[^1]

*Abstract*\-- A recent failure of Galileo's magnetic recorder was
attributed to LED degradation. Annealing the culprit OP133 proved
successful: irreplaceable data, taken during the encounter with
Jupiter's previously unvisited moon Amalthea, was recovered. Annealing
test data for both proton and electron displacement damage combined with
a simple device temperature model explains some details of the failure
and the partial recovery of recorder motor function.

# INTRODUCTION

[The]{.smallcaps} Galileo spacecraft to Jupiter was launched via the
space shuttle on October 18, 1989, and the mission has been very
successful in spite of a number of technical obstacles. These range from
the impact of the Challenger explosion (delaying launch and causing a
major propulsion-systems re-design) to the failed unfurling of the high
gain antenna (efforts to maximize data delivery over the low gain
antenna were successful) and the sticking tape in the magnetic data
recorder (slower speed operational procedures were developed that have
limited re-occurrences). Now in its second mission extension and having
far exceeded expectations for delivering invaluable science data about
the Jovian system, the aging spacecraft is showing signs of impending
failure of critical electronics due to the cumulative effects of the
trapped radiation belts through which it regularly passes \[1, 2\].

Currently, Galileo is on its final orbit which will culminate in its
intentional destruction in Jupiter's atmosphere on September 21, 2003.
During its previous orbit, while recording data during its only
encounter with the moon Amalthea, Galileo abruptly stopped collecting
data about ten minutes after closest approach. This was caused by the
spacecraft entering its "safe" configuration apparently the result of
repeated proton-induced single-event transients causing loss of lock in
all four phase-locked loops (PLLs) of the command and data system (CDS).
Safeholds have truncated data collection occasionally before; while not
unexpected, they are certainly inconvenient. Fortunately, essentially
all of the primary-goal data and much of the secondary-goal information
for the Amalthea encounter had been recorded on magnetic tape. However,
efforts to play back this data after exiting the safehold revealed that
the recorder would not work and this time it was not because the tape
had stuck again. The signature of the failure indicated the motor drive
electronics as the likely source of failure. Because the Amalthea
encounter brought the spacecraft deeper into the trapped electron and
proton belts than ever before, radiation was immediately suspected as
the proximate cause of the problem. This paper tells the story of the
investigations that followed and how an understanding of the specific
failure allowed an effective annealing scheme to be carried out in
flight. Thus, this story has a happy ending: the collected Amalthea
encounter data was successfully downlinked to Earth.

# Background and In-flight Failure

## Recorder Description

The Galileo magnetic reel-to-reel tape recorder was manufactured by
Odetics and is shown in the photograph in Fig. 1 (encased in a
transparent box). On the top face, one of the tape reels can be seen
(the other reel is immediately underneath) as well as the nearby rollers
(white) that guide the tape as it winds its way around four (two read
and two write) heads in a convoluted path between the reels. In the
center foreground, an angled guide roller (white) can be seen; it serves
to direct the tape between the two planes of the stacked reels. The
capstan drive motor's axis is perpendicular to the plane of the reels
and has an encoder wheel attached near the bottom face.

The encoder wheel is shown in Fig. 2, a cross section through the center
of the drive motor. Transparent sections of the glass wheel allow a
light path between the LED/phototransistor pairs, one of which is shown
in Fig. 2. There are two others and between them they select which of
the motor's three stator coils is energized to move the rotor. The three
LEDs are electrically connected in series, driven by a constant current
source of about 25 mA.

## Failure of the Recorder

The recorder was working normally about ten minutes after the encounter,
recording data from back-looking instruments, when it was shut down as a
consequence of the fault protection software ordering the spacecraft
into a "safehold" configuration. Spacecraft telemetry indicates that all
four PLLs in the CDS were unable to maintain lock causing the fault
protection software to command a safehold. One consequence of a safehold
is the immediate, commanded cessation of recorder activity. Later, when
Galileo had moved out of the more intense part of the radiation belts,
the recorder was commanded to play back its data. However, no data was
forthcoming and the telemetry data indicated that the tape and motor had
not moved *at all* from the position reached at the time of the
safehold.

