General considerations

The charged particle detectors need to cover as much of $4\pi$ solid angle as possible. Of course, this ensures the most efficient use of the weak radioactive beam intensity, but in addition it minimizes the reliance on Monte-Carlo efficiency calculations for coincidence experiments. Such calculations can present considerable problems for reactions in massively inverse kinematics such as transfer on proton or deuteron target nuclei. To avoid this problem, the TIARA design features an annular detection system with mimimal dead area. In order to deal with pickup and stripping reactions on hydrogen targets, with transfer on heavier target nuclei such as ¹³C, and also with Coulomb excitation, the full range of scattering angles must be covered. The provision of a full angular coverage without the need to move detectors offers several important advantages in terms of space, set-up and alignment time, versatility and efficient use of the beam. Silicon diode detectors are proven to have excellent energy resolution for heavy ions and make a convenient choice for a compact particle detection array. The resolution will in most cases be limited by target thickness effects, so there is no dramatic advantage in cooling the detectors and they can be very conveniently operated at room temperature. Lithographic techniques are now well established, and allow the fabrication of many essentially independent detectors on a single silicon wafer. This allows a close-packed geometry of many individual detectors, thus avoiding any significant loss of efficiency arising from dead space between detectors. The required thicknesses of silicon to stop the charged particles of interest are less than 1 mm and hence they ensure minimal attenuation of gamma-rays from reactions in the target. In summary, a high geometrical efficiency and compactness will be achieved using a barrel of detectors around the target and annular detectors at forward and backward angles. Each detector will be further segmented using lithographic techniques.

  

Figure 1: Schematic side view of TIARA: (a) the back-angle annular detector, (b) the ``barrel'' around the target, in the form of an octagonal box, (c) the forward angle annular detector. The dimensions may be changed in the final design; they serve to illustrate a possible implementation using reasonably sized detectors.