NEUTRON IMAGING
Please visit our Neutron Detector Products page for information on our neutron detector products.
NOVA Scientific is actively engaged in the development of imaging detectors for neutrons. For radiography, there is increasing interest in the use of neutrons as a complement to X-rays for a number of important applications. This is due to the enhanced sensitivity to light elements such as hydrogen, carbon, and nitrogen, as well as resonance imaging of certain other elements, which cannot be seen with X-rays. For a visual example of neutron versus X-ray radiography, visit the Paul Scherrer Institut web site.
Despite the rapid advance in the past decade of other imaging technologies, due mainly to progress in solid state imagers such as CCDs, neutron imaging has seen only minor improvement in either the resolution or detection efficiencies of existing detector schemes. Because neutrons do not directly ionize atoms, detection schemes must rely on an intermediate conversion step to charged particles or photons. These reaction products then generate a detectable signal - a more involved two-step process which tends to give lower detection efficiencies than would occur with, say, photons, electrons, or ions. For example, in the past the neutron detection efficiency of conventional microchannel plate (MCP) detectors has typically been well below 1% or essentially negligible, regardless of energy. Detection has been further complicated by the presence of gamma ray photons, either within the neutron beam itself or from scattering and radiative capture from surrounding detector materials. Researchers have enhanced the neutron detection efficiency of conventional MCPs through use of special foils (e.g., Gd) at the MCP front surface. However, this has only very limited effectiveness, and is clearly not an intrinsic improvement or advance in the basic detector itself.
NOVA's Approach to Enhanced Detection Efficiency
The approach to improved neutron detection efficiency is structured around the incorporation of neutron-sensitive materials into the base glass of the electron multiplier structure itself, as originally suggested a decade ago by our colleagues Fraser and Pearson at the University of Leicester (UK). The neutron-sensitive materials are selected to greatly enhance neutron capture cross-section within the bulk of the detector, and then the reaction product escape probability is optimized to result in creation of an electron avalanche pulse. This sequence provides for exceptional neutron detection efficiencies leading to unparalleled imaging resolution and counting capabilities. The additions of elements such as 6Li, 10B, 155Gd, and 157Gd are tailored to the glass requirements and to the energy spectrum of the primary neutron beam.
An entire family of neutron to electron multipliers can therefore benefit from this novel approach. Derivatives of MCP technology such as Microsphere Plates (MSPs), and now NOVA's revolutionary new MicroReticulated Plate (MRP) and MicroFiber Plate (MFP) technologies, can be used as powerful neutron detectors, with the MRP/MFP approach perfectly suited to very large format (up to 1 m2) neutron detection and imaging problems. The MicroFiber Plate is capable of being inexpensively manufactured into almost arbitrarily large, arbitrarily shaped formats. (For more information on the MRP or MFP approach, please contact NOVA directly.)
The following figure provides an example of how the neutron detection process works in an MCP. The very well known 10B(n,α) 7Li capture conversion process for thermal neutrons within an MCP channel wall yields a short-range alpha particle, which in turn liberates free secondary electrons into the adjacent evacuated channel. An electron cascade develops along the channel and is amplified by as much as 105 into a detectable signal. The burst of electrons emitted from the very small diameter channels are then electronically registered and processed to construct a digital image. The resultant intensity map or image corresponds to the variation in neutron flux striking the detector surface. Contrast differences within the image of a sample can be used to infer physical and chemical properties. In contrast to film images, electronic images acquired by the MCP technique are linear over a broad scale, and can be post-processed and archived.
NOVA has undertaken extensive development of neutron-sensitive electron multiplier-based detectors, including a number of proprietary neutron-sensitive multiplier glasses. Work on detector construction and hardware modifications has been carried out, followed by comprehensive, ongoing testing at the national NIST Center for Neutron Research (NCNR) of the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. Additional testing for gamma sensitivity has been carried out as well, with the devices repeatedly found to be essentially insensitive to the gamma ray component. Furthermore, neutron pulse counting at megahertz rates over the area of the detector has been demonstrated.
The High Resolution Imaging Features
High resolution imaging requires exceptionally small features in the intrinsic detector structure - here, down to the level of the basic electron multiplier structure. Any electronic readout device which registers the output electron pulse, is in turn required to have exceptionally small features as well. For high resolution, MCPs are the detector of choice, with both MRPs and MFPs better suited for somewhat coarser spatial resolution requirements (~100 microns or greater).
