IEC TS 62396-1:2006

TECHNICAL IEC
SPECIFICATION TS 62396-1
First edition
2006-03
Process management for avionics –
Atmospheric radiation effects –
Part 1:
Accommodation of atmospheric radiation effects
via single event effects within avionics electronic
equipment
Reference number
IEC/TS 62396-1:2006(E)
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TECHNICAL IEC
SPECIFICATION TS 62396-1
First edition
2006-03
Process management for avionics –
Atmospheric radiation effects –
Part 1:
Accommodation of atmospheric radiation effects
via single event effects within avionics electronic
equipment
© IEC 2006 ⎯ Copyright - all rights reserved
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– 2 – TS 62396-1 ¤ IEC:2006(E)
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope and object.7
2 Normative references .7
3 Terms and definitions .7
4 Abbreviations used in the document .14
5 Radiation environment of the atmosphere.15
5.1 Radiation generation .15
5.2 Effect of secondary particles on avionics.16
5.3 Atmospheric neutrons.16
5.4 Secondary protons .21
5.5 Other particles.21
5.6 Solar enhancements.22
6 Effects of atmospheric radiation on avionics .22
6.1 Types of radiation effects .22
6.2 Single event effects.23
6.3 Total Ionising Dose (TID) .26
6.4 Displacement damage .27
7 Guidance for system designs.27
7.1 Overview .27
7.2 System design.30
7.3 Hardware considerations .31
7.4 Parts characterisation and control .32
8 Determination of avionics single event effects rates .34
8.1 Main single event effects.34
8.2 Single event effects with lower event rates .34
8.3 Single event effects with higher event rates - Single event upset data.36
8.4 Calculating SEE rates in avionics .42
9 Considerations for SEE compliance.42
9.1 Compliance .42
9.2 Confirm the radiation environment for the avionics application .42
9.3 Identify system development assurance level .42
9.4 Assess preliminary electronic equipment design for SEE.43
9.5 Verify that the system development assurance level requirements are met for
SEE.43
9.6 Corrective actions .43
Annex A (informative) Thermal neutron assessment .44
Annex B (informative) Methods of calculating SEE rates in avionics electronics.45
Annex C (informative) Review of test facility availability.50
Annex D (informative) Tabular description of variation of atmospheric neutron flux
with altitude and latitude .54

TS 62396-1 ¤ IEC:2006(E) – 3 –
Figure 1 – Energy Spectrum of Atmospheric Neutrons at 40 000 Feet (12 160 m),
latitude 45 degrees .17
Figure 2 – Variation of the Atmospheric Neutron Flux with Altitude (see Annex D) .18
Figure 3 – Distribution of vertical rigidity cut offs around the world.19
Figure 4 – Variation of the 1 to 10 MeV atmospheric neutron flux with latitude .20
Figure 5 – Energy Spectrum of Protons within the Atmosphere .21
Figure 6 – System Safety Assessment Process .28
Figure 7 – SEE in relation to System and LRU effect. .30
Figure 8 – Variation of RAM SEU cross section as function of neutron/proton energy .38
Figure 9 – Neutron and proton SEU bit cross-section data .39
Figure 10 – SEU cross section in SRAMs as function of manufacture date.40
Figure 11 – SEU cross section in DRAMs as function of manufacture date .41
Table 1 – Nomenclature Cross Reference.29
Table B.1 – Sources of high energy proton or neutron SEU cross section data .46
Table B.2 – Some Models for the Use of Heavy Ion SEE Data to Calculate Proton
SEE Data.47
Table D.1 – Variation of 1 to 10 MeV neutron flux in the atmosphere with altitude.54
Table D.2 – Variation of 1 to 10 MeV neutron flux in the atmosphere with latitude.54

– 4 – TS 62396-1 ¤ IEC:2006(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –
Part 1: Accommodation of atmospheric radiation effects via
single event effects within avionics electronic equipment
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
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Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• The subject is still under technical development or where, for any other reason, there is
the future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 62396-1, which is a technical specification, has been prepared by IEC technical
committee 107: Process management for avionics.

