Comparison between PHITS and GEANT4 Simulations of the Heavy Ion Beams at the BEVALAC at LBNL and the Booster Accelerator at BNL

Heavy charged particles have been discussed for clinical use due to their superior dose-depth distribution compared to conventional radiation such as X-rays. In addition, high-charge and energy (HZE) ions in galactic cosmic rays (GCR) present important health risks for crewed space missions to the Earth's moon or Mars. Experiments at heavy ion accelerators are used in radiobiology studies; however, numerical simulations of track segment or Bragg peak irradiations are complicated by the details of the beam-line and dosimetry systems. The goal of the present work is in support of biophysics modeling of historical radiobiology data at Lawrence Berkeley National Laboratory (LBNL) and more recent results from the Brookhaven National Lab (BNL) facility (NASA Space Radiation Lab (NSRL)). In this work, the Spread-Out Bragg Peak (SOBP) of 4He, 12C, and 20Ne particles, and a Bragg curve of 56Fe ion have been simulated numerically in the geometries of LBNL and BNL using the Monte-Carlo based PHITS and GEANT4 simulation toolkits. The dose contributions of primary particles and secondary particles, including neutrons and photons, in the target material are computed and discussed as well. Comparisons suggest more contributions of secondaries in GEANT4 simulations compared to PHITS simulations, and less statistical fluctuation and better prediction of neutrons in PHITS simulations. Neutrons and gamma-rays are estimated to make minor contributions to absorbed doses for these beams.

Twenty years of space radiation physics at the BNL AGS and NASA Space Radiation Laboratory

Highly ionizing atomic nuclei HZE in the GCR will be a significant source of radiation exposure for humans on extended missions outside low Earth orbit. Accelerators such as the LBNL Bevalac and the BNL AGS, designed decades ago for fundamental nuclear and particle physics research, subsequently found use as sources of GCR-like particles for ground-based physics and biology research relevant to space flight. The NASA Space Radiation Laboratory at BNL was constructed specifically for space radiation research. Here we review some of the space-related physics results obtained over the first 20 years of NASA-sponsored research at Brookhaven.

Influence of track directions on the biological consequences in cells irradiated with high LET heavy ions

PURPOSE

The impact of the damage distribution to cellular survival and chromosomal aberrations following high Linear Energy Transfer (LET) radiation was investigated.

MATERIALS AND METHODS

High LET iron-ions (500 MeV/n, LET 200 keV/μm) were delivered to G1-phase synchronized Chinese Hamster Ovary (CHO) cells located at a vertical or horizontal angle relative to the ion beam in order to give same dose but different fluence and damage distribution.

RESULTS

Horizontal irradiation produced DNA double-strand break (DSB) along each ion track represented as clustered lines, and vertical irradiation produced a greater fluence. The initial damages measured by premature chromosome condensation were equal per dose in both irradiation types. Horizontal irradiation proved to be less effective in cell killing than vertical at doses of more than 3 Gy. Vertical irradiation produced a higher number of metaphase chromosomal aberrations compared to horizontally irradiated samples. In particular, formation of exchange-type aberrations was the same, but that of deletion-type aberrations were significantly higher after vertical irradiation than horizontal irradiation.

CONCLUSIONS

Therefore, we concluded that high fluence per dose is more effective than low fluence per dose to produce radiation-induced chromosomal aberrations and to kill exposed cells following high LET heavy-ion exposure.

Validation of the HZETRN code for laboratory exposures with 1A GeV iron ions in several targets

A new version of the HZETRN code capable of validation with HZE ions in either the laboratory or the space environment is under development. The computational model consists of the lowest order asymptotic approximation followed by a Neumann series expansion with non-perturbative corrections. The physical description includes energy loss with straggling, nuclear attenuation, nuclear fragmentation with energy dispersion and downshift. Measurements to test the model were performed at the Alternating Gradient Synchrotron and the NASA Space Radiation Laboratory at Brookhaven National Laboratory with iron ions. Surviving beam particles and produced fragments were measured with solid-state detectors. Beam analysis software has been written to relate the computational results to the measured energy loss spectra of the incident ions for rapid validation of modeled target transmission functions.

