1. INTRODUCTION
The HIE-ISOLDE facility at CERN accelerates over 1000 isotopes from around 70 elements, reaching collision energies up to approximately 10 MeV/A, making it an excellent platform for exploring nuclear theories through optimal (N, Z) combinations. The ISRS collaboration has proposed an innovative high-resolution recoil separator, the “ISOLDE Superconducting Recoil Separator” (ISRS), aiming to expand the physics program to study more exotic isotopes generated in the secondary target. This involves combining focal plane spectroscopy with particle and photon detection using various detectors at HIE-ISOLDE.
The aim of the ISRS Collaboration is to develop a Proof-of-Concept study as described in the LOI-INTC-228 (2021). The R&D activity will cover the beam dynamics, buncher, magnet and cryogenics design, injection/extraction system, test bench, physics detectors and prototyping. This study includes a review of the physics requirements, and the delivery of a Whitebook. A summary of the tasks is given in the Appendix 2 of the LoI.
2. PHYSICS PROGRAM
Nuclear structure of exotic nuclei can be investigated at HIE-ISOLDE by several reaction mechanisms that will benefit from the use of the recoil separator ISRS. Transfer reactions in inverse kinematics have high cross sections at HIE-ISOLDE beam energies. The angular distribution of the cross-sections is very sensitive to the details of the nuclear wave-functions, such as energy, angular momenta, and the spectroscopic factors. The evolution of the nuclear structure along the nuclear chart can be used to test theoretical models, but also to probe reactions relevant for nucleosynthesis around the important region of closed shells N ≈ 82 and N ≈ 126. Multinucleon transfer processes through deep inelastic, quasi-elastic, and quasi-fission reactions can populate nuclear levels of interest in exotic nuclei, including regions near the drip line around 78Ni and the N = 126 closed-shell region, which are crucial for studying shell-quenching and the astrophysical r-process. Charge-exchange reactions enable the investigation of spin and isospin excitations, the structure of particle-hole states, and scalar and isovector components of nuclear matrix elements relevant for nuclear beta decay, thus connecting the physics of strong and weak interactions. On the other hand, Fusion-evaporation reactions can yield very exotic residues, providing an opportunity to perform lifetime measurements using plungers. Both direct and inverse kinematics with light or heavy targets can be carried out, which necessitates the spectrometer’s feature to rotate around the target position, so that to cover a range from zero degree to typical heavy-ion grazing angles (∼50° – 70° Lab), which are relevant for e.g., multi-nucleon transfer reactions.
3. CONCEPTUAL DESIGN
The ISRS spectrometer design departs from traditional linear magnet arrays, addressing limitations like drift lengths and dispersive plane constraints. It avoids issues such as iron-induced nonlinearities, hysteresis, magnetization, and ohmic losses. Instead of dispersive planes, it employs a unique approach involving a particle storage system with iron-free superconducting multifunction magnets (SCMF) cooled by cryocoolers in a compact storage mini-ring using Fixed Field Alternating Gradient focusing (FFAG).
The spectrometer layout includes four corners, each bending the beam 90°. They are composed of multifunction (nested) iron-free Canted Cosine Theta (CCT) magnets, either a single unit or split in few straight sections, with dipole and quadrupole functions (sextuple corrections are also considered), a set of quadrupoles for focusing, an injection/extraction system, and a focal plane detector. The reduced iron content eliminates non-linearities and hysteresis, leading to a significantly lighter and more compact design than traditional spectrometers, while maintaining high resolving power and efficiency.
The injection of the HIE-ISOLDE beam into the ISRS ring requires a compact bunch structure, so a Multi-Harmonic Buncher device is proposed for this task. The MHB will operate at a frequency of 10.128 MHz, which is a 10% of the linac frequency, and would be installed before the RFQ.The MHB is desgined as a two electrodes system, and the MHB signal, composed for the first four harmonics of the fundamental frequency, is fed into the electrodes that are connected to the central conductor of a coaxial waveguides. The design of the MHB includes electromagnetic optimization of the electrode shape, optimization of the weights of each of the harmonic contribution, mechanical and thermal design of the structure, and the RF generation and electronics to power up the device.