TS 62396-1 ¤ IEC:2006(E) – 5 –
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
107/41/DTS 107/46/RVC
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 62396, as currently conceived, consists of the following parts, under the general title
Process management for avionics – Atmospheric radiation effects:
Part 1: Accommodation of atmospheric radiation effects via single event effects within
avionics electronic equipment
Part 2: Guidelines for single event effects testing for avionics systems
Part 3: Guidelines to optimize avionics system design to reduce single event effects rates
Part 4: Guidelines for designing with high voltage aircraft electronics and potential single
event effects
Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics systems
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
———————
Under consideration.
– 6 – TS 62396-1 ¤ IEC:2006(E)
INTRODUCTION
This industry-wide technical specification informs avionics systems designers, electronic
equipment, component manufacturers and their customers of the kind of ionising radiation
environment that their devices will be subjected to in aircraft, the potential effects this
radiation environment can have on those devices, and some general approaches for dealing
with these effects.
The same atmospheric radiation (neutrons) that is responsible for the radiation exposure that
crew and passengers acquire while flying is also responsible for causing the Single Event
Effects (SEE) in the avionics electronic equipment. There has been much work carried out
over the last few years related to the radiation exposure of aircraft passengers and crew.
A standardised industry approach on the effect of the atmospheric neutrons on electronics
should be viewed as consistent with and an extension of the on-going activities related to the
radiation exposure of aircraft passengers and crew.
Atmospheric radiation effects are one factor that could contribute to equipment hard and soft
fault rates. From a system safety perspective, using derived fault rate values, the existing
methodology described in ARP4754 (accommodation of hard and soft fault rates in general)
will also accommodate atmospheric radiation effect rates.
In addition, this technical specification is related to the JEDEC Standard JESD89, which
relates to soft errors in electronics by atmospheric radiation at ground level (at altitudes less
than 10 000 feet (3 040 m)).
TS 62396-1 ¤ IEC:2006(E) – 7 –
PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –
Part 1: Accommodation of atmospheric radiation effects via
single event effects within avionics electronic equipment
1 Scope and object
This Technical Specification is intended to provide guidance on Atmospheric Radiation effects
on Avionics electronics used in aircraft operating at altitudes up to 60 000 feet (18,3 km). It
defines the radiation environment, the effects of that environment on electronics and provides
design considerations for the accommodation of those effects within avionics systems.
This Technical Specification is intended to help aerospace equipment manufacturers and
designers to standardise their approach to Single Event Effects in Avionics by providing
guidance, leading to a standard methodology.
Details of the radiation environment are provided together with identification of potential
problems caused as a result of the atmospheric radiation received. Appropriate methods are
given for quantifying Single Event Effect (SEE) rates in electronic components. The overall
system safety methodology should be expanded to accommodate the Single Event Effects
rates and to demonstrate the suitability of the electronics for the application at the component
and system level.
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.
IEC 62239, Process management for avionics – Preparation of an electronic components
management plan
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
NOTE Users of this technical specification may use alternative definitions consistent with convention within their
companies.
3.1
aerospace recommended practice
these documents relating to avionics are published by the Society of Automotive Engineers
Inc
3.2
avionics equipment environment
is, for aeronautical equipment, the applicable environmental conditions (as described per the
equipment specification) that the equipment shall be able to withstand without loss or
degradation in equipment performance during all of its manufacturing cycle and maintenance
life (the length of which is defined by the equipment manufacturer in conjunction with
customers)
– 8 – TS 62396-1 ¤ IEC:2006(E)
3.3
capable
term used to indicate that a component can be used successfully in the intended application
3.4
certified
indicates assessment and compliance to an applicable third party standard and maintenance
of a certificate and registration (i.e. JAN, IECQ- CECC)
3.5
characterisation
process of testing a sample of components to determine the key electrical parameter values
that can be expected of all produced components of the type tested
3.6
component application
process that assures that the component meets the design requirements of the equipment in
which it is used
3.7
component manufacturer
organisation responsible for the component specification and its production
3.8
critical charge
smallest charge that will cause a SEE if injected or deposited in the sensitive volume
NOTE Units: picoCoulomb (pC). For many devices, this is now measured in femtoCoulombs (fC) rather than pC.
3.9
cross section (σ)
in radiation terms for proton and neutron interactions, this is combination of sensitive area
and probability of an interaction depositing the critical charge for a SEE.
