The linear energy transfer (LET) spectra measured by plastic (CR-39) detectors in Exp. P0006 on LDEF are much higher at high LET than expected from methods commonly used to predict LET spectra produced by the space ionizing radiation environment. This discrepancy is being investigated by examining modeling approximations used in the predictions, and some interim results are presented

Armstrong, T. W.

Benton, E. V.

Colborn, B. L.

English

NASA Technical Reports Server

N95- 23815
PREDICTIONS OF LET SPECTRA MEASURED ON LDEF
T. W. Armstrone and B. L. Colborn
Science Applications International Corporation 1
Route 2, Prospect, TN 38477
Phone: 615/468-2603, Fax: 615/468-2676
E. V. Benton
Physics Research Laboratory, University of San Francisco 1
2130 Fulton St., San Francisco, CA 94117-1080
Phone: 415/666-6281, Fax: 415/666/2469
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ABSTRACT
The linear energy transfer (LET) spectra measured by plastic (CR-39) detectors in Exp. P0006 on
LDEF are much higher at high LET than expected from methods commonly used to predict LET spectra
produced by the space ionizing radiation environment. This discrepancy is being investigated by
examining modeling approximations used in the predictions, and some interim results are presented.
INTRODUCTION
The P0006 Experiment on LDEF (re£ 1) contained plastic detectors (CR-39) for measuring linear
energy transfer (LET) spectra. Analyses of these data reported to date, Benton, et al. (ref. 2), show
observed spectra that are quite different than expected from commonly-used LET prediction methods.
Since LET spectra are fundamental in predicting a variety of radiation effects of practical importance (e.g.,
biological damage, electronics upsets) in spacecraft and mission design, it is important to investigate the
reason for this discrepancy, and reported here are some interim results of such work.
The problem addressed is illustrated by Fig. 1. Shown here is the measured LET spectrum (re£2)
in one of the CR-39 sheets located 6.5 g/cm 2 from the space end of the main detector stack in the P0006
experiment. Also shown is a pre-recovery LET prediction made by Derrickson (ref. 3) using the NRL
CREME code of Adams (ref. 4), which is commonly used for predicting LET spectra in performing
assessments of space radiation effects on microelectronics. Since this pre-recovery prediction was of a
scoping nature to obtain a quick estimate, several approximations were involved -- e.g.: (a) the spacecraft
and detector shielding is approximated as an aluminum sphere, (b) the calculated LET spectra are for
silicon, whereas the CR-39 data have been converted to LET in water, (c) the calculated spectra are for the
space environment at the LDEF insertion altitude and not averaged over the LDEF mission, and (d) the
calculation neglects the effects of secondary particles created in the detector and spacecraft, including both
"projectile fragments" (secondaries from the breakup of incident ions during nuclear collisions) and
"target fragments" (residual nuclei and secondary particles from collisions with detector material nuclei).
Discussed below are calculations which remove some (but not all) of the approximations in the pre-
recovery LET predictions.
lWork supported by NASA Marshall Space Flight Center, Huntsville, Alabama
181
https://ntrs.nasa.gov/search.jsp?R=19950017395 2020-06-16T08:32:22+00:00Z
LET PREDICTIONS
ShieldingEffects
Sincea detailed3-Dmassmodelof theLDEFspacecraft,experimentrayF2 contentscontainingthe
P00_ experiment,andtheP0006detectorstackhasbeendeveloped(ref. 5) for LDEF radiationanalyses,
theeffectsof shieldingon theLET spectrapredictionsCanbetreatedaccurately.Therefore,theLET
spectrumat apointin thecenterof theCR-39layercorrespondingto thelocationof themeasured
spectrumhasbeencalculatedusingtheLDEF3-Dshieldingmodel. Radiationtransportcalculationswere
