Dark signal subtraction and conversion to photon count rate

The x-ray flux (photons per second) into one detector segment is calculated as follows:

$$\Phi = \frac{F}{C} \frac{U_{\mbox{out}} - U_0 - C \times I... ... \times t_{\mbox{dwell}}}{t_{\mbox{dwell}} - t_{\mbox{dead}}}$$ (10.4)

[1, Eq. 3.9], where

• $F$ is the photon-charge conversion factor (number of photons to generate a certain charge). E.g., $41.7 \mbox{fC}^{-1}$ for 540 eV x rays in silicon (the material of our detector chip). The number used to be stored in sm_par as sidet_photons_per_femtocoulomb until file version 6. Starting from file version 7, we store sidet_ev_per_femtocoulomb in the datafile, which has a constant value of 22594.25 (corresponding to 3.62eV creating one electron-hole pair in silicon at room temperature). photons_per_femtocoulomb is then calculated after reading the file from ev_per_femtocoulomb by dividing by the x-ray energy. It will be constant for regular images but depend on the pixel in scans where one axis is the monochromator.

• $C$ is the electronics calibration constant (output voltage of the instrumentation amplifier per input charge). It is stored in sm_par as sidet_mvolts_per_femtocoulomb.

• $U_{\mbox{out}}$ is the output voltage of the detector channel as measured by the ADC.

• $U_0$ is a voltage offset (extrapolated voltage for zero dwell time - no physical meaning, but comes out as a linear fit parameter in the calibration). It is stored in sm_par as sidet_u0_mvolts.

• $I_{\mbox{dark}}$ is the dark (leakage) current of the silicon chip. The product $C \times I_{\mbox{dark}}$ is stored in sm_par as sidet_c_idark_mvolts_per_ms.

• $t_{\mbox{dwell}}$ is the pixel dwell time of the microscope.

• $t_{\mbox{dead}}$ is the dead time of the detector as determined by the setting of the S2 pulse (delay plus width on the Quantum Composers timing module). It is stored in sm_par as sidet_deadtime_ms.

The calibrated signal (count rate) is calculated from the raw data automatically by SM_GUI or read_stxm5.pro. Make sure you look at the correct signal (see Chap. 9)! If you want to do it yourself, apply the two formulas listed above.