The Antarctic Impulsive Transient Antenna ultra-high energy neutrino detector: Design ...

50 Pages·2008·4.13 MB·English

SLAC-PUB-16278
The Antarctic ImpulsiveTransient Antenna Ultra-highEnergy Neutrino Detector
Design,Performance, andSensitivity for2006-2007BalloonFlight
P.W. Gorham1,P.Allison1,S.W.Barwick2,J.J.Beatty3,D.Z.Besson4,W.R.Binns5,C.Chen6,
P. Chen6,13, J. M.Clem7, A. Connolly8, P. F. Dowkontt5, M. A. DuVernois10, R. C. Field6,
D.Goldstein2, A. Goodhue9 C.Hast6, C. L.Hebert1, S.Hoover9, M.H. Israel5, J. Kowalski,1
J.G.Learned1,K.M.Liewer11,J.T.Link1,13,E.Lusczek10,S.Matsuno1,B.C.Mercurio3,C.Miki1,
P.Miocˇinovic´1,J.Nam2,12,C.J.Naudet11,R.J.Nichol8,K.Palladino3,K.Reil6,A.Romero-Wolf1
8
0 M.Rosen1,L.Ruckman1, D.Saltzberg9, D.Seckel7,G.S.Varner1, D.Walz6,Y.Wang12, F.Wu21
0 (ANITA Collaboration)
2
1 Dept. of Physics and Astronomy, Univ. of Hawaii, Manoa,

c HI 96822. 2Univ. of California, Irvine CA 92697. 3Dept. of Physics,
e Ohio State Univ., Columbus, OH 43210. 4Dept. of Physics and Astronomy,
D Univ. of Kansas, Lawrence, KS 66045. 5Dept. of Physics,
Washington Univ. inSt. Louis, MO 63130. 6Stanford Linear Accelerator Center,
0
Menlo Park, CA, 94025. 7Dept. of Physics, University of Delaware,
1
Newark, DE 19716. 8Dept. of Physics, University College London, London,

United Kingdom. 9Dept. of Physics and Astronomy, Univ. of California,
]
h LosAngeles, CA 90095. 10School of Physicsand Astronomy, Univ. of Minnesota,
p Minneapolis, MN 55455. 11Jet Propulsion Laboratory, Pasadena,
- CA 91109. 12Dept. of Physics, National Taiwan University, Taipei,
o
Taiwan. 13Currentlyat NASA Goddard Space Flight Center, Greenbelt, MD, 20771.
r
t
s We present a detailed report on the experimental details of the Antarctic Impulsive Transient Antenna
a (ANITA)longdurationballoonpayload,includingthedesignphilosophyandrealization,physicssimulations,
[
performanceoftheinstrumentduringitsfirstAntarcticflightcompletedinJanuaryof2007, andexpectations

