AD-A283 144

DETAILED COMPONENT DESIGN FOR A COMPACTENVIRONMENTAL ANOMALY SENSOR (CEASE)

John 0. McGarity Hugh AndersonAlan C. Huber Douglas PotterJohn A. PantazisDavid Sperry

AMPTEK, INC.6 De Angelo Drive *. PT I CBedford, MA 01730 E CTLE

25 March 1994 -4

•AcP94-23449Scientific Report No. 3 I 11t11111ill 111111

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++ PHILLIPS LABORATORYDirectorate of GeophysicsAIR FORCE MATERIEL COMMANDHANSCOM AFB, MA 01731-3010

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* IDetailed Component Design for a Compact Environmental PE 6341 OFAnomaly Sensor (CEASE) PR 2823 TA 01 WU AC

6. AUTHOR(S)

John 0. McGarity David J. Sperry Contract F19628-90-C-0159Alan C. Huber Hugh AndersonJohn A. Pantazis Douglas Potter

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER

AMPTEK, Inc.6 De Angelo DriveBedford, MA 01730

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Phillips Laboratory29 Randolph RoadHanscom AFB, MA 01731-3010 PL-TR-94-2132

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13. ABSTRACT (Maximum 200 words)

The outer space environment experienced by a modem, electronically sophisticated spacecraft can bevery hostile due to interactions between its complex, sensitive electronics systems and the naturallyoccurring energetic particle population indigenous to the solar system. The Compact EnvironmentalAnomaly Sensor (CEASE) is being developed as a small, low-power device to monitor space "weather"and provide autonomous warnings of conditions that may cause operational anomalies in a hostspacecraft. CEASE uses a two-element solid-state telescope and two radiation dosimeters to samplecritical energetic particle fluxes and uses a sophisticated real-time processing program that canforecast hazardous environmental conditions before they effect the spacecraft. The spacecraft, in turn,can re-prioritize its operations, inhibit any anomaly sensitive operations such as attitude adjustments,or take any other prudent action suggested by the potential of erratic conditions. The CEASEinstrument is forecast to be packaged as a 4 inch cube weighing 2 pounds and dissipating a maximumof 2 watts.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Compact Environmental Anomaly Sensor, CEASE, Surface Charging, 74Deep Dielectric Charging, Single Event Upsets, Radiation Dose Effects. 16. PRICE CODE

117. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION I 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACTOF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified SAR

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)Prescribed bv ANSI Std. Z39-16298-102

CONTENTS1. INTRODUCTION 12. ENVIRONMENTS 3

3. MODELED SENSORS 73.1 Requirements 7

3.1.1 Telescope 7

3.1.2 Dosimeter 7

3.2 Geometrical Design 73.2.1 Coincidence Telescope 83.2.2 Dosimeter 8

3.3 Model Response 103.4 Preliminary Warning Logic 13

3.4.1 SEUs 163.4.2 Dielectric Charging (DDC and SDC) 163.4.3 Dose (Sdose and Tdose) 16

3.4.4 Converting Output to Warning Flags 16

4. HARDWARE 17

4.1 HUTSAT 184.2 STEP 4 18

5. SOFTWARE 19APPENDIX A. MODELED RESPONSE OF INSTRUMENT SUITE IN FIVE ENVIRONMENTS 20

Tables1. - ENVIRONMENTS 32. - ANOMALY CAUSES AND THRESHOLDS 43. CEASE DETECTOR ABSORBER DEPTHS 104. CEASE DETECTOR FORWARD AND REVERSE GEOMETRIC FACTORS 105. COUNTING RATES IN TELESCOPE DUE TO PROTONS PENETRATING THE SIDES 126. COUNTING RATES THROUGH APERTURE OR FRONT (F) AND SHIELD (R) 4.1 G/CM 2 137. COMBINATION OF TELESCOPE BOXES INTO ANOMALY BOXES 148. DOSIMETER BOXES 159. COUNTING RATES IN ANOMALY BOXES 15

Figures1. ELECTRON SPECTRA OF SELECTED ENVIRONMENTS 52. ION SPECTRA OF SELECTED ENVIRONMENTS 63. SOLID STATE TELESCOPE ASSEMBLY 84. DOSIMETER 1 ASSEMBLY - THIN ABSORBER 95. DOSIMETER 2 ASSEMBLY - THICK ABSORBER 96. TELESCOPE RESPONSE TO INNER ZONE PARTICLES ENTERING THE APERTURE 117. CEASE TELESCOPE ANOMALY BOX BOUNDARIES 148. CEASE CONCEPT PACKAGE WITH TELESCOPE AND TWO DOSIMETERS 179. STEP 4 DOSIMETER (REMOTE OPERATION) 1810. STEP 4 MAIN ELECTRONICS BOX 19

