Wondwosen Mengesha

Experimental Results from an Antineutrino Detector for Cooperative Monitoring of Nuclear Reactors

Our collaboration has designed, installed, and operated a compact antineutrino detector at a nuclear power station, for the purpose of monitoring the power and plutonium content of the reactor core. This paper focuses on the basic properties and performance of the detector. We describe the site, the reactor source, and the detector, and provide data that clearly show the expected antineutrino signal. Our data and experience demonstrate that it is possible to operate a simple, relatively small, antineutrino detector near a reactor, in a non-intrusive and unattended mode for months to years at a time, from outside the reactor containment, with no disruption of day-to-day operations at the reactor site. This unique real-time cooperative monitoring capability may be of interest for the International Atomic Energy Agency (IAEA) reactor safeguards program and similar regimes.

Improved fast neutron spectroscopy via detector segmentation

Organic scintillators are widely used for fast neutron detection and spectroscopy. Several effects complicate the interpretation of results from detectors based upon these materials. First, fast neutrons will often leave a detector before depositing all of their energy within it. Second, fast neutrons will typically scatter several times within a detector, and there is a non-proportional relationship between the energy of, and the scintillation light produced by, each individual scatter; therefore, there is not a deterministic relationship between the scintillation light observed and the neutron energy deposited. Here we demonstrate a hardware technique for reducing both of these effects. Use of a segmented detector allows for the event-by-event correction of the light yield non-proportionality and for the preferential selection of events with near-complete energy deposition, since these will typically have high segment multiplicities.

Distance Dependent Quenching and Gamma-Ray Spectroscopy in Tin-Loaded Polystyrene Scintillators

In this work, we report the synthesis and inclusion of rationally designed organotin compounds in polystyrene matrices as a route towards plastic scintillators capable of gamma-ray spectroscopy. Tin loading ratios of up to 15% w/w have been incorporated, resulting in photopeak energy resolution values as low as 10.9% for 662 keV gamma-rays. Scintillator constituents were selected based upon a previously reported distance-dependent quenching mechanism. Data obtained using UV-Vis and photoluminescence measurements are consistent with this phenomenon and are correlated with the steric and electronic properties of the respective organotin complexes. We also report fast scintillation decay behavior that is comparable to the quenched scintillators 0.5% trans-stilbene doped bibenzyl and the commercial plastic scintillator BC-422Q-1%. These observations are discussed in the context of practical considerations such as optical transparency, ease-of-preparation/scale-up, and total scintillator cost.

Lithium alkali halides - New thermal neutron detectors with n-γ discrimination

We are investigating two promising new families of materials derived from proven, low-cost, well-understood, versatile scintillator hosts CsI and NaI. These are modified by incorporating Li ions to achieve the desired spectroscopic properties, producing high quality, combined n/γ sensors. Specifically we report on the synthesis and characterization of Li3Cs2I5 (LCI) and LixNa1-xI (LNI), both of which demonstrate pulse shape discrimination (PSD) and pulse height discrimination (PHD) for effective suppression of gamma background from neutron signals. In the case of LCI, the primary decay time for thermal neutron interactions is faster than for gamma interactions, and is on the order of 250 ns for neutrons and 500 ns for gamma rays. LNI is opposite to LCI in this respect, which shows slower, 210 ns, decay for neutron interactions and relatively faster, 180 ns, decay for gammas. The measured light yield for LCI is ~40,000 to 55,000 photons/neutron, which corresponds to an electron equivalent energy of 2.1 to 2.8 MeV. Whereas LNI demonstrates a much brighter yield of over 100,000 photons/neutron and electron equivalent energy per neutron interaction of over 4.5 MeV, very close to the 6Li(n,α) Q value of 4.7 MeV. Relatively narrow emission bands with a peak at 450 nm for LCI:Eu and at 420 nm for LNI:Tl make these sensors well matched to the quantum efficiency of conventional photodetectors such as PMTs. Our data show that significant red shift in emission can be achieved by doping LCI with Tl and LNI with Eu, making these materials well suited for use with such sensors as solid state photomultipliers (SSPMs). In addition to their excellent scintillation properties, the use of widely available, proven host materials, and a possible vapor deposition method for their synthesis, are promising features of this development. This combination allows mass production of large-area, high-performance neutron sensors in a uniquely time efficient manner.

Modeling of a Directional Scintillating Fiber Detector for 14 MeV Neutrons

The GEANT4 Monte Carlo simulation tool was used to model a prototype 14 MeV neutron fiber detector. Detail features of the prototype were implemented in the modeling to assess the directionality and detector performance. The prototype was built using plastic fibers consisting of a core scintillating material and an acrylic outer cladding. A total of 64 square fibers were used in parallel with a fiber pitch of 2.3 mm. Recoil protons scattered by an incident mono-directional 14 MeV neutrons were tracked to enable reconstruction of a two-dimensional (2-D) direction of the incident neutrons. Simple kinematics of neutron scattering together with a backprojection technique was implemented. Reconstructed direction has a peak with a full width at half maximum (FWHM) of 10degrees and rests on a pedestal of uniform counts. Results from the present GEANT4 simulation have demonstrated promising directionality of a prototype scintillating fiber detector

Fast neutron resonance tomography using double scatter spectroscopy for materials identification

Fast neutron based inspection systems are of interest in many Homeland Security applications because they offer the potential for elemental identification particularly for low Z elements which are the prime constituents of explosives. We are investigating a resonance tomography technique which may address some of the current problems found in fast neutron based inspection systems. A commercial off-the-shelf DT generator is used with an array of detectors to probe materials simultaneously over a large energy range and multiple viewing angles allowing for simultaneous 3-D imaging and materials identification. A prototype system has been constructed and we present here recent results for the identification of materials with differing H, C, N, O compositions.

