Test design parameters for accelerated dose rate radiation effects tests for spacecraft parts and subsystems used in long term mission (years) are detailed. A facility for use in long term accelerated and unaccelerated testing is described

Miller, C. G.

Parker, R. H.

Price, W. E.

English

NASA Technical Reports Server

DESIGN CONSIDERATIONS AND TEST FACILITIES FOR
ACCELERATED RADIATION EFFECTS TESTING*
William E. Price, Charles G. Miller and Richard H. Parker
Jet Propulsion Laboratory, Pasadena, California
Test design parameters for accelerated dose rate radiation effects tests
for spacecraft parts and subsystems used in long term missions (years) are
detailed. A facility for use in long term accelerated and unaccelerated
testing is described.
INTRODUCTION
Outer planet missions require electronics and
science instruments that have a useful lifetime of
up to ten years. During this time, they are con-
tinuously within the gamma and neutron fields of
the radioisotope thermoelectric generator (RTG),
which provides electrical power to the spacecraft.
Additionally, these electronics and science in-
struments are subjected to a number of other
radiation stresses: Jovian radiation belts,
solar flares, solar wind and cosmic radiation.
One means of verifying the adequacy of components
and of design for the ten-year useful lifetime is
by accelerated testing in an adequately simulated
environment. The intent is to simulate, in a
short testing period, the effects to be expected
in the real spacecraft mission, taking into
account all of the stresses involved in radiation-
cryogenic-vacuum environments.
RADIATION SOURCE CONSIDERATIONS
Jovian Radiation Belts
The Jovian radiation belt simulation testing
need not be accelerated. This is because the
belts may be traversed in a few hours on flyby
missions. Both electrons and protons are expected
to be trapped in the Jovian magnetic field, al-
though the only direct estimates that have been
made imply only an electron flux. It appears that
readily available particle accelerators can simu-
late the Jovian fluxes in real time for both
electrons and the theoretical models for protons.
Solar Flares
Solar flare rates will be lower than the Jovian
radiation belts and the total doses can almost
certainly be obtained by allowing 0.5 h/flare
(acceleration by a factor of about 1OO). A
twelve-hour test will conservatively simulate a
ten-year mission.
Cosmic Radiation
The flux of cosmic ray particles is small and
the expected damage is too low to he considered
as an important area for environmental testing.
Solar Wind
The solar wind, whose effects are manifested
as surface problems, has not interfered with past
missions. The average flux of solar wind par-
ticles over a twelve-year period, with an assumed
average energy of 3 keV, is 107_protons/cm2-s.
Although at 1 AU the flux is lO _ protons/cm2-s,
the 1/r 2 decrease means that at Jovian distances,
the flux is reduced by a factor of 1/25 A Since
3 keV protons have a range of 10-5 g/cm c, if we
assume a target with a thickness equal to the
proton range, the delivered power density is
4.8 x lO -G W/cm 2 at Jupiter orbit. For those com-
ponents that rely on radiative cooling for their
heat loss, typically unsupported thin films, too
great an acceleration of the above given natural
solar wind rates may lead to annealing effects in
the test, which are not present in the real space
env_ ron_ent
The power radiated from a b_ack body (in
watts/cm 2) is equal to (T/645) _ where T is the
temperature in degrees Kelvin. Table 1 gives the
expected equilibrium temperature of a radiatively
cooled thin film, subjected to various accelerated
dose rates of solar wind, calculated for various
initial component temperature and power densities.
In programming a test, larger accelerations (that
lead to larger temperature rises) can be tolerated
for systems that operate at low temperatures than
for systems that operate at high temperatures,
because the annealing temperature is related to
the Debye temperature for the material under test.
These restrictions are further relaxed when more
efficient means of cooling may be employed (e.g.,
conductive cooling to the spacecraft for non-thin-
film components).
*This paper presents the results of one phase of
research carried out at the Jet Propulsion
Laboratory, California Institute of Technology,
under Contract No. NAS 7-1OO, sponsored by the
National Aeronautics and Space Administration.
570
https://ntrs.nasa.gov/search.jsp?R=19720010025 2020-03-17T03:45:14+00:00Z
Table 1. Equilibrium temperature of a radiatively cooled 
thin film, subiected to various doses of (accelerated ratel 
solar wind 
calor- 
tion, 
imos 
minai 
Equilib- 
rium 
tompor- 
aturo, 
O K  
Initial 
P o w r  
