the nsbri/apl neutron energy spectrometer · in 1997, nasa founded the national space bio-medical...

56 JohnshopkinsApLTechnicALDigesT,VoLume27,number1(2006)

r. h. mAurer et al.

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Thensbri/ApLneutronenergyspectrometer

Richard H. Maurer, David R. Roth, James D. Kinnison, Dennis K. Haggerty, and John O. Goldsten

ong-term interplanetary space travel and habitation produce enhanced risks toastronautsasaresultofthehigh-energycosmicraysandprotonsunmitigatedbyanythickplanetaryatmospheresandprotectivemagneticfieldssuchasthoseofearth.Thehigh-energycollisionsofthesecosmicraysandprotonswiththickstructuresandthinatmo-spheresproduceasecondary“beam”ofchargedparticlefragmentsandneutronsthataremorenumerousthantheincidentprimaryparticles.since1997,ApLhasbeendevelopingacompact,real-timeneutronenergyspectrometertohelpmonitortheradiationenviron-mentinsideinterplanetarytransportvehiclesandonplanetarysurfaces.

BACKGROUNDin 1997, nAsA founded the national space bio-

medicalresearchinstitute(nsbri),1aconsortiumof12institutions,tostudyhumanhealthproblemsasso-ciated with prolonged space travel and countermea-sures to those risks.ApLandthehopkinsschoolofmedicinearechartermembersofnsbri.TheinstituteworkedwithnAsAonthebioastronauticsroadmap2thatidentifiesabout55criticalrisksofspaceflightandcategorizestherisksinto12riskareas.ApLhasbeeninvolved in research in a number of these risk areasassociatedwith longspaceflights includingbone loss3andotherphysiologicalchanges.4Thisarticlediscussesanother problem for human space travel—radiationeffects—andApL’srole intheevolutionofaneutronenergyspectrometertomonitortheradiationenviron-mentlikelytobeexperiencedbyhumanswithintrans-portvehiclesandonotherplanetarysurfaces.

SCOPE OF INVESTIGATIONhigh-energychargedparticlesofextra-galactic,galac-

tic,andsolarorigincollidewithspacecraftstructuresinearth orbit outside the atmosphere and in interplan-etary travel beyond the earth’s magnetosphere. Theseprimaryparticlescreateanumberofsecondaryparticlesinsidethestructuresthatcanproduceasignificantion-izingradiationenvironment.Thisradiationisathreatto astronauts and others on long-term space missionsandproducesanincreasedriskofcarcinogenesis,acuteandlatecentralnervoussystemrisks,DnAdamage,andchronicanddegenerativetissuerisks.2,5specifically,theprimary high-energy cosmic rays and trapped protonscollidewithspacecraftmaterialssuchasaluminumandsilicon to create secondary particles that are chargedparticle fragmentsandneutrons inside structures.Theeffectoftensofgramspersquarecentimeterofstructureor atmosphere is to convert and multiply the primary

JohnshopkinsApLTechnicALDigesT,VoLume27,number1(2006) 57

Thensbri/ApLneuTronenergYspecTromeTer

proton“beam”intoasecondaryenvironmentdominatedbyneutrons.chargedprotonsarereadilydetected,andinstrumentsarealreadyinexistenceforthistask.neu-tronsareelectricallyneutralandmuchmoredifficulttodetect.Theseneutronsareestimatedtocontribute10–30%oftheradiationdoseinsidespacestructuressuchasthe international space station (iss)6 and cannot beignored.currentlynocompact,portable,real-timeneu-trondetectorinstrumentationisavailableforuseinsidespacecraftoronplanetarysurfaces.

similar to the very low-energy thermal neutrons(0.025eV)atsealevelonearth,themuchhigher-energyneutronsinspacecanreadilypenetrateintothehumanbodywheretheyaremoderatedorslowedbybodytissue,which is primarily water. Whereas thermal neutrons,whichmustbecapturedbyelementssuchaslithiumorboron (rare in the human body) to deposit significantenergy, the high-energy neutrons are readily scatteredandslowed.Theythenhaveamuchgreaterprobabilityofinteractingbyelasticorinelasticreactionsanddepos-itingsignificantenergyinthesoftorgantissuenearthecenterofthebody(e.g.,liverandspleen).Therefore,thecarcinogenesisriskissignificantlygreaterforsuchcriti-calbodyorgansthanonearth’ssurfacewheretheenvi-ronmentisdominatedbythermalneutrons.

otherbiologicalradiationeffectsofconcernforastro-nautsarethepossiblebreakageofDnAdoublestrandscausedbysinglehitsofneutrons,protons,andmassiveheavyions,whichcouldproducelatentgeneticdefects,anddamagetolocalizedareasofthebrain,whichmaycauselatentperceptualproblemsorimmediateseizures.2Thislatterbiologicaleffectisakintothedamageexperi-encedinintegratedcircuitmemoriesbythesingle-eventeffects resulting from single neutron, proton, or heavyionstrikesinspace.