madefor shieldingin eachof 720solidanglebinsaroundthedetectorpoint. A simplifiedrepresentation
of theshieldingdistributionis shownin Fig.2. Thetransportcalculationsalongeachshieldingdirection
weremadeusingtheBurrell transportcode(ref.6) for incidenttrappedprotonsandthe CREMEcode
(ref. 4) for galacticprotonsandheavyions. TheLDEFexposureto trappedprotonspredictedby Watts,
etal. (ref. 7) wasused,whichtakesintoaccountthetrappedprotonanisotropyaswell asaltitudeand
solarcyclevariationsduringtheLDEFmission.Incidentgalacticcosmicrayspectrafor theLDEForbit
werecalculatedusingtheCREMEcode.AveragegalacticspectraoverLDEFaltitudeandsolarcycle
variationswerecomputed,but theaverageresultsarenotsignificantlydifferentfrom thesolarminimum
spectraat theLDEFinsertionaltitudeassumedin thepre-recoverypredictions,asillustratedin Fig. 3for
protons.TheLET spectrumin wateris calculatedto correspondto thedata,asopposedto LET in silicon
for thepre-recoverypredictionof Fig. 1.
Resultsfrom thiscalculationarecomparedwith measurementsin Fig. 4. Thereis some
improvementcomparedto Fig. 1whenshieldingeffectsaretakenintoaccount,but thelargedifference
for thehigh-LET"tail" (_>1500MeV. cm2/g)still exists.Thedifferenceatlow LET (5300MeV. cm2/g)
isunderstandablebecauseof theinherentinsensitivityof CR-39atlow LET andbecauseof theparticular
etchingprocessused.Thus,theCR-39hasvery low detectionefficiencyfor trappedprotons.This is
illustratedin Fig. 5, which is thesameasFig.4 butindicatesthepredictedtrappedprotonandgalactic
components.
SEPIron Contribution
Frommeasurementsmadeby theHIISexperimentof Adams,et al.onLDEF, it wasfoundthatthe
largesolarenergeticparticle(SEP)eventsduringOct.1989madealargecontributionto theobservediron
spectrain theenergyrangefrom -- 200-800MeV/nucleon(ref. 8). Sinceiron_>350MeV/ncanpenetrate
the6.5g/cm2 minimumshieldingof theCR-39layerof interestin Exp.P0006,andsincetheLET
calculationsaboveneglectSEPevents,wehavecheckedthecontributionof SEPiron to theLET.
ThesecalculationsweremadebymodifyingtheCREMEcodeto incorporatetheFespectra
measuredby HIIS onLDEF. LET spectraarecomparedin Fig.6 with andwithout theSEPiron
included.TheseresultsshowthatSEPiron makessomecontributionathighLET, butnotnearlyenough
to accountfor thepredictedvs.observeddiscrepancyin Exp.P0006.
Contributionof HeavyIon Fragmentation
To checkthecontributionathighLET from secondaryparticlesgeneratedwhenincidentheavyions
breakupinto lower-Zionsdueto nuclearcollisions,theUPROPcodeof Letaw(ref. 9) wasused.This
codeaccountsfor theproductionandsubsequenttransportof all secondaryparticlesfrom ion breakupin
nuclearcollisions. Theresultsof thiscalculation(madefor asphericalaluminumshield)showthat,even
for thecaseof ratherthickshielding(50g/cm2),thesecondariesfrom ion fragmentationdonot
significantlyincreasetheLET spectrum(Fig.7).
182
SUMMARY
TheLET calculationsdescribedaboveremovesomeof theapproximationsmadein initial, pre-
recoverypredictions,but theydonotexplainthelargedifferenceat highLET betweenpredictionsand
measuredspectrafor Exp.P0006.Thecalculationsto datehavenot takenintoaccounttargetnuclei
fragmentsandelasticrecoilsfrom nuclearcollisionsproducedby trappedprotons,which issuspectedas
beingthemostlikely causeof the largeunderpredictionathighLET.
To accountfor theeffectsof nuclearinteractionproductsfrom trappedprotoncollisionswith the
CR-39constituents,amoredetailedradiationtransportcalculationis requiredthanpossiblewith the
codesusedfor theabovepredictions.A calculationalapproachfor accuratelysimulatingtheCR-39
measurementsi underdevelopment,but resultsarenotyetavailable.Theapproachconsistsof two steps
in theradiationtransport:First, thetrappedprotonflux in thedetectoriscomputedusingastandard
protontransportcode(e.g.,ref. 6) andthe3-DLDEF spacecraft/detectormodel. Thisprocedure,which
hasbeenusedextensivelyfor doseandactivationpredictionsto comparewith LDEF data(e.g.,ref. 10),