donrity, 
Wlrm2 
Dumtion componont 
of tort tomporaturo, 
OK 
I I I 
Earth orbit 
Mission avorago 
300.1 
300.5 
308 
Mission avomgo 
150.6 
155 
Jupitor orbit 
3650.days 1.78 X 1 O-g 150  (arrumed) 
36.5 days 1.78 X lo-' 150 (orrumed) 
150.1 
5.2 min 1.78 X 10-4 150  (assumed) 
lo6 0.52 min 1.78 X 150 (assumed) 169 
RTG Radiation 
Gama Radiation 
The spectral distribution and intensity of the 
gamma field from the RTG, although varying with 
time and dependent upon the initial fuel composi- 
tion and construction of the RTG capsule, have 
been extensively studied at JPL. 
The design considerations given in this paper 
indicate the practical limits of acceleration Of 
testing, and take into account costs, radiation 
safety needs and requirements for Spatial uni- 
formity of accelerated test fields. 
The analysis is based on a generalized outer 
planet spacecraft configuration in which the RTG- 
science-instrument spacing is taken as 180 in., 
and the Rn; electronics spacing is taken as 72 in. 
Actual design geometry and environmental condi- 
tions will be determined as the program proceeds. 
The analysis is based on the simulation of the 
radiation of an 8000 W (thermal) source, as Was 
envisioned for an earlier JPL TOPS configuration. 
Newer configurations may have somewhat different 
radiation levels, but such changes will not change 
the conclusions of the study. 
complished by increasing the dose rates delivered 
to the test object by either of the two methods 
described below, or by the use of both simultan- 
eously. 
Acceleration of delivery of total dose is ac- 
Loading 
Increasing the gama flux by using larger 
amounts of nuclear material or a satisfactory 
nuclear species that gives the same gamma 
spectrum. The apparent limitations are in cost 
and safety necessary for large sources. 
tical acceleration of X 25 (over the output of an 
8000 W (thermal source) can be had for a moderate 
cost. The X 25 acceleration source could be used 
in the JPL 25-ft space simulator (see Fig. 1). 
A prac- 
-_ 
Fig. 1. JPL 25-Ft Space Simulator 
An alternate loading acceleration has been 
designed that would use a fast decaying, but in- 
tense source of sodium-24 as the principal 
nuclide. This design allows a loading accelera- 
tion of X 250 at slightly more than twice the 
cost of the X 25 source. However, this source 
could not be ready for use in less than approxi- 
mately eight months. 
Geometrical 
An increase in the gama flux can be had by 
bringing the source closer to the test objects 
than the design value of 15 feet for the science 
instruments, and six feet for the electronics. 
When the source real or simulated, is placed 
closer than its normal separation distance from 
the test object the field strength is increased 
by the ratio $Id2 or 62/d2 for science instru- 
ments or electronics, respectively. The uni- 
formity of the exposure from the near-face to 
the far-face of the test object, however, de- 
creases as the distance d from the source to the 
test object decreases. Practical geometry 
increases are from X 10 t o  X 100, depending 
57 1 
onthesizeof thetest objectandtheuniformity
of flux required.
Becauseof thedeeppenetrationof thegammas,
the heatinputperunit volume of the test object
is low, and annealing is not expected to be a
problem, even for a X 2500 acceleration.
Figure 2 shows the uniformity of dose (in
terms of tolerance from the centerline dose) for
varying distances up to 200" from the source and
for test objects whose size (extent in the beam
direction), is 2, 4, 8, or 16 in. The figure
shows that a geometrical acceleration of X 10
may be had for a 4-inch-thick object with a uni-
formity of + _ of centerline intensity, while a
geometric acceleration of X I00 yields a variation
of + 20_. Similarly, Fig. 3 shows the uniformity
of _ose (from centerline dose) and geometrical
acceleration possibilities for electronic bay
equipment, which is normally six feet from the
s ource.
j ioox C_OA_Ta_AL ACCE_TION _TE
_--50x
J /--Z0x
2I"
./
\ \
*/o_
THn, TEST
OIJECTDEPTH
nl
0 40 0o 120 i,Io _
OOSTAt_CEFIU3Msouwce, _..
Fill, 2. Uniformity of dose as a function of distance from the source for vaflQut hJst Qb_-t$
"H
z"
o
0
C_
60
50
40
30
20
(_
I
\
(_
\
I t
( ) /- 50x GEOMETRICAL
ACCELERATION RATE
16-in. TEST
J OBJECT DEPTH
" I
'1
_ _-'--_ 4-i..
20 40 60 80 100 120
DISTANCE FROM SOURCE, in.
Fig. 3. Uniformity of dose as a function of distance
from the source for electronic bay equipment
Neutron Radiation
It does not seem practical, at present, to
make large loading accelerations for the neutron
flux components of the RTG radiation. Therefore,
any acceleration using isotopic sources will be
dependent on increases via geometrical means.
Studies are underway to determine if larger geo-
