# modeling radiation effects and dosage on a commercial mobile router

*Prepared by*:

Alan Chmiel[^1]

David Carek[^2]

Allison Hess^\*^

**Abstract**

> *The report details the modeling effort of a commercial mobile router
> for space applications. This report should be used as a reference if
> radiation testing of the Cisco Mini Router is performed in the future.
> In addition the report adds to the body of knowledge study of the
> radiation effects on commercial mobile routers.*

# Introduction

NASA Glenn Research Center is currently involved in defining
next-generation communication architectures for space. These
architectures are attempting to utilize Internet Protocols to ensure
interoperability between terrestrial and space based systems. Current
research efforts are examining the use of mobile networking devices to
support Internet Protocols in future space networks^\[^[^3]^\]^. One
such device is a commercial-off-the-shelf (COTS) mobile router, which
includes hardware and software to support conventional network routing,
and software to support network mobility. To be fully successful in
space, COTS mobile router hardware and software will require rigorous
analysis and testing to determine if the device can be used or modified
for use in extreme space environments.

Mobile router technology offers the potential for transparent end-to-end
connectivity with standardized protocols. However, this capability comes
at the expense of more complicated software that may be difficult to
certify for space flight use. The demands for software verification and
validation of space rated systems will likely limit the use of many COTS
products in high reliability space applications. Determining the
feasibility for qualifying mobile router software is an area for future
research and not included within the scope of this investigation. For
the purposes of this analysis, only hardware considerations are
evaluated. However, to be fully realizable in space applications,
software qualification issues must be addressed.

This report provides background into the sources of radiation, potential
susceptibility testing options, and some of the tools available to help
model and analyze the effects radiation has on electronics and other
hardware.

This report derives numerical models for several mission scenarios for
the mobile router technology. The scenarios derived are: low earth orbit
(LEO); ISS orbit (external to the craft); ISS orbit (internal to the
craft); and lunar orbit. This report presents data on the environment
for each scenario. This data is used to help define testing levels for
potential future radiation failure mitigation testing. In addition this
data is also used to model the dosage and flux through each integrated
circuit on a Cisco Mini Router. The primary card examined within this
analysis is the Mobile Access Router card (CISCO3220TMARC). However, it
should be noted that two additional cards are typically used with the
bundled Cisco Mini Router product. The bundled product includes a Serial
Mobile Interface Card (CISCO3220TSMIC) that provides two sets of serial
signals, and a Fast Ethernet Mobile Interface Card (CISCO3220TFESMIC)
that provides two sets of 10/100 Fast Ethernet signals. These additional
cards have not been examined within this report.

Electronics employed during space missions must be able to operate in
the radiation environment of space for the duration of the mission. The
two mechanisms by which radiation can create damage in electronics are
ionization and atomic displacement. Ionization is caused by the
collision of charged particles with matter. The charged particle passing
through the medium may lose some or all of its kinetic energy. Atomic
displacement is damage that results when high energy particles displace
atoms from their usual site within the medium.

The goal of radiation susceptibility testing is to determine the effects
of this ionizing radiation on microelectronics. The purpose of testing
is to estimate ionizing radiation-induced functional interrupt rates and
other error rates that can be expected in space.

Modeling performed on hardware that may be deployed into the high
radiation environment can assist in engineering design, risk mitigation
testing, reliability analysis, or failure analysis. The results
presented here aid in the development of suitable test definitions to
assess the Mini Routers for space flight applications.

The energy imparted by the ionization process is most commonly referred
to as Total Ionizing Dose (TID). The TID accumulated by hardware is a
function of orbit, shielding, and time. This dose is a cumulative total
of the energy of all of the incident particles, causing the device to
break down. A TID level is established by analysis. If that level
appears to be of concern, a test should be performed to that level.

Single event effects (SEE) testing typically requires the operation of
hardware in a high-energy radiation environment, such as a proton
stream. Once errors are observed, an analysis is done to make an
estimate of the expected error rates. This testing is typically quick as
high-energy particles can be used to achieve an accelerated testing
profile.

# definitions

**[absorbed dose]{.underline}** -- the absorbed dose (D) is the quotient
of ∆E~D~ by ∆m, where ∆E~D~ is the energy imparted by ionizing radiation
to the matter in a volume element and ∆m is the mass of matter in the
volume element.

**[apoapsis]{.underline}** -- the point in an orbit farthest from the
body being orbited.

**[apogee]{.underline}** -- the apoapsis in earth orbit; the point in
its orbit where a satellite is at the greatest distance from the
earth.**[fluence]{.underline}** -- the time integrated particle flux
(also know as integral flux).**[flux]{.underline}** -- The amount of
particles passing through an area per unit time.

**[functional interrupt (FI)]{.underline}** -- an event requiring a
software reboot or a power cycle.

**[integral flux]{.underline}** -- the number of protons, particles, or
incident energy per unit area integrated over time.

**[ionization]{.underline}** -- the separation of a normally
electrically neutral atom or molecule into electrically charged
components.

**[ionizing radiation]{.underline}** -- any radiation consisting of
directly or indirectly ionizing particles or a mixture of both.

**[linear energy transfer (LET)]{.underline}** -- generally described
as:
$L_{\Delta} = \left( \frac{\text{dE}}{\text{dl}} \right)_{\Delta}$where
dE is the energy lost in traversing distance dl and ∆ is an upper bound
on the energy transferred in any single collision.

**[periapsis]{.underline}** -- the point in an orbit closest to the body
being orbited.

**[perigee]{.underline}** -- periapsis in earth orbit; the point in its
orbit where a satellite is nearest to the earth.**[single event burnout
(SEB)]{.underline}** -- an event where the device has an abnormal
conduction path established by the ionizing radiation and is destroyed
almost immediately.

