Dosimetry for exposures to cosmic radiation in civilian aircraft
BS EN ISO 20785-2:2020 pdf free.Dosimetry for exposures to cosmic radiation in civilian aircraft Part 2: Characterization of instrument response.
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 levell-I5ll-ll. Galactic cosmic radiation (GCR) can have energies up to 1020 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 1015 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 on the fluence basis 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 ionsllZJll8J.
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, electrons, positrons, protons, photons and muons are the most measurable 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 statistically 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 usually far greater than the dose from SPEs. Intense SPEs can disturb the Earth’s magnetic field and often leads to a reduction of the GCR dose rates.
Detailed consideration of the measurements to be made and the radiation field are given in ISO 20785-hal.
The radiation field at aviation altitudes is complex. Thus, its dosimetry requires specialized techniques of measurement and calculation. The preferred approach would be to use devices that have an ambient dose equivalent response that is independent of the energy and the direction of the total field, or the field component to be determined. It is generally necessary to apply corrections to the results of measurements, using data on the energy and direction characteristics of the field and the energy and angle ambient dose equivalent response of the device.
The field comprises mainly photons, electrons, positrons, muons, protons and neutrons. There is not a significant contribution to dose equivalent from energetic primary heavy charged particles 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 (together with 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 strong force interactions.
The particle fluence energy distributions are shown in Annex A. The contributions of different particle types to ambient dose equivalent are shown in Figure 1 for a representative value of cut-off and solar modulation[.12i. As a guide, at normal flight altitudes, the rounded percentage contributions to total ambient dose equivalent at temperate latitudes are: electrons and positrons 25 %, muons 5 %, photons 10 %, neutrons SO % and protons 10 %(2OJ.
For dosimetric purposes, it is convenient to divide the radiation field into low-LET (<10 keV’pm-l) and high-LET (10 keVpm-1) components, LET being the commonly used abbreviation for linear energy transfer. This definition is based on the dependence of the quality factor on LET, which is unity below 10 keV lim-’. This separation between low- and high-LET particles can be applied to tissue-equivalent proportional counters and to other materials and detectors, but the low-LET/high- LET threshold can vary between 5 keV•m-1 and 10 keVim-1. The low-LET component comprises the following components:
a) directly ionizing electrons, positrons and muons;
b) secondary electrons from photon interactions;
c) most of the energy deposition by directly ionizing interactions of protons;
d) part of the energy deposition by secondary particles from strong interactions of protons and neutrons.
The high-LET component is from relatively short range secondary particles from strong interactions of protons and neutrons. The relative contributions to the total ambient dose equivalent of low LET and high LET are not necessarily the same, but are generally similar in magnitude.
Another common approach to classifying the components of a radiation field is to distinguish between neutron and non-neutron components. This approach is based on the detection technique applied, since many measurement systems are not sensitive to neutron radiation. There are similarities between the neutron and high-LET components and between the non-neutron and low-LET components. But at neutron energies above 20 MeV, the neutrons produce, in addition, an increasing low-LET contribution as well.
The low-LET and the non-neutron component can be measured using an ionization chamber, a silicon-based detector or scintillation detector, or a passive luminescence or ion storage detector. The neutron component can be measured using an extended-range neutron survey meter or a multi-sphere spectrometer, or a passive etched track detector, bubble detector or fission foil with damage track detector. A passive etched-track detector can be used to determine this component using one or both of two approaches: as a LET spectrometer and/or through the simple counting of the secondary particles mentioned above.
The summed components, low-LET plus high-LET, or non-neutron plus neutron, with suitable calibration and normalization, give the total ambient dose equivalent. Therefore, it is essential for the measurement of the complex radiation fields that the instruments used be fully characterized at national metrology institutes where possible, and by taking response characteristics and uncertainties properly into account, in order to ensure the best possible estimate of the measurand.BS EN ISO 20785-2 pdf download.Dosimetry for exposures to cosmic radiation in civilian aircraft