IEC TS 62396-5:2008

IEC/TS 62396-5
Edition 1.0 2008-08
TECHNICAL
SPECIFICATION
Process management for avionics – Atmospheric radiation effects –
Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics
systems
IEC/TS 62396-5:2008(E)
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IEC/TS 62396-5
Edition 1.0 2008-08
TECHNICAL
SPECIFICATION
Process management for avionics – Atmospheric radiation effects –
Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics
systems
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
R
ICS 03.100.50; 31.020; 49.060 ISBN 2-8318-9959-1
– 2 – TS 62396-5 © IEC:2008(E)
CONTENTS
FOREWORD.3
1 Scope.5
2 Normative references .5
3 Terms and definitions .5
4 Overview of thermal neutron single event rate calculation .5
5 Thermal neutron flux inside an airliner.6
5.1 Definition of thermal neutron .6
5.2 Overview .7
5.3 Background on aircraft measurements.7
5.4 Calculational approach .8
5.5 Processing of in-flight neutron flux data.9
6 Thermal neutron SEU cross sections .12
6.1 Overview of the issue .12
6.2 Mechanism involved .13
6.3 Thermal neutron SEU cross sections and Ratio-2.14
7 Recommendation for devices in avionics at present time .16

Bibliography.17

Figure 1 – Atmospheric neutron spectra measured in four aircraft.9

Table 1 – Tabulation of the various atmospheric neutron measurements used .9
Table 2 – Comparison of thermal and high energy neutron fluxes and their ratios .11
Table 3 – Calculated neutron fluxes in the Boeing 747 structure .12
Table 4 – SRAM SEU cross sections induced by thermal and high energy neutrons.15

TS 62396-5 © IEC:2008(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –

Part 5: Guidelines for assessing thermal neutron
fluxes and effects in avionics systems

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. In
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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/TS 62396-5, which is a technical specification, has been prepared by IEC technical
committee 107: Process management for avionics.

– 4 – TS 62396-5 © IEC:2008(E)
This standard cancels and replaces IEC/PAS 62396-5 published in 2007. This first edition
constitutes a technical revision.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
107/82/DTS 107/89/RVC
Full information on the voting for the approval of this standard 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.
A list of all parts of the IEC 62396 series, under the general title Process management for
avionics – Atmospheric radiation effects, can be found on the IEC website.
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 edition of this document may be issued at a later date.

TS 62396-5 © IEC:2008(E) – 5 –
PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –

Part 5: Guidelines for assessing thermal neutron
fluxes and effects in avionics systems

1 Scope
The purpose of this technical specification is to provide a more precise definition of the threat
that thermal neutrons pose to avionics as a second mechanism for inducing single event
upset (SEU) in microelectronics. There are two main points that will be addressed in this
technical specification:
a) a detailed evaluation of the existing literature on measurements of the thermal flux inside
of airliners and
b) an enhanced compilation of the thermal neutron SEU cross section in currently available
SRAM devices (more than 20 different devices).
The net result of the reviews of these two different sets of data will be two ratios that we
consider to be very important for leading to the ultimate objective of how large a threat is the
SEU rate from thermal neutrons compared to the SEU threat from the high energy neutrons
(E >10 MeV). The threat from the high energy neutrons has been dealt with extensively in the
1)
literature and has been addressed by two standards (IEC/TS 62396-1 in avionics and [1] in
microelectronics on the ground).
2 Normative references
The following referenced documents are indispensable for the application of this document,
only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
IEC/TS 62396-1, Process management for avionics – Atmospheric radiation effects – Part 1:
Accommodation of atmospheric radiation effects via single event effects within avionics
electronic equipment
3 Terms and definitions
For the purpose of this document, the terms and definitions of IEC/TS 62396-1 apply.
4 Overview of thermal neutron single event rate calculation
The two ratios that this technical specification considers to be so important are:
a) the ratio of the thermal neutron flux inside an airliner relative to the flux of high energy
(> 10 MeV) neutrons inside the airliner and
b) the ratio of the SEU cross section due to thermal neutrons relative to that due to high
energy neutrons.
These ratios are considered to be important because with them, once we know what the SEU
rates are from the high energy neutrons for an avionics box, a topic which has been dealt with
___________
1)
Numbers in square brackets refer to the bibliography.