Influence of fragment reaction of relativistic heavy charged particles on heavy-ion radiotherapy

The production of projectile fragments is one of the most important, but not yet perfectly understood, problems to be considered when planning for the utilization of high-energy heavy charged particles for radiotherapy. This paper reports our investigation of the fragments' fluence and linear energy transfer (LET) spectra produced from various incident ions using an experimental approach to reveal these physical qualities of the beams. Polymethyl methacrylate, as a substitute for the human body, was used as a target. A deltaE-E counter telescope with a plastic scintillator and a BGO scintillator made it possible to identify the species of fragments based on differences of various elements. By combining a gas-flow proportional counter with a counter telescope system, LET spectra as well as fluence spectra of the fragments were derived for each element down from the primary particles to hydrogen. Among them, the information on hydrogen and helium fragments was derived for the first time. The result revealed that the number of light fragments, such as hydrogen and helium, became larger than the number of primaries in the vicinity of the range end. However, the greater part of the dose delivered to a cell was still governed by the primaries. The calculated result of a simulation used for heavy-ion radiotherapy indicated room for improving the reaction model.

Benchmark studies of the effectiveness of structural and internal materials as radiation shielding for the international space station

Accelerator-based measurements and model calculations have been used to study the heavy-ion radiation transport properties of materials in use on the International Space Station (ISS). Samples of the ISS aluminum outer hull were augmented with various configurations of internal wall material and polyethylene. The materials were bombarded with high-energy iron ions characteristic of a significant part of the galactic cosmic-ray (GCR) heavy-ion spectrum. Transmitted primary ions and charged fragments produced in nuclear collisions in the materials were measured near the beam axis, and a model was used to extrapolate from the data to lower beam energies and to a lighter ion. For the materials and ions studied, at incident particle energies from 1037 MeV/nucleon down to at least 600 MeV/nucleon, nuclear fragmentation reduces the average dose and dose equivalent per incident ion. At energies below 400 MeV/nucleon, the calculation predicts that as material is added, increased ionization energy loss produces increases in some dosimetric quantities. These limited results suggest that the addition of modest amounts of polyethylene or similar material to the interior of the ISS will reduce the dose to ISS crews from space radiation; however, the radiation transport properties of ISS materials should be evaluated with a realistic space radiation field.

Monte Carlo transport model comparison with 1A GeV accelerated iron experiment: heavy-ion shielding evaluation of NASA space flight-crew foodstuff

Deep-space manned flight as a reality depends on a viable solution to the radiation problem. Both acute and chronic radiation health threats are known to exist, with solar particle events as an example of the former and galactic cosmic rays (GCR) of the latter. In this experiment Iron ions of 1A GeV are used to simulate GCR and to determine the secondary radiation field created as the GCR-like particles interact with a thick target. A NASA prepared food pantry locker was subjected to the iron beam and the secondary fluence recorded. A modified version of the Monte Carlo heavy ion transport code developed by Zeitlin at LBNL is compared with experimental fluence. The foodstuff is modeled as mixed nuts as defined by the 71st edition of the Chemical Rubber Company (CRC) Handbook of Physics and Chemistry. The results indicate a good agreement between the experimental data and the model. The agreement between model and experiment is determined using a linear fit to ordered pairs of data. The intercept is forced to zero. The slope fit is 0.825 and the R2 value is 0.429 over the resolved fluence region. The removal of an outlier, Z=14, gives values of 0.888 and 0.705 for slope and R2 respectively.

Approach and issues relating to shield material design to protect astronauts from space radiation

One major obstacle to human space exploration is the possible limitations imposed by the adverse effects of long-term exposure to the space environment. Even before human spaceflight began, the potentially brief exposure of astronauts to the very intense random solar energetic particle (SEP) events was of great concern. A new challenge appears in deep space exploration from exposure to the low-intensity heavy-ion flux of the galactic cosmic rays (GCR) since the missions are of long duration and the accumulated exposures can be high. Since aluminum (traditionally used in spacecraft to avoid potential radiation risks) leads to prohibitively expensive mission launch costs, alternative materials need to be explored. An overview of the materials related issues and their impact on human space exploration will be given.