Cosmic Radiation
Michael Bagshaw
Francis A. Cucinotta
Cosmic rays were discovered in 1911 by the Austrian physicist, Victor Hess. The planet Earth is continuously bathed in high-energy galactic cosmic ionizing radiation (GCR), emanating from outside the solar system, and sporadically exposed to bursts of energetic particles from the sun referred to as solar particle events (SPEs). The main source of GCR is believed to be supernovae (exploding stars), although occasionally a disturbance in the sun’s atmosphere (solar flare or coronal mass ejection) leads to a surge of radiation particles with sufficient energy to penetrate the Earth’s magnetic field and enter the atmosphere. Inhabitants of planet Earth gain protection from the effects of cosmic radiation from the Earth’s magnetic field and the atmosphere, as well as from the sun’s magnetic field and solar wind. These protective effects extend to the occupants of aircraft flying within the Earth’s atmosphere, although the effects can be complex for aircraft flying at high altitudes and high latitudes.
Travelers in space do not have the benefit of this protection and are exposed to an ionizing radiation field very different in magnitude and quality from the exposure of individuals flying in commercial airliners. The higher amounts and distinct types of radiation qualities in space lead to a large need for understanding the biological effects of space radiation. It is recognized that although there are many overlaps between the aviation and the space environments, there are large differences in radiation dosimetry, risks and protection for airline crewmembers, passengers, and astronauts. These differences affect the application of radiation protection principles of risk justification, limitation, and the principle of as low as reasonably achievable (ALARA). This chapter accordingly is divided into three major sections, the first dealing with the basic physics and health risks, the second with the commercial airline experience, and the third with the aspects of cosmic radiation appertaining to space travel including future considerations.
FUNDAMENTAL PHYSICS OF COSMIC RADIATION
Ionizing Radiation
Ionizing radiation refers to subatomic particles that, on interacting with an atom, can directly or indirectly cause the atom to lose an electron or break apart its nucleus. It is when these events occur in tissues of the body that health effects may result if the human body’s self-repair mechanisms fail. Ionizing radiation types and their properties are shown in Table 8-1.
Outside the Earth’s atmosphere, GCR consists mostly of fast-moving protons (hydrogen nuclei), α-particles (helium particles), and high charge and energy (HZE) nuclei ranging from lithium to uranium. GCR is 98% atomic nuclei and 2% electrons (1). Of the energetic nuclei, 87% are protons, 12% are helium ions, and 1% are heavier ions. The energy of GCR is expressed as megaelectron volt per atomic mass unit (1 mMeV/u = 9.64853336 × 1013 m2/s2). The energies range from a few MeV/u to more than 10,000 MeV/u peaking near 1,000 MeV/u. The higher energy ions move close to the speed of light.
As charged particles pass through shielding or the atmosphere and tissue, they lose energy and undergo nuclear interactions. Energy loss is caused by electromagnetic interactions transferring energy to electrons leading to ionization and excitation. The rate of energy loss increases rapidly with increasing charge of the particle and decreasing speed (2). The distance traveled depends on the energy, and massive particles are more penetrating than lighter particles of the same charge and speed. Uncharged particles have longer free paths and, for neutrons, larger energy transfers per event result in energy losses that appear as isolated occurrences along the particle’s path.
Nuclear interactions produce lower charge and mass nuclei from a primary GCR nucleus and secondary radiation from the material being hit (3). The mean free path for nuclear collision is on the order of 10 cm and after
several mean free paths the primary GCR heavy ions are converted largely into protons and neutrons. On entering the Earth’s atmosphere, the particles collide with the nuclei of nitrogen, oxygen, and other atmospheric atoms, generating additional (secondary) ionizing radiation particles. At normal commercial aircraft flight altitudes, this GCR consists mainly of neutrons, protons, electrons, positrons, and photons.
several mean free paths the primary GCR heavy ions are converted largely into protons and neutrons. On entering the Earth’s atmosphere, the particles collide with the nuclei of nitrogen, oxygen, and other atmospheric atoms, generating additional (secondary) ionizing radiation particles. At normal commercial aircraft flight altitudes, this GCR consists mainly of neutrons, protons, electrons, positrons, and photons.
TABLE 8-1 | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Figure 8-1 illustrates the production of secondary particles as a primary particle penetrates the Earth’s atmosphere and interacts with an atmospheric nucleus.
