IEEE Ttansaction6 on Nuxceo.A Science, Vot.NS-25, No.6, Decembet 1978

EVALUATION OF RADIATION INTERFERENCE IN THE VOYAGERSUN SENSOR'S CADMIUM SULFIDE DETECTOR

Theodore C. Clarke'" and Edward L. DivitaeAbstract

The simulation of radiation interference effects andthe results of a radiation interference test on twoVoyager Sun Sensor prototype detector assemblies arereported. The derivation of test levels and require-ments are discussed and show that cobalt 60 gammaradiation is an effective and practical simulator ofthe ionization dose rate effects induced by high-energy electron flux incident on the spacecraft at arate of 3. 7 x 108 e/cm2 -s (10 rad(Si)/s) during closestapproach to Jupiter. The test results provide infor-mation that is used to confirm an analytic correlation,and to predict satisfactory performance of a space-craft Sun sensing device having stringent angularresolution requirements. The measured detectorresponse shows that at dose rates incident on thedetector elements of 2 rad(Si)/s, which is four timesthat expected during Jupiter encounter, the radiation-induced angle error is almost an order of magnitudeless than that allowed by the acceptance criteria.

Introduction

With impending Jupiter encounters by two Voyagerspacecraft in March and July, 1979, satisfactory per-formance of the spacecraft attitude control subsystemand its associated assemblies in the Jovian radiationbelts is crucial. In particular, the Sun Sensor (SS)assembly must continue to perform within specifica-tions to satisfy the pointing accuracy requirements ofthe science scan platform. The SS uses recrystallizedphotoresistive cadmium sulfide (CdS) detectors as itssensing elements. CdS is an ideal Sun-sensing mate-rial for a spacecraft mission to the outer planetsbecause of its sensitivity and dynamic range.

This paper reports on the results of radiation inter-ference testing of two Voyager SS prototype detectorassemblies. The detectors were exposed to radiationdose rates of 0. 24 to 20 rad(Si)/s using gamma raysfrom a cobalt 60 radiation source to simulate theelectron flux in the Jupiter radiation belts. The testresults support predictions of successful performanceof the Voyager spacecraft Sun-sensing device understringent angular resolution requirements.

Cobalt 60 gamma radiation is used as an effective andpractical simulation of the ionization dose rate effectsinduced by high-energy electrons. Exposure fluxes aslarge as 7. 5 x 108 e/cm2-s (3 MeV equivalent), corre-sponding to a dose rate of 20 rad(Si)/s, were simu-lated. Peak fluxes expected at the spacecraft surfacefor the closest approach trajectory (5 Jupiter radii(RJ)) may be as large as 3.7 x 108 e/cm -s (3 MeVequivalent), or 10 rad(Si)/s. The estimated Jupiterelectron flux at the detector elements will be approxi-mately 1/25th of that expected at the spacecraft sur-face because of SS housing and detector packagingattenuation.

The measurement repeatability requirements for theSS detector at Jupiter encounter are demonstrated bythe test results to be satisfied within established cri-teria. An analytic model of the detector response isdeveloped, and calculations based on the model indi-cate good agreement with the measured detectorresponse. The model and the findings are shown tosupport the photoconductivity physics of the radiationinteractions.

A comparison of measured detector response with thatcalculated using the analytic model shows that, at thedetector elements, dose rates even four times that

Jet Propulsion Laboratory, Pasadena, Calif.

expected at Jupiter encounter produce radiation-induced angle errors an order of magnitude lower thanthat allowed by the acceptance criteria.

Developmental Tests Background

Developmental radiation tests performed on the Voy-ager SS breadboard model in 1974 induced unacceptablyhigh angle errors that were traced to radiation effectsin the SS detector. 1 An analysis of the developmentaltest results showed that the detector was overtested inboth dose rate and total dose by a factor about 40 timesgreater than the level the detector would actuallyreceive. The dose rate tests, conducted using gammasfrom a cobalt 60 radiation source, resulted in interfer-ence effects 20 times greater than the acceptance cri-teria. The total dose tests, however, that were con-ducted using 10 MeV electrons from a pulsed linearaccelerator resulted in permanent damage in the detec-tor which only moderately exceeded the acceptance cri-teria, and then only in an anomalous area of the detec-tor. As a result, a new, detector-only, radiationqualification test was designed, with emphasis on thedose rate tests, to resolve questions raised by thedevelopmental tests. The results of the new tests arepresented in this paper.

