Journal of Imaging Article Boron-Based Neutron Scintillator Screens for Neutron Imaging William Chuirazzi 1,*, Aaron Craft 1, Burkhard Schillinger 2, Steven Cool 3 and Alessandro Tengattini 4,5 1 Advanced Post-Irradiation Examination Department, Idaho National Laboratory, Idaho Falls, ID 83401, USA; [email protected] 2 Heinz Maier-Leibnitz Zentrum (FRM II) and Faculty for Physics E21, Heinz Maier-Leibnitz Zentrum (FRM II), Technische Universität München, 85748 Garching, Germany; [email protected] 3 DMI/Reading Imaging, Reading, MA 01867, USA; [email protected] 4 Le Centre National de la Recherche Scientifique (CNRS), Université Grenoble Alpes, Grenoble INP, 3SR, 38000 Grenoble, France; [email protected] 5 Institut Laue-Langevin (ILL), 71 Avenue des Martyrs, 38000 Grenoble, France * Correspondence: [email protected] Received: 27 October 2020; Accepted: 16 November 2020; Published: 19 November 2020 Abstract: In digital neutron imaging, the neutron scintillator screen is a limiting factor of spatial resolution and neutron capture efficiency and must be improved to enhance the capabilities of digital neutron imaging systems. Commonly used neutron scintillators are based on 6LiF and gadolinium oxysulfide neutron converters. This work explores boron-based neutron scintillators because 10B has a neutron absorption cross-section four times greater than 6Li, less energetic daughter products than Gd and 6Li, and lower γ-ray sensitivity than Gd. These factors all suggest that, although borated neutron scintillators may not produce as much light as 6Li-based screens, they may offer improved neutron statistics and spatial resolution. This work conducts a parametric study to determine the effects of various boron neutron converters, scintillator and converter particle sizes, converter-to-scintillator mix ratio, substrate materials, and sensor construction on image quality. The best performing boron-based scintillator screens demonstrated an improvement in neutron detection efficiency when compared with a common 6LiF/ZnS scintillator, with a 125% increase in thermal neutron detection efficiency and 67% increase in epithermal neutron detection efficiency. The spatial resolution of high-resolution borated scintillators was measured, and the neutron tomography of a test object was successfully performed using some of the boron-based screens that exhibited the highest spatial resolution. For ome applications, boron-based scintillators can be utilized to increase the performance of a digital neutron imaging system by reducing acquisition times and improving neutron statistics. Keywords: neutron imaging; neutron radiography; digital imaging; neutron scintillator; scintillator screen; boron scintillator; scintillator characterization; scintillator detection efficiency; epithermal neutron imaging; neutron sensor 1. Introduction 1.1. Background Neutron imaging is a nondestructive examination technique that measures neutron transmission to examine a sample’s internal structure. This technique has been used in a variety of studies, including those of nuclear fuels and materials [1–6], cultural heritage objects [7–9], fuel cells [10–14], turbine blades [15–17], and many others. Digital neutron imaging techniques are now an essential capability J. Imaging 2020, 6, 124; doi:10.3390/jimaging6110124 www.mdpi.com/journal/jimaging J. Imaging 2020, 6, x FOR PEER REVIEW 2 of 22 J. Imaging 2020, 6, 124 2 of 22 essential capability for any state‐of‐the‐art neutron imaging facility [18]. The most common digital neutron imaging system consists of a digital camera optically coupled to a neutron‐sensitive for any state-of-the-art neutron imaging facility [18]. The most common digital neutron imaging system scintillator screen. consistsA critical of a digital component camera of optically digital neutron coupled imaging to a neutron-sensitive systems is the scintillatorneutron scintillator screen. , because the scintillatorA critical dictates component the neutron of digital capture neutron efficiency imaging and systems is a islimiting the neutron factor scintillator, in spatial becauseresolution. the scintillator dictates the neutron capture efficiency and is a limiting factor in spatial resolution. Common Common scintillator screens for thermal and cold neutron imaging utilize either a 6Li(n,α)3H reaction 6 α 3 scintillatordue to its large screens neutron for thermal absorption and cold cross neutron‐section imaging and relatively utilize eitherlarge areactionLi(n, )energyH reaction, or a dueGd(n,γ to its + large neutron absorption cross-section and relatively large reaction energy, or a Gd(n,γ + ICe ) reaction ICe−) reaction because of gadolinium’s high neutron absorption cross‐section [19–23]. Although− these