Octagon/Spectrometer Interference Drawings

These drawings show the areas of interference between octagon and spectrometer. All are in color Postscript.

The first drawing shows the ends of all 8 cooling frames and the octagon. The orange rectangle extends out 5 mm from the OUTER edge of the octagon rib (NOT from the inner edge where the readout boards actually sit). Clearly, only Frame 1 is a problem.

The second drawing is an end view of the octagon and Frame 1.   The orange rectangle here extends out 6.5 mm from the inner edge of the octagon slot where the readout boards sit. This was the original proposal from Birger for the location of the back face of the readout board (this fills the full depth of the slot in the octagon ribs).  NOTE that this shows the slot and readout board as they would be if my proposal to move the slot away from beam height by 3 mm was accepted and also the board is moved in the slot as far from beam height as possible. You can see that this 3 mm shift reduces the potential interference with the module cover (red rectangle), and with the module cover screw (green knob on the cover).  You can still see potential interference between the readout board and both the module mounting screws (green one holding the module to the cooling frame) and the spectrometer small plate (black rectangle).  Note also the potential interference between the module cover screw and the rib itself. An updated version shows Birger's latest proposal to move the readout boards in by several mm.

The third drawing is a top view of the octagon and Frame 1 in the same area shown in the previous drawing. The orange rectangle shows the readout board as it would be if it's back surface was 6.5 mm from the inner edge of the octagon rib slot (same as in the previous drawing). Note that the locations of the screws  (green knobs) on the module cover and for module support are very unfortunately right at the octagon rib. Also note that the end of the readout board at rib 10 (light blue bar just past the end of the orange rectangle) is in a region of the spectrometer small plate which cannot be cut away due to the reinforcing bar underneath (the black bar seen merging with the black edge of the small plate right after rib 10). An updated version shows Birger's latest proposal to move the readout boards in by several mm.

NOTE: All screw heads are shown as 4 mm high which is probably larger than they really are by 1-1.5 mm.

Proposed solutions:

1. Octagon readout boards and module covers: Move slot holding readout board about 3 mm away from the beam as shown in the second drawing. Position readout board as far away from module cover as possible.
2. Octagon readout board and module mounting screw: Use a smaller head screw. Also, move readout board into slot as far as possible (as shown in modified second drawing) and possibly make slot slightly deeper at this rib so board can bend in slightly.
3. Octagon readout board and spectrometer small plate: Cut away spectrometer small plate as shown very roughly in the fourth drawing. This doesn't interfere with the small plate support bar underneath but it does leave a small section of Frame 1 sticking out over the edge. We will place small bumpers to prevent collisions between the module and the octagon. There is still a problem of the readout board near rib 10 as shown here and in both versions of the third drawing.
4. Octagon rib and module cover screw: Use much smaller screw or possibly support cover without a screw or file a notch in the rib.

Contingency Plans:

I've generated two drawings showing what we can accomplish by moving either the spectrometer or the octagon if the above solutions fail to solve our problems or we run into unexpected interferences. Unfortunately, small shifts don't help very much. On the other hand, due to the large spread of the vertex locations, even moves of order a centimeter in Z are not so significant for our physics.

1. My preferred option is to move the octagon by about 1 cm in the -Z direction. This moves rib 9 away from the screws and moves the end of the readout board into a region of the spectrometer small plate which can safely be cut away. One disadvantage of this move is that it removes the symmetry of the multiplicity counter (octagon and rings considered as a unit) but I think that's only significant for the few collisions very near Z=0.
2. Less attractive to me is to move the spectrometer by 1 cm backwards. Because the spectrometer moves at 30 degrees you need a large motion along the rails to get significant distance from the octagon. My main objection to this is that it moves the spectrometer planes relative to the magnetic field.

Entry posted by George Stephans.

Last edited: Friday, February 04, 2000 01:38:48 PM