Collimators and Sieve Slits

Both spectrometers have front-end devices for calibrating the optical properties of the spectrometers. These are known as the collimator boxes. These boxes are positioned between the scattering chamber and the first quadrupoles (Q1). Each box is carefully aligned and rigidly attached to the entrance flange of the Q1 of the respective spectrometer. The boxes are part of the vacuum system of the spectrometer.

Inside each box a ladder is mounted which is guided by a linear bearing and moved up and down by a ball screw. On this ladder 3 positions are available to insert collimators. Below this ladder a special valve is mounted that can isolate the vacuum in the spectrometer from the target system. This valve should be activated when it is moved in front of the holes connecting the box with spectrometer and target chamber. A schematic view of the collimator box is shown in Figure 4.12.

Figure 4.12: Schematic layout of the collimator box.

1.

Vacuum requirement is $10^{-6}$ Torr. The material for the box is aluminum. It is possible to open one side of the box so that collimators can be exchanged. The reproducibility of collimator positions after moving the ladder and/or after replacing a collimator is better than 0.1 mm in horizontal and vertical direction. The dimensions of the box are roughly height=175 cm , width=35 cm and depth=15 cm. The tolerance in the dimension of the 7 msr collimator hole is $\pm0.5$ mm in each direction. The tolerance in the position of each of the sieve-slit holes is $\pm0.1$ mm in each direction.

Figure 4.13: Sieve slit collimator for optics calibration.

1.

A typical sieve slit collimator is shown in figure 4.13. It consists of a plate of roughly 14 cm x 20 cm containing 49 holes positioned in a regular 7x7 pattern. This slit is made out of 5 mm thick tungsten. The holes have a diameter of 2 mm except for the central one and one positioned off-diagonal which have a diameter of 4 mm. The horizontal distance between the holes is 12.5 mm while the vertical distance is 25.0 mm.

To get the latest information on the dimensions and locations of the collimators see the Hall A homepage on the www. (http://www.jlab.org/HaLL-A/)