Telemetry also revealed that, during playback attempts, the motor was
drawing all the current available to it. This signature clearly reveals
that this problem is different from the tape-sticking problem previously
encountered (where the motor turned but the tape didn't move). The cause
of the new problem was identified as the drive electronics not moving
through the sequence of energizing the motor's three windings, but
rather stuck energizing only one. This could happen (a) if the rotor
were unable to move into alignment with the energized winding, or (b) if
the position encoder wheel were not moving with the rotor, or (c) if the
electronics were not reporting the encoder wheel position correctly. The
first two (mechanical) events were deemed unlikely because of the low
probability that a catastrophic, no-warning jam or wheel breakage would
have occurred simultaneously with the safehold (motor moving) or in the
time interval before the playback command (motor stopped). Thus,
scrutiny turned to the encoder electronics.

## The Suspect LED: OP133

The encoder wheel position is detected by three LED-phototransistor
pairs. If any of the three phototransistors does not detect its LED's
illumination when a window in the encoder wheel is in position, then the
sequence would stop and the motor would draw either a high current or no
current -- high current and no motion are exactly the symptoms initially
observed. Radiation could cause this by causing (a) a drop in LED light
output, (b) a drop in the phototransistor's gain, or (c) darkening of
the elements in the optical path, i.e. the LED and phototransistor
lenses and/or the glass wheel. The latter possibility is easily ruled
out because of the extremely high doses needed for significant darkening
of most transparent materials. The LED is an OP133 made by Optek and the
phototransistor is a silicon device, the MD300 from Motorola; both have
focusing lenses built into their package. The LED is an amphoterically
doped GaAs diode (wavelength = 935 +/- 35 nm). Previous work with this
type of LED \[3\] has shown that, although the light output is
practically insensitive to ionizing dose, the output is very sensitive
to displacement damage. Previous work with optocouplers made from
amphoterically doped LEDs and Si phototransistors \[4\] showed that the
phototransistor contribution to reduced optocoupler gain resulting from
proton irradiation is less than 10% of the LED's contribution. Thus,
displacement damage to the OP133 LED was identified as the prime suspect
in causing the recorder failure.

# The Radiation Environment

The Jovian environment has a stronger magnetic field than Earth and
thus, the corresponding trapped particle belts have significantly higher
energy and populations. Most of the moon-encounter orbits descend into
the higher altitude belts of Jupiter, and the largest contribution to
total dose is from high-energy electrons. Displacement damage from these
energetic electrons was expected to cause significant degradation of
light output from the OP133 LEDs because amphoterically doped LEDs (like
the OP133) have been shown to be extremely sensitive to displacement
damage effects \[3, 4\] in *proton* experiments. The electron
irradiations reported in Section IV demonstrate that this is, in fact,
the case.

In this section, both the proton and electron environments are described
in terms of equivalent 50-MeV protons, using standard GaAs NIEL values
\[5, 6\] to relate the integrated displacement damage for the particle
spectra behind shielding. The NIEL dose in units of MeV per g/cm${}^{2}$
can be obtained by multiplying by the given equivalent fluence by
3.75 x 10${}^{- 3}$. Note that proton NIEL values used may overestimate
the actual damage for energies above 30 MeV as first noted by Barry et
al. \[7\] based on their irradiations of GaAs LEDs. Others have also
seen this problem, as is summarized in Srour's recent displacement
damage review paper \[8\].

The LEDs' location in Galileo is heavily shielded. In the *least*
shielded direction, the LEDs are protected by about 300 mil (7.6 mm) of
aluminum-equivalent in the direction perpendicular to the shear plate.
In the opposite direction, they are shielded by a great deal of
spacecraft structure. The computer code NOVICE \[9\] was used to
calculate the proton and electron spectra and the resulting NIEL dose
behind a 400-mil aluminum spherical shell, a reasonable lumped
approximation and simplification of the actual spacecraft structures
providing shielding.

