Overview

The TIARA target chamber is designed specifically to allow charged particle and gamma-ray coincidences to be studied efficiently for a large range of reactions using radioactive beams. The primary design objectives may therefore be summarized as:

• the charged particle array should be as compact as possible, to fit inside the most efficiently configured gamma-ray array available,
• the charged particle coverage should be as close to $4\pi$ as possible,
• the charged particle detectors should not interfere with the gamma-ray detection,

and these criteria have largely determined the rest of the general design. It has been a guiding principle that any compromises necessarily introduced by engineering constraints should be in other properties of the array and not in these main objectives. This, in turn, introduces major challenges into the detailed design, for example with target changing, beam alignment, access to detectors, signal connections, flange connections, etc. In an initial design study, it has been possible to identify solutions in principle to all of these problems and to provide an estimate of the construction costs for the chamber. In summary, therefore, the main component of the array is a small chamber to house the central part of the silicon array, to fit inside the most closely packed geometry of the Exogam gamma-ray array. This chamber is shown in fig. 5.

  

Figure 5: Main vacuum vessel for the TIARA array, viewed from the back.

The forward and rear parts of the silicon array are allowed to fill a larger volume as they are further from the target and out of the way of the gamma-ray detectors. A plunger that enters through a small gap in the silicon array is used for target changing. In order to protect the fragility and alignment of the central vacuum vessel and silicon detectors, and of course also to protect the heavy gamma-ray detectors themselves, a substantial framework is necessary to support the Exogam detectors. The best way to assure the safety of the Exogam detectors during their operation with TIARA was to use Exogam's own detector support structure and to mount TIARA within in. However, the design study revealed that this was not possible owing to some specific properties of the Exogam support. As an alternative, a close copy of the framework for supporting the ring of $90^\circ$ gamma-ray detectors was adapted for TIARA. As an added benefit, this structure will allow the possibility of setting up TIARA independently, so for example if Exogam were set up on the VAMOS spectrometer with its maximum complement of 12 detectors for that application, then TIARA could be set up on an adjacent beam line with the remaining 4 Exogam detectors mounted in the gamma-cube geometry.