ISO 20785-1:2006

INTERNATIONAL ISO
STANDARD 20785-1
First edition
2006-04-01
Dosemetry for exposures to cosmic
radiation in civilian aircraft —
Part 1:
Conceptual basis for measurements
Dosimétrie de l'exposition au rayonnement cosmique dans l'aviation
civile —
Partie 1: Fondement théorique des mesurages

Reference number
©
ISO 2006
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ii © ISO 2006 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols. 1
3.1 Quantities and units . 1
3.2 Atmospheric radiation field . 5
4 General considerations. 7
4.1 General description of the cosmic radiation field in the atmosphere. 7
4.2 General calibration considerations for the dosemetry of cosmic radiation fields in aircraft. 9
4.3 Conversion coefficients . 11
5 Dosemetric devices . 11
5.1 Introduction . 11
5.2 Active devices . 11
5.3 Passive devices . 14
Annex A (informative) Representative particle fluence energy distributions for the cosmic
radiation field at flight altitudes for solar minimum and maximum conditions and for
minimum and maximum vertical cut-off rigidity. 17
Bibliography . 21

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 20785-1 was prepared by Technical Committee ISO/TC 85, Nuclear energy, Subcommittee SC 2,
Radiation protection.
ISO 20785 consists of the following parts, under the general title Dosemetry for exposures to cosmic radiation
in civilian aircraft:
⎯ Part 1: Conceptual basis for measurements
A Part 2 dealing with the characterization of instrument response is in preparation.
iv © ISO 2006 – All rights reserved

Introduction
Aircraft crew are exposed to elevated levels of cosmic radiation of galactic and solar origin and secondary
radiation produced in the atmosphere, the aircraft structure and its contents. Following recommendations of
[1]
the International Commission on Radiological Protection in Publication 60 , the European Union (EU)
[2]
introduced a revised Basic Safety Standards Directive , which included exposure to natural sources of
ionizing radiation, including cosmic radiation, as occupational exposure. The Directive requires account to be
taken of the exposure of aircraft crew liable to receive more than 1 mSv per year. It then identifies the
following four protection measures: (i) to assess the exposure of the crew concerned; (ii) to take into account
the assessed exposure, when organizing working schedules with a view to reducing the doses of highly
exposed crew; (iii) to inform the workers concerned of the health risks their work involves; and (iv) to apply the
same special protection during pregnancy to female crew in respect of the “child to be born” as to other female
workers. The EU Council Directive has already been incorporated into laws and regulations of EU Member
States and is being included in the aviation safety standards and procedures of the Joint Aviation Authorities
and the European Air Safety Agency.
For regulatory and legislative purposes, the radiation protection quantities of interest are equivalent dose (to
the foetus) and effective dose. The cosmic radiation exposure of the body is essentially uniform and the
maternal abdomen provides no effective shielding to the foetus. As a result, the magnitude of equivalent dose
to the foetus can be put equal to that of the effective dose received by the mother. Doses on board aircraft are
generally predictable, and events comparable to unplanned exposure in other radiological workplaces cannot
normally occur (with the rare exceptions of extremely intense and energetic solar particle events). Personal
[2], [3]
dosemeters for routine use are not considered necessary . The preferred approach for the assessment of
doses of aircraft crew, where necessary, is to calculate directly effective dose per unit time, as a function of
geographic location, altitude and solar cycle phase, and to fold these values with flight and staff roster
information to obtain estimates of effective doses for individuals. This approach is supported by guidance from

