IEC 62396-5:2014

IEC 62396-5 ®
Edition 1.0 2014-08
INTERNATIONAL
STANDARD
colour
inside
Process management for avionics – Atmospheric radiation effects –
Part 5: Assessment of thermal neutron fluxes and single event effects in
avionics systems
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IEC 62396-5 ®
Edition 1.0 2014-08
INTERNATIONAL
STANDARD
colour
inside
Process management for avionics – Atmospheric radiation effects –

Part 5: Assessment of thermal neutron fluxes and single event effects in

avionics systems
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
S
ICS 03.100.50; 31.020; 49.060 ISBN 978-2-8322-1821-1

– 2 – IEC 62396-5:2014  IEC 2014
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 . 7
5.1 Definition of thermal neutron . 7
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 . 13
6.1 Overview of the issue . 13
6.2 Mechanism involved . 13
6.3 Thermal neutron SEU cross-sections and Ratio-2 . 15
7 Recommendation for devices in avionics at the present time . 17
7.1 General . 17
7.2 Ratio-1 . 17
7.3 Ratio-2 . 18
7.4 Thermal neutron upset rate . 18
8 Determining thermal neutron SEE rates for use in equipment assessments . 18
8.1 Demonstration of thermal neutron immunity to SEE . 18
8.1.1 Thermal neutron test . 18
8.1.2 Absence of boron 10 (B10) . 18
8.2 Determination of thermal neutron SEE where there is no evidence of thermal
neutron immunity . 19
8.2.1 Results from thermal neutron testing . 19
8.2.2 Conservative estimation . 19
8.2.3 High voltage devices . 19
9 Single event burn out in high voltage devices due to thermal neutrons . 19
Bibliography . 20

Figure 1 – Atmospheric neutron spectra measured in four aircraft . 10
Figure 2 – Neutron cross-sections for boron-10 including total cross-section (red), total
elastic cross-section (green) and (n,α) nuclear capture cross-section (blue) . 15

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 . 16

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –

Part 5: Assessment of thermal neutron
fluxes and single event effects in avionics systems

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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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.
International Standard IEC 62396-5 has been prepared by IEC technical committee 107:
Process management for avionics.
This first edition cancels and replaces the first edition of IEC/TS 62396-5 published in 2008.
This edition constitutes a technical revision.
This edition includes the following technical changes with respect to the previous technical
specification:
a) Change to title.
b) Updated references and bibliography.
c) Subclause 6.2 expanded to consider smaller geometry devices.

– 4 – IEC 62396-5:2014  IEC 2014
d) Table 4 neutron cross-sections expanded to add more recent data and the ratio between
thermal and high energy neutron cross-section amended to 2,42 from 2,77.
e) Addition of reference section on thermal neutron high voltage burn out.
f) New clause on determination of thermal neutron SEE rates for use in equipment
assessments.
g) Document aligned as an IEC standard.
The text of this international standard is based on the following documents:
FDIS Report on voting
107/237/FDIS 107/242/RVD
Full information on the voting for the approval of this international 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 the parts in the IEC 62396 series, published 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 stability 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
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

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PROCESS MANAGEMENT FOR AVIONICS –
ATMOSPHERIC RADIATION EFFECTS –

Part 5: Assessment of thermal neutron
fluxes and single event effects in avionics systems

1 Scope
The purpose of this part of IEC 62396 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 part of
IEC 62396:
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 are
considered 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 literature and has been addressed by two standards (IEC 62396-1 in
avionics and JESD89A [1] in microelectronics on the ground). Neutrons with E > 1 MeV are
considered for parts with geometries below 150 nm.
NOTE Reference is made to IEC 62396-1:2012, 5.3.2, for smaller geometry parts below 150 nm which provides
the neutron flux for energies above 1 MeV.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62396-1:2012, Process management for avionics – Atmospheric radiation effects – Part 1:
Accommodation of atmospheric radiation effects via single event effects within avionics
electronic equipment
IEC 62396-4, Process management for avionics – Atmospheric radiation effects – Part 4:
Design of high voltage aircraft electronics managing potential single event effects
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62396-1 apply.
4 Overview of thermal neutron single event rate calculation
The two ratios that this part of IEC 62396 considers to be important are:
___________
Numbers in square brackets refer to the Bibliography.

