Radiation Doses for Human Exposed to Galactic Cosmic Rays and Their Secondary Products on the Lunar Surface

Abstract

On the lunar surface, every human being would be exposed to galactic cosmic rays (GCRs) and their secondary products such as gamma rays and neutrons. For the human activity on the lunar surface in the future, it is important to estimate the effect of these particles on radiation doses. The annual ambient dose equivalent on the lunar surface was estimated on the basis of the latest observational data of GCRs. It is found that the annual ambient dose equivalent amount to about 570 mSv/yr during the intermediate period between the maximum and the minimum phases of the solar activity. This amount of dose is mainly produced from primary components of GCRs heavier than proton and helium nuclei. The annual ambient dose equivalent due to iron nuclei during this period is about 130 mSv/yr, more than 20% of the total dose on the lunar surface. Moreover, the dose due to these neutrons among the secondary particles reaches 50 mSv/yr, suggesting that the dose due to neutrons must be considered from the viewpoint of the human activity on the lunar surface.

Neutron production in the lunar subsurface from alpha particles in galactic cosmic rays

“…Since Simpson's first report on the GCR spectra (Simpson, 1983), many researchers have made calculations based on his spectra. However, some recent measurements of GCR spectra performed within the framework of the AMS, CAPRISE98, and BESS balloon experiments reveal a number of important discrepancies from Simpson's data (Alcaraz et al, 2000;Boezio et al, 2003;Shikaze et al, 2007;Hayatsu et al, 2008). For example, according to Hayatsu et al (2008), the total fluxes of protons and alpha particles calculated from the GCR spectra (10 MeV/n-20 GeV/n) by BESS at φ = 600 MV are 3.37 and 0.346 particle/cm 2 /s, which are 28 and 82%, respectively, higher than the fluxes used in this paper (see Table 2).…”

“…8. Comparison of the GCR proton and alpha particle spectra used in this work and given by Hayatsu et al (2008).…”

“…Consequently, the shielding of human beings from intense radiation exposure is one of major problems to be solved to secure the safety of future human activities on the lunar surface. The unshielded dose equivalent rate on the Moon is currently estimated to be 300-400 mSv/year (Adams et al, 2007), depending on the solar cycle, but it has recently been estimated to be as high > 800 mSv/year during the solar minimum of the solar cycle (Hayatsu et al, 2008; in comparison, the unshielded dose equivalent rate is about 2.4 mSv/year on the Earth (UNSCEAR report, 1988). This variability in dose estimation mainly results from uncertainties in the observational data of GCR spectra, composition of the lunar soil, and the particle and heavy ion transportation code.…”

HZE Particle and Neutron Dosages from Cosmic Rays on the Lunar Surface

“…These results were already reported in our previous work. 5) On the other hand, the dose from SEP protons reachs about 90% in the total primary SEP dose and that from SEP heavies such as oxygen and iron is several % or less, though the flunece of SEP protons is by orders of 4 or 5 higher than that of SEP irons. Because a stopping power of charged particle is basically proportional to square of its charge, which are dependent on LET correponding to its stopping power.…”

“…Using these data and the Geant4 code, 4) Hayatsu et al 5) have pointed out the importance of GCR heavy ions for the dose on the lunar surface. However, the doses from heavy components in SEPs was not discussed yet since the composition of heavy ions in large SEP events had not been observed in detail.…”