We are presently utilizing 5 micron channels spaced on 6 micron centers for our MCP detectors, which are then coupled to one of several optional readout schemes. A commonly used approach is to accelerate the MCP output electron pulse onto a high quality phosphor screen, and to observe the image on the screen using a video camera focused on a first surface mirror. Since video cameras cannot operate directly in the neutron beam, the camera is placed off-axis, viewing the mirror at 45O. We have successfully used this technique with cold and thermal neutrons in our testing procedures at NIST.
The accompanying figures are neutron radiographs taken with this off-axis viewing setup. The first is a cadmium metal strip that contains 1 mm and 250 micron holes, while the second is a radiograph of a house fly. The measurements made by the MCP-based detector demonstrate that features having a separation of about 30 microns are clearly resolved in the raw images. The theoretical resolution of this configuration is approximately 20 microns, depending on the divergence of the neutron beam and the particular readout used.
Below is a photograph of a No. 2 pencil, and beside it, a neutron radiograph of the pencil for comparison, showing the metal sleeve, rubber eraser, graphite shaft, and wood sheathing. The neutron radiograph clearly delineates the four different materials within the same picture.
 
Software post-processing of the events and images can improve the resolution still further. Furthermore, because the images are collected at 30 frames per second, the detector can easily be used for dynamic processes. Indeed, as MCPs are capable of sub-nanosecond timing, the readout method becomes the limiting factor for speed.
The neutron-sensitive MCPs can be packaged with a high quality phosphor screen and aluminum front window, into a sealed image intensifier. When accompanied with a small power supply, this simple system provides a highly portable and user-friendly approach to neutron radiography, beam monitoring, neutron leak detection, and scientific experimentation.
The Move to Radiation-Hard Readouts
As one electronic readout option, NOVA has already integrated MCP neutron imagers with a Charge Injection Device (CID) camera made by Thermo CIDTEC (Liverpool, NY). CIDs can be read out nondestructively, and are radiation hardened. NOVA has teamed with Thermo CIDTEC to offer an effective high resolution imaging detector for radiography, beam profiling and general reactor usage. One important application would be in the area of fuel cell technology. Since the imager can resolve very small features with excellent contrast - and in real time - one can monitor hydrogen diffusion across very thin membranes and observe liquid movement through porous media.
It is also possible to combine the CID camera with the image intensifier to provide moderate to low resolution imaging over very large areas using MSP or MFP detectors, a large format phosphor screen and lens coupling.
Very Large Neutron Detector Formats
Through the support of the Department of Energy (DOE), NOVA has developed processing options to provide very large imaging detector formats for neutron energies from cold to epithermal. With the larger format, the resolution requirements are typically reduced, with a targeted spatial resolution on the order of 250 microns. However, the total detection area sought approaches 1 m2, or even larger. Imaging detectors meeting such requirements would have immediate application for neutron scattering studies, BNCT medical studies, as well as cargo and container inspection at ports of entry.
NOVA achieves such large formats by combining our proprietary neutron-sensitive glass compositions with highly unique electron multiplier structures using very small microspheres or microfibers. This microsphere plate, or MSP, was originally introduced and commercialized by El-Mul Technologies of Israel. The MSP is constructed by sintering an array of microscopic glass beads or spheres forming an irregularly-packed and porous disk with similar dimensions to that of an MCP. MSP plates operate according to similar principles as MCPs, except for the fact that the electron avalanche develops in the gaps between the spheres and is therefore not laterally constrained.
MSPs have seen limited application for imaging due to spreading of the electron avalanche as it moves through the plate. This moderate resolution of a few hundred microns would, however, be quite acceptable for a number of neutron detection applications. Both MSPs and MFPs could have significant application for neutrons if these approaches could be extended to rather large formats. Ideally such a detector should be capable of being shaped with a cylindrical or hemispherical radius, or even into arbitrary shapes.
NOVA has now established the performance of an enhanced neutron-sensitive MFP plate and a US patent based on this approach has been awarded. Our intermediate scale-up objectives are approximately 250 mm square format with nanosecond temporal resolution. An appropriate and expandable readout system is being matched to the larger areas to digitally image the signal in real time, as well as and permit post-processing and archiving.
Both the MSP and MFP electron multipliers provide a viable scheme to assemble very large format detectors either as single units, or through tiling or assembling a mosaic of smaller area detectors with integrated readout.
Imaging of Fast Neutrons
The energy region of 'fast' neutrons is typically about an MeV and higher, thus the neutron reaction cross-sections of boron, lithium, and gadolinium are much smaller and simply not as effective for detection, as compared with less energetic neutrons. Nature does not cooperate as easily! The detection efficiency of conventional 3He gas filled systems for fast neutrons is also low for the same reason, and the spatial resolution is poor. There appear to be few if any imaging detectors that are effective within this energy range.
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