Shuzo Uehara

Cross-sections for water vapour for the Monte Carlo electron track structure code from 10 eV to the MeV region

Electron cross-sections were summarized for an accurate simulation of the track structure of high energy electrons up to a few MeV in water vapour. Elastic scattering was described by the Rutherford formula taking the screening effect into account. Partial and total ionization cross-sections were calculated using Seltzers formula based on the Weizsacker-Williams method. The total excitation cross-section was evaluated by a so-called Fano plot with parameters given by Berger and Wang. The validity of the cross-sections was examined by comparison with experimental and calculated data. Use of these cross-sections enabled the development of a Monte Carlo track structure code KURBUC encompassing the energy range between 10 eV and 10 MeV. Finally the tracks generated by KURBUC were compared with those generated by MOCA8B in terms of radial distributions of interactions, point kernel and frequencies of energy deposition in small cylindrical targets pertaining to biological macromolecules such as DNA.

Comparison and assessment of electron cross sections for Monte Carlo track structure codes.

The purpose of this study was to make an intercomparison and assessment of cross sections for electrons in water used in electron track structure codes. This study is intended to shed light on the extent to which the differences between the input data and physical and chemical assumptions influence the outcome in biophysical modeling of radiation effects. Ionization cross sections and spectra of secondary electrons were calculated by various theories. The analyses were carried out for water vapor cross sections, as these are more abundant and readily available. All suitable published experimental total ionization cross sections were fitted by an appropriate function and used for generation of electron tracks. Three sets of compiled data were used for comparison of total excitation cross sections and mean excitation energy. The tracks generated by a Monte Carlo track code, using various combinations of cross sections, were compared in terms of radial distributions of interactions and point kernels. The spectrum of secondary electrons emitted by the ionization process was found to be the factor that has the most influence on these quantities. A different set of cross sections for excitation and elastic scattering did not affect the electron track structure as much as did ionization cross sections. It is concluded that all codes, using different cross sections and in different phase, currently used for biophysical modeling exhibit close similarities for energy deposition in larger size targets while appreciable differences are observed in B-DNA-size targets. We recommend fitted functions to all available suitable experimental data for the total ionization and elastic cross sections. We conclude that most codes produce tracks in reasonable agreement with the macroscopic quantities such as total stopping power and total yield of strand breaks. However, we predict differences in frequencies of clustering in tracks from the different models.

Heavy charged particles in radiation biology and biophysics

Ionizing radiations induce a variety of molecular and cellular types of damage in mammalian cells as a result of energy deposition by the radiation track. In general, tracks are divided into two classes of sparsely ionizing ones such as electron tracks and densely ionizing tracks such as heavy ions. The paper discusses various aspects and differences between the two types of radiations and their efficacies in radiation therapy. Biophysical studies of radiation tracks have provided much of the insight in mechanistic understanding of the relationship between the initial physical events and observed biological responses. Therefore, development of Monte Carlo track-structure techniques and codes are paramount for the progress of the field. In this paper, we report for the first time the latest development for the simulation of proton tracks up to 200MeV similar to beam energies in proton radiotherapy and space radiation. Vital to the development of the models for ion tracks is the accurate simulation of electron tracks cross sections in liquid water. In this paper, we report the development of electron track cross sections in liquid water using a new dielectric model of low-energy electrons accurate to nearly 10% down to 100eV.

Development of a Monte Carlo track structure code for low-energy protons in water