– 6 – IEC 62396-5:2014  IEC 2014
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 the SEU rates are
defined from the high energy neutrons for an avionics box, a topic which has been dealt with
extensively, such as in IEC 62396-1 and [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).
2 2
SEU rate
= 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)
Φ (neutron flux)
thermal
= (3)
Ratio-1
( )
Φ neutron flux
Hi
σ (therm SEU X - Sctn.)
= (4)
Ratio-2
σ (Hi E SEU X - Sctn.)
SEU rate
(thermal neutron, SEU rate (Hi E neutron upset/dev∙h) × Ratio-1 × Ratio-2 (5)
upset/dev∙h)
The objective of this part of IEC 62396 is to provide the 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. The 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.
___________
Hi E = High Energy.
5 Thermal neutron flux inside an airliner
5.1 Definition of thermal neutron
Thermal neutrons are so called because although most atmospheric neutrons are produced at
high energies following cosmic ray interactions, after a sufficient number of in-elastic
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.
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 around 0,025 eV is usually taken to be the
definition of thermal neutrons, for purposes of this part of IEC 62396, neutrons with energies
< 1 eV will be considered as 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, neutrons with E < 1 eV are considered as thermal neutrons.
However, the more accurate definition of thermal neutrons is neutrons with energies close to
0,025 eV (equivalent to those at room temperature, hence the term “thermal”). The thermal
neutron flux in the atmosphere is relatively low due to absorption by atmospheric molecular
nitrogen. By contrast, both calculation and measurement show that the high energy neutron
flux inside an aircraft structure is very similar to that in the atmosphere. The presence of the
airplane structure and its contents produce far more thermal neutrons inside the aircraft than
that 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 being 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 high energy 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 information 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 key parameter is not
latitude, however, but rigidity. Geomagnetic cut-off rigidity is a measure of the momentum a

– 8 – IEC 62396-5:2014  IEC 2014
charged particle requires in order to penetrate the Earth’s magnetic field at a given location.
The vertical rigidity cut-off 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
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 is expected within a large airliner. In this case, the main interest lies 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, it is believed that the data collected by this detector system and
contained in [9] cannot 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 neutron fluxes are much higher everywhere
inside the aircraft compared to outside within the atmosphere, so there is 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, the results from [10] will be used, but they will also be compared 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.
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]

– 10 – IEC 62396-5:2014  IEC 2014
+7
+6
+5
+4
Thermal
+3
neutrons
+2
+1
High energy
neutrons
−1
−2
−3
−4
−5
−6
−8 −7 −6 −5 −4 −3 −2 −1 +1 +2 +3
10 10 10 10 10 10 10 10 1 10 10 10
Neutron energy  (MeV)
IEC
Key
Hewitt NASA Ames 40 000 ft (12,2 km) western USA
Nakamura, 37 000 ft (11,3 km) over Japan
EADS, 34 800 ft (10,6 km) over Atlantic
Goldhagen, ER-2 40 000 ft (12,2 km) western USA
Armstrong calculation 39 000 ft (11,9 km)
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
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 longitudinal dependence is less acute than latitudinal
dependence, the effect is still such that two locations with very similar latitudes can have
significantly different vertical rigidity cut-offs. In the case of these two cities, the rigidity cut-off
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 cut-off 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 US (factor of 3 compared to San Jose, CA, and factor of 4 for Denver, CO). This
is based on altitudes of 37 000 ft (11,3 km) for Nagoya, Japan and 40 000 ft (12,2 km) for the
western US locations. The altitude difference accounts for an increase of 24 % in the flux
calculated using Annex D of IEC 62396-1:2012. The QARM model [13] can be used to
determine the nominal flux at any particular location.
Thus, if the spectrum in Figure 1 was increased by a factor of 3 to 4 to make the
measurement over Japan be equivalent to that over the western US, the Nakamura curve
would lie right within the NASA-Ames and Goldhagen curves. For this reason, the Nakamura
Differential neutron flux  (n/cm MeVs)

