Correction For Detector Dark Currents

Since the silicon detector segments have (varying) dark currents, the integrated signal output is dependent on the integration (i.e., dwell) time. To remove the influence of this dark contribution, a time dependent pedestal is subtracted from the detector output.

To perform the calibration with the IDL routine det_cal.pro, you need to collect a few images with the detector in dark conditions.

To do this

• Make sure that the microscope is well shielded from visible light and that the illuminator for the visible light microscope is turned off.

• In the Shutter Control section of the scanning microscope program, select closed to keep the shutter closed during the scan (no x-rays).

• Set up a piezo scan with 1000$\times$10 pixels, and a step size of 0.001 $\mu$m. Take scans for 1, 2, 5, and 10 msec dwell times and note the name of the first file. One way to do this is with

  sm_script_detcal @detcal_script.dat
  

  which runs the script file that reads

  # Line 1 is repeated before every scan (why?)
  # Line 2 is repeated before every scan (why?)
  scan,XPZT,0.001,1000,YPZT,0.001,10,1.00
  scan,XPZT,0.001,1000,YPZT,0.001,10,2.00
  scan,XPZT,0.001,1000,YPZT,0.001,10,5.00
  scan,XPZT,0.001,1000,YPZT,0.001,10,10.00
  

Start an IDL session on the microscope computer. Start the detector calibration routine:

IDL> det_cal, n_darkfiles=4

The program will perform a linear fit to the dark signal, characterize the noise by taking a Gaussian fit of voltage distributions (hopefully showing the spread expected by the statistics of the number of electron-hole pairs created per photon), and do a Fourier analysis of the noise to see what time frequencies have significant noise. If your first file is x1ais_08feb2004_0025.sm, as an example, it will make files with that same file header but extensions of _histogram.ps, _dark_noise.ps, _dark_signal.ps, and _fourier.ps. It will also create the file /mnt/x1a/sm_gui_disp_settings_stxmi.dat, which contains the latest best estimate of detector voltage scalings and offsets.