LiF/CsI:Tl Scintillator for High Resolution Neutron Imaging (#3556)
S. R. MIller1, M. S. J. Marshall1, M. J. More1, J. Crespi1, S. Waterman1, V. V. Nagarkar1, R. Riedel2
1 RMD, Inc., Watertown, Massachusetts, United States of America
We have developed a novel scintillator for neutron imaging by combining enriched 6LiF with the well-known CsI:Tl scintillator films. The CsI:Tl scintillator is attractive for imaging applications due to its light yield of 54,000 photons/MeV, and highly columnar form that provides high spatial resolution by collimating the scintillation light to the detector. To make the scintillator sensitive to neutrons, 6LiF (95% enriched) was applied directly to the CsI:Tl film using two different approaches. One was to apply the 6LiF in powder form and the other was to deposit a film via thermal evaporation. The 6LiF thickness was approximately 30 microns deposited on 30 micron CsI:Tl films. Further studies to optimize performance are underway varying the thickness of both layers.
The films were tested by coupling to CCD cameras at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL), as well as at the NIST Center for Neutron Research (NCNR). Excellent images were acquired with a spatial resolution of 15 lp/mm (33 µm) observed. When coupled to a PMT, a gamma equivalent energy (GEE) of 2.6 MeV was observed, indicating a light signal of 140,000 photons/neutron. Details of the imaging results including tomographic imaging will be presented here.
Keywords: microcolumnar scintillator, neutron radiography, neutron computed tomography
Field-Deployable LiF:ZnS Neutron Detectors for Fission Source Detection and Identification (#3836)
I. Cullen1, M. Foster1, M. Schear2, M. Dallimore2, T. Garcia2
1 Symetrica Security Ltd., Southampton, United Kingdom of Great Britain and Northern Ireland
This paper describes a mobile neutron coincidence counter that can be used to categorize random, spontaneous or induced fission neutron sources based on their measured coincident-to-total neutron count rate (D/S) as a function of their total neutron count rate (S). The counter consists of two hinged banks of polyethylene containing 8 thermal neutron detectors per bank. These slim-profile detectors utilize thin screens of LiF:ZnS around a thin plane of wavelength shifter, read out via silicon photomultipliers. Each blade in the system is equipped with individual channel readout. When detector banks are deployed with a 90-degree angle between them, to form “L” configuration, the 16 blades constitute a sensitive area corresponding to an 8x8 (50x50cm2) grid. It is possible to estimate source position within the detector plane using the relative individual channel count rates. Using a Gaussian and Uniform prior in conjunction with a deconvolution algorithm, we have been able to resolve two, equal strength, sources to within 10cm near the corner where the two banks are hinged. For a single point source, we show that our simulated efficiency is in good agreement (<3.5%) with the measured efficiency of a point Cf-252 source, at various positions in the detector grid. Source localization is a key factor in correcting for detection efficiency variations present during in-field deployments, where the source location, source moderation, and background count can vary. Since the doubles rate is proportional to the square of the efficiency, small changes in efficiency may cause large variations in the doubles-to-singles neutron rate. Similarly, source moderation and measurement environment can also affect the detection efficiency. An extension of previous work was performed using PuO2, PuGa, AmLi and AmBe sources, together with the quantification of various moderator configurations in an attempt to simulate real world scenarios.
Keywords: fissile material detection, He-3 free neutron detectors, neutron source characterization, neutron coincidence counting, illicit trafficking of nuclear materials
Fibre coupling ZnO:Zn/6LiF for position sensitive thermal neutron detection (#3224)
G. J. Sykora1, E. Humphreys1, E. M. Schooneveld1, Q. Mutamba1, N. J. Rhodes1
1 STFC Rutherford Appleton Laboratory, ISIS, Harwell Oxford, Please select, United Kingdom of Great Britain and Northern Ireland
Recent advances in neutron scattering facilities have led to a search for detectors that can cope with increasing count rates. Current ZnS:Ag/6LiF based detectors are count rate limited by the long lifetime afterglow in the scintillator. ZnO:Zn/6LiF has recently been considered as an alternative scintillator technology with equivalent simplicity to ZnS:Ag/6LiF. Fibre coupling ZnS:Ag for position sensitive detectors is routinely done with either transparent optical fibres or green wavelength shifting fibres. ZnO:Zn however has its main emission centred at 505 nm making it a poor match for standard green wavelength shifting fibres. In this work, fibre coupling with both transparent optical fibres and orange wavelength shifting fibres from Kuraray is investigated. A detailed study of excitation-emission spectra as well as light attenuation mechanisms is presented and compared with standard green wavelength shifting fibres. The effect of the fibres on overall light collection and gamma sensitivity is addressed. Single photon counting pulse shape discrimination is used to separate neutron from gamma events. Fibre coupling of ZnO:Zn/6LiF represents the next step in count rate capability improvements over ZnS:Ag based neutron detectors.