Inputs to NOVICE in the form of unshielded spectra, shown in Fig. 3,
were taken from the recently published average GIRE (Galileo Interim
Radiation Electron) model \[2\] for electrons and the earlier
Divine-Garrett particle model \[1\] for protons. The average GIRE model
replaces the electron model of Divine-Garrett for low energy electrons
(\<30 MeV) based on measurements taken by Galileo; at energies above 100
MeV, it uses a power law to extrapolate the low energy data into the
high energy regime and predicts somewhat higher flux than the Divine
model. Because electron NIEL increases with increasing energy, the
difference could be important, especially for heavily shielded locations
like that of interest here. It should be noted that, if the Divine model
is more correct at higher electron energies, then the electron
calculations here also overestimate the actual displacement damage.

NOVICE was used to calculate the shielded electron and proton spectra
inside the recorder using the spherical shell representation described
above. These shielded integral spectra are shown in Fig. 4a and 4b.

Galileo has made 35 orbits around Jupiter, all but two with moon
encounters (orbit JOI was the insertion into orbit). Before the recent
pass near Amalthea (dubbed A34), the mean per-orbit 50 MeV proton
*equivalent* fluence (97+% from high-energy electrons) was \~6.5 x
10${}^{9}$ p/cm${}^{2}$. Thus, the total 50-MeV equivalent fluence for
the preceding 34 passes is 2.2 x 10${}^{\text{11}}$ p/cm${}^{2}$, mainly
due to electrons. This is a sufficiently high fluence to severely
degrade the LEDs. However, the design used for Galileo has a great deal
of margin. Further, normal operations would cause significant
current-induced annealing, particularly at the slow rate necessitated by
"no stick" procedures and low communication bandwidth. After filling the
tape with data during a typical encounter, it would take 24 hours of
playback to transmit all the data. That is, over 2000 Coulombs of charge
passed through the LEDs after each dose increment that accompanied an
encounter when the data is played back. Because most of the anneal-able
damage in an LED has recovered after only a few hundred Coulombs, the
recorder continued to operate normally up to the A34 orbit.

The last pass towards Amalthea moved Galileo to lower altitudes and,
thus, into more intense regions of the radiation belts. That orbit not
only included the largest single-orbit fluence of high energy electrons,
but also went through four times as much proton fluence as had been
encountered in all previous orbits combined. The equivalent fluence
produced by this pass was \~5 x 10${}^{\text{10}}$ p/cm${}^{2}$, with
about 60% of the damage produced by electrons. Thus, orbit A34 increased
the total displacement damage by more than 20%; about 91% of the total
(2.7 x 10${}^{\text{11}}$ equiv-p/cm${}^{2}$) from electrons.

# LED Damage and Annealing

## Background and Previous Work

Displacement-induced light output reduction in Optek OP130 LEDs from
protons was measured previously \[3, 10, 11\]. According to Optek, the
OP130 is identical to the OP133 except that a post-manufacturing sort
labels high output (5 mW at I${}_{f}$=100 mA) devices as OP133s and low
(\<3 mW) devices as OP130s. Indeed, they are described on the same spec
sheet differing only in radiant power output.

The amount of damage produced by displacement effects in LEDs is greater
at low current compared to the damage that occurs at higher currents. In
addition, to avoid significant injection-enhanced annealing, short
pulsed measurements are required. Thus, it is important to use LED
characterization data that is taken at about the current used in the
application. Using test data at 1 mA for an application at 25 mA will
overestimate the damage by a considerable factor while DC measurements
will anneal some damage, thus resulting in an underestimate.

Some of the damage will anneal after an LED is irradiated, provided
current flows through it. (Note that thermal annealing is not
significant until temperatures are raised to about 200 degrees C.) The
amount of annealing after a given amount of charge has been passed
through an LED is almost independent of the current (or rate of charge
injection). Indeed, without charge injection almost no recovery should
be expected \[3\]; this was confirmed experimentally for both proton and
electron damage on the OP133.

After irradiation with protons, it takes a total charge of approximately
100 Coulombs for half of the recoverable part of the damage to anneal
\[11\]. Annealing continues as current is applied until nearly the
entire recoverable fraction is attained after approximately 1000
coulombs have passed through the device.