The cross section may be calculated using the following formula:
σ = number of errors/particle fluence
NOTE The units for cross section are cm per device or per bit.
3.10
electron
elementary particle having a mass of approximately 1/1 840 atomic mass units, and negative
–19
charge of 1,602 × 10 C
3.11
Electronic Components Management Plan
ECMP
equipment manufacturer's document that defines the processes and practices for applying
components to an equipment or range of equipment. Generally, it addresses all relevant
aspects of controlling components during system design, development, production, and post-
production support
3.12
electronic components
electrical or electronic devices that are not subject to disassembly without destruction or
impairment of design use. They are sometimes called electronic parts, or piece parts
NOTE Examples are resistors, capacitors, diodes, integrated circuits, hybrids, application specific integrated
circuits, wound components and relays

TS 62396-1 ¤ IEC:2006(E) – 9 –
3.13
electronic equipment
item produced by the equipment manufacturer, which incorporates electronic components
NOTE Examples are end items, sub-assemblies, line-replaceable units and shop-replaceable units.
3.14
Electronic Flight Instrumentation System
EFIS
example of an avionics electronic system requiring system development assurance level A
type II and for which the pilot will be within the loop through pilot/system information
exchange
3.15
expert
has demonstrated competence to apply knowledge and skill to the specific subject
3.16
firm fault
term used at the aircraft function level. It is a failure that cannot be reset other than by
rebooting the system or by cycling the power to the relevant functional element. Such a fault
could impact the value for the MTBF of the LRU and provide no fault found during subsequent
test
3.17
Fly By Wire
FBW
example of avionics electronic system requiring system development assurance level A type I
and for which the pilot will not be within the aircraft stability control loop
3.18
Functional Hazard Analysis
FHA
assessment of all hazards against a set of defined hazard classes
3.19
GeV
radiation particle energy giga electron volts (thousand million electron Volts)
NOTE The SI equivalent energy is 160,2 picoJoule.
3.20
gray
Gy
SI unit of ionising radiation dose and is the energy deposited as ionisation and excitation (J)
per unit mass (kg)
NOTE Related units centigray (cGy) and rad. 1 cGy is equal to 1 rad.
3.21
hard error
permanent or semi-permanent damage of a cell by atmospheric radiation that is not
recoverable even by cycling the power off and on
3.22
hard fault
term used at the aircraft function level. It refers to the permanent failure of a component
within an LRU. A hard fault results in the removal of the LRU affected and the replacement of
the permanently damaged component before a system/system architecture can be restored to
full functionality. Such a fault could impact the value for the MTBF of the LRU repaired

– 10 – TS 62396-1 ¤ IEC:2006(E)
3.23
heavy ions
positively charged nuclei of the elements other than hydrogen
3.24
in-the-loop
test methodology where an LRU is placed within a radiation beam that provides a simulation
of the atmospheric neutron environment and where the inputs to the LRU would be from an
electronic fixture external to the beam to enable a closed loop system
NOTE The electronic fixture would contain a host computer for the aircraft simulation model. The electronic fixture
would also contain appropriate signal conditioning for compatibility with the LRU. In the case of an automatic
control function, the outputs from the LRU could be, in turn, sent to an actuation means or directly to the host
computer. The host computer would automatically close a stability loop (as in the case of a fly-by-wire control
system). In the case of a navigation function, the outputs from the LRU could be sent to a display system where the
pilot could then close the navigation loop.
3.25
Integrated Modular Avionics
IMA
implement aircraft functions in a multitask computing environment where the computations for
each specific system implementing a particular function are confined to a partition that is
executed by a common computing resource (a single digital electronic circuit)
3.26
latch-up
triggering of a parasitic pnpn circuit in bulk CMOS, resulting in a state where the parasitic
latched current exceeds the holding current, this state is maintained while power is applied
3.27
Linear Energy Transfer
LET
energy deposited per unit path length in a semiconductor along the path of the radiation
NOTE Units: MeV cm /mg.
3.28
Linear Energy Transfer threshold
LETth
for a given component is the minimum LET to cause an effect at a particle fluence of
7 2
1×10 ions/cm
3.29
Line Replaceable Unit
LRU
piece of avionics electronic equipment that may be replaced during the maintenance cycle of
the system
3.30
may
indicates a course of action that is permissible within the limits of this document
3.31
MeV
radiation particle energy Mega electron volts (million electron Volts)
NOTE The SI equivalent energy is 160,2 femtojoule.

