Leisa Townsley and
Patrick Broos
Penn State University
INTRODUCTION
The Advanced CCD Imaging Spectrometer (ACIS) is a CCD camera that
operates
in photon-counting mode to record both spatial and spectral information
from celestial X-rays. Each photon that is stopped by
photoabsorption
produces a cloud of secondary electrons; this charge cloud is pixelized
and recorded as an ``event'' with well-defined characteristics,
including
position, amplitude, and a measure of its spatial concentration
(``grade'').
We have developed a Monte Carlo CCD simulator for use in characterizing and calibrating the X-ray CCD detectors in the ACIS instrument on the Chandra X-ray Observatory. It contains machinery for simulating all typical kinds of X-ray CCDs: bulk front-illuminated, epitaxial front-illuminated, and back-illuminated.
The work discussed here was conducted by Leisa
Townsley and Patrick
Broos
(with help from several others) to support their involvement in the
analysis
of ACIS data. This work is NOT an official product of the
ACIS Team
and has NOT been endorsed by the ACIS Team or the Chandra X-ray Center
(CXC).
THE
TECHNIQUE
Our Monte Carlo simulations are used to augment ACIS calibration data;
once the simulator is tuned to reproduce monochromatic calibration
data,
it can be used to predict the instrument's spectral response at
energies
unavailable in the laboratory. This ability is used to
generate the
ACIS response matrix, the mathematical representation of the ACIS
spectral
redistribution function. The tool can also be used to
simulate photon
pile-up, which can occur when bright sources are observed. It
also
incorporates a
model
for charge transfer inefficiency (CTI), a phenomenon that
spectrally
and spatially degrades events from ACIS detectors.
The basic aspects of the data that we are attempting to reproduce are the detected CCD event spectrum, the quantum efficiency, and the event grade distribution (branching ratios) resulting from a monochromatic incident X-ray flux. The current model relies on new solutions of the diffusion equation to predict the radial charge cloud distribution in field-free regions of CCDs (Pavlov and Nousek 1999, "Charge Diffusion in CCD X-ray Detectors," Nuclear Instruments and Methods in Physics Research A, 428, 348). By adjusting the size of the charge clouds, we can attempt to reproduce the grade distribution seen in ACIS calibration data event lists. We have built into our code a model for channel stops and an interpretation of the MIT/ACIS group's model for the insulating layer under the gate structure (Prigozhin et al. 2000, "The Physics of the Low Energy Tail in the ACIS CCD. The Spectral Redistribution Function," Nuclear Instruments and Methods in Physics Research A, 439, 582). These components are necessary to explain subtle redistribution features in the spectra.
Three papers have been published describing our work. You can get to these from ADS if you are careful to check the Physics box in "Databases to query" at the top of the page.
Mitigating
Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced
CCD Imaging Spectrometer
\bibitem[Townsley et al.(2000)]{2000ApJ...534L.139T} Townsley, L.~K.,
Broos, P.~S., Garmire, G.~P., \& Nousek, J.~A.\ 2000, \apjl,
534, L139
Simulating CCDs
for the Chandra Advanced CCD Imaging Spectrometer
\bibitem[Townsley et al.(2002)]{2002NIMPA.486..716T} Townsley, L.~K.,
Broos, P.~S., Chartas, G., Moskalenko, E., Nousek, J.~A.,
\& Pavlov, G.~G.\ 2002, Nuclear Instruments and Methods in
Physics Research A, 486, 716
Modeling charge transfer
inefficiency in the Chandra Advanced CCD Imaging Spectrometer
\bibitem[Townsley et al.(2002)]{2002NIMPA.486..751T} Townsley, L.~K.,
Broos, P.~S., Nousek, J.~A.,
\& Garmire, G.~P.\ 2002, Nuclear Instruments and Methods in
Physics Research A, 486, 751
THE CODE
CCD simulation code and a CTI
correction tool (both in the IDL language) are provided in
the hope that they
may assist other investigators interested in simulating ACIS devices or
other X-ray CCDs.
You must have IDL and Wayne Landsman's IDL Astro Library .
Choose a directory to hold our software and arrange for it to be in your IDL path. The procedure for doing this may depend on how your IDL site is administered. If you have a personal IDL startup file you could put a statement similar to this in it:!PATH = '+/home/townsley/idl/acis:' + !PATH
Download two compressed tarballs to the directory you've chosen, one containing our TARA software and one containing the CCD simulator, then expand the tarballs, e.g.
tar -xzvf
tara2008nov18.tar.gz
tar -xzvf ccdsim2008nov18.tar.gz
EXAMPLE
The most straightforward way to run the CCD simulator is to supply it
with X-ray photons generated by MARX.
For example
idl
.run detect_marx_rays
InitializeDomainDataset, GUI_ACTIVE=0, /UNDEFINE_PROPERTIES, VERBOSE=0
detect_marx_rays, 'marx', 'sim', 3, 50, TSTART=279591231.8, TEMPERATURE=-120, /WRITE_EVENTS
In this example, a MARX simulation of a source on CCD 3 previously created in the directory 'marx' is passed through the CCD simulator and an event list is written to the directory 'sim'. CTI levels appropriate for the specified mission time and the nominal -120C temperature are included in the simulation, and the resulting events are then passed through the Penn State CTI corrector. For efficiency, only a 50x50 pixel region of the CCD around the source is actually simulated.