Terrestrial Protection from Galactic Cosmic Radiation
Protection from cosmic radiation for the Earth’s inhabitants is provided by three variables:
 FIGURE 8-1 Production of secondary particles in the atmosphere. |
The sun’s magnetic field and solar wind (solar cycle)
The Earth’s magnetic field (latitude)
The Earth’s atmosphere (altitude)
1. The sun has a varying magnetic field with a basic dipole component that reverses direction approximately every 11 years. Recently solar maximum period peaked around 2000 to 2002 and the next one is expected around 2011. Near the reversal, at “solar minimum” (around 2006 in the current cycle), there are few sunspots and the magnetic field extending throughout the solar system is relatively weak and smooth. At solar maximum, there are many sunspots and other manifestations of magnetic turbulence, and the plasma of protons and electrons ejected from the sun (the solar wind) carries a relatively strong and convoluted magnetic field with it outward through the solar system (4).
When the solar magnetic field is stronger, the paths of the electrically charged ions are deflected further and less GCR reaches the Earth. Therefore, solar maximum causes a radiation minimum and, conversely, solar minimum is the time of radiation maximum. The effect of this depends on the other two variables, altitude and geomagnetic latitude. At the altitudes flown by commercial jet aircraft and at polar latitudes, the ratio for GCR at solar minimum to that at solar maximum is in the region of 1.2 to 2 and increases with altitude (5,6).
2. The Earth’s magnetic field has a larger effect than the sun’s magnetic field on cosmic radiation approaching the atmosphere.
Near the equator, the geomagnetic field is almost parallel to the Earth’s surface. Near the magnetic poles, the geomagnetic field is nearly vertical and the maximum number of primary cosmic rays can reach the atmosphere. At extremes of latitude, there is no further increase in GCR flux with increasing latitude and this is known as the polar plateau.
As a result, cosmic radiation levels are higher in polar regions and decline toward the equator, the size of this effect being dependent upon altitude and the point in the solar cycle. At the altitudes flown by commercial jet aircraft, at solar minimum, GCR is 2.5 to 5 times more intense in polar regions than near the equator, with larger latitude dependence as altitude increases (7).
3. Life on Earth is shielded from cosmic radiation by the atmosphere.
Charged cosmic radiation particles lose energy as they penetrate the atmosphere by ionizing the atoms and molecules of the air (releasing electrons). The particles also collide with the atomic nuclei of nitrogen, oxygen, and other atmospheric constituents.
Ambient radiation increases with altitude by approximately 15% for each increase of approximately 2,000 ft (˜600 m) (dependent on latitude), with certain secondary particles reaching a maximum at approximately 65,000 ft (20 km) (the Pfotzer maximum). Primary heavy ions and secondary fragments become important above this point.
In addition to providing shielding from GCR, the atmosphere contributes different components to the radiation flux as a function of atmospheric depth. Accordingly, the potential biological effects of cosmic radiation on aircraft occupants are directly altitude dependent. Dose rate increases with both altitude and latitude. The effect of increasing latitude at a constant altitude is greater than that of increasing altitude at a constant latitude.
Figure 8-2 shows the calculated effective dose rate from each of the secondary components produced by GCR (and the total effective dose) as a function of altitude for a location at the edge of the polar plateau during solar minimum (radiation maximum) (4).
It can be seen that the total effective dose rate at 30,000 ft is approximately 90 times the rate at sea level. It increases by a factor of 2 between 30,000 ft and 40,000 ft, and by another factor of 2 between 40,000 ft and 65,000 ft. It should be noted that at all altitudes from 10,000 ft to more than 80,000 ft (3 to 25 km) neutrons are the dominant component. They are less dominant at lower latitudes, but still contribute 40% to 65% of the total dose equivalent rate.
 FIGURE 8-2 Calculated effective dose rate as a function of altitude for various component particles of galactic cosmic radiation in the atmosphere near the plar plateau (cutoff = 0.8 GV) at solar minimum (June 1997). Data are courtesy of K. O’Brien, calculated using his LUIN-98F radiation transport code, but with WR for protons set equal to 2 (NCRP 1993) rather than 5. (Reproduced from Goldhagen P. Overview of aircraft radiation exposure and recent ER-2 measurements. Health Phys 2000;79(5):586-591, the journal Health Physics with permission from the Health Physics Society and the National Council on Radiation Protection and Measurements.) |
Solar Flares
Occasionally a disturbance in the sun’s atmosphere, known as a solar particle event, leads to a surge of radiation particles. These are produced by sudden sporadic releases of energy in the solar atmosphere (solar flares) and by coronal mass ejections (CMEs), and are usually of insufficient energy to contribute to the radiation field at aviation altitudes. However, on occasions proton particles are produced with sufficient energy to penetrate the Earth’s magnetic field and enter the atmosphere. These particles interact with air atoms in the same way as GCR particles. Such events are comparatively short lived and vary with the 11-year solar cycle, being more frequent at solar maximum.
