Concluding Recommendations  Facilities and Instrumentation: How  European Radioactive Beam Facilities

Special instrumentation

The continuing development of new instrumentation has been of vital importance to nuclear structure physics. Progress in research with exotic radioactive beams will be intimately linked to a continuous collaborative effort of the European laboratories concerned. Such co-ordinated R&D activities will be especially important to the key issues of recoil spectrograph, ion traps and storage devices and ultra-sensitive high-resolution detection systems. For example further progress in nuclear spectroscopy is intimately connected with the availability of high-resolution $\gamma$-ray detection systems. Only Ge-technology can provide suitable detectors for the next decade, although new solid state materials and other technologies, such as liquid Xe detectors, show promise of new, better instruments in the long run. The new 4$\pi$ EUROBALL array, presently installed at LNL Legnaro, constitutes today's state-of-the-art $\gamma$-spectrometer for studies of reactions with high $\gamma$-ray multiplicities. It is optimised for the highest resolving power and its total photopeak efficiency is about e_ph=10%. Evolving from this project, segmented Ge-detectors are currently being developed. Segmented Cluster, segmented Clover and segmented true coaxial detectors will be employed in the MINIBALL, EXOGAM and MARS arrays, respectively, which are dedicated to studies of reactions with low multiplicities, in particular with radioactive beams. Given their low beam intensities maximum efficiency is of prime importance. High granularity of the individual detectors is highly desirable because it allows the correction of the Doppler effects which determine the effective energy resolution in in-beam experiments. At the same time the detectors must be made insensitive to the increased radiation background introduced by the beam or the system must at least be capable of processing and rejecting its effects. All exotic beam facilities are scheduled, from the very beginning, to be used at a European level; collaboration in construction and use of these detection systems, due to their increased complexity, should also be at a European level. These arrays are planned to be available from 1998 on and will provide efficiencies well in excess of e_ph=10 Concerning charged-particle detection, be it stand-alone or ancillary to a photon detection system, highly efficient arrays of detectors which cover most of the relevant solid angle of the reaction space are needed. There are two main areas for design considerations, detectors that surround the immediate reaction target and detectors for forward focused fragments. Ion implanted silicon detectors with thicknesses between 30-1000$\mu$m can now be manufactured with areas as large as 30 cm². Position sensitivity of 10$\mu$m is obtained by segmentation (``strips''). This allows us to select very weak reaction channels. Determining the direction of $\gamma$-emitting nuclei with such highly segmented detectors can be employed to reduce Doppler effects. For very rare events, one powerful technique is to use the 4$\pi$ detectors in coincidence with ejectiles analysed by means of a recoil spectrometer. Neutron detectors are also powerful ancillary systems in connection with neutron emitting reaction channels. They are of vital importance for the study of neutron-rich nuclei, either to provide a trigger or for complete kinematics experiments. For these types of investigation good granularity with high overall efficiency is at a premium. The Franco-Belgian detector DEMON is a good example of such an array, and the LAND detector at GSI presents the state-of-the-art for relativistic neutrons. Complementary detector systems such as a high resolution 2$\pi$ electron-solenoid in conjunction with a 2$\pi$ Ge-shield are also important investments for future nuclear spectroscopy studies. A requirement for many future experiments with rare exotic beams is high beam quality, i.e. low transverse and longitudinal emittance as well as the suppression of contaminants with very similar charge-to-mass ratios. A flexible variable time structure in the beam is also important. For these reasons the development of techniques for bunching, cooling, storing, and purification e.g. with lasers, traps, radiofrequency and buffer gas methods, are of vital importance for future success in nuclear physics and have potential applications in a number of different fields.