[ RadSafe ] ICRP - Assessment of Radiation Exposure of Astronauts in Space

Fred Dawson fd003f0606 at blueyonder.co.uk
Wed Jul 4 11:00:51 CDT 2012

Draft report: Assessment of Radiation Exposure of Astronauts in Space
The draft ICRP report “Assessment of Radiation Exposure of Astronauts in Space” is now available for public consultation. ICRP welcomes comments from individuals and groups. The draft document can be downloaded, and comments submitted, through the ICRP web site.

Comments must be submitted through the ICRP web site no later than August 31, 2012.

Draft Executive Summary
Astronauts are living and working in low Earth orbits for extended periods of time, especially during the operation and maintenance of the International Space Station (ISS) and scientific investigations. Furthermore, plans are already discussed for outer space missions of astronauts.

In ICRP 103 it is stated that “in exceptional cases of cosmic radiation exposures, such as exposure in space travel, where doses may be significant and some type of control warranted, should be dealt with separately from the conventional approach of occupational exposure”. Therefore, although astronauts are exposed to ionizing radiation during their occupational activities they are usually not classified as being occupationally exposed in the sense of the ICRP system for radiation protection.

The report contains 7 Chapters. The first one is an introduction describing the specific situation of astronauts in space and the differences of the radiation field in space compared to fields on Earth, thereby showing areas where approaches applied in radiological protection measures on Earth need to be modified.

In Chapter 2 the radiation fields in space are described in detail. The solar system with the Sun at its centre is embedded in a complex mixture of galactic cosmic radiation (GCR) - protons, alpha particles and heavy ions - which continuously enters the heliosphere from all directions. Inside the heliosphere, the GCR fluence rate and particle energy distributions are modulated by the interplanetary magnetic field produced by the charged particles continuously emitted by the Sun, the so-called solar wind. In addition to the solar wind, the Sun occasionally emits unusually large pulses of energetic particles – mostly protons and electrons – called solar-particle events (SPEs). Celestial bodies equipped with a magnetic moment like the Earth are surrounded by toroidal belts of particulate radiation. Such radiation belts constitute an important third primary exposure source. Fluence rates of cosmic radiation are not constant; they vary between two extremes which correspond in time with the maximum and minimum solar activity. Solar activity and cosmic radiation fluence rates are inversely correlated. In Sections 2.2 to 2.5 the various components of the radiation field in space are presented and the influence of the Earth´s magnetic field is described.

Chapter 3 is dealing with the quantities used in radiological protection. In the first part the system of dose quantities as given in Publication 103 (2007) is described and secondly the relative biological effectiveness (RBE) is discussed especially with respect to the large contribution of heavy ions and the very high energies. A single wR-value of 20 for all heavy ions of all energies is not appropriate for space radiation fields. Hence, for space applications the concept of a quality factor, Q(L), is applied also to the protection quantities. In Section 3.3 the approach for applications in space is described in detail.

In Chapter 4 the methods of fluence and dose measurements in space are described. This includes instrumentation for fluence measurements, radiation spectrometry, area dosimetry, and individual monitoring. Passive and active devices are mentioned and also the use of biomarkers for the assessment of mission doses is described. Some advice for quality control and the assessment of uncertainties is also given in this Chapter.

In Chapter 5 the methods of determining quantities describing the radiation fields within a spacecraft are given. Radiation transport calculations are the most important tool for an assessment of radiation fields inside a spacecraft starting from the radiation field in free space and considering the walls and further equipment of the spacecraft. In this chapter some physical data used in radiation transport codes are presented and the various codes used for calculations in high-energy radiation fields as in space described. Results of calculations of radiation fields in spacecrafts are given. A discussion of shielding possibilities is included in this Chapter, too.

Chapter 6 is dealing with methods of determining mean absorbed doses and dose equivalent s in organs and tissues of the human body. Calculated conversion coefficients of fluence to mean absorbed dose in an organ or tissue are given for heavy ions up to Z=58 for energies from 10 MeV/u to 100 GeV/u. For the same set of ions and ion energies mean quality factors in organs and tissues are presented using on the one hand the Q(L) function defined in Publication 60 of the Commission and on the other hand a Q(Z,E) function proposed by NASA. In Sections 6.4 assessment of doses in the body by measurements are described and results are compared with calculations. In Section 6.5 biodosimetric measurements for the assessment of mission doses are presented.

In Chapter 7 operational measures with regard to the assessment of the exposure of astronauts during space missions. This includes pre-flight mission design, area and individual monitoring during flights in space and dose recording. The importance of considering uncertainties in dose assessment is also mentioned.

In an Annex numerical data of conversion coefficients and mean quality factors for protons, neutrons, alpha particles and heavy ions (2 < Z ≤ 28) are given.


Fred Dawson
New Malden

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