Mechanical assembly

Each crystal is wrapped in a diffuse reflector sheet for the best collection of scintillation light at the end of the crystal. After testing several candidates, we have chosen a single layer of 200 $\mu$m thick Goretex teflon as the wrapping material because it gives a good light output, is easy to handle, and is expected to be stable against aging. The mechanical assembly of a single CsI($Tl$) counter is shown in Fig. . All sides of the crystal, except for the end face for light readout, are covered by a single layer of 200 $\mu$m thick porous teflon. It is then covered by a laminated sheet of 25 $\mu$m thick aluminum and 25 $\mu$m thick mylar for light and electrical shielding. An aluminum-shielded preamplifier box is attached on the aluminum base plate with screws.

Figure: Mechanical assembly of the ECL counter.

Two photodiodes, each having an active area of 10 mm $\times$ 20 mm, are glued at the center of crystal end surface via acrylite plate of 1 mm in thickness. The acrylite plate is used because direct glue joints between the photodiode and the CsI were found to fail after temperature cycling, probably due to the different thermal expansion coefficients of silicon and CsI. ECCOBAND 24 glue from Grace Japan Co. is used.
The barrel crystals were installed in a honeycomb-like structure formed by 0.5 mm-thick aluminum septum walls stretched between the inner and outer cylinders. The outer cylinder, the two end rings, and the reinforcing bars are made of stainless steel and form a rigid structure that supports the weight of the crystals. The inner cylinder is made of 1.6 mm-thick aluminum to minimize the inactive material in front of the calorimeter. Except for the barrel end, eight crystals, four rows in $\theta$ and two columns in $\phi$, were inserted in each cell and fixed to the reinforcing bars through a fixing jig at the back. In each cell at the barrel end, ten crystals, five rows in $\theta$ and two columns in $\phi$, were inserted. The whole barrel structure was installed in a thick stainless steel cylinder supported from the magnet. Finite element calculations indicate that the whole barrel structure would sag $\sim$200 $\mu$m in the final assembly that is considered to be safe for the contained crystals. The overall support structure is made gas tight and flushed with dry air to provide a low-humidity (5 %) environment for the CsI($Tl$) crystals. The preamplifier heat, a total of 3 kW, is removed by a liquid cooling system. An operating temperature of lower than 30$^o$C with $\pm 1^o$C stability is required for the stable operation of the electronics.
The end-cap support structure is similar to that of the barrel. Here the crystal weight is transmitted via a front plate, thin septa, a side wall, inner/outer walls to thick back-reinforced bars that are supported from blocks mounted on a support cylinder. The end-cap calorimeter can be slided out using an additional end-cap mover with rails when access to the inner detectors is necessary.