inaFebruary2004 interview,Dr.Frankcucinotta,the chief radiation officer at nAsA Johnson spacecenter, stated that nAsA weighs radiation danger inunitsofcancerrisk.Ahealthy40-year-oldnonsmokingmalehasa20%chanceofeventuallydyingfromcancer.Theaddedriskofa1000-daymarsmissionissomewherebetween1%and19%,withthelargeuncertaintyaresultofthelackofknowledgeabouthowthehumanbodywillreacttoincreasedlevelsofionizingradiation.Theoddsareworse,nearlydouble, for femaleastronautsbecauseofthesensitivityofthebreastsandovariestoradiation.obviously, a40%chanceof a life-ending cancer afterreturn to earth is unacceptable to Dr. cucinotta andnAsA.

in the nAsA bioastronautics roadmap,2 radiationeffectsonhumansareofthegreatestconcernforamarsmission.risksaddressedbymonitoringtheastronauts’environmentsareasfollows:

• risk28:carcinogenesis, i.e., increasedcancermor-talitycausedbyradiation—priority1

• risk 29: Acute and late radiation damage to thecentralnervoussystem,includingchangesinmotorfunctionandneurologicaldisorders—priority1

• risk30:Degenerative tissuedisease, includingcar-diacanddigestivediseasesandcataracts—priority1

• risk 31: Acute radiation syndromes from intensesolarparticleeventsorsynergisticeffectsfromexpo-suretoradiation,microgravity,andotherspacecraftenvironmentalfactors—priority1

There are two critical physical questions related tothesebiologicalrisks:

1. howdothethickness,design,andmaterialcompo-sitionofspacevehiclesaffecttheinternalradiationenvironmentandbiologicalassessment?

2. What are the risks from energetic solar particleevents,andwhatimpactdotheyhaveonoperations,extra-vehicularactivity,andsurfaceexploration?

monitoring of the neutron environment helps supplydata to answer these questions and to enable radio-biologists to assess the increased risk of cancer forastronauts.

Analliedtopicofinvestigation,inadditiontomon-itoringtheexistingneutronenvironmentinsidespacevehicles or on planetary surfaces, is how to designvehiclesandhabitatssoastominimizethetransfor-mationandmultiplicationoftheprimarycosmicraybeam.7,8Aftercollisionwiththickstructures,thetotalsecondaryradiationenvironmentofchargedparticlefragmentsandneutronshasahigherfluxthanthepri-marycosmicraybeam.inaddition,thelowerenergiesofthesesecondariesincreasetheirprobabilityofinter-actioninastronauts’bodies.structuralandshieldingmaterials shouldbechosen tominimize theproduc-tionofneutronsandheavyionfragmentsintheenergyrangesthatposethegreatestthreatfordepositionofradiationdoseordoseequivalentinthehumanbody.recentacceleratorexperimentshaveshownthatalu-minum,themostcommonlyusedspacecraftmaterial,istheworstwithrespecttotheproductionofsecond-aryhigh-energyneutrons.9since it is impractical tocreate a habitat thickness that is equivalent to therangeof the incidenthigh-energycosmic rays(hun-dredsof centimeters inwateror aluminum), currenteffortsforspacehabitatdesignconcentrateonmateri-alswithhighhydrogencontentforefficientprotectionfromneutronsandprotonsandhighatomicnumber/atomicmassratioforelectronicscatteringofchargedparticles.

Dr.TracyYangofJohnsonspacecenterstatedthatnAsA’sgreatestneed forknowledgeof theastronaut’slong-termradiationenvironmentwasacompactneutronenergyspectrometer.instrumentdevelopmentbeganinmay1997.


r. h. mAurer et al.

NEUTRON SPECTROMETER HARDWARE DESIGN

ApLoriginallyproposed todesignandbuild apor-table, low-power, robust neutron spectrometer thatwould measure the neutron spectrum from 20 keV to500meVwithatleast10%energyresolution.coveringsuchalargespectrum,however,requiresmorethanonedetectorsystem.ourinitialconceptwasfortwodetec-tor systems: a helium-3 (3he) proportional counter forlow-energyneutrons(10keV–5meV)andasiliconsolid-statedetectorforneutronsintherangeofseveralmeVto500meV. itbecameapparent that the lowneutroncapturecrosssectionofthe3hetubeabove1meVandthe inherent noise floor in the silicon detector below10meVmadeitdesirabletouseathirddetectorsysteminthe“fast”neutronregionbetween1and10meV.Weselectedaplasticorganicbicron454scintillatordetec-torforthefastneutronregion.sincethebicron454isascintillator,aphotomultipliertubemustbematedtoittodetectandcollectthelightproducedbytheneutroninteractions.

inFebruary2003ourconceptforamarsLanderneu-tron spectrometer was published.10 Figure 1 shows our

proposedinstrumentconceptforthesubsequentlycan-celledmars2003Lander.