[3]
the European Commission and the ICRP in Publication 75 .
The role of calculations in this procedure is unique in routine radiation protection and it is widely accepted that
the calculated doses should be validated by measurement. Effective dose is not directly measurable. The
operational quantity of interest is ambient dose equivalent, H*(10). In order to validate the assessed doses
obtained in terms of effective dose, calculations can be made of ambient dose equivalent rates or route doses
in terms of ambient dose equivalent, and values of this quantity determined by measurements traceable to
national standards. The validation of calculations of ambient dose equivalent for a particular calculation
method may be taken as a validation of the calculation of effective dose by the same computer code, but it
can be necessary to confirm this step in the process. The alternative is to establish, a priori, that the
operational quantity ambient dose equivalent is a good estimator of effective dose and equivalent dose to the
foetus for the radiation fields being considered, in the same way that the use of the operational quantity
personal dose equivalent is justified for the estimation of effective dose for radiation workers. Ambient dose
equivalent rate as a function of geographic location, altitude and solar cycle phase, is then calculated and
integrated with flight and staff roster information. The calculations of ambient dose equivalent rates or route
doses can then be validated by measurements traceable to national standards.
The radiation field in aircraft at altitude is complex, with many types of ionizing radiation present, with energies
ranging up to many GeV. The determination of ambient dose equivalent for such a complex radiation field is
difficult, and the purpose of this International Standard is to give the conceptual basis for performing such
measurements and for the calibration of instruments used for this purpose. In many cases, the methods used
for the determination of ambient dose equivalent in aircraft are similar to those used at high-energy
accelerators in research laboratories. Therefore, it is possible to recommend dosemetric methods and
methods for the calibration of dosemetric devices, as well as the techniques for maintaining the traceability of
dosemetric measurements to national standards. Dosemetric measurements taken to evaluate ambient dose
equivalent must be performed using accurate and reliable methods that ensure the quality of readings
provided to workers and regulatory authorities. The future Part 2 of ISO 20785 will give procedures for the
characterization of the response of instruments for the determination of ambient dose equivalent in aircraft.
Requirements for the determination and recording of the cosmic radiation exposure of aircraft crew have been
introduced into the national legislation of EU Member States and other countries. Harmonization of methods
used for determining ambient dose equivalent and for calibrating instruments is desirable to ensure the
compatibility of measurements performed with such instruments.
This International Standard is intended for the use of primary and secondary calibration laboratories for
ionizing radiation, by radiation protection personnel employed by governmental agencies, and by industrial
corporations concerned with the determination of ambient dose equivalent for aircraft crew.

vi © ISO 2006 – All rights reserved

INTERNATIONAL STANDARD ISO 20785-1:2006(E)

Dosemetry for exposures to cosmic radiation in civilian
aircraft —
Part 1:
Conceptual basis for measurements
1 Scope
This part of ISO 20785 gives the conceptual basis for the determination of ambient dose equivalent for the
evaluation of exposure to cosmic radiation in civilian aircraft and for the calibration of instruments used for this
purpose.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
NOTE The documents published by the ICRP (International Commission on Radiological Protection) and the ICRU
(International Commission on Radiation and Measurements) are recognised by the ISO committee as having a wide
acceptance and authority status.
ICRU Report 60:1998, Fundamental Quantities and Units for Ionizing Radiation
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
3.1 Quantities and units
3.1.1
particle fluence
fluence
Φ
quotient of dN by da, where dN is the number of particles incident on a sphere of cross-sectional area da
dN
Φ =
da
−2 −2
NOTE 1 The unit of the particle fluence is m , a frequently used unit is cm .
NOTE 2 The energy distribution of the particle fluence, Φ , is the quotient dΦ by dE, where dΦ is the fluence of
E
particles of energy between E and E + dE. There is an analogous definition for the direction distribution, Φ , of the particle
Ω
fluence. The complete representation of the double differential particle fluence can be written (with arguments)
Φ E, Ω (E, Ω ), where the subscripts characterize the variables (quantities) for differentiation and where the symbols in the
brackets describe the values of the variables. The values in the brackets are needed for special function values, e.g. the
energy distribution of the particle fluence at the energy, E = E is written as Φ (E ). If no special values are indicated, the
,
0 E 0
brackets can be omitted.
3.1.2
particle fluence rate
fluence rate
i
Φ
quotient of dΦ by dt, where dΦ is the increment of particle fluence in the time interval dt:
i
ddΦ N
Φ==
ddta⋅dt
−2 −2
NOTE The unit of the particle fluence rate is m /s, a frequently used unit is cm /s.
3.1.3
energy imparted
ε
energy imparted by ionizing radiation to the matter in a given volume
NOTE The unit of the energy imparted is J.
3.1.4
mean energy imparted
ε
expectation value of the energy imparted by ionizing radiation to the matter in a given volume
NOTE The unit of the mean energy imparted is J.
3.1.5
specific energy imparted
z
quotient of ε by dm, where ε is the energy imparted by ionizing radiation to matter of mass dm:
dε
z =
dm
NOTE The unit of specific energy is J/kg, with the special name gray (Gy).
3.1.6
absorbed dose
D
quotient of dε by dm, where dε is the mean energy imparted to matter of mass dm:
dε
D =
dm
NOTE The unit of absorbed dose is J/kg, with the special name gray (Gy).
3.1.7
kerma
K
quotient of dE by dm, where dE is the sum of the initial kinetic energies of all the charged particles liberated
tr tr
by uncharged particles in the mass, dm, of material:
dE
tr
K =
dm
NOTE The unit of kerma is J/kg, with the special name gray (Gy).
2 © ISO 2006 – All rights reserved