**[single event effect (SEE)]{.underline}** - an event caused by atomic
displacement that results in permanent or temporary damage in a device.

**[single event latchup (SEL)]{.underline}** -- an event where the
device has an abnormal conduction path established by the ionizing
radiation and as indicated by a primary power supply current change.
Power must be recycled to regain control and/or to save the device from
destruction.

**[single event upset (SEU)]{.underline}** -- an event like a bit flip
resulting in a data error only.

**[total ionizing dose (TID)]{.underline}** - the total dose absorbed by
a device over its operational life.

# 

# SOurces of radiation

Charged particles emanate from three sources: terrestrial, solar, and
galactic.

Terrestrial sources refer to those particles trapped by the earth and
are generally referred to as the Van Allen Belts. The Van Allen Belts
are divided into two toroidal belts, an outer belt and an inner belt.
The inner belt, discovered by Van Allen, is made up of ions from
galactic sources that are trapped by the earth. The inner belt extends
to about 45° north and south geomagnetic latitudes and from about 800 km
to about 8000 km in altitude. The inner belt can produce a flux of about
3x10^4^ protons/(cm²∙s) at energies greater than 30MeV. The second,
outer belt is made up of plasma trapped by the magnetosphere. The outer
belt extends to about 70° geomagnetic latitudes, north and south, and to
altitudes of 130,000km. The outer belt can produce a flux of about
1x10^7^protons/(cm²∙s) at energies greater than 1MeV.

The second source, solar, is divided into four major contributing parts:
Solar particle events (energetic protons), flares, prominence eruptions,
and coronal mass ejections. Solar radiation is about 88% protons, 2%
electrons, and 8% alpha particles; the remainders are various heavier
nuclei. Solar radiation can produce a flux of about
2x10^13^protons/(cm²∙s) at energies around 20keV. Periods of solar
activity can last between 10 to 100 hours and can occur two to fourteen
times a year. The number of significant events is typically one to two
times a year.^\[^[^4]^\]\[^[^5]^\]^

The third source of radiation is Galactic. Galactic Cosmic Rays (GCR's)
originate outside the solar system. GCR's are 98% atomic nuclei and 2%
electrons. Of the atomic nuclei about 90% are protons. Energies range
from 10^7^ to 10^19^ eV with an average of about 10^12^ eV.

# Types of Radiation

The types of radiation that dominate space are protons and heavy charged
particles, electrons, neutrons, and electromagnetic radiation. The
following is a very brief description of each type of radiation. A more
detailed explanation can be found is NASA CR-1871 in the section titled
"Effects of the Interaction of Radiation with Matter."^\[^[^6]^\]^

## Protons and Heavy Charged Particles

A proton is a positively charged hydrogen ion. Most of the radiation
that a spacecraft encounters is proton radiation. Energies in space will
range from 2000eV to 10^19^ eV. The primary process by which a proton
imparts energy is Coulomb-force interaction with atomic electrons.

## Electrons

An electron is a particle with a mass of 1/1800 that of a proton and has
a negative charge. Electrons in space are primarily found in the Van
Allen belts and as secondary particles that result from collisions with
other charged particles.

## Neutrons

Neutrons are uncharged particles whose mass is almost equal to a proton.
They are typically produced by nuclear reactions, but can also be
produced when alpha particles interact with light elements such as
lithium.

## Electromagnetic Radiation

Electromagnetic radiation is a discrete quantity of energy, which
travels in a straight line at the speed of light. High-energy
electromagnetic radiation is usually gamma rays that emanate from the
nucleus of an atom.

# Mobile Router Use Scenarios

This section identifies several mission scenarios with the potential to
use mobile router technology. These mission scenarios are used to define
radiation characteristics that the device must withstand. While other
mission scenarios are plausible, this subset represents a reasonable
cross section of potential mission applications.

[Scenario 1 -- Polar Orbit Science Satellite]{.underline}

A mobile router onboard a science satellite could serve several
purposes. First, the device could act as a gateway to isolate and rate
limit data traffic between various onboard subsystems, such as command
and data handling, attitude control, data storage, sensor data
acquisition, and external communications interfaces^\[^[^7]^\]^. In
addition, the device could use mobility aspects in software to maintain
end-to-end connectivity to a single ground processing site through a
disperse ground station network. The operating environment for this
scenario is an unpressurized volume requiring an active or passive
thermal control system.

[Scenario 2 -- International Space Station (external)]{.underline}

NASA Glenn Research Center has investigated concept architectures for a
direct to ground communications system to augment the downlink of
International Space Station science payload data (the Advanced
Communications Architecture Demonstration). The location for this
scenario is an external pallet adapter. Similar to scenario 1, a router
device could be used to isolate and rate limit subsystem communications
as well as provided end-to-end connectivity to disperse ground stations.
The operating environment for this scenario is an unpressurized volume
requiring an active or passive thermal control system.

[Scenario 3 -- International Space Station (internal)]{.underline}

Two uses are envisioned for this scenario. The first is the use of the
device within a single payload to route data among various computers
associated with the experiment. Similarl to scenario's 1 and 2, the
ability to isolate and rate limit data traffic allows mitigation of
problems associated with competing traffic flows. The second use is to
route and isolate data among different payload racks. Current
International Space Station hardware uses a payload Ethernet hub gateway
(PEHG) to perform this function. In a hub configuration all payloads see
each other's traffic. A router in this scenario could isolate data among
different legs of the network as well as provide rate limits to avoid
exceeding bandwidth limitations for downlink paths. These scenarios
would not necessarily need the mobility aspect of the router for return
links that go through the Tracking Data Relay Satellite System (TDRSS)
network. However, if a direct to ground option was available, the
mobility aspect could potentially be used. The operating conditions for
this scenario are a pressurized environment internal to the Space
Station, allowing for air-cooling[^8] or cold plate cooling through rack
cooling loops.