Purpose : The development of a new generation of Monte Carlo track structure code is described, which simulates full slowing down of low-energy proton history tracks (lephist) in the range 1 keV-1 MeV in water. Material and methods : All primary protons are followed down to 1 keV and all electrons to 1 eV. All primary interactions, including elastic scattering, ionization, excitation and charge exchange processes by protons and neutral hydrogen were taken into account. Cross-sections for proton and hydrogen impact were obtained from experimental data for water. Where data were lacking, the existing experimental data were fitted and extrapolated. The tracks of secondary electrons were generated using the electron track code kurbuc. The cross-sections and the energy transfer data were individually evaluated for the principal interactions induced by protons and hydrogen atoms in water. The analysis starts with the published cross-section data for water using a semi-empirical model including contributions from the neutral hydrogen atoms. For excitation cross-sections, the original Miller-Green analytical formula was used. For ionization by neutral hydrogen atoms, the same energy spectrum was assumed for secondary electrons as for protons. The total cross-sections were taken from the experiment of Blorizadeh and Rudd (1986b, c). For the stripping of charge by neutral hydrogen the data of Toburen et al. (1968) were used. Results : Data are presented on total and differential elastic crosssections as a function of energy and scattering angle respectively; single and double differential cross-sections for secondary electrons ejected by various energy proton impact; total cross-sections due to proton and hydrogen impact on water; stopping power cross-sections; and fraction of stopping power for water for protons as a functions of proton energy. Conclusions : Tracks were analysed to provide confirmation on the reliability of the code and information on physical quantities, such as range, W, restricted stopping power, radial dose profiles and some microdosimetric parameters. Model calculations show good agreement with the experimental and calculated data.

Calculations of electronic stopping cross sections for low-energy protons in water

Abstract Calculations of electronic stopping cross sections for low-energy protons were performed using available interaction cross sections and information on the mean energy loss per collision. The results were compared with published stopping power data to assess the general consistency and reliability of the results. The calculated stopping cross sections include contributions from excitation by both the proton and neutral-hydrogen components of the charge-equilibrated beam and the mean energy lost per interaction. The calculated electronic stopping cross sections for swift charge-equilibrated hydrogen particles in water are in agreement with previous tabulations of stopping power to within 20% (approx.) for particles energies less than 0.3 MeV.

Microdosimetry of low-energy electrons

Abstract Purpose: To investigate differences in energy depositions and microdosimetric parameters of low-energy electrons in liquid and gaseous water using Monte Carlo track structure simulations. Materials and methods: KURBUC-liq (Kyushu University and Radiobiology Unit Code for liquid water) was used for simulating electron tracks in liquid water. The inelastic scattering cross sections of liquid water were obtained from the dielectric response model of Emfietzoglou et al. (Radiation Research 2005;164:202–211). Frequencies of energy deposited in nanometre-size cylindrical targets per unit absorbed dose and associated lineal energies were calculated for 100–5000 eV monoenergetic electrons and the electron spectrum of carbon K edge X-rays. The results for liquid water were compared with those for water vapour. Results: Regardless of electron energy, there is a limit how much energy electron tracks can deposit in a target. Phase effects on the frequencies of energy depositions are largely visible for the targets with diameters and heights smaller than 30 nm. For the target of 2.3 nm by 2.3 nm (similar to dimension of DNA segments), the calculated frequency- and dose-mean lineal energies for liquid water are up to 40% smaller than those for water vapour. The corresponding difference is less than 12% for the targets with diameters ≥ 30 nm. Conclusions: Condensed-phase effects are non-negligible for microdosimetry of low-energy electrons for targets with sizes smaller than a few tens of nanometres, similar to dimensions of DNA molecular structures and nucleosomes.

Physical and biophysical properties of proton tracks of energies 1 keV to 300 MeV in water.

Purpose: To investigate physical and biophysical properties of proton tracks 1 keV–300 MeV using Monte Carlo track structure methods. Materials and methods: We present model calculations for cross sections and methods for simulations of full-slowing-down proton tracks. Protons and electrons were followed interaction-by-interaction to cut-off energies, considering elastic scattering, ionisation, excitation, and charge-transfer. Results: Model calculations are presented for singly differential and total cross sections, and path lengths and stopping powers as a measure of the code evaluation. Depth-dose distributions for 160 MeV protons are compared with experimental data. Frequencies of energy loss by electron interactions increase from ∼3% for 10 keV to ∼77% for 300 MeV protons, and electrons deposit >70% of the dose in 160 MeV tracks. From microdosimetry calculations, 1 MeV protons were found to be more effective than 5–300 MeV in energy depositions greater than 25, 50, and 500 eV in cylinders of diameters and lengths 2, 10, and 100 nm, respectively. For lower-energy depositions, higher-energy protons are more effective. Decreasing the target size leads to the reduction of frequency- and dose-mean lineal energies for protons <1 MeV, and conversely for higher-energy protons. Conclusions: Descriptions of proton tracks at molecular levels facilitate investigations of track properties, energy loss, and microdosimetric parameters for radiation biophysics, radiation therapy, and space radiation research.