takesintoaccountthetrappedprotondirectionalityandaccuratelytreatsshieldingeffects. In thesecond
step,theprotonflux in theCR-39layeris usedasthesourcefor a3-DMonteCarlotransportwithin the
dosimeter.A modifiedversionof theHETCcode(ref. 11)canbeusedfor theMonteCarlocalculationto
takeintoaccountheproductionandtransportof nuclearrecoilsandsecondaryparticlesin thedetector
region.
183
REFERENCES
.
.
.
.
,
.
.
.
.
10.
11.
Benton, Eugene V. and Parnell, Thomas A.: Linear Energy Transfer Spectrum Measurement
Experiment (P0006): in The Long Duration Exposure Facility (LDEF) Mission 1 Experiments,
Clark, Lenwood G. et al. (Eds), NASA SP-473, 1984.
Benton, E. V.; Csige, I.; Oda, K.; Henke, R. P.; Frank, A. L.; Benton, E. R.; Frigo, L.A.;
Parnell, T. A.; Watts, J. W., Jr. and Derrickson, J. H.: LET Spectra Measurements of Charged
Particles in the P0006 Experiment on LDEF, Second LDEF Post-Retrieval Symposium, NASA
CP-3194, 1993.
Derrickson, J. H.: Linear Energy Transfer for the LDEF Mission: in Ionizing Radiation Exposure
of LDEF (Pre-Recovery Estimates), E. V. Benton, et al. (Eds), Nucl. Tracks Radiat. Meas. 20,
No. 1, 75 (1992).
Adams, James H., Jr.: Cosmic Ray Effects on Microelectronics, Part IV. NRL Memorandum
Report 5901, December 1986.
Colborn, B. L. and Armstrong, T. W.: Development and Application of a 3-D Geometry/Mass
Model for LDEF Satellite Ionizing Radiation Assessments. Second LDEF Post-Retrieval
Symposium, NASA CP-3194, 1993.
Bun'ell, M.O.: The Calculation of Proton Penetration and Dose Rates. George C. Marshall Space
Flight Center, Huntsville, AL., NASA TM X-53063, August 1964.
Watts, J. W., Jr.; Armstrong, T. W. and Colborn, B. L.: Revised Predictions of LDEF Exposure
to Trapped Protons. Second LDEF Post-Retrieval Symposium, NASA CP-3194, 1993.
Adams, J. H., Jr.; Beahm, L. P.; Boberg, P. R.; Kleis, T. and Tylka, A. J.: HIIS Observations
During the Large Energetic Particle Events of October 1989 (preprint).
Letaw, John R.: UPROP: A Heavy-Ion Propagation Code. Severn Communications Corp. Report
SCC 89-02, August 1989.
Armstrong, T. W.; Colborn, B. L.; Harmon, B. A.; Parnell, T. A.; Watts, J. W., Jr. and Benton,
E.V.: Comparison of Model Predictions with LDEF Satellite Radiation Measurements. Adv.
Space Res. 14, No. 10, 17 (1994).
Armstrong, T. W. and Colborn, B. L.: A Thick-Target Radiation Transport Code for Low-Mass
Heavy Ion Beams, HETC/LHI. NucL Instr. Meth. 169, 161 (1980).
184
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Fig. 1. Comparison of LET spectra measured (ref. 2) by LDEF Exp. P0006
plastic track detector with pre-recovery predictions (ref. 3).
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20 30 40 50 60 70
Shielding (g/cm 2)
Shielding distribution for P0006 detector.
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186
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LET [MeV/(g/cm2)]
Fig. 4. Predicted LET using 3-D shielding model.
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Fig. 5. Contribution of trapped protons to LET.
Predicted, Trapped Protons + GCR
f • including 3-D ShieldingforLDEF Spacecraft and Detector
"'""•.... • nuclear recoils neglected
o
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187
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108 .I= ,, ,,,,,,,, ,,,,,,_! ........ ! ........ ! I, : ...... I ....... n
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LET (MeV cm2/mg)
Fig. 6. Influence of SEP iron ions on LET.
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Fig. 7.
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28.5 °, 500km x 500km, solar min
Without
Projectile
Fragmentation
With
Projectile
Fragmentation
10 .2 10-1 10 0 101 10 z
LET (MeV cm2/mg)
Influence of ion ("projectile") fragmentation on LET.
10 3
188


Predictions of LET spectra measured on LDEF

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