Rose and Barnes \[12\] showed that the damage could be parameterized by
using the following relationship with n= 2/3 for the case of constant
injection:

> $\left\lbrack \left( \frac{I_{o}}{I} \right)^{n} - 1 \right\rbrack = K\Phi$
> \[1\]

where I${}_{o}$ is the initial light output, I is the reduced light
output after irradiation, n = 2/3 for an LED operating at constant
current, K is the damage constant, and $\Phi$ is the particle fluence.
This equation was used to evaluate damage to Optek OP130 LEDs and to
extend the data by approximately a factor of two because the maximum
fluence used for earlier radiation tests was lower than the equivalent
proton fluence experienced on Galileo.

Figure 5 shows the dependence of damage (as defined by Eq. 1) on fluence
for OP130 LEDs at three different currents. At the highest current,
negligible damage occurs at the lower fluences, and the apparent
nonlinearity at low fluence is simply the result of measurement
uncertainty. The values in this figure were used initially to estimate
the net change in light output at the higher equivalent fluence
encountered by Galileo.

## Damage Results for the OP133

Representing damage as a function of 50-MeV equivalent protons (or as a
function of NIEL) is reasonable for proton-induced LED damage. However,
no recent work has been done on electron damage in LEDs, and the NIEL
concept may not be accurate because electron damage is dominated by
vacancy-interstitial pairs, not cascade damage. Therefore, it was
important to investigate electron damage and annealing effects from
electrons for these devices because most of the damage on the Galileo
mission was from electrons. However, proton tests were also conducted on
the OP133 for completeness (and as a check on the applicability of the
older OP130 data).

Some caveats are in order. The OP13x family devices have been made "in
the same way" for almost 30 years. Galileo recorder devices are over 20
years old while the test samples were purchased recently (and likely
recently manufactured as well). Further, due to the manufacturing
process, these devices are somewhat inconsistent in light output from
sample-to-sample and lot-to-lot. Unfortunately, barring a discovery of a
cache of old LEDs, studying modern devices is the best that can be done.

All LED testing was conducted at the recorder operating current of 25
mA. Electron irradiations were done at the Rensselear Polytechnic
Institute's Gaerttner LINAC Laboratory and proton irradiations at the
UC-Davis cyclotron. Both facilities perform calibration when no device
is in the beam. The electron LINAC uses TLDs (thermoluminescent
dosimeters) and the proton facility uses a Faraday cup. When irradiating
a device, the LINAC facilty counts pulses to get fluence while the
proton facility measures their beam continuously by passing it through a
SEM (secondary emission) detector; the SEM/Faraday cup ratio is
re-calibrated when the beam is re-tuned. Test results on OP133s
irradiated at these two facilities are shown in Fig. 6a and 6b.

## In-flight Annealing Results

A special operating mode was utilized that allowed current to be
continually applied to the three LEDs without actually attempting to
move the motor. After six hours of current-enhanced annealing of the
LEDs followed by an inactive period for cooling, the recorder was
commanded to move the tape. The tape moved for only 1/2 second but this
was an improvement compared to the totally non-operating condition that
existed before the annealing was attempted. More recovery was apparent
after another six hours of current-enhanced annealing: tape motion
persisted for 1.8 seconds. This series of annealing and operational
sequences was continued for more than 100 hours, as shown in Fig. 7.
Each time the recorder operating period was longer, and after the
next-to-last annealing cycle, it was able to function for more than one
hour. Data playback was accomplished by interspersing 20 minute periods
of operation with 80 minutes of cooling until the entire data set was
recovered.

The normal operating current in the LED is 25 mA, which is sufficiently
high to raise the temperature in the LED due to self-heating. The LED
output decreases about 0.8% per degree C according to Optek's
specifications. Output decrease from self-heating is the most likely
reason that the recorder cannot operate indefinitely. Thus, it is likely
that the percentage of damage that recovered was only a small amount
above the threshold condition required in the circuit. Earlier work on
annealing in the OP130 showed that only a fraction of the total damage
would recover \[10\], so the annealing approach has diminishing returns
after current has passed through the LED for extended time periods.
Diminishing returns are clearly seen in Fig. 7 where the last anneal
period only produced a small incremental improvement in recorder run
time.

## Annealing Results for the OP133

Devices that were irradiated in the electron experiments were annealed
for about a week using 25 mA of current. As mentioned earlier, the total
charge, that is, the integrated current, works reasonably well in
collapsing different anneal currents to the same curve. Although not
needed here (because the actual application current was used), the
annealing data for electron damage are plotted in Fig. 8 with total
charge as the x-axis.

