm level. Furthermore, depth of interaction error in the 1 cm or greater crystal depth required causes severe degradation of off-axis resolution. This project seeks to develop enhanced high resolution PET through the highly novel approach of the lead walled straw (LWS). In a Phase I NIH project, feasibility of application of this high energy physics spinoff technology has been proven, and in fact it has been demonstrated that considerably enhanced imaging characteristics can be achieved. A 2 mm LWS modular unit has been developed which has produced 1.0 mm FWHM axial spatial resolution. When utilized in ring arrays, such a module will produce a reconstructed volumetric spatial resolution of less than 2 /spl mu/l, which is a factor of 8 improvement compared to the best commercial camera. Furthermore axial sensitive field of view can be readily extended to as large as 20 to 40 cm, and very high sensitivity can be achieved at modest cost.