Release notes for the S2 (w182c4r) detector quantum efficiency curve w182c4r_eff_997.qdp

Release notes for preliminary quantum detection efficiency curve
for ACIS front-illuminated detector S2 = w182c4r.  
12 Sept 97 M. Bautz

Filename: 
w182c4r_eff_997.qdp

Format: 
ASCII table suitable for use with qdp. There are two columns:
1) Energy (keV)
2) Quantum Efficiency (for ASCA grades 0,2,3,4,6)

Description:
This file provides a preliminary estimate of the quantum efficiency
of the ACIS S2 CCD detector (MIT Lincoln Laboratory model ccid17, 
serial number w182c4r.)  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 w182c4r at 4 energies (525 eV, 677 eV, 1740 eV and 2015 eV), 
using reference detector w103c4 as a quantum efficiency standard.

The spatially averaged relative efficiencies used in the model
fit are 1.009, 0.965, 0.987,and 1.017 at these four energies, as presented
in the the ACIS Preliminary Calibration Report. Formal statistical
errors in these relative quantum efficiency measurements are about +-4e-4.
Systematic errors in these relative measurements are thought to be less 
than 3%.

The detection efficiency of the reference detector w103c4 was determined from
fits to measurements of the response of the detector 
to undispersed synchrotron radiation data obtained at PTB/BESSY synchrotron 
storage ring. The assumed model parameters for the reference detector were:
	Estimated Gate Structure Parameters for w103c4 Reference Detector
	Si	0.284 (micron) (fit)
	Si02	0.215 (micron) (fit)
	SiN3	0.033 (micron) (fit)
	CSSiOx	0.450 (micron) (fixed,sem)
	CSWidth	4.1   (micron)  (fixed,sem)
	CSSi	0.35 (micron)  (fixed, mesh)
	Depl.   57.9 (micron) (fixed, branching ratio)
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 w182c4r used to derive this quantum efficiency
curve are: 
	Si	0.230 (micron)	(fit)
	Si02	0.243 (micron)	(fit)
    	Si3N4	0.048 (micron)	(fit)
    	CSSiOx	0.45 (micron) (fixed,sem)
  	CSWidth 4.1 (micron) (fixed,sem)
    	CSSi  	0.35 (micron) (fixed,mesh)
	Depl.	75.0  (micron)  (fixed high-energy branching ratios)

Formal errors on the first three of these parameters are relatively 
large: single-parameter 90% confidence intervals are +-0.04 microns,
+-0.015 microns, and +-0.0.018 microns for Si, SiO2 and Si3N4, respectively.
The errors on these parameters are highly correlated.

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 75 +-2 microns.  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.

Intended use:
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.rmf (see http://www.acis.mit.edu), this quantum 
efficiency
curve may be used for estimating  counting rates and simulating
spectra obtained from  proposed ACIS/AXAF observations. 

Notes/Bugs: 
1. Errors in this
efficiency curve are believed to be no greater than 15%, except at energies
in the immediate vicinity (within 20 eV) of characteristic absorption edges
of N, O and Si, where near edge structure (which has been measured but not
included in this prediction) is known to cause quantum efficiency variations
of order 20-30%.  XAFS will be included in subsequent releases of ACIS
quantum efficiency curves.

2. The model for absolute standard used to calibrate this detector, w103c4,
was fit to Bessy data without performing pileup corrections. This will be
remedied in subsequent releases.

3. The model for the relative quantum efficiency of w182c4 with respect to
w103c4 in the 0.5-2 keV range does not account for differences in the 
depletion depth of these two devices. Since this difference is nearly
20 microns, this omission will cause the efficiency of w182c4 to be over-
estimated at energies just below the Si K edge (1.84 keV); the magnitude of
the overestimate is less than 2%.

4. This quantum efficiency relation  is  expected to be fairly representative 
of all ACIS FI detectors. However, w182c4r has slightly higher
quantum efficiency than the other front-illuminated detectors in the ACIS
focal plane, particularly at energies above 6keV, where the chip-to-chip 
differences in QE can be as high as 15%. Spatial variations in quantum 
efficiency within one ACIS FI detector are generally less than 5% when 
averaged on 32x32 pixel 
scales.

5. The gate structure models assumed here (slab and stop) are too simple and
will be elaborated in subsequent releases.

6. This data in this file (w182c4r_eff_897.qdp) are replica of
/suiko/d3/ti/Response/w182c4r/mwb/w182c4rfkb_wrt_w103c4mjp.qdp as of 
Sep 12 17:01