INTRODUCTION
Ionising Radiation is both electromagnetic and particle radiation that has an energy high enough to cause ionisation. For EM radiation this means frequencies in the UV and higher (ie X-rays and gamma-rays). Particle radiation involves sub-atomic particles with energies in the keV level and above. Ionisation means the removal of electrons from the atoms in matter leaving ions.
This process can be detrimental to both physical and biological materials.
UNITS OF RADIATION
Radiation is measured in different ways according to the situation. For inanimate matter such as electronic components radiation absorbed dose is measured in Grays (Gy). For biological materials (eg living organisms such as man) radiation is measured in Sieverts (Sv).
Background radiation dose = 1 - 3 mSv/year
Lowest clinically detectable single dose = 200 mSv
Single whole body dose to produce illness = 1 - 2 Sv
Medical Xray = 50 μSv - 1 mSv
LD50-30 = 4 Sv
These levels are an average. The background radiation dose comes from radioactive materials in the environment and from galactic cosmic radiation. The first factor varies widely across the world. Parts of France and India have radioactive levels way above the average indicated here. LD in the lethal dose for 50% of an exposed human population to die within 30 days.
BIOLOGICAL EFFECTS OF RADIATION
The ions produced by radiation are known as free radicals in biological systems. These free radicals may cause damage to cells in living things. Direct damage to biological molecules may also occur from GCR The damage may be either somatic or genetic. Somatic damage is damage to the cell's cytoplasm. If considerable the cell will die. If sufficient cells die organism becomes ill. In the extreme, death may occur. Genetic damage is to the nucleus of the cell. In a mature cell this may cause cancer (usually needs repeated damage over a long period). In a developing organism (eg fetus) mutation may occur and organism then does not grow in the normal way.
At the low radiation levels encountered in aviation, only genetic damage is significant – eg slightly increased cancer risk or risk to unborn child in the first 3 months of pregnancy.
ICRP RECOMMENDATIONS
The ICRP is the International Commission on Radiological Protection. (ARPANSA is the Australian Radiological Protection and Nuclear Safety Agency).
The current ICRP recommendations are:
Notes:
1 These new limits were adopted in 1990 using the ALARA principle (As Low As Reasonably Achievable) rather than from any scientific evidence, and are considerably lower than the pre-1990 limits.
2 The radiation received by astronauts in the International Space Station exceeds these limits.
SOURCES OF AVIATION RADIATON
In aviation, only two sources of radiation contribute significant doses to crew and passengers:
The main type of radiation from both these sources is high energy protons.
GCR consists of primary and secondary particles. Primary particles are mostly very energetic protons. Secondary particles are created when primary protons hit atoms of air. This creates a shower or cascade of particles.
Aircraft are affected only by secondary GCR
GALACTIC COSMIC RADIATIION
GCR comes from outside the solar system.
GCR is present all the time.
It is isotropic (arrives from all directions)
It is very penetrating radiation (because of its high energy)
It varies with:
GCR increases with latitude (at airline altitudes).
This is because of the Earth's magnetic field.
The magnetic field shields the equatorial regions more effectively than the polar regions.
Radiation dose increases by a factor of about two as you move from the equator to a pole.
Coronal Mass Ejections or CMEs are giant clouds of plasma that are thrown out of the Sun. Magnetic fields carried along in the plasma will deflect the lower energy GCR primaries so that they do not reach the Earth. CME occurrence is very loosely correlated with sunspot number. The reduction in GCR dose rate at high sunspot number is about a factor of two.
GCR increases with altitude. At 35,000ft (~11 km) the dose from GCR is about 30 times the dose at ground level. The rate decreases above 65,000 ft (20 km). This is because of the GCR secondary showers.
At 35,000 ft GCR dose may vary from 1 to 10 uSv / hour
MEASURED AVIATION GCR DOSE
Various measurements have been made by different airlines of the actual radiation dose to aircrew. The histogram below shows the results of individual dosimetry of aircraft crew by a Czech air company. Average dose over the year of 2004 was 2.1 mSv, approximately one tenth of the recommended maximum dose.
