IEC PAS 62396-5:2007

Process management for avionics - Atmospheric radiation effects - Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics systems

Provides a more precise definition of the threat that thermal neutrons pose to avionics as a second mechanism for inducing single event upset in microelectronics.

General Information

• Status: Replaced
• Publication Date: 17-Sep-2007
• ICS: 03.100.50 - Production. Production management
• ICS: 31.020 - Electronic components in general
• ICS: 49.060 - Aerospace electric equipment and systems
• Technical Committee: TC 107 - Process management for avionics
• Current Stage: DELPUB - Deleted Publication
• Completion Date: 19-Aug-2008

Relations

• Replaced By: IEC TS 62396-5:2008 - Process management for avionics - Atmospheric radiation effects - Part 5: Guidelines for assessing thermal neutron fluxes and effects in avionics systems
• Effective Date: 05-Sep-2023

Standards Content (Sample)

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
...