Factors affecting performance of the micro-layered fast-neutron detector
Not provided.
Instruments & Instrumentation↗
Engineering topics
Harrison, Mark J.
Publications and source records attributed to Harrison, Mark J..
A high-efficiency, low-Ĉerenkov Micro-Layered Fast-Neutron Detector for the $\mathrm{TREAT}$ hodoscope
Here this paper presents an enabling technology for high-efficiency fast-neutron detection for the Transient Reactor Test (TREAT) Facility hodoscope. The hodoscope is currently outfitted with 25.4-mm long Hornyak buttons for fast-neutron detection. However, they require improvements, as they suffer from low detection efficiency, Ĉerenkov radiation contaminating the signal, and non-linearity of detector response at large transients. To address these issues, a layered ZnS:Ag device has been constructed. The layered design provides various advantages: it incorporates greater scintillation volume, Ĉerenkov background is significantly reduced by eliminating the need for additional light guides, and the detection process is simplified by achieving gamma-ray rejection using only pulse-height discrimination. The result is a greater number of fast neutron counts with reduced background noise contaminating the signal. On being irradiated with a 252Cf source (whose neutron energy spectrum closely mimics a reactor fission spectrum), a 25.4-mm long layered device was found to exhibit an intrinsic fast-neutron detection efficiency of 8.1% for neutrons with an average energy of 2 MeV, while a 43-mm long device allows for a greater number of fast neutron to be detected, increasing the efficiency to 9.1%. Further lengthening of the device up to 20 cm has been numerically predicted to exhibit even higher efficiencies. The detectors can be mass-produced easily and inexpensively.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Preliminary Analysis of Damage to MPFDs Caused by Reactor Pulses
The need for next-generation, accident-tolerant fuels has generated significant interest in the restart of domestic fuels testing at the Transient Reactor Test (TREAT) facility. The TREAT reactor is designed to produce large, transient fluxes that replicate the effects of a wide variety of accident conditions on an irradiated fuel sample. Peak powers generated during short pulses approach 20 GW with a neutron flux of approximately 10{sup 17} n cm{sup -2} s{sup -1}. As part of the restart effort, new vehicle designs are being explored for sample irradiation, including the Static Environment Rodlet Transient Test Apparatus (SERTTA) and variants. As part of the SERTTA designs, micro-pocket fission detectors (MPFDs) are being explored for localized flux measurements before, during, and after transient operations. These detectors are also being considered as part of future, core-wide instrumentation for the TREAT reactor. However, the high fluxes associated with the pulses in the TREAT core present certain thermodynamic concerns given their intensity and the amount of energy they will release within the MPFDs within such a short amount of time. Assuming each fission that occurs in a MPFD results in a fission fragment depositing 85 MeV of energy in the substrate on average and each MPFD has 0.5 μg of natural uranium deposited on it, a 100 ms long pulse in the TREAT reactor at 1017 n cm{sup -2} s{sup -1} flux will produce approximately 4.6 x 10{sup 6} fissions in an MPFD. This pulse deposits 6.3 x 10{sup -5} J in the substrate over a 100 ms duration. That amount of energy is sufficient to melt roughly 33% of the Pt under the UO{sub 2} deposition or a circular spot with a diameter of approximately 860 μm. Of course, the above estimate makes a number of simplifying assumptions and ignores heat transfer, but does serve to illustrate the point that this rapid heating effect could be detrimental to the integrity and life of these in-core MPFDs. To study how these layers are affected by large transient neutron fluxes, five samples were prepared, pre-analyzed, and then subjected to a pulse in the central thimble of the Kansas State University TRIGA Mark II reactor to a peak power of 1.58 GW. After being allowed to decay to safer radiation levels, the sample substrates and coatings were inspected using scanning electron microscopy (SEM) to identify any changes to their surface morphology. Indeed, certain changes were found and are discussed in greater detail in this paper.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
Event Localization in Bulk Scintillator Crystals Using Coded Apertures
The localization of radiation interactions in bulk scintillators is generally limited by the size of the light distribution at the readout surface of the crystal/light-pipe system. By finding the centroid of the light spot, which is typically of order centimeters across, practical single-event localization is limited to ~2 mm/cm of crystal thickness. Similar resolution can also be achieved for the depth of interaction by measuring the size of the light spot. Through the use of near-field coded-aperture techniques applied to the scintillation light, light transport simulations show that for 3-cm-thick crystals, more than a five-fold improvement (millimeter spatial resolution) can be achieved both laterally and in event depth. At the core of the technique is the requirement to resolve the shadow from an optical mask placed in the scintillation light path between the crystal and the readout. In this paper, experimental results are presented that demonstrate the overall concept using a 1D shadow mask, a thin-scintillator crystal and a light pipe of varying thickness to emulate a 2.2-cm-thick crystal. Spatial resolutions of ~ 1 mm in both depth and transverse to the readout face are obtained over most of the crystal depth.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
LiCaAIF 6 :Eu Scintillator Family Neutron Detection (Final Report)