curve is considered accurate and reliable. The EADS spectrum in Figure 1 is another matter.
This measurement was made at 35 000 ft (10,7 km), which represents the lowest altitude of
all the in-flight data. Using the model in [12], the spectrum at 35 000 ft (10,7 km), is expected
to be lower than that at 40 000 ft (12,2 km) by a factor of 1,5, but this applies over the entire
spectrum. As seen in Figure 1, over the (1 to 10) MeV portion of the spectrum, the EADS
curve is similar to all of the other curves. However, especially at the highest energies,
> 10 MeV, the EADS spectrum is far too low, by about an order of magnitude, so a factor of
1,5 to account for the difference between 35 000 ft (10,7 km) and 40 000 ft (12,2 km) will not
improve the situation very much. One of the reasons given in [9] is that the spectrometer was
only calibrated up to 20 MeV and therefore data indicated above 20 MeV could not be used as
reliable information for the calculation of Ratio-1.
In addition, in looking at Figure 1 carefully and the other three spectra, the two in-flight
measurements by Goldhagen and NASA-Ames and the calculated spectrum from Armstrong,
there appears to be relatively good agreement except at the higher energies, E > 10 MeV.
Above 10 MeV, the NASA-Ames spectrum appears to be noticeably too high.
Using the actual spectra shown in Figure 1 in terms of the differential neutron flux, each of
them was integrated to obtain the high energy portion of the neutron flux (E > 10 MeV) and
the thermal neutron portion of the flux (E < 1 eV), both in units of n/cm s. In addition, the high
energy and thermal neutron fluxes were included as calculated by Armstrong and also as
calculated by Dyer-Lei in [10]. The results are shown in Table 2, and the last column of the
table contains the ratio of the thermal neutron flux to the high energy (E > 10 MeV) neutron
flux. This is Ratio-1 that was defined in Equation (3). In two cases, the original results were
multiplied by a specified factor to make them applicable to 40 000 ft (12,2 km) altitude over
the western US, like the measurements of Goldhagen and NASA-Ames.
Table 2 – Comparison of thermal and high energy neutron fluxes and their ratios
Neutron flux, Neutron flux, Ratio-1,
Altitude Hi E thermal thermal /
Researcher Condition
ft (km) (> 10 MeV) (< 1 eV) Hi E
2 2
n/cm s n/cm s E
Hubert 34 800 (10,6 km) A300 over Atlantic 0,13 0,063 0,5
a
34 800 (10,6 km) A300 over Atlantic 0,19 0,09 0,5
Hubert (× 1,5)
Goldhagen 40 000 (12,2 km) ER-2, California 0,74 0,18 0,24
NASA Ames 40 600 (12,6 km) C-141, western US 1,52 0,21 0,14
Nakamura 37 000 (11,3 km) Over Nagoya, Japan 0,23 0,11 0,49
b
Nakamura 37 000 (11,3 km) Over Nagoya, Japan 0,80 0,41 0,49
Armstrong 39 000 (11,9 km) Calc. -atmosphere 0,94 0,19 0,20
Dyer-Lei 33 000 (10,1 km) Calc. -atmosphere 0,70 0,15 0,21
Calc, Boeing 747,
Dyer-Lei 33 000 (10,1 km) 1,0 1,75 1,75
cockpit
Calc, Boeing 747,
Dyer-Lei 33 000 (10,1 km) 1,0 1,70 1,70
window
−3 −3
Goldhagen Ground Bonner Sphere 3,2 × 10 2,4 × 10 0,75
a
Multiplied by factor of 1,5 to make equivalent to altitude of 40 000 ft (12,2 km).
b
Multiplied by factor of 3,5 (equivalent to location over western US at altitude of 40 000 ft (12,2 km)).

In looking at Table 2, it is clear that the ratios derived from all of the in-flight measurements
are much lower than what was calculated for the cockpit of a Boeing-747 by Dyer-Lei in [10].
The high values of Ratio-1 based on the Dyer-Lei calculations were mentioned in 5.4. In terms
of the flight measurements, the data from Hubert and the EADS group, although it gives the
highest ratio of all of the in-flight measurements, cannot really be used. It was already

– 12 – IEC 62396-5:2014  IEC 2014
remarked that the high energy portion of this spectrum seems abnormally low, due to the fact
that the EADS spectrometer was only calibrated up to 20 MeV. Thus, the Ratio-1 value from
the EADS measurements cannot be relied on.
With regard to Goldhagen’s measurements in the ER-2, the high energy neutron flux appears
to be consistent with other measurements, but the thermal neutron flux may be
underestimated due to the location within the aircraft of the detector used for the
measurements. This was already discussed in 5.3 and so this results in a value for Ratio-1
that is too low.
The NASA-Ames measurement for the high energy neutron flux appears to be too high. This
can be seen in Table 2 and was already commented on above based on looking at the curve
for this spectrum in Figure 1. It is not clear why the high energy portion of the spectrum is too
high, but the most likely reason is a problem with the data reduction of the entire set of
Bonner sphere measurements. It could be that the Bonner sphere detectors were not
calibrated carefully enough or that the process for reducing the data, which involves
convolution of the Bonner sphere response functions, was not carried out carefully enough.
Goldhagen, whose ER-2 data is considered by far to be the best set of in-flight airplane
measurements, spent several years in reducing his data before publishing them.
The last set of in-flight measurements is that due to Nakamura. The Ratio-1 that his data
gives is 0,5. Table 2 shows that when his values are multiplied by a factor of 3,5 (a value
between 3 and 4) to adjust the measurements to a location over the western US and at
40 000 ft (12,2 km), high energy fluxes (that are consistent with what Goldhagen measured
and what Armstrong calculated) are obtained. The thermal neutron flux is higher than what
Goldhagen measured, but it has been already explained why Goldhagen’s thermal neutron
flux is too low to be applicable to an airliner. Thus, based on the most applicable set of in-
flight measurements, there is a value of the ratio of the thermal to high energy neutron flux
within an airplane of 0,5 for civil aircraft; the figure 0,5 is proposed though it is noted there
remains significant uncertainty around this figure.
Table 2 also contains the results of the calculations by Dyer-Lei. It was used at the two most
appropriate locations for which there are data, the cockpit and the window, and it also
included a point outside the airplane. However, neither the cockpit nor the window is a
location where most of the avionics are located. It is noted that for the external point, there is
relatively good agreement between their results and the calculation of Armstrong. For both
internal locations, the cockpit and a window, Ratio-1 is high, a value of 1,75 and 1,7
respectively, much higher than the value of 0,5 that was obtained from the in-flight
measurements by Nakamura. Additional calculation work is included in the article by Dyer et
al. [14] and demonstrates the potential variation in different aircraft locations; the table with
Ratio-1 added is reproduced here as Table 3.
Table 3 – Calculated neutron fluxes in the Boeing 747 structure
Calculated neutron fluxes at 10 km, 1 GV cut-off
–2 –1
(units = n cm s )
Cockpit Mid-fuselage Fuel tank Window External
TOTAL 3,94 3,10 1,17 3,78 4,07
< 1 eV (thermal) 1,80 1,47 0,277 1,73 0,146
1 eV to 1 MeV 0,687 0,444 0,083 3 0,691 2,79
1 MeV to 10 MeV 0,345 0,260 0,073 6 0,280 0,434
10 MeV to 1,03 0,860 0,679 0,997 0,696
1 000 MeV
Neutron flux ratio of < 1 eV to above 10 MeV
Ratio-1 1,75 1,71 0,408 1,74 0,210