– 6 – TS 62396-5 © IEC:2008(E)
extensively, such as in [1], then the additional SEU rate due to thermal neutrons can be
obtained with these ratios. Thus, given the SEU rate from high energy neutrons, multiplying
this by the two ratios gives the SEU rate from the thermal neutrons. The total SEU rate will be
the combination of the SEU rates from both the high energy and thermal neutrons.
The process for calculating the SEU rate from the thermal neutrons is shown in the following
set of equations, (1) to (5).
SEU Rate
2 2
= Φ (neutron flux = 6 000 n/cm h) × σ(Hi E, SEU X-Sctn. cm /dev) (1)
Hi
(Hi E, Upset/dev·h)
SEU Rate
Φ (neutron flux)
σ(therm SEU X-Sctn.)
therm
=×SEU Rate (Hi E) ×  (2)
(thermal neutron,
Φ (neutron flux) σ(Hi E SEU X-Sctn.)
Hi
Upset/dev·h)
Ratio-1
Φ (neutron flux)
thermal
= (3)
Φ (neutron flux)
Hi
Ratio-2
σ (therm SEU Cross Section)
= (4)
σ(Hi E SEU Cross Section)
SEU Rate
SEU Rate (Hi E neutron Upset/dev·h) × Ratio-1 × Ratio-2 (5)
(thermal neutron,
Upset/dev·h)
The objective of this technical specification is to provide values of Ratio-1, the ratio of the
thermal to high energy neutron flux within an airplane, and of Ratio-2, the ratio of the SEU
cross section due to thermal neutrons relative to that due to high energy neutrons. We believe
that Ratio-1 should be relatively similar in various types of commercial airliners, but it could
vary significantly in other types of aircraft, such as military fighters. However, in the larger
type of military aircraft, such as AWACS (Advanced Warning and Command System, E-3,
which is based on either a Boeing 707-320-B or 767) and JSTARS (Joint Surveillance Target
Attack Radar System, E-8C, which is based on Boeing 707-300 airframe), the ratio should be
very similar to that in airliners.
With regard to the ratio of the thermal neutron SEU cross sections, until recently, not very
many such SEU cross sections were reported in the literature. There were a few, and these
were cited in [1]. Due to the data that has recently become available, the number of devices in
which the thermal neutron SEU cross section has been measured has increased significantly.
This additional data allows us to have good confidence on the values that have been
measured and the resulting average value of the ratio.
5 Thermal neutron flux inside an airliner
5.1 Definition of thermal neutron
Thermal neutrons have been given this name because while most neutrons start out with
much higher energies, after a sufficient number of collisions with the surrounding medium, the
neutron velocity is reduced such that is has approximately the same average kinetic energy
as the molecules of the surrounding medium. This energy depends on the temperature of the
medium, so it is called thermal energy. The thermal neutrons are therefore in thermal
equilibrium with the molecules (or atoms) of the medium in which they are present.