The high-energy spectrometer (hes) detects neu-tronsintheenergyrangefromseveralmeVto500meV.itsprimarydetectorisa5-mm-thicklithium-driftedsili-condetectorthatproducesapulseheightproportionalto the charge or energy deposited by neutron–siliconatom elastic and inelastic collisions. From the start itwas recognized that an anti-coincidence shield/circuitwould be necessary to discriminate against chargedparticles depositing energy in the thick silicon detec-torandthatitwouldhavetobequiteefficienttokeepthe signal-to-noise ratio reasonable. consequently, wedesignedandfabricatedacesiumiodide(csi)cylindri-caldetectorscintillatingtubethatsnuglycontainedthe5-mm-thickdetector.Wewereabletosuccessfullyoper-ate and evaluate the combineddetector systemat theindiana university cyclotron Facility (iucF) in sep-tember2001using200-meVprotonsandatthecolum-biauniversityradiologicalresearchAcceleratorFacil-ity (rArAF) innovember2001with10- to20-meVneutrons. We found that the detector system and theassociated front-end electronics could operate up to amaximumdatarateof3khz.

Themedium-energyspectrometer(mes,notshownin Fig. 1) detects neutrons in the energy range of1–10meV.Thedetectorconsistsofthebicron454plas-ticorganic scintillatorasmentionedearlier.Themesusesasimpletechniquetoensurethataneventinthedetectoriscausedbyaneutron.Tofirstorder,aneutronenters the detector and scatters off multiple hydrogenatoms.Thisscatteringthermalizestheneutrons.Afterashorttimetheseverylow-energyneutronsmaybecap-turedbytheboron-loadedscintillator.Theneutroncap-turehasastandardpulseheightassociatedwithit.Thetime between the thermalizing and capturing of theneutronisrelatedtotheamountofboroninthedetec-torandcanbecharacterizedatrArAFusingneutronsofknownenergies.Therefore,thetechniquetoensurethat a neutron caused an event is a time-correlateddoublepulse from thedetector,with the secondpulsehavingthecharacteristicsofacapture.Thistechniquehasa fewparasiticproblems.one,notall thermalizedneutronsarecaptured;wecallthese“escapes.”Two,itispossiblefortwoeventswithinthedetectortosimulateaneutronevent.modelingandcalibrationareusedtocharacterizetheescapesandaliasing.

Thelow-energyspectrometer(Les)detectsneutronsfrom 10 keV up to 1 meV and consists of a 3he pro-portional counter tube. rise-time techniques are usedontheoutputpulseof the3hetubetodetermine ifaneutron created the event within the detector. Therearetwodominantreactionmodesofaneutronwith3he(ref.11).oneisanabsorptionmodethatbreaksthe3heinto a proton and triton, releasing 764 keV of kineticenergy to the products of the reaction. Therefore,

Figure 1. Proposed neutron spectrometer instrument design for the subsequently cancelled Mars 2003 Lander mission. The two detectors shown are a 3He gas proportional counter tube for the low-energy neutrons and a pair of thick lithium-drifted (Si(Li)) sili-con detectors inside the cesium-iodide tellurium-doped (Csi(T,)) scintillator veto enclosure for the high-energy neutrons. The scintil-lator light is collected by a pair of silicon (Si-PiN) photodiodes. The electronics module contains high-energy spectrometer (HES) and low-energy spectrometer (LES) analog processing, a data pro-cessing unit (DPU), a spacecraft interface, power switching and conversion, and low- and high-voltage power supplies (HVPS).

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thermal or epi-thermal neutrons will produce a pulseheightproportionalto764keV,whichservesasausefulcalibration tool. higher-energy neutrons can also beabsorbed within the gas, releasing 764 keV plus theincomingneutronenergyinthereactionproducts.Thesecondmainreactionmodeis3hetargetrecoils.neu-trons or charged particles can elastically scatter froma3heatomanddepositenergyinthedetector’s sensi-tive volume. helium-3 absorption reactions normallyproducelongerrisetimes,soknowingtherisetimeforeacheventisimportant.knownincidentneutronener-giesareusedatrArAFtocalibraterisetimesandpulseheights.modelingisalsousedtocorrecttherawdata.

THICK SILICON DETECTOR RESPONSE FUNCTION

We performed experiments with high-energy neu-trons (20–800 meV) at Los Alamos neutron sciencecenter(LAnsce)inAugust2000anddeterminedtheresponsefunctionofthe5-mm-thicksilicondetectorfortheseneutrons.Weobservedanefficiencyenhancementinthethicksilicondetectorasaresultofpioncreationaboveneutronenergiesof300meVasexpected.Aftersuccessfullydeconvolvingtheenergydepositionspectrainthesilicondetectortodeducethemostprobableinci-dentneutronenergyspectraandcomparingthemtotheLAnscebeammonitorresults,weranablindexperi-mentinwhichwehadnoknowledgeofthebeamchar-acteristics. our deconvolution procedure successfullydeducedtheblindexperimentneutronenergyspectrumestablishedbyLAnscecalculations(Fig.2).

