Neutron Tomography schematic diagram
Neutron imaging has wide industrial and scientific significance and can provide detailed information concerning the inner structure and composition of objects. The principle of neutron imaging is based on the attenuation, through both scattering and absorption, of a directional neutron beam by the matter through which it passes. Since different materials vary in their ability to attenuate neutrons, then both composition and structure can be probed. The neutron beam can be generated either in a reactor, from a neutron emitting isotope or from a target in a proton accelerator. The technique also non-destructive in nature, and has been effectively applied to artefacts of archaeological significance. The neutron imaging technique, rather than being in competition with X-ray imaging, is entirely and ideally complimentary to it. Whereas X-rays are scattered and absorbed by the electrons, and as such atoms with greater electron shells interact more strongly, neutrons on the other hand interact with the atomic nuclei. Furthermore there is no real periodic regularity dictating the degree of this interaction, and even isotopes of the same element may differ markedly in their attenuation ability.Of particular practical significance is the high contrast between hydrogen, which interacts very strongly with neutrons, and most metals, which offer effective transmission of neutrons. This is directly opposed to X-ray imaging, and offers the means to effectively visualize the dynamics of organic hydrogen-containing substances in metal containers, such as the ability to visualize fuel within engines. It can also equally be used to view plastic seals or lubricants embedded within metal structures. The degree of attenuation for some materials can sometimes depend on the neutron energy, fast or thermal neutrons, such is the case with iron.
Neutron Radiography involves placing an object in the path of the neutron beam, and measuring the shadow image of the object that is projected onto a neutron detector, often consisting of scintillator optically coupled to a CCD or EMCCD. Neutron Tomography takes this a step further and entails rotating the sample in the beam and recording multiple 2D images through an angular range of 180°. From the data set, a 3D representation through the object can be constructed.
Andor's iKon-L CCD range is a revolutionary large area CCD platform, offering back illuminated (> 90% QE) full frame sensors up to 4 Mpixel (2k x 2k) and unparalleled priority TE cooling down to -100°C. For neutron imaging we recommend the iKon-L DW436 BV or DZ436 BV. These back-illuminated 2k x 2k cameras combine low noise readout of 2 to 3 electrons rms, > 90% QE, and exceptionally low darkcurrent enabled by Andor's exclusive 5-stage large area Thermoelectric (TE) cooler.
Andor's iKon-M CCD range offers affordable, yet unmatched sensitivity for neutron detection. The iKon-M platform houses a full frame and frame transfer sensors, both front-illuminated and back-illuminated (>90% QE) varieties.For neutron detection we recommend the iKon-M DU-934N-BV. This back-illuminated 1k x 1k camera, with 13μm2 pixels, offers low readout noise of 2 to 3 electrons rms, >90% QE and -100°C TE cooling, a sensitivity performance that remains unmatched in the market.
One of the traditional limitations of CCDs for neutron detection, was that they were useful only for monitoring stationary objects or slow processes, given that the pixel readout rate of standard slow scan CCDs is limited to 1 MHz, in order to reduce readout minimize noise during acquisition. However new state-of-the-art EMCCD detectors are emerging as a means to preserve > 90% QE while offering single photon sensitivity at frame rates of > 30 full frames/sec. This is a significantly improved performance over intensified CCD cameras that exhibit markedly lower QE and are used typically only for measuring dynamic periodic processes where multiple short gates of the intensifier can be integrated on a longer CCD exposure. The dynamic range is also better than ICCDs in that EM gain can readily be tuned to regions of maximal dynamic range. Dynamic range can also be markedly extended through accumulation of multiple rapid exposures, without summation of noise!
EMCCD-enhanced sensitivity and speed opens the door on the ability to follow dynamic processes in real time and to perform faster 3D tomography (or a 4D (3D + time)) approach offering fast tomography of dynamic processes). Despite inherent optical photon losses through the lens coupling of scintillator screen to sensor, these properties make EMCCDs compete favourably with non-cooled "direct detection" flat panel detectors, in terms of enhanced sensitivity, speed, dynamic range and resolution. EMCCDs also have the flexibility to be operated as conventional highly sensitive slow-scan CCD detectors, making them ideally suited to longer exposure measurements also.Furthermore, interline EMCCDs are now available that can readily be adopted for time-gated measurement of periodic (stroboscopic) processes, without intensifier, making use of a microsecond electronic on-chip gating method. Microsecond gating is more than sufficient time gate resolution for stroboscopic neutron imaging, in that time-resolution is ultimately limited by the decay time of the scintillation light of the neutron detection screen. For example, a ZnS:6LiF screen has a decay time of 85μs to 10%.
iXonEM+ and LucaEM imaging EMCCD platforms each displays single photon sensitivity combined with high QE at multi-MHz rapid readout speeds.
Andor's pioneering iXonEM+ is a revolutionary camera range that provides Single Photon Detection sensitivity, highest QE (> 90% available), and –100°C cooling at rapid frame rates, utilizing Andor's pioneering and award-winning EMCCD technology. For neutron imaging, Andor recommends the iXonEM+ DU-897 BV, DU-888 BV or DU-885 VP.
Andor's LucaEM is the latest EMCCD innovation, a highly cost-effective EMCCD technology option. Operate "gain off" for conventional CCD operation under brighter conditions - turn on the EM gain when the photons become scarce.For dynamic neutron imaging, Andor recommends LucaEM DL-658M, providing Single Photon Detection sensitivity and ~ 52% QE at 30 full frames/sec, in a cooled USB 2.0 platform. The interline sensor is ideal for microsecond stroboscopic probing of periodic motion, without the QE and resolution restriction of a photocathode intensifier tube.
QE curves relevant to Neutron Radiography/Tomography
"Mercure from Thalwil"
Images kindly supplied by Eberhard H. Lehmann of the NEUTRA facility.
The figure on the right shows a virtual slice through a famous bronze sculpture “Mercure from Thalwil”, exhibit of the Swiss National Museum, Zurich, Switzerland.
With the help of neutron tomography it becomes possible to study inner structure and casting failures completely non-destructively. Neutrons are required because the commonly used X-ray cannot penetrate the alloy, which contains high amounts of lead.
The figures below show a sprinkler nozzle, which contains a tube filled with liquid.The sealing rings around were inspected in-situ (as installed) for quality assurance and functionality check. Rubber and liquids deliver high contrast for neutrons compared to the metallic structure around.
The figures below show a crab with outer size of 20 cm which was investigated to see the inner organs within the shell. Neutrons have high contrast for organic materials.
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