TS 62396-5 © IEC:2008(E) – 7 –
In a medium that has only a small probability of absorbing, rather than scattering, neutrons,
the kinetic energies of the thermal neutrons is distributed statistically according to the
Maxwell-Boltzmann law. Therefore, based on this Maxwell-Boltzmann distribution, the neutron
kinetic energy that corresponds to the most probable velocity is kT, where T is the absolute
temperature of the medium and k a constant. For a temperature of 20 °C, room temperature,
this is 0,025 eV. This is based on a highly idealized model of elastic collisions between two
kinds of particles, nuclei and neutrons, within a gaseous medium, and so there are departures
from it in the real world.
Therefore, even though a neutron energy of 0,025 eV is officially taken to be the true
definition of thermal neutrons, for purposes of this technical specification, we will consider
neutrons with energies < 1 eV to be thermal neutrons. Additional details on this are found in
6.2.
5.2 Overview
In a modern airliner, we know that the thermal neutron flux inside the aircraft should be higher
than the thermal neutrons outside of the airplane because of the presence of all of the
hydrogenous materials within it (fuel, plastic structures, baggage, people, etc.). The
hydrogenous materials “slow down” the high energy neutrons through nuclear collisions,
primarily with the hydrogen atoms. After a large number of such interactions, the high energy
neutrons (energy > 10 MeV) have had their energy reduced by about seven orders of
magnitude. For practical purposes, we consider neutrons with E < 1 eV as thermal neutrons.
However, the more accurate definition of thermal neutrons are neutrons with energies close to
0,025 eV (equivalent to those at room temperature, hence the term “thermal”). Thus, we
expect, and have seen it verified by measurements, that the high energy neutrons inside an
airliner and outside it within the atmosphere would be very similar. However, for thermal
neutrons, this is not true. The presence of the airplane structure and its contents produces far
more thermal neutrons inside the aircraft than are present in the atmosphere just outside the
airplane.
5.3 Background on aircraft measurements
The thermal neutron flux inside an airliner is a rather elusive quantity that has not been
measured very often despite the fact that hundreds and in fact thousands of ionizing radiation
measurements have been and are currently been made inside of aircraft. Firstly, most of the
thousands of measurements are of the dose equivalent that passengers and crew accumulate
during flight. Although it varies depending on the location of the flight path, in general, the
dose equivalent is approximately (50 to 60) % from the neutrons, about (25 to 35) % from
electrons and the remainder from other charged particles, mainly protons (10 to 20) %,
gamma rays (<10 %) and muons (<10 %) [2]. Most of these kinds of instruments measure the
combined dose rate from all of the charged particles present in the atmosphere.
Thus, to measure only the neutrons in the atmosphere required a detector system that was
sensitive only to neutrons. The early systems that were flown in the 1960s consisted of
detectors that were optimized to measure mainly neutrons in the energy range of (1 to 10)
MeV. This data was used to develop the simplified Boeing model [3] based on the variation of
the (1 to 10) MeV neutron flux with altitude and latitude. The original variation was not with
latitude but rather as a function of the vertical rigidity cutoff, a parameter indicating how
effective the earth’s magnetic field is at any location in allowing the primary cosmic rays to
reach the atmosphere. The vertical rigidity cutoff varies mainly with latitude, but there is also
a variation due to longitude. Similarly, NASA-LaRC developed a more elaborate model [4] that
was also based on the (1 to 10) MeV measurements.
Since that time, there have been more recent flight measurements made with neutron-specific
instruments that respond to the entire neutron spectrum. These have been primarily a series
of Bonner spheres, a set of instruments with a detector that measures thermal neutrons
surrounded by varying thicknesses of moderating material. The moderating material, generally
polyethylene, is used to “slow down” the high energy neutrons which constitute most of the
neutrons, through nuclear interactions with the hydrogen within it. The larger the sphere of

– 8 – TS 62396-5 © IEC:2008(E)
surrounding polyethylene, the more thermal neutrons are produced and the larger the signal
by the detector. Careful calibrations are needed of the set of Bonner sphere detectors before
a collection of in-flight measurements can be transformed into neutron fluxes within specific
energy ranges. This is a painstaking process and therefore is undertaken by a limited number
of research groups.
Two such sets of measurements have been made, one by a NASA-Ames group [5], and the
other by a Japanese group [6], and these are used in this evaluation. In addition, the most
highly regarded set of such measurements [7] were made by P. Goldhagen of the
Environmental Measurements Laboratory (formerly part of DOE, now a part of the Homeland
Security Administration). Unfortunately, Goldhagen’s measurements were made in an ER-2
aircraft.
The ER-2 is drastically different from a modern airliner. Exacerbating the situation even more,
the detector that Goldhagen relied upon for the thermal neutron measurement was located in
the very tip of the nose of the ER-2 [8]. For all practical purposes, this detector was located in
a part of the airplane that is almost indistinguishable from the atmosphere outside of the
airplane. Thus, the thermal neutron flux measured by Goldhagen in the ER-2 is too low
compared to what we expect within a large airliner. In this case, we are mainly interested in
Ratio-1, i.e., the ratio between the thermal neutron flux and the high energy (E > 10 MeV)
neutron flux.
A more recent paper by a group at EADS [9] that used a simpler detector system, again
Bonner spheres, but specifically designed to be used in an airliner was examined.
Unfortunately, the high energy neutron fluxes from this paper are considered to be far too low
to be realistic. Thus, we do not believe that the data collected by this detector system and
contained in [9] can be considered to be accurate enough and consistent enough to be used
for our purposes of obtaining a reliable and representative value for Ratio-1.
5.4 Calculational approach
There is one paper in the literature [10] that represents a very significant step forward. It is
based on applying an elaborate calculational method to a geometry consisting of a large
airliner (a Boeing 747) and the atmosphere around it. The gross take-off weight of a large
Boeing 747 is close to 450 000 kg (1 million lbs) and the overall internal volume is
3 3
approximately 850 m (30 000 ft ) (based on the cargo capacity of cargo versions of the
Boeing 747). The actual size is therefore enormous (length of aircraft is ~76 m (~250 ft) and
wingspan of ~69 m (~225 ft) compared to most structures or vehicles that are modelled for
purposes of radiation transport calculations. Out of necessity, the calculation had to simplify
the true geometry by orders of magnitude in order to be able to develop the model and carry
out the calculations in a relatively short time. As a result, the full aircraft is described as being
comprised of approximately 30 smaller volumes, into which the different proportions of the full
1 million lbs are distributed, using gross approximations for the various materials (fuel,
baggage, aluminium structure, interior, etc.).
Thus, it is unclear how accurate the results of these calculations are, especially for the
thermal neutrons. For the high energy neutrons, it is clear that for most locations the neutron
flux should be very similar inside the airplane as it is outside the airplane, and that is true in
the results of [10], so this serves as a consistency check. However, for the thermal neutrons,
there are no consistency checks. The thermal neutrons are much higher everywhere inside
the aircraft compared to outside within the atmosphere, so we have no idea of how accurate a
result [10] represents. It may be correct, but it also may be that especially for locations where
the electronics are located, a much smaller model, greatly reduced in overall size, but much
more detailed in terms of the internal structures and the mass distribution that is used, would
be needed to calculate the thermal neutron flux accurately.
Therefore, we will use the results from [10], but we will also compare them to the
measurements from [5] and [6], to obtain Ratio-1. The results from [10] will represent the
upper bound and the results from the in-flight measurements will represent a lower bound.