-.i Ab Lt . 1JtSf lW1

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1. Introduction

The instrument CEASE (-Compact Environmental Anomaly Sensor) is designed to detectconditions that may cause anomalies, that is, occurrences that may adversely affectspacecraft operation. It will do so before these conditions become so severe that anomaliesactually occur, and will provide warnings or flags on which spacecraft may act to protectthemselves. These anomalies are:

Body char&&in occurs when the spacecraft chassis accumulates electrical charge and thewhole space frame must rise to a significant potential (with respect to the surroundingplasma) in order to null the net current to it. This charging occurs even when the wholespacecraft surface is conducting and electrically connected. Body charging is caused byspacecraft emitting charge, such as in a particle beam, or when they are bombarded bykilovolt electrons such as the auroral electron flux or the hot plasma in GEO. The chargingcan become significant in the absence of neutralizing currents such as cooler plasma orphotoelectrons ejected by sunlight.

Differential charging occurs under the same conditions as body charging if the spacecraftexterior has sections of electrically isolated conductors. As different portions of the surfaceexperience different currents, these separate sections may reach different equilibriumpotentials.

Surface dielectric charging occurs when somewhat penetrating particles bombard exposeddielectrics and stop in them. If the rate of charge buildup exceeds leakage through thedielectric, the resulting charge density may produce a very strong electric field thateventually breaks down the dielectric. Leakage depends upon the dielectric material; hence,charging may be cumulative over minutes up to many hours. Leakage increases asconductivity is enhanced by the effect of the bombarding particles. Consequently, protonsare not very effective in causing dielectric charging because they readily increase dielectricconductivity. In addition, sunlight may help discharge very shallow deposits of charge. Theresult of these factors is that surface charging occurs when the electron flux exceeds athreshold value and sums to a fluence that exceeds a threshold.

Deep dielectric charging is produced in the same way as surface dielectric charging exceptthat the causative electrons have enough energy to penetrate some distance into the dielectric.In some cases they may reach dielectrics that are behind conductors. An example of a burieddielectric is the insulator within a coaxial cable. Obviously, the energy boundary that dividessurface and deep charging particles is somewhat arbitrary. Sunlight does not mitigate deepdielectric charging.

Total radiation dose is measured by the energy deposited in materials by charged particlespassing through them. Dosage results from the totality of ionizing radiation that reaches thesite of interest and is thus strongly affected by surrounding shielding. Sufficiently highlevels can damage most materials, including dielectrics and optical materials. In most casessemiconductors are the most sensitive elements on a spacecraft. Some devices experiencedegradation at tens of kilorads, while radiation hard devices can tolerate 10 to 100 timesmore. Dosage is mostly cumulative, as semiconductors self heal only a little.

Single event upsets (SEUs) result when an ionizing particle leaves enough charge(equivalently energy) in the sensitive volume of a semiconductor device to flip the logical

I

state of the device. Generally, there is no permanent damage. The sensitive volumes are ofmicron size, and the required energy is 1 - 100 MeV depending upon the device. Neitherelectrons or protons have high enough LET (Linear Energy Transfer) to leave so muchenergy in a small volume. Only more highly charged particles such as alphas (in some cases)and Z > 2 particles can do this. However, protons that have nuclear collisions (called starsfrom their appearance in photographic film) in the sensitive volume can break up a nucleusand the resulting fragments leave enough energy to cause SEUs. Protons are so much morenumerous than heavier particles that these nuclear collisions are the principal cause of SEUs.

The strategy that has evolved for a successful CEASE instrument consists of making alimited number of high-energy particle measurements using a pair of dosimeters and a solid-state telescope. We believe that if the right radiation parameters are sampled, then these datamay be interpreted to identify and forecast environmental conditions that may affect theoperation or longevity of a spacecraft. The issue is what parameters to measure with whataccuracy and dynamic range and how to make those measurements.

A review of previous research including experimental measurements and theoreticalmodeling has identified five different environments that are occasionally characterized byextreme conditions that may damage or disrupt spacecraft operations. We have examinedrecords of these environments and used theoretical models to augment existing data bases todevelop spectral characteristics. These data provided the dynamic range and temporalbehavior of the environments. We then knew what and how much was needed to bemeasured and had some idea of how accurate the measurements needed to be. We also usedthis information to create five test vectors (energy/flux models) that could be used toevaluate the performance of our instrument design in each critical environment.

