Two state-of-the-art instruments, GLAD and COCOTIER, were designed and built at Irfu in the last few years and are now operational in the R3B experimental room of the GSI heavy ion accelerator (Darmstadt, Germany). Both are intended to be part of the equipment that will be used at FAIR, the new machine under construction at the GSI site. GLAD is a large acceptance spectrometer for the analysis of relativistic radioactive heavy ion beam reactions. It was installed on site in 2015 and saw the beam for the first time in the fall of 2018. In some experiments, these beams will have interacted upstream on the COCOTIER liquid hydrogen target. The latter, funded in part by the Agence Nationale de la Recherche, has just been used for the first time in an experiment in March 2021. These two pieces of equipment are key elements for measuring the properties of nuclei at the limit of nuclear stability and allow current nuclear models to evolve towards more predictive ones.
After the successful testing of the cold mass (screen, vacuum chamber) of the magnet, (22 t at 4.5 K) at Saclay in a cryogenic test station of the DACM, GLAD has been installed in its cryostat (its shell) and transported to GSI in October 2015.
For a year and a half, the magnet was installed in one of the experimental halls of the GSI accelerator. It was positioned, connected to its power supply and connected to its refrigerator by GSI teams.
By the end of 2018, the magnet had reached its nominal current, even a little more: 3590 A. One quench (the phenomenon of transition from the superconducting to the resistive state) at 3546 A was enough to reach its nominal operating regime. However, a thermal overload on the busbar (superconducting lines connecting the feet of the current leads to the magnet, in blue on the diagram) required a greater flow of helium to cool it. This led to an excessive consumption of liquid helium and the nominal current could only be maintained for a few hours.
The physics experiments in 2019 were therefore carried out only with a current limited to 2400 A. Although reduced, the level of magnetic field produced was more than sufficient for the first physics experiments.
At the beginning of 2020, the liquefier was improved by adding liquid nitrogen to its first heat exchangers, which significantly increased its performance. It can now fully compensate for the overconsumption of helium by the magnet and allowed in March 2020 to reach the nominal current of the magnet without quenching, and to leave it in operation for 16 hours without any complication. This was not yet achieved after a complete thermal cycling of the magnet.
The Glad spectrometer is now fully operational for the current experimental campaigns that started in March 2021.
After a beam test in the fall of 2018, GLAD was successfully used in the spring 2019, 2020 and 2021 R3B campaigns. Figure 5 shows the Z atomic number identification of the two fission fragments, made possible by GLAD combined with the SOFIA focal plane detectors. This experiment led by J. Taieb (CEA/DAM), J. Benlliure (University of Santiago de Compostela), D. Muecher (University of Guelph) was particularly complex because we used for the first time the COCOTIER target (see the following paragraph)
This liquid hydrogen target is designed to perform quasi-free scattering experiments where the nucleus to be studied, in the form of a beam, impacts on a target of protons that will selectively eject a proton or a neutron from the nucleus in question.
To compensate for the low intensity of the exotic beams, we use dense (hence the need to liquefy hydrogen) and very thick (up to 15 cm) proton targets.
It is therefore necessary to reconstruct the position of the reaction vertex inside the target using a tracking detector. This information is necessary to perform the spectroscopy of the studied nuclei in order to correct the trajectories and the energy loss of the measured particles.
Fig 6 : The target followed by the tracking detectors and surrounded by the CALIFA calorimeter (credits: GSI)
To liquefy hydrogen at pressures close to atmospheric pressure, it must be cooled to cryogenic temperatures (21° K). The hydrogen is liquefied in a condenser cooled by a Gifford-McMahon type cryocooler and flows by gravity into the target cell made of Mylar©. A turbomolecular pumping allows to obtain a high vacuum (10-6 mbar in the cryostat and in the target chamber) in order to limit the convective flows. The principle is similar to that of the MINOS target (see FM 2013), but the integration into the constrained R3B setup posed many challenges that we can guess from Fig. 1. The target in fact is placed in the middle of the CALIFA calorimeter, far from the vertical of the cryostat.
The target cell is wrapped in several 5 µm thick multi-layer insulation sheets in order to reduce the radiation heat flux, especially from the tracking detectors placed at 25 mm in the same reaction chamber and which allow to reconstruct the position of the reaction vertex inside the target (see Fig. 6).
Three target lengths of 15 mm, 50 mm and 150 mm were produced to meet the requirements of the experiments approved by the GSI experiment committee (see Fig. 7)
The system is controlled by the MUSCADE supervision system (developed at CEA/Irfu/DIS) which centralizes the information coming from the programmable logic controller (PLC) and the various controllers. The system allows a connection and a remote piloting via a secured internet client (VPN layers, Dongle, rights, ...). This allowed in particular to perform all filling and monitoring operations of the target remotely, due to the absence of the Irfu team on site because of the pandemic.
The target system was funded in 2017 by a grant from the French Research Agency (ANR JCJC COCOTIER) with the aim of pursuing the study of short-range correlations in exotic nuclei (see actuality "A glimpse of nuclear couple through transparent nuclei"). It was designed and built at CEA/Irfu and installed at the end of 2019 at GSI by the Irfu teams (see Fig. 8).