Fig. 8 shows that the annealing rate of electron damage is higher for
damage from lower energy electrons. This result holds for proton
annealing as well: damage from lower energy protons anneals faster
\[11\]. Comparing the annealing rate of the electron damage as shown in
Fig. 8 with that of similar levels of proton damage as reported in Ref.
10, one can conclude that electron damage anneals somewhat faster than
proton damage. This is probably due the fact that electron damage is
dominated by nearby vacancy-interstitial pairs, not cascade damage, like
proton damage and more localized damage should be easier to repair.

# Modeling the In-flight Recorder Behavior

## Calculating the LED Damage

It seems natural to combine proton and electron damage by using the
expected 50 MeV proton-equivalent fluences of each with the experimental
data on the modern samples. (This is exactly the same as combining NIEL
doses - except for the proportionality constant - and is a practice well
accepted for ionizing dose.) The proton irradiations stopped a little
beyond the expected on-orbit fluence, but the electron irradiations are
short of the mark and require extrapolation. The Rose and Barnes
equation \[Eq. 1\] was used to extrapolate electron damage. In the
absence of annealing, the result is that the original light output of
the LEDs is reduced to 62% by the protons and to about 6% by the
electrons by the end of I33 (before the Amalthea encounter). Combining
these leaves only 4% of the original light output, under the assumption
of no annealing. This is far too low for the recorder to be working
approaching Amalthea. Fortunately, the large amount of charge put
through the LEDs in the course of normal operation of the recorder has
been annealing the LEDs' electron damage all along.

## LED Temperature Model

The self-heating of the LED can be calculated using data measured or
gleaned from the data sheet. The OP133 is a 200 mW device and, in
operation, it is being run at 34 mW. Given a thermal resistance of \~500
degrees C per Watt, the expected LED temperature rise is 17 degrees C.
Using a bang on / bang off model for the temperature rise, the
temperature history for the LED starting with the Amalthea encounter is
given in Fig. 9.

## Combined Results

In an unirradiated OP133, the 17 degree temperature rise results in a
decrease of light output of about 13%. Assuming that this also applies
to an irradiated device and combining the temperature model with the
electron annealing results, a combined model of the failure and recovery
is reached. This model of the events is shown in Fig. 10. While there is
uncertainty in the absolute fidelity of this model, it is clear that the
playback attempts are stopped when the device heats up and its output
drops. More annealing extends the time until less than the minimum
required output is available and, thus, the recorder run time. It also
shows how the diminishing effects of annealing occur before the recorder
is out of the range where LED self-heating will stop the motor.

# Conclusions

The rapid and successful response to the Galileo recorder failure
required (1) identification of the recorder components and circuit
elements capable of causing the failure signature, (2) recognition of
the role of radiation damage, and (3) development and deployment of
remote repair operations. When the LEDs rose to the top of the suspect
list and current-enhanced annealing was identified as a possible
solution, it was not clear whether or not too much damage had occurred
for that strategy to work. The identification of a mode of recorder
operation that would fairly conveniently and unintrusively run current
through the LEDs allowed annealing to begin as an in-flight
"experiment." Early hints of some success encouraged continued
annealing. Fortunately, although the recovery hit diminishing returns,
it was at a point where recorder operation was restored to a reasonable
operating regime.

In addition, the experiments conducted to calibrate the understanding of
the failure and recovery are significant. Surprisingly, the earlier
studies of the OP130 with protons turned out to be less helpful
quantitatively than originally thought, even though they are essentially
identical to the OP133. Perhaps most important are the electron
irradiation and annealing results (since the literature has so few
electron displacement studies). Similar to the case for protons, damage
from lower energy electrons annealed somewhat easier compared to devices
irradiated with higher energy. Finally, the most interesting new
experimental result here is that electron damage in these LEDs annealed
considerably easier and to a much greater extent than proton damage.
Thus, it turns out that Galileo was fortunate (from the standpoint of a
working recorder) that most of the particles encountered in its long
mission were electrons.

# Acknowledgment

The authors gratefully acknowledge the experimental work of Tetsuo F.
Miyahira. His careful approach and attention to detail is much
appreciated. Also, thanks to Odetics for supplying a few older devices
for testing.

