Nuclear Instruments and Methods in Physics Research A 591 (2008) 530–533 Thermal neutron detection with pyrolytic boron nitride D.S. McGregor à , T.C. Unruh, W.J. McNeil S.M.A.R.T. Laboratory, Department of Mechanical and Nuclear Engineering, 3002 Rathbone Hall, Kansas State University, Manhattan, KS 66506-2503, USA Received 4 January 2008; received in revised form 21 February 2008; accepted 4 March 2008 Available online 7 March 2008 Abstract Commercially acquired samples of pyrolytic boron nitride (pBN) were tested as potential thermal neutron detectors. The pBN material has a high content of 10 B (19.9% natural abundance), allowing for the excitation of charge carriers from the 10 B(n,a) 7 Li reaction. Samples 5 mm  5 mm  1 mm thick with conductive contacts applied to both sides were operated as traditional planar semiconductor detectors. Being a solid, the material is attractive as a possible solid-state neutron detector. The devices were tested in a diffracted neutron beam from a nuclear reactor. Experimental results from the prototype devices are reported. r 2008 Published by Elsevier B.V. PACS: 29.40.Wk Keywords: Semiconductor neutron detectors; Solid-state neutron detectors 1. Introduction There has long been interest in the development of solid- state neutron detectors. Although neutron-sensitive scin- tillators such as LiI(Eu) and neutron-sensitive thermo- luminescent dosimeters (TLDs) such as LiF are solid-state materials, it is the development of semiconductor materials for neutron detection that is usually meant when consider- ing solid-state neutron detectors. There are two basic types of semiconductor neutron detectors, those being thin-film- coated diodes and solid form devices [1,2]. Thin-film- coated devices are configured as junction diode detectors, often from Si or GaAs, with a conversion film of neutron- reactive material attached to the rectifying junction surface (details on the basic operation found elsewhere [1]). Solid form semiconductor detectors consist of a compound semiconducting material, fashioned into a detector, with one or more constituents being a neutron-reactive material. The most popular neutron reaction of interest is the 10 B(n,a) 7 Li reaction, which yields two possible de-excita- tion branches from the excited 11 B compound nucleus, namely 1 0 n þ 10 5 B ! 7 3 Li  ð1:4721 MeVÞþ 4 2 Heð0:8398 MeVÞð93:7%Þ 7 3 Lið1:7762 MeVÞþ 4 2 Heð1:0133 MeVÞð6:3%Þ ( where the Li ion in the 93.7% branch is ejected in an excited state, which de-excites through the prompt emission of a 480 keV gamma ray. For thermal neutrons, the two charged particle reaction products are ejected in opposite directions. Fully enriched 10 B has a microscopic absorption cross-section for thermal neutrons (2200 m s 1 ) of 3840 b. With a mass density of 2.15 g cm 3 , the solid structure of 10 B has a macroscopic thermal neutron absorption cross- section of 500 cm 1 . The absorption cross-section for 10 B follows a 1/v dependence [3,4]. Solid-state neutron detectors based on boron semicon- ductors are attractive as compact high-efficiency devices. Boron-based compounds that have been studied include boron monophosphide (BP), boron arsenide (BAs), boron nitride (BN) and various forms of boron carbide (B x C) [5–10], with patents for such devices dating back decades [11,12], as well as being recently allowed [13–16]. Cubic BP ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.03.002 à Corresponding author. Tel.: +1 785 532 5284; fax: +1 785 532 7057 E-mail address: mcgregor@ksu.edu (D.S. McGregor). URL: http://https://www.mne.ksu.edu/research/centers/SMARTlab (D.S. McGregor).