TS 62396-1 ¤ IEC:2006(E) – 11 –
3.32
Mean Time Between Failure
MTBF
is a measure of reliability requirements and is the mean time between failure of equipment or
a system in service
3.33
Mean Time Between Unscheduled Removals
MTBUR
is a measure of reliability requirements and is the mean time between unscheduled removal of
equipment or a system in service
3.34
Multiple Bit Upset
MBU
occurs when the energy deposited in the silicon of an electronic component by a single
ionising particle causes upset to more than one bit
3.35
neutron
elementary particle with atomic mass number of one and carries no charge. It is a constituent
of every atomic nucleus except hydrogen
3.36
particle fluence
is for a unidirectional beam of particles the number crossing unit surface at right angles to
beam. For isotropic flux, this is number entering sphere of unit cross-sectional area
NOTE Units: particles/cm .
3.37
particle flux
fluence rate per unit time
NOTE Units: particles/cm ⋅s.
3.38
pion or pi-meson
sub atomic particle. The charge possibilities are (+1, –1, 0) and they are produced by
energetic nuclear interactions
3.39
Preliminary System Safety Assessment
PSSA
evaluation of the planned architecture to determine the reasonableness of the architecture to
meet the system safety requirements
3.40
proton
elementary particle with atomic mass number of one and positive electric charge. It is a
constituent of all atomic nuclei
3.41
risk
measure of the potential inability to achieve overall program objectives within defined cost,
schedule, and technical constraints

– 12 – TS 62396-1 ¤ IEC:2006(E)
3.42
Single Event Burn Out
SEB
occurs when a powered electronic component or part thereof is burnt out as a result of the
energy absorption triggered by an individual radiation event
3.43
Single Event Effect
SEE
response of a component caused by the impact of a single particle (for example galactic
cosmic rays, solar energetic particles, energetic neutrons and protons)
NOTE The range of responses can include both non-destructive (for example upset) and destructive (for example
latch-up or gate rupture) phenomena.
3.44
Single Event Functional Interrupt
SEFI
upset in a usually complex device, for example, a microprocessor, such that a control path is
corrupted, leading the part to cease to function properly
NOTE This effect has sometimes been referred to as lockup, indicating that sometimes the part can be put into a
“frozen” state (see 6.2.6).
3.45
Single Event Gate Rupture
SEGR
occurs in the gate of a powered insulated gate component when the radiation charge
absorbed by the device is sufficient to cause gate rupture, which is destructive
3.46
Single Event Latch Up
SEL
occurs in a four layer semiconductor device when the radiation absorbed by the device is
sufficient to cause a node within the powered semiconductor device to be held in a fixed state
whatever input is applied until the device is de-powered, such latch up may be destructive or
non-destructive
3.47
Single Event Transient
SET
spurious signal or voltage, induced by the deposition of charge by a single particle that can
propagate through the circuit path during one clock cycle (see 6.2.4)
3.48
Single Event Upset
SEU
occurs in a semiconductor device when the radiation absorbed by the device is sufficient to
change a cell’s logic state
NOTE After a new write cycle, the original state can be recovered.
3.49
Single Hard Error
SHE
single event induced hard error
occurs when in a single event the radiation absorbed by the device is sufficient to cause
permanent stuck-bit in the device, and a hard error within the equipment

TS 62396-1 ¤ IEC:2006(E) – 13 –
3.50
Single word Multiple-bit Upset
SMU
occurs when the energy deposited in the silicon by a single ionising particle causes upset to
more than one bit in a single memory word
3.51
soft error
also known as a single event upset and is the change of state of a latched logic state from
one to zero or vice-versa, it is non-destructive and can be rewritten or reset
3.52
soft fault
is a term used at the aircraft function level that refers to the characteristic of invalid digital
logic cell(s) state changes within digital hardware electronic circuitry
NOTE It is a fault that does not involve replacement of a permanently damaged component within an LRU but it
does involve restoring the logic cells to valid states before a system/system architecture can be restored to full
functionality. Such a fault condition has been suspected in the "no fault found" syndrome for functions implemented
with digital technology and it would probably impact the value for the MTBUR of the involved LRU. If a soft fault
results in the mistaken replacement of a component within the LRU, the replacement could impact the value for the
MTBF of the LRU repaired.
