Release notes for the S3 (w134c4r) detector quantum efficiency curve w134c4r_pre_eff_997.qdp

Notes for pre-release quantum detection efficiency curve
for ACIS back-illuminated detector S3 = w134c4r.  
Note: these data represents work in progress; substantial
uncertainties pertain. See "intended use" and "notes/bugs" below.
on "intended use" below.
14 Sept 97 M. Bautz/

Filename: 
w134c4r_eff_pre_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  the best currently available  estimate of the 
quantum efficiency
of the ACIS S3 CCD detector (MIT Lincoln Laboratory model ccid17, 
serial number w134c4r, a back-illuminated device.)  
The data are for ASCA grades 0,2,3,4,6, with
a split-event threshold of 15 electrons (13 adu; adu= analog-to-digital
converter units) and an event threshold of 45 electrons (20 adu).

These estimates were derived from MIT CSR subassembly calibration 
measurements and from XRCF Phase I (flat-field) measurements
and reflect the spatially averaged detection efficiency of the device.

A simple three-component model of the device response
was fit to relative quantum efficiency measurements. The model includes
uniform dead layers of silicon and silicon dioxide; the total thickness
of the photosensitive region is the third model parameter. 

Two data sets were used. Relative quantum efficiency 
 (QE)  data were obtained at CSR with respect to
a standard detector (w203c2, a front-illuminated device for
 calibrated at PTB/Bessy)
To allow joint fits with
XRCF Phase I data, the MIT relative QE data were converted to relative
efficiencies with respect to the S2 detector, w182c4r, using the
relative QE of w182c4r and reference detector w103c4r. The BESSY
calibration of w203c2 was not used in this conversion.

XRCF (ACIS telemetry) data from Phase I provided additional 
measurements of the QE of w134c4r relative to S2 (w182c4r). At four
of the six energies sampled by both the XRCF and CSR data 
(viz., 1.74, 2.1, 4.5 and 8.0 keV), there
is agreement in the efficiency with respect to S2 within 6%. At
lower energies (0.53 and 0.7 keV) there are disagreements in the
the two data sets that are as large as 25%. The source of this
discrepancy is unknown. 
Somewhat arbitrarily, we have chosen to ignore the XRCF data at 
0.525 and 0.705 keV, although we include XRCF phase I data at 0.277 keV,
in the process of modeling the QE of w134c4r.

The relative QE of w134c4r (S3)  with respect to w182c4r(S2) was
converted to an absolute detection efficiency using the best-fit
model for the absolute detection efficiency of w182c4r contained
in w182c4r_eff_997.qdp.  Data used in the fit are listed below:

Absolute QE of w134c4r obtained from relative QE measurements at MIT
follows:
	Energy	QE 
	(kev)   (g02346)
	0.525 1.003 
	0.677 0.888 
	1.74 0.9844 
	2.015 0.9636 
	4.509 0.96 
	5.894 0.7497 
	6.399 0.6536 
	8.04 0.415368 

The absolute QE of w134c4r obtained from relative QE measurements 
with respect to S2 at XRCF follows:
	Energy	QE 
	(kev)   (g02346)
	0.277 0.9639 
	0.525 1.1424  (excluded from w134c4r model fit)
	0.705 1.2382  (excluded from w134c4r model fit)
	1.489 0.9856 
	1.74 1.0028 
	2.166 0.9782 
	4.509 0.9024 
	8.04 0.43416 

The best-fit model parameters for w134c4r used to generate this
curve follow:						90% 1-parm conf. int.
	Deadlayer Silicon Thickness: 	1.e-4 microns 	
	Deadlayer Si02 Thickness:	2.3e-2 microns	(-2.e-3,+7.3e-3)
	Photo-sensitive Thickness	39.7 microns	(+-4 microns)

Intended use:
This curve is intended for use by ASC to illustrate the performance of
ACIS back-illuminated detectors. Given the (yet-to-be-resolved) 
inconsistencies
in data analyzed to date, it is difficult to assign an accurate errors
to this QE estimate. See notes for an attempt. In any event, 
THIS CURVE SHOULD NOT BE USED FOR PURPOSES  REQUIRING ERRORS LESS
THAN 25% BELOW 1.5 keV.

Notes/Bugs: 
1. Given as-yet unexplained inconsistencies between MIT and XRCF
data below 1.5 keV (see above), it is not possible to form reliable error 
 estimates for this qe curve in this spectral region. Taking the magnitude 
of the
inconsistencies as a measure of the likely uncertainties suggests
errors no greater than  10% at energies exceeding 1.5 kev.  Errors at 
lower energies are undoutedly  larger, and 
may be  as large as 25% in the immediate vicinity of the oxygen edge.

2. The relative QE data from MIT CSR should be reanalyzed and related 
 directly  to BESSY-calibration of w203c2. Additional data (comparing 
w134c4r to w147c3, a BESSY-calibrated, back-illuminated  device) should
be used to constrain the quantum efficiency model.

3. The quantum efficiency model effectively assumes branching ratios
which are independent of energy for both front- and back-illuminated devices.
This assumption is not strictly correct.

4. The quantum efficiency w134c4r is similar to that of  
the other back-illuminated device in the ACIS focal plane, w140c4r(S1) at 
energies (less than 4 keV), but S3 is considerably 
more efficient than S1 at higher energies (25% more efficient at 8 keV).

5. Spatial variations in detection efficiency across w134c4r can be as large 
as 15-20%, RMS. 

6. The model qe data in this file (w134c4r_eff_pre_997.qdp) are replica of
/benz/h4/mwb/asc/obs_guide/qe/newbi/w134c4r_model_data_rev.qdp of
 0637 Sep 15