Optimizing Cs 2 LiYCl 6 for fast neutron spectroscopy N. D’Olympia a,n , P. Chowdhury a , C.J. Guess a , T. Harrington a , E.G. Jackson a , S. Lakshmi a , C.J. Lister a , J. Glodo b , R. Hawrami b , K. Shah b , U. Shirwadkar b a Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, MA 01854, United States b Radiation Monitoring Devices Inc., Watertown, MA 02472, United States article info Article history: Received 24 April 2012 Received in revised form 9 June 2012 Accepted 9 July 2012 Available online 24 July 2012 Keywords: Fast neutron detector CLYC N-g discrimination 35 Cl(nP) 6 Li(n a) Cs 2 LiYCl 6 abstract Cs 2 LiYCl 6 (CLYC) has generated recent interest as a thermal neutron detector due to its excellent n=g-ray pulse-shape discrimination and energy resolution. Here, the capabilities of CLYC as a fast neutron detector and spectrometer are reported. A 1 in.  1 in. CLYC detector was used to measure the response of mono-energetic neutrons over a range of 0.8–2.0 MeV produced via the 7 Li(p,n) reaction at the University of Massachusetts Lowell 5.5 MV Van de Graaff accelerator. A broad continuum from the 6 Li(n, a) reaction was observed, as well as additional peaks below the thermal capture peak. Based on possible reactions in CLYC, the additional peaks are determined to be due to the 35 Cl(n,p) 35 S reaction, with a Q-value of þ615 keV, and corroborated in simulations using MCNPX. The average resolution of 9% for these peaks makes CLYC a promising candidate for a fast neutron spectrometer. Published by Elsevier B.V. 1. Introduction Measurement and spectroscopy of fast neutrons is a funda- mental tool in many areas of research, including nuclear energy, stewardship science, nuclear safeguards, fusion plasma diagnos- tics, and fundamental nuclear physics. Fast neutron spectroscopy, however, continues to be a challenge due to limitations in existing detector technology. While time-of-flight techniques can produce accurate energy spectra, they are limited to situa- tions in which a start signal, such as a correlated g-ray or accelerator beam burst, is available. A compromise must also be made between improving resolution (with a longer flight path and thin detector) and increasing efficiency (with a shorter flight path and larger detector). Other spectrometers, such as proton recoil scintillators or 3 He proportional counters, involve unfolding measured spectra or the presence of a recoil continuum. Over the past few years, Cs 2 LiYCl 6 (CLYC) has been studied by several groups, including ours, as a thermal neutron and g-ray detector. The optical properties of CLYC were first reported by the Delft group [1] in an effort to investigate chlorides as high light output scintillators. CLYC has since been used primarily as a thermal neutron detector through the 6 Li(n, a) 3 H reaction, which has a Q-value of 4.78 MeV. One of its primary advantages is its excellent n=g discrimination capabilities, due to the large differ- ence in decay times for pulses produced by electrons and those produced by the a-triton pair formed in neutron capture. Pulse- shape discrimination (PSD) with CLYC is typically done using the charge comparison method. Details on recent work describing PSD and thermal neutron detection with CLYC can be found in Refs. [2,3]. The thermal neutron peak resolution has been measured to be typically 2.5–3.1%. Therefore, though the 6 Li(n, a) cross-section drops from 940 b for thermal neutrons to 0.24 b at 1 MeV [4], it may be possible to distinguish fast neutron induced reactions from thermal neutrons in a pulse height spectrum. For fast neutrons, the kinetic energy would be added to the Q-value of the reaction and shared between the a-particle and triton, depositing a total energy of E n þ Q in the crystal. In the past, this concept was applied to studying 6 LiI:Eu as a potential fast neutron spectrometer [5,6]. However, the resolution of the fast neutron peaks were found to be very broad (18%) at room temperature, and required that the crystals be cooled with liquid nitrogen. More recently, similar attempts were made using LiBaF 3 :Ce [7], which again suffered from poor energy resolution (26% at 1.5 MeV). Prompted by the excellent thermal neutron peak resolution in CLYC, a study of its potential fast neutron detection and spectro- scopy capabilities have been carried out at the University of Massachusetts Lowell. Nearly mono-energetic neutron beams between 0.81 MeV and 2.02 MeV were produced at the 5.5 MV Van de Graaff via the 7 Li(p,n) 7 Be reaction and detected with a 1 in.  1 in. CLYC crystal. In analyzing the resulting neutron Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nima.2012.07.021 n Corresponding author. Tel.: þ1 978 934 4373. E-mail addresses: nathan_dolympia@student.uml.edu, ndolympia@gmail.com (N. D’Olympia). Nuclear Instruments and Methods in Physics Research A 694 (2012) 140–146