3.53
Solar Energetic Particle (SEP) events
during these periods there is enhancement of solar particles (protons, ions and some
neutrons) caused by solar flare activity or coronal mass ejections
NOTE The enhancement can last from a few hours to several days. A small fraction has sufficiently energetic
spectra to produce significantly enhanced secondary neutron fluxes in the atmosphere.
3.54
substitute component
component used as a replacement in equipment after the equipment design has been
approved
NOTE In some contexts, the term “alternate component” is used to describe a substitute component that is “equal
to or better than” the original component.
3.55
System Safety Assessment
SSA
performed to verify compliance with the safety requirements
3.56
system
collection of hardware and software elements that implement a specific aircraft function or set
of aircraft functions
3.57
Total Ionising Dose
TID
cumulative radiation dose that goes into ionisation that is received by a device during a
specified period of time
3.58
validation
method of confirmation of component radiation tolerance by the equipment manufacturer,
when there is no in-service data from prior use or radiation data from a test laboratory

– 14 – TS 62396-1 ¤ IEC:2006(E)
3.59
will
expresses a declaration of intent when used in the context of being compliant to this
document
4 Abbreviations used in the document
AC Advisory Circular
AIR Atmospheric Ionizing Radiation
ARP Aerospace Recommended Practices
ASIC Application Specific Integrated Circuit
BIT Built-In Test
BPSG Borophosphosilicate glass
CECC CENELEC Electronic Components Committee
CMOS Complimentary Metal Oxide Semiconductor
COTS Commercial Off The Shelf
D-D Deuterium-Deuterium
DOE Department Of Energy (USA)
DRAM Dynamic Random Access Memory
DSP Digital Signal Processor
D-T Deuterium-Tritium
DTS Draft Technical Specification
E Energy
ECMP Electronic Components Management Plan
EDAC Error Detection And Correction
EFIS Electronic Flight Instrumentation System
ESA European Space Agency
eV electron Volt
FBW Fly-By-Wire
FHA Functional Hazard Analysis
FPGA Field Programmable Gate Array
GCR Galactic Cosmic Rays
GeV Giga electron Volt
GLE Ground Level Event
GV Giga Volt (Rigidity unit)
HW Hardware
IBM International Business Machines
ICE Irradiation of Chip and Electronics
IECQ IEC Quality Assessment System for Electronic Components
IEEE Trans. Nucl. Sci. IEEE Transactions on Nuclear Science
IGBT Insulated Gate Bipolar Transistor
IMA Integrated Modular Avionics
IUCF Indiana University Cyclotron Facility (USA)
JAN Joint Army Navy (USA Department of Defence)
JEDEC JEDEC Solid State Technology Association
JESD JEDEC Standard
JPL Jet Propulsion Laboratory
LET Linear Energy Transfer
LETth Linear Energy Transfer threshold
LRU Line Replaceable Unit
TS 62396-1 ¤ IEC:2006(E) – 15 –
MBU Multiple Bit Upset
MeV Mega electron Volt
MOSFET Metal Oxide Semiconductor Field Effect Transistor
MTBF Mean Time Between Failure
MTBUR Mean Time Between Unscheduled Removals
NASA National Aeronautical and Space Agency
PCN Product Change Notification
PSI Paul Scherrer Institute (Switzerland)
PSSA Preliminary System Safety Assessment
PWM Pulse Width Modulator
RADECS RADiations, Effets sur les Composants et Systèmes.
