Release notes for preliminary quantum detection efficiency curve for ACIS front-illuminated detectors.
29 August 1997
ASCII table suitable for use with qdp. There are two columns:
This file provides a preliminary estimate of the quantum efficiency of the ACIS I3 CCD detector (MIT Lincoln Laboratory model ccid17, serial number w215c2r.) The data are for ASCA grades 0,2,3,4,6, with a split-event threshold of 14 electrons (13 adu; adu= analog-to-digital converter units) and an event threshold of 40 electrons (38 adu).
These estimates were derived from MIT CSR subassembly calibration measurements, and reflect the spatially averaged detection efficiency of the device.
A simple "slab and stop" model of the gate structure (see K. Gendreau, MIT PhD. Thesis, 1995) was fit to relative quantum efficiency measurements of w215c2r at 4 energies (525 eV, 677 eV, 1740 eV and 2015 eV), using reference detector w190c3 as a quantum efficiency standard. The spatially averaged relative efficiencies used in the model fit are 1.002, 1.018,1.000,and 0.993 at these four energies; formal statistical errors in these relative quantum efficiency measurements are about ±4x10-4. Systematic errors in these relative measurements are thought to be less than 3%.
The detection efficiency of the reference detector w190c3 was determined from fits to white-light data obtained at PTB/BESSY synchrotron storage ring. The assumed model parameters for the reference detector were:
See ACIS Team preliminary calibration report (in preparation) for an explanation of the model parameters and other details. The error in the absolute efficiency of the reference detector is believed to be less than 5% in the energy range 0.3-4 keV.
The best-fit parameters for w215c2r used to derive this quantum efficiency curve are:
The depletion depth, which determines the response at higher energies, was constrained using the 5.9 keV branching ratio method of Prigozhin; see the ACIS Team preliminary calibration report (in prepartion) for details. The best fit depletion depth is 65 ±2 µm. This estimate agrees, within 5%, with measurements made relative to the solid state beam normalization detector at the MSFC X-ray Calibration Facility during ACIS flat-field testing.
When combined with ancillary response functions (arf) representing the HRMA effective area and ACIS optical blocking filter transmission as functions of energy, and the normalized response function matrix previously release (w215c2r_norm.html), this quantum efficiency curve may be used for estimating counting rates and simulating spectra obtained from proposed ACIS/AXAF observations.