IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013 1111 Boron-Coated Straw Detectors for Backpack Monitors Jeffrey L. Lacy, Member, IEEE, Athanasios Athanasiades, Member, IEEE, Christopher S. Martin, Member, IEEE, Liang Sun, Member, IEEE, and Gerson J. Vazquez-Flores, Member, IEEE Abstract—The limited inventory and minute natural abundance of He gas on Earth necessitate the adoption of new technologies for the detection of neutrons, especially in homeland security applications, where large volume deployments are required. We investigate the neutron detection efficiency of a backpack radiation detector based on an unmoderated array of boron-coated straws (BCS). A neutron module was configured that would be light and small enough to fit inside a regular-sized backpack. The module consisted of 36 tubes, arranged in two rows, for a total number of BCS detectors equal to 36 7 252. The overall dimensions of the neutron module were 3.18 cm 26.7 cm 38.1 cm, and its weight was 1.62 kg (3.57 lbs). The module was fitted immediately inside the back side of the backpack. The detectors are thus very close to the torso of the operator, which, together with a plastic scintillator for gamma detection, act as moderators for high-energy neutrons. In order to maintain a low weight, no other moderating material was installed inside the backpack. The weight of the pack containing both neutron and gamma detector panels, along with foam packing, electronics, and battery, was 5.36 kg (11.8 lbs). The compact dimensions of the design afforded use of a computer style backpack of minimal dimension providing significant additional operational advantages. The testing results indicate that, through use of high-density straw cluster detectors, the proposed design meets both neutron and gamma response government requirements for backpack monitors. Index Terms—Backpack radiation detector, boron-coated straws, neutron/gamma source detection, radiation detectors. I. INTRODUCTION H ELIUM-3 He gas is the material of choice for neu- tron detection because of its high-capture cross section, effective discrimination between neutrons and gamma rays, and because detectors can be readily constructed with the large di- mensions required. Over the past five years, the demand for He-based detectors has increased greatly, while the supply con- tinues to decrease. The primary reason for the increase in de- Manuscript received June 15, 2012; revised November 05, 2012 and De- cember 31, 2012; accepted January 25, 2013. Date of publication March 19, 2013; date of current version April 10, 2013. This work has been supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office (DNDO), under competitively awarded contract HSHQDC-11-C-00087. This support does not constitute an express or implied endorsement on the part of the Government. This work was additionally supported by the U.S. Defense Threat Reduction Agency (DTRA) , under contract DTRA-01-02-D-0067. The authors are with Proportional Technologies, Inc., Houston, TX 77054 USA (e-mail: jlacy@proportionaltech.com; aathanasiades@proportional- tech.com; cmartin@proportionaltech.com; lsun@proportionaltech.com; gvazquez@proportionaltech.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2013.2245915 mand is the expanded use of He in neutron detectors for na- tional security, and neutron science applications. The limited inventory and minute natural abundance of He gas on Earth necessitate the adoption of new technologies for the detection of neutrons, especially in homeland security applications, where the large volume deployments that are being considered would exhaust the entire world supply. The boron-coated straw (BCS) detector technology offers a replace- ment for He neutron detection solutions [1]–[3]. Previous developments focused on large detectors for portal monitors. This work demonstrates the flexibility of the technology by presenting a neutron detector having substantial advantages for backpack systems containing both neutron and gamma detectors. This flexibility derives from the ability of straws to be easily manufactured in small diameters, after the coating of a thin foil, and be packed in dense arrays. We have developed and tested a BCS-based backpack radia- tion detector (BRD) in order to investigate the potential of this technology in portable detectors for security applications. Be- cause straws can be readily fabricated with a small diameter (4 mm), they can be close-packed to form a compact panel, having high detection efficiency. The panel, when packaged inside a backpack, is in close proximity to thermal neutrons emerging from the operator’s body. No additional moderator is thus needed, allowing for low weight and compact size. The proposed design takes advantage of these straw characteristics to effectively replace high-pressure, high-efficiency He tubes currently used in backpack radiation monitors [4]. The large diameter of conventional boron-lined coun- ters (BLCs) limits their ability to achieve high detection efficiency in compact forms. The sensitivity of a boron-lined detector array is approximately proportional to coated area. If a detector module with similar sensitivity to the proposed BCS panel is formed with 2.54-cm (1-in) tubes, then the module becomes too thick for a backpack. GE Reuter Stokes has re- cently employed such BLCs in portal monitors [5], but critical detector design details have not been published, including coating thickness and purity. Centronic, Ltd., Houston, TX, USA, has developed a mono- lithic design with multiple “tubelets” machined from an alu- minum core [6]. The core has a diameter up to 6.35 cm (2.5 in), accommodating up to 14 tubelets, each 1.27 cm (0.5 in) in diam- eter, and lined with B (neither coating thickness nor purity are reported). This design has been tested in portal monitors [7]. If the Centronic tubelet design were to be employed in a backpack module, then several units would be required to achieve a sim- ilar coated area. These would have to be stacked in two layers 0018-9499/$31.00 © 2013 IEEE