The Radiation Field In Space

In the interplanetary space, the radiation field is composed mainly of the SCR and the GCR. In the vicinity of the Earth, a third radiation component, trapped by the Earth's magnetosphere, is present, the so-called Van Allen belts (McCormack et al. 1988, Reitz et al. 1995). Typical integral energy spectra for these radiation components in the vicinity of the Earth are shown in Figure 7-03.

SCR consist of the low energy solar wind particles that flow constantly from the sun and the SPEs that originate from magnetically disturbed regions of the sun which sporadically emit bursts of charged particles with high energies. These events are composed primarily of protons with a minor component (5-10%) being helium nuclei (alpha particles) and an even smaller part (1%) heavy ions and electrons. SPEs develop rapidly and generally last for no more than some hours, however some proton events observed near Earth may continue over several days. The emitted particles can reach energies up to several GeV (Figure 7-03). In a worse case scenario, doses as high as 10 Gy could be received within a short time. Such strong events are very rare, typically about one event during the 11 -year solar cycle. Concerning the less energetic, though still quite intensive events, e.g., in cycle 22 (1986-1996), there were at least eight events for proton energies greater than 30 MeV. For LEO, the Earth's magnetic field provides a latitude dependent shielding against SPE particles. Only in high inclination orbits and in interplanetary missions, SPEs create a hazard to humans in space, especially during extravehicular activities.

10-' 10° 10» 102 103 104 105 10« 107 10s Particle Energy (MeV)

GCR originate outside the solar system in cataclysmic astronomical events, such as supernova explosions. Detected particles consist of 98% baryons and 2% electrons. The baryonic component is composed of 85% protons (hydrogen nuclei), with the remainder being alpha particles (helium nuclei) (14%) and heavier nuclei (about 1 %). The latter component comprises particles of High charge Z and high Energy (HZE), which are defined as cosmic ray primaries of charges Z>2 and of energies high enough to penetrate at least 1 mm of spacecraft or of spacesuit shielding.

Although they only contribute to roughly 1% of the flux of GCR, they are considered as a potential major concern to living beings in space,

Figure 7-03. The energy spectra of the components of the radiation field in space in the vicinity of the Earth: (a) electrons (belts); (b) protons (belts); (c) solar particle events; (d) heavy ions of galactic cosmic radiation; (dl) during solar minimum; (d2) during solar maximum.

10-' 10° 10» 102 103 104 105 10« 107 10s Particle Energy (MeV)

Figure 7-03. The energy spectra of the components of the radiation field in space in the vicinity of the Earth: (a) electrons (belts); (b) protons (belts); (c) solar particle events; (d) heavy ions of galactic cosmic radiation; (dl) during solar minimum; (d2) during solar maximum.

especially for long-term missions at high altitudes or in high inclination orbits, or for missions beyond the Earth's magnetosphere. Reasons for this concern are based on one hand on the inefficiency of adequate shielding and, on the other hand, on the special nature of lesions produced by HZE particles (see Section 4.1). If the particle flux (flow rate) is weighted according to the energy deposition, iron nuclei will become the most important component although their relative abundance is comparatively small.

Figure 7-04. Filled circles: Percent contributions from individual GCR nuclei to the particle flux. Open triangles: Radiation dose, weighted by the square of the charge Z of the particle. Filled squares: Dose equivalent at solar minimum. Adapted from Cucinotta et al. (2003).

Figure 7-04. Filled circles: Percent contributions from individual GCR nuclei to the particle flux. Open triangles: Radiation dose, weighted by the square of the charge Z of the particle. Filled squares: Dose equivalent at solar minimum. Adapted from Cucinotta et al. (2003).

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The fluence3 of GCR is isotropic and energies up to 1020 eV can be present (Figure 7-03). When GCR enter our solar system, they must overcome the magnetic fields carried along with the outward-flowing solar wind, the intensity of which varies according to the about 11-year cycle of solar activity. With increasing solar activity the interplanetary magnetic field increases, resulting in a decrease of the intensity of GCR of low energies. This modulation is effective for particles below some GeV per nucleón. Hence the GCR fluxes vary with the solar cycle and differ by a factor of approximately five between solar minimum and solar maximum with a peak level during minimum solar activity and the lowest level during maximal solar activity (Figure 7-03). At peak energies of about 200-700 MeV/u during solar minimum, particle fluxes reach 2xl03 protons per 100 urn2 per year4 and 0.6 Fe-ions per 100 (im2 per year.

3 The fluence is the product of particle flux and time, expressed in units of particles or energy per square centimeter.

4 100 ¡rm2 is the typical cross-section of a mammalian cell nucleus.

Figure 7-04 shows the frequency distribution of the GCR nuclei. Although iron ions are one-tenth as abundant as carbon or oxygen, their contribution to the GCR dose is substantial, since dose is proportional to the square of the charge. This is visualized by the curve with the open triangles in the figure where the abundances of the GCR nuclei are weighted by the square of the charge of the particle to give a measure of the "ionizing power", the radiation dose.

The fluxes of GCR are further modified by the geomagnetic field. Only particles of very high energy have access to low inclination orbits. Towards higher inclination particles of lower energies are allowed. At the pole, particles of all energies can impinge in the direction of the magnetic field axes. Due to this inclination dependent shielding, the number of particles increases from lower to higher inclination.

In the vicinity of the Earth, the Van Allen belts are a result of the interaction of GCR and SCR with the Earth's magnetic field and the atmosphere. Two belts of radiation are formed, comprising electrons and protons, and some heavier particles trapped in closed orbits by the Earth's magnetic field. The main production process for the inner belt particles is the decay of neutrons produced in cosmic particle interactions with the atmosphere. The outer belt consists mainly of trapped solar particles. In each zone, the charged particles spiral around the geomagnetic field lines and are reflected back between the magnetic poles, acting as mirrors. Electrons reach energies of up to 7 MeV and protons up to about 200 MeV. The energy of trapped heavy ions is less than 50 MeV although their radiobiological impact is very small (Figure 7-03). The trapped radiation is modulated by the solar cycle: proton intensity decreases with high solar activity, while electron intensity increases, and vice versa.

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