ACIS UV/Optical Blocking Filter Calibration at the Advanced Light Source

The transmission of the ACIS optical blocking filters between 50 and 700eV was measured at the Advanced Light Source at Berkeley Labs. The measurements were performed on beamline 6.3.2 which contains a variable-line spaced plane grating monochroator that provides high resolution (E/$\Delta$E $\sim$ 2000 at 550 eV with 50$\mu$m exit slit and 1200 l/mm grating) and a triple mirror ``order-suppressor'' that in combination with filters provides high spectral purity.

The high flux levels obtainable ($\sim$ 1 $\times$ 10¹² photons/sec/0.1% BW) on this beamline were comparable to the levels obtained at NSLS. Additionally, the stability of the source beam, the high spectral purity and wavelength accuracy at the ALS allowed us to obtain high quality transmission data which clearly resolve the near and extended absorption fine structure of the Al-L, C-K, N-K and O-K absorption edges (see Figures 5.6 - 5.9).
 

Figure 5.6: Al-L XAFS.

Figure 5.7: C-K XAFS.

 
 
 

Figure 5.8: N-K XAFS.

Figure 5.9: O-K XAFS.

 
 

As an indicator of the absolute accuracy of the transmission values a comparison of the ALS and NSLS transmission values away from absorption edges indicates differences of less than 1% of the measured value. Figure 5.10 shows the X-ray transmission function of the ACIS-I and S filters between 0.05 and 3 keV. Transmission values below 0.7 keV are from the ALS measurements, transmission values between 0.7 and 1.8 keV are from the NSLS measurements.
 
 

Figure 5.10: X-ray transmission of the ACIS filters in the 0.05-3keV band.

We have created a transmission function of the ACIS OBF's between the energies 0.05-10 keV as a composite of measurements and models. (Fig 5.10) In particular in the energy range between 0.05 - 0.7 keV we use the X-ray transmission values derived from the measurements of the OBF's at the ALS at Berkeley Laboratories, in the energy range 0.7 - 1.8 keV we use the transmission values obtained at the NSLS at Brookhaven Laboratories and between 1.8 - 10 keV we have used a model that incorporates results from fits to the measured transmission data.