PITS: Positive Ion Track Structure
Simulated 1 MeV proton track segment
Introduction
Computer simulation of charged particle tracks has contributed significantly to our understanding of the micro- and nanodosimetry of radiations in tissue-like matter. Historically, the focus has been on water as a surrogate for soft tissue, inasmuch as tissue is over eighty percent water. Because of the need for radiation track simulations in additional materials, such as DNA, bone, etc., and because input source data and computer hardware and software have advanced considerably, we have rewritten many of the earlier simulation tools. Also, we are extending the simulations to radiations not previously included, such as Bremsstrahlung production and low energy photon interactions (so the 'PITS' acronym is now somewhat of a misnomer).
PITS derives from the ion-transport modules of the MOCA series of detailed-histories charged particle transport codes (Paretzke, 1987); we have modified the modules to allow ion track simulation in a variety of media and optionally to forego delta-ray transport. There has been considerable evolution in computer hardware and software since the introduction of MOCA over two decades ago (Paretzke, 1973). Modernization of the codes, to bring them into conformance with current coding practices makes for easier maintenance and documentation. Parallel versions of the codes are being developed to run, for example, on a Beowulf cluster of workstations.
Documentation
The theoretical basis of the ion simulations is presented in:
W. E. Wilson, J. H. Miller and H. Nikjoo, "PITS: A Code Set for Positive Ion Track Structure",
In Computational Approaches in Molecular Radiation Biology, (Ed. M. N. Varma and A. Chatterjee) Plenum Press, New York, (1994),
and in:
W. E. Wilson and Hooshang Nikjoo, "A Monte Carlo code for positive ion track simulation", Radiat. Environ. Biophys, (1999) 38:97-104. (As a 630KB pdf file, updated and corrected).
Comparison of model algorithm with experimental DDCS, plot (50KB jpg file).
Comparison of simulated DDCS with model algorithm, plot (49KB jpg file).
Documentation of the low energy photon simulations is in preparation.
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PUBLICATIONS
J. H. Miller, W. E. Wilson, D. J. Lynch, M. Resat and H. E. Trease, Computational Dosimetry for Electron Microbeams: Monte Carlo Track Simulation Combined with Confocal Microscopy, Radiat. Res., 156, 438-439, (2001).
W. E. Wilson, D. J. Lynch, K. Wei and L. A. Braby, Microdosimetry of a 25 keV Electron Microbeam, Radiat. Res., 155, 89-94 (2001).
J. H. Miller, M. Sowa Resat, N. F. Metting, K. Wei, D. J. Lynch, and W. E. Wilson, Monte Carlo Simulation of Single-Cell Irradiation by an Electron Microbeam, Radiat. Envir. Biophysics, 39, 173-177 (2000).
REPORTS
Progress Report, (May, 2003), (Oxford0303.pdf, 400KB file)
Progress Report, (May, 2002), (rpt0205, 132KB pdf file)
"Dosimetry Calculations for Electron Microbeams", D. J. Lynch, W. E. Wilson, R.R. Lewis, M. Kameya and J. H. Miller, presented at the 49th annual meeting of the Radiation research Society, April 20-24, 2002, Reno, Nevada, (pstrB0204, 850 KB pdf file).
"Soft X-ray Microbeam Dosimetry", W. E. Wilson, J. H. Miller, D. J. Lynch, and K. Wei, presented at the 49th annual meeting of the Radiation research Society, April 20-24, 2002, Reno, Nevada, (pstrA0204, 1.1MB pdf file).
"Low-LET Microbeam Dosimetry", W. E. Wilson, J. H. Miller, D. J. Lynch, K. Wei and M. Kameya, presented at the Low-Dose Radiation Research Program Workshop, March 25-27, 2002, Rockville MD, (pstr0203, 350KB pdf file).
"Low-LET Microbeam Dosimetry", W. E. Wilson, J. H. Miller, D. J. Lynch, K. Wei and A. Kurtulus, presented at the DOE/NASA Radiation Investigator's Workshop, June 27-30, 2001, Washington, DC, ( Abstract ).
"Low-LET Microbeam Dosimetry", W. E. Wilson, J. H. Miller, D. J. Lynch, K. Wei and A. Kurtulus, a poster paper presented at the 13th Symposium on Microdosimetry, 27 May - 1 June, 2001, Stresa, IT, ( pstr0106, 914KB pdf file).
Progress Report, (January, 2001), ( rpt0101, 16KB pdf file).
"Microdosimetry of a 25 keV Electron Microbeam", by W.E. Wilson, D. J. Lynch, K. Wei and L. A. Braby, ( drft0101, 104KB pdf file).
"Microdosimetry for Low-Dose Low-LET Selected Cell Irradiations", by W. E. Wilson, D. J. Lynch, K. Wei and J. H. Miller, ( drft0006, 384KB pdf file).
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RECENT RESULTS
Electron Microbeam Dosimetry:
Track structure simulations were carried out for monoenergetic electrons with energies between 25 and 80 keV injected into a water medium at a fixed point and perpendicular to the electron-gun exit window. One million tracks were scored at each electron energy to calculate the probability that energy is deposited in a 1 micrometer diameter sphere with a center at depth h in the medium and distance r from the beam axis. We refer to this deposition of energy as an 'event' at the location (h,r). The mean energy deposited in an event was also calculated. The dependence of event frequencies and sizes on h and r was similar at all of the electron energies investigated (25, 30, 50, 60, and 80 keV). This suggests that an analytical representation can be determined that will allow the spatial dependence of these quantities to be calculated at any beam energy between 25 and 80 keV. Progress toward this objective is described in the full progress report (130KB pdf file).
Soft X-ray Microbeam Dosimetry:
Extensive simulations have been made at three X-ray fluorescence energies, (278, 1487, and 4509 eV) and the density distributions in energy imparted evaluated for a series of virtual scoring spheres of diameters (2, 5, 10, 20, 50 and 100 nm). Mapping the penetration and radial extent of soft X-ray microbeam energy deposition follows the same scheme described above for the electron beams, but with necessary differences. Owing to the much lower energies, the appropriate virtual scoring spheres are considerably smaller than for 25 to 100 keV electrons (nanometers rather than micrometers in diameter). Preliminary results are described in the full progress report (130KB pdf file).
The Monte Carlo soft X-ray module (XPITS) has been extended to optionally allow for a Bremsstrahlung 'contamination' of the primary photon source. When enabled, the feature randomly samples from a simple theoretical continuum spectrum at a user-specified average frequency. No self-absorption or filtering is presently assumed for the Bremsstrahlung rays, so the feature provides an estimate of worst-case scenarios. A focused-beam source geometry feature has also been added to XPITS and is being tested; a parallel-code version of XPITS for running on our pc-cluster is under development.
Electron Fluence Calculations:
The spatial variation of electron slowing-down spectra for 50 keV electrons injected into a homogeneous water medium is being calculated for comparisons with photodiode measurements at the PNNL microbeam facility. Slowing-down spectra were integrated over energy with allowance for diode efficiency in order to compare with preliminary experimental data that measured only the total planar fluence of electrons. Corrections were made for the large density difference between measurement done in air and calculations, which assumes unit density water vapor. Good agreement is achieved (full report).
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USAGE
For further information contact the author at the email address below.
Comments and questions: wwilson@tricity.wsu.edu.
Technical Assistance: richarde@tricity.wsu.edu.
Last update, 22 November, 2002. Copyright © Washington State University.
Disclaimer , URL: http://www.tricity.wsu.edu/htmls/eecs/cs/PITS/index.html