Test Purpose

The purpose of the radiation qualification test was two-fold: (1) to determine the extent of interference effectsinduced in an SS detector when exposed to radiationdose rates spanning those expected at Jupiter encoun-ter, and (2) to determine the extent of permanent dam-age effects induced in the detector by radiation dosesspanning those expected at Jupiter encounter.

Simulation of Jupiter ElectronRadiation Environment

Experimental simulation is necessary when analyticevaluation cannot accurately predict radiation effectsin materials and electronic parts and potential conse-quences in subsystems. However, limitations of exist-ing facility capabilities make it necessary to simulateeffects rather than to reproduce the expected radiationenvironment. To establish test requirements appro-priately, analysis must be performed to determine thecompatibility of the actual environment with the simu-lated environment for the overall test design. Forexample, the test may require rate acceleration abovethe actual expected rate, and the spectrum energy orflux level may require simulation. In these testsacceleration is required, and an electron energy spec-trum equivalent gamma dose level is used to simulatethe anticipated electron ionization damage.

The Jupiter electron environment contains electronshaving an energy spectral distribution primarily in therange of 0. 1 MeV to 100 MeV. Near the peak of thespectrum, at 3 to 6 MeV, monoenergetic electronsmay be utilized to simulate the damage and interfer-ence effects of the dominant part of the spectrum. Ifthe nature of the damage mechanism can be determineda priori, then the electron effect may be simulatedrealistically with other types of radiation. For exam-ple, in this case, where ionization rate effects are

expected to dominate, cobalt 60 gammas were used tosimulate the ionization effects of the electron environ-ment. The ionization dose conversion used to deter-mine the test levels were derived from energy lossdata in Refs. 2 and 3.

Figure 1 presents the flux profile and dose used indesigning the Voyager mission.4 5 The peak fluxshown in Fig. 1 occurs at llRJ, near closest approach.The total electron exposure during the mission is

0018-9499/78/1200-1324$00.75 © 1978 IEEE1324

about 2 x 105 rad(Si). Figure 2 shows the electron fluxand its corresponding exposure dose rate incident onthe SS as the spacecraft encounters Jupiter. Thecobalt 60 test levels are derived from these analyticmodels of the Jupiter electrons. The levels areselected to simulate the ionization interference effectsin the SS piece parts, components, and circuits. Theexposure times for the irradiation were chosen basedon a rate acceleration selected for practical simula-tion of the flux occurrence within an acceptable mar-gin. The rate was applied over these time exposuresto also accumulate an ionization dose at levels nearand above that expected to estimate the ionization dam-age effects.

E1I

z0

-

0

-5 0 5

TIME FROM PERIJOVE, hr

equivalent fluence. However, because of the substan-tial inherent shielding provided by the detector packag-ing, the detector elements themselves are expected tosee only a simulated 5 x 1011 e/cm2, 3 MeV equiva-lent fluence, during actual Jupiter encounter.

Test SetupThe test setup is shown schematically in Fig. 3. Theactual environment at the detector at Jupiter encounteris an equivalent maximum dose rate of approximately0. 4 rad(Si) / s and a total dose of approximately 8. 0 x103 rad(Si). The maximum electron dose rate at thespacecraft surface is approximately 10 rad(Si)/s.

a0

z0

u

Fig. 1 Unshielded Electron Flux and Dose DuringJupiter Encounter Trajectory Profile(Cutoff, Ee > 0.4 MeV)

10'

10R

E

x5-

z0

u

*u

cL

-6

z

106

l-5

104 I_ 1 _10 1 10° 10I 10- 103

ELECTRON ENIERGY, MeV

Fig. 2 Integral Peak Electron Flux at JupiterEncounter (Cutoff, Ee > 0. 4 MeV)