# References

1.  N. Divine and H. B. Garrett, "Charged particle distributions in
    Jupiter's magnetosphere," *J. Geophys. Res.*, vol. 88, no. A9, pp.
    6889-6903, Sept. 1983.

2.  H. B. Garrett, I. Jun, J. M. Ratliff, R. W. Evans, G. A Clough,
    and R. W. McEntire, *Galileo Interim Radiation Electron Model*, JPL
    Pub. 03-006, 72 pp., Feb. 2003.

3.  A. H. Johnston, B. G. Rax, L. E. Selva, and C. E. Barnes, "Proton
    degradation of light-emitting diodes," *IEEE Trans. on Nucl. Sci.*,
    vol. 46, no. 6, pp. 1781-1789, Dec. 1999.

4.  A. H. Johnston and B. G. Rax, "Proton Damage in Linear and Digital
    Optocouplers," *IEEE Trans. on Nucl. Sci.*, vol. 47, no. 3, pp.
    675-681, June 2000.

5.  G. P. Summers, E. A. Burke, M. A. Xapsos, C. J. Dale, P. W.
    Marshall, and E. L. Petersen, "Displacement damage in GaAs
    structures," *IEEE Trans. on Nucl. Sci.*, vol. 35, no. 6, p.
    1221-1226, Dec. 1988.

6.  G. P. Summers, E. A. Burke, P. Shapiro, S. R. Mesenger, and R. J.
    Walters, "Damage correlation in semiconductors exposed to gamma,
    electron and proton irradiation," *IEEE Trans. on Nucl. Sci.*,
    vol. 40, no. 6, p 1372-1379, Dec.1993.

7.  A. L. Barry, A. J. Houdayer, P. F. Hinrischsen, W. G. Letourneau,
    and J. Vincent, "The energy dependence of lifetime damage constants
    in GaAs LEDs for 1-500 MeV protons," *IEEE Trans. on Nucl. Sci.*,
    vol. 42, no. 6, p 2104, Dec. 1995.

8.  J. R. Srour, C. J. Marshall, and P. W. Marshall, "Review of
    displacement damage effects in silicon devices," *IEEE Trans. on
    Nucl. Sci.*, vol. 50, no. 3, pp. 653-670, Dec. 2003.

9.  T. M. Jordan, "NOVICE, a radiation transport shielding code,"
    Experimental and Mathematical Physics Consultants, Aug. 2000
    version.

10. A. H. Johnston and T. F. Miyahira, "Characterization of proton
    damage in light-emitting diodes," *IEEE Trans. on Nucl. Sci.*, vol.
    47, no. 6, pp 2500-2507, Dec. 2000.

11. A. H. Johnston and T. F. Miyahira, "Energy Dependence of Proton
    Damage in Optical Emitters," *IEEE Trans. on Nucl. Sci.*, vol. 49,
    no. 3, pp 1426-1431, June 2002.

12. B. H. Rose and C. E. Barnes, "Proton damage effects in
    light-emitting diodes," *J. Appl. Phys.*, vol. 53, no. 3, p. 1772
    (1982).

    ![](media/image1.png){width="3.4in" height="4.346527777777778in"}

Fig. 1. Photograph of the tape recorder model that was used on Galileo.

![](media/image2.png){width="3.420138888888889in"
height="2.3743055555555554in"}

Fig. 2. Cutaway mechanical drawing of the Galileo tape recorder's motor
drive assembly showing one of the OP133/MD300 LED/phototransistor pairs.

![](media/image3.png){width="3.4583333333333335in"
height="2.390277777777778in"}

Fig. 3a. Cumulative unshielded integral spectra for *electrons* that
Galileo encountered orbiting Jupiter \[2\]. The cumulative spectrum up
through the orbit (I33) preceding the Amalthea orbit (A34) is compared
to the spectrum including A34. Note that, although the A34 orbit
encountered more electrons above 15 MeV than any other, it represents
only a small increment to the total accumulation.

