Design and operation of a 2-D thin-film semiconductor neutron detector array for use as a beamport monitor Troy C. Unruh a , Steven L. Bellinger a , David E. Huddleston b , Walter J. McNeil a , Eric Patterson a , Tim J. Sobering b , Douglas S. McGregor a,Ã a SMART Laboratory, Kansas State University, Mechanical & Nuclear Engineering Department, Manhattan, KS 66506, USA b Electronics Design Laboratory, Kansas State University, Manhattan, KS 66506, USA article info Available online 30 January 2009 Keywords: Semiconductor thin-film neutron detector 2-D imaging array Beamport monitor abstract Silicon-based diodes coated with a thin film of neutron reactive materials have been shown to produce excellent low-efficiency neutron detectors. This work employs the same technology, but groups 25 equally sized and spaced diodes on a single 29 mm by 29 mm substrate. A 5 Â 5 array was fabricated and coated with a thin film of 6 LiF for use as a low-efficiency neutron beam monitor. The 5 Â 5 neutron detector array is coupled to an array of amplifiers, allowing the response to be interpreted using a LabVIEW FPGA. The 5 Â 5 array has been characterized in a diffracted neutron beam. This work is a part of on-going research to develop various designs of high- and low-efficiency semiconductor neutron detectors. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Coated semiconductor diodes have been used as neutron detectors for many decades. The typical design consists of a Schottky barrier or pn junction diode that has a neutron reactive coating, such as 10 B or 6 LiF, applied over the top of the device. Low cost, compact size, low power requirement, ruggedness and adaptability for the end user are advantages that coated diodes have over many other types of neutron detectors [1–3]. The following work describes the development of a low- efficiency 2-D silicon-based neutron detector array for use as a nuclear reactor beam port monitor. The detector is operated upstream of other instrumentation in order to monitor the neutron beam in real time while performing experiments in the beam. The low efficiency allows the neutron beam to remain relatively unperturbed as it proceeds through the detector to the experiment station, hence neutron beam attenuation is negligible. The detection array yields valuable real time and time-averaged information about the neutron flux as a function of position. 2. Theory Thin-film-coated semiconductor neutron detectors are gener- ally configured as planar semiconductor diodes with a thin film of neutron reactive material deposited upon the diode surface. Detector operation is straightforward. Neutrons are absorbed in the neutron reactive film, which spontaneously releases ionizing reaction products. The most commonly used ‘‘converter’’ films rely either on the 10 B(n,a) 7 Li reaction or the 6 Li(n,t) 4 He reaction. The 10 B(n,a) 7 Li reaction releases a 1.47MeV a particle and a 840 keV 7 Li ion with 94% branching, or a 1.78 MeV a particle and a 1.05 MeV 7 Li ion with 6% branching. Pure 10 B has a microscopic thermal neutron cross-section of 3840b, which translates into a macroscopic thermal neutron cross-section of 500 cm À1 . The 6 Li(n,t) 4 He reaction releases a 2.73 MeV triton and a 2.05 MeV a particle. Pure 6 Li has a microscopic thermal neutron cross-section of 940b. Unfortunately, pure 6 Li is hygroscopic and highly reactive, hence not easily handled as a neutron converter material. However, the stable compound LiF has proven to be easily handled, and can be applied to the surface of a diode by a variety of methods [1]. Pure 6 LiF has a mass density of 2.54 g cm À2 , which translates into a macroscopic thermal neutron cross- section of 57.5 cm À1 [1]. Of the two main candidate films, 6 LiF has the most energetic ionizing reaction products, thereby, being easier to detect and discriminate from background radiations. For thermal neutrons, conservation of mass and energy demands the reaction products to be ejected in opposite directions, hence only one of the particles can enter the diode active region (see Fig. 1). The overall thermal neutron detection efficiency varies with the film thicknesses, reaching an optimum value of approximately 4.5%, depending upon the lower- level discriminator setting of the detection system [1]. In the present case, the device is designed to operate with low efficiency, hence application of the 6 LiF thin film is straightforward. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.087 Ã Corresponding author. Tel.: +1785 532 5284; fax: +1785 532 7057. E-mail address: mcgregor@ksu.edu (D.S. McGregor). Nuclear Instruments and Methods in Physics Research A 604 (2009) 150–153