The External Calibration Source (ExtCalSrc)

The External Calibration Source (hereafter abbreviated ExtCalSrc) consists of three separate radioactive sources held in a common holder. While all three sources contain ⁵⁵Fe, they each predominately produce lines of different energies. Two widely collimated sources use the Mn K$\alpha$ and Mn K$\beta$ photons to fluoresce either Ti or Al and excite those characteristic K emission lines. The third source does not fluoresce another metal and mainly emits only Mn K and L photons. Some of the Mn photons pass through the Al and Ti sources without being absorbed, so these sources also produces Mn K and L lines. The ExtCalSrc illuminates the ACIS focal plane only when the HRC-S instrument is in the telescope focus position. The various emission lines produced by the ExtCalSrc will allow an accurate gain measurement for each node of each ACIS CCD. With sufficiently long integrations, the ratio of the Mn K$\alpha$ flux to the Mn L-Fe L flux can be measured and used to monitor the potential accumulation of contaminants on ACIS (see Section 4.11.2 for the details of this technique). Below, we discuss the spectrum of the ExtCalSrc, the expected count rate in the Al, Ti and Mn K$\alpha$ lines, and the spatial distribution of the flux.