The Space Radiation Environment

THE SPACE RADIATION ENVIRONMENT

A Brief Review

In space and in a space station, respectively, as well as in high altitude aircraft humans are exposed to a complex mixed radiation field.

The Space Radiation Environment is composed of geomagnetically trapped radiation, solar particle event radiation and galactic cosmic radiation.

· Geomagnetically Trapped Radiation (Van Allen Belts) consists of electrons with E > 0.5 MeV, protons with E > 10 MeV and a few helium nuclei (Fig. 1.). The radiation belts vary in altitude over various parts of the earth. Over the south atlantic region, the geomagnetic field draws particles closer to the earth. This region is known as the South Atlantic Anomaly (SAA). The orbit parameters of a spacecraft (inclination and altitude) determine the number of passes made per day through this region. At an orbit below an altitude of about 550 km a considerable part of absorbed radiation dose is caused by passing the SAA (about 30 % in the space station MIR).

Fig. 1. The Van Allen Belts

· Solar particle-event radiations are in general large clouds of charged particles (mainly protons and helium nuclei in a wide range of energy) released from the sun by gigantic eruptions during solar storms. During the Apollo programme, it was estimated that one of the largest solar particle-events on record (August 4-9, 1972) would have caused a skin dose of 3.6 Gy and 0.35 Gy in the blood-forming organs of the crew in the well-shielded command module. Radiation doses to the crew inside the thinly shielded lunar module or during extravehicular activities during such an event would have been extremely serious. Within geomagnetic shielding the effect of solar particle-events is reduced to some percent (J. V. Bailey, Dosimetry during space missions. IEEE Trans. Nucl. Sci. 23 (4) (1976)).

· Galactic Cosmic Radiation consists of completely ionised atomic nuclei (from protons up to high Z). Heavy Charged Particles (HCP) have their origin outside the solar system and are accelerated to extremely high energies. Average dose rates in absence of geomagnetic shielding are about 15 µGy/h and vary by a factor of 2 with the solar cycle because of solarmagnetic shielding of the central planets.

Interaction processes with matter, e.g. the hull of a spacecraft, transform this primary space radiation into secondary radiation consisting of charged particles, neutrons, gamma- and X-rays. A considerable part of the resulting radiation is characterised by a high Linear Energy Transfer (LET). Radiation with different LET causes different biological effects and defects in matter. For determination of the biologically relevant equivalent dose and for estimation of radiation damages in matter, information about the LET spectrum is required.

In a similar way, interaction of the primary cosmic radiation with the atoms and molecules of the atmosphere produces a broad spectrum of different secondary particles with varying energy and LET: protons, neutrons, electrons, muons, pions, gamma-quanta and bremsstrahlung. With increasing depth in the atmosphere, the primary cosmic radiation component decreases, whereas the secondary radiation component increases. This complex situation results in a maximum of the dose rate at an altitude of 20 to 25 km, the so-called Pfotzer maximum. From this altitude to earth surface, the dose rate decreases continuously. Besides of the dependence of the radiation field on the altitude, its parameters vary with the geomagnetic latitude and the solar cycle.