BS ISO 20785-3:2015 pdf download – Dosimetry for exposures to cosmic radiation in civilian aircraft Part 3: Measurements at aviation altitudes
4 General considerations
4.1 General description of the cosmic radiation field in the atmosphere
The primary galactic cosmic radiation (and energetic solar particles) interact with the atomic nuclei of atmospheric constituents, producing a cascade of interactions and secondary reaction products that contribute to cosmic radiation exposures that decrease in intensity with depth in the atmosphere from aviation altitudes to sea level [6] . Galactic cosmic radiation (GCR) can have energies up to 10 20 eV, but lower-energy particles are the most frequent. After the GCR penetrates the magnetic field of the solar system, the peak of its energy distribution is at a few hundred MeV to 1 GeV per nucleon, depending on solar magnetic activity, and the spectrum follows a power function of the form E –2,7 eV up to 10 15 eV; above that energy, the spectrum steepens to E –3 .
The fluence rate of GCR entering the solar system is fairly constant with time, and these energetic ions approach the Earth isotropically. The magnetic fields of the Earth and Sun alter the relative number of GCR protons and heavier ions reaching the atmosphere. The GCR ion composition for low geomagnetic cut-off and low solar activity is approximately 90 % protons, 9 % He ions and 1 % heavier ions; at a vertical cut-off of 15 GV, the composition is approximately 83 % protons, 15 % He ions and nearly 2 % heavier ions [7] .
The changing components of ambient dose equivalent caused by the various secondary cosmic radiation constituents in the atmosphere as a function of altitude are illustrated in Figure 1. At sea level, the muon component is the most important contributor to ambient dose equivalent and effective dose. At aviation altitudes, neutrons, protons, electrons/positrons, photons and muons are the most significant components. At higher altitudes, nuclear ions heavier than protons start to contribute. Figures showing representative normalized energy distributions of fluence rates of all the important particles at low and high cut-offs and altitudes at solar minimum and maximum are shown in Annex A.
The Earth is also exposed to bursts of energetic protons and heavier particles from magnetic disturbances near the surface of the sun and from ejection of large amounts of matter (coronal mass ejections — CMEs) with, in some cases, acceleration by the CMEs and associated solar wind shock waves. The particles of these solar particle events, or solar proton events (both abbreviated to SPE), are much lower in energy than GCR, generally below 100 MeV and only rarely above 10 GeV. SPEs are of short duration, a few hours to a few days, and highly variable in intensity. Only a small fraction of SPEs, on average one per year, produce large numbers of high-energy particles which cause significant dose rates at high altitudes and low geomagnetic cut-offs and can be observed by neutron monitors on the ground. Such events are called ground level enhancements (GLEs). For aircraft crew, the cumulative dose from GCR is far greater than the dose from SPEs. Intense SPEs can affect GCR dose rates by disturbing the Earth’s magnetic field in such a way as to change the galactic particle intensity reaching the atmosphere.
The field comprises mainly neutrons, protons, electrons/positrons, photons and muons. There is not a significant contribution to dose equivalent from energetic primary heavy charged particles (HZE) or fragments. The electrons/positrons and muons are directly ionizing radiation, and together with indirectly ionizing photons and secondary electrons, interact with matter via the electromagnetic force. Neutrons (and a small contribution from pions), interact via the strong interaction producing directly ionizing secondary particles. Protons are both directly ionizing via the electromagnetic force and indirectly via neutron-like strong interactions.BS ISO 20785-3 pdf download.