What is EXPACS?

EXPACS represents "EXcel-based Program for calculating Atmospheric Cosmic-ray Spectrum". It can instantaneously calculate terrestrial cosmic ray fluxes of neutrons, protons, ions with charge up to 28 (Ni), muons, electrons, positrons, and photons nearly anytime and anywhere in the Earth's atmosphere. It is based on the PARMA model: PHITS based Analytical Radiation Model in the Atmosphere, which comprises numerous analytical functions with parameters whose numerical values were fitted to reproduce the results of the extensive airshower simulation performed by Particle and Heavy Ion Transport code System (PHITS) [1]. Based on the calculated cosmic-ray fluxes, EXPACS can also deduce the effective dose, ambient dose equivalent, and absorbed dose in air due to cosmic-ray exposure. The detailed description of the model is given in Refs [2-4]. The calculated dose rates can be visualized on the map of Google Earth^TM using EXPACS-V, which can be also downloaded from this web site.

Introduction

Fig.1 Schematic view of the atmospheric propagation of cosmic-ray

People are subjected to be exposed to various kinds of natural radiations, and it is estimated that roughly 20% of their doses are attributed to the terrestrial cosmic-ray exposure. However, the cosmic-ray dose rates significantly depend on various parameters such as altitude and solar modulation potential, and none of the existing models are able to predict the values with satisfactory accuracy.
Furthermore, radiation protection for aircrews against the terrestrial cosmic-rays has been intensively discussed since the publication of ICRP60 in which the aircrew exposure is recognized as occupational hazard. Development of aircrew-dose calculation code is the one of the most important issues in the discussion. Establishment of reliable model for calculating the cosmic-ray spectra for any global conditions is the key issue in the development. It should be also noted that estimation of the spectra is of great importance from the viewpoint of geoscience.

Monte Carlo Simulation

With these situations in mind, we have calculated the cosmic-ray spectra by performing Monte Carlo (MC) particle transport simulation in the atmosphere based on the Particle and Heavy Ion Transport code System PHITS. In the simulation, cosmic-rays with charges up to 28 (Ni) with their spectra calculated by DLR model [5] were considered as the source particles. The results of the simulation are depicted in Fig. 2 in comparison with the corresponding experimental data [6-12]. Excellent agreements can be observed between the data, indicating the accuracy of our simulation.

Fig.2 Cosmic-ray spectra at various locations obtained by our simulation (PHITS) in comparison with the experimental data [6-12]. (Data are calculated by EXPACS version 2.0)

Analytical Model: PARMA

It is extremely time-consuming to perform the MC simulation of the cosmic-ray propagation for each route-dose calculation even using the latest computer. Hence, assumption or parameterization is thus required to allow the MC obtained spectra to be used in a route dose calculation code. Based on a comprehensive analysis of the simulation results, we proposed an analytical model for estimating the atmospheric cosmic-ray spectra for neutrons, protons, He nuclei, muons, electrons, positrons and photons applicable to any global conditions. The model was designated "PARMA": PHITS based Analytical Radiation Model in the Atmosphere.
The Figs. 3 and 4 shows the cosmic-ray spectra and the altitude dependence of cosmic-ray dose rates at Tokyo (latitude 35.7N, longitude 139.7E), respectively, obtained by PARMA and the MC simulation. It is evident from the figure that PARMA enables us to calculate the cosmic-ray doses instantaneously with precision equivalent to that of the MC simulation that requires much computational time.

Fig.3 Calculated cosmic-ray spectra at Tokyo. (Data are calculated by EXPACS version 2.0)

Fig.4 Altitude dependence of cosmic-ray dose rates at Tokyo. (Data are calculated by EXPACS version 2.0)

Conclusion

EXPACS adopts the PARMA model to calculate the atmospheric cosmic-ray spectra. It can also estimate the the radiation doses and ionization rates of air due to cosmic-ray exposure. EXPACS-V is designed for visualizing the cosmic-ray dose rates by EXPACS. Please refer the user's manual / download page to learn how to use / obtain EXPACS as well as EXPACS-V.