Beam tracking

Radioactive beams produced as secondary beams in fragmentation cannot be focussed as well as traditional beams and the beam spot could typically be 5mm in diameter (for a case when LISE was tuned to its best performance). For re-accelerated beams (i.e. for SPIRAL beams) it is possible to attain better focussing, closer to that expected from a traditional cyclotron. Especially for the fragmentation beams, but in some cases also for the SPIRAL beams since it will assist with the beam focussing itself, it is necessary to have tracking detectors in the path of the beam. Then, every beam particle has its (x,y) position measured twice and its position of impact on the target can be reconstructed. Depending on the tracking detectors and their location, a resolution of better than 1 mm can be attained for the target hit position. In our previous experiments we have used position sensitive, parallel plate avalanche counters borrowed from other groups. Improved performance would be obtained if dedicated tracking detectors could be mounted inside the TIARA vacuum vessels and used in all the experiments where they were required. Note that PPAC's such as we have used previously are limited to use with beams up to a few by 10⁴ particles/sec. TIARA could operate with beams more intense than this, for example when using the VAMOS spectrometer at zero degrees. However, in such cases of beams that are too intense to track, the focussing will tend to be better than average either because SPIRAL is being used or because the slits on the fragment separator can be partially closed to better define the beam. Hence, the count rate limitation on the PPAC's poses no serious limitation on the operation at present. We propose to build three PPAC detectors either to the specification of the presently used LPC detectors, or to a new design developed at Daresbury. The Daresbury experts will investigate the possibility of improving the position resolution, which in the present design is limited to the pitch of the anode wires in the PPAC. Two of the three PPAC's will be deployed for beam tracking, in containment vessels mounted in front of the main TIARA chamber. The third PPAC is intended to operate in place of the zero degree telescope when TIARA is used together with VAMOS or with a magnetic beam-dump device. It will provide timing information for particle identification plus a direct measurement of the entrance angle into the spectrometer or beam filter.