3.1.8
linear energy transfer
LET
L
quotient of dE by dl, where dE is the energy lost in a material by a charged particle due to electronic collisions
in traversing a distance dl:
dE
L =
dl
NOTE The unit of the linear energy transfer is J/m, a frequently used unit is keV/µm.
3.1.9
dose equivalent
H
product of Q and D at the point of interest in tissue, where D is the absorbed dose, Q is the quality factor at
that point, D is the distribution of the dose D in linear energy transfer L, and Q(L) is the quality factor as a
L
function of L in water:
H=⋅QD= Q()L D dL
L
∫
NOTE 1 The unit of dose equivalent is J/kg, with the special name sievert (Sv).
NOTE 2 Values for the relationship Q(L) are given in ICRP Publication 60:1998.
3.1.10
dose-mean specific energy per event
single-event dose-mean specific energy
z
D
∞
expectation value z = zd ()z dz , where d (z) is the dose probability density of z
D 1 1
∫
NOTE The dose probability density of z is given by dz() =dD (z)/dz, where D (z) is the fraction of absorbed dose
11 1
per event delivered with specific energy less than or equal to z.
3.1.11
lineal energy
y
quotient of ε by l where ε is the energy imparted to the matter in a given volume by a single energy

s s
deposition event and l is the mean chord length in that volume:
ε
s
y =
l
NOTE The unit of the lineal energy is J/m, a frequently used unit is keV/µm.
3.1.12
dose-mean lineal energy
y
D
∞
expectation value yy= d()y dy , where d(y) is the dose probability density of y.
D
∫
NOTE 1 The dose probability density of y is given by dy() =dD(y)/dy, where D(y) is the fraction of absorbed dose
delivered with lineal energy less than or equal to y.
NOTE 2 Both y and the distribution d(y) are independent of the absorbed dose or dose rate.
D
3.1.13
ambient dose equivalent
H*(10)
dose equivalent at a point in a radiation field, that would be produced by the corresponding expanded and
aligned field, in the ICRU sphere at 10 mm depth on the radius opposing the direction of the aligned field
NOTE The unit of the ambient dose equivalent is J/kg, with the special name sievert (Sv).
3.1.14
particle fluence-to-ambient dose equivalent conversion coefficient
*
h
Φ
quotient of the particle ambient dose equivalent, H*(10), and the particle fluence, Φ :
*
H (10)
*
h =
Φ
Φ
NOTE The unit of the particle fluence-to-ambient dose equivalent conversion coefficient is J⋅m /kg with the special
2 2
name Sv⋅m , a frequently used unit is Sv⋅cm .
3.1.15
indication
reading
M
〈of a measuring instrument〉 value of a quantity provided by a measuring instrument
NOTE 1 The value read from the displaying device can be called the direct indication; it is multiplied by the instrument
constant to give the indication.
NOTE 2 The quantity can be the measurand, a measurement signal, or another quantity to be used in calculating the
value of the measurand.
NOTE 3 It is necessary to document whether the indication (reading) is normalized to the reference conditions to
account for influence quantities and is corrected for intrinsic background and other factors.
3.1.16
response characteristic
response
R
quotient of the indication (reading), M, of the instrument, by the value of the quantity, X, to be measured by the
instrument, for a specified type, energy and direction distribution of radiation:
M
R =
X
NOTE 1 VIM says, as a note in the definition of “measurement signal”, that “the input signal to a measuring system
may be called the stimulus, the output signal may be called the response.” The term “response characteristic” is defined
as the “relationship between a stimulus and the corresponding response, for defined conditions”. In radiation metrology
generally, the term “response” is an abbreviated form of “response characteristic”.
NOTE 2 It is necessary, in order to avoid confusion, to state the type of response, e.g. fluence response (response with
respect to Φ):
M
R =
Φ
Φ
or, dose equivalent response (response with respect to dose equivalent H)
M
R =
H
H
4 © ISO 2006 – All rights reserved