becausescintillator of gadolinium’sscreens are the high state neutron of the art absorption in the digital cross-section neutron [imaging19–23]. Although community, these the scintillator continual screensimprovement are the stateof neutron of theart scintillators in the digital are neutron necessary imaging to meet community, the ever the‐higher continual user improvement demands for of neutronincreased scintillators spatial resolution are necessary to toexamine meet the smaller ever-higher samples user demandsand increased for increased detection spatial efficiency resolution to todecrease examine measurement smaller samples time. and increased detection efficiency to decrease measurement time. Boron-based screens offer a promising alternative to existing 6Li- and Gd-based neutron scintillator Boron‐based screens offer a promising alternative to existing 6Li‐ and Gd‐based neutron screens for a number of reasons. First, the thermal neutron cross-section of 10B (3840 b) is approximately scintillator screens for a number of reasons. First, the thermal neutron cross‐section of 10B (3840 b) is four times greater than that of 6Li (980 b), as shown in Figure1, which should increase the neutron approximately four times greater than that of 6Li (980 b), as shown in Figure 1, which should increase detection efficiency for a comparable 6Li-based screen. Second, the daughter products created by the neutron detection efficiency for a comparable 6Li‐based screen. Second, the daughter products neutron absorption of 10B have less total energy (2.31 MeV or 2.79 MeV) than those of 6Li (4.78 MeV) created by neutron absorption of 10B have less total energy (2.31 MeV or 2.79 MeV) than those of 6Li 10 α 7 and(4.78 Gd MeV) (7.937 and MeV Gd or(7.937 8.536 MeV MeV), or and8.536 the MeV) lower, and energy the fromlower a energyB(n, )fromLi reaction a 10B(n,α) is distributed7Li reaction on is 6 α 3 heavierdistributed daughter on heavier products daughter compared products to a comparedLi(n, ) H to reaction. a 6Li(n,α) The3H lowerreaction energy. The lower and larger energy size and of thelarger daughter size of products the daughter reduces products the range reduces the daughter the range products the willdaughter travel, products a fundamental will travel, limiting a factorfundamental of a screen’s limiting spatial factor resolution, of a screen’s potentially spatial enabling resolution, borated potentially scintillator enabling screens borated to exhibit scintillator higher 6 10 spatialscreens resolutionto exhibit compared higher spatial to the resolution screens created compa withred toLi the or Gd.screens Additionally, created withB has6Li aor lower Gd. γ Additionally,-ray sensitivity 10B thanhas gadoliniuma lower γ‐ray due sensitivity to its lower than atomic gadolinium number. due Scintillator to its lower screens atomic with number. a boron 6 converterScintillator can screens theoretically with a boron be made converter much can thinner theoretically than current be madeLi- much and Gd-basedthinner than screens, current while 6Li‐ 6 γ exhibitingand Gd‐based superior screens, neutron while detection exhibiting efficiency superior to Li neutron scintillator detection screens, efficiency improved to-ray 6Li insensitivity scintillator 10 overscreens, Gd improved scintillator γ screens,‐ray insensitivity and improved over Gd spatial scintillator resolution. screens, The and combination improved ofspatialB’s resolution. improved γ neutronThe combination detection eofffi ciency,10B’s improved spatial resolution, neutron detection and -ray efficiency, insensitivity spatial could resolution, potentially and provide γ‐ray a high-performanceinsensitivity could neutronpotentially scintillator. provide a high‐performance neutron scintillator. 1.E+06106 BB-1010 101.E+055 LiLi66 101.E+044 101.E+033 101.E+022 1.E+0110 Cross Section(b) 1.E+001 101.E-01-1 101.E-02-2 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 102 1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 Energy (MeV) Figure 1.1. Microscopic absorption cross-sectioncross‐section of 1010B and 6LiLi from the Evaluated Nuclear Data File (ENDF(ENDF//B-VIII)B‐VIII) cross-sectioncross‐section libraries.libraries. Boron-basedBoron‐based neutron scintillators scintillators for for neutron neutron imaging imaging applications applications were were conceived conceived as asearly early as asthe the1950s 1950s [24,25] [24, 25and] andtheir their superior superior neutron neutron detection detection efficiency efficiency has been has documented been documented [26]. Glass [26]. Glassscintillators scintillators [27,28] [27, scintillators,28], scintillators with with a mixture a mixture of neutron of neutron absorbing absorbing materials materials [29,30]