Abstract
Neutron scatter cameras are a type of directional neutron detectors that rely on multiple neutron scatters to generate images that can reveal the direction and distribution of neutron sources. Fast neutron cameras which have recently been developed rely on multiple detector volumes and make use of neutron time-of-flight measurements. These designs, though effective in localizing the source direction, relies on a large amount of detection and electrical equipment, thus increasing size, cost, and complexity of the systems to unreasonable levels for some applications. This project seeks to develop a compact scatter camera that is less expensive than systems relying on multiple detector volumes. Crucially, two components and capabilities are needed to achieve this: fast scintillation detection materials and picosecond electrical pulse timing. Utilizing such electronics, distinguishing between scintillation light pulses generated by the same neutron within one detector volume is possible. An MCNPX-PoliMi model of such a system has been developed to guide prototype designs. A cube of EJ-230 fast plastic scintillator and six photomultiplier tubes (PMTs) were used to construct the prototype camera that localizes neutron sources based on the principle of cone back projection. Prototypes of the system in one, two, and three dimensions have shown promising initial results when coupled with a script that algorithmically identifies candidate neutron double scatter events and back projects probability cones in the direction of possible sources. Imaging resolution/quality, double scatter efficiency, and cost for the system are quantified. Paths forward for further improvement of a future system based on this camera’ operating principles are discussed.
Highlights
The field of nuclear safeguards seeks to stop the spread of nuclear weapons by developing institutional, legal, and technical mechanisms intended to prevent the misuse of nuclear materials and technology [1,2]
The overall goal of the algorithm used in tandem with the simplified Neutron scatter cameras (NSCs) is to read in voltage pulse arrays produced by the system, determine parameters of the pulses based on their shape, height, timing, and relative prominence, use these parameters to estimate the positions and energy depositions associated with neutron double scatter events, and use these scattering parameters to generate neutron images based on the principle of back-projection
A guess vector can predict a likely direction of a neutron source by comparing the skew in the distribution of opposite photomultiplier tubes (PMTs) light ratios for all events detected in the scintillator volume during the measurement time
Summary
The field of nuclear safeguards seeks to stop the spread of nuclear weapons by developing institutional, legal, and technical mechanisms intended to prevent the misuse of nuclear materials and technology [1,2]. Much of the focus of nuclear material measurement technologies for safeguards is on destructive analysis (DA) and nondestructive assays (NDA) of nuclear material Both DA and NDA techniques seek to determine properties of nuclear material through chemical, spectroscopic, or other means to make quantities known for material and accounting purposes. The kinematic principle involved in determining the original particle trajectory from two consecutive scattering events is similar to the operating principle of Compton cameras, though NSCs use fast neutrons rather than gamma rays [4]. The region of space where the surfaces of the all the back projected cones overlap is interpreted to be the most likely direction of the neutron source Advances in both fast light pulse plastic scintillator materials and sub-nanosecond digitizer have allowed for the construction of compact or single volume scatter cameras with high efficiencies and accurate angular imaging resolutions.
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