The proposed work was based on a learning cycle format with three parts, (1) Modeling, (2) Growth, (3) Characterization, to investigate the family of colquiriite scintillators and various activating dopants as efficiently and effectively as possible. Under this research methodology, each of these functions would feed information to the next. Each iteration around the cycle was expected to improve results and provide clues to the best path forward for the next iteration. However, it was realized that this research model is not robust and unable to handle setbacks at any one of the functions. Unfortunately, the project suffered setbacks in the “Growth” function and progress in the other functions was severely restricted. Of course, numerous alternative paths were trialed within the growth function to enable progress, but these required addition time and resources which in the end only provided limited progress. After reviewing the project as a whole, it can be concluded that the research model used was indeed too fragile for this sort of research and a more robust plan should be utilized in the future. As a whole though, significant progress was made toward understanding the barriers toward realizing the benefits of this family of scintillators through modeling. Various paths forward were postulated, but limited growth and synthesis capabilities prevented experimental validation. Experimentally, it was found that the synthesis and growth of colquiriite scintillators requires special considerations for its thermodynamic and chemical properties. These were overcome in the end and a new, lower cost growth method realized, but too late to have a significant impact in validating modeling hypotheses.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
High Temperature Fission Chamber for He- and FLiBe-cooled Reactors
We have evaluated candidate technologies for in-core fission chambers for high-temperature reactors to monitor power level via measurements of neutron flux from start-up through full power at up to 800°C. This research is important because there are no commercially available instruments capable of operating above 550 °C. Component materials and processes were investigated for fission chambers suitable for operation at 800 °C in reactors cooled by molten fluoride salt (FLiBe) or flowing He, with an emphasis placed on sensitivity (≥ 1 cps/nv), service lifetime (2 years at full power), and resistance to direct immersion in FLiBe. The latter gives the instrument the ability to survive accidents involving breach of a thimble. The device is envisioned to be a two-gap, three-electrode instrument constructed from concentric nickel-plated alumina cylinders and using a noble gas–nitrogen fill-gas. We report the results of measurements and calculations of the response of fill gasses, impurity migration in nickel alloy, brazing of the alumina insulator, and thermodynamic calculations.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Investigation of the LiCaAlF 6 :Eu Scintillator Family for Next-Generation Neutron Detection (Annual Report)
LiCaAlF 6 :Eu (LiCAF:Eu) is a promising scintillator that possesses several fundamental advantages over CsLiYCl 6 :Ce (CLYC). This scintillator, however, is just one member of a much broader family of materials known as colquiriites, designated as LiMe II Me III F 6 , where Me II and Me III can be any number of divalent and trivalent cations, respectively. The LiCAF system has incredible potential for a wide variety of substitutions in the Me II and Me III sites, and its members are recognized as excellent hosts for luminescing activator dopants (e.g., Ce 3+ , Eu 2+ , Cr 3+ , Pb 2+ and Yb 2+ ) in order to tune the nuclear and luminescence performance in several ways. We intend to fully exploit this versatility by investigating the effects of substituting other alkali, alkali-earth, rare-earth, Group III, and Group XIII elements. The objective of this work is to explore the colquiriite system of scintillating materials, most notably LiCaAlF 6 :Eu, to identify and characterize new scintillators with improved scintillation performance, simplified crystal growth, and tunable neutron and gamma sensitivities.
Harrison, Mark J.↗
Crystal Compton Camera
Stand-off detection is one of the most important radiation detection capabilities for arms control and the control of illicit nuclear materials. For long range passive detection one requires a large detector and a means of “seeing through” the naturally occurring and varying background radiation, i.e. imaging. Arguably, Compton imaging is the best approach over much of the emission band suitable for long range detection. It provides not only imaging, but more information about the direction of incidence of each detected gamma-ray than the alternate approach of coded-aperture imaging. The directional information allows one to reduce the background and hence improve the sensitivity of a measurement. However, to make an efficient Compton imager requires localizing and measuring the simultaneous energy depositions when gamma-rays Compton scatter and are subsequently captured within a single, large detector volume. This concept has been demonstrated in semi-conductor detectors (HPGe, CZT, Si) but at ~ $1k/cm 3 these materials are too expensive to build the large systems needed for standoff detection. Scintillator detectors, such as NaI(Tl), are two orders of magnitude less expensive and possess the energy resolution required to make such an imager. However, they do not currently have the ability to localize closely spaced, simultaneous energy depositions in a single large crystal. In this project we are applying a new technique that should, for the first time ever, allow cubic-millimeter event localization in a bulk scintillator crystal.
98 NUCLEAR DISARMAMENT, SAFEGUARDS, AND PHYSICAL P↗