Pixellated NaI (Tl) Detector for Light Yield Nonproportionality Investigation

An 8 × 8 pixellated Nal(Tl) detector (1.7 cm × 1.7 cm × 1.0 cm) coupled to a multi anode PMT read out system was modeled to study the impact of light yield nonproportionality due to multiple photon scatterings and assess the contribution of delta rays in Nal(Tl) energy resolution. Geant4 simulation tool with modified nonproportional optical photons sampling routine was used in the study. A deconvolution technique was implemented to allow decoupling of the multiple photon scatterings contribution from the total energy resolution. Results from the simulation demonstrated that the effect of nonproportionality due to multiple photon scatterings is too small to be observed. However, the contribution of the delta rays was found to be a significant component of the evaluated Nal(Tl) energy resolution.

14-MeV neutron directional fiber detector modeling using GEANT

GEANT4 Monte Carlo simulation tool was used to model a prototype 14 MeV neutron fiber detector under development at the Sandia National Laboratories (SNL). Detail geometric design features of the prototype fiber detector were implemented in the modeling to assess directionality and performance of the detector. BCF-12, plastic fiber material, produced by Saint-Gobain, was used in the prototype development. The fiber consists of a core scintillating material of polystyrene with 0.48 mm times 0.48 mm dimension and an acrylic outer cladding of 0.02 mm thickness. A total of 64 square fibers, each with a cross-sectional area of 0.25 mm2 and length of 100 mm positioned parallel with a fiber pitch of 2.3 mm, were used in the tracking of 14-MeV neutron induced recoil proton (n-p) events. Neutron induced recoil proton events, resulting energy deposition in two collinear fibers, were considered in reconstructing a two dimensional (2D) direction of incident neutrons. Energy resolution of the fiber detector was also considered to account uncertainty in direction reconstruction. Reconstructed direction has a limiting angular resolution of 3deg due to fiber dimension. Energy resolution of the fiber, which was estimated to be 10% at 14 MeV proton energy, resulted in further broadening of the reconstructed direction and the simulated angular resolution was 20deg. These values were determined when incident neutron beam makes an angle of 45 degrees relative to the front surface of the detector. Comparable values were obtained at other angles of incidence. Results from the present simulation have demonstrated promising directionality of the scintillating fiber detector under development

Performance and design study of a directional scintillating fiber detector for 14-MeV neutrons using GEANT

A directional scintillating fiber detector for 14-MeV neutrons was simulated using the GEANT4 Monte Carlo simulation tool. Detail design aspects of a prototype 14 MeV neutron fiber detector under development were used in the simulation to assess performance and design features of the detector. Saint-Gobain produced, BCF-12, plastic fiber material was used in the prototype development. The fiber consists of a core scintillating material of polystyrene with 0.48 mm × 0.48 mm dimension and an acrylic outer cladding of 0.02 mm thickness. A total of 64 square fibers, each with a cross-sectional area of 0.25 mm2 and length of 100 mm were positioned parallel to each other with a spacing of 2.3 mm (fiber pitch) in the tracking of 14-MeV neutron induced recoil proton (n-p) events. Neutron induced recoil proton events, resulting energy deposition in two collinear fibers, were used in reconstructing a two dimensional (2D) direction of incident neutrons. Blurring of recoil protons signal in measurements was also considered to account uncertainty in direction reconstruction. Reconstructed direction has a limiting angular resolution of 3° due to fiber dimension. Blurring the recoil proton energy resulted in further broadening of the reconstructed direction and the angular resolution was 20°. These values were determined when incident neutron beam makes an angle of 45 degree relative to the front surface of the detector. Comparable values were obtained at other angles of incidence. Results from the present simulation have demonstrated promising directional sensitivity of the scintillating fiber detector under development.

Directional neutron detectors for use with 14 MeV neutrons :fiber scintillation methods for directional neutron detection.

Current Joint Test Assembly (JTA) neutron monitors rely on knock-on proton type detectors that are susceptible to X-rays and low energy gamma rays. We investigated two novel plastic scintillating fiber directional neutron detector prototypes. One prototype used a fiber selected such that the fiber width was less than 2.1mm which is the range of a proton in plastic. The difference in the distribution of recoil proton energy deposited in the fiber was used to determine the incident neutron direction. The second prototype measured both the recoil proton energy and direction. The neutron direction was determined from the kinematics of single neutron-proton scatters. This report describes the development and performance of these detectors.