[Scenario 4/5 -- Lunar Orbit Communications Satellite]{.underline}

A lunar communications relay satellite may need to communicate with
multiple lunar surface assets in addition to Earth. Data transmission
from one lunar surface asset to another, or from a surface asset to
Earth could be established through the relay satellite. A mobile router
device onboard the lunar relay satellite could direct the data to the
appropriate RF device based on the destination address. The mobility
aspect of the router for the lunar/earth link would have limited
application due latency problems with standardized protocols (e.g.
transmission control protocol) over those distances. However, mobility
features might be used among lunar surface assets. The operating
environment for this scenario is an unpressurized volume requiring an
active or passive thermal control system.

# Orbital Parameters

## Scenario 1

Scenario 1 is a 90° orbit at an apogee of 400km. The solar weather is
quiet. The model assumes no shielding, case shielding is added using the
MNCP modeling package.

## Scenario 2

Scenario 2 is a 51.6° orbit at an apogee of 450km (This is the standard
ISS orbit). The solar weather is quiet. The model assumes no shielding
and the location is assumed to be the ISS exposed facility.

## Scenario 3

Scenario 3 is the same 51.6° orbit at an apogee of 450km (ISS orbit),
with the solar weather quiet. The model assumes no case shielding. The
location is assumed to be internal to the US-lab.

## Scenario 4

Scenario 4 is a lunar orbit at an apoapsis/periapsis of 85 km. The solar
weather is quiet. The model assumes no shielding.

## Scenario 5

Scenario 5 is identical to scenario 4 but uses the peak flux model for
solar flare activity. This model is based on the peak five-minute
averaged fluxes observed on GOES in October 1989. This model is not
realistic for a whole orbit transmission model, but is used in this
analysis to bound worst-case limits.

# environment Models

Cosmic Ray Effects on Micro Electronics - 1996 Revision (CREME96) is
used for creating the numerical model of the ionizing radiation
environment in near-earth orbits. CREME was originally developed in
1981^\[^[^9]^\]^ by the Naval Research Laboratory. The CREME96 update
takes into account additional knowledge collected experimentally about
the ionizing environment of space. CREME96 is the DOD standard for
radiation environment modeling (MIL-STD-809.) CREME96 has been found to
be in good agreement with measured near-earth
data.^\[^[^10]^\]\[^[^11]^\]^ Additional updates to CREME96 include the
introduction of a trapped proton model.

For clarification, several items should be explained before the data is
presented. The solar radiation environment is not taken into account
explicitly in the requirements. For completion, a fifth scenario is
added to help account for the addition of solar particles. To accomplish
this AP-8, Trapped Proton Environment Model^\[^[^12]^\]^ is used. This
report does not include the trapped electron environment, which is
defined in AE-8^\[^[^13]^\]^.

AP-8 is a model that predicts min and max proton environments

AE-8 same but for electrons

For each scenario the target material is assumed to be silicon. Each
model assumes one orbit. The range of elements is assumed to be
Z[\<]{.underline}28, or all elements with an atomic number less than or
equal to 28 (nickel). For most applications this is sufficient.
Energetic particles with Z\>28 are rare and are generally negligible. If
the target device has a very high SEE threshold, or if the mission
scenario has a requirement for a very low SEE rate then Z will be
expanded to 92. All of the results presented start at an LET of 0.1MeV.
LET's less than 0.1MeV are excluded because they are generally absorbed
by the cover layer.

Each scenario is presented below. Using the CREME96 model described
above a numerical model was generated for each scenario. Figures 1
through 5 presented in Appendix A show the relationship between flux and
LET or energy for each scenario.

The left-hand plot shows the relationship between integral flux and LET.
The center plot represents the relationship between the trapped proton
flux and the particle energy. Scenarios 4 and 5 do not have a trapped
proton flux plot. The lunar position is outside the Earth's
Magnetosphere, therefore there is no significant contribution of trapped
protons. The right-hand plot shows the flux of the first five elements
Z[\<]{.underline}5. Some plots do not show all five elements as the
excluded elements make minimal contributions to the total flux.

The model results for scenario 2 show a much higher flux than scenario
1; this is the result of the South Atlantic Anomaly.

  ----------------- -------------------- -------------------- -------------------- -------------------- -------------------- ----- -----------
                    **Scenarios**                                                                                                  

                    **1**                **2**                **3**                **4**                **5**                      

  **Orbiting Body** Earth                Earth                Earth                Lunar                Lunar                      

  **Apoapsis**      400                  450                  450                  85                   85                         **km**

  **Periapsis**     400                  450                  450                  85                   85                         **km**

  **Inclination**   90                   51.6                 51.6                 0                    0                          **°**

  **Shielding**     0.00                 0.00                 0.63                 0.00                 0.00                       **cm**

  **Proton Model**  AP8MIN               AP8MIN               AP8MIN               AP8MIN               AP8MIN                     

  **Solar Weather** Quiet                Quiet                Quiet                Quiet                Peak                       

  **Elements**      [\<]{.underline}28   [\<]{.underline}28   [\<]{.underline}28   [\<]{.underline}28   [\<]{.underline}28         **Z**

  **Min. Energy**   0.1                  0.1                  0.1                  0.1                  0.1                        **MeV**
  ----------------- -------------------- -------------------- -------------------- -------------------- -------------------- ----- -----------

*Table I - Operational Scenarios*

# SHIELDING

# Absorbed dose is a function of the material and the shielding between the radiation environment. Doses in silicon at the center of an aluminum sphere, representative of doses to electronic devices shielded by an equivalent thickness of aluminum, are given in Figure 8. These doses are based on the proton environment presented in scenario 2 and are given for a range of aluminum shield thicknesses. All doses are calculated from the trapped proton /electron environment using the SHIELDOSE model. The SHIELDOSE model is documented in NBS Technical Note 1116. The data shows the importance a good shielding philosophy has on the total dose.

