Data Acquisition and Analysis

The transmission mode XAFS typically employs a gas ionization chamber as the detector. Two ionization chambers with identical light intensity response are used to separately detect the incident light intensity and the transmitted light intensity, and eliminate the temporal fluctuations of incident light. For ionization chambers with a fixed length, the absorption ratio can be altered by varying the composition of the filling gas. For instance, mixtures of N₂/He, N₂/Ar, or Ar/Kr can be used as filling gases for ionization chambers operating at different energy ranges, from low to high (5~30 keV). The beamline station offers a selection of ionization chambers with different lengths, and when paired with an efficient and automatic gas mixing system, it can quickly fill chambers with different gas mixtures. Transmission mode XAFS is suitable for measuring samples with higher concentrations, typically requiring a mass ratio of the element greater than 5wt%.

For low-concentration samples (0.1~5wt%), the Lytle gas ionization chamber is typically employed for fluorescence mode XAFS measurements. In addition, corresponding filters are used to suppress scattered signals. During the actual testing process, signal amplification is required based on the strength of the fluorescence signal, with current intensities ranging from 10pA to 10nA.

For even lower-concentration samples (ranging from 0.1wt% to ppm levels), high-sensitivity solid-state detectors are generally used for fluorescence XAFS detection. The beamline station is equipped with a liquid nitrogen-cooled 36-element Ge solid-state detector with an array-type probe arrangement. This detector offers an excellent energy resolution that can be better than 160 eV@5.9 keV.

The beamline is equipped with a QXAFS rapid measurement system, which utilizes a channel-cut monochromator and a high-speed response gridded ionization chamber system. By driving the channel-cut crystal to swing rapidly with a high-power torque motor, a time resolution capability ranging from milliseconds to seconds can be achieved. Both the gridded ionization chambers with a fast response time (<10 μs) and the parallel-plate ionization chambers with a slow response time (~220 μs) can be used to carry out QXAFS measurement within the resolution of 100 ms ~ 1 s.