X-ray imaging is a widely used technique to image the interior of materials – anyone who has had their teeth or another part of their body X-rayed will be familiar with the images it produces. Less used is neutron imaging, which is better than X-ray imaging in some cases, for example imaging the interior of dense metals. The reason is that neutron beams that are intense enough for imaging are not easy to produce and are available at only a few facilities worldwide.
The n_TOF facility at CERN has two intense neutron beams and normally uses them to study interactions between neutrons and atomic nuclei. However, the facility has recently started to explore the feasibility of also using one of its beams for imaging. And the first results from this exploration look good: imaging of particle-producing targets that have been used or are designed to be used at the neighbouring Antiproton Decelerator (AD) to produce antiprotons (the antiparticles of protons) has shown that the beam can reveal the samples’ internal structure.
Neutron imaging is based on recording the attenuation of a neutron beam as it passes through a sample. The quality of the resulting image depends on several factors, including the energy of the neutrons at the sample’s position and the distance between the sample and the collimator that focuses the beam. Using a commercially available neutron-imaging camera, the n_TOF researchers set up a neutron-imaging station at n_TOF and analysed some of these factors. They then set out to test the imaging station with five antiproton-producing targets: two targets from the AD, which produces antiprotons by taking an intense proton beam from the Proton Synchrotron accelerator and firing it into a target made of dense metal; and three potential new targets for the AD that had previously been tested at the HiRadMat facility.
One of the two AD targets was a spare, never used, whereas the other AD target and the three HiRadMat targets had been subjected to intense proton beams that could have damaged them. The n_TOF imaging of the targets showed their internal structure with good contrast and, in the case of the targets that had been exposed to proton beams, revealed deformation, bending or cracking of their interior. For two of the targets, the damage observed was confirmed by opening the target, and for one of these targets the damage was also confirmed by imaging at a neutron-imaging station at the Paul Scherrer Institute.
Neutron image of one of the AD targets studied, showing damage to the target’s core (uneven boundaries of the thin black strip). Neither the damage nor the core itself can be seen with X-ray imaging (Image: n_TOF collaboration)
The results served two purposes: they demonstrated the feasibility of using n_TOF’s neutron beam for imaging and they offered two-dimensional images of the inside of the antiproton-producing targets that would otherwise have been more difficult to obtain. Conventional imaging techniques such as X-ray imaging cannot penetrate the dense metals from which the targets are made to reveal their internal state and, if they were to be imaged with specialised imaging facilities outside of CERN, the targets would need to be transported and subjected to inspection before being handled.
The next steps towards developing a full-fledged imaging station at n_TOF include improving the collimation system, which would lead to higher-resolution images, and adding equipment that would allow three-dimensional rather than two-dimensional images to be obtained.