IEC PAS 62396-5:2007

IEC/PAS 62396-5
Edition 1.0 2007-09
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
Process management for avionics – Atmospheric radiation effects –
Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics
systems
IEC/PAS 62396-5:2007(E)
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IEC/PAS 62396-5
Edition 1.0 2007-09
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
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
P
ICS 03.100.50; 31.020; 49.060 ISBN 2-8318-9206-6

– 2 – PAS 62396-5 © IEC:2007(E)

CONTENTS
FOREWORD.3

1 General .5

2 Thermal neutron flux inside an airliner.6

2.1 Definition of thermal neutron .6

2.2 Overview .6
2.3 Background on aircraft measurements.7
2.4 Calculational approach .8
2.5 Processing of in-flight neutron flux data.8
3 Thermal neutron SEU cross sections .11
3.1 Overview of the issue .11
3.2 Mechanism involved .12
3.3 Thermal neutron SEU cross sections and Ratio-2.13
4 Recommendation for devices in avionics at present time .15

Bibliography.16

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

Table 1 – Tabulation of the various atmospheric neutron measurements used .8
Table 2 – Comparison of thermal and high energy neutron fluxes and their ratios .10
Table 3 – SRAM SEU cross sections induced by thermal and high energy neutrons.14

PAS 62396-5 © IEC:2007(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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A PAS is a technical specification not fulfilling the requirements for a standard but made
available to the public.
IEC-PAS 62396-5 has been processed by IEC technical committee 107: Process management
for avionics.
The text of this PAS is based on the This PAS was approved for publication
following document: by the P-members of the committee
concerned as indicated in the following
document:
Draft PAS Report on voting
107/58/NP 107/70/RVN
Following publication of this PAS, which is a pre-standard publication, the technical committee
or subcommittee concerned will transform it into an International Standard.

– 4 – PAS 62396-5 © IEC:2007(E)

This PAS shall remain valid for an initial maximum period of three years starting from 2007-09.

The validity may be extended for a single three-year period, following which it shall be revised

to become another type of normative document or shall be withdrawn

IEC/PAS 62396 consists of the following parts, under the general title Process management

for avionics – Atmospheric radiation effects:

• Part 2: Guidelines for single event effects testing for avionics systems

• Part 3: Optimising system design to accommodate the Single Event Effects (SEE) of

atmospheric radiation
• 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
PAS 62396-5 © IEC:2007(E) – 5 –

PROCESS MANAGEMENT FOR AVIONICS –

ATMOSPHERIC RADIATION EFFECTS –

Part 5: Guidelines for assessing thermal neutron fluxes

and effects in avionics systems

1 General
The purpose of this PAS 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 PAS: 1) a detailed

evaluation of the existing literature on measurements of the thermal flux inside of airliners and
2) 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 literature and has been addressed by two
standards ([2] in avionics and [1] in microelectronics on the ground).
The two ratios that this PAS considers to be so important are: 1) the ratio of the thermal
neutron flux inside an airliner relative to the flux of high energy (> 10 MeV) neutrons inside
the airliner and 2) 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 extensively, such as [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 = 6000 n/cm hr) × 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)
Numbers in square brackets refer to the bibliography.

– 6 – PAS 62396-5 © IEC:2007(E)

SEU Rate
SEU Rate (Hi E neutron Upset/dev·h) × Ratio-1 × Ratio-2 (5)
(thermal neutron,
Upset/dev·h)
The objective of this PAS 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], but they were relatively few. 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.
2 Thermal neutron flux inside an airliner
2.1 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.
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 PAS, we will consider neutrons with
energies < 1 eV to be thermal neutrons. Additional details on this are found in 3.2.
2.2 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

PAS 62396-5 © IEC:2007(E) – 7 –

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.
2.3 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 (10-20) %, gamma
rays (<10 %) and muons (<10 %) [3]. 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 [4] 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 [5] 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 [6], and the
other by a Japanese group [7], and these are used in this evaluation. In addition, the most
highly regarded set of such measurements [8] 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 [9]. 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 [10] 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

– 8 – PAS 62396-5 © IEC:2007(E)

contained in [10] 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.

2.4 Calculational approach
There is one paper in the literature [11] 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 747) and the atmosphere around it. The gross take-off weight of a large 747 is

close to 1 million pounds (450 000 kg) and the overall internal volume is approximately 30 000
cubic feet (850 cubic metre) (based on the cargo capacity of cargo versions of the 747). The

actual size is therefore enormous (length of aircraft is ~250 ft (~76 m) and wingspan of ~225

ft (~69 m) 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
pounds 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 [11], 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 [11] 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 [11, but we will also compare them to the
measurements from [6] and [7], to obtain Ratio-1. The results from [11] will represent the
upper bound and the results from the in-flight measurements will represent a lower bound.
2.5 Processing of in-flight neutron flux data
For the comparison of in-flight measurements data is taken from four groups, [6, 7, 8 and 10],
and in addition the calculations from two other groups, [11] and Armstrong [12] are used. First
the measured spectra from the four aircraft measurements are shown in Figure 1, along with
the calculated spectrum from [12]. 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

Researcher Organization Detector Aircraft Year Altitude, Ft Ref.
Goldhagen EML Bonner sphere ER-2 1997 40 000 (12,2 km) [8]
Hubert EADS 7-detect A300 2004 34 800 (10,6 km) [10]
spectrometer
Hewitt NASA-Ames Bonner sphere C-141 1974 40 600 (12,4 km) [6]
Nakamura Tohoku U. Bonner sphere DC-8 1985 37 000 (11,3 km) [7]
Armstrong ORNL Calculation Atmosphere 1973 39 000 (11,9 km) [12]

PAS 62396-5 © IEC:2007(E) – 9 –

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)
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 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 [13] 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 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.
Thus if we were to increase the spectrum in Figure 1 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, we believe the
Nakamura curve is accurate and r
...