Wepublishedourbasicspectruminversionmethod-ologyinaFebruary2003article.12Themethodologyis

basedonmeasurementofthedetectorresponsefunctionforhigh-energyneutronsandinversionofthisresponsefunctionwithmeasureddepositiondatatodeduceneu-tronenergyspectra.

neutrons, being electrically uncharged, do notdirectlydepositenergyindetectors.instead,somereac-tionwiththeneutronmustoccurthatcreateschargedsecondaryparticlesorenergeticphotonsfortheneutrontobedetected.Forhigh-energyneutrons,thesesecond-aryparticlesdepositadistributionofenergiesinadetec-torforagivenincidentneutronenergy.Theresponseofthedetectorcapturesthedepositedenergydistributionsformono-energeticneutronexposuresandcanbeusedtopredictenergydepositioninthedetectorfromagen-eralexposureusing

C c A Ei ij ij j jjj

= = ∆∑∑ w , (1)

where Ci is the number of events depositing energyin thedetector for the ithdepositedenergybin,Aij isthe matrix form of the detector response, and wjDEjis the total number of neutrons in the jth incidentenergybin.

Tomeasureaneutronspectrum,thecountspectrumCi for a sourceenvironment ismeasured, andeq.1 isinvertedwiththeknownresponsematrixtogivewjDEj,the unknown neutron energy spectrum. The silicondetector spectrum inFig.2wasdeterminedusing thisprocess.Wehavefoundthatstandardmatlabinversionroutinesusing iterativetechniquesgivegoodestimatesofneutronspectrafromeq.1.

While the LAnsce experiments provided a goodmeasureoftheresponsematrixforthe5-mmsidetector,theresultsareuniquetothatdetector.Therefore,weusedtheclemsonuniversityprotoninteractionsinDevices(cupiD)computermodeltopredicttheresponseofageneric5-mm-thickdetector.13cupiDusesacascade-evaporationmodelof theunderlyingnuclear reactionstopredictsecondaryparticlegeneration,trackthesec-ondariesthroughthevolume,andcalculatetotalenergydepositionfromallparticlesassociatedwithindividualreactions.Thehighestenergydepositionfromeachneu-tron energy bin predicted by cupiD is shown in Fig.3,alongwiththeactualhighestdepositionmeasuredintheLAnsceexperiments for the sameneutronener-gies.clearly, thismodel fails to accuratelypredict theresponse.

pions are generated in nuclear reactions when theincidentneutronenergy is aboveabout250meV,andanygeneratedpionsinourdetectorwillnotdepositsig-nificantenergyinthereaction.Therefore,piongenera-tionrepresentsalossofdepositedenergynotaccountedforinthecupiDmodel.Aspartofthisresearcheffort,anewversion,calledpion(laterpicupiD),wasdevelopedthatdidincludepiongeneration.AsseeninFig.3,pion

Figure 2. Results from the Los Alamos blind experiment. The neutron fluence spectrum was modified by changing the switch-yard moderators without the experimenters’ knowledge. The sili-con detector spectrum was determined by our response function methodology, while the Los Alamos calculation is based on Monte Carlo N-particle code calculations for the particular switchyard moderator.

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doesfaithfullypredictthedetectorresponse.picupiDhasalsobeenusedsuccessfullytogeneratetheresponsematrix foramuchsmallersilicondetectorusedinthecrres(combinedreleaseandradiationeffectssat-ellite) micro-electronics package pulse height Ana-lyzer14toexploretherelativecontributionoflow-energyprotons, high-energy protons, and galactic cosmic rayenvironments in near-earth orbit and to evaluate theaccuracyofstandardnAsAtrappedprotonmodels.

Theeffectofpionsdemonstratedbythecomparisonwith experimental data has a larger implication. Thehigh-energy particle transport codes being developedby nAsA for simulation of the environments insidelarge manned space vehicles and planetary habitatsmust accurately use the correct partition of energy ineachgenerationofinteractionduringtransportthroughmaterialcombinations.ourresultsemphasizetheneedtotreatthepioncomponentcorrectly.

FLIGHT TESTING

Aircraft FlightsAs a product of our initial nsbri funding during

fiscal years 1998–2001 we designed and fabricated anengineering prototype neutron spectrometer that wasflown on F-15 and F-18 aircraft from nAsA DrydenFlight center. The spectrometer consisted of bothlow- and high-energy subsystems. Low-energy neu-trons (0.025 eV–1 meV) were detected using the 3hegastubeandincludedthermalandepithermalneutrons.

high-energyneutrons(5–800meV)weredetectedusinga5-mm-thicklithiumdriftedsiliconsolid-statedevice.both low- and high-energy spectrometers underwentground-basedevaluationandcalibrationusingradioac-tivesourcesandacceleratorfacilities.

Theaircraftneutronspectrometerwasflownontwoflights on 13 and 14 August 2001 in a pod under thewingoftheF-18.Athirdsuccessfulflightinthesamepod under the fuselage of the F-15 was executed inoctober2001.Asexpected,theneutronfluxincreaseddramaticallyastheaircraftachieveda40,000-ftaltitude(Fig.4).Themainresultfromthethreeflightswastheverificationofourengineeringdesign,not the limiteddataobtainedbecauseof the short duration (≈2h)oftheflights.Thevalueforourhardwarewastheprovenapproachinhandlinghighvoltageatthehigh-altitudecoronaregiontobeusedforballoonflights.