3.1.17
atmospheric depth
X
v
mass of a unit-area column of air above a point in the atmosphere
2 2
NOTE The unit of atmospheric depth is kg/m ; a frequently used unit is g/cm .
3.1.18
magnetic rigidity
rigidity
momentum per unit charge (of a particle in a magnetic field)
NOTE 1 The unit of rigidity is T⋅m. A frequently used unit is V (or GV) in a system of units where momentum, p, is given
in eV/c (or GeV/c), and where rigidity is c times momentum per unit charge, p⋅ c/Q.
NOTE 2 Rigidity is especially useful in characterizing charged particle trajectories in magnetic fields. All particles
having the same magnetic rigidity have identical trajectories in a magnetic field, independent of particle mass or atomic
charge.
3.1.19
geomagnetic cut-off rigidity
cut-off rigidity
r
c
minimum magnetic rigidity an incident particle can have and still penetrate the geomagnetic field to reach a
given location above the Earth
NOTE Cut-off rigidity depends on angle of incidence. Often vertical incidence is assumed.
3.1.20
vertical geomagnetic cut-off rigidity
vertical cut-off
cut-off
minimum magnetic rigidity a vertically incident particle can have and still reach a given location above the
earth
3.2 Atmospheric radiation field
3.2.1
cosmic radiation
cosmic rays
cosmic particles
ionizing radiation consisting of high-energy particles, primarily ionized nuclei, of extra-terrestrial origin and the
particles they generate by interaction with the atmosphere and other matter
3.2.2
primary cosmic radiation
primary cosmic rays
cosmic radiation incident from space
3.2.3
secondary cosmic radiation
secondary cosmic rays
cosmogenic particles
particles that are created, directly or in a cascade of reactions, by primary cosmic radiation interacting with the
atmosphere or other matter
NOTE Important particles with respect to radiation protection and radiation measurements in aircraft are neutrons,
protons, photons, electrons, positrons, muons, and to a lesser extent, pions and nuclear ions heavier than protons.
3.2.4
galactic cosmic radiation
galactic cosmic rays
GCR
cosmic radiation originating outside the solar system
3.2.5
solar cosmic radiation
solar cosmic rays
solar particles
cosmic radiation originating from the sun
3.2.6
solar particle event
SPE
large fluence rate of energetic solar particles ejected into space by a solar eruption, or the sudden increase of
cosmic radiation observed when such particles arrive at Earth
3.2.7
ground level event
GLE
sudden increase of cosmic radiation, observed on the ground and at flight altitudes associated with a solar
particle event, having a high flux of particles with high energy (greater than 500 MeV)
NOTE GLEs are rare, occurring on average about once per year.
3.2.8
solar modulation
change of the GCR field (outside the earth's magnetosphere), caused by change of solar activity and
consequent change of the magnetic field of the heliosphere
3.2.9
solar cycle
period during which the solar activity varies with successive maxima separated by an average interval of
about 11 years, the so-called solar cycle
NOTE 1 If the reversal of the Sun’s magnetic field polarity in successive 11 year periods is taken into account, the
complete solar cycle may be considered to average some 22 years.
NOTE 2 The sunspot cycle, as measured by the relative sunspot number, has an approximate length of 11 years, but
this varies between about 7 years and 17 years. An approximate 11-year cycle has been found or suggested in
geomagnetism, frequency of aurora, and other ionospheric characteristics. The u index of geomagnetic intensity variation
shows one of the strongest known correlations to solar activity.
3.2.10
relative sunspot number
measure of sunspot activity computed from the expression k(10 g + f), where f is the number of individual
spots, g is the number of groups of spots, and k is a factor that varies with the observer's personal experience
of recognition and with observatory (location and instrumentation)
NOTE The relative sunspot number is also known as the “Wolf number”.
3.2.11
solar maximum
time period of maximum solar activity during a solar cycle, usually defined in terms of the relative sunspot
number
6 © ISO 2006 – All rights reserved

