THE PSU CCD SIMULATOR
A MONTE CARLO METHOD FOR SIMULATING X-RAY CCDs

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

(The various parameter files used by the simulator must stay with the .pro files.)

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.