The development of a Monte Carlo code simulating electron-photon showers and its evaluation by various transport benchmarks

Abstract The author has developed a Monte Carlo program which simulates electron-photon showers. The physical processes necessary for accurate calculations have been considered. The slowing down process of electrons consists of two components; continuous energy loss while being deflected and discrete energy loss. The discrete electron reactions included inelastic scattering, bremsstrahlung production and positron annihilation in-flight. All the photon interactions including coherent and incoherent scattering, and the emission of characterisitc X-rays have been taken into account. The shower code has been completed by linking the treatments for electrons and photons. Choosing the optimal parameter set, calculations of miscellaneous transport problems have been carried out for the examination of code. The response functions of photon and electron detectors, the electron transport in an Al foil and the electron depth-dose in water have been compared to the benchmarks obtained with the ETRAN and the EGS. The validity of the developed shower code has been confirmed in the energy range up to 30 MeV.

Effect of energy resolution on scatter fraction in scintigraphic imaging: Monte Carlo study.

The effect of energy resolution in detector systems on scatter fractions in scintigraphic imaging through Monte Carlo simulation is investigated. A 10-cm Tc-99m line source within a cylindrical water phantom 20 cm in diameter and 20 cm in length was modeled and energy spectra were calculated at three different line source positions. The energy resolution was changed from 8% to 16% FWHM at 140 keV and a symmetrical energy window width was varied from 8% to 23% on the photopeak of 140 keV in energy spectra corresponding to each energy resolution. The relationship between the scatter fraction and the symmetrical energy window width, and the relationship between the scatter fraction and the primary counts were presented for all energy resolutions investigated. Furthermore, the effect of the asymmetrical energy window on reducing the scatter fraction was also studied and compared with the narrow symmetrical energy window. The results quantitatively showed that improved energy resolution can considerably decrease the scatter fraction with a narrow symmetrical energy window or an asymmetrical energy window without significant primary count-loss compared to that obtained with lower quality energy resolution. The asymmetrical energy window could reduce scatter fraction as compared with the narrow symmetrical energy window when the same number of primary counts was required for both energy windows. Knowing the relationship between the scatter fraction and the primary counts is important in scintigraphic imaging to select the optimum energy window corresponding to the energy resolution.

A database of frequency distributions of energy depositions in small-size targets by electrons and ions

Linear energy transfer (LET) is an average quantity, which cannot display the stochastics of the interactions of radiation tracks in the target volume. For this reason, microdosimetry distributions have been defined to overcome the LET shortcomings. In this paper, model calculations of frequency distributions for energy depositions in nanometre size targets, diameters 1-100 nm, and for a 1 μm diameter wall-less TEPC, for electrons, protons, alpha particles and carbon ions are reported. Frequency distributions for energy depositions in small-size targets with dimensions similar to those of biological molecules are useful for modelling and calculations of DNA damage. Monte Carlo track structure codes KURBUC and PITS99 were used to generate tracks of primary electrons 10 eV to 1 MeV, and ions 1 keV µm(-1) to 300 MeV µm(-1) energies. Distribution of absolute frequencies of energy depositions in volumes with diameters of 1-100 nm randomly positioned in unit density water irradiated with 1 Gy of the given radiation was obtained. Data are presented for frequency of energy depositions and microdosimetry quantities including mean lineal energy, dose mean lineal energy, frequency mean specific energy and dose mean specific energy. The modelling and calculations presented in this work are useful for characterisation of the quality of radiation beam in biophysical studies and in radiation therapy.