Optimization of {sup 6}LiF:ZnS(Ag) Scintillator Light Yield Using Geant4

Conference · Wed Jul 01 04:00:00 EDT 2015

OSTI ID:22531432

Yehuda-Zada, Y. ^[1]; Pritchard, K.; Ziegler, J. B.; Cooksey, C.; Siebein, K. ^[2]; Jackson, M.; Hurlbut, C. ^[3]; Kadmon, Y.; Cohen, Y.; Maliszewskyj, N. C. ^[1]; Ibberson, R. M.; Majkrzak, C. F. ^[2]; Orion, Y. ^[4]; Osovizky, A. ^[2]

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1. Nuclear Research Center Negev, Beer-Sheva (Israel)
2. National Institute of Standards and Technology, Gaithersburg, Maryland (United States)
3. Eljen Technology, Sweetwater Texas (United States)
4. Ben-Gurion University (Israel)

Neutrons provide an effective tool to probe materials structure. Neutron diffraction is a method to determine the atomic and magnetic structure of a material based on neutron scattering. By this method a collimated incident beam of thermal neutrons heat the examined sample and based on the obtained diffraction pattern information on the structure of the material is provided. Research for developing a novel cold neutron detector for Chromatic Analysis Neutron Diffractometer Or Reflectometer (CANDOR) is underway at the NIST center for neutron research. The system unique design is aimed to provide over ten times fold faster analysis of materials than conventional system. In order to achieve the fast analysis a large number of neutron detectors is required. A key design constraint for this detector is the thickness of the neutron sensitive element. This is met using {sup 6}LiF:ZnS(Ag) scintillation material with embedded wavelength shifting (WLS) fibers conducting scintillation light to silicon photomultiplier photo-sensors. The detector sensitivity is determined by both the neutron capture probability ({sup 6}Li density) and the detectable light output produced by the ZnS(Ag) ionization, the latter of which is hindered by the fluorescence absorption of the scintillation light by the ZnS. Tradeoffs between the neutron capture probability, stimulated light production and light attenuation for determining the optimal stoichiometry of the {sup 6}LiF and ZnS(Ag) as well as the volume ratio of scintillator and fiber. Simulations performed using the GEANT4 Monte Carlo package were made in order to optimize the detector design. GEANT4 enables the investigation of the neutron interaction with the detector, the ionization process and the light transfer process following the nuclear process. The series of conversions required for this detector were modelled: - A cold neutron enters the sensor and is captured by {sup 6}Li in the scintillator mixture ({sup 6}Li (n,α) {sup 3}H reaction). The study of investigating the capture process probability for neutron energy of 5.1 meV to 2.27 meV (4 - 6 A) is presented. - Alpha particles and tritons travel for a few microns in the scintillation material (α ∼0.007 mm, T ∼0.04 mm) losing energy and ionizing the ZnS. The mean free path of the two particles in each of the component materials and the complete compound was investigated. - The ionization of the ZnS(Ag) scintillation material produces blue light photons with luminescence wavelength of 450 nm. The amount of light output produced for different grain sizes of ZnS is discussed. - A large portion of the scintillation photons are reabsorbed during their passage through the scintillation material. - The blue photons that reach the WLS fibers are absorbed by fluorescent dye and are re-emitted as green photons, conducted by the fiber to the SiPM photo-sensor. This work presents the CANDOR unique design and its design constrains, the results measured by the ultra-thin {sup 6}LiF:ZnS(Ag)-based neutron detector versus the simulation results for several binder concentrations. The light measurement attenuation results along with the measured stopping power were utilized to predict the sensitivity results of configuration with different ZnS grain size, weight ratios and fibers geometry (number and location). The simulations enable to optimize the final sensor design. This design successfully achieved both the high gamma rejection with a sensitive and accurate neutron event detection of 80 percent. (authors)

• Research Organization: Institute of Electrical and Electronics Engineers - IEEE, 3 Park Avenue, 17th Floor, New York, N.Y. 10016-5997 (United States)
• OSTI ID: 22531432
• Report Number(s): ANIMMA--2015-IO-43
• Country of Publication: United States
• Language: English

Related Subjects

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
ALPHA PARTICLES
COLD NEUTRONS
CONCENTRATION RATIO
FLUORESCENCE
GRAIN SIZE
LITHIUM 6
LITHIUM FLUORIDES
MEV RANGE
MONTE CARLO METHOD
NEUTRON DETECTORS
NEUTRON DIFFRACTION
NEUTRON DIFFRACTOMETERS
NEUTRON REACTIONS
SCINTILLATION COUNTERS
SENSITIVITY
SENSORS
STOPPING POWER
THERMAL NEUTRONS
TRITONS
ZINC SULFIDES