Balloon Flightseffortsin2002and2003weredirectedatdesigning

andfabricatinganeutronspectrometerforhigh-altitudeballoonflights.Theelectronicsweremademorerobustand compact for the balloon flight instrument. Thedetector suitewaschangedto includeanmesfor thefastneutronsinthe1-to20-meVenergyrangeinaddi-tiontothethicksilicondetectorforthe>20-meVneu-tron energies. The 3he tube was not included for thelow-energyneutrons(10keV–1meV)sincethisconven-tionalsystemhadbeenvalidatedontheaircraftflightsandwillbereadilyavailableforfutureflightefforts.Themes,asnotedearlier,wasabicron454plasticscintilla-tordetectorsystemthatborrowsfromthedevelopmentofa similar system for theApLunmannedmessen-germissiontomercury.sophisticatedenergydeposi-tionsignaltimediscriminationallowedbothscatteringandcapturepeaksoftheneutronsinthebicrondetec-tortobeobservedforindividualcountsandenergies.

balloon flights were executed from Fort sumner,new mexico, at an altitude of 85,000 ft on two

Figure 3. Comparison of two models (CUPID and Pion) of the detector response matrix with results from the LANSCE experiments.

Figure 4. Aircraft flight 3He tube counts versus DPU time. Note that the counts are the count rate integrated for 30 s.

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occasions: 9 october 2002 and 9 october 2003. The85,000-ft altitude was chosen since the amount ofatmosphere remaining (≈20 g/cm2) is the same as theamountofcarbondioxideatthesurfaceofmarsandwasexpectedtoyieldareasonablesimulationofthedown-ward neutron spectrum on mars. The october 2002flightdidnotyieldanyusefulscientificdataasaresultof engineering problems with the high-voltage con-nectionto thesilicondetectorandground loop issuesbetweentheelectronicsandthealuminumcontaineroftheinstrument.

DuringJanuarytoApril2003weimprovedourbal-loonflightinstrumentdesignandmadeitmorerobustand reliable. We qualified it for flight at the nationalscientificballoonFacilityatFortsumnerinmay2003.Theoctober2003balloonflightat85,000ftforapprox-imately20hoffloattimeyieldedusefulscientificdatafromthethicksilicondetectorforhigh-energyneutrons.TheballoonflightinstrumentisshowninFig.5.Figure6isaphotographofthelaunchwithourpayloadatthebottomona gondola.bothballoon andpayloadwererecovered.

initial analysis showed that we obtained a highlymoderatedneutronenergyspectrum,withthemajorityofneutronsintheenergyrangebetween20and35meV.modelingof thedetector shielding geometry isneces-sarytodeducethespectrumincidentontheinstrumentcontainerrelativetothatmeasuredatthedetectorloca-tion.Thismodelingincludestheinherentshieldingofouraluminuminstrumentpackage,the1atmosphereofairpressure inside thecontainer,and thecsichargedparticle discrimination scintillator that surrounds thesilicondetector.Figure7isadiagramofthecomponentsof the radiationenvironment thatmustbeconsideredwhendeducingtheneutronspectrumincidentuponthe

Figure 5. October 2003 balloon flight instrument.

Figure 6. October 2003 launch of the helium-filled balloon. The parachute enables a safe payload (on the gondola at the bottom) return. The balloon started east from Fort Sumner, New Mexico, then turned around, headed west, and was brought down near Soccoro, New Mexico, approximately 120 mi from the launch site, after 24 h.

spectrometerfromtheoneobservedatthethicksilicondetector.

preliminarysimulationswiththegeAnT (geometry And Track-ing) code15 are aimed at produc-inganenergydeposition spectrumfor the thick silicon detector withtransportthroughallthesurround-ing materials taken into consider-ation. The incident spectrum thatproduces the observed depositionspectra is the most probable neu-tronspectrumat85,000ftorunder20g/cm2ofatmosphere.

Thedeconvolvedneutronenergyspectrumforthe5-mm-thicksilicondetectorinferredusingthemeasureddetector response function is themost probable spectrum observedat thedetector location.modeling

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willenableustoestimatethecontributionsofsecond-aryneutronsproducedbytheinstrumentpackagesowecandistinguishtheseneutronsfromthoseproducedbytheatmosphere.modelingwillalsoallowustocorrectfor the number of primary and secondary cosmic rayions,ionfragments,andphotonsthatmayhaveescapedthecsianti-coincidencedetectorintheballoonflightconfiguration.

