Target-based experiments are plentiful at CERN, be it at the Antiproton Decelerator, the ISOLDE facility or the North Area. They provide the Laboratory with a variety of secondary particles through the interaction of the target's components with high-energy proton beams from the accelerator complex. One example is the n_TOF (Neutron Time-Of-Flight) facility, where a spallation target is used to produce a neutron beam. After ten years of service, the old n_TOF neutron spallation target was removed and a third-generation target successfully installed in the facility this month. This achievement marks the culmination of four years of development led by the Sources, Targets and Interactions (STI) group in the Systems (SY) department, which is responsible for the operation of the n_TOF facility.
The n_TOF collaboration (numbering more than 120 physicists) hopes to find answers to the questions posed by the processes of nucleosynthesis (how are chemical elements produced outside of nuclear fusion during Big Bang nucleosynthesis and within stars, and what role do neutrons play in this phenomenon?), as well as to much more pragmatic issues, such as nuclear waste disposal. To achieve this, n_TOF scientists work with a high-quality neutron beam produced by the collision of high-energy protons (20 GeV/c, 7 ns wide) from the Proton Synchrotron (PS) with the lead nuclei of the spallation target. The neutrons "knocked" from the target assembly by the proton beam fly towards and collide with experimental samples, after being moderated by water, doped with enriched boron. Their time of flight and the number of decay products allows the calculation of the probability of interaction (cross section). This makes it possible to take unprecedented measurements of isotopes of elements such as osmium, thulium and beryllium, to name just a few, which help to shed light on nucleosynthesis processes.
The old spallation target, a 1.2-tonne water-cooled monolithic lead cylinder, had to retire after ten years of receiving high-energy protons. Its replacement is made up of six separate U-shaped lead blocks - weighing a total of 1.5 tonnes - a new design that offers several logistical advantages. First of all, it allows the beam-heated lead to be cooled with gaseous nitrogen at ambient pressure instead of with water, which will significantly reduce the pollution of the circuit by removing the erosion and corrosion mechanisms induced by the water in direct contact with lead. Secondly, the new target was designed to house an additional demineralised water moderator tank on its top, across one of the two neutron beam tracks. This new moderator tank will improve the resolution of the measurements of a neutron's time of flight in the vertical flight path, a crucial aspect of n_TOF's research. Thirdly, it further improves the physics performance of the facility.
Finally, new modified target shielding was installed in order to provide access to the target area for inspection and operational purposes, as well as to irradiate materials in a field representative of CERN's accelerator systems and evaluate their long-term behaviour within the framework of the Radiation to Materials component of the R2E (Radiation to Electronics) project at CERN. In addition, it would make it possible - if required - to develop an experimental test station much closer to the spallation target than the two existing ones, significantly increasing the measurable number of neutrons per proton pulse. While the construction of this additional experimental station is still under review, the new spallation target leaves n_TOF scientists poised for at least ten more years of world-class neutron research at CERN.
