Complementing X-ray imaging with Neutron Radiography
Neutron Radiography (NR) is a useful non-destructive imaging system that uses thermal energy neutrons to probe the internal sections of various materials. NR is a contrast to X-ray imaging as unlike X-rays, neutrons only interact with atomic nuclei.
This difference in interaction means the attenuation pattern of thermal neutrons is different as X-rays are attenuated solely based on material density. Denser materials are able to stop more X-rays passing through. With neutrons, the sample’s ability to attenuate them is not related to density.
Light materials such as boron absorb neutrons, hence boron’s use in nuclear reactor control rods. Hydrogen will also normally scatter neutrons, and many common metals allow neutrons to pass through their structure. Because neutrons interact with materials in a different way, neutron imaging can be regarded as complementary to X-ray imaging.
Since various materials have different attenuation behaviours the neutron beam passing through a sample can be interpreted as signal carrying data about its composition and structure. In a simple way you can X-ray an iron casting and see a radiopaque shape but a neutron image could well give details of gas bubbles or micro-cracks inside the iron.
A useful imaging technique used more and more is computed tomography, which applies to neutron as well as X-ray imaging. This technique allows the user acquire proportional 3D data on the structure in a sample. By using radiographic projections from many views, the method effectively takes image slices through the sample which are reconstructed by a computer. The resulting images can then show the distribution of materials in a sample in a 3D fashion.
Neutron imaging applications
Neutron radiography and tomography (3D radiographic imaging) are highly useful techniques for examining the inside of metal castings to check for voids and weakness. They are also good general techniques to determine the integrity of internal areas that cannot be exposed without destroying the item.
A good example is inspecting a rubber component O-ring for position and integrity inside a metal component. Neutron radiography can also be used to examine complex processes, for example, tracing the propagation of minute quantities of hydrogen containing compounds within metal castings such as an internal combustion engine or even rock and soil matrices.
In another example, a partially water-saturated compacted silica sand with two different grain morphologies was examined. The dual imaging method showed an improved ability to distinguish solid silica, liquid water, and gas phases and to obtain void ratio, void percentage variation, and particle size distribution data.
Short exposure neutron radiographs can be used to follow rapid periodic processes. Some examples are examining water flow in an operating proton exchange membrane fuel cell, measuring the lithium ion diffusion coefficients in Li-containing electrodes, imaging fission neutrons in special nuclear materials.
Dynamic processes can pose a challenge to the acquisition of tomographic data. This is because a sample is usually not allowed to change during the scan as this might cause motion artefacts. However, by acquiring projections in a set order, it is still possible to obtain good spatio-temporal data from a sample that changes during a scan.
Most neutron radiography uses thermal neutrons, which are defined as neutrons with energy of about 0.025eV. There are two reasons for using thermal neutrons. Firstly, neutrons within this energy range exhibit the most useful attenuation. Secondly, thermal neutrons are easily obtained by moderation.
Using neutrons from point sources, e.g. a nuclear reactor, usually means they have a higher energy than thermal neutrons and diverge in direction. It is therefore necessary to slow down fast neutrons and collimate the neutron beam to generate a sharp radiograph with high resolution.
Obtaining neutron-based images
Detectors for Neutron Imaging (NI) are those able to measure the neutron field in two dimensions perpendicular to the beam direction. The detector area needs to be in the order or larger than the beam cross-section. Also, boundary conditions include the spatial and time dependent resolution of the detector, which can be very different among detector systems.
Charge-coupled device (CCDs) are generally used as imaging cameras for neutron tomography. The Andor scientific range of imaging CCD cameras, e.g. the iKon L-936, are ideal with their extremely low noise, -100 °C cooling and highest QE delivering optimal performance.
However, a limitation of CCDs for some neutron detection applications, such as dynamic processes in real time provides an effective readout speed of only 3-5 MHz. This is extremely effective for monitoring stationary objects or slow processes but for the faster framing requirements, or to perform faster 3D tomography other cameras/detectors are available.
The Zyla from Andor is a camera that delivers a low noise, a large target field of view and 100 full frames per second and faster rates for smaller regions of interest
If the application also requires single photon sensitivity, then an electron-multiplying charge coupled device (EMCCD) detector should be selected such as the iXon Ultra 888 with its fast 30 MHz and 26 fps readout capability. EMCCD detectors generally offer >90% QE, as well as single photon sensitivity at frame rates of around 30 fps. EMCCD enhanced sensitivity allows faster 3D tomography and overcomes the inherent optical losses of the system.
Although up to now neutron and X-ray imaging have been used as separate entities there are instances where they have been combined in a hybrid fashion for a new dual-modal contrast imaging technique. Neutron tomography and X-ray images are processed and combined to provide a new insight. To do this, Andor Technologies provides the ideal cameras to solve any of your applicational needs and requirements.
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