# Testing

To successfully conduct testing and establish the effects of radiation
on a device an understanding of available test facilities and potential
test methods is required. This section briefly elaborates on potential
test facilities and a possible approach to testing.

## Facilities

Facilities able to mimic the environment in space are required for
testing because the heavy ion and proton flux on Earth is much smaller
than is encountered in space. A survey of facilities available for
testing electronics that will undergo space flight are detailed in the
attached tables. Table 1 lists some of the accelerators that are being
used for heavy ion research. Table 2 lists some facilities that are
exclusively proton accelerators. Heavy ion facilities are useful if
engineering concerns persist about a devices susceptibility to heavy
ions. As detailed earlier, protons make up the great majority of the
environment to which most hardware is exposed. Typically proton testing
is sufficient and the approach method discussed below is centered on
that approach.

## Approach

Of all electronic components, semiconductors are the most sensitive to
radiation. For most applications semiconductor-device performance will
determine the maximum radiation flux that an electronic circuit will
tolerate. Radiation can cause both permanent and temporary damage to the
device. Permanent effects are attributed to bulk damage. The term "bulk
damage" is used to describe changes in the properties of structures
caused by atomic displacement as a result of exposure to a radiation
environment. Temporary effects are generally attributed to the
generation of excess free carriers in the junction regions as resulting
from exposure to high energy particles.

The most effective way to determine the effects of radiation on
microelectronics is through testing the devices in radiation
environments.

Several methods exist for conducting radiation testing. Not all of these
methods are discussed in detail in this report. The omitted methods are
described in references [^14], [^15], and [^16]. Each method of
radiation susceptibility testing requires use of SEU test data, which is
measured at a testing facility.

During testing, electronics are exposed to a uniform particle beam to
extract failure rate information. A general method for this type of
testing is to use the beam test results to determine a failure rate
(failures/unit time). The failure rate divided by the particle flux
gives the SEE cross section and is usually defined as

$\sigma_{\text{SEE}}(E) = \frac{\text{dN}_{f}}{\text{dt}} \cdot \frac{1}{\varphi_{0}}$,

where dN~f~/dt is the failure rate, and $\varphi$~0~ is the particle
flux. This data can now be used to calculate the soft error rate (SER).
To determine the SER, the product of the differential energy flux and
SEU cross section are integrated over the energy spectra of interest.
The SER is expressed as

$\frac{\text{dF}}{\text{dt}} = \int_{}^{}{\frac{d\varphi(E)}{\text{dE}}\sigma_{\text{SEE}}(E)\text{dE}}$.

This result gives a number of failures per unit time for a given range
of particle energies.

For the purpose of data taken, the SER cannot be determined for any
energy other than beam energies. Therefore, data taken during testing is
reduced statistically, using the data from the modeling effort rather
than numerically, to give an approximate failure rate. This statistical
approach can be calculated using the Bendel A method.^\[^[^17]^\]^
Efforts have been made by NASA to create computer codes to perform this
analysis from test data. ^\[^[^18]^\]^

Another method, sometimes referred to as the Burst Generation Rate (BGR)
method, was developed by Ziegler and Lanford ^\[^[^19]^\]^ to
numerically determine the SEU rate induced by proton/neutron
interactions with microelectronics. The BGR method hinges on the
statistical theory that only recoil reactions cause upsets. According to
the BGR method the SEU rate can be approximated by

$$\frac{\text{dF}}{\text{dt}_{\text{BGR}}} = C\sum_{i}^{N}{t\Delta\sigma\int_{\text{Ep}}^{}{\text{BGR}(E_{p},E_{\text{ri}})\frac{\text{dJ}}{\text{dE}}\text{dE}_{p}}}$$

where C is the collection efficiently, t is the collection depth in µm,
Δσ~i~=σ~i~-σ~i-1~ where σ~i~ is the heavy ion SEU cross section for the
i^th^ portion of the curve expressed in cm², BGR(E~p~,E~ri~) is the
burst generation rate is cm²/µm^3^, E~p~ is the energy of the incoming
particle in MeV, E~ri~ is the ith recoil energy, (E~ri~ = t  0.23 
LET~i~ \[MeV\]) and dJ/dE is the differential flux in
particles/cm²∙sec∙MeV. ^\[^[^20]^\]^

The BGR method is effective for older devices, but does not handle
modern devices with smaller sensitive volumes. The method also assumes
that the charge collection region is constant, which in actuality it is
not. The charge collection region changes dimensions depending on the
total deposited energy and location because the depletion region
collapses if the energy is sufficiently high. Modern devices have
complex charge collection regions because diffused charge in the
substrate, well beyond the depletion region, can be collected by a
reverse-biased junction. ^\[^[^21]^\]^

Another method of testing is to determine the threshold values for both
failure modes (permanent and temporary). This test requires a source
where the ion species and energies can be changed. Once the threshold is
observed experimentally the expected SEU rate can be determined
numerically by integrating the fluence from the threshold to infinity.
This method has proven to be more costly than the previous method. In
general each of these methods is considered a reliable predictor of
error rates, but the last method is more valuable to the reliability
analysis effort.