Keywords: Thermal neutron, ZnO:Zn, Wavelength shifting fiber, position sensitive neutron detector, fiber coupling, pulse shape discrimination
Investigations into the Effect Detector Thickness Has on CLYC Performance (#1852)
C. Allwork1, M. Ellis1
1 AWE, Reading, Berkshire, United Kingdom of Great Britain and Northern Ireland
Research into the use of detectors that have the ability to simultaneously detect neutron and gamma radiation has become more common place. One such detector is the inorganic scintillator Cs2LiYCl6 (CLYC), which has the ability to detect thermal neutrons, fast neutrons and gamma radiation. One significant disadvantage of CLYC is that current growth techniques preclude large crystal sizes and costs are still high. As part of an investigation to limit the amount of scintillator material required, a series of measurements and simulations were undertaken to assess the effect CLYC crystal thickness has on detector performance with respect to pulse shape discrimination and thermal neutron detection efficiency. For a given volume of CLYC scintillator, pulse shape discrimination performance was found to be best in more symmetric crystal sizes. No plateauing in the number of detected neutrons with crystal thickness was observed when scatter-corrected thermal neutron efficiency measurements were undertaken. MCNP6 simulations of a 252Cf source positioned within a high density polyethylene sphere found that an increase in crystal thickness from 1 to 2 inches resulted in only a 20 % increase in the observed number of detected thermal neutrons despite a 54 % increase in cost.
Keywords: Pulse Shape Discrimination, Neutron Detection, Gamma Detection, Homeland Security
A High-Resolution Neutron Anger Camera Using Silicon Photo Multipliers (#2498)
R. Riedel1, C. Donahue1, T. Visscher1, C. Montcalm1, H. Cao1
1 Oak Ridge National Laboratory (ORNL), Oak Ridge, United States of America
We report on the performance of a neutron sensitive Anger Camera using arrays of Silicon Photo Multiplier (SiPM) sensors. The 116mm x 116 mm active area Anger Camera was developed using current electronics and software from a Photo Multipliers tube (PMT) based system by replacing the PMTs with a set of four SiPMT arrays, each consisting of sixty-four 7.2mm center to center sensors. We find the FWHM resolution was improved from a typical value near 1.2 mm for the PMT version to 650um for the SiPM version. Minimization of the gaps between the SiPM arrays also removed distortions seen in the PMT based Anger Camera. When tested in a magnetic field, we found no effect on SiPM Anger Camera performance up to the 500 Gauss limit of the test apparatus. Results of testing at the HFIR HB-3a (Four-Circle) beam line confirmed other important parameters such as background, efficiency and refinement residuals were equivalent to the current PMT based Anger Camera.
Keywords: neutron, detector, Silicon Photo Multiplier
Development of a 3-D position sensitive neutron detector based on organic scintillators with double side SiPM readout (#2315)
Y. Tian1, 2, Y. Fu1, 2, Y. Li1, 2, Y. Li1, 2
1 Tsinghua University, Department of Engineering Physics, Beijing, China
A 3-D position sensitive neutron detector is being developed based on organic scintillator arrays. A double side SiPM readout is used to determine the depth of interaction (DOI) in each scintillator unit. In the preliminary test, the DOI in a 25.4 × 0.6 × 0.6 cm3 SP101 plastic scintillator is measured at different positions using a collimated 60Co source. The SiPM (KETEK PM6660) signals are recorded by a 2.5 GS/s digital oscilloscope. The DOI results are calculated using both the temporal and the amplitude information. Position resolutions (FWHM) of 6.6 cm and 2.5 cm are realized, respectively. Different types of SiPMs and readout circuits are being studied to improve the performance. The pulse shape discrimination will be carried out using the EJ309 liquild scintillator or EJ299 plastic scintillator in the future work. A detector based on a 2-D array is capable of recording the 3-D position information of the incident neutron (10B or 6Li is needed for thermal neutrons). This type of detector is very useful in some applications, e.g., the neutron scatter camera, neutron detection in the inverse beta decay.
Keywords: fast neutron imaging, SiPM, depth of interaction, organic scintillator