Long-distance radio communications are sometimes disrupted because of increased ionization of the Earth’s upper atmosphere by x-rays, protons or ultraviolet radiation from the sun. This can occur in the absence of excessive ionizing radiation levels at commercial flight altitudes. Similarly the Aurorae Borealis and Australis
(northern and southern lights), while resulting from the interaction of charged particles with air in the upper atmosphere, are not an indication of increased ionizing radiation levels at flight altitudes.
(northern and southern lights), while resulting from the interaction of charged particles with air in the upper atmosphere, are not an indication of increased ionizing radiation levels at flight altitudes.
When primary solar particle energies are sufficient to produce secondary particles detected at ground level by neutron monitors, this is known as ground level enhancement (GLE). GLEs are rare, averaging approximately 1/yr grouped around solar maximum, and the spectrum varies between events (8). Any rise in dose rates associated with an event is rapid, usually taking place in minutes. The duration may be hours to several days.
The strong magnetic disturbance associated with SPEs can lead to significant decreases in GCR dose rate over many hours as a result of the enhanced solar wind (Forbush decrease). The disturbance to the geomagnetic field can allow easier access to cosmic rays and solar particles. This can give significant increases at lower latitudes particularly for SPEs. Therefore, the combined effect of an SPE may be a net decrease or increase in radiation dose, and further work is needed to understand the contribution of SPEs to dose. Prediction of which SPEs will give rise to significant increases in radiation dose rates at commercial aircraft operating altitudes is not currently possible, and work continues with this aspect of space weather.
GLEs have been recorded and analyzed since 1942, and are numbered sequentially (9). Of the 64 GLEs observed up to 2003, with the exception of GLE5 (February 1956), none has presented any risk of attaining an annual dose of 1 mSv (the International Commission on Radiological Protection [ICRP] recommended public exposure limit) (10). For GLE60, which occurred in April 2001, the total contribution to radiation dose from the SPE was measured as 20 μSv (11).
GLE42, which occurred in September 1989, was the most intense observed since that of 1956 (GLE5) with a recorded magnitude of 252%. However, this represented approximately 1 month of GCR exposure only, which would not have given an annual dose in excess of 1 mSv (12). Concorde supersonic transport aircraft of British Airways were flying during this solar event and the on-board monitoring equipment did not activate a radiation warning alert, which is triggered at 0.5 mSv/hr. However, it should be cautioned that the latitude effect exceeds the altitude effect for SPEs and Concorde did not reach very high magnetic latitudes.
It has been reported (10) that a number of airlines have changed flight plans to avoid high geomagnetic latitudes during periods of predicted solar flare ground level events, with significant cost and delays to service. Data indicate that these actions were unnecessary in terms of radiation dose protection.
RADIOBIOLOGY
Biological Effects of Ionizing Radiation
Very high levels of ionizing radiation, such as that from a nuclear explosion, will cause severe cell damage or cell death. Adverse health impacts include early death, within days or a few weeks, as a result of acute exposure whereas longer-term consequences include development of cancer, or genetic maldevelopment as a result of damage to the reproductive cells. It is more difficult to predict the effects of low-level doses of ionizing radiation such as cosmic radiation or medical x-rays because of the individual variability in the body’s self-repair processes. Indeed, several health effects have been suggested at low doses and dose rates, including that the effect of radiation on human health is not linear, but is either a J-shaped curve with exposure being beneficial at low doses (13,14); or in contrast is increased due to nontargeted effects where cells not directly traversed by radiation tracks are responsible for malignancy (15,16).
Biological effectiveness depends on the spatial distribution of the energy imparted and the density of the ionizations per unit path length of the ionizing particles. The energy loss per unit path length of a charged particle is referred to as the stopping power, whereas the energy deposited is referred to as linear energy transfer (LET).
The ionization process in living tissues consists of atomic and molecular excitations, and ejecting bound electrons from the cellular molecules, leaving behind chemically active radicals that are the source of adverse changes. Many of the radicals resulting from radiation injury are similar to those produced in normal metabolic processes, for which the cell has developed recovery mechanisms needed for long-term survival (17). The number of ionization events per particle passage is related to the physical processes by which particle kinetic energy is transferred to the cellular bound electrons (2). The rate at which ions produce electrons in isolated cells is important because repair of a single event is relatively efficient unless many events occur within the repair period (14).
