N2B3  Neutron Detectors: Thermal Capture Scintillation Detectors

Tuesday, Nov. 3  10:30-12:30  Golden West

Session Chair:  Ralf Engels, Forschungszentrum Juelich GmbH, Germany; Scott Kiff, Sandia National Laboratories, United States

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(10:30) N2B3-1, An Energy Dispersive Neutron Detector Based on 6LiF:ZnS(Ag) Scintillator Coupled with Wavelength Shifting Fiber Readout

N. C. Maliszewskyj1, A. Osovizky2,3, K. M. Pritchard1, J. B. Ziegler1, P. Tsai1, E. Binkley1, N. Hadad1, C. F. Majkrzak1

1Center for Neutron Research, NIST, Gaithersburg, MD, USA
2University of Maryland, College Park, MD, USA
3Rotem Industries Ltd, Beer Sheva, Israel
The NIST Center for Neutron Research is developing a chromatically analyzing neutron detector as part of the development of a white beam reflectometer. The detector for the reflectometer uses an array of 54 highly oriented graphite crystals set at a range of takeoff angles to diffract the polychromatic scattered neutrons into ultrathin (~ 1mm) proportional counters comprised of 6LiF:ZnS(Ag) scintillator, wavelength shifting fibers, and silicon photomultipliers (SiPMs) as photosensors. It will then be possible to simultaneously energy analyze neutrons within a 4 to 6 angstrom wavelength range with a fractional wavelength resolution of approximately one percent.
Work in recent years has centered on the development of the ultrathin neutron proportional counters. Refinements to scintillator composition, fiber geometry, SiPM selection, and discrimination electronics and algorithms have led to the realization of stopping power exceeding 90%, absolute neutron sensitivity above 75%, and gamma rejection ratios on the order of 107.
This year we have built a limited scale (8 channel) prototype of a full array which demonstrates the validity of the detector concept as well as the function of a collection of ultrathin neutron proportional counters.

(10:50) N2B3-2, Large Area Wavelength Shifting Fibre Thermal Neutron Detectors Using 64 Channel Flat Panel PMTs

G. J. Sykora, E. M. Schooneveld, N. J. Rhodes

Instrumentation Division, STFC - ISIS, Harwell Oxford, United Kingdom
Large area thermal neutron detectors are needed in fields such as nuclear safeguarding and neutron scattering science. In neutron scattering, position sensitive 3-He detectors have historically been employed on instruments requiring large area detectors. Cost-effective alternatives to large area 3-He detectors are currently being sought due to 3-He becoming rare and expensive. ZnS:Ag/6-LiF scintillator detectors are currently in use at ISIS, the SNS and J-PARC and are being investigated as the next generation of large area detectors at ISIS. Wavelength shifting fibre (WLSF) technology has resulted in a cost effective light transport option. WLSF have previously been coupled to single anode PMTs, 16 channel multi-anode PMTs, standard 64 channel MAPMTs and Si-PMs. Each of these have their own advantages and disadvantages with the main purpose being to economically readout scintillation events from the ZnS:Ag/6-LiF without sacrificing detector performance. Recent developments in flat panel PMT (FP-PMT) technology have made single photon detection feasible allowing a new, cost-effective, solution for WLSF detectors. Large, 6mm x 6mm, pixel size combined with 64 channels allows for a high degree of fibre coding while drastically reducing the high voltage, electronics and cabling requirements. The performance of 64 channel FP-PMTs in large area WLSF coupled ZnS:Ag/6-LiF detectors is presented here. Current ISIS WLSF detector technology was used to evaluate the performance of the FP-PMT. Performance of the 64 channel FP-PMT is discussed in terms of readout method, pixel to pixel cross-talk and pixel to pixel uniformity. A new position determination algorithm is shown to eliminate the effect of pixel to pixel cross-talk. A factor of up to 2.5 in gain variation from pixel to pixel is shown to have less than 10% effect on the uniformity of the neutron detector response. The 64 channel FP-PMT is shown to be a viable option for large area wavelength shifting fibre detectors.

(11:10) N2B3-3, Optimization of ZnS/6LiF Scintillators for Wavelength-Shifting-Fiber Neutron Detectors

C. L. Wang, M. L. Crow, B. W. Hannan, J. P. Hodges, R. A. Riedel

Instrument and Source Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
Wavelength-shifting-fiber (WLSF) neutron imaging detectors have been successfully deployed in two time-of-flight (TOF) beam lines (POWGEN, VULCAN) at Spallation Neutron Source (SNS), due to their large detector coverage (0.3 m2 per module), reasonable spatial-resolution (~5 mm) for powder diffraction, low gamma-sensitivity (10-6-10-7), and relative low cost. However, the detection efficiency is reduced for epithermal neutrons (wavelength <1.8 Å, the main wavelength band for POWGEN) compared to the thermal and cold neutrons. Increasing the neutron detection efficiency at shorter wavelengths without a loss in spatial resolution poses a serious challenge for the present versions (V.2, and V.3) as well as the next generation higher-resolution WLSF detectors (V.4). To solve this issue, we measured the light output and photon travel lengths of flat ZnS/6LiF scintillators with the thickness d=0.2-0.8 mm using photon counting and pulse-height analysis. We found that the 0.8 mm-thick scintillator has the highest light output, which is 52% higher compared with a grooved scintillator of similar thickness as explained by a photon potential-well model. The average photon travel length is decreased with d in the range of d=0.2-0.8 mm as measured from TOF diffraction patterns of diamond and vanadium powders using four V.3 detector modules at VULCAN and POWGEN. These results contrast with traditional Swank diffusion model for microcomposite scintillators, but they may be explained by photon diffusion under weak Anderson localization. The flat 0.8 mm-thick scintillators or thicker ones are expected to improve the detection efficiency of epithermal neutrons without sacrificing spatial resolution.