As explained in 5.4, it is not clear how accurate the calculated values are. Our main concern
is that out of necessity, the geometric modelling was done on such a gross basis, mixing
materials (fuel, baggage, structural members, etc.) within very large volumes within the
airplane, that there is a lot of uncertainty as to the accuracy of the final results at much more
localized positions. Therefore, it is proposed that an approximate average between the two
values of 0,5 and 1,75 be used, which is taken as 1,1, for the value of Ratio-1. However, it
could just as easily be taken as 1,0 to simplify matters. Furthermore, based on the
calculations, it is certainly possible that there can be some locations in an airliner where
Ratio-1 can be as large as 2 or higher.
Table 2 also contains the results of earlier measurements made by Goldhagen at ground level.
From this neutron spectrum, the ratio of thermal neutrons to high energy neutrons is 0,75,
which is not too different from the value of ratio of 1,1 that was indicated which applies to an
airliner. However, it is known that at ground level the thermal neutron flux can vary by a factor
of 2 due to local conditions (weather, i.e., rain, bodies of water, nature of the surrounding
buildings, etc.).
6 Thermal neutron SEU cross-sections
6.1 Overview of the issue
It has been known for about 25 years that thermal neutrons can cause single event upsets in
microelectronics (see [15] for the earliest reference). About ten years later, this topic was
again investigated as part of a way to use a nuclear reactor to simulate the SEU environment
posed by atmospheric neutrons to avionics [16]. After that, as the feature size of IC
technologies continued to decrease, resulting in a continuing decline in the critical charge of
devices, the threat of thermal neutrons to induce SEUs in the devices became more of a
problem. This thermal neutron threat was recognized as a potential problem for ICs being
used in both avionics and ground level applications. However, only within the last ten years
have researchers tested devices in both a thermal neutron environment and a high energy
neutron/proton environment to allow the two types of SEU cross-sections, due to high energy
and thermal neutrons, to be compared. More of this kind of neutron SEU data is currently
available and will be utilized in Clause 6.
6.2 Mechanism involved
In larger geometry devices thermal neutrons cause single event upsets because of their
interaction with the boron-10 isotope within the IC (glassivation layer over the silicon), rather
than with the silicon atoms. About 20 % of naturally occurring boron is B-10, with the
remaining 80 % being B-11 which does not interact with the thermal neutrons. The boron is
usually present as borophosphosilicate glass, BPSG, which is also used as the dielectric
between the metallization layers in the overlayer that covers the silicon transistors.
In more modern devices with smaller feature size and power devices there may be sufficient
boron-10 to cause SEE. Boron-10 can be found in IC features such the p-dopant, and
although this is at lower atomic concentrations, it may be significant for small geometry and
high voltage devices (see Clause 8). In addition, there can be internal IC structures at higher
atomic densities (> 10 at/cc), such as in the boron nitride coating applied to tungsten plugs
(in 65 nm and 45 nm technology ICs). This presence of boron-10 near the transistors results
in SEU rates from the thermal neutrons comparable to the rates from the high energy
neutrons [17, 18]. Further, in the 45 nm and 65 nm SRAMs, approximately 20 % to 50 % of all
the upsets induced by the thermal neutrons are multiple cell upsets (one neutron upsets more
than one cell [17]. Thus, the additional SEU rate in avionics due to thermal neutrons should
be accounted for by either increasing the high energy neutron SEU rate by an appropriate
factor (Clause 7), or by demonstrating i
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