RAM Random Access Memory
RVC Result of Voting (IEC)
SAFETI Systems and Airframe Failure Emulation Testing and Integration
SC Stacked Capacitance
SDRAM Synchronous Dynamic Random Access Memory
SEB Single Event Burn-out
SECDED Single Event Correction Double Event Detection
SEDR Single Event Dielectric Rupture
SEE Single Event Effect
SEFI Single Event Functional Interrupt
SEGR Single Event gate Rupture
SEL Single Event Latch
SEP Solar Energetic Particles
SER Soft Error Rate
SET Single Event Transient
SEU Single Event Upset
SHE Single event induced Hard Error
SMU Single word Multiple-bit Upset
SRAM Static Random Access Memory
SSA System Safety Assessment
SSEEM Segmented Secondary Electron Emission Monitor
SW Software
TIC Trench Internal Capacitance
TID Total Ionizing Dose
TRIUMF Tri-University Meson Facility (Canada)
TSL Theodor Svedberg Laboratoriet (Sweden)
WNR Weapons Nuclear Research (Los Alamos USA)
5 Radiation environment of the atmosphere
5.1 Radiation generation
The atmosphere is penetrated by a flux of various charged and neutral particles that in
combination create a complex ionising radiation environment. These particles are created by
the interaction of the continuous stream of primary cosmic ray particles with the atoms in the
atmosphere (mainly nitrogen and oxygen), and so are called secondary cosmic rays. The
primary cosmic rays are usually referred to as the galactic cosmic rays (GCR), indicating that
their origins are beyond that of the solar system.

– 16 – TS 62396-1 ¤ IEC:2006(E)
The galactic cosmic radiation is composed of atomic nuclei that have been completely ionised
(fully stripped of their electrons) and subsequently accelerated to very high energies. Galactic
cosmic rays consist of about 83 % protons, 16 % alpha particles and <2 % heavy ions
(particles with atomic number Z > 2). As the primary cosmic rays, mainly very high-energy
protons, bombard the atmosphere, they create a cascade of secondary, tertiary, etc. particles
from their interactions with the atoms of the atmosphere. Thus, for each primary cosmic ray
entering, many more secondary particles are created. At a very approximate level, the flux of
incoming primary cosmic rays at the top of the atmosphere is 3 particle/cm ⋅s, and at aircraft
altitudes, the flux of all secondary particles is about 10 particle/cm ⋅s. The density of the
lowest portion of atmosphere is so high, that most of the flux of particles is absorbed, so that
at sea level the nominal flux of secondary particles is less than 0,1 particle/cm ⋅s.
The flux of secondary particles is not uniform around the earth due to the effect of the earth’s
magnetic field that is at right angles to the particle direction at the equator. Particles cross
field lines at right angles at the equator and are bent away while at the poles they travel
parallel to the field and are not deflected. As a result the primary cosmic rays are able to
penetrate into the atmosphere more readily near the magnetic poles and they interact with the
atoms in the atmosphere creating larger numbers of cascade particles.
5.2 Effect of secondary particles on avionics
Some of the secondary particles can interact with microelectronic devices within aircraft
avionics systems and cause single event effects (SEE) in the devices. These secondary
particles deposit enough charge through the recoils they create within a sensitive portion of a
device to result in a malfunction of the device. It has been found that neutrons, protons and
pions are the main particles that can cause these effects.
5.3 Atmospheric neutrons
5.3.1 General
Neutrons are the secondary cosmic ray particles that have been shown to be mainly
responsible for causing single event upsets (SEUs) in memories and other devices on aircraft
since the early 1990s. This identification of the neutrons as the main cause of the SEUs was
based on several different kinds of correlations:
1) the variations of the upset rates against altitude and geographic latitude followed the
variation of the neutron flux with altitude and latitude,
2) neutron induced SEU rates calculated using SEU cross sections measured in a laboratory
and integrated with the neutron flux in the atmosphere agreed with measured in-flight SEU
rates and
3) upset rates at ground level, due to secondary neutrons are proportional to rates at aircraft
altitudes. For the neutrons, as well as all of the secondary particles within the atmosphere,
the variation of the particle flux with three parameters is most important, (energy, altitude
and latitude), for understanding the variation of the SEU rate.
5.3.2 Energy spectrum of atmospheric neutrons
The energy variation of the atmospheric neutrons is usually presented by plotting the
differential flux (flux per unit energy interval) as a function of energy, which is often called the
spectrum, see Figure 1. Monte Carlo generated spectra have produced the following fractions
<1 MeV 0,53; 1 to 10 MeV 0,2; >10 MeV 0,27. The fits quoted below may give slightly
different values. Measurements of the energy spectrum of the cosmic ray neutrons have been
made since the 1950s using a variety of techniques. In Figure 1 we plot four neutron spectra
at an altitude of approximately 40 000 feet (12,2 km). These include the original
measurements made by Hess in 1959 (Ref. 1) , a calculation by Armstrong in 1973 (Ref. 2), a
fit to measurements by Hewitt et al at NASA in 1977 (Ref. 3) and the recent DOE
measurements by a fit to measurement by Goldhagen in an ER-2 aircraft during 1997 (Ref 4).
