Octagonal barrel of detectors around the target

The provisional design for the barrel around the target is shown in figs. 1 and 2. As shown in fig. 2, it envisages an octagonal box of silicon strip detectors with a length of 142 mm and with the width of each detector being 36mm. Each detector will have four resistive strips of width 9mm and length 140mm. The strips, which will run axially, will automatically offer measurement and pixellation of the azimuthal angle in 32 bins of approximately 11 degrees. The measurement of the position along the strip for scattered particles will be achieved by resistive division and should have a precision of better than 0.4 mm. This will give the scattering angle to better than one degree (limited by a 1 mm beam spot; better beam tracking would improve this by a factor of between 2 and 4). Resistive strip detectors of this general design have been used successfully by members of the TIARA team for over 8 years. Edge-connector technology which exists already will be able to deal with the extraction of the signals from both ends of the detectors via one end of their mounting board. One side of the barrel has been sacrificed from the angular coverage in the present design. This is because it is necessary to be able to insert and change targets during the experiment. In view of the tight space constraints, a relatively simple plunger mechanism for target changing has been chosen. A more sophisticated remote controlled arrangement could be considered in the future, and may allow the eventual completion of the barrel, but in the first stage this is not included. This partly explains the specific choice of an octagonal geometry for the barrel. It represents a balance between lost solid angle and the complexity of building the barrel from many separate detectors and getting their signals out. The other important reason for the octagonal choice is that it provides a good match between the geometries of the charged particle detectors and the gamma-ray detectors.

  

Figure 2: Strip detectors for TIARA: (a) the rectangular detectors each have 4 resistive strips and are single-sided, (b) they are arranged with their long sides touching to form an octagonal box, (c) the annular detectors are double-sided 6-inch wafers with annular strips at 1 mm pitch on one side and radial segments on the other side