The radiation test environment used to simulate theJovian electrons consisted of 1. 17 MeV and 1. 33 MeVgamma rays generated by a 14, 000 Curie cobalt 60radiation source. The dose rates and doses used inthe test are given in Tables 1 and 2, respectively.Each level in the dose rate tests was run 15 times,while each level in the total dose tests was run twice.The total dose to which the detector was exposed dur-ing the test was equivalent to 5 x 101L e/cm2, 3 MeV

Fig. 3 Detector Radiation Test ConfigurationThe test configuration was designed to test all fourdetector elements, two angle detectors (ADETs) andtwo illumination detectors (IDETs) within the detectorpackage. The parameters to be measured includedlight resistance RL and dark resistance RD for all fourelements before, during, and after irradiation. Theseparameters were to be determined with the detectorilluminated by a lamp at either -10 deg, 0 deg, or +10deg with the lamp intensity equivalent to the Sun's inten-sity as seen from Jupiter. This test setup and theanalytic model developed for these tests provide amethod to evaluate the radiation-induced angle error.In addition, the tests were designed to run with theADETs configured for angle measurements to obtain adirect measurement of angle accuracy degradationinduced by the irradiation.Test points monitored during the tests include the plusand minus detector excitation voltages and the outputvoltages from the detector element resistance measur-ing circuits and the ADET angle circuits. Initial abso-lute value measurements were made with a digital volt-meter before and after each irradiation. Changes tothe detector element resistance data or ADET angledata during irradiation were continuously recorded byan eight-channel brush recorder starting from one min-ute before the start of irradiation and continuingthroughout the irradiation to up to five minutes afterthe conclusion of irradiation for each test. Thedetector excitation voltages were also recorded duringthe irradiation to ensure that any changes observed inthe resistance or angle data were not in fact caused bychanges in the excitation voltages.

AnalysisSun Sensor OperationA cutaway view of the Voyager SS showing the detectorassembly optical configuration is given in Fig. 4. Aschematic representation of the detector arrangementwithin a detector assembly is shown in Fig. 5. The

1325

rstL

TABLE 1

RADIATION DOSE RATE TEST LEVELS

Dose Dose Per Accumulated Equivalent Equivalent 3 MeVRate, Exposure Irradiation, Dose, 3 MeV Electron Electron Accumulated

Rad(Si)/s Time, s Rad(Si) Rad(Si) Flux, e/cm2-s Fluence, e/cm2

0.24 60 14.4 2.16 x 102 9.84 x106 8.61 > 1090.72 60 43.2 8.64 x 102 2.95 x107 3.54 x 10101.8 60 108.0 2.48 x 103 7.38 x107 1.02 x 10113.6 60 216.0 5.72 x 103 1.48 x108 2.34 x 1011

TABLE 2

RADIATION TOTAL DOSE TEST LEVELS

Dose Dose Per Total Equivalent Equivalent 3 MeVRate, Exposure Irradiation, Accumulated 3 MeV Electron Electron Accumulated

Rad(Si)/ s Time, s Rad(Si) Dose, Rad(Si) Flux, e/cm2-s Fluence, e/cm

7.5 1,500 1. 13 x 104 2.82 x 104 3.08 x 108 1. 16 x 101214.0 1,500 2.10x 104 7.02 x 104 5.74 x 108 2.88 x 101220.0 1,300 2.60 x 104 1. 22 x 105 8.20 x108 5.0 x 1012

YAW AXIS PITCH AXIS

Fig. 4 Voyager Sun Sensor Optical Configuration

Fig. 5 Redundant ADET and IDET ArrangementWithin the Yaw Detector Assembly

ADETs generate angle-error voltage information foruse in maintaining spacecraft attitude control, whilethe IDETs perform the Sun acquisition function. TheSS ADETs function, after Sun acquisition, when an

image of the Sun-illuminated 0. 076 cm wide SSentrance slit falls across the primary and redundant

1326

ADETs. The illumination on the detectors induces anarrow, low-resistance current bridge across the nor-mally dark, high-resistance CdS detector. The posi-tion of the slit image along the long dimension of thedetector elements depends on the angle error, 0, theangle between the source of light (the Sun) and the SSboresight.

