Beam Current Measurement and Beam Drift
The nominal beam current is used as a scaling factor for the beam drift correction. Any non-zero value will suffice, however to have the count rates reported close to the actual count rates, a nominal beam current should be used that is similar to the actual beam current used during the acquisition. That is, the counts displayed in the log and Analyze! Window is the beam drift corrected intensities (normalized to the nominal beam current in the Count Times dialog).
Microprobes which use a faraday cup (JEOL and Cameca) for the beam current measurement, require an extremely stable and regulated beam current since the beam current measurement occurs after the x-ray count acquisition. The benefit of the faraday cup beam current measurement method is that the electron optics geometry (i.e., sample current) can be changed during the quantitative run without affecting the beam drift calibration.
The beam drift correction calculation is shown here :
Where :
is the beam drift corrected unknown intensity
is the
uncorrected unknown intensity
is the
nominal beam current
is the
count time for the unknown intensity
is the
count time for the nominal beam current measurement
is the beam
current for the unknown intensity
If you do not want a beam drift correction (it is recommended to always use a beam drift correction for the most quantitative work), one can either enter a nominal beam current of zero or disable the beam drift correction from the Analytical | Analysis Options menu.