TS 62396-5 © IEC:2008(E) – 9 –
5.5 Processing of in-flight neutron flux data
For the comparison of in-flight measurements data is taken from four groups, [5], [6], [7] and
[9], and in addition the calculations from two other groups, [10] and Armstrong [11] are used.
First, the measured spectra from the four aircraft measurements are shown in Figure 1, along
with the calculated spectrum from [11]. A tabulation of the main features concerning where
the measurements were taken and which aircraft were used is given in Table 1.
Table 1 – Tabulation of the various atmospheric neutron measurements used
Altitude
Researcher Organization Detector Aircraft Year Ref.
ft (km)
Goldhagen EML Bonner sphere ER-2 1997 40 000 (12,2 km) [7]
Hubert EADS 7-detect A300 2004 34 800 (10,6 km) [9]
spectrometer
Hewitt NASA-Ames Bonner sphere C-141 1974 40 600 (12,4 km) [5]
Nakamura Tohoku U. Bonner sphere DC-8 1985 37 000 (11,3 km) [6]
Armstrong ORNL Calculation Atmosphere 1973 39 000 (11,9 km) [11]

KEY
Hewitt NASA Ames 40 000 ft
(12,2 km) western USA
Nakamura, 37 000 ft
(11,3 km) over Japan
EADS, 35 000 ft (10,7 km)
over Atlantic
Goldhagen, ER-2 40 000 ft
(12,2 km) western USA
Armstrong calculation
38 000 ft (11,6 km)
Neutron energy  (MeV)
IEC  1369/08
Figure 1 – Atmospheric neutron spectra measured in four aircraft
All of the spectra have relatively similar shapes over 11 orders of magnitude, however two of
the spectra seem to be lower than the other three, and these are the in-flight measurements
by Nakamura over Japan and by the EADS group over the Atlantic. The differential neutron
flux spectrum by Nakamura is lower than the others across the entire spectrum. The reason
for this is that the measurements were made in an airplane over Japan. The simplified Boeing
model of the neutron flux as a function of latitude and longitude is not adequate to deal with
this situation. Taking San Jose, CA as the approximate location for the ER-2 flights, the
Differential neutron flux  (n/cm MeVs)

– 10 – TS 62396-5 © IEC:2008(E)
latitude for San Jose is approximately 37° which is similar to that for Nagoya, Japan, the
approximate location for Nakamura’s measurements. The earth’s magnetic field varies with
longitude as well as latitude. Although the variation is small in most locations, for other sites it
can be large, with the result that two locations very similar latitudes can have significantly
different vertical rigidity cutoffs. In the case of these two cities, the rigidity cutoff over Nagoya,
Japan is much larger than it is in California, meaning that the cosmic rays are deflected much
more over Japan than California and so the atmospheric neutron flux is much lower. Using a
recent model by Gordon and Goldhagen for the variation of the atmospheric neutron flux with
location [12] that is based on the vertical rigidity cutoff parameter, the net result is that the
neutron flux over Japan is a factor of 3 to 4 lower compared to the flux over the western
...