Taking Care of the Segmented Silicon Detector

As mentioned in Sec. 1.4.2, the inboard microscope by default uses a segmented silicon detector with phase contrast capabilities. Remember two things if you want to get meaningful data:

1. This detector is sensitive to visible light. Therefore, you have to cover the chamber properly with black cloth. Otherwise, you have a strong noisy background in your images (the fluorescent light bulbs on the NSLS experimental floor give a very strong 120Hz noise component!).

   It turns out that you need at least three, better five layers of cloth over the camber to shield the detector properly. There is one cloth with a reddish edge which is good for one layer. Another black one can be folded to provide two layers. If necessary, there is also a dark blue one around which provides another two.

   Don't forget to cover the viewports on the side of the chamber with Aluminum foil!

   After moving the cover around, it makes sense to recalibrate the detector - see below.

   If you want to check the frequency components in the signal, you can do Extra $\rightarrow$ Plot Noise per Line in iTool if you run SM_GUI in a fully licensed IDL version.

2. The detector's dark signal (caused by the leakage current in the silicon chip) can vary with ambient temperature, so that it requires regular recalibration.

   When running the microscope from the SM_GUI graphical user interface, just click the "Calibrate Now" button at regular intervals (say, every half hour). It takes only a few moments to finish.

   When setting up a stack (see Sec. 2.11.1), you'll have the option to tell how often the calibration should be done if the silicon detector is used in that scan. When setting up your own script file, you can use the command calibrate_sidet whenever you want.

   Also note that any stray visible light contributes to the dark (non-x-ray) signal. Therefore, cover the chamber properly as described above, and recalibrate the detector after moving the cover around.