3.2.12
solar minimum
time period of minimum solar activity during a solar cycle, usually defined in terms of the relative sunspot
number
3.2.13
cosmic ray neutron monitor
ground level neutron monitor
GLNM
large detector used to measure the time-dependent relative fluence rate of high-energy cosmic radiation, in
particular the secondary neutrons generated in the atmosphere, but can also include protons, other hadrons,
and muons
NOTE Installed worldwide at different locations and altitudes on the ground (and occasionally placed on ships or
aircraft), cosmic radiation neutron monitors are used for various cosmic radiation studies and to determine solar
modulation.
4 General considerations
4.1 General description of the cosmic radiation field in the atmosphere
The earth is continually bombarded by high-energy particles that come from outside the solar system, called
galactic cosmic radiation (GCR), and sporadically exposed to bursts of energetic particles from the sun. This
primary cosmic radiation interacts 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
[4], [5], [6]
intensity with depth in the atmosphere from aircraft altitudes to sea level . Solar cosmic radiation is
discussed at the end of this section; doses are far smaller than from GCR. About 98 % of the GCR are ionized
atomic nuclei and about 2 % are electrons. Of the nuclei, about 90 % are protons (hydrogen nuclei), 9 % are
[7] 20
helium ions, and 1 % are heavier ions . They can have energies up to about 10 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,
−2,7 15
and the spectrum follows a power function of the form E eV up to 10 eV; above that energy, the
−3
spectrum steepens to E eV. The fluence rate of the GCR entering the solar system is fairly constant in time,
and these energetic ions approach the earth from all directions about equally.
Since cosmic radiation particles are electrically charged, their paths are bent when they cross a magnetic field.
The lower the momentum per unit charge (magnetic rigidity) of an ion, the more its path is bent. The magnetic
field that permeates the solar system and the stronger magnetic field of the earth act as shields that lower the
number of low-momentum GCR striking the atmosphere and consequently lower atmospheric cosmic-ray
dose rates.
The Sun has a varying magnetic field with a basic dipole component that reverses direction approximately
every 11 years. Near reversals, 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. At a solar maximum,
the GCR are maximally deflected as they enter the solar system, causing a radiation minimum at Earth.
Between reversals, at a “solar minimum”, there are few sunspots, and the heliosphere's magnetic field is
relatively weak and smooth, so a solar minimum is the time of radiation maximum. The relative change in
dose rates from this solar modulation depends on the other two main variables affecting cosmic radiation in
the atmosphere: geomagnetic latitude and altitude.
The Earth’s magnetic field has a larger effect than the Sun’s on cosmic radiation particles approaching the
atmosphere. Near the equator, where the geomagnetic field is nearly parallel to the ground, vertically incident
particles with magnetic rigidity less than about 15 GV are bent back into space. The minimum rigidity that a
vertically incident particle can have and still reach a given location is called the vertical geomagnetic cut-off
rigidity (cut-off) for that point. Near the magnetic poles, where the geomagnetic field is nearly vertical, the
vertical cut-off approaches zero, and the maximum number of primary cosmic radiation can reach the
atmosphere. As a result, cosmic radiation exposure rates are highest in polar regions and decline toward the
equator. The size of this effect depends on the altitude and the phase in the solar cycle.
The magnetic fields of the Earth and the Sun also alter the relative number of GCR protons and heavier ions
reaching the atmosphere. The GCR ion composition entering the atmosphere, as given above (90 % protons,
9 % He ions, 1 % heavier nuclei), is true for low geomagnetic cut-off and low solar activity. At a vertical cut-off
of 15 GV, the composition is approximately 83 % protons, 15 % He ions, and nearly 2 % heavier ions.
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 low altitudes, the muon
component is the most important contributor to ambient dose equivalent and effective dose; at aircraft
altitudes, neutrons, electrons, positrons, photons, and protons 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, or 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 SPEs), are of 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 particle fluence rate. Only a small fraction of SPEs, on average one per year,
produce large numbers of high-energy particles that 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 events (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 GCR particle fluence rate reaching the atmosphere.

Key
X altitude (km)
−1
Y ambient dose equivalent rate, (µSv⋅h )
[8]
Conditions: 1 GV cut-off and solar minimum (deceleration potential, φ, of 465 MV)
Figure 1 — Calculated ambient dose equivalent rates as a function of standard barometric altitude
for various atmospheric cosmic radiation component particles
8 © ISO 2006 – All rights reserved

