Maximising the Ion Chamber Readings on Station 14.1

Maximising Ion-Chamber Readings The usual back ion chamber reading measured with the back slits set at 200 x 200 microns and with the SRS running at 200 mA, is 2.5.

There are two ion chambers that produce a reading on the display in the User Area outside the Hutch. The front ion chamber is situated immediately after the first set of slits on the camera, on the input side, closer to the source. Similarly, the back ion chamber is situated after the second set of slits, on the crystal side, and is the one that should normally be chosen since it is nearer the crystal. If the display shows no reading, check that the port and station shutter are both open. If there is still no reading, you should open up the camera slits to around 0.5 mm before proceeding with optimisation.

The PXGEN Program

A PC bl141vig4, running LINUX is used to drive the station motors. Start by looking at the back ion chamber reading. Maximise it using all the motors. Visually switch to the front reading, maximise using all the motors. Finish on the back ion chamber reading. Keep the back ion chamber optermised during data collection.

The table motors are driven using the PXGEN program. To start this up type:

pxgen &

in a shell window of the computer. This will launch the program and start the interface which allows movement of the table motors. It should look like this:

From this interface select HUTCH MOTORS, CARRIAGE, MOVE BY and SLOW drive speed. Move in steps of 50 initially then 10 for fine tuning. A good strategy for aligning the table in the beam is as follows,

Click on CARRIAGE FRONT to drive the motors of the front of the table. Use a step size of about 10 click on one of the left or right arrow buttons and watch the ion chamber reading as the table moves sideways. Use the left/right arrow buttons to move the table and optimise the ion chamber reading.
Now use the up and down arrow buttons to move the table vertically and obtain an optimum ion chamber reading.
Click on CARRIAGE BACK to drive the back motors of the table. Use the left/right arrow buttons to optimize horizontally and the up/down arrow buttons to optimize vertically.
Cycle between the FRONT and BACK a couple of times until there is no observed increase in the ion chamber reading (It is best to finish on the BACK motors).
Note down the ion chamber readings, SRS current and the slit settings in the Station Log Book.

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