Koji Niita

Improvement of Three-dimensional Monte Carlo Code PHITS for Heavy Ion Therapy

The general purpose particle and heavy ion transport code, PHITS, was modified for improved analysis of dose distribution in carbon therapy systems. We added two new functions into PHITS, one for an energy dispersion calculation and the other for transport in an AC magnetic field, which enabled 3-dimensional modelling of a carbon therapy system for the first time. With this code we calculated the dose distribution in a carbon therapy system, and these results showed good agreement with experimental data. This improved version of PHITS is a valuable tool for the design of carbon therapy aperture or for the estimation of the dose distribution in treatment planning.

Evaluation of dose rate reduction in a spacecraft compartment due to additional water shield

The dose reduction rates brought about by the installation of additional water shielding in a spacecraft are calculated in the paper using the particles and heavy ion transport code system PHITS, which can deal with transport of all kinds of hadrons and heavy ions with energies up to 100 GeV/n in three-dimensional phase spaces. In the PHITS simulation, an imaginary spacecraft was irradiated isotropically by cosmic rays with charges up to 28 and energies up to 100 GeV/n, and the dose reduction rates due to water shielding were evaluated for 5 types of doses: the dose equivalents obtained from the LET and linear energy spectra, the dose equivalents to skin and red bone marrow, and the effective dose equivalent. The results of the simulation indicate that the dose reduction rates differ according to the type of dose evaluated. For example, 5 g/cm2 water shielding reduces the effective dose equivalent and the LET dose equivalent by approximately 14% and 32%, respectively. Such degrees of dose reduction can be regarded to make water shielding worth the efforts required to install it.

Benchmarking for High-energy Physics Applications Using by PHITS code

Research Organization for Information Science – Technology (RIST), 2-4 Shirane, Tokai-mura, Ibaraki, 319-1106, Japan Particle and Heavy Ion Transport code System (PHITS) support a broad range of research activities: radiation shielding and dosimetry, radiotherapy and space science as well as the high-energy physics. In this paper, various benchmark calculations based on high-energy collision experiments are carried out using the PHITS code: particle production (positive and negative pion) on thin or thick targets (hydrogen, carbon and aluminum), energy deposition in target and peripheral equipment. On the whole, the good agreement between PHITS calculations and experimental data is shown for many cases.

Improvements and developments of physics models in PHITS for space applications

Precise predictions of the radiation environment inside space vehicles, and inside the human body, are essential when planning for long term deep space missions. Since these predictions include complex geometries, as well as the contributions from many different types of radiation, including neutrons, 3-D Monte Carlo codes with precise physics models are needed. In this paper, we present improvements and developments of some physics models used in the general purpose 3-D Monte Carlo code PHITS [1]. The total reaction cross section (σR) and the decay lifetime of a projectile particle are the first essential quantities in MC calculations, since these determine the mean free path of the transported particles and the probability function according to which a projectile particle will collide within a certain distance in the matter depends on the σR. This will also scale the calculated partial fragmentation cross sections. In this paper we present comparisons of calculated and measured σR using the Kurotama Hybrid σR, model [2] which is incorporated into PHITS. The prediction of the fragmentation reactions of relativistic heavy ions is also essential for ensuring radiation safety of astronauts. The default model for nuclear-nuclear reactions is JQMD in PHITS. However, JQMD cannot accurately enough describe the nucleon and d, t, 3He and 4He induced reactions. Therefore the Intra-Nuclear Cascade of Liège (INCL) [3] has been selected as the default model for these reactions. Moreover, it has been realized that the production of light fragments is underestimated by conventional simulation codes based on a combination of intranuclear cascade and statistical decay models. This is because this combination cannot reproduce the high multiplicity events that are responsible for the production of light fragments. To better reproduce high multiplicity events, we have simulated fragmentation cross sections using a combination of JQMD/INCL, statistical multi-fragmentation model (SMM) [4,5] and the generalized evaporation model (GEM). Examples of these simulations will be presented. A new approach to describe neutron spectra of deuteron-induced reactions in the Monte Carlo simulations has also been developed by combining the INCL and the Distorted Wave Born Approximation (DWBA) calculation [6]. We have incorporated this combined method into PHITS and applied it to estimate (d,xn) spectra on light targets at incident energies ranging from 10 to 40 MeV. In this paper, we will show that the double differential cross sections obtained by INCL and DWBA successfully reproduced broad peaks and discrete peaks, respectively.

Profile of Energy Deposition in Human Body Irradiated by Heavy Ions

Radiological protection against high energy heavy ions has been an essential issue in the planningof long term space missions. A detailed analysis of profi1es of energy deposition inside a human bodyirradiated by such heavy ions is indispensable for a proper determination of the radiation qualities of those particles. We therefore perfomed Monte Carlo calculations of dose distributions in temns ofthe linear energy transfer of ionizing particles (dose-LET distribution) using a newly developed particles transport code PHITS for incidences of various kinds of heavy ions with energies up to 3 GeV/A. We found from results that the radiation qualities of heavy ions directly reflect profiles of primaryparticles ,opposite to the hadron incident cases in which the radiation qualities significantly depend on profiles of secondary particles. The maximum value of the mean quality factors derived from the Q-L relationship is approximately 20,which coincides with the radiation weighting factor for heavy ions.

CALCULATION OF SECONDARY NEUTRON FIELDS GENERATED BY HIGH-ENERGY HEAVY-ION REACTIONS USING MONTE-CARLO CODE PHITS

The neutron fields generated by high-energy heavy ions in a production target are of key importance as they are used as source terms for further transport calculations. Various calculations of secondary neutron fields generated by high-energy heavy-ion reactions, such as benchmarks with systematic data, neutron energy spectra and doses compared with data measured recently in GSI, and calculations for the Super-FRS project, are done. Comparison of measured neutron fields with calculations performed by the PHITS code will be included for more precise predictions for secondary neutron productions.

Calculation of energy-deposition distributions of a 9C beam using the PHITS code

Carbon-9 beams represent one of the possible alternatives (presently under consideration and investigation) to conventional 12C beams for heavy-ion cancer treatment. Interest in this exotic isotope stems from the expected boost in biological effectiveness due to the β-delayed emission of two α particles and a proton that takes place at the ion stopping site. Experiments have been performed [1] to characterise 9C beams physically and models have been developed [2] to estimate quantitatively its biological effect. In this work, we have used the PHITS code [3] to calculate energy-deposition and LET distributions for a 9C beam in water and we have compared the results with some published data [1]. Even though PHITS fails to reproduce some of the features of the distributions, its result is that decay gives a negligible contribution to the energy-deposition distributions, thus contradicting the previous interpretation of the measured data