Dose rate on a Qantas flight from Johannesburg to Sydney is shown below. (data provided by air crewman Ian Getley). The average dose rate is 5 μSv per hour. The white curves indicate the expected geomagnetic cutoff in GeV. A lower cutoff means less GCR shielding by the Earth's magnetic field.
ARPANSA have indicated the following from their modelling studies: "Measurements and modelling of Australian aircrew exposures have indicated an additional dose from airflight of around 1.8 mSv per year for those involved in domestic routes, and around 4 mSv per year for those involved in international flight routes. "
ARPANSA also made estimates of radiation dose received by aircrew over specific routes.
These show that a crew member can thus make 140 return flights Sydney to Johannesburg in one year before exceeding his/her occupational limit (on the basis of these estimates).
SOLAR PARTICLE EVENTS
Solar particle events (SPEs) are transient bursts of mainly high energy protons from the Sun. They may last from hours to days. Energies range from several MeV to occasionally several GeV. Only the high MeV and GeV events pose a hazard to aviation. These are very rare – approximately one per 5 to 10 years. Because energies are lower than GCR, the Earth's magnetic field and atmosphere provide good shielding.
| The exact production and acceleration mechanisms of SPE's are still hotly debated. Some but not all are associated with CME's. |  |
In the latter 50 years of the twentieth century, only 5 large SPEs had sufficient energy and intensity to produce a significant radiation increase to terrestrial aviation:
23 February 1956 17 July 1959 13 November 1960 09 August 1972 20 October 1989 24 March 1991
It is thought that there may be one super SPE about every 100 years. The last two may have been in 1859 and 1956. This is still very speculative.
The two graphs below show measured dose rates on two flights in 2001, a day apart, between Prague and New-York (route goes near the geomagnetic north pole). On the 15th an SPE occurred with a peak dose rate of 14 uSv per hour. The background GCR rate is just above 4 uSv/hr as can be seen on the following day (16th) when no SPE was present.
Estimates have also been made of dose rates from past SPEs over the route from London(Heathrow) to Los Angeles.
Of these, the only really significant event is that in 1956 and many researchers question the levels specified as there was no calibration with satellite data at that time.
By comparing dose rate with total dose we can see that most of the dose is delivered in a short time, often in less than one hour. This occurs near the start of the event.
Although we can predict when no SPEs will occur (ie when no sunspots are present), it is very difficult to predict when they will occur. The only significant predictor with one or more days lead time is a compact sunspot group whose major axis is north-south. False alarm rates are very high. Certain radio burst signatures indicate SPE's but these only give a few minutes lead time. Satellite sensors monitor high energy protons at geosynchronous orbit and allow nowcasting. Major SPEs are not correlated with the sunspot cycle.
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The US Air Force issues a solar radiation alert map when appropriate.
AVIATION RADIATION REGULATION
Only the European Union has mandated regulations covering aviation radiation (other countries have adopted an advisory approach):
AVIATION RADIATION MANAGEMENT
The dose rate due to GCR is well known and understood and can be modelled by computer. CARI is a free program available from the US FAA.
Flight crew assignments can thus be managed to ensure that annual doses are well within the occupational limits. Very frequent flyers might be considered for assessment.
Significant SPEs are very rare events. The worst case scenario is unlikely to place flight-crew over the allowable 50 mSv one year max. However, if this occurs, it must be averaged to ensure no more than 20mSv/year over any adjacent 5 year period.
An SPE at high-altitude (>30,000ft) and high-latitude may exceed a passenger's 1 mSv limit. Prompt (minutes) notification of a major SPE and prompt altitude reduction (<20,000ft) may alleviate this problem. Risk is very low.
CONCLUSION
"Large studies involving the health of pilots and aircrew have generally shown no significant association with an increased risk of cancer and, in particular, with the types of cancer that might be expected to arise from radiation exposure. This observation fits with the likely risks of low exposure, and the scientific basis upon which exposure standards are underpinned." (ARPANSA)
Australian Space Academy