Assuming the use of the Bendel A method described above the following
may be a testing option. Using the Midwest Proton Radiotherapy Institute
facility at Indiana University (Described in Appendix A) each scenario
can be tested, scenario 1 will be detailed here. All the data for each
scenario is presented in table form at the end of this section. From the
data in Appendix A the average integral flux, discounting solar flares,
for the entire spectrum of scenario 1 can be calculated to be
0.652x10^6^ protons/cm^2^·day. For this example the total mission
duration will be assumed to be ten years. Therefore for the entire
mission the total integral flux will be 2.38x10^9^ protons/cm^2^. Each
cm^2^ of the device under test will be exposed to 2.38x10^9^ protons.
Given the capabilities at the IU Cyclotron Facility the exposure of each
square centimeter will take approximately two minutes. Therefore for
each test position the total run time should not exceed three hours.
This total test time can be reduced by only testing critical components,
but a critical component analysis is beyond the scope of this report.
The testing will yield a number of failures, hard and soft, permanent or
temporary, for the mission duration. The Bendel A method can then be
applied to predict a mean time between failures for this mobile router.

  ------------- --------------- ------------- --------------- -------------
                **Fluence /\    **Fluence/\   **Test Time per **Total Test
                Day**           10 Year**     component       Time
                                              \[minutes\]**   \[Hours\]**

  **Scenario    652.00E+03      2.38E+09      2               3
  1**                                                         

  **Scenario    2.63E+06        9.60E+09      6               10
  2**                                                         

  **Scenario    3.79E+06        13.83E+09     9               15
  3**                                                         

  **Scenario    9.09E+03        33.18E+06     1               2
  4**                                                         

  **Scenario    67.00E+09       \-\--         41              70
  5**                                                         
  ------------- --------------- ------------- --------------- -------------

Scenerio 5 does not have a calculation for fluence per mission life
since during any 11 year cycle the peak solar activity should only occur
during a single day.

# Reliability

As stated above data gathered from testing is used to perform analysis
to estimate the SEE susceptibility of the device under test. The results
collected for a given particle and energy can be used in conjunction
with available analysis tools to generate a composite MTBF number due to
atomic displacement for a device. In general a failure can be defined as
one of the following:

• Single Event Upset (SEU) -- an event like a bit flip resulting in a
data error only.

• Functional Interrupt (FI) -- an event requiring a software reboot or a
power cycle.

• Single Event Latchup (SEL) -- an event where the device has an
abnormal conduction path established by the ionizing radiation and as
indicated by a primary power supply current change. Power must be
recycled to regain control and/or to save the device from destruction.

• Single Event Burnout (SEB) -- an event where the device has an
abnormal conduction path established by the ionizing radiation and is
destroyed almost immediately.

Microelectronics can experience a reduction in reliability due to
TID^\[^[^22]^\]^. A relationship between component life reduction in a
transistor is generally given as:

> **τ~Φ~= τ~o~+ (1/Κ~τ~φ)**

Where:

**τ~Φ\ ~**is the component life after exposure

**τ~o\ ~**is the component life before exposure

**Κ~τ\ ~**is the lifetime damage constant (cm^2^/particles)

**Φ** is fluence (particles/cm^2^)

Each of the dependant variables can be observed directly through an
elaborate test program.

Damage from atomic displacement­­---as previously discussed---can occur
from primary or secondary effects. If a particle enters a material and
is of high enough energy to impart recoil energy it will displace an
atom by the primary collision and several more through secondary
effects. The number of atoms displaced by secondary collisions is given
through the equation:

> ***N~s~(E)=fE/2E~d~***

Where:

***N~s~(E)*** is the number of atoms displaced

***f*** is the fraction of recoil atoms energy that will be consumed by
ionization

***E*** is the displacement energy

***E~d~*** is average over all directions of the displacement energy

This relationship was theorized by Lindhard^\[^[^23]^\]^ and has been
confirmed experimentally for silicon by Sattler ^\[^[^24]^\]^.

These atoms create vacancies in the structure which, if not annealed,
can cause contamination in materials. It is not known how contamination
can effect long term reliability.^\[^[^25]^\]^ However, a device that
has been recovered through low-temperature annealing, may tend to be
more sensitive to further radiation.^\[13\]^

There exists a particle of high enough energy to permanently damage
microelectronics in an SEE, thereby affecting reliability numbers. Since
this particle is undetermined by theory, the reliability numbers cannot
be devalued for the purposes of analysis without direct observation
during testing.

# Radiation Modeling 

Modeling the effects of radiation on matter is a complex task that
requires the understanding of both the environment and the exposed
material. The modeling effort must include the effects of neutrons and
protons and give both the initial energy of the particles as well as the
energy deposited by recoiling atoms and their directions. The physical
environment, material composition, physical size, and other variables
must also be included in the models. For this task MCNP^\[^[^26]^\]^
will be used. MCNP is a good general application particle modeling
toolkit. Modeling is effective in providing absorbed dose rates, and
fluence at locations within the electric circuit.

## Modeling

The collision of energetic particles into electronic devices can cause
two types of damage to the device. Both types of damage result from
incident energetic particles imparting energy upon the atoms in the
silicon. The damage is called ionization damage when an electron-hole
pair is produced. The operation of semiconductor devices is highly
dependent on the number of carriers. The current through the device can
become too great if the number of carriers is too high, thus damaging
the device. The modern silicon chips are especially susceptible to
damage of this variety as the physical dimensions are small. As
mentioned in previously the damage is referred to as displacement damage
when an atom is displaced from its normal position to an interstitial or
another lattice site. ^\[^[^27]^\]^ The displacement of an atom
inevitably leaves a vacancy in its former position. The problem with
this type of damage is that one displaced atom displaces other atoms and
creates a disordered region.^\[^[^28]^\]^ This damage can be reversed by
annealing. This reduces the defects in the material and reunites
vacancies and interstitials.