metrical accelerations can be tolerated for
neutrons than for gammas, because of the different
nature of the scattering and interaction mech-
anisms involved.
An alternate possibility exists for accelerated
neutron testing in the use of a machine acceler-
ator as a neutron source. While this has some
limitations on size of component accepted, the
energy spectrum can be grossly simulated and
fluxes as high as 10 ll n/cm2-s can be attained in
small areas at JPL.
COMBINED ENVIRONmeNTS AND TESTING CONDITIONS
Simultaneous application of other space environ-
ments (space vacuum, solar and ultraviolet irra-
diation, and control temperatures down to those
of liquid nitrogen) will also be possible in the
JPL facilities.
Table 2 shows a selected set of operating con-
ditions that are available using a combination
of loading and geometrical accelerations. In
each case, it is possible to increase the irra-
diation uniformity (decrease the variation) from
that presented in Figures 2 and 3 and in Table 2,
to a small fraction of the given values by mount-
ing the test objects on a rotating table, or a
table that oscillates through 180 degrees, so
that the two faces of test objects alternate in
their exposure to the source.
572
Table 2. Operating conditions for accelerated testing
(10-yr mission length)
Acceleration, limos nominal
10
25
10
25
250
250
Geometdcal Total
10 100
I0 250
I00 1000
100 2500
10 2500
30 7500
Required
total test
dumtlen,
dc,.
36
t4
3.6
1.5
!.5
0.5
Remarks
-----!2% uniformity for
8-in.-thick test
object
_"! 2% uniformlty for
8-in.-thick test
obiect
--4"--10%uniformity for
2-1n.-thJck test
object
+10% uniformity for
2-in.-thlck test
object
ACCELERATED RADIATION EFFECTS TESTING FACILITIES
Comparatively little work has been done to
determine if long-term (years), of low level
radiation environments such as encountered by
spacecraft on long missions, will affect the life-
time and reliability of spacecraft parts. Most of
the existing radiation effects studies have con-
centrated on material property studies, weapons
effects, and reactor irradiations which are of
little value to radiation effect studies for
outer planet missions. Space radiation simulation
has been done on a limited number of device types
used in present day spacecraft (Ref. i).
Accelerated testing is important to obtain
data in a practical time. To be useful, however,
a relationship must be established between parts
testing in real time using combined environments
(under varied test conditions) and accelerated
testing to take the rate effects, if any, into
account. Such a program of studies is indispen-
sable for carrying out present outer planet
missions. An initial simple program is described
below. As the requirements become clear, addi-
tional variables may be added at the expense of
increased complexity and cost.
The major object of the program described is
to determine for a large number of types of parts,
whether accelerated testing is a v_lid procedure
to predict long-life, real time operation in a
low level radiation field, and if so, to define
for various parts and components what rate of ac-
celeration can be tolerated in testing. Standard
testing procedures must be developed and validated
for the parts to be tested.
An Accelerated Radiation Effects Laboratory
has been put into operation at JPL in a group of
former explosive test cells. A source of radia-
tion, 5 curies of Ir 192, was placed in a collimator
with the radiation beam directed toward the ceiling
(see Fig. 4). Circuit boards of parts and
components are suspended in the cone of radiation
at two or three distances from the source to ob-
tain different radiation dose rates. Some of the
parts will be non-operating and some will be
operating to simulate conditions on a spacecraft
(where some systems are continually in operation
while others are used intermittently or only
at planet encounters). There will be control
specimens subject to the same environmental con-
ditions except for the radiation, and these will
be operated by the same power supplies as those
powering the irradiated samples.
Fig. 4. JPL Accelerated Radiation Effects
Laboratory
Table 3 shows the acceleration rates available
with a 5 curie Irlg 2 source. To obtain a X 1
rate at the farthest distance from the source
(at 2.5 meters) the radiation must be attenuated.
That was accomplished using several thin lead
sheets (0.033 cm each). This turns out to be a
slight advantage in that a single sheet may be
removed periodically to compensate for the change
in dose rate due to the decay of the source (74
day half-life). However, a change to a longer-
life isotope, Cs 137 (30 year half-life), is anti-
cipated in order to have a more stable dose rate