SPACECRAFT SHIELDING MATERIAL STUDIES

inJanuary2000wewerenotifiedthattheproposalwehad submitted tonAsA, “Developmentof aneu-tron spectrometer to Assess biological radiationDamagebehindspacecraftmaterials,”wouldbefundedfor3.5years,frommay2000tonovember2003.Fortheevaluationofspacecraftstructuralandshieldingmateri-alswebuiltatwo-detectorstackversionoftheneutronspectrometercompatiblewithground-basedacceleratorresearch.16sinceanacceleratorbeamisunidirectionalcompared to the omnidirectional space environment,the charged particle veto detector can just be a thinsilicontransmissiondetectorinfrontofthe5-mm-thicksilicondetector.Weverifiedits successfuloperationattherArAF innovember2001, thenproceededwithspacecraftshieldingexperimentsusing200-meVprotonbeamsattheiucFinnovember2002andnovember2003and500-meVprotonbeamsatTriumFinVan-couver,canada,inseptember2003.(TriumF,which

originally stood forTri-universitymesonFacility,hassinceexpandedto include many more members.)Aluminum, carbon, and polyethyl-eneblocktargetswereusedtosimu-latespacecraftmaterials.

beamcurrentsatbothTriumFand iucF were generally on theorderof0.1–0.2nA,yieldingprotonfluxes in the range of 53106 to23107protonspercm2?s.sufficientshieldmaterialwasusedsothatthe200-meV primary proton beam atiucF was completely absorbed atnormal incidence or 0° and couldnot interfere with the neutrondetection by excessively triggeringthefrontsilicontransmissiondetec-tor,therebycausingahighchargedparticlevetorate.

The experiments were carriedout by putting detectors at evenlyspacedanglesintheforwardhemi-sphere a distance of 31.8 cm fromtheexit faceof the shieldmaterialblock,withthedetectorstackaligned

Figure 7. Diagram of the neutron energy spectrometer showing the components of the natural environment at an altitude of 85,000 ft that can contribute to events in the thick silicon detector (PMT = photo-multiplier tube).

onthecenterlinefortheappropriateangle.Theresult-ingthicknessesofaluminum,polyethylene,andgraphitewereinthe12.7-to33.0-cmrange,resultinginarealden-sitiesof34.4g/cm2foraluminum,30.9g/cm2forgraph-ite,30.7g/cm2forpolyethylene,and36.9g/cm2forthealuminum/polyethylene combination. Fortunately, thisrange of areal densities encompasses the amount ofshieldingpresentinsidetheiss,sotheneutronspectraproducedby theprotons should be representative andrelevanttotheissradiationenvironment.

Theresults9yieldedneutronproductionenergyspec-trashowingthereducedyieldfromcarbon-basedmate-rialsandthemoderating(scattering)effectsofpolyeth-ylenewhencomparedtoaluminum.ourdatavalidatetherecentconclusionaboutaluminumbeingthe leastsuitable spacecraft material with respect to increasedneutronproductionandhumanradiationeffects.

As an example, Fig. 8 shows the normalized (neu-tronsproducedoverasmallsolidanglecenteredat0°or30°perincidentprotononthefrontfaceofthematerialblock)integralneutronproductionenergyspectraatornear the forwarddirection for aluminum,carbon, andpolyethyleneat200and500meV.Therearenosignifi-cantdifferencesbetweenmaterialsat500meV,whereamajorportionoftheprotonbeamcanpassthroughthethicktargetwithoutinteracting.however,at200meV,wheretheprotonbeamisstoppedevenat0°,aluminumproducesafactorof5–20moreneutronsabove100meV.Atthehigherneutronenergies,carbonbyitself is theequivalentofpolyethylene;below50meV,polyethylene

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moderates and reduces the neutron production com-paredtobothaluminumandcarbon.

similar data on polyethylene at large angles (60°fromthebeam-targetaxis)showthesignificantmoder-ating effectofpolyethylene,which results inneutronsbeing scattered from the forward direction to lateraldirections. At 60° there is a reduction in neutrons inthe 45- to 135-meV energy range, with a consequentincrease in the number of neutrons in the 14- to 45-meVenergyrange.

Thus, the effect of angle is complex and exhibitsa strong interaction with the type of material. Whilepolyethylene reduces the neutron flux in the forwarddirection,itincreasesthefluxatlargeanglesfromthebeamdirectionbelowabout50meV.notethatneutronswithenergylessthan20meVareconsideredmoredam-agingforbiologicaleffects.Theultimatesolutiontothespacehabitatshieldingproblemwillbeanoptimizationofthematerial(s)andthicknessthatminimizestheflux,or fluence, of charged particles, charged particle frag-ments,andneutronsintheappropriateenergyrange(s)andconsequentlyminimizesthedoseequivalenttotheastronauts.modelingwithsophisticatedparticletrans-portcodeswillbeamajorpartofthiseffort.

CURRENT R&D

Combined Ion and Neutron Spectrometer inmarch2004wewerenotifiedbynsbrithatour

proposaltodevelopacombinedionandneutronspec-trometer (cins) for apotential compactglobal radia-tionmonitorwouldbefundedfor4years.Thespecificobjectivesareto

• Design and fabricate a prototype cins for spaceapplications

• calibrateandevaluatetheresponsefunctionsofthecinsdetectorsystemsusingground-basedaccelera-torbeamsofappropriatetypeandenergy

Figure 8. Neutrons produced over a small solid angle per inci-dent 200- or 500-MeV proton for three different spacecraft mate-rial blocks near the forward or beam direction.