The slit image thus acts like the sweeper of a poten-tiometer, as seen in the equivalent electrical circuitof the ADET shown in Fig. 6(a). Using this equivalentcircuit, an angle error dependent voltage, VL, isdeveloped from the excited nichrome resistor stripadjacent to the CdS, and the angle error voltage, Vo,is output to the SS signal processing electronics. TheE5 V ADET output signal corresponds to the ADET lin-ear field of view of +20 deg. Within the SS electronicsa bias voltage is beat against the ADET output signalto shift the zero to a position on the detector corre-sponding to the angular separation between the Sun andthe Earth as subtended at the spacecraft. In this waythe spacecraft high-gain antenna is maintained pointingat Earth.

Physics of the Radiation Interactions

In Fig. 6(a), RD, the dark resistance, represents theelectrical sum of many resistors, in parallel with eachother and also with RL, the resistive representation ofthe slit-image illuminated portion of the detector. Foran electrically symmetric detector, VD, the electricalcenter of gravity, coincides with the physical center ofthe detector, for which VD equals zero volts.

RL LIGHTED CdS RESISTANCERD DARK CdS RESISTANCEVL VOLTAGE PICKED OFF NICHROME RESISTOR

ADJACENT TO CdS IN ADETVD VOLTAGE AT ELECTRICAL CENTER-OF-GRAVITY

OF DARK CdSV0 ANGLE ERROR VOLTAGE OUTPUT FROM ADET

CURRENT

+5V

VD-5V VD

(a) (b)

Fig. 6 Circuits for the Sun Sensor ADET:(a) Electrical Equivalent,(b) Analytical Equivalent

VL

RL

itIt 0 o~v

The expected radiation rate test response from thedetector is a negative change in the angle error voltagewhen the preirradiated ADET output signal is positive,such as when the +10 deg lamp is on, and a positivechange in the angle error voltage when the preirradi-ated ADET output signal is negative. These expectedresponses are based on the following argument.

In the dark part of the detector, the charge carrierconcentration is much less than the trap concentration.6The leakage current in the dark CdS corresponds tothose parts of the detector photoconductive responsecurves below the knees, shown in Fig. 7. Thepresence of ionizing radiation will partially empty thetraps as well as generate carriers directly by ionizingthe copper donor recombination centers and the CdSitself. Although additional traps are also generated inthe dark CdS due to the irradiation, their increase issmall compared to the existing trap population. Theconductivity of the dark CdS will, therefore, increase,and its resistance will decrease, during the presenceof ionizing radiation.

In the lighted portion of the detector, the carrier con-centration is much greater than the trap concentra-tion. 6 The photocurrent in the lighted CdS correspondsto those parts of the photoconductive response curvesabove the knees in Fig. 7. Bombardment by ionizingradiation introduces additional trapping sites below theenergy level of the conduction band, thus increasingthe probability that a charge carrier will fall out of theconduction band and into a trap. Although additionalcarriers are generated in the lighted CdS, as in thedark CdS, due to the irradiation, the percentageincrease in carrier concentration over the initial car-rier concentration is small compared to the percentageincrease in the trap concentration. The net effect is adecrease in carrier concentration, and, thus, adecrease in conductivity and an increase in resistancein the lighted CdS. The predicted response of both thedark and lighted CdS is supported by the data.

The decrease indark resistance is clearly much greaterthan the increase in light resistance. For an electri-cally symmetric CdS ADET, in the limit, ionizing rad-iation of sufficient flux would always result in VD, andtherefore Vo, going to zero. This explains the expect-ed response. Even for electrical nonsymmetry, theexpected polarity of the response would not change.

