The recently commissioned Spallation Neutron Source (SNS) facility (Oak Ridge, TN) will push available thermal neutron flux at least an order of magnitude above that achievable at any other neutron science facility, offer a broad bandwidth of neutrons, and high wavelength resolution. Current neutron detector technologies cannot satisfy the requirements of SNS instruments. These requirements, include high sensitivity and spatial resolution, a very high count rate capability, a low gamma-ray efficiency, and no parallax errors.
Current limitations. Scintillator detectors are efficient gamma ray converters, and thus fail the low gamma ray background requirement. Pressurized 3He tubes, widely used in neutron detection, can provide the needed spatial resolution, sensitivity and gamma ray discrimination, however, this technology cannot achieve high rate operation, without fundamental developments. Furthermore, large detection areas are costly, because of the complexity of the pressure vessels required, and parallax errors limit the time-of-flight accuracy of the instrument.
New detector design. We propose a detector technology based on thin-walled straws, lined with a 1 μm-thick sputter coating of enriched boron carbide (10B4C). Neutrons converted with the 10B(n,α)7Li reaction generate heavy 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 square-meter panel detector consisting of several thousand, close-packed, independently read-out straws, as shown above, offers a large detection area, achieves a high detection efficiency, supports high event rates, discriminates effectively against gamma rays, and generates fully 3D position data (no parallax errors).
Straw detectors have become popular over the past decade in high energy physics applications where they are employed in dense arrays to track the complex trajectories of elementary particles. One of the more significant examples is the Transition Radiation Tracker (TRT) straw system developed for the ATLAS experiment station at CERN. Because of the density of the instruments in this experiment and the high radiation levels imposed by hadron colliding beams, the detectors in this station must operate for a full decade under extremely harsh conditions with minimal if any service. As a result of such imposing requirements of reliability a tremendous body of research has gone into life time issues including maintenance of wire tension, mechanical stability, accumulated count degradation, and effects of materials of construction.
Thermal neutron detection. Thermal neutrons captured in 10B are converted into secondary particles, through the 10B(n,α) reaction:
10B + n → 7Li + α
The energy released by the reaction is 2.31 MeV in 94% of all reactions (2.79 MeV in the remaining 6%), and equals the energy imparted to the two reaction products (the energy of the captured neutron is negligible by comparison). The reaction products, namely an alpha particle (α) and a lithium nucleus (7Li) are emitted isotropically from the point of neutron capture in exactly opposite directions and, in the case of the dominant excited state, with kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictated by the conservation of energy and momentum). Since the boron carbide layer is only a few micrometers thick, one of the two charged particles will escape and ionize the gas contained within the straw. Using a reasonably deep stack of straws, neutron detection efficiencies up to 80% can be achieved on the 1-10 Å neutron wavelength range, as determined in Monte Carlo simulations (see figure below).
Boron-lined proportional detectors have been employed for many years, but achieve at most a few percent efficiency, due to the fact that, if the foil thickness exceeds the range of reaction products, no escape occurs. Thus only conversions in the very thin layer near the surface are detected. This layer captures only a small percentage of the incident neutrons. The 10B4C-coated straw removes the low efficiency barrier, by providing many layers of very thin converters, each providing efficient reaction product escape.
Prototype development & testing. A prototype detector based on the proposed technology was developed and evaluated in a thermal neutron beam at the Texas A&M University Nuclear Science Center (College Station, TX). The detector, described in some detail below, was used to assess the performance of the proposed 1 m2 panel detector.
The prototype detector consisted of three identical modules. Ea ch module contained 50 close-packed straws in a 5×10 array. Straws were made from aluminum foil ribbon, 9.5 mm wide, that was sputter coated with natural B4C to a desired thickness of 1 μm (actual thickness was about 0.8 μm). All straws were threaded with resistive wires (43 Ω/cm), 20 μm in diameter. The sensitive face area in the orientation shown on the right was 400 cm2, and the depth in the direction of irradiation was 5.2 cm. Such close packing can be carried to as many modules as desired to produce a continuous detection volume within a secondary housing.
Gamma discrimination. A very important requirement for detectors operating at the SNS is very high discrimination against gamma rays. Gamma rays readily interact with the straw walls producing Compton electrons that ionize the straw gas and make pulses. However the charge delivered into the gas by the very densly ionizing alpha and Li particles is far greater than that delivered by the minimum ionizing electrons. A typical pulse hight spectrum is shown below for the straw system irradiated simultaneously by high energy gamma rays and by thermal neutrons. By setting a threshold of 30 keV for neutron detection the discrimination level for gamma rays is ~1x107. This very high discrimination is achieved because for an electron to deposit 30 kev in the detector gas the path length must be 16 mm, which rarely occurs because of the very narrow straw geometry.
Position Resolution. The spatial resolution of the prototype detector in the direction parallel to the straw axis was investigated through the use of a slit collimator. The figure below shows the spectrum of interaction positions in each of the three meter-long modules. The modules were placed behind a 4.5 mm thick borated aluminum collimator (10B areal density of 45 mg/cm2, 99.9% attenuation) with nine 1 mm wide slits, 10 cm apart from one another. The mean full-width-at-half-maximum (FWHM) of all peaks in module no. 3 (front) was 6.4 mm. The minimum value was measured in the center of the module and was 5.5 mm. The average resolution recorded in module nos. 2 and 1 were 6.4 mm and 6.5 mm, respectively.
Funding
This technology has been funded by the following grants-
NIH under the project name "High Resolution Neutron Detector for Protein Crystallography"-RR018053
DOE under the project name "High Rate Large Area Enriched Boron Neutron Detector for SNS" - DE-FG02-08ER84997
DOE under the project name "Novel Large Area High Resolution Neutron Detector for the Spallation Neutron Source" - DE-FG02-05ER84251
DOE under the project name "Novel Neutron Detector for High Rate Imaging Application" - 65611S01-1
References
More details on neutron imaging and the prototype development and testing can be found in the following publications:
- J.L. Lacy, A. Athanasiades, N.N. Shehad, C.S. Martin, and L. Sun. “Performance of 1 Meter Straw Detector for High Rate Neutron Imaging”, IEEE 2006 Nuclear Science Symposium Conference Record, vol. 1 (2006), pp. 20-26. **(download)
- Athanasiades, N.N. Shehad, C.S. Martin, L. Sun and J.L. Lacy. “Straw Detector for High Rate, High Resolution Neutron Imaging”, IEEE 2005 Nuclear Science Symposium Conference Record, vol. 2 (2005), pp. 623-627. **(download)
- J.L. Lacy, A. Athanasiades, N.N. Shehad, R.A. Austin, and C.S. Martin. “Novel Neutron Detector for High Rate Imaging Applications”, IEEE 2002 Nuclear Science Symposium Conference Record, vol 1 (2002), pp. 392-396. **(download)
- P. Convert and J. B. Forsyth, “The principles of thermal neutron detection,” in Position-sensitive detection of thermal neutrons, P. Convert and J. B. Forsyth, Eds. London: Academic Press, 1983, pp. 1–22. (osti.gov)
- G. F. Knoll, Radiation detection and measurement, 3rd ed. John Wiley & Sons, Inc., 2000. (request book)
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