• usecinsinaground-basedacceleratorcomparisonwithtraditionallyusedspaceinstrumentationsuchasthetissueequivalentproportionalcounter

• examine the effects of baseline and new radiationshieldingmaterialsascountermeasuresbymeasuringcharged and neutral secondary particle generationfromasimulatedspaceenvironment

The prototype instrument will be developed usingheritage from the neutron energy spectrometer devel-oped under previous nsbri grants described above,instrumentelectronicsfromunmannedspacecraftmis-sionssuchasmessenger,andthedesignandopera-tionofthemarsodysseyorbitermArie(marsradia-tionenvironmentexperiment)instrument.

mAriehas threesignificantproblems.(1)pream-plifiergainsaretoohigh,sothathitsfromanyheavyionwithlinearenergytransfer>35keVpermicrometerofpath length result in saturationof theelectronics.(2) The trigger detectors are noisy, so the thresholdsmustbekepthightoavoidanexcessiveeventrateduetospurioustriggers;thehighthresholdsthencausethedetector to be blind to protons with energies aboveabout 100 meV, and the exact energy cutoff is notwell known or easily determined. (3) The event rateis limitedtoaboutthreepersecond,adequateduringtimeswhenthesunisnotactiveorevenmoderatelyactive, but inadequate during intense solar particleevents.

Thecinswilluse silicon stacks todetect chargedparticles similar to those used for purposes of dosim-etryflyingon the issandon the2001marsodysseyorbiter. itwill also incorporate several improvements,themostimportantofwhichwillbeintheelectronics.mAriecurrentlycanprovideadynamicrangeofonlyabout100:1;weintendtoexpandthisrangebyafactorof10to20throughbetterdesignoftheelectronicsandmorecarefulattentiontoamplifiergains.Thenextmostimportantimprovementwillbeinthecapabilityofthesystemtohandlethehighratesthatoccurduringsolarparticleevents.ThemAriedesignislimitedbycpuhardwareandsoftwaretoadatarateofaboutthreetrig-gerspersecond;weexpecttoincreasethisbyatleastafactorof10byimplementingamoremoderncpuanddatabus.

Antarctic Balloon Flight Instrument in June 2004 we were notified that we would be

fundedunderasubcontractfromnAsAmarshallspaceFlight center to develop and supply a balloon-borneneutronspectrometer(bbns)fortheDeepspaceTestbedmissionstobeflownonhigh-altitude(≈125,000ft)balloons from Antarctica over the next several years,beginning inDecember2005. inaddition toverifyingtheperformanceofengineeringsystems,thesemissionsof several weeks each will simulate the interplanetary

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radiation environment by simultaneously monitoring(1) the incident cosmic ray spectrum at the top of theatmospherewiththeminimalmagneticfieldshieldingofthe south pole and (2) the “interior” spectra producedbehind different common and innovative spacecraftshieldingmaterials.Wearedesigningand fabricatingathird-generationballoonflightinstrumentforthesemis-sions.Thebbnswillincludeallthreedetectorsystemsfor thefirst time(the3he tube, thebicron454 scintil-lator, and the 5-mm-thick silicon inside a csi chargedparticlediscriminator).itwillmonitorthecompleteneu-tronenergyspectrum,undertheseveraldifferenttypesofshielding,rotatingona4-ft-dia.carouselabovethebbnsdetectorhousingonthemultideckballoongondola.

severalengineeringchangesfrompastversionsoftheneutronspectrometerwererequired inordertoenablethismission.spaceon thedeckbelow thecarousel islimited;therefore,theneutronspectrometerwasbrokenupintotwoboxes,adetectorboxandanelectronicsbox(tobeplacedonalowerdeck).Thebbnsinstrumentwill need to function with more autonomy than pastversions.payloadrecoveryisnotguaranteed,sothedataloggingprocedureswillchange,andtheoperatingtem-perature range for thebbnswillbehigher thanpastversions.significantattentionisbeingpaidtomechani-calandsystemissuestomeetthedifferentrequirementsofthismission.

SUMMARYThe nsbri/ApL neutron energy spectrometer

is built with three detector systems: a thick lithium-driftedsolid-statedetector,solid-statescintillators,andagastubetocoverthedesiredenergyrangeofneutronsof interest to radiobiologists aswell as todiscriminateagainst charged particles. The instrument is designedwith robustness for aircraft and balloon flights. it hasevolvedsince1997fromevaluationsofseveraltypesofindividualdetectorswithradioactivesourcesandaccel-eratorbeams.Thenextstepistoproduceaspaceflightinstrument suitable for interplanetarymissions.Alliedground-based investigations using high-energy accel-eratorbeamson thick targetshavecontributed to thesearch for better spacecraft structural and radiationshieldingmaterials.

AcknoWLeDgmenT: This work was supportedunder nAsA cooperative Agreement 9-58 with thenational space biomedical research institute andnAsAresearchgrantnAg8-1695with themarshallspaceFlightcenter.

reFerences 1nsbri:http://www.nsbri.org. 2bioastronautics roadmap, A risk reduction strategy for human

exploration:http://bioastroroadmap.nasa.gov/index.jsp(updatedFeb2005).