The substantive target of radiation injury is considered to be the DNA structure that may be changed or injured directly by a passing ionizing particle (2). DNA damage consists of simple types with a single base damage or break in the DNA sugar-phosphate backbone, termed a single strand break, to complex types where two or more damages occur in a single helical turn of DNA. The spectrum of DNA damages shifts from simple to more complex as the LET is increased (18). Double-strand breaks (DSB), defined as one or more breaks on opposing sides of the DNA sugar-phosphate backbone within 20 base pairs of each other, are expected to be the most detrimental form of DNA damage leading to various forms of mutation including gene deletion and chromosomal aberrations. For high-LET radiation, most DSB are highly complex involving base damage and other breaks near a DSB.
The ability of the cell to repair the effects of ionization depends on the class of DNA lesion (simple or complex) and in part on the number of such events occurring within the cell from the passage of a single particle, and the rate at which such passages occur. There are two major pathways of DSB repair in vertebrae (19): (i) nonhomologous end-joining (NHEJ) and (ii) homologous recombination (HR). NHEJ is an error-prone form of repair and is dominant in
the prereplication phase of the cell cycle and in resting cells. This process involves removal of damaged regions near the initial break and ligation of the remaining DNA ends. HR is a high-fidelity form of DNA damage repair, acting during DNA replication and mitosis, and requires a sister chromatid to act as a template for the synthesis of DNA during repair.
the prereplication phase of the cell cycle and in resting cells. This process involves removal of damaged regions near the initial break and ligation of the remaining DNA ends. HR is a high-fidelity form of DNA damage repair, acting during DNA replication and mitosis, and requires a sister chromatid to act as a template for the synthesis of DNA during repair.
In recent years, there has been increased focus on non-DNA targets for harmful biological effects of radiation (15, 16). These include oxidative damage in the cytoplasm and mitochondria, and aberrant cell signaling processes that disrupt normal cellular processes such as the control of cellular growth factors, the tissue microenvironment, and DNA replication. These so-called nontargeted effects can be both mutagenic and carcinogenic.
Chromosome Aberrations
Tissue cells may be damaged by physical agents such as heat, cold, vibration, and radiation. Throughout life, there is a continuous ongoing cycle of cell damage and repair utilizing the body’s self-repair mechanisms. During the repair process, gene translocation and other chromosome aberrations may occur.
A number of studies have identified an increased rate of unstable chromosome aberrations such as dicentrics and rings in flight crewmembers, and related these to cosmic radiation exposure (20, 21, 22). Nicholas et al. noted that unstable aberrations decrease with time and therefore do not serve as good indicators of cumulative exposure to GCR. They postulate that structural chromosome aberrations such as translocations may be a better marker because they are relatively stable over time since exposure (23). Nicholas et al. also showed that the mean number of translocations per cell was significantly higher among the airline pilots who were studied compared to controls. However, within the radiation exposure range encountered in the study, observed values among the pilots did not correspond to the dose-response pattern expected on the basis of available models for chronic low-dose radiation exposure. In addition, this study does not determine the role of radiation in the induction of translocations and so far, no epidemiologic evidence links these aberrations with the development of cancers.
Studies of chromosome aberrations with high-LET radiation, including heavy ions, show that the complexity of chromosome aberrations also increases with LET (24). These studies are made using multicolor fluorescence in-situ hybridization (FISH), where chromosome-specific probes are used to label individual chromosomes, and aberrations between two or more chromosomes are observed after irradiation as illustrated in Figure 8-3. The number of chromosomes involved in chromosomal aberrations appears to increase with the LET of the radiation field. George et al. (25) reported the number and types of chromosomal aberrations in astronauts on the International Space Station (ISS).
The biological effect of ionizing radiation depends upon whether it is high or low LET. Early studies of the effect of identical doses of different types of radiation on biological systems showed that different amounts of damage were produced. This led to the concept of “relative biological effectiveness” (RBE), which is defined as the ratio of a dose of a particular type of radiation to the dose of γ rays or x-rays that yield the same biological endpoint.
The dose equivalent to the tissue (DE) is the product of the absorbed dose (D) and the quality factor (Q or QF), Q being dependent upon LET. The numerical value of Q depends not only on appropriate biological data but also on the judgment of the ICRP. It establishes the value of the absorbed dose of any radiation that engenders the same risk as a given absorbed dose of a reference radiation (26). The radiation weighting factor (WR) takes account of quality factor, and recommendations are published from time to time by the ICRP (26).
