Calibration Strategy

The transmission of the filters is determined by moving the filter in and out of the beam and using a ``measurement'' detector to measure the X-ray flux attenuated and non-attenuated by the filter respectively. The purpose of the monitor detector measurements is to take into account possible flux variations of the beam between the time the filter is in vs. when it is out of the beam. Both detectors are silicon PIN photodiodes. One detector is exposed to the X-ray beam at a time allowing a simultaneous background signal level measurement from the other unexposed detector. If I_i,j represents a measured signal from the i'th detector (i=0 for the monitor and i=1 for the measurement detector), taken when the filter is located in the j'th position ( j=in, for filter in beam, j=out, for filter out of beam), then the X-ray transmission at the X-ray energy E is determined using the expression,
 

$${T(E)} = {{{I_{1,in} - B_{1,in}}\over{I_{1,out} - B_{1,out}}} {{I_{0,out} - B_{0,out}}\over{I_{0,in} - B_{\0,in}}}}$$ (5.1)

where B_i,j is the background measurement of detector i with a filter in the j position.

Measurements with the filters out of the beam provide cross calibration of the monitor and measurement detector at all energies selected for the transmission measurements. A cross calibration between measurement and monitor detectors was performed before and after each transmission measurement. As an estimate of the errors in our measurements, we took the percent change in the two cross calibration runs. For the data points considered in the present analysis the percent change in normalization was below 0.5%. There is a delay of about 20 sec between measurements from each detector and one expects there to be an additional calibration error due to changes in the beam intensity, shape and direction that could occur during that time interval.