Shielding can prevent damage to electronics. All electronics have a
threshold LET above which damage is done to the device. Shielding the
device with some other material will allow the particle to expend its
energy prior to reaching the electronics of the device in question.
Hence, when the particle does reach the electronics, the energy imparted
is not enough to cause damage. The energetic particles that this will
still have enough energy to cause damage are very few in number and are
unlikely to be encountered.

The modeling was completed with MCNP. MCNP is a general-purpose Monte
Carlo N-Particle code that can be used for particle transport. Specific
areas of application include, but are not limited to, radiation
protection and dosimetry, radiation shielding, radiography, medical
physics, nuclear eriticality safety, Detector Design and analysis,
nuclear oil well logging, Accelerator target design, Fission and fusion
reactor design, decontamination and decommissioning. The code treats an
arbitrary three-dimensional configuration of materials in geometric
cells bounded by first- and second-degree surfaces and fourth-degree
elliptical tori.

The five scenarios were all set up using the Visual Editor for MCNP5.
This editor was useful for providing an input deck window as well as for
providing a graphical interface for the geometry being described in the
input deck. The Visual Editor was not useful for running the scenarios
and returned many errors. A DOS command window was used to run the
scenarios. MCNP5 has three possible particle modes: neutron, photon, and
electron. All of the scenarios were set up to run under the neutron mode
to ensure that no errors were made in the geometry or other cards in the
input file.

Setting up the geometry required the dimensions for the fiberglass
circuit board, each silicon chip, and any other components of interest.
Each surface is represented by a single input card by a plane oriented
perpendicular to either the x-,y-, or z-directions. An additional
spherical surface was added in order to provide a surface from which the
particles are originating. The cells were then defined on the input deck
cards by defining the surfaces that enclose them. Also defined on the
cell cards are the material of the cell, as well as the density of that
material. The volume inside the sphere, but outside of the mini router
also had to be defined as a cell. As all space has to be accounted for
and be a part of a cell, the space outside of the sphere was also
designated as a cell.

The data cards were the next to be defined in the input deck. The
material cards were defined on input deck cards by defining each element
in the material, the abundance of that element, and the location of the
cross section for that element. The mode was classified to be the proton
mode. The importance of protons in each non-void cell was given an
importance of 1, while those in the void outside of the sphere was given
an importance of 0. The PHYS card was used to define the maximum energy
for a given proton, which is approximately 30,000 MeV.

The SI and SP cards differentiate between the five scenarios. While the
SI, or source information cards, are the same for all five scenarios,
the corresponding SP, or source probability, cards are different for
each scenario. The SI card defines the energy bins for the protons and
the SP card defines the probability of the incident proton having an
energy in a given energy bin. It was important to note that an energy of
less than 1 MeV is not permissible on the SI card.

The reason that low energy particles are negligible is that local case
shielding on the electronics would be sufficient to fully shield these
low energy particles.

The tally cards are use to find the proton flux through a given cell or
the number of protons passing over a given surface. The useful data from
the scenario is the same as the data from the tallies. Calculations can
be made using the result of these tallies to get other desired
quantities---fluence and dose in this case.

The five scenarios were run using MCNPX 2.4.0. The advantage of MCNPX
2.4.0 over MCNP5 was that it has more particle modes than does MCNP5.
Proton energy data was provided, and therefore the proton mode was
necessary for accurate data. The relative error given with the tally
data is dependent on the number of incident protons being aimed at the
mini router.

The data can come in two forms under the tally counts. It can either
tally the number of protons, or it can tally the cumulative energies of
the incident protons. In this problem, it was most beneficial to use the
energy tally for the surface count and the number tally for the cell
flux.

Also important to the problem is the number of particle histories run,
as designated by the NPS card. The higher the number of particle
histories run, the lower will be the relative error for the tallies. The
writers of the MCNP5 manual emphasize that the relative errors and
tallies cannot be considered reliable unless the relative error is less
than 10%. A combination of trial and error and calculation led to the
NPS being set to 120,000,000 particles. Each scenario took approximately
20 hours to run and only one scenario could be run at a time.

Upon completion of running the scenarios, calculations where performed
to find the fluence and dose of radiation entering the silicon chips. In
order to do the calculations, it was noted that the output of the \*F4
tally is in the units of MeV/cm^2^ when the input is in MeV. Since the
LET data was given in MeV•cm^2^/g, the output of the tally is in MeV/g.
Using the mass and surface area of the cells of interest, the data was
manipulated in order to achieve the units consistent with fluence (flux
across some surface over time) and absorbed dose (energy deposition per
mass over time).

The fluence and dose data, combined with data on the tolerance of the
silicon chips to radiation can be used to estimate the lifetime of the
mini router in any of the given radiation environments.

## RESULTS

All modeling was completed and the results are displayed in appendix A.
Figures 7 and 8 show the models used by MCNP. The visual representations
are rendered with Autocad using a program written in visual basic to
transfer the geometry data from MCNP to Autocad. Colors and labels were
added in Autocad for orientation, Figures 9 and 10 show actual images of
the Cisco Mini Router for comparison.

Figure 11 shows top and bottom layouts of the Cisco Mini Router and how
the each Cell was mapped into MCNP (Numbers in Black with Leaders.) The
Surfaces in the X and Y planes are also identified (in red.)

Energy loss as a function of penetration is commonly characterized by
Bragg's Curve as shown in Figure 1.

![](media/image1.wmf){width="3.1194444444444445in"
height="2.339583333333333in"}

Figure 1. Bragg's Curve

Energy distribution in a substrate is a function of the number of
particles and the energy of each particle. This was demonstrated
experimentally[^29] and is shown in Figure 2. E is the particle energy,
f is distribution and X is distance.

