Experimental Configuration

The 0.26 to 3 keV X-ray transmission of the AXAF CCD Imaging Spectrometer (ACIS) UV/optical blocking filters was measured at the National Synchrotron Light Source at Brookhaven National Laboratories. The use of the NSLS synchrotron source for the calibration was necessitated by the required high accuracy for these measurements. The NSLS provided a monochromatic X-ray beam allowing us to measure the transmission of the ACIS filters at fine energy steps with sufficient intensity for good counting statistics and fair beam stability to allow accurate detector normalization corrections.

The X-ray ring at NSLS in normal operation mode accelerates electrons to energies of about 2.5 GeV for optimized X-ray emission in the range of 1 to 10 keV. A table of the operating parameters of the NSLS X-ray storage ring is provided at their Web site (http://www.nsls.bnl.gov/). Our experiment was set up on beamline X8A, one of 56 X-ray beamlines attached to the X-ray storage ring. The beam energy incident on the filters was selected by adjusting the orientation of the crystal element of a double crystal monochromator located upstream from the vacuum chamber. Each crystal element spans a certain energy range. In particular a W/Si multilayer covers the energy range 0.26-2.0 keV with energy resolution ($\Delta E / E$) of 2x10^-2 a Si(111) crystal covers the range 2.1-5.9 keV with a resolution of 5x10^-4 and a Beryl(1010) crystal covers the range from 0.8-2.0 keV with a resolution of 8x10^-4. In Table 5.1, we list the energies surveyed and the monochromator crystal elements used for the transmission measurements of the ACIS-I and ACIS-S UV/Optical blocking filters. Detailed fine energy scans were performed above the absorption K edges of C, N, O, and Al to investigate extended X-ray absorption fine structure (EXAFS) and shifts in the location of the Al-K edge due to possible Al₂O₃ contamination.

 

Energy Range (eV) Step Size (eV) Monochromator Beam Filter Absorption Edge

260-360 1 W/Si Multilayer Titanium C-K(284eV)

300-600 5 W Si Multilayer Chromium N-K(401.6eV)

500-660 5 W/Si Multilayer Nickel O-K(543.1eV)

660-860 20 W/Si Multilayer Nickel

800-1300 25 W/Si Multilayer Magnesium

1200-1600 20 W/Si Multilayer Aluminum Al-K(1559eV)

1450-1700 30 W/Si Multilayer Nickel Al-K(1559eV)

1700-2000 40 W/Si Multilayer Nickel Si-K(1838.9eV)

1450-1700 2 Beryl(1010) Titanium Al-K(1559eV)

1540-1590 0.5 Beryl(1010) Titanium Al-K(1559eV)

1700-2000 2 Beryl(1010) Titanium Si-K(1838.9eV)

2000-2600 20 Si(111) Titanium

2600-3000 5 Si(111) Titanium Cl-K(2833.0eV)

The configuration of the test set up is shown in Figure 5.1. The ACIS filters were placed into the UC/SAO spectrometer-reflectometer chamber and mounted onto a mechanical fixture. The fixture was made to support both image and spectrometer filters and was mounted on a rotary stage that allowed for the selection of the portion of the filter to be intercepted by the X-ray beam. The X-ray beam was collimated with a set of vertical and horizontal entrance slits to a size of 2 mm by 2 mm. The beam flux was monitored using a detector that was moved in and out of the X-ray beam with a computer controlled actuator.

 

Figure 5.1: Schematic of the test configuration for the measurement of the X-ray transmission of the ACIS filters.