4.2 General calibration considerations for the dosemetry of cosmic radiation fields
in aircraft
4.2.1 Approach
The general approach necessary for measurement and calibration is given in 4.2. Details of calibration fields
and procedures will be given in the future Part 2 of ISO 20785.
4.2.2 Considerations concerning the measurement
[9]
The ambient dose equivalent cannot be measured directly by conventional dosemetric techniques . The
experimental determination of ambient dose equivalent for the complex radiation field considered here (see
Figure 1) is particularly difficult. An approximate approach is to use a tissue equivalent proportional counter
(TEPC) to measure dose equivalent to a small mass of tissue, by measuring the absorbed dose distribution in
lineal energy (which is an approximation for LET), with corrections applied, and directly applying the
LET-dependent quality factor. However, this measurement still does not realize the quantity.
Dosemetry of the radiation field in aircraft requires specialized techniques of measurement and calculation.
The preferred approach is 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 using data on the energy and direction characteristics of the field and the
energy and angle ambient dose equivalent response characteristics of the device.
4.2.3 Considerations concerning the radiation field
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 (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 force producing directly ionizing secondary particles. Protons
are both directly ionizing via the electromagnetic force and indirectly via neutron-like strong interactions.
The directly ionizing component and the secondary electrons from indirectly ionizing photons, comprise the
non-neutron component. The neutrons plus the neutron-like interactions of protons comprise the neutron
component. Alternatively for dosemetric purposes, the field can be divided into low-LET (< 10 keV/µm) and
high-LET (> 10 keV/µm) components. This definition is based on the dependence of the quality factor on LET.
The quality factor is unity below 10 keV/µm. This separation between low- and high-LET particles can be
applied to TEPCs, and to other materials and detectors, but the low-LET/high-LET threshold can vary between
5 keV/µm and 10 keV/µm. The low-LET component includes the directly ionizing electrons, positrons and
muons; the secondary electrons from photon interactions, which contribute most of the energy deposition by
directly ionizing interactions of protons; and 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 non-neutron component, and high-LET and neutron and neutron-like component
are not necessarily the same, but are generally similar in magnitude.
The operational dose quantity relevant for these determinations, the ambient dose equivalent, is reasonably
approximated, assuming suitable calibration and normalization, by the response of a tissue equivalent
proportional counter (TEPC), recombination ionization chamber or semiconductor spectrometer. The low-LET
or non-neutron energy deposition can be determined using an ionization chamber, silicon-based detector or
scintillation detector; or a passive luminescence or ion storage detector. The high-LET or neutron component
can be measured using an extended range neutron survey meter or multi-sphere spectrometer; or a passive
etched track detector, bubble detector or fission foil with damage track detector. The summed components,
low-LET plus high-LET, or non-neutron plus neutron and neutron-like, with suitable calibration and
normalization, give the total ambient dose equivalent. It is essential for the measurement of the complex
radiation fields that the instruments used are fully characterized at national standards laboratories, where
possible, and thus that full traceability is established.
Definitions of terms and details of normal procedures used in the calibration and use of measurement devices
[85] [88]
are given in various ISO and ICRU documents (for instance, ISO 4037–3 , ISO 8529–3 ,
[9]
ICRU Report 66 ). The determination of the uncertainties associated with any set of measurement is an
important part of dosemetry. Uncertainties associated with specific methods of dosemetry are frequently not
statistically independent. Even when they are independent, the total uncertainty is frequently not simply the
root mean square of the individual uncertainties but depend upon the procedure for measurement and
analysis. Details are given in GUM.
4.2.4 Considerations concerning calibration
In terms of ambient dose equivalent, the main contributions to the radiation field at aviation altitudes are from
neutrons from a few hundred keV up to a few GeV, protons from a few tens of MeV to a few GeV, electrons,
positrons and photons from a few MeV to a few GeV. The determination of the response characteristics, both
energy and angle dependence, of devices used for the determinations of ambient dose equivalent for the
cosmic radiation fields in aircraft should be carried out, where possible, in ISO reference radiations. However,
ISO reference radiations do not fully cover the energy range of photons, neutrons and electrons to account for
the majority of the contributions to total ambient dose equivalent. Thus, additional calibration fields are
required, including, for some devices, proton radiation fields.
To determine the response characteristics to high-energy low-LET radiation field components for which
reference fields are not available, it can be demonstrated by measurement and calculation for particular
devices, for example the tissue equivalent proportional counter (TEPC), that the details of the energy
deposition distribution in the sensitive volume of the device are similar for these components to those for
the ISO high-energy photon reference field R-F. This addresses the particular problems associated with the
[10] to [12]
setting of the low-LET threshold of TEPCs and other devices . Quasi-monoenergetic neutron fields are
[13] to [17]
available for energies up to about 200 MeV . For the determination of the neutron response
characteristics of devices for higher energies, measurements may sometimes be made in mono-energetic
proton beams in combination with calculation, or in broad energy distribution neutron fields, also in
combination with calculation.
For non ISO fields, a traceable technique can be used to measure the particle fluence and convert it to
ambient dose equivalent by applying fluence to dose conversion factors.
4.2.5 Simulated aircraft fields
4.2.5.1 Accelerator based fields
Instrument response measurements and intercomparisons can be made in the simulated cosmic radiation
neutron field, which has been designed and provided at CERN. The facility has been developed and
characterized jointly with the European Commission and is known as CERF (CERN-EU high-energy
[18] to [20]
Reference Field facility) . The fields are created by beams of high energy protons and pions with
momenta of either 120 GeV/c (positive or negative) or 205 GeV/c (positive) incident on a copper target. There
is massive concrete shielding at the side of the beam at the target positions, and, depending on target position,
either iron or concrete shields above. The areal mass of the 80 cm concrete shields is almost equal to the air
layer above for flight altitudes of 10 km to 15 km. Well-characterized neutron fields are located both at the side
of the target area and on the roof shields. The neutron component (plus other hadrons) of the radiation field in
[21]
each calibration position has been calculated by using the Monte Carlo code FLUKA . A number of
multisphere spectrometry measurements has also been made. At present, the metrology of the field is not
traceable to national standards.
4.2.5.2 Cosmic radiation fields on mountains
The cosmic radiation fields on the ground at high elevations are the radiation fields closest to those in
[22] to [25]
aircraft , but, as with accelerator-produced simulated aircraft fields, differences between these fields
and the aircraft fields can affect the intercomparisons and/or evaluations of some instruments and should be
taken into account. The composition and spectral fluence of the cosmic radiation field on the ground, even at
altitudes as high as 4 km, are not exactly the same as the cosmic radiation field at aviation altitudes. The
fraction of the dose from muons is higher at lower altitudes. The cosmic radiation neutron spectrum on the
ground has relatively more neutrons with energies above 10 MeV, fewer with energies from 1 eV to about
10 © ISO 2006 – All rights reserved

