Shower Detector Structure

 

Shower detector has a shape of trapezium and consists of 3 chambers (preshower and 2 postshowers) and 2 lead converters. All three chambers have the same basic construction and differ only slightly in dimensions. The detector dimensions are adjusted to cover the same solid angle with respect to the target. The preshower gas chamber consist of:

1. Cathode pad plate, 1.5 mm thick fiber glass, with 20 mm thick aluminum support window.Pads are organized in rows and columns. Pads are aligned in each sector with respect to the target, such that there is a one-to-one correspondence between pads in a particular row and column in all 3 detectors.
2. Two rigid frames made from epoxy resins, each 4 mm thick, with integrated printed board circuits for wire connections.
3. 0.5 mm thick stainless steel cathode plate with 20 mm aluminum support window.

Side cuts of the preshower detector and first lead converter. Upper panel shows detector side cut at the position of gas inlets and the lower at the position of fixing bolts.
(1)-wire plan, (2,3)-Aluminium windows (4)-Lead converter
(5)-Pad plane (6)-stainless steel cathode plate

One full cathode pad plane is produced from 8 fiber glass plates (1.5 mm thick FR-4 laminate from Ferrozell covered with thin copper layer). Pads are manufactured using etching technique. The shapes of the pads have been optimized with respect to two conditions:

- to minimize pad double hit probability
- to maintain reasonable area for shower integration.

Final pad configuration is shown on below picture. There are 32 pad rows in azimuthal direction (rows are parallel with respect to the parallel sides of trapezium). Number of columns (columns are defined in polar direction) varies from 32 in the uppermost to 20 in the lowermost row. Pad height's changes from 4.5 cm (upper rows) to 3 cm (7 lowermost rows) and are aligned with chamber cells (i.e. 4.5 cm pads cover exactly 3 chamber cells). This configuration results in total number of 942 pads for one detector. However, in order to simplify read-out and trigger processor algorithm identical number of 32 rows and columns are foreseen. Pads from each row are connected via twisted-pair (one cable is grounded) flat cable to one 32 channel Front-End ASIC. Thus the total numbers of 32 ASIC's are needed to read-out one chamber. FE cards are located along longer base of the trapezium.
The preshower detector
cathode plane pad structure
The pad plane is glued to the aluminum support window (below picture). This window guarantees planarity of the pad plane and additionally increases rigidity of the epoxy resins frames to which chamber wires are connected. The aluminum window consists of 3 horizontal and 1 vertical ribs joined to the trapezium side frames. The thickness of the aluminum frame is 20 mm. The ribs are located inside sensitive area of the detector and therefore for the preshower detector are made from low radiation length material ((Durstone EPFU-2 fiber reinforced plastic with elasticity module E=33 000 N/mm squared). For the postshower detectors these ribs are constructed from aluminum. There are two gas inlets and two gas outlets in each gas chamber.

Aluminum support window. Side (right) and front (left) view. One half of the frame is shown. The right half is symmetric.

The wire plane is composed from sense (anode-25 µm golden tungsten) and field wires (125 µm Cu-Be) which are spanned on two separate frames made from ®Stesalit 4411W rigid epoxy resins. T This scheme ensures very good isolation of sense wires on high potential (+ 3400 V) from field wires (0 V) what has been demonstrated with the prototype chambers. There are 89 sense and field wires spanned with 50G and 150 G force, respectively. Selected wire tensions guarantee small gravitational wire sagging ( about 0.08 mm) and minimize dislocations due to the high electric field inside the chamber. The printed boards are produced on FR-4 laminate (1.6mm thickness) and glued with Araldit glue with the frames. There are 3 printed boards for sense (and also 3 HV connections) and field wires on one frame. The below picture shows one half of the one wire frame. Besides places for printed boards the gasket groove 1x 2 mm for 1.5 mm Veton O'Ring is foreseen.

One half of the wire frame with printed boards. Side views (left) and a front view (right) as indicated.

The second wire frame is identical with the exception for O'Ring. The wires are spanned on the eliminates sagging Stesalit frames after they are assembled with 20 mm thick aluminum frames. This practically of the frames due to wire spanning force and therefore the pre-deformation of the stesalit frames is not necessary. The second cathode of the detector is made from 0.5mm thick polished stainless steel plate. The selection of flat cathode instead of multiwire plane simplifies detector construction. It has been demonstrated flat stainless steel cathodes have very good gas aging properties. Additional advantage of flat cathode is more uniform field configuration at the cathode surface. The mechanical stability of the chamber has been investigated using finite element calculations performed with the ALGOR code. In this calculation detector consisting of 2 aluminum, 2 epoxy frames, pad and stainless steel planes and 178 wires have been modeled. The construction has been divided on 2111 finite elements. The worst case scenario for the horizontally positioned detector has been considered. The detector has been supported at four corners. Deformations of the frames due to the gravity and wires tension forces and tensile strengths inside construction have been calculated. The main results can be summarized as follows:
- Maximal horizontal deflection of side frames amounts to 0.0024 mm.
- Maximal vertical deflection of the whole detector amounts to 0.55mm. However, the maximal relative displacement of the cathode plane with respect to the wire plane amounts to 0.25 mm and has been found on the crossing of the vertical and second long horizontal rib.
- Maximal calculated tension strength amounts only to 15% of the elasticity limit value for aluminum.


Identical calculations have been performed for the lead converter. Several types of sandwich type constructions have been considered. The final solutions for the first converter are:
1. 2mm thick stainless steel plate
2. 9.9 mm thick lead layer.
3. mm thick stainless steel plate.
Again, it has been assumed that lead converter is positioned horizontally and supported at four corners. The maximal vertical deflection amounts to 0.44 mm.