Particle Radiation Sources, Propagation and Interactions in Deep Space, at Earth, the Moon, Mars, and Beyond: Examples of Radiation Interactions and Effects

Abstract

Particle radiation has significant effects for astronauts, satellites and planetary bodies throughout the Solar System. Acute space radiation hazards pose risks to human and robotic exploration. This radiation also naturally weathers the exposed surface regolith of the Moon, the two moons of Mars, and other airless bodies, and contributes to chemical evolution of planetary atmospheres at Earth, Mars, Venus, Titan, and Pluto. We provide a select review of recent areas of research covering the origin of SEPs from coronal mass ejections low in the corona, propagation of events through the solar system during the anomalously weak solar cycle 24 and important examples of radiation interactions for Earth, other planets and airless bodies such as the Moon.

Appendices

Appendix A: Acronyms

Table 1 Acronyms and Models used throughout the paper

Appendix B: Components of EMMREM and PREDICCS

Throughout parts of this paper, we have used results from the Earth–Moon–Mars Radiation Environment Module (EMMREM, Fig. 18) and PREDICCS (Predictions of Radiation from Release, EMMREM, and Data Incorporating the CRaTER, COSTEP and other SEP measurements, http://prediccs.sr.unh.edu). EMMREM includes a suite of modules is designed to predict the radiation environment at Earth, the Moon, Mars and throughout the interplanetary medium in the inner heliosphere (Schwadron et al. 2010). PREDICCS is an online system that provides characterization of the radiation environment of the inner heliosphere in near real-time. PREDICCS utilizes data from satellites (ACE, Wind, GOES) in conjunction with EMMREM to produce dose rate and particle flux data at the Earth, Moon and Mars (Joyce et al. 2013). Validation has been performed through detailed comparisons between dose rates produced by PREDICCS with those measured by CRaTER.

Fig. 18

The Earth–Moon–Mars Radiation Environment Model (EMMREM) provides a link from the Space Science to Space Exploration programs by characterizing time-dependent radiation exposure from observed and simulated particle radiation events. EMMREM provides a series of primary and secondary transport modules for the prediction and validation of particle radiation environment. Key components of EMMREM include: the Energetic Particle Radiation Environment Module (EPREM) solves for the propagation and acceleration of energetic particles in the evolving magnetic fields of the inner heliosphere with input based on observations from satellites; the Baryon Transport Module (BRYNTRN), which is a deterministic, coupled proton-neutron space radiation transport model that transports incident protons and their secondary products (protons, neutrons, deuterons, tritons, helions, and alphas) through shields of arbitrary composition and thickness (Wilson et al. 1991); the High-charge (Z) and Energy (HZE) Transport (HZETRN) model (Nealy et al. 2007) and the high-energy transport code (HETC-HEDS) (Townsend et al. 2005) to estimate associated dose rate from galactic cosmic rays (Wilson and Badavi 1986; Wilson et al. 1991, 2003; Shinn et al. 1991; Cucinotta 1993; Nealy et al. 2006)

The EMMREM modules consists of four primary pieces:

• The Energetic Particle Radiation Environment Module (EPREM) solves for the propagation and acceleration of energetic particles in the evolving magnetic fields of the inner heliosphere with input based on observations from satellites.

• The Baryon Transport Module (BRYNTRN) is a deterministic, coupled proton-neutron space radiation transport model that transports incident protons and their secondary products (protons, neutrons, deuterons, tritons, helions, and alphas) through shields of arbitrary composition and thickness (Wilson et al. 1991). BRYNTRN is used primarily in the modeling of solar proton events.

• The High Energy Transport Code for Human Exploration and Development in Space (HETC-HEDS) is a Monte Carlo based algorithm that was specially designed to address space radiation problems, and, in particular, with the secondary particle distributions that are produced when the high-energy GCRs or SPEs interact with shielding and/or body organs (e.g., bone marrow or the central nervous system). HETC-HEDS has been extensively verified and validated, particularly against available laboratory beam data. HETC-HEDS models the nuclear elastic and inelastic interactions, decay and atomic interactions (Townsend et al. 2005). It accomplishes this using a Monte Carlo approach for computing the trajectories of particle cascades that result from the nuclear collisions. The particles included in HETC-HEDS are: protons, neutrons, $\pi^{+}$, $\pi^{-}$, $\mu^{+}$, $\mu^{-}$, and heavy ions. The code is fully three-dimensional and the particles can be distributed in angle, energy, and space. Each particle in the cascade is tracked until it: (1) escapes from the system; (2) undergoes nuclear collision or absorption; (3) comes to rest, as the result of energy losses from ionization and excitation of atomic electrons; or (4) in the case of pions and muons, decays. HETC-HEDS has been used in EMMREM to describe the interaction of GCR distributions with planetary atmospheres.

• GCR and SEP flux distributions are fed in to the High-charge (Z) and Energy (HZE) Transport (HZETRN) model (Nealy et al. 2007) to estimate associated GCR dose rate. The HZETRN code transports the incident charged ions and their nuclear reaction secondary particles (protons, neutrons, deuterons, tritons, ³He, ⁴He, and heavier ions) generated from nuclear collisions. The code outputs, include particle fluences, dose, dose equivalent, effective dose, and linear energy transfer (LET) distributions. The HZETRN model results are used to derive effective dose and organ doses for the assumed aluminum shield configurations.

In model calculations, organ doses ($D$) are in units of centiGray (cGy) where $1~\mbox{cGy} = 1~\mbox{rad}$ and $100~\mbox{cGy} = \mbox{Gy} = 1~\mbox{J}/\mbox{kg}$. Organ dose equivalents ($H$), which are the product of dose with a quality factor, $Q$ ($H = Q D$), are in centiSievert (cSv) where $1~\mbox{cSv} = 1~\mbox{rem}$ and $100~\mbox{cSv} = 1~\mbox{Sv} = 1~\mbox{J}/\mbox{kg}$. The units of effective dose ($E$) are also cSv. For HZETRN effective dose is calculated from

$$\begin{aligned} E = \sum_{T} w_{T} H_{T} \end{aligned}$$

(1)

where $H_{T}$ is the organ dose equivalent for the organ specified by $T$ (e.g., skin, eye, lens, etc.). The tissue (organ) weighting factors $w_{T}$ are the proportionate detriment of the organ when the whole body is irradiated and are tabulated in Table 5.1 of National Council on Radiation Protection and Measurements (NCRP) [1993].