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TS 62396-1 ¤ IEC:2006(E) – 17 –
1 × 10
1 × 10
1 × 10
–1
1 × 10
–2
1 × 10
–3
1 × 10
Boeing model - Fit to 1974 NASA ames flight data
Hess measured spectrum, 1959
–4
1 × 10
Armstrong calculated spectrum, 1973
1997 ER-2 measurements, Bonner spheres
–5
1 × 10
–3 –2 –1 0 1 2 3 4
1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10 1 × 10
Neutron energy  MeV
Figure 1 – Energy spectrum of atmospheric neutrons
at 40 000 Feet (12 160 m), latitude 45 degrees
A fit to the NASA Ames data (Energy (E) up to 300 MeV), that had been used in the past has
been modified for Energy > 300 MeV using the more recent measurements. The modified
spectrum is given, with Energy in MeV, as
–0,922 2
dN/dE = ª0,346E × exp (–0,0152(lnE) ) E <300 MeV n/cm ⋅s⋅MeV (1)
–2,2
¬340E E >300 MeV
It should be noted that when this differential flux is integrated for Energy > 10 MeV, the
2 2
integrated neutron flux is ~5 600 n/cm per hour, which can be rounded up to 6 000 n/cm per
hour. This nominal high energy neutron flux 6 000 n/cm per hour at 40 000 feet (12,2 km)
and geographic latitude 45° may be treated as a typical in flight envelope and scaled for
different avionics applications (for example, for altitude variation per 5.3.3 and for latitude
variation per 5.3.4). This flux of 6 000 n/cm per hour is conservative by a factor of
approximately 2 compared to the ER-2 measurements. At ground level the flux is
approximately a factor of 300 lower than at 40 000 feet (12,2 km), thus on the ground, the flux
for Energy > 10 MeV is 20 n/cm per hour, (Ref 5) and this agrees with an independently
derived calculation for New York City (Ref 6).
5.3.3 Altitude variation of atmospheric neutrons
The altitude variation of the atmospheric neutron derives from the competition between the
various production and removal processes that affect how the neutrons and the initiating
cosmic rays interact with the atmosphere. The result is a maximum in the flux at about
60 000 feet (18,3 km), called the Pfotzer maximum that can be seen in Figure 2. The Figure
compares the altitude variation of the 1 to 10 MeV neutron flux as given by two models as a
function of altitude. Of the two models, the simplified Boeing model was developed utilising
two sets of 1 to 10 MeV neutron flux measurements from balloons, and is based on a latitude
of 45°. (Refs. 7, 8). A much more rigorous approach was taken by NASA-Langley in
developing a model that is currently called AIR (Ref. 9). It utilised measurements made on
aircraft during the 1960s and 70s, and developed a model that gives the 1 to 10 MeV neutron
flux as a function of three parameters, the atmospheric depth (g/cm ), vertical rigidity cut off
(GV) and solar weather conditions.
Neutron diff. flux  n/cm ⋅ s MeV

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1 × 10
1 × 10
Simplified Boeing model, 45°
NASA-Langley model, R = 5 GV
–1
1 × 10
–2
1 × 10
–3
1 × 10
3 4 5
1 × 10 1 × 10 1 × 10
Altitude Feet
Figure 2 – Variation of the atmospheric neutron flux with altitude (see Annex D)
A tabular description of the variation of atmospheric neutron flux with altitude and also with
latitude as given by the Boeing model is provided in Annex D to enable calculation of neutron
flux at various flight locations.
5.3.4 Latitude variation of atmospheric neutrons
The latitude variation is expressed in terms of the vertical rigidity cut off, R, in units of GV.
The rigidity cut offs indicate the required rigidity (essentially the particle momentum divided by
its charge) of primary cosmic ray particles needed to penetrate to a given location above the
atmosphere. At the equator, where the magnetic field is at right angles to particle direction,
it requires particles with the highest rigidity, R ~ 15 GV, to penetrate to this region, and where
it is par
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