The nature of solid-state detectors and telescopes is that there are possible ambiguities intheir measurements. Discriminating electrons from protons or heavier masses can beimpossible under some circumstances. Particles that stop entirely in the first detector or flythrough both detectors or enter through the sides or back of the telescope confuse simpleinterpretation of the detector outputs. Appropriate selection of absorber thickness, detectorthickness and area, and pulse processing electronics can minimize confusion. Theelectro/mechanical design of the telescope and dosimeter are critical to their usefulness.

A thorough knowledge of the spectral characteristics of the environment may also be used toguide the detector design toward placing regions of possible detector ambiguity at placeswhere the natural environment would not contain components that trigger that uncertainty.For example, we may not be able to distinguish electrons from protons in a certain energyregion, but we know that the natural environment consists of only one of the two species inthat range.

It is also possible to design these ambiguity regions into areas where it is not relevant todiscriminate the triggering event since it either doesn't portend a possible anomaly or in thecase of SEUs (Single Event Upsets) the precise identification of the energetic ion doesn'tmitigate or influence the SEU probability.

Lastly, a knowledge of the natural spectral shape of the environment may allow theidentification of an ambiguous detector response by designing the electro/mechanical systemsuch that naturally occurring events that may not be unambiguously resolved also trigger

2

unambiguous responses at other energies. An intelligent data processing algorithm may bedesigned that integrates this natural spectral knowledge and uses it to interpret detectorresponses. Knowledge of the probable spectral shape of an environment may be used to sumcount rates into quantatized total-dosage estimates without extensive knowledge of theindividual event characteristics.

It is the deductive and selective nature of CEASE that distinguishes it from a scientificexperiment. It is designed to measure and discriminate only those environment parametersthat are needed to forecast potential anomaly conditions.

2. Environments

Table 1 lists five environments. The first three represent extreme cases and use Inner(Trapped Radiation) Zone at the equator for maximum proton intensity, and Outer Zone, alsoat the equator, for maximum energetic electron intensity. Inner and Outer Zoneenvironments include proton and electron intensity from AP8Min and AE6Max/AE8Max atlow/high altitude. Present in the transient in GEO environment are anomalously large solarprotons from CREME, and maximum storm electrons using a bi-maxwellian as listed in theMil Std 1809, Space Environment for USAF Space Vehicles. Of all the environments thesestorm electrons produce the greatest counting rate in the front detector of the telescope if itsabsorber admits >30 keV electrons. The last two environments test the ability of CEASE todetect anomaly-causing conditions. Anomalously large solar protons scaled to the thresholdof threatening flux in the presence of Outer Zone electrons allow us to examine the device'sability to warn of SEUs. Maximum storm electrons scaled to threshold intensity in thepresence of Inner Zone protons verify the ability to warn of dielectric charging. Table 2gives threshold conditions. If need be, other combinations may be used.

Table 1. - Environments

Environment Source Description

Inner Zone AP8Min, AE6Max at equator Represents max energetic protons

Outer Zone APSMin, AE8Max at equator Represents max energetic electrons

Transient in CREME solar protons; Anomalously large solar protons;GEO Magnetic storm electrons from Maximum storm electrons from a bi-

Mil Std 1809 maxwellian

Solar Protons Scaled protons and outer Anomalously large solar protons >20electrons MeV scaled to SEU threshold

intensity; Outer zone electrons

Storm Electrons Scaled electrons and inner Maximum storm electrons >30 keVprotons scaled to dielectric charging

threshold; Inner zone protons

3

Table 2. - Anomaly Causes and Thresholds

Anomaly Cause Threshold

Surface Dielectric Charge Electrons 30 - 250 keV Intensity > 5 x 105/cm2-sec and(SDC)

Deep Dielectric Charge Electrons > 250 keV Fluence > 2 x 101°/cm2 in last 10(DDC) hrs

SEUs Protons > 25 MeV or 5 x 10-5 SEU/bit-day: >120High LET protons or >2.4 x 10- 3/cm2.sec high

LET

Surface Dose (SDose) Protons > 2 MeV 10 krad

Total Dose (TDose) Protons > 20 MeV 10 kradElectrons > 1 MeV

The sources listed in Table I were used to compile electron and proton energy intensity testvectors for each environment. Figures 1 and 2 show the test vectors that are used to validatecandidate detector models. (Please see the next two pages)

4

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3. Modeled Sensors

3.1 Requirements

For all detectors, the maximum counting rate of the detector-preamplifier and logic chainshould be as great as possible to achieve a large dynamic range. This affords maximumsensitivity and response to smaller fluxes. In the geometrical designs, we scaled thegeometric factors to yield 2.5 x 105 counts per second in the most intense environments.

