Collimation System

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The GlueX active collimator consists of 8 separate tungsten wedges placed symmetrically in two concentric rings around a 5-mm diameter clearance hole. It can be placed over any primary collimator aperture that is 5mm in diameter or smaller, and provide 8 independent signals which come into balance when the photon beam spot is properly centered on the collimator aperture. The wedges are mounted as shown inside a cylinder made of boron nitride, a white soft ceramic material that is an excellent insulator as well as being machinable and mechanically robust. The placement of the wedges on the boron nitride substrate are shown below.

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Figure 1. Images of the GlueX active collimator during assembly. There are 4 identical inner wedges and another set of 4 identical outer ones. The rows of pins on the inner radius of each wedge are critical to their performance as a local photon flux monitor.

After the wedges are installed, a mating aluminum frame is lowered into the cylinder. It is designed with separate compartments for each wedge, and serves as an electrostatic shield and isolates the return currents. Fig. 2 shows this frame before and after installation in the boron nitride housing. The fully assembled detector is shown in Fig. 3, where the right panel shows it installed and cabled up in front of the primary collimator in the Hall D collimator cave.

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Figure 2. Images of the GlueX active collimator during assembly. The aluminum frame fits down over the tungsten wedge assembly and provides electrostatic shielding and isolation for them.

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Figure 3. Images of the GlueX active collimator after assembly (left) and installed in the Hall D collimator cave (right).

The precise dimensions of the wedges and details of the pin geometry are shown in the following drawings.