Design study

The specification of the TIARA chamber has been undertaken at Daresbury as a design study. This enabled a detailed costing to be achieved and the general construction problems to be addressed. The solution to the problem of supporting the detectors in a very restricted space was to mount the forward annular detectors and the barrel detectors from the rear flange of the main vacuum vessel that will fit inside the gamma-cube. The target is changed using a plunger that enters through the top of the barrel but does not interfere with the gamma-cube. The main vacuum vessel for TIARA is shown in fig. 5, looking from an angle of approximately $45^\circ$ to the beam direction. Just visible inside the back of the box are the support legs that mount off the back plate and support the silicon array and the zero degree PPAC detector. The Exogam detectors are supported around the target chamber using a modified version of the original Exogam support frame. This saves on design effort and provides a safe and secure support for the Exogam detectors which is assured of being fully compatible. The frame must not only support the gamma-ray detectors in their run-time positions, but also it must allow them to be pulled away to provide access to the silicon array for alignment and setting up. The unmodified design has many interlocking components so that there is not sufficient space to support an inner chamber such as TIARA in a reliably aligned position. The modified version of the frame is designed to allow the whole ring of 8 detectors to be used at 90 degrees if desired, though the main emphasis for the transfer work will be to employ the close-packed gamma-cube geometry. It will use the mechanism developed for the original frame to allow individual gamma-ray detectors to be wound in and out from the target on lead-screws. Fig. 7 shows a close-up of the target chamber assembled in the beam line in front of VAMOS with sample Exogam detectors supported around it. At the left, the second of two beam-tracking PPAC's can be seen, whilst at the right hand side of the view it is possible to see the angled pole-faces from the first quadrupole in VAMOS. The cryostats of the Ge detectors (which are all that is shown of them) are supported within the modified framework (which is mostly omitted her, for clarity, but is shown in outline more fully in fig. 6). The framework also supports the weight of the TIARA vacuum vessel and the PPAC vessels.

  

Figure 7: Close-up of the target chamber assembled in the beamline with the Exogam-style support stand around it. For clarity, only two detector cryostats are shown (not, in fact, those for the gamma-cube detectors), and the detectors themselves are not shown.

The inside arrangement of TIARA provided the biggest design challenges and will require the most effort in order to specify in detail suitable for final construction. The solution that was found gives compact support for the silicon detectors inside the close-packed gamma-cube, whilst ensuring good alignment is maintained with the beam axis. The signals for the barrel and all of the forward angle detectors are brought out on the back flange in space that does not conflict with the VAMOS entrance. A close-up of the support structure for the silicon detectors inside the TIARA vessel is shown in fig. 8.

  

Figure 8: Support of detector system inside of the TIARA vacuum vessel.

The back plate of the main vacuum vessel can be adapted to a top-hat component that is used in stand-alone mode (not coupled to VAMOS or any other large zero-degree device). The top-hat section allows a zero-degree telescope to be mounted beyond the back plate itself. The hole in the back plate subtends an angle of $\pm 10$ degrees, matched to the VAMOS acceptance, and will easily allow beam-like particles that travel through the centre of the annular detector to be recorded. An overhead view including the back of the TIARA array, set up for stand-alone operation, is shown in fig. 9.

  

Figure 9: TIARA array set up for stand-alone operation with a full complement of 8 Exogam detectors. The zero-degree beam telescope is mounted in the top-hat section which allows it to be further removed from the target than rest of the array. The whole array and the zero-degree telescope are mounted from the large VAMOS adaptor plate which is mounted solidly and is aligned into position on the beam axis.