The Cosmic Veto Sidewalls

The original design of the whole Cosmic Veto House consists of the Top, two sidewalls, a front- and a backwall. Up to now only the most important part - the top counter - was installed. In addition we have received the material (scintillator paddels from Dubna and WLS from Bicron) to continue building and integrating new counters. The two sidewall counters we have built in the beam break time early August 2000.

The principal design is the same as for the top counter (see figure 1). The metal box is filled with scintillator paddels, 300 mm wide and on the upper and on the lower sides - parallel to the beam axis has each sidewall counter two wavelengthshifter bars. The wrapping is the same as for the top counter.

Figure 1: Here you can see the alignment of the 300 mm wide scintillator pieces and the Wavelength Shifters.

Meanwhile we have received the missing parts to finish building up the sidewalls completely: a few lighguides and the mechanical parts to hold the PM tubes and to cover them lighttight, which have been produced externally.

Photomultiplier Coupling

The Photomultiplier coupling to the WLS is not as easy here as for the top counter, because at the sidewalls space is more limited.

Lower Downstream Counters

The lower downstream counters are the only counters where we have no problems. Here we can mount directly the new PMT mechanics. This mechanics consists 2 pieces (figure 2, originally one piece, which was cut) with an external thread which holds the WLS and a tubular body (figure 3) which contains the PM tube (together with a soft iron shield and Teflon insulating). The tube has an internal thread to couple to the WLS by the 2-part external thread. Figure 3 shows the mounted PM tube.

Figure 2: Workshop drawing for the PMT mechanics.

Figure 3: Workshop drawing for the PMT mechanics.

Figure 4: Lower PMT downstream beam right mounted.

Lower Upstream Counters

The lower counters on upstream side have the limitation, that the PMT cover is wider than the WLS itself and so there is a conflict when you have to move the Leadhouse. The distance between the WLS and the Leadhouse Frontwall is only 7 mm. One has to bring the photomultiplier a little bit off to prevent drive with the house without destroying the tubes.

We have decided to bring the PMT a little bit off by a lighguide extension under an angle of 10 degrees (see the drawing in figure 5 and the photograph in figure 6). The PMT will be mounted parallel to the Wavelengthshifter, not as shown as an alternative under this 10 degree angle which needs more space on the other side where we are limited too.

Figure 5: Search for the correct lighguide parameters - the angle should not be higher to obtain good light collection efficiency. One can see the green light coming out from the wavelength shifters (a light is shining through from the other side).

Figure 6: The lighguide extension from above after glueing it to the WLS. One can see the green light coming out from the wavelength shifters (a light is shining through from the other side).

Upper Counters

The upper edges show up another problem. There we are getting an interference between the CV top photomultplier mechanics and the sidewall mechanics. Therefore we think, it's best to dismount the huge Philips 2'' tubes, cut the top wavelength shifters and polish them and mount 1'' Burle photomultiplier as on the sidewalls. This will give us a second advantage of easier move of the platform without dismounting the CV top PMs.

On figure 6 one can see the interference between the counters. The sidewall PM would be mounted where you can see the Aluminium foil. The kind of mounting would be the same as for the

Figure 7: Interference between the meachanical parts of the readout of the top and sidewall counters.

Send a mail to the author

H.P. Wirtz , 11-Sep-2000
last update:  11-Sep-2000

 Paul Scherrer Institut (PSI) WWW Information Services, Home Page
 Pion Beta Experiment Home Page