3charles,h.k.Jr.,chen,m.h.,spisz,T.s.,beck,T.J.,Feldmesser,h.s., et al., “AmpDXAforprecisionboneLossmeasurementsonearthandinspace,” Johns Hopkins APL Tech. Dig.25(3),187–200(2004).

4coolahan,J.e.,Feldman,A.b.,andmurphy,s.p.,“simulationofintegrated physiology based on an Astronaut exercise protocol,”Johns Hopkins APL Tech. Dig.25(3),201–213(2004).

5nationalresearchcouncil,Radiation Hazards to Crews of Interplan-etary Missions: Biological Issues and Research Strategies,nationalAcad-emypress,Washington,Dc(1996).

6badhwar,g.D.(ed.),Proceedings of the Workshop on Predictions and Measurements of Secondary Neutrons in Space Workshop,Johnsonspacecenter,houston,TX(sep1998).

7Wilson, J. W., miller J., konradi, A., and cucinotta, F. A. (eds.),Shielding Strategies for Human Space Exploration, nAsA cp-3360(1997).

8Wilson, J. W., cucinotta, F. A., miller, J., shinn, J. L., Thibeault,s.A.,etal.,“ApproachandissuesrelatingtoshieldmaterialDesigntoprotectAstronauts fromspaceradiation,”Mater. Des.22,541–554(2001).

9maurer,r.h.,kinnison,J.D.,androth,D.r.,“neutronproductionfrom200–500meVprotoninteractionwithspacecraftmaterials,”inProc.ICRS 10/RPS 2004,madeiraisland,portugal(may2004);also,Radiat. Prot. Dosim. (inpress,2005).

10maurer,r.h.,roth,D.r.,kinnison, J.D.,goldsten, J.o.,gold,r. e., and Fainchtein, r., “mArtian neutron energy spectrometer(mAnes): An instrument for the mars 2003 Lander,” Acta Astro-naut.52,405–410(2003).

11knoll,g.F.,Radiation Detection and Measurement, 3rded.,JohnWileyandsons,newYork(2000).

12kinnison,J.D.,maurer,r.h.,roth,D.r.,andhaight,r.c.,“highenergyneutronspectroscopywithThicksiliconDetectors,”Radia-tion Res.159,154–160(2003).

13kinnison, J. D., maurer, r. h., roth, D. r., mcnulty, p. J., andAbdul-kader, W. g., “neutron-induced pion production insilicon-based circuits,” IEEE Trans. Nucl. Sci. 50, 2251–2255(2003).

14mcnulty, p. J., kinnison, J. D., maurer, r. h., roth, D. r., reed,r. A., and Abdul-kader, W. g., “energy-Deposition events mea-suredbythecrresphAexperiment,”IEEE Trans. Nucl. Sci. 51, 3381–3387(2004).

15geAnTcode:http://wwwasd.web.cern.ch/wwwasd/geant4/geant4.html.16maurer,r.h.,roth,D.r.,kinnison,J.D.,Jordan,T.m.,heilbronn,

L.h.,etal.,“neutronproductionfrompolyethyleneandcommonspacecraft materials,” IEEE Trans. Nucl. Sci. 48, 2029–2033(2001).



THE AUTHORS

Johno.goldsten

Dennisk.haggerty

JamesD.kinnison

richardh.maurer

Davidr.roth

Richard H. MaurerofApL’sspaceDepartmentisaprincipalprofessionalstaffmemberandtheprincipalinvestigator(pi)andprojectmanageroftheneutronenergyspectrometerteam.Dr.maurerprovidesthenuclear/particlephysicsbackgroundfortheproject.David R. RothisaseniorphysicistinApL’sspaceDepartment.Dr.rothsuppliesthedetectorandelectronicsdesigns,overseestheelectronicsfabricationoftheneutronspectrometer,andcoordinatesthemechanicaldevelopment.James D. Kinnison,aprincipalprofessionalstaffphysicistinthespaceDepartment,was

responsibleforthecalibrationandestablishmentofthesolid-statesilicondetectorresponsefunctionfromexperimentsthathedesignedandanalyzed.Dr.kinnisonwasalsoresponsiblefortheoriginaldetectorevaluationsanddataanalysis.Dennis K. Haggertyisaseniorphysi-cistinthespaceDepartment.Dr.haggertyprovidesthemodelingusingthegeAnTcodefor theneutronspectrometerflown inatmosphericballoonflights so thatmodeledenergydepositionspectracanbecomparedtoexperimentallymeasuredspectra.hehasalsomodeledthe first concept of the cins charged particle telescope. John O. Goldsten is a principal

professional staff electrical engineer in the space Department. heassistsondetectorandelectronicsdesigns,scintillatorevaluations,andsystems engineering considerations and was the system engineer forthesuccessfulmAnesproposalforthemars2003Lander.Forfurtherinformation,[email protected].

the nsbri/apl neutron energy spectrometer · in 1997, nasa founded the national space bio-medical...

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