Detection of neutrons, at high total efficiency, with greater resolution in kinetic energy, time and/or real-space position, is fundamental to the advance of subfields within nuclear medicine, high-energy physics, non-proliferation of special nuclear materials, astrophysics, structural biology and chemistry, magnetism and nuclear energy. Clever indirect-conversion geometries, interaction/transport calculations and modern processing methods for silicon and gallium arsenide allow for the realization of moderate- to high-efficiency neutron detectors as a result of low defect concentrations, tuned reaction product ranges, enhanced effective omnidirectional cross sections and reduced electron–hole pair recombination from more physically abrupt and electronically engineered interfaces. Conversely, semiconductors with high neutron cross sections and unique transduction mechanisms capable of achieving very high total efficiency are gaining greater recognition despite the relative immaturity of their growth, lithographic processing and electronic structure understanding. This review focuses on advances and challenges in charged-particle-based device geometries, materials and associated mechanisms for direct and indirect transduction of thermal to fast neutrons within the context of application. Calorimetry- and radioluminescence-based intermediate processes in the solid state are not included.

https://iopscience.iop.org/article/10.1088/0953-8984/22/44/443201/pdf

The physics of solid-state neutron detector materials and geometries

Neutron imaging is a highly successful experimental technique ever since adequate neutron sources were available. In general, neutron imaging is performed with a wide wavelength spectrum for best flux conditions in transmission geometry. Neutrons provide outstanding features in the penetration of many structural materials, which often makes them more suited for bulk sample studies than other forms of radiation, often in particular as they are also highly sensitive to some light elements, especially Hydrogen. In contrast to neutron scattering applications, imaging resolves macroscopic structures, nowadays down to, in the best case, below 10 micrometre, directly in real space. However, since more than a decade there is a growing number of techniques and applications in neutron imaging that – supported by powerful neutron sources – are taking advantage of wavelength resolved measurements. In this review we summarize and discuss this outstanding development and how wavelength resolved transmission neutron imaging is successfully exploiting diffraction mechanisms to access crystal structure information in the Angstrom regime, which conventionally is probed in reciprocal space by diffraction techniques. In particular the combination of information gained in real space and on crystallographic length scales makes this neutron imaging technique a valuable tool for a wide range of new applications, while it also qualifies neutron imaging to fully profit from the new generation of powerful pulsed neutron sources.

Diffraction in neutron imaging—A review

Conventional neutron radiography can be strongly enhanced by obtaining Bragg-edge information spatially correlated with the attenuation coefficient. This can now be achieved through time-of-flight techniques at pulsed neutron sources, utilising a neutron counting detector with high-spatial and high-temporal resolution. In these measurements, the positions of Bragg edges can in principle be obtained for each 55 × 55 μm2 pixel of the radiographic image. The combination of both Bragg-edge and attenuation information enables high spatial resolution studies to be carried out on material composition, phase transitions, texture variations, as well as residual strain mapping. In this article, we present the results of high-resolution strain maps of a ferritic steel cantilever sample measured at different loads by both transmission and conventional diffraction modes, as well as strains in an austenitic steel compact-tension (CT) crack sample. The proof of principle experiments performed on the ENGIN-X beamline on a bent cantilever arrangement resulting in a uni-axial stress field verified that the strain values measured in diffraction and transmission mode are in good agreement. The characteristics of the transmission mode detector as well as the measured strain maps and future possibilities of this technology are discussed.

High-Resolution Strain Mapping Through Time-of-Flight Neutron Transmission Diffraction with a Microchannel Plate Neutron Counting Detector

Neutron counting detectors with boron or gadolinium doped microchannel plates (MCPs) have very high detection efficiency, spatial and temporal resolution, and have a very low readout noise. In this paper we present the results of both theoretical predictions and experimental evaluations of detection efficiency and spatial resolution measured at cold and thermal neutron beamlines. The quantum detection efficiency of a detector (not fully optimized) was measured to be 43% and 16% for the cold and thermal beamlines, respectively. The experiments also demonstrate that the spatial resolution can be better than 15 μm—highest achievable with the particular MCP pore dimension used in the experiment, although more electronics development is required in order to increase the counting rate capabilities of those

Detection efficiency, spatial and timing resolution of thermal and cold neutron counting MCP detectors

Due to the absence of efficient neutron focusing optics the spatial resolution needed for neutron imaging requires either a high-resolution detector, as in the case of direct projection radiography, or else a bright point source as in the case of magnification imaging geometry. The very limited neutron flux in the latter makes it unacceptably slow and thus puts more emphasis on improving the detector resolution. In this paper, we present the first experimental evidence of imaging both thermal and cold neutrons with a spatial accuracy better than 15 μm FWHM. This resolution is possible due to some unique features of neutron sensitive microchannel plates (MCPs). The position of the neutron capture event can be determined with high accuracy because the event is contained within a single or two channels of the MCP. The products of neuron capture have ranges of only a few microns in the MCP glass. The channel has a typical diameter of 6–10 μm and the inter-channel wall thickness averages 2–3 μm. Once the neutron capture initiates an electron avalanche in the MCP, the event position encoding is identical with photon/charged particle detection, for which the MCP technology has been widely used for several decades. Images demonstrating single channel resolution were taken using cold neutrons incident on a Gd pinhole mask and one with 20–50 μm B4C particles. The proof-of-principle images were obtained at a low count rate limited by the present version of the readout electronics. Development of a new fast readout schemes is an active area of research and the rate capability of this detector should benefit from these efforts.

On the possibility to image thermal and cold neutron with sub-15 μm spatial resolution

Microchannel plate (MCP) event-counting imaging detectors with very high spatial resolution ( ∼ 10 μ m ) and timing accuracy ( ∼ 100 ps ) are widely employed for the detection and imaging applications of electrons and ions, as well as UV and X-ray photons. Recently, it was demonstrated that the many advantages of MCPs are also applicable to neutron detection with high two-dimensional spatial resolution. Boron, enriched in the isotope 10B, was added to the MCP glass structure to enhance the neutron interaction within the MCP through the B 10 ( n , α ) 7 Li reaction. The energetic charged particle reaction products release secondary electrons directly into MCP channels, initiating an electron avalanche and a subsequent strong output pulse. In this paper we present a detailed model for calculating the quantum detection efficiency of MCP neutron detectors incorporating 10B, for the specific case of square channel MCP geometry. This model predicts that for thermal neutrons (0.025 eV), MCP detection efficiencies of up to 78% are possible using square channels. We also show theoretically that square channel MCPs should have a very sharp ( ∼ 17 mrad ) angular drop in sensitivity for detection of normal incidence neutrons, opening up new possibilities for angle-sensitive neutron imaging as well as collimation. The calculations can be used to optimize MCP neutron detection efficiency for a variety of applications. In a subsequent companion paper, the model will be extended to the case of hexagonally packed circular channels.

/pdf/efficiency-optimization-of-microchannel-plate-mcp-neutron-3m85b2nrdl.pdf

R. Gregory Downing

Papers

Efficiency optimization of microchannel plate (MCP) neutron imaging detectors. I. Square channels with 10B doping