NOVA Scientific has developed a series of high resolution imaging detector systems for neutrons based upon NOVA’s neutron-sensitive MicroChannel Plate (MCP) technology, in conjunction with Surface Concept in Germany. Neutrons can provide information to scientists and researchers not otherwise measurable using X-rays, due to the neutron’s great advantage over X-rays in having enhanced sensitivity to hydrogen and other light elements. For example, measurements with neutrons can be used in areas where X-rays are simply ineffective, such as in studies of water moving through a fuel cell membrane, for identification of composite materials, or in a wide variety of other important applications (as described in the Applications section).
NOVA’s Neutron-Sensitive MCPs.
NOVA Scientific has been the developer and provider, worldwide, of neutron-sensitive MCP technology, through its patent-protected innovations in MCP materials composition and design. Our advances in this area have resulted in exceptional MCP neutron detection efficiencies, in a revolutionary new type of neutron 2-D detector component having simultaneous high spatial (~10 µm) and timing resolution (~100 ns epithermal), and at very high intrinsic event rates of up to 10 MHz/cm2.
The following animation provides an example of how the neutron detection process works in an MCP. The 10B(n,?) 7Li capture conversion process for thermal neutrons within an MCP channel wall yields short-range (3-4 µm) alpha particles and lithium nuclei, which in turn, through the secondary emitting surface, 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 for each singular event.
Once an electron pulse is triggered, it will be amplified sufficiently within a single microchannel to a level where it can create a detectable light pulse on a phosphor screen and then coupled to a CCD camera, or else generate a measurable charge pulse using a suitable electronic readout (e.g., delay line anode, Medipix/Timepix, cross-grid or cross-strip, etc.). If additional pulse amplitude is required, a common practice is to place a second ‘amplifier’ MCP in tandem with the front neutron-converting MCP. The resulting 2-D intensity map or image will the accurately correspond to variations in the neutron flux striking the detector. Once digitized, images can be post-processed to draw out and enhance information, and can be electronically archived.
The burst of electrons is contained within the very small diameter channels and subsequently registered on a phosphor screen where they can be recorded by camera or 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.
High Resolution Radiographic Imaging Capabilities.
High resolution 2-D imaging generally requires small features to be inherent within the detector structure itself. Any associated readout device which registers the output electron pulse from the main neutron converter, must in turn have exceptionally small features as well. NOVA’s NeuView™ neutron-sensitive MCPs are typically constructed with 8 µm channels spaced on 10 µm centers.
The simplest radiographic system will utilize one or two MCPs proximity-focused onto a phosphor screen – all sealed into an evacuated image tube. A CCD or CMOS camera is then focused onto the phosphor screen through a front-surface mirror, with the camera placed off-axis (thus out of the direct neutron beam), viewing the mirror at a 45o angle.
Pixelated electronic readouts are essentially a matrix of metal anodes in various formats, whereby the image can be registered with either high (20-50 µm) or moderate (100-200 µm) spatial resolution. NOVA has teamed up with Surface Concept in Germany to provide the readout and electronics, to provide a delay line readout system capable of extremely fast time-tagging of individual neutron events, as well as high resolution imaging, simultaneously. This forms the operating basis of our demountable neutron imaging detector product.
A schematic diagram is shown below of the Surface Concept delay line operation.
Sealed image tube readout – Alternatively, when NeuView™ MCPs are to be employed in a vacuum-sealed image tube configuration – so that no active or external ultrahigh vacuum pumping is required – the 2-D delay line readout anode can be externally mounted to the detector itself. In this adaptation, the anode is capacitively-coupled to the output of the MCP, through the rear ceramic wall of the sealed detector. MCP output charge pulses first fall onto a simple germanium film anode coated onto the thin ceramic rear window, and then this 2-D charge pulse ‘footprint’ gets coupled externally onto the delay line anode, which is attached in proximity. This proven readout innovation was invented by particle physicist Schmidt-Bocking in Germany in the 1990’s, and later developed into the so-called Image Charge technique by Jagutzki, Lapington, and others in Europe.
This MCP readout advance, where the readout anode itself is moved outside the vacuum, provides enhanced simplicity and reliability. It eliminates the requirement that the readout be ultrahigh vacuum-compatible, and thus eliminates the previously very challenging cleanliness issues. Sealed tube detector manufacturing is thus simplified, leading to improved yields, and the readout hardware can be easily detached and repaired or modified if so desired.
NOVA’s neutron-sensitized MCPs have a long track record of demonstrating high neutron detection efficiency within the thermal (.025 eV) and cold (~.005 eV) neutron energy ranges, with tests carried out at major neutron facilities worldwide such as PSI (Switzerland), NIST (Maryland, USA), and the Oak Ridge National Lab (Tennessee, USA). Other labs where our MCP neutron detectors were employed in cold and thermal neutron experiments, are Los Alamos National Laboratory in the USA, J-PARC in Japan, the Institut Laue-Langevin in France, Helmholtz-Zentrum Berlin and TU-Munich’s FRMII in Germany, as well as the Rutherford Appleton Laboratory in the UK. Currently, the US Department of Energy is funding our efforts to expand the sensitivity range into the higher energy ‘epithermal’ energy band as well (0.1 eV to 100 eV), where there is increasing interest.
Installed neutron detectors enabled by NOVA MCPs are currently providing ongoing user support in the USA at both the Oak Ridge National Laboratory’s High Flux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS), as well as the NIST Center for Neutron Research (NCNR). In addition, a number of university research reactors have ordered and utilized our neutron detectors and systems.