The Collimator GUI (Figure 4.6) controls the position of the two sets of collimators located after the Tagger Magnet (See Figure 2.11. These collimators are used to clean the photon beam after its generation by the radiator. They are simple cylinders with a hole drilled in the middle. A translation stage moves these cylinders in and out of the beam (Figure 4.8).
Launching the GUI is very easy once the GUI_launch GUI (see section 4.2) is up. Simply click on the left popup menu and select 'scaler'. A window (Figure 4.6) should shortly pop-up on the screen.
If it's a photon run, then the collimators have to be precisely positioned. So first check if the warning string ``check Calibration'' is displayed in the lower message box. If it is, then click on the second left button in the main button bar to move a collimator in, then click on the ``out'' button to move the collimator out. It should reset itself against the limit switch and display ``Calibration OK''.
| [1] | 2l|The Position Control Panel | |
| Buttons | Move the chariot to put the requested device into the beam. ``Out'' take everything out of the beam. ``N/A'' is not available. |
| [2] | 2l|The Help Panel | |
| [2.a] | This button is to be clicked when the system seems no more to respond to the commands. It exit as nicely as possible every loops the SNL program could be locked in. | |
| [2.b] | This popup-menu will allow you to open the help window or the ``expert'' GUI window (the last one is not documented). |
| [3] | 2l|The Calibration Panel | |
| String | This string can be :
|
| [4] | 2l|Position and Switches Panel | |
| [4.a] | The position of the chariot in inches. Zero is the ``out'' position, against the limit switch. | |
| [4.b] | This is the switch indicator, it can display :
|
| [5] | 2l|Graphic Position Indicator Panel | |
| A graphic position indicator, it is. |
The Collimators clean up the photon beam, by removing particles
that are not on it's axis. The beam spot (70% of the
photons) at the level of the target will have a divergence of
. It's well inside of the target at 4
GeV, but 30% of the photons would still hit the frame if it
wasn't for the collimators.
A Collimator box (Figure 4.8) has four different positions. Three of them are carrying a Collimator, with different inside diameter. The latest position is not used, and is a rest position when the Collimator is not needed. Each collimator is a nickel cylinder with a thin hole along the beam axis. Nickel is used because it is the material that will be the less likely to produce neutrons when stopping photons. Neutrons can travel a long way in matter and are able to create noticeable background when reaching the detector. Each collimator box is moved by a motor under EPICS control.
After the primary collimator, a magnet will clean the beam from charged particles generated by collision of photons with the collimator.
But still some secondary photons made inside the first Collimator are not removed and a second Collimator is placed after the sweeping magnet. Its inside diameter is far greater than the first one and is not intended to touch the photon beam, it's purpose is only to remove things far outside of the beam-line (figure 4.7).
To move precisely enough in position, the routine that moves the Collimator make it in three steps :
The typical speed diagram for a move is displayed in Figure 4.9.
The application is under CVS monitoring and is known as collimator.To
check it out, type cvs checkout collimator. Then build it by typing
make in the main directory.
There are several files of interest :
