Of all the protons accelerated at CERN, only 0.1% reach the LHC. Yet up to 60% smack into targets at the ISOLDE facility, creating radioactive isotopes for the many experiments housed there. To maximise the potential of this plethora of protons, scientists and engineers at ISOLDE have been repairing and upgrading the centrepiece of HIE-ISOLDE (High Intensity and Energy Isotope Separator On-Line Device): a compact linear accelerator (linac) for heavy isotopes.
The HIE-ISOLDE linac is a prowess of engineering: in the tight space left between the beamlines that carry the radioactive nuclei to the ISOLDE experiments, ISOLDE engineers have been able to squeeze in an accelerator composed of 20 superconducting radiofrequency cavities grouped into four cryomodules, each cooled at 4.9 kelvins. This accelerator brings the radioactive isotopes to speeds reaching 10% that of light. At this energy, the nuclei can merge or exchange nucleons (protons and neutrons) with the atoms of the experimental targets that they smash into after their race inside the linac. The high energy of the re-accelerated ISOLDE beams, which reached their highest energy in 2018, has led to interesting discoveries, such as the discovery of pear-shaped radium nuclei at HIE-ISOLDE. Embedding these radioactive isotopes into molecules could shed light on physics beyond the Standard Model.
These achievements were made possible by the HIE-ISOLDE machine, despite some technical issues experienced after the last cryo-module had been installed during the 2017–2018 winter break. One of the 20 radiofrequency cavities was found defective after its installation. Thus, up to now, the beam has “only” been accelerated by the remaining 19 cavities. Additionally, some unexplained beam losses hampered the operation of the accelerator to full specifications. The second long shutdown (LS2) provided an opportunity to fix these issues and also to upgrade the electron gun of the charge breeder, which strips some electrons from the isotopes before injection into the linac for more effective acceleration.
Some of you may remember road closures in January 2020 for a truck driving at an average speed of 0.5 km/h. This truck was bringing back to ISOLDE the entire cryomodule containing the problematic cavity that had just been repaired at SM18 with the utmost care. “A cryomodule is like a spaceship: they consist of some 10 000 pieces each, all assembled in a dedicated cleanroom in the SM18 assembly hall. Knowing that the slightest grain of dust or the slightest bump on the road on the way back could ruin the superconductivity or the alignment of the cavities inside the cryomodule, we are extremely cautious when installing these devices in the machine. Once installed, there is no easy way back,” explains Erwin Siesling, technical coordinator for HIE-ISOLDE.
Despite these challenges and after the excruciatingly slow ride, the cryomodule reached HIE-ISOLDE, along with the new diagnostics boxes that have been installed along the path of the old normal conducting linac (REX-ISOLDE), which serves as a pre-injector to the HIE-ISOLDE linac. These new diagnostics boxes have already begun cracking the mystery of the beam losses.
Before protons resume flooding the ISOLDE facility from the Proton Synchrotron Booster, a stable neon beam from an independent source has been injected into the upgraded machines to tune the cavities: so far, everything works like a charm. Armed with the extra energy provided by the repaired cavity and a higher beam intensity thanks to the brand-new electron gun, HIE-ISOLDE is ready to accelerate heavier elements to higher energies to push the frontiers of nuclear physics.