with time at the experimental areas.
573
Table 3. Acceleration and Dose Rates Obtained with a 5 ci Irlg2 Source 
Distance 
From Source 
Meters 
2.5 
1.5 
1.0 
0.75 
0.49 
Radiation Acceleration Rate Acceleration 
Dose Rate Above that at the Rate With 
TOPS Electronics Bay Attenuation 
by Lead Sheets 
456 x 4  x 1  
2750 X 24 x 6  
4560 X 40 x 10 
11400 x 100 X 25 
1140 x 10 X 2.5 
A larger source is planned for an adjoining 
test cell which will allow accelerated testing 
up to X 1000, so that accelerated rates may be 
chosen for testing parts between X 1 and X 1000 
of the radiation dose rate expected on the elec- 
tronics of the outer planet mission spacecraft 
presently planned. 
used for long-term testing, possibly up to ten 
years. Therefore, the tests chosen, the test 
plan adopted and the environmental conditions 
must be planned in great detail so that useful 
and statistically significant data will result. 
The parts which are now considered the most 
sensitive to radiation induced changes are semi- 
conductor devices, since minor changes in the 
trapping centers and in surface ionization can 
produce major changes in operating conditions in 
these devices. Therefore, semiconductor devices 
of all comonly used types should be included in 
the test. 
resistor types, capacitors and other active 
electronic components, can also be considered for 
long-term testing. 
shown that these device types are more radiation 
resistant than transistors, and they should not 
be included until a later phase of the long-term 
experiment unless they are of unorthodox con- 
struction or involve thin-film elements. 
lists should be obtained in sufficient numbers to 
allow a test to be set up which has a statistical 
significance for the expected low rates of failure. 
Data analysis should include Weibull plots, since 
we are interested in a determination of failure 
predictions down to low doses and low probabilities 
Investigators have shown this to be a valid 
approach (Ref. 2). 
one year in order to determine applicable test 
procedures for subsequent longer term testing. 
Elements of the initial tests should include the 
following: 
It is anticipated that the test cells w i l l  be 
Representative samples of various 
However, past testing has 
Transistors chosen from the most recent parts 
Initial tests should be set up for times up to 
1. Testing at room temperature. 
2. Gam radiation exposure. 
3. Powered and unpowered operating transistors 
in the radiation environment; and control 
devices in a similar, but radiation-free 
environment. 
Each test group should be mounted on 
separate printed circuit boards to allow 
rapid parameter measurements. 
remote measurements should be determined. 
The test design should incorporate adequate 
statistical planning, so that with a 
limited number of specimens the most mean- 
ingful test can be carried out. 
In the JPL Accelerated Radiation Effects 
4. 
5. The validity of removing the boards for 
6. 
Laboratory (AREL), the initial work is being done 
at the X 1 rate, because these tests take longer 
times. 
mator locations in relationship to the X 1 sample 
locations (on the ceiling) and the X 10 location 
(at the table top). 
Figures 5 and 6 show the source and colli- 
Fig. 5. antrance to the Low Acceleration Exposure 
Room 
57 4 
Fig. 6. Inside Low Acceleration Exposure Room 
Figure 7 shows the power supplies and the test 
points for the integrated circuits under test and 
the control samples located on the wall mounting 
in the AREL control room. 
control room with the source operating mechanism, 
safety system and the window labyrinth for wiring 
Figure 8 shows the AREL 
Fig. 8. Mi., Control R o n  
CONCLUDING REMARKS 
The Accelerated Radiation Effects Laboratory 
was constructed utilizing the design principles 
described in this paper and in order to confirm 
these principles. 
for testing spacecraft piece parts, the versatility 
allows a number of other types of experimentation 
to be considered, such as testing survivability of 
micro-organisms at l o w  dose rates over long periods, 
and in vacuum. 
lants is also practical in this facility and ar- 
rangements are being made to this end. 
While the initial purpose was 
Testing of pyrotechnic$ and propel- 
57 5 
REFERENCES
io
2.
J. E. Drennan and D. J. Hamman, "Space-Radiation Damage to Electronic Components
and Materials", REIC Report #39, Battelle Memorial Institute, January 1966.
W. E. Horne and J. A. Folsom, "Total-Dose Survival Probability for Bipolar
Transistors", IEEE Trans. on Nuclear Science, Vol. NS-17, No. 6, Pp. 167-172,
December 1970.
57 6


Design considerations and test facilities for accelerated radiation effects testing

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