Low-LET radiation, all with a weighting factor of 1, includes photons, x- and γ rays, as well as electrons and muons. Electrons are the low-LET radiation of prime concern at aircraft operating altitudes.
Neutrons, α-particles, fission fragments and heavy nuclei are classified as high LET, with neutrons providing approximately half the effective dose at high altitudes.
At all altitudes from 10,000 ft to more than 80,000 ft (3-25 km) neutrons are the dominant component of the cosmic radiation field. They are less dominant at lower latitudes, but still contribute 40% to 65% of the total dose equivalent rate. Because neutron interactions produce lowenergy ions, neutron radiation is more effective in inducing biological damage than γ radiation. However, there are no adequate epidemiologic data to evaluate to what extent neutrons are carcinogenic to humans (27).
At all altitudes from 10,000 ft to more than 80,000 ft (3-25 km) neutrons are the dominant component of the cosmic radiation field. They are less dominant at lower latitudes, but still contribute 40% to 65% of the total dose equivalent rate. Because neutron interactions produce lowenergy ions, neutron radiation is more effective in inducing biological damage than γ radiation. However, there are no adequate epidemiologic data to evaluate to what extent neutrons are carcinogenic to humans (27).
 FIGURE 8-3 Observation of chromosomal aberrations in human lymphocyte cells exposed to 300 mGy of γ rays or 1 GeV/u iron ions. (Durante M, George K, Wu H, et al. Karyotypes of human lymphocytes exposed to high-energy iron ions. Radiat Res 2002;158:581-590.) |
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The current weighting factors are shown in Table 8-2. The weighting factor for neutrons depends on the energy of the incident neutrons. ICRP Publication 92 proposes that the means of computation of the factor should be a continuous function of energy rather than the step function given in Publication 60 (26).
These proposals are based on current knowledge of biophysics and radiobiology, and acknowledge that judgments about these factors may change from time to time.
(ICRP recommends that no attempt be made to retrospectively correct individual historical estimates of effective dose or equivalent dose in a single tissue or organ. Rather the revised weighting factor should be applied from the date of adoption.)
Radiation Units of Measurement
The standard unit of radioactivity is the Becquerel (Bq), which is defined as the decay of one nucleus per second.
When considering cosmic radiation the practical interest is in the biological effect of a radiation dose, the dose equivalent being measured in Sievert (Sv). The ICRP has recommended a number of quantities based on weighting absorbed dose, to take account of the RBE of different types of radiation. Dose equivalent (Sv) is one of these.
Dose equivalent (H) is defined as:
H(LET) = Q(LET) × D(LET)
where Q is the quality factor and is a function of LET, and D is the absorbed dose.
The effective dose is obtained by the use of absorbed dose, D, along with different weighting factors for organs and tissues.
Doses of cosmic radiation are of such a level that values are usually quoted in microSievert (μSv) per hour or milli-Sievert (mSv) per year (1 mSv = 1,000 μSv).
The Sievert has superseded the rem as the unit of measurement of effective dose (1 Sv = 100 rem, 1 mSv = 100 mrem, 1 μSv = 0.1 mrem).
Other Terrestrial Sources of Ionizing Radiation
There is a constant background flux of ionizing radiation at ground level. Terrestrial background radiation from the Earth’s materials contributes 2.6 mSv/yr in the United Kingdom and 3 mSv/yr in the United States (28). This flux is dominated by the low-LET component (93%).
Inhaled radon gas contributes approximately 2 mSv/yr to the total overall background ionizing radiation level (28).
Medical x-rays are delivered in a concentrated localized manner, and usual doses are of the order (28):
Chest x-ray | 0.1 mSv (100 μSv) |
Body computed tomographic (CT) scan | 10 mSv |
Chest CT scan | 8 mSv |
Intravenous pyelogram (IVP) | 1.6 mSv |
Mammogram | 0.7 mSv (700 μSv) |
These are effective doses averaged over the entire body, accounting for the relative sensitivities of the different tissues exposed.
Doses received from radiotherapy for cancer treatment range from 20 to 80 Sv (29). These are all average figures with wide individual variations.
Radiological Protection
Workers in the nuclear industry and those who work with medical x-rays may be designated as “classified workers” and have their occupational radiation exposure monitored and recorded. For classified workers, the ICRP recommends maximum mean body effective dose limits of 20 mSv/yr (averaged over 5 years, with a maximum in any 1 year of 50 mSv), with an additional recommendation that the equivalent dose to the fetus should not exceed 1 mSv during the declared term of the pregnancy. This limit for the fetus is in line with the ICRP recommendation that the limit for the general public should be 1 mSv/yr (30).
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