All of the detectors will mount on plates mounted on the rail system owned by the BNL Chemistry group. These base plates are 12 by 25 inches with three holes on each side to hold the rails. Four (or possibly 5) of these plates can be mounted simultaneously. The nominal center of the beam will be centered horizontally on these plates and located 6.90 inches (175 mm) above the top surface of the plates. This height is determined by the Phobos Si setup.

A drawing of one possible testbeam setup shows the likely arrangement of the detectors for the Geant test. The target and phototubes for the scintillators will be supported by ring clamps. The large scintillator in front will be used to monitor the total beam rate and also as a multi-particle veto. The small scintillator mounted in the rear will be used in the trigger for the straight-through tests and might be usable as a veto in the trigger for the Geant Test. The HODO (not shown) will be located as close as possible behind the rear of this setup. For the tests of the Phobos Si alone, the Taiwan telescope may be moved farther forward to allow the Phobos box to rotate.

A rough sketch shows the base plate for the WA98 detector and the Taiwan telescope that will sit directly behind it. The WA98 stand should sit as close to the front edge of this plate as possible and the telescope stand should be as close as possible behind it. The footprint of the WA98 detector is only a guess at this point and so the 5 inch dimension for the location of the first set of telescope holes is only a guess. All other dimensions are correct and will not change. We will probably not use all four sets of leg clamps for the telescope but should have all four machined. If we are interested in rotating the WA98 detector, a second set of telescope table holes could be machined allowing the telescope to move back.

The Taiwan telescope boxes will sit on stands which were built for last year's testbeam. These stands were designed assuming that the active area of the telescope was 60 mm above the base (the information we had at that time). Shims will be added to the legs to raise the stand for this year's telescope which is claimed to have its active area 55 mm above the bottom. The legs are held down to the base plate and positioned using a clamp which can include both a dowel pin and a 6-32 thumb screw. It was decided to use these clamps, rather than bolting directly into the legs from below the base plate for ease of changing the configuration.

The WA98 detector mounts on a ring held down by a clamp ring (no picture). A detail view shows the ceramics and other pieces of the WA98 detector held between the two rings. The entire assembly has 16 large holes through it. It is held by two of these which are not diametrically opposite (see WA98 ring drawing). This choice was dictated by the desire to have the seams in the detector vertical and horizontal. For the 97 testbeam, we will probably choose to have one quadrant centered on the beam so that the seams are at roughly 45 degree angles from the horizontal. The support stand will attach to the WA98 detector using two pegs which go through the holes. There may be some of these pegs left over but if not, new ones can easily be machined.

After the pegs are attached to the WA98 detector, they are screwed to the stand. There was a version of this made for WA98 but it was lost. This stand will need to be designed and machined at UIC. The only critical dimension on the stand is the horizontal spacing of the screw holes for attaching the WA98 assembly, which are 9.05 inches apart. For flexibility, I have shown this stand as having a vertical height adjustment which allow movement of the nominal beam center from hole in the center of the WA98 detector to a point near the outer edge of the detector. Note that the center of the detector is 0.90 inches below the height of the mounting holes in the stand. If the detailed Geant study design converges in time, we may select one fixed height. Another open question is whether we want to mount the WA98 detector off-center relative to the beam.

The WA98 detector needs to be covered to keep out light. In previous tests, this was accomplished by attaching foil to the two mounting rings using double sided tape. We should also use this procedure since it allows having the target or other detectors very close to the WA98 sensor.

One side benefit of this testbeam effort will be a recalibration of the WA98 detector itself. For this purpose, it may be useful to rotate the detector to allow particles to go through at an angle. The mount drawing shows this capability. The Phobos detector assembly will be modified to allow it to mount on one of the standard base plates and also rotate about its center.

G.Stephans 3-Apr-1997