![](media/image2.wmf){width="3.183333333333333in" height="2.3875in"}

Figure 2. Substrate energy distribtuion

The energies imparted by the energetic particle into the substrate is
modeled with MCNP.

Table 3 shows the flux across each cell during the run for each
scenario. Table 4 shows the total absorbed dose in Rads(Si) for the Mini
Router in each run configuration. Please note that the results for
scenario 5 are not presented per year as the duration of a solar flare
typically last for only minutes or hours.

Figure 12 shows the flux versus energy for Cell 8 in Scenario 1. For the
sake of brevity not all of the flux data is shown for each cell in
graphical form. The data is presented in the appendix for each cell and
each scenario. Figure 12 does show some very interesting results. It
shows that the particles of low energy are absorbed either by the air or
by the cover layer and as a result does not contribute to the total flux
through the cell, this result was expected, and supports the decision to
exclude low energy particles from the model.

The results show that all parts are dosed evenly as a function of
surface area, and suggests that all parts should be tested to the flux
levels presented here. The results of testing can be analyzed using one
of the methods outlined earlier and a reliability analysis can be
performed.

# 

# Conclusion

This preliminary report presents operational scenarios for the use of a
mobile router in space applications. These operational scenarios were
translated in to specific mission classes with orbital parameters to
prepare environmental models. The results and background information
presented here will be used along with further analysis to present a
study of the effects of radiation on a specific implementation of a
commercial mobile router.

This report details an approach, method and options for using testing
combined with analysis and modeling to generate error rates and
reliability data for microelectronics based on an operating radiation
environment. Future research used in conjunction with the test method
detailed above and the operations scenarios defined previously can help
mitigate the effects of radiation on microelectronics.

The modeling completed here suggests that the Cisco mini router, with
proper testing is a candidate for space applications in a high radiation
environment. The Cisco mini router should be tested to the flux levels
presented here. The results must then be analyzed before an MTBF for
radiation damage can be assessed.

# APPENDIX A -- Figures and REsults

# 

![](media/image3.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 3 - Scenario 1 Environment Numerical Model. Integral flux over
one orbit.

![](media/image4.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 4 - Scenario 2 Environment Numerical Model

![](media/image5.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 5 - Scenario 3 Environment Numerical Model

![](media/image6.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 6 - Scenario 4 Environment Numerical Model

![](media/image7.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 7 - Scenario 5 Environment Numerical Model

![](media/image8.wmf){width="6.495833333333334in"
height="4.871527777777778in"}

Figure 8 - Dose versus Thickness from SHIELDOSE Model

# 

  --------------------------------------- ----------- --------------- -------------------
  **Facility**                            **Zproj**   **Eproj**       **Eproj(^56^Fe)**

                                                      **(MeV/nuc)**   **(MeV/nuc)**

  Alternating Gradient Synchrotron (AGS)  1--79       600--30K        600--1K

  Brookhaven National Laboratory (BNL)                                

  Brookhaven, New York, USA                                           

  NASA Space Radiation Laboratory (NSRL)  1--79       100--3K         100--1K

  Brookhaven National Laboratory                                      

  Brookhaven, New York, USA                                           

  Centro Nazionale di Adrotera Oncologica 1,6         250             ---

  (planned)                                                           

  Italy                                                               

  88\" Cyclotron                                                      

  Lawrence Berkeley National Laboratory   1--8        55              ---

  Berkeley, California, USA                                           

  Grand Accelerateur National D'Ions      6--92       25--95          ---
  Lourds                                                              

  Caen, France                                                        

  Heavy Ion Medical Accelerator at Chiba  1--54       100--800        500

  National Institute for Radiological                                 
  Sciences                                                            

  (Chiba, Japan)                                                      

  Tandem-ALPI                             1--8        8--20           ---

  Laboratori Nazionali di Legnaro (LNL)                               

  Legnaro, Italy                                                      

  Superconducting Cyclotron               1--6        70              ---

  Laboratori Nazionali del Sud (LNS)                                  

  Catania, Italy                                                      

  ETOILE                                  1,6         50--400         ---

  \(2007\)                                                            

  Lyon, France                                                        

  National Superconducting Cyclotron Lab. 1--92       90              ---

  Michigan State University                                           

  East Lansing, Michigan, USA                                         

  Nuclotron                               1--26       6K              6K

  Joint Institute for Nuclear Research                                
  (JINR)                                                              

  Dubna, Russia                                                       

  Ring Cyclotron                                                      

  Inst. for Physical and Chemical         6           137             ---
  Research                                                            

  Wako Saitama, Japan                                                 

  (Wako Saitama, Japan)                                               

  SIS-18 Heavy Ion Synchrotron            1--92       50--2K          1K

  Gesellschaft für Schwerionenforschung                               

  Darmstadt, Germany                                                  

  Synchrophasotron                        1--16       4K              ---

  Joint Institute for Nuclear Research                                

  Dubna, Russia                                                       
  --------------------------------------- ----------- --------------- -------------------

Table 1 - Heavy Charge Particle Accelerators

# 

  ------------------------------------------------------------- ------------
  **Facility**                                                  **E~max~**

                                                                **(MeV)**

  Brookhaven National Laboratory Linear Accelerator             200

  Brookhaven, NY USA                                            

                                                                

  Crocker Nuclear Laboratory Cyclotron                          70

  University of California                                      

  Davis, California, USA                                        

                                                                

  Loma Linda Proton Treatment Center                            250

  Loma Linda University Medical Center                          

  Loma Linda, California USA)                                   

                                                                

  iThemba Laboratory for Accelerator-Based Sciences             200

  Medical Radiation Group                                       

  Capetown, South Africa                                        

                                                                