[26]
2 MeV, and more at thermal energies . Different materials of the “ground”, e.g. soil, water or snow, concrete,
or other building materials, scatter neutrons differently and can affect the shape of the neutron spectrum.
4.3 Conversion coefficients
The fluence-to-ambient dose equivalent conversion coefficients depend on the particle type and their energy.
The data up to 20 MeV are well established and part of international recommendations (ISO, IEC,
[27]
ICRU 57 ). A compilation of fluence to ambient dose equivalent conversion factor for all particles and
energies of relevance can be found in Reference [28].
Recommended conversion coefficients will be given in Part 2 of ISO 20785.
5 Dosemetric devices
5.1 Introduction
The types of detectors that can be used for measurements to determine ambient dose equivalent onboard
aircraft are similar to those devices used at accelerator laboratories. They can be categorized as active or
passive, or by the component of the field measured (see, for example, Reference [29]). This part of ISO 20785
gives the basis for instruments’ use in the determination of ambient dose equivalent.
5.2 Active devices
5.2.1 Devices to determine all field components
5.2.1.1 Energy deposition spectrometers
The two main types of energy deposition spectrometers are gas-filled devices, in particular tissue equivalent
proportional counters (TEPCs), and solid state (normally silicon) devices.
5.2.1.1.1 Tissue equivalent proportional counters
A tissue equivalent proportional counter (TEPC) is sensitive to directly ionizing particles and to indirectly
ionizing particles via the charged secondary particles created by them in the walls of the counter. The
sensitive volume is filled with a gas of chemical composition similar to tissue, at a low pressure in order to
simulate a biological site of a few microns. Although ideally of spherical symmetry, TEPCs are often cylindrical.
Incident radiation produces electrons in the gas that are collected on the central anode when an electric field
is applied between the anode and the wall of the detector. Each event (or particle track through the gas)
produces an output signal whose magnitude is proportional to the initial energy deposited. Each event
detected is analysed using a pulse height analysis method and stored to produce the lineal energy distribution
spectrum, d(y), where y is the energy deposited divided by the average chord length of the detector. For many
practical purposes, y is used as an approximation to LET. The sum of the deposited energy for each event
divided by the mass of gas provides the absorbed dose. The dose equivalent may be calculated by folding the
absorbed dose distribution with the quality factor.
The ambient dose equivalent is determined by a calibration in reference fields, but note that for use, a
cor
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