AFTER ENCAPSULATION-5.0 Vdc DIAS0.051-cm SLIT WIDTH0 ANGLE DET 2

10 A ANGLE DET SLOPE=1.09

SLOPE 1.05

10 6

z10-7°'°0dS/Xg~~~~~~~~Iu t 1 SATURN LEVEL JUPITER LEVEL

O 10 SLOPE = ( 39 Et cd) (466 ft cdfL 1.84 I

104

Fig. 7 Photo Current vs Illuminance forCdS Angle Detector

Analytic Model DevelopmentFrom the above arguments, and using Fig. 6(b), whichis a valid representation of the essential elements ofthe CdS electrical equivalent circuit shown in Fig. 6(a),an analytic model is developed to predict the voltagechanges observed at the output of the ADETs.Applying simple circuit equations to the analytic cir-cuit, the ADET output voltage is

VDRL +VLRDV =DL Dv0 R + RL D

Differentiating Eq. (1) yields

(VD VL)

(RL +RD

(1)

RL(VL - VD)L

+2R2 D(RL + RD)

+ RD V RLt VR + R AL R + R ADL D L D(2)

The first and second terms in Eq. (2) are the contribu-tions to AVo caused by changes in the light and darkresistances, respectively, of the ADETs. The thirdterm in Eq. (2) is the contribution to AVO caused by achange in the voltage picked off the nichrome resistoradjacent to the CdS ADET. It is clear that VL changesonly if the excitation bias voltages change, or if achange occurs in the nichrome resistor itself. Test-ing conducted at the highest flux levels while monitor-ing excitation bias voltages show that they remainedstable. Nichrome resistor material is considered tobe several orders of magnitude harder to radiationthan CdS. For these reasons AVL is taken as zero inthe model.

The fourth and final term in Eq. (2) is the contributionto AVO caused by a shift in the electrical center ofgravity of the dark resistance of the ADET due to irra-diation. The ADETs tested were electrically symmet-rical, and thus their electrical centers of gravity wereat the physical center of the detector. Because of theelectrical symmetry, it was found that AVD = 0 voltseven for the highest flux levels used. RD was gener-ally orders of magnitude greater than RL at the illu-mination levels used. With the above considerations,then, Eq. (2) becomes

(VD - VL)ARL (VL - VD)(RL + ARL)ARD0 D +AD (RD + ARD)2 3

AV0 is the radiation-induced angle error voltage, andARL and ARD are the radiation-induced light and darkresistance changes, respectively. Equation (3) is theanalytic model as it pertains to an electrically sym-metric detector.

The R's and AR's are determined from the output volt-ages and characteristics of the logarithmic amplifierresistance circuits associated with the testinstrumentation.

Test Results

The allowed contribution to the specified SS pointinginaccuracy of ±0. 034 deg per axis due to radiationeffects at Jupiter has been set at ±0. 02 deg per axis.To provide a safety margin of 2, the test acceptancecriteria was established at ±0. 01 deg. To test theradiation sensitivity of different regions of the detec-tor elements, light sources used to simulate the Sun'sintensity at Jupiter were placed at -10 deg, 0 deg, and+10 deg from the detector boresight, as shown inFig. 3.

The analysis of the dose rate tests used the measuredR and AR data and Eq. (3) to compute radiation-

1327

TABLE 3

SUMMARY OF RADIATION-INDUCED ABSOLUTEANGLE ERRORS FOR THE -10 DEG

LAMP-ON CONDITION

Radiation-Induced Angle Error, AGRadiation Computed, Deg Measured, Dega

Rate,Rad(Si)/s ADET 1 ADET 2 ADET 1 ADET 2

0.24 0. 000 0.000 0.000 0. 000

0.72 0.000 0.000 0. 000 0.000

1.8 0.001 0.000 0.002 0.001

3.6 0.002 0.002 0.003 0.001

7.5 (b) (b) 0. 004 0.010

14.0 0.021 0.019 0.008 0.024

aMeasurement accuracy 40. 001 deg.Insufficient data to make computation.