  Midwest Proton Radiotherapy Institute                         210

  Indiana University Cyclotron Facility                         

  Bloomington, Indiana, USA                                     

                                                                

  Northeast Proton Therapy Center                               230

  Massachusetts General Hospital                                

  Boston, Massachusetts, USA                                    

                                                                

  Paul Scherrer Institut Proton Therapy Facility                270

  Villigen, Switzerland                                         

                                                                

  Proton Medical Research Center                                500

  University of Tsukuba                                         

  Tsukuba, Japan                                                
  ------------------------------------------------------------- ------------

Table 2 - Proton-Only Accelerators

# 

![](media/image9.png){width="4.520833333333333in"
height="3.3020833333333335in"}

Figure 9 - Rendered Model Top View

![](media/image10.png){width="4.364583333333333in"
height="3.1041666666666665in"}

Figure 10 - Rendered Model Bottom View

![](media/image11.jpeg){width="3.4305555555555554in"
height="2.5881944444444445in"}

Figure 11 - Cisco Mini Router Top View

![](media/image12.jpeg){width="3.4305555555555554in"
height="2.5881944444444445in"}

Figure 12 - Cisco Mini Router Bottom View

![](media/image13.png){width="4.072916666666667in"
height="5.759027777777778in"}

Figure 13 - Cell and Surface Mapping to MCNP

  -------------- ------------------------- ------------------------- ------------------------- ------------------------- ------------------------
  **Scenario**   **1**                     **2**                     **3**                     **4**                     **5**

  ** **          **Flux**                  **Flux**                  **Flux**                  **Flux**                  **Flux**

  **cell**       **(Particles/Cell∙yr)**   **(Particles/Cell∙yr)**   **(Particles/Cell∙yr)**   **(Particles/Cell∙yr)**   **(Particles/Cell∙s)**

  3              9.49E+08                  3.84E+09                  2.94E+09                  1.23E+07                  2.04E+06

  4              1.28E+07                  5.19E+07                  3.98E+07                  1.67E+05                  2.76E+04

  5              4.74E+07                  1.92E+08                  1.47E+08                  6.15E+05                  1.02E+05

  6              1.30E+08                  5.27E+08                  4.04E+08                  1.69E+06                  2.80E+05

  7              1.30E+08                  5.27E+08                  4.04E+08                  1.69E+06                  2.80E+05

  8              6.86E+07                  2.77E+08                  2.12E+08                  8.91E+05                  1.47E+05

  9              2.68E+08                  1.08E+09                  8.30E+08                  3.48E+06                  5.76E+05

  10             1.49E+08                  6.03E+08                  4.62E+08                  1.94E+06                  3.21E+05

  14             6.75E+06                  2.73E+07                  2.09E+07                  8.76E+04                  1.45E+04

  15             4.75E+07                  1.92E+08                  1.47E+08                  6.16E+05                  1.02E+05

  16             4.32E+07                  1.75E+08                  1.34E+08                  5.61E+05                  9.28E+04

  17             1.40E+08                  5.65E+08                  4.33E+08                  1.81E+06                  3.00E+05

  18             5.87E+06                  2.37E+07                  1.82E+07                  7.62E+04                  1.26E+04

  19             1.69E+07                  6.83E+07                  5.24E+07                  2.20E+05                  3.63E+04

  20             1.40E+08                  5.65E+08                  4.33E+08                  1.81E+06                  3.00E+05

  21             5.19E+07                  2.10E+08                  1.61E+08                  6.74E+05                  1.12E+05

  22             2.70E+07                  1.09E+08                  8.35E+07                  3.50E+05                  5.79E+04

  23             1.49E+08                  6.03E+08                  4.62E+08                  1.94E+06                  3.21E+05

   ?????         **2.38351E+09**           **9.63446E+09**           **7.38409E+09**           **3.09543E+07**           **5.12106E+06**
  -------------- ------------------------- ------------------------- ------------------------- ------------------------- ------------------------

Table 3 - Flux per Cell

  -------------- --------------------- --------------------- --------------------- --------------------- ----------------------
  **Scenario**   **1**                 **2**                 **3**                 **4**                 **5**

  ** **          **Absorbed Dose**     **Absorbed Dose**     **Absorbed Dose**     **Absorbed Dose**     **Absorbed Dose**

  ** **          **\[Rads(Si)/yr\]**   **\[Rads(Si)/yr\]**   **\[Rads(Si)/yr\]**   **\[Rads(Si)/yr\]**   **\[Rads(Si)/min\]**

                 **60.7**              **245.4**             **188.1**             **0.8**               **7.8**
  -------------- --------------------- --------------------- --------------------- --------------------- ----------------------

Table 4 - Dose per Scenario

![](media/image14.wmf){width="6.497916666666667in"
height="3.6395833333333334in"}

# Figure 14 - Cell 8 - Scenario 1 Flux versus Energy 

# APPENDIX B -- REFERENCES

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[^19]: Ziegler J. F. and Lanford W. A., "Effect of Cosmic Rays on
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[^20]: Normand, Eugene, "Extensions of the Burst Generation Rate Method
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[^21]: Johnston, Allan, "Scaling and Technology Issues for Soft Error
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[^24]: Sattler, A., Phys. Rev., 138, A1815 (1965).

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[^26]: LA-UR-03-1987 "General Monte Carlo N-Particle Transport Code,
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[^27]: Johnston, A.H. and Guertin, S.M., "The Effects of Space Radiation
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[^28]: J.R. Srour, Cheryl J. Marshall, and Paul W. Marshall, "Review of
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[^29]: B. Wilken, T.A. Fritz, *Nucl. Instrum. Meth.* 138, 331 (1976).