The Advanced X-ray Astrophysics Facility (AXAF) is an orbiting X-ray telescope being developed as part of NASA's Great Observatories program for launch in 1998.[1] AXAF will provide high-angular-resolution imaging spectroscopy and transmission grating spectroscopy in the 0.1-10 keV band. The MIT Center for Space Research, together with the MIT Lincoln Laboratory and Pennsylvania State University, is developing the AXAF CCD Imaging Spectrometer (ACIS), one of AXAF'S two scientific focal plane instruments.[2]
To unfold accurately the X-ray spectra of astrophysical sources, careful calibration of the ACIS instrument is required. The AXAF mirror assembly and science instruments are being calibrated at both the instrument and system levels. AXAF scientific objectives demand extremely accurate knowledge of instrumental response; for example, the goal for detection efficiency knowledge accuracy is of order 1%. The ACIS instrument calibration program, summarized elsewhere in these proceedings[3], includes measurements of the CCD energy scale and spectral response function, as well as absolute measurements of X-ray CCD detection efficiency as a function of X-ray energy and detector position.
Detection efficiency is measured by comparing the response of a flight CCD with that of a reference CCD when exposed to laboratory X-ray sources described elsewhere in these proceedings[4]. The reference CCDs serve as absolutely calibrated transfer standards and are in turn calibrated against two overlapping standards: The first is a synchrotron light source whose spectral radiant intensity is known absolutely for energies up to 4 keV (flux is limited at higher energies). We have used the facilities of the Physikalisch-Technishe Bundesanstalt (PTB) laboratory at the Berlin Electron Storage ring BESSY[5] for this purpose. The synchrotron beam is passed through a monochromator producing a tunable, highly monochromatic X-ray beam ideal for calibration measurements[6]. A full discussion of the CCD calibration at PTB/BESSY will be presented elsewhere.
The second standard is a Si(Li) solid state detector (SSD), utilized
as a standard for energies above 4 keV and as a comparison to the
BESSY calibration for energies from 2 - 10 keV. The response of the
SSD in the range 2 - 10 keV is calculable to reasonable accuracy
(better than 
) from first principles, given that the
thickness and composition of all windows and front surface layers are
known. Ultimately, we plan to calibrate our SSD against another
transfer standard detector for the AXAF program.
In this paper, we focus on initial results on the calibration of several reference CCDs relative to the Si(Li) SSD. The use of the reference CCDs is first discussed, after which the SSD is described in detail. The geometry of the calibration setup and X-ray sources utilized are then discussed, followed by a discussion of analysis methods. The paper closes with a presentation of sample data and initial results, along with plans for further measurements and analysis.