TABLE 4

SUMMARY OF RADIATION-INDUCED ABSOLUTEANGLE ERRORS FOR THE 0 DEG

LAMP-ON CONDITION

Radiation-Induced Angle Error, AGRadiation Computed, Deg Measured, DegaRate, -_-

Rad(Si)/s ADET 1 ADET 2 ADET 1 ADET 2

0.24 0.000 0.000 0.000 0.000

0.72 0.000 0.000 0.000 0.000

1.8 0.000 0.000 0.002 0.002

3.6 0.000 0.000 0.002 0.002

aMeasurement accuracy ±0. 001 deg.

TABLE 5

SUMMARY OF RADIATION-INDUCED ABSOLUTEANGLE ERRORS FOR THE +10 DEG

LAMP-ON CONDITION

Radiation-Induced Angle Error, AGRadiation Computed, Deg Measured, DegaRate,__ _ _ _ _ _ _ _

Rad(Si)/s ADET 1 ADET 2 ADET 1 ADET 2

0.24 0.000 0. 000 0.000 0.000

0.72 0. 000 0.000 0. 000 0.000

1.8 0.001 0.001 0.000 0.000

3.6 0.002 0.003 0.003 0.002

aMeasurement accuracy ±0. 001 deg.

induced angle errors. These computed angle errors,along with directly measured radiation-induced angleerrors are summarized in Tables 3, 4, and 5. Direct

angle error measurements were accurate to ±0. 001

1328

deg. The computed and measured radiation-inducedangle errors for both ADETs and for the -10 deglamp-on case are plotted in Fig. 8. It is seen in thefigure that for dose rates as much as four times thatexpected at the ADETs during Jupiter encounter, theinduced angle errors are about an order of magnitudeless than the maximum allowed errors. Data for theother lamp-on conditions also satisfy the acceptancecriteria (Tables 4 and 5), and show equally goodagreement between measured and modeled responses.

10-'

Z

z0 1U -

0<

--II ,,I

FOUR TIMES EXPECTED DOSERATE AT DETECTOR ELEMENTSDURING JUPITER ENCOUNTER

/MAXIMUM ALLOWEDERROR

.1

CALCULATED/ --*0-0 ADET I

I Z// ---A ADET 2

/ MEASURED' / *-** ADET

A Ah ADET 2

1 L

2 4 6 8 10

GAMMAO DOSE RATE, rod(St)/s12 14

Fig. 8 ADET Response to Gamma Radiation RateTests, -10 Deg Lamp-On Conditions

The results of the total dose measurements were allwithin the errors of the measurement repeatability ofthe test instrumentation, thus verifying the inferencefrom the 1974 tests that total dose effects in the SSCdS would be negligible.

Acknowledgment

The research described in this paper was carried outat the Jet Propulsion Laboratory, California Instituteof Technology, under NASA Contract No. NAS 7-100.

References

1. T. C. Clarke, "Report on the August 1974 Radia-tion Tests of the MJS '77 Cruise Sun Sensor,JPL Publication EM 343-203, Oct. 21, 1974,Jet Propulsion Laboratory, Pasadena, Calif.(JPL internal document).

2. J. H. Hubbell, "Photon Cross Sections, Attenua-tion Coefficients and Energy Absorption Coeffi-cients from 10 KeV to 100 GeV, " NSRDS-NBS 29,National Bureau of Standards, Washington, D. C.,Aug. 1969.

3. M. J. Berger and S. M. Seltzer, "AdditionalStopping Power and Range Tables for Protons,Mesons, and Electrons, " NASA SP 3036, NationalAeronautics and Space Administration, Washington,D. C., 1966.

4. E. L. Divita, et al., "Jupiter Radiation Belts andTheir Effects on Spacecraft, " Technical Memoran-dum 33-708, Jet Propulsion Laboratory, Pasadena,Calif., Oct. 15, 1974.

5. T. N. Divine, "Jupiter Radiation Belt Models(July 1974), "Technical Memorandum 33-715,Jet Propulsion Laboratory, Pasadena, Calif.,Nov. 15, 1974.

6. C. Kittel, Introduction to Solid State Physics,2nd Edition, Wiley & Sons, New York, 1963,pp. 512-521.

_ _ I _; n-6n1

d