Introduction

The complexity of experiments that can be attempted using nuclear beams is a function of the beam intensity that can be obtained. Using stable beams, with increasing complexity in the apparatus, the selection of very rare reaction products has been pursued and it has been shown at this conference how recoil decay tagging has taken this to new levels. For example, studies in the neutron deficient Pb-Po-Rn region can access nuclei produced with cross sections of order $\sigma = 0.1~\mu$b, using typical experimental parameters and beams of order $6 \times 10^{10}$ particles/sec (pps), i.e. 10 pnA. With a radioactive beam of an interesting isotope, quite removed from stability, it is realistic to suppose that a beam of order 10^-6 of this intensity could be produced at present. Thus, reactions of order 0.1 b can be considered, and if a highly efficient detection system were developed then it could be supposed that a cross section of 10 mb would be accessible. These are the typical magnitudes of nucleon transfer cross sections, so it can be seen that these reactions - a proven means for studying single-particle structure and hence shell structure - become accessible at beam intensities in excess of 1 - 10 $\times 10^4$ pps. Proven traditional spectroscopic tools such as (p,d) and (d,p) reactions, when applied to radioactive beams, must be performed using inverse kinematics. This introduces certain experimental constraints that are characteristic of the reaction geometry and vary only a little between different specific reactions. In general, it is not possible to achieve resolutions better than typically 200 keV for the excitation energy of the final particles, using just the detection of the outgoing charged particles. Gamma-ray detection, which is possible for excited bound states, is required to improve the achievable resolution. Fortunately, the kinematics are very similar for a wide range of beam masses and incident energies, and this allows the construction of a dedicated detection system with wide applicability. One such system is the UK's TIARA array, which is presently under construction. The need to develop new spectroscopic techniques to deal with the extremely low intensity of radioactive beams, compared to traditional stable beams, has led to the study of nucleon removal via a knockout mechanism which was not widely used previously. This has successfully been applied to very exotic beams with extremely low intensities, of the order of 1 particle/min or less, and is an interesting complement to the study of transfer reactions.