1 forthelimitingneutrinodetectionsensitivity.Neutrinophysicsresultswillbereportedseparately.
v
0
2 I. INTRODUCTION
9
1
CosmicneutrinosofenergyintheExavoltandhigher(1EeV=1018 eV)energyrange,thoughasyetundetected,
.
2 areexpectedtoarisethroughahostofenergeticaccelerationandinteractionprocessesatsourcelocationsthroughout
1
theuniverse. However,in onlyoneof thesesources–thedistributedinteractionsofthe ultra-highenergycosmicray
8
flux–doesthecombinationofobservationalevidenceandinteractionphysicsleadtoastrongrequirementforresulting
0
: highenergyneutrinos. Whateverthesourcesofthehighestenergycosmicrays, theirobservedpresencein thelocal
v
universe,combinedwiththeexpectationthattheirsourcesoccurwidelythroughouttheuniverseatallepochs,leadsto
i
X theconclusionthattheirinteractionswiththecosmicmicrowavebackgroundradiation(CMBR)–theso-calledGZK
process(afterGreisen,Zatsepin,andKuzmin[1])–mustyieldanassociatedcosmogenicneutrinoflux,asfirstnoted
r
a by Berezinsky and Zatsepin [2]. These neutrinos are often called the GZK neutrinos, as they arise from the same
interactions of the ultra-high energy cosmic rays (UHECR) that cause the GZK cutoff, but they are perhaps more
properlyreferredtoastheBZneutrinos.
InBZneutrinoproductionscenarios,currentexperimentalUHECRmeasurementsinvariablypointtothepresenceof
anassociatedultra-highenergyneutrinoflux.ForUHECRaboveseveraltimes1019eV,intergalacticspaceisoptically
thicktoUHECRpropagationthroughtheCMBRatadistancescaleofseveraltensofMpc. EachUHECRsourceat
allepochsisthussubjecttolocalconversionofitshadronicfluxtosecondary,lowerenergyparticlesoveradistance
scaleoforder100MPcinthecurrentepoch. Neutrinosaretheonlysecondaryparticlethatmayfreelypropagateto
cosmicdistances,andtheresultingneutrinofluxatearthisthusrelatedtotheintegraloverthehighest-energycosmic
rayhistoryoftheuniverse,totheearliestepochatwhichtheyoccur.
AlthoughlocalsourcesmayalsocontributetotheEeV-ZeVneutrinofluxatearth,thebulkofthefluxisgenerally
believed to arise from a much wider spectral convolution, and will thus be imprintedwith the cosmologicalsource
distributioninadditiontoeffectsfromlocalsources. ThisleadstostrongmotivationstodetecttheBZneutrinoflux:
first,itisrequiredbystandardmodelphysics,andthusitsabsencecouldsignalnewphysicsbeyondthestandardmodel.
Second,itistheonlywaytodirectlyobservetheUHECRsourcebehaviorovercosmicdistancescales. Finally,once
established, the spectrum and absolute flux of such neutrinos may afford a calibrated “test beam” for both particle
This material is based upon work supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515 and HEP.
2
physics and astrophysics experimentation, providing center-of-momentumenergies on target nucleons of 100-1000
TeV,anenergyscalenotlikelytobereachedbyothermethodsinthenearfuture.
TheAntarcticImpulsiveTransientAntennawasdesignedwiththegoalofmeasuringtheBZneutrinofluxdirectly,
orlimitingitatalevelwhichwouldprovidecompellingandusefulconstraintsontheearlyUHECRsourcehistory.The
BZneutrinofluxispotentiallyverylow–oforder1neutrinopersquarekilometerperweekarrivingover2p steradians
is a typical estimate. This flux presents an extreme challenge to detection, since the low neutrino interaction cross
section also means that any targetvolume will have an inherently low efficiencyfor convertingany given neutrino.
ANITA’smethodologycentersonobservingthelargestpossiblevolumeofthemosttransparentpossibletargetmate-
rial: Antarcticice,whichhasbeendemonstratedtoprovideextremelylow-losstransmissionofradiosignalsthrough
itsbulkovermuchofthecontinent. ANITAthenexploitstheAskaryaneffect[3],coherent,impulsiveradioemission
fromthechargeasymmetryintheelectromagneticcomponentofahighenergyparticlecascadeinadielectricmedium.
ANITAsearchesforcascadesinitiatedbyaprimaryneutrinointeractingintheAntarcticicesheetwithinitsfieldof
viewfromtheLong-DurationBalloon(LDB)altitudeof35-37km. Theobservedareaoficefromthese altitudesis
oforder1.5Mkm2. Combiningthiswiththeelectromagneticfield attenuationlengthoficewhichisoforder1km
atANITA’sobservationfrequencyrange,ANITAissensitivetoatargetvolumeoforder1-2Mkm3. Theacceptance,
however,isconstrainedbythefactthatatanylocationwithinthetarget,theallowedsolidangleofarrivalforaneutrino
tobedetectableattheseveral-hundred-kmaveragedistanceofthepayloadisasmallfractionofasteradian. Folding
intheseconstraints,thevolumetricacceptanceisstilloforderhundredstothousandsofkm3steradiansovertherange
ofenergyoverlap–1018.5 20eV–withtheBZneutrinospectrum.Thislargeacceptance,whiletemperedbythelimited
−
exposure in time provided by a balloon flight, still yields the largest sensitivity of any experiment to date for BZ
neutrinos. InthisreportwedocumenttheANITAinstrumentandourestimatesofitssensitivityandperformancefor
thefirstflightofthepayload,completedinJanuaryof2007. Aseparatereportwilldetailtheresultsontheneutrino
flux.
II. THEORETICALBASISFORANITAMETHODOLOGY.
Theconceptofdetectinghighenergyparticlesthroughthecoherentradioemissionfromthecascadetheyproduce
canbetracedbackover40yearstoAskaryan[3],whoarguedpersuasivelyforthepresenceofstrongcoherentradio
emission from these cascades, and even suggested that any large volume of radio-transparentdielectric, such as an
ice sheet, a geologic saltbed, or the lunar regolith could provide the target material for such interactions and radio
emission. Infactalloftheseapproachesarenowbeingpursued[4,5,6].
Althoughsignificantearlyeffortsweresuccessfulindetectingradioemissionfromhighenergyparticlecascadesin
theearth’satmosphere[7],itisimportanttoemphasizethatthecascaderadioemissionthatANITAdetectsisunrelated
totheprimarymechanismforairshowerradioemission. ParticlecascadesinducedbyneutrinosinAntarcticiceare
verycompact,consistingofa“plug”ofrelativisticchargedparticlesseveralcmindiameterand 1cmthick,which
∼
develops at the speed of light over a distance of several meters from the vertex of the neutrino interaction, before
dissipating into residual ionization in the ice. The resulting radio emission is coherent Cherenkov radiation with
a particularly clean and simple geometry, providing high information content in the detected pulses. In contrast,
the radio emission from air showers is a complex phenomenonentangled with geomagnetic and near field effects.
Attemptsto understandand exploitthis formofair showeremission forcosmic raystudieshavebeenhamperedby
thiscomplexitysinceitsdiscoveryinthemid-1960’s,althoughthisefforthasseenarecentrenaissance[? ].
Surprisinglylittle workwas doneon Askaryan’ssuggestionsthatsolidssuchas ice couldbe importantmediafor
detection until the mid-1980’s, when Gusev and Zheleznykh [10], and Markov & Zheleznykh [11] revisited these
ideas. MorerecentlyahostofinvestigatorsincludingZheleznykh[12],Dagkesamansky&Zheleznykh[13],Frichter,
Ralston, & McKay [17], Zas, Halzen, & Stanev[14], Alvarez-Mun˜iz& Zas [15], andRazzaque et. al [19] among
others have taken up these suggestions and confirmed the basic results through more detailed