Introduction

The study of the structure of exotic nuclei poses unique experimental challenges, raised both by the constraints arising from the production of such nuclei and also by the experimental tools necessary to explore their properties. A general constraint is that such nuclei are typically produced with low intensities, which needs a detection system with very high efficiency. Among the many experimental tools employed in their study, direct reactions are the most powerful and incisive to elucidate their spectroscopic and structural characteristics. Such information is most readily extracted using reactions in which one of the participants has a relatively simple structure, e.g. ¹H or ⁴He. In this instance the use of radioactive beams necessitates the use of ¹H or ⁴He targets and inverse kinematics. The information of interest can then be deduced by measuring either the kinematical characteristics of the heavy residue and/or of the light fragment. In the case of the heavy residue, the detection efficiency is increased by the forward focusing of the reaction. In addition the large velocity allows the use of relatively thick targets. However, the detection of the heavy fragment is possible only for those reactions where it is bound or it has a lifetime long enough to reach the detection system. Moreover, the angular centre-of-mass resolution which can be obtained becomes rather poor as soon as the mass of the projectile exceeds a few mass units. In these cases, the measurement of the energy and emission angle of the light recoil fragment, preferably in coincidence, allows the reconstruction of the kinematics of the reaction, and in principle more accurate determination of the Q-value of the reaction can be achieved. However, the recoil velocities are often very low, so very thin targets are needed, though the use of coincident gamma-ray detection can allow thicker targets to be used, but this does not permit a measurement of the angular distributions.

If one wants to extend the studies very far away from stability, all the above experimental problems are enhanced. The low intensities will imply the use of thick targets to achieve a reasonable yield given the small intrinsic gamma-ray detection efficiency, and thus the gamma detection rates will be very small making coincident gamma detection impractical. A very promising solution to such experimental challenges is the use of an active target detector, where the gas constitutes both the target and the detection medium. Such a detector may have a very high efficiency, very low particle detection thresholds and eliminates the problems associated with the use of thick targets as the interaction point of the beam with the gas is determined on an event-by-event basis. Such a detector needs very good position resolution in three dimensions for track reconstruction and a large dynamic range. This may necessitate the incorporation of magnetic fields in the detector design. It also leads to a very large number of readout channels implying the development of high-density electronics using, for example, the recently developed ASIC technology.

Below are described briefly the key types of experimental studies that such a detector is ideally suited for:

Inelastic Scattering

Examples of such reactions are A(⁴He, ⁴He´)A*, A(³He, ³He´)A*, A(d, d´)A* etc. Inelastic scattering is a very sensitive probe of collective nuclear properties, in general, for example it may be used to infer the degree of nuclear deformation. Here we are mainly interested in inelastic scattering leading to giant resonances, in particular to the giant monopole resonance. It is well known that the giant monopole resonance is a privileged tool to measure the nuclear compressibility. The variation of nuclear compressibility as a function of isospin is an open and interesting question. The study of such reactions is very difficult experimentally, because the angular domain where the separation of the monopole resonance from other contributions, such as the dipole or the quadrupole resonances, is possible only at very small centre-of-mass angles, in the domain of 0-5 degrees. This implies very low recoil energies of the order of 1 MeV. Beam energies of 50-150 MeV/nucleon are well suited for these studies.

Charge Exchange Reactions

An example is ^AZ(³He, ³H)^A(Z-1). At high energy, mainly the Isobaric Analogue Resonance State (IAS) and the Gamow-Teller (GT) resonance are populated. At energies lower than about 50 MeV/nucleon, the cross-section for the IAS is dominant, whereas at higher energies the GT transition is dominant. The GT resonance strength can be compared to large-scale shell model calculations that should become possible for nuclei of mass up to 100 in the near future. The GT-strength is also of astrophysical interest. The IAS is a powerful spectroscopic tool to study single particle properties of neutron rich nuclei. Consider as an example the reaction ¹³³Sn(³He, ³H)¹³³In(IAS). The IAS is the analogue of ¹³³Sn. The Coulomb displacement energy, determined by the Q-value of the reaction, is directly related to the radius. The IAS will decay by a strong branch via proton emission to ¹³²Sn. The recoil energy of the triton is very low, in the 1 MeV region.

Transfer Reactions

Examples of such reactions are ^AZ(p, d)^A+1Z, ^AZ(³He, d)^A+1(Z+1), ^AZ(d, ³He)^A-1(Z-1). Transfer reactions are a well-known tool to explore single particle properties of nuclei. They can be used very generally to study the angular momentum and spectroscopic factors associated with specific single particle states. In the context of secondary beams, it will be one of the main tools to study shell closures far from stability and to explore the evolution of single particle structure with isospin. The 4pi solid angle coverage and the very high efficiency of the active target make it ideally suited for such studies. Additionally, the ability of the active target to determine the interaction point of the reaction will allow thick targets to be used, increasing the reaction yield.

Break-Up Reactions

Breakup reactions are a powerful method to study cluster structure in exotic nuclei, properties which are predicted to be of increased importance in extremely neutron-rich systems. The multiparticle tracking capacity together with the 4pi solid angle coverage and the very high efficiency of the active target make it ideally suited for such studies. More generally, resonance formation during the deceleration of the beam in the target may be used to study, in considerable detail, the structure of nuclei above particle-decay thresholds. For example, excitation energies, spins and total and partial widths may be extracted.