Fish, thermal and cold

In a typical neutron imaging experiment a sample is irradiated with a polychromatic neutron beam. The sample is placed as close as practically possible to a scintillator-based detector, ensuring maximum resolution. The scintillator screen is made up of a ZnS/6LiF material, which converts neutrons into light. This light is directed towards a camera where a  transmission radiograph is recorded. The measurement time for each radiograph can range from 0.5 to 60 seconds, depending on the desired contrast level while ensuring the scintillator screen is not saturated.  

To produce final normalized radiograph of a sample, the raw radiographic is normalized against both open beam and dark current measurements. This process corrects for the beam intensity fluctuations, removes the neutron guide structure and  background noise, resulting in a more accurate representation of the sample's internal structure.
In 3D computed tomography (CT) measurements, the sample is rotated incrementally, typically from 0 to 360°. At each rotational step, a radiograph is captured. Once all radiographs are collected, the 3D structure of the sample is reconstructed using softwares such as OCTOPUS and MuhRec. For visualization and further analysis, a variety of software tools are available, both commercial and open-source, including AVIZO, VG-Studio, Paraview, Dragonfly, and Fiji (ImageJ). These tools provide powerful capabilities for exploring, quantifying and analyzing the reconstructed 3D data. A typical CT measurement can take several hours, depending on the desired resolution and quality, which can be estimated based on Nyquist-Shannon sampling theorem. 

Neutrons offer a unique contrast between elements and isotopes (e. g. Li6 and Li7), combined with high penetration power makes  them highly  complementary to X-rays. While the X-ray contrast significantly increases for heavier elements, neutrons exhibit strong attenuation for light elements such as hydrogen or boron. Many heavy metal objects appear more transparent to neutrons than to X-rays.
Consequently, neutrons are particularly sensitive to light elements like hydrogen, boron, and lithium, and they penetrate metals (e.g., bronze and steel) more effectively than X-rays. The two imaging instruments are designed in complement with each other, therefore exploring these capabilities further. The thermal FISH situated in the reactor hall utilizes a thermal spectrum, making it ideal for thick metal objects, due to its a high penetration depth albeit at the expense of lower contrast. While, the Cold FISH situated in the experimental hall utilizes the cold spectrum, providing higher contrast, but with lower penetration depth.

Key Features / Advantages for neutron imaging

1. Deep Penetration: Neutrons allow the study of bulk and internal structure of materials, and can be combined with complex sample environments like, cryostats, ovens or high pressure equipment
2. Sensitivity to Light Elements: Unlike X-rays, neutrons can be highly sensitive to light elements such as H, Li.
3. Isotope Differentiation: Neutrons can distinguish between different isotopes of the same element (such as, H and D), for instance allowing to “play” with the contrast by replacing H₂O with D₂O.
4. Magnetic Structure Analysis: Neutrons possess a magnetic moment, enabling them to interact with magnetic moments in materials. This unique property allows us to see internal magnetic features, although this needs a special approach with polarized neutrons as a probe.