The primary collimation of LAD is designed to deliver a beam which is 50mm high and 40mm wide. However, the size of the beam is normally reduced by installing a final aperture, and most experiments use a beam size 40mm high and 15mm wide. In order to change the aperture it is first necessary to remove the shielding inserts from the sample tank. It is then possible to remove the aperture holder, which is made of B₄C-loaded polythene, from between the two backward-angle detector banks. There are a number of different sized aperatures, made of crispy-mix (B₄C boron carbide grains held together with small amount of resin), which can be inserted into the holder. A list of the dimensions of the apertures which are available is given in Table 2. It is also possible to make other sizes by using glue to stick pieces of B₄C-loaded polythene onto the standard apertures. Previous experience shows that these apertures can be highly successful in tailoring the beam to pass precisely through a gap in a piece of equipment. Note that a beam width greater than 20mm leads to a large background in the low angle detectors due to the penumbra of the straight-through beam.

Section 3.5 gives further information of use in selecting a suitable beam size.

Last Updated 09 Nov 1998