The ACIS reference CCDs were calibrated against the SSD at five primary energies, corresponding to the K lines of Si (1.74 keV), P (2.01), Ti (4.51), Mn (5.89), and Cu (8.03). The Mn lines are generated by radioactive decay of a 10 Ci Fe source with an Al collimator followed by a 2 mm Al aperture. When used with the calibration aperture, this produces a circular spot of irradiation on the CCD approximately 320 pixels (7.7 mm) in diameter.
The other lines are produced by irradiating pure samples of the target element with continuum X-ray emission from a commercial X-ray tube with a molybdenum anode, operated at 15 kV. This radiation efficiently ionizes the target material's inner shell, yielding fluorescent X-ray emission at the characteristic K lines of each material used. This source, referred to as the High Energy X-ray Source (HEXS) is described elsewhere in these proceedings. Depending on the angle between the Mo anode and the surface normal of the target element, the radiation pattern reaching the CCD has a width varying from approximately 13 to 17 mm. The current was adjusted on the commercial X-ray tube to optimize the flux rate for efficient counting with losses due to pulse pile-up limited to or less. Pile-up corrections have been measured as a function of count rate, assuming the X-ray flux is linear with tube current.
For each reference CCD, the calibrated points above 4 keV will be used to model the depletion depth, which is the key parameter governing detection efficiency for the higher energy X-rays. The depletion depth has been estimated to be 47 m for one reference CCD (device w34c3). The flight chips will be operated with higher gate voltages and are estimated to have a depletion depth of order 70 m, increasing the detection efficiency for the higher energies. Other detection parameters such as the CCD oxide layer, which are more important for low energy X-rays (4 keV), are determined from the PTB/BESSY calibrations.