The neutron beam

Our neutrons are generated by a nuclear reactor, and transported to the instrument via neutron guides coated with neutron-reflective supermirrors.

In order to increase the achievable spatial resolution, the beam is then collimated, i.e., its size is reduced by a pinhole, so that only the neutrons traveling parallel to a specified direction are allowed through (since it is not currently possible to focus radiation with lenses), as sketched below. Filters are often added to modify the energy spectrum of the beam or increase its homogeneity.

The spatial resolution achievable with a beam depends the collimator geometry and is expressed by the L/D ratio, where L is the collimator length (i.e. the distance from the pinhole to the scanned object) and D is the pinhole diameter. The resolution also depends on the distance between the imaged object to the detector l, which should be as small as possible. At a first approximation, the highest gemetrical resolution achievable for a given beamline setup d = (D*l)/L.

Considering that a larger pinhole D and a smaller collimation distance L increase the neutron flux, it is evident that a higher spatial resolution comes at the price of a reduced flux, and therefore termporal resolution (slower acquisiton times).

The beam is then transmitted through the object of interest and measured by a detector, which records a projection of the object on the detector plane.

If multiple images at different angles (radiograms) are acquired, it is possible by means of dedicated algorithms to reconstruct 3D images of the object (tomographies). For parallel beams (for example neutron beams) a 180 degrees range of angles suffices, while for other gemetries (e.g. conic beams as for our microfocus x-ray source) a 360 degrees range is necessary.

Neutron (and x-ray) interaction with matter

The neutron is one of the constituent nucleons of the atomic nucleus (together with the proton). It has no electric charge, but has a magnetic moment and a mass about 1840 times higher than an electron.

Since neutrons are electrically neutral, they interact only weakly with matter, allowing for deeper penetration than x-rays. Unlike x-rays, which interact prevalently with the outer electron shell of the atoms, neutrons interact with their nuclei.

It is therefore unsurprising that the more electrons an atom has (i.e. with increasing atomic number Z), the higher the probability for x-ray to be absorbed. This means that light atoms like hydrogen, oxygen, etc. are relatively invisible to x-rays, while quite visible with neutrons. Conversely, several metals (which highly absorb x-rays) are comparatively transparent to neutrons thus allowing quite high penetration depth, for example in the thick metal casing required to impose extreme thermo-hydro-mechanial conditions.

The high complementarity of these two techniques is further highlighted in the image below, showing indicative neutron and x-ray cross sections (i.e. their contrast in an image, see below) for all the elements on the periodic table.

Unlike for x-rays, neutrons can be captured by nuclei, which get activated and produce instable (mostly short lived) radioactive isotopes. The radioactivity level of the tested samples need therefore to be checked before they can leave the beamline.

One of main differences between neutron and x-ray imaging is their flux and therefore maximum resolution. Thanks to very high brightness of synchrotron sources, in fact, it is possible to achieve very high spatial resolutions (e.g. in the nanometer range) and very fast scans. Also laboratory x-ray sources (µ-focus x-ray tubes) allow for high resolution, as it is possible to take advantage of the diverging geometry of their beam to achieve high resolutions (e.g. 1 µm and below), although the scan speed is in some cases comparable to that of neutron imaging. In neutrons the more modest flux (even at the ILL, which privides the world's highest flux), the parallel geometry of the beam as well as the thickness of the available scintillators, limit the maximum achievable resolution and the scanning speed.

For both neutron and x-ray transmission, the law of attenuation of radiation passing through matter (Beer-Lambert law, sketched below) is generally valid.

The cross-section σ represents the probability of neutrons to interact with matter, which is particularly high for cold neutrons, as in our instrument. Tables for the cross-section of the different elements can be found at the links below. Thanks to the above law it is straightforward to extract quantitative measures of the material composition (e.g. hydrogen content), from radiograms if the sample shape and dimension d are known.

This simple exponential attenuation law does not nonetheless hold in all situations. For example in neutron imaging, thick samples of strongly scattering (e.g. hydrogen) or absorbing materials (e.g. containing boron or gadolinium) deviate from this law because of multiple scattering effects or because of the shift in neutron energy spectrum.

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