(11:30) N2B3-4, Measurement and Comparison of the Light Output of Ni-Doped 6LiF/ZnS for Use in Neutron Multiplicity Counting

S. Behling1, M. Bliss1, C. Cowles1, R. Kouzes1, A. Lintereur2, S. Robinson1, E. Siciliano1, S. Stave1, Z. Wang1

1Pacific Northwest National Laboratory, Richland, WA, USA
2University of Utah, Salt Lake City, UT, USA
Alternatives to 3He for neutron detection have recently become attractive for safeguards applications. Pacific Northwest National Laboratory is developing a neutron multiplicity counter that is based on 6LiF/ZnS. Some of the properties of this material, such as the scintillation light decay time, can be tuned by doping the material with a small amount of nickel. This doping affects other properties of the material, in particular the time dependence of the scintillation light output. To determine whether the nickel-doped or undoped 6LiF/ZnS material would better suit the neutron multiplicity counter system, a series of experiments compared the use of undoped 6LiF/ZnS and a Ni-doped variant using both small samples and full-scale detectors made using the two materials. Both materials were manufactured by Eljen Technology of Sweetwater, Texas. The Ni-doped variant produced less light than the undoped material and had a shorter decay time. For all choices of detection threshold above the electronic noise, the decrease in light output did not affect the detection efficiency of the system.

(11:50) N2B3-5, Experimental Results and Applications of 6Li Glass Particles Heterogeneous Neutron Detector

K. D. Ianakiev, M. T. Swinhoe, M. P. Hehlen, A. Favalli, M. L. Iliev

Los Alamos National Laboratory, Los Alamos, USA
Most of the scintillator based neutron detectors have overlapping neutron-gamma pulse-height distributions, which limits their usefulness and performance. Different techniques are used to mitigate this shortcoming, including Pulse Shape Discrimination (PSD) for heavily overlapped or threshold settings that suppress all gamma rays as well as much of the neutrons. As a result, count-rates are limited and dead times are high when PSD is used, and the detection efficiency for neutron events is reduced due to need to set the high threshold. This challenge is especially severe for neutron multiplicity counting and Differential Die Away (DDA) measurements that have numerous conflicting requirements such as high detection efficiency, short die-away time, short dead time, and high stability. In this paper we present the experimental results and application aspects of scalable neutron detector prototype with separates neutron and gamma distribution based small 6Li scintillator glass particles embedding in a non-scintillating organic light-guide medium. The detector prototype comprises of 8200 cubes with 1.5 mm size of GS20 neutron glass scintillators cubes uniformly distributed by supporting structure in 3” dia. by 10” length detector volume of mineral oil with matched refraction indexes. The prototype detector was characterized using 252Cf ,137Cs PuO2 radiation sources, as well as pulsed DT neutron generator. The experimental results show plateau of the counting characteristic, less than 15 ?s short die away time and dead time less than 150 0 ns. The detector volume contain about 4x1020 atom/cm3 density of 6Li neutron absorber and about 7x1022 atom/cm3 hydrogen , which is equivalent to a 3He atom density of 16 atm. 3He gas in the volume, but in our developed detector the 6Li particle are in a moderator medium. The application of this technology for advanced neutron multiplicity counter with 4? detection volume and DDA measurements will be discussed

(12:10) N2B3-6, Simulation of Light Transport in a Thermal Neutron Detector

M. P. Hehlen1, K. D. Ianakiev1, M. Iliev1, T. C. Lin2, M. T. Barker3, M. T. Swinhoe1, A. Favalli1

1Los Alamos National Laboratory, Los Alamos, NM, USA
2University of California at Los Angeles, Los Angeles, CA, USA
3Sandia National Laboratories, Albuquerque, NM, USA
Scintillator-based gamma and neutron detectors play a key role in a wide range of nuclear nonproliferation, safeguards, emergency response, medical, and scientific applications. The efficiency of light transport from the scintillator to the photosensor plays a key role in the performance of such detectors. Monte Carlo methods can be applied to modeling the light transport by tracing light rays through the optical subsystem. We use raytracing simulations to study the light transport in emerging heterogeneous thermal neutron scintillators, which consist of thousands of millimeter-sized particles of 6Li glass scintillator dispersed in an organic matrix, where the latter acts as both a moderator for the incident neutrons and a lightguide for the scintillation photons. Raytracing simulations of such materials is particularly challenging because of the large number of elements and associated surfaces combined with the complex interplay of absorption, reemission, scattering, reflection, and refraction processes. We will present our work towards modeling a thermal neutron detector consisting of 8200 6Li glass (GS20) cubes in a mineral oil matrix. We show that the light transport efficiency is sensitive to the reflectance of the coating surrounding the scintillator, while the absorption of scintillation light by the mineral oil matrix is insignificant. The model also includes wavelength-dependent reabsorption and reemission of light by the scintillator, an effect that can become relevant at greater scintillator volumes.