3.1 Telescope

In the telescope, the front detector D1 must be fully depleted to minimize the energy range inwhich electrons and protons are indistinguishable. For the same reason Dl should be as thinas possible consistent with measuring minimum ionizing particles. The rate of energy lossby a minimum ionizing (relativistic) singly charged particle is about 2 MeV per g/cm2. Thisrequires 150 Pg/cm2 or about 65 Pm of silicon to exceed 30 keV energy loss. As a practicalmatter, fully depleted ion implant detectors are available down to only 150 pm thick; surfacebarrier detectors are made much thinner. The back detector D2 should be thicker than 500gpm, preferably substantially thicker, to separate protons from electrons over as wide anenergy range as possible. The areas of Dl and D2 are not as critical because the collimatorand aperture limit entrance of particles. However, their respons t to particles scattering andpenetrating the shield on the back and sides depends on size. It is of advantage to mount Dland D2 as close as possible to minimize their sizes.

There is no particular reason to push the minimum energy-loss threshold as low as possible.The detector-preamp noise-counting rate above this threshold must remain insignificantthroughout the life of a CEASE instrument. As the instrument may be stored for extendedperiods, and will not be cooled or closely temperature controlled, we choose a 30 keV energyloss (pulse-height) threshold.

3.1.2 Dosimeter

The size of the detector in the dosimeter is critical as there is no aperture. The sensitivity toincident intensity depends therefore-upon area and volume. It is necessary to know the totalsensitive volume accurately in order to convert energy loss to rad-Si.

3.2 Geometrical Design

We have modeled geometrical designs of the telescope and dosimeter includingrecommended detector types. We choose the telescope aperture and dosimeter detector sizesso that in the most extreme environment counting rates do not exceed 250 kHz. These sizesmay change if tests of the prototype show that a different maximum rate is appropriate or toadapt CEASE to various missions that encounter differing environments.

7

3.21 Coincidence Telscope

Figure 3 shows the design for the coincidence telescope. It has a 1.5 cm thick (4.1 g/cm2)aluminum shield and collimator that accepts particles from 90° field of view. Following is asmall aperture and thin absorber before the detectors D I and D2. We intend that the front beremovable so that the aperture size and absorber may be changed. We have carried outcalculations for two absorber thicknesses, thick and thin. CEASE will use only onetelescope with an absorber between these two limits. With thicker absorber, the maximumrate due to storm electrons is less, which allows a larger aperture. Both detectors are 1502

mm area and 150 and 700 pm thick, respectively. If subsequent analysis shows that athicker D2 is required, we can stack these detectors.

stacionaColimaterAperture

A=fInsulation Except for absorber and detectors, all paets

phave mnterial removed from center.e r 1 All dimensions are m centin eters.

SpacerInetector2SShield 1,84 DIA - 5.4-

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Figure 3. Solid State Telescope Assembly

3.2.2 Dosimeter

The dosimeters use a single solid state detector close behind a planar absorber with 1.5 cm ofaluminum surrounding the rest of the dosimeter. The detector has -170* FOV through theabsorber. Dosimeters #1 and 2 have absorbers 0.43 and 1.7 g/cm 2 thick respectively, whichcorresponds to electron/proton minimum energies of 1.1/18 MeV and 3.2/37 MeV. Figure 4shows the design for dosimeter 1, with the thin absorber and smaller detector. Figure 5 isDosimeter 2 with the thicker absorber and large detector. CEASE uses both dosimeters.

8

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Figure 5. Dosimeter 2 Assembly - Thick Absorber

If counting rates with these detectors prove too high, smaller areas can be made on specialorder. Table 3 shows the absorber cross section thicknesses. Table 4 tabulates the geometricfactors of all of the sensors.

9

Table 3. CEASE Detector Absorber Depths

Device Part DecriptionDosimeter I Abs: 430 mg,/m2 Al

Det I 115 mg/can SiDosimeter 2 Abs: 1.7 g/cmn2 AI

Det 1 115 mg/cm 2 Si

Thick SST Abs: 13.5 mg/cm2 AlDet 1 34.5 mg/cm2 SiDet 2 161 mg/cOn2 Si

Thin SST Abs: 1.6 mg/=? AlDet 1 34.5 mg/cm 2 Si

Det 2 161 mg/cm 2 Si

Table 4. CEASE Detector Forward and Reverse Geometric Factors

Device Dir Area Opening Geometric Factor(cm2) Angle (deg) (cmz-sr)

Dosimeter I F 0.25 170 0.7794R 0.25 170 0.7794

Dosimeter 2 F 2 170 6.2355R 2 170 6.2355

Thick SST F 0.016 90 0.0251R 2x1.5 90 2x2.3562

Thin SST F 0.0016 90 0.0025R 2x1.5 90 2x2.3562

Note that the aperture for the thick SST has ten time the area of the thin SST. The geometricfactors of the two reflect that ratio, and this results in the counting rates that are tabulated onTable 6. Under some circumstances the thicker absorber results in higher count rates, whichat first seems counter-intuitive until the larger aperture area is considered.

