NOVA Scientific is actively engaged in ongoing development of high resolution imaging detectors for neutrons. For an increasing number of radiography applications, there is interest in the use of neutrons as a complement to X-rays. This is due to the enhanced sensitivity neutrons can provide 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 comparing neutron versus X-ray radiography, showing the relative elemental cross-sections, visit the NIST Neutron Imaging Facility website.
For neutron radiographic images, visit the Paul Scherrer Institut web site.
NOVA's Approach to Enhanced Detection Efficiency
The approach to improved neutron detection efficiency is structured around incorporation of neutron-sensitive materials into the base glass of the electron multiplier structure itself, as originally suggested during the 1990's 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 real-time imaging resolution and direct digital event registration at high count rates. The additions of elements such as 10B, 155Gd, and 157Gd are tailored to the requirements of the detector glass material 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 NOVA's novel microfiber plate (MFP) approach, can also be exploited as moderate spatial (~100 microns) resolution and high time resolution neutron detectors. Utilizing the MSP/MFP approach, much larger format sizes (100 to 1000 cm2) are possible for neutron detection and imaging applications. The MFP is capable of being inexpensively manufactured into large, even arbitrarily shaped formats. (For more information on the MFP approach, please contact NOVA directly.)
The following animation 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 short-range (3-4 microns) alpha particles and lithium nuclei, which in turn liberate free secondary electrons into the adjacent evacuated channel. An electron cascade develops along the channel and can be amplified by as much as 106 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.
The High Resolution Imaging Capabilities
High resolution imaging requires correspondingly small features in the intrinsic detector structure - here, down to the level of the basic electron multiplier itself. 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).
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 electronic readout schemes. Initially, the most expedient approach was to accelerate the MCP output electron pulse onto a phosphor screen, and to observe the image on the screen using a CCD camera focused onto 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 in the past with cold and thermal neutrons in our testing procedures at NIST.
The accompanying figures are neutron radiographs taken with this initial 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 FWHM are clearly resolved in the raw images. The limiting resolution of this configuration obviously depends upon the divergence of the particular neutron beam employed.
Software post-processing of the events and images can enhance the information gained. 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.
Electronic Readout Technology
Our more recent approach is to replace the phosphor screen with an electronic readout permitting exceptionally high resolution radiography for fuel cell evaluation, biological specimens, low flux measurements, etc. NOVA has worked with several different versions of readouts including cross delay-line anodes and cross-strip anodes available from our colleagues at Sensor Sciences (Walnut Creek, CA), as well as Medipix II, a development of the CERN consortium including our colleagues at University of California-Berkeley Space Sciences Lab. Each of these is basically a variation of small pixilated metal anodes and format sizes whereby the image can be determined with very high resolution. For the cross strip-approach the counting rates can be >1MHz at very high spatial resolution, and for the Medipix, target count rates are > 1 GHz, but al somewhat lower resolution of 55 microns. Moreover, imaging measurements can be carried out in real-time.
An example of the Medipix neutron imaging is shown below of a Gd mask having 50 µm pinholes in a regular pattern. This image was taken at the HFIR facilities at Oak Ridge National Laboratory (ORNL) with a cold beam (4.75 meV) and a beam flux of 105 η/cm2/sec.
Imaging at high count rates can be employed to evaluate differing materials. Shown following is a radiographic image at ORNL of nylon washers, polyetheretherketone (PEEK) screws, and stainless steel screws. Note the comparison of the stainless image to that of the hydrogen-containing materials.
A comparison of cold and thermal beam imaging is shown for measurements at the Paul Scherrer Institute (PSI) in Switzerland. Note the improved contrast in the cold beam over that of the thermal beam. Here the detection efficiency is higher the lower the neutron energy.
A radiograph was taken of a watch, both visually and with the cold beam at PSI. One can clearly distinguish very minute features within the watch structure.
For a detailed description of the radiographic imaging capabilities and procedures please visit the publications section. Several of the published papers are directly available by clicking on the citation.
NOVA Scientific has several neutron radiographic systems operating at Universities and NIST. Present development is targeting 10 to 15 µm resolution with high (MHz) count rates.