![](media/image4.png){width="3.4319444444444445in"
height="2.4319444444444445in"}

Fig. 3b. Cumulative unshielded integral spectra for *protons* that
Galileo encountered orbiting Jupiter \[1\]. Again, the cumulative
spectrum up through the orbit (I33) preceding the Amalthea orbit (A34)
is compared to the spectrum including A34. Orbit A34 encountered many
more protons above 10 MeV than the total of all the previous orbits.

![](media/image5.png){width="3.4166666666666665in"
height="2.3333333333333335in"}Fig. 4a. Shielded integral spectrum for
*electrons* summed through the Amalthea orbit (A34) compared to the
spectrum from one orbit earlier (I33). The shielding considered here is
a 400 mil Al spherical shell and approximately represents the OP133
shielding in the Galileo recorder. Note that the increase in electrons
due to A34 is small.

![](media/image6.png){width="3.4034722222222222in"
height="2.598611111111111in"}

Fig. 4b. Shielded integral spectrum for *protons* summed through the
Amalthea orbit (A34) compared to the spectrum from one orbit earlier
(I33). The shielding considered here is a 400 mil Al spherical shell and
approximately represents the OP133 shielding in the Galileo recorder.
Note that the A34 protons represent a very large increase in proton
fluence.

![](media/image7.png){width="3.448611111111111in"
height="2.5930555555555554in"}

Fig. 5. Linearized damage function vs. proton fluence for the OP130 LED.
This relationship was used to extrapolate the data set of Ref. 10 to
higher fluences.

![](media/image8.png){width="3.4451388888888888in"
height="2.515277777777778in"}

Fig. 6a. Reduction in OP133 output from electron displacement damage as
a function of fluence. Pulsed measurements at I${}_{f}$=25 mA were taken
to minimize current-induced annealing. Note that converting the x-axis
to NIEL units would collapse the two distinct curves into one.

![](media/image9.png){width="3.448611111111111in"
height="2.2680555555555557in"}

Fig. 6b. Reduction in OP133 light output from 50 MeV proton displacement
damage as a function of NIEL. The 50 MeV proton fluences were multiplied
by the factor 3.75 x 10${}^{- 3}$ to convert to NIEL units.

![](media/image10.png){width="3.4451388888888888in"
height="2.4868055555555557in"}

Fig. 7. In-flight annealing is shown by the increasing times that the
recorder motor moved after successive intervals when the LEDs were
operated continuously in order to cause current-induced annealing.

![](media/image11.png){width="3.375in" height="2.1354166666666665in"}

Fig. 8. Results for current annealing of OP133 LEDs damaged to about 22%
of their original output with two energies of electrons (10 and 60 MeV).
The vertical bars overly the in-flight anneal points from Fig. 7; the
anneal times have been converted to charge on the x-axis and the
resulting cool recorder run times are given the intersecting boxes.

![](media/image12.png){width="3.2784722222222222in"
height="2.0569444444444445in"}

Fig. 9. Temperature history of LED through the failure and recovery,
including read attempts, modeled by a 17 degree C temperature increase
when the LED is operated. The star indicates the safehold occurrence
shortly after the Amalthea encounter. The cross-hatched regions
represent the periods of current annealing. The read attempts are
flagged with the recorder run times which are growing as LED damage is
annealed.

![](media/image13.png){width="3.375in" height="2.365972222222222in"}

Fig. 10. The combined damage, temperature, and annealing model showing
the LED output following the Amalthea encounter through the safehold and
seven read attempts interspersed with periods of current-induced
annealing.

[^1]: ^^Manuscript received July 22, 2003 and revised August 29, 2003.
    The research done in this paper was carried out at the Jet
    Propulsion Laboratory, California Institute of Technology, under
    contract with the National Aeronautics and Space Administration
    (NASA) and was partially sponsored by the NASA Electronic Parts and
    Packaging Program (NEPP), Code AE.

    Reference herein to any specific commercial product, process, or
    service by trade name, trademark, manufacturer, or otherwise, does
    not constitute or imply its endorsement by the United States
    Government or the Jet Propulsion Laboratory, California Institute of
    Technology.

    The authors are with the Jet Propulsion Laboratory / California
    Institute of Technology, Pasadena, CA 91109 USA (telephone:
    818-354-5059, e-mail: <gary.m.swift@jpl.nasa.gov>).