3.3 Model Response

From the five standard environments and the geometries listed earlier in this report, theprogram detresp computes the energy loss in the detectors from range-energy tables forprotons and Integrated detresp TIGER Series (ITS) Monte Carlo (See Appendix A) routinesfor electrons. The detresp program takes account of incoming intensity, geometric factor,and the effect of oblique transmission through the instruments. Briefly, detresp calculatesthe energy loss per unit energy interval in each detector for the given environment.

10

Figure 6 is the result for the telescope from Inner Zone electrons and protons entering thefront aperture. The two axes are the energy losses (pulse heights) in the two detectors. Theshading represents the amount of response according to the scale on the right. Starting withlow incident energy, the protons register in the lower left and with increasing incident energyextend to the right along the bottom of the box until they penetrate DI and start producingresponse in D2. The response then rises in D2 while slightly dropping in DI until theprotons penetrate D2. Then, the response drops in both detectors as it merges into theelectron "blob." To some extent, electrons follow the same pattern except the response islower and considerably blurred by energy straggling and angular scattering.

Pulse Height CoincidenceInner Zone, L=1.4-1.8

105 . ... I I . "'

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The text in the upper left describes the device. The grid superimposed on DI-D2 space arethe boxes that correspond to where we set electronic thresholds in the detector system andthe count rates that accumulate in each box. The grid also shows the values of thethresholds. Appendix A contains is the complete set of these plots along with plots that showthe count rate from electrons and protons in each box.

We assume that particles with pulse height below 30 keV are not detected. Hence, the box inthe lower left comer has no corresponding counting channel. The boxes along the bottom of

11

the plot correspond to count with anti-coincidence in D2. The boxes along the left are anti-coincidence with DI.

Figure 7 shows the boxes along with the range of incident energies in each box. The valueof the energy range for electrons is not precise because the electrons of a given input energygo into several boxes. Hence, there are boxes that clearly have electron response that do nothave an energy range in Figure 7. Appendix A shows these thresholds for both telescopecombinations. The detresp program also calculates the response independently in thedetectors.

For each device and particle input spectra, we calculate both forward through the primaryabsorber and backwards through the shield. Forward particles are those that enter throughthe telescope aperture and collimator. Table 6 shows the total rates for each detector.

The dosimeters have planar absorbers and accept particles from a cone of -l170* full openingangle. Their effective geometric factor are therefore -n- x (area of detector). (It is it, not 2w,because integration is carried out with the projected area of the flat detector.)

An aluminum shield surrounds the back and sides of the dosimeters and telescope; the sameshield covers the front of the telescope except in the collimator. Using iTs, we ran electronsthrough various thicknesses of aluminum. At 1.5 cm (4.1 g/cm2), essentially no electronspenetrate the detector. Thus, we chose that thickness for the shield of the telescope. Thepenetrating protons within an acceptance angle of somewhat more than 900 can producecoincidences whether moving forward or backward. The geometric factor for such particlesis the area of the detector times the solid angle. The factor x2 accounts for both forward andbackward motion through the shield. This mechanism enhances the telescope's sensitivity toprotons that cause SEUs. In addition, protons can enter the shield on the sides but notproduce coincidences.

We ran each environment from the back of the devices through the shield. Over a 900 FOV,detresp calculates the total counting rate in each detector as well as the pulse heightdistribution, which gives the count rate in each box.

Table 6 shows the total counting rates in the detectors that result from each environmententering the front aperture of the instrument and also penetrating the shield. In the case ofthe dosimeter, this calculation includes the entire shield. For the telescope it is protons fromdirections that can cause coincidence. Table 5 shows the counting rates in the individualtelescope detectors caused by protons from the side. These numbers are estimated from thearea of the detectors and the integral proton fluxes and are not calculated by detresp; noenergy loss distribution is available. To suppress accidental coincidences, the CEASEcoincidence time gates should be as small as possible.

In the geometrical designs the apertures and dosimeter sizes are adjusted to yield a maximumcounting rate of 2.5 x 105/sec. The counting rates from particles penetrating the shield donot change as the telescope aperture changes, but do scale with detector size. We can makemore detailed calculations of the shield's effect after the prototype is built and calibrated in aparticle beam.

12

Table 5. Counting Rates in telescope due to protons penetrating the sides

IDetector Inner Outer Trans GEO Solar Protons IStorm ElectronsSST DI 8 x 103 40 119 8 x 101 8 x 103SSTD2 9 x 10 45 135 9 x 103 9 x 10i

Table 6. Counting Rates through aperture or front (F) and shield (R) 4.1 g/cm2

Device Det Dir Inner Outer Trans Solar StormGEO Protons Electrons

Dosimeter 1 F 13,551 52,753 8,455 52,778 4,0021

R 986 0 1,285 3 986Dosimeter I F 12,190 3,438 24,877 3,508 12,072

2

R 7,890 0 10,278 27 7,890

Thick SST 1 F 234,821 59,330 231,646 59,302 1,831R 7,516 94 11,657 125 7,503

2 F 60,717 25,655 2,115 25,658 1,282R 7,660 201 12,171 234 7,632

Thin SST 1 F 48,667 14,092 360,557 11,504 372R 7,516 94 11,657 125 7,503

2 F 8,597 3,139 709 3,140 141R 7,660 201 12,171 234 7,632

3.4 Preliminary Warning Logic

We combine the boxes into anomaly boxes that warn of various threats. For the telescopeand referring to Figure 7, we number the boxes from the lower left, with the lower left boxbeing (0,0), the box above it being (0,1), and so forth. Table 7 shows the makeup of thetelescope anomaly boxes. For the dosimeters, the figures in Appendix A show the boxes,which number from the bottom with 0 being the box below 30 keV. Table 8 shows themakeup of the dosimeter anomaly boxes.

13

Table 7. Combination of Telescope Boxes into Anomaly Boxes

(Electron energies are approximate.)

Anomaly Boxes Detected Particles

SDC* (1,0)+(2,0)+(3,0) 30-250 keV electrons

DDC (1,1)+(l,2)+(l,3)+(2,1)+(3,0) >250 keV electrons+(3,l)+(3,2)+(4,0)

SEUs (4,4)+(0,4) >25 MeV protons

SDose, protons (5,0)+(5,4)+(6,0)+(6,1)+(6,2) 2-25 MeV protons+(6,3)+(6,4)

* With thicker absorber, electrons below 100 keVare not admitted Very shallow surface

charging would not be sensed.

Energy Thresholds

Threshold: 0.01/MeV2

10 . 61 6 31

5 0 2o 4 2 4 I4

4

104 Pulse

Height 2 1"21

(keV) 310 __._

2 210.o

21 10

31.2

00 1 2 3

10 10 10 10

Pulse Height 1 (keV)

Bin Number 0 1 2 3 4 5 6 7

Figure 7. CEASE Telescope Anomaly Box Boundaries

The particle energies and types match the discussion of anomalies at the beginning of thisreport. Note that with the thicker absorber, essentially no electrons below 100 keV aredetected, so that some surface dielectric charging is not sensed. Similarly, the absorbers onthe two dosimeters preclude measuring electrons that cause surface dielectric charging.Table 9 shows count rates in the anomaly boxes from the five environments.

14

The SEU channel responds only to penetrating protons. In the outer zone, there is no SEUresponse. In the presence of threshold SEU fluxes (Solar Protons), there is sufficientresponse in the SEU channel to detect. Note that several environments cause SEUs anddielectric charging. Table 8 tabulates the anomaly derivation from the Dosimeter data.

For the charging channels, we cannot distinguish between protons and electrons that stop inDI. The electrons that stop there are significant for charging and cannot be ignored. TheStorm Electron spectrum is threshold electrons combined with high proton flux and, hence,corresponds to the worst case for detecting charging. For the thick SST, of the 60 DDCcounts, 31 are from protons; of the 122 SDC counts, 13 are from protons. For the thin SST 6of 22 counts in DDC and 2 of 173 counts are from protons. Thus, even under the worstconditions, we can determine threshold charging.

Table 8. Dosimeter Boxes

Anomaly Boxes Detected Particles

DDC 1 Electrons >1.1or3.7 MeV

SEUs 3 Protons and Heavier

TDose 1+2+3 All particles

Table 9. Counting Rates in Anomaly Boxes

Anomaly Device Inner Outer Trans Solar StormGEO Protons Electrons

DDC Dosimeter 1 694 3,551 8 3,551 5

Dosimeter 2 75 600 0 600 0

Thick SST 135,697 32,977 60,203 32,965 60

Thin SST 24,716 7,073 34,052 5,261 22

TDose Dosimeter 1 14,537 52,753 9,739 52,781 4,989

Dosimeter 2 20,080 3,438 35,155 3,536 19,962

SDC Thick SST 171,715 33,412 229,298 33,406 122

Thin SST 39,884 9,929 359,790 8,448 173

SDose Thick SST 2,027 24 2,504 13 2,024

Thin SST 643 297 1,773 6 643

SEUs Dosimeter 1 1,383 0 2,730 8 1,383

Dosimeter 2 2,775 0 7,817 23 2,775

Thick SST 1,638 1 4,649 15 1,638

Thin SST 1,566 0 4,470 13 1,566

15

For each anomaly, we can now construct a threshold value that corresponds to a thresholdthreat and a means of determining how much the threat from that anomaly exceeds thatthreshold.

3.4.1 SEUs

Recalling that we constructed the Solar Protons spectrum to correspond to thresholdconditions for SEUs, we set the threshold levels in each device to the values for SEUs underSolar Protons in Table 6.

3.4.2 Didkctric Charging (DDC and SDOWe constructed the Storm Electron spectrum to correspond to threshold dielectric chargingfor both Deep (DDC) and Surface (SDC) Dielectric charging. Therefore, the thresholdlevels in each device are the values for DDC and SDC under Storm Electrons in Table 6.

As Table 2 indicates, dielectric charging requires both that the electron intensity exceed athreshold and that the fluence accumulated at these intensities in the preceding -10 hours(4 x 10 s) exceed the threshold intensity times 4 x 10 s. Thus, we have a threshold for bothfluence and flux.

3.4.3 Dose (Sdose and Tdose)

The relationship between count rate and dose is

Dose Rate = 2 x 10- 3 rads/hr (R( EeV)0.03 g) (1)

where R is the count rate, E is the mean energy-loss of particles (or energy loss of eachparticle that could be measured), m is the mass of the detector, which is 0.03 g for Dosimeter1, 0.23 g for Dosimeter 2, 0.05 g for telescope DI, and 0.24 g for telescope D2.

Hence, we get different doses for each detector. For each box, we multiply counts by thegeometric mean pulse height in the box, sum these, and multiply by the detector mass to getthe dose.

3.4.4 Convetng Output to Warning FlagsWe define a base level for each anomaly at 10 ½2times the threshold level. If we use a 4 bitflag for each value, we compute the 4 bit flag by 2 logI0(level/base level). Thus, one flagoccupies one-half byte and a value of two corresponds to the threshold level.

This leads to a total of 15 flags, two for dose for each of the dosimeters, two for surface dosein the two detectors in the telescope, three for all devices times two (flux and fluence) fordeep dielectric charging, two for surface dielectric charging from the telescope, and three forSEUs from each device. These 15 flags should be available to all clients on request.

However, most clients won't want that much information on threats. We want to reduce theflags to five summary flags to correspond to the five threats. The final determination on howto do this must await some experience during testing, calibration, and ultimately operation.

16

For the summary SEU and dose (TDose and SDose) flags, our initial cut is simply to averagethe individual parts. For charging, we must deal with flux vs. fluence. We suggest usingprimarily the flux flag. If the fluence flag is less than threshold (2), subtract 2 and add thefluence flag:

' JK >_2Charging Flag (SDC, DDC)-- jJ-2 +K 2 (2)

where J is the flux flag and K is the fluence flag. It would seem like a good idea todisseminate time averages of these flags over the last hour or so.

An operational plan for CEASE should include a plan on how to disseminate these flags aswell as plans for interrupts and other data. Alsoit can further address how to combine flagsas well as their definition.

4. Hardware

The baseline hardware that was projected for CEASE in the second annual report (ReportPL-TR-92-2307;ADA261729) is still valid. Figure 8 depicts the current configurationincluding the two dosimeter sites. Note that the telescope has a collimator (900) built intothe housing.

•.---.8dd SODl TnCOM•

4.0(r

Figure 8. CEASE Concept Package with Telescope and Two Dosimeters

The current designed dimensions of the dosimeters and solid state telescope allow bothmounting and are within the weight budget. We are working toward hybridizing theelectronics to accommodate this enclosure size.

17

4.1 HUTSAT

The Helsinki University of Technology is assembling a small satellite, HUTSAT. Its smallsize demands tight experiment constraints of power and volume. The CEASE Dosimeter hassimilar constraints, and its development and fabrication schedule parallel the HUTSATschedule. It may be possible to use the HUTSAT as a test bed for the CEASE dosimeter.This possibility is being explored. At present, little activity is occurring as we awaitactivation of the Finish portion of the program.

4.2 STEP 4

STEP 4 is one of a series of Air Force Satellites design as test beds for small satellitetechnology. Amptek, Inc. is presently developing designs for an instrument on STEP 4which will use a CEASE type dosimeter and a microproceessing system patterned after theCEASE hybrid architecture. We have provided preliminary mechanical housing drawings,weight and power estimates. TRW is the integrating contractor. Figures 9 and 10 depict thecurrent STEP 4 design envelopes.

LZQ] 1.00

.500

2.00

3.00

STEP 4 Dosimeter

Figure 9. STEP 4 Dosimeter (Remote operation)

18

.50 5.00 .50

r-6&00 -

4.00

oC5o J3.(=. JZ

H CD. J1 -

Conltrolfricpu Assenfl VUth RPXS .2_

Figure 10. STEP 4 Main Electronics Box

The above drawings are preliminary, but are providing base lining for TRW engineers.

Specifics of projected design are:Dosimeter Box 0.75 poundsDrift meters and electronics 3.62Cable 0.58Engineering margin 0.55

5. Software

Most of the software work this year has been targeted toward developing and testingalgorithms to convert CEASE diagnostics data into warning flags and hazard notices. Somethought has been given to optional data packets that contain scientific data as well as thehazard alerts. This feature would make in-flight testing of the CEASE much easier and moreaccurate.

19

ApRendix A. Modeled Response of Instrument Suite in Five Environments

Pulse height coincidence and bin accumulation for the thick and thin absorbertelescope for three environments, both forward and reverse propagation.

hImer Zone, L=1.4-1.8 Forward Thick SST Pulse Height Coincidence 22Reverse Thick SST Pulse Height Coincidence 23

Outer Zone, L=4-6, QuietForward Thick SST Pulse Height Coincidence 24Reverse Thick SST Pulse Height Coincidence 25

Outer Zone and GEO, Transient Forward Thick SST Pulse Height Coincidence 26Reverse Thick SST Pulse Height Coincidence 27

Inner Zone, L=1.4-1.8 Forward Thick SST Bin Accumulation 28Reverse Thick SST Bin Accumulation 29

Outer Zone, L=4-6, Quiet Forward Thick SST Bin Accumulation 30Reverse Thick SST Bin Accumulation 31

Outer Zone and GEO, Transient Forward Thick SST Bin Accumulation 32Reverse Thick SST Bin Accumulation 33

Inner Zone, L= 1.4-1.8 Forward Thin SST Pulse Height Coincidence 34Reverse Thin SST Pulse Height Coincidence 35

Outer Zone, L=4-6, Quiet Forward Thin SST Pulse Height Coincidence 36Reverse Thin SST Pulse Height Coincidence 37

Outer Zone and GEO, Transient Forward Thin SST Pulse Height Coincidence 38Reverse Thin SST Pulse Height Coincidence 39

Inner Zone, L=1.4-1.8 Forward Thin SST Bin Accumulation 40Reverse Thin SST Bin Accumulation 41

Outer Zone, L=4-6, Qumt Forward Thin SST Bin Accumulation 42Reverse Thin SST Bin Accumulation 43

Outer Zone and GEO, Transient Forward Thin SST Bin Accumulation 44Reverse Thin SST Bin Accumulation 45

CEASE Solid State Telescope Energy Thresholds, Forward and Reverse; two absorberthicknesses.

Energy Thresholds Forward Thick SST 46Reverse Thick SST 47

Energy Thresholds Forward Thin SST 48ReverseThin SST 49

CEASE Dosimeter Response Figures, five environments, both forward and reversepropagation

Inner Zone, L=1.4-1.8. Forward Dosimeter 1 (thin absorber) Response 50Reverse Dosimeter I (thin absorber) Response 51

Outer Zone, L--4-6, Quiet Forward Dosimeter 1 (thin absorber) Response 52Reverse Dosimeter I (thin absorber) Response 53

Outer Zone and GEO, Transient Forward Dosimeter I (thin absorber) Response 54Reverse Dosimeter I (thin absorber) Response 55

Solar Protons Anomalous Forward Dosimeter I (thin absorber) Response 56Reverse Dosimeter I (thin absorber) Response 57

Scaled Storm Electrons Forward Dosimeter I (thin absorber) Response 58Reverse Dosimeter I (thin absorber) Response 59

Inner Zone, L=1.4-1.8 Forward Dosimeter I (thick absorber) Response 60Reverse Dosimeter 1 (thick absorber) Response 61

Outer Zone, L-4-6, Quiet Forward Dosimeter I (thick absorber) Response 62

20

Reverse Dosimeter I (thick absorber) Response 63Outer Zone and GEO, Transient Forward Dosimeter 1 (thick absorber) Response 64

Reverse Dosimeter I (thick absorber) Response 65Solar Protons Anomalous Forward Dosimeter I (thick absorber) Response 66

Reverse Dosimeter I (thick absorber) Response 67Scaled Storm Electrons Forward Dosimeter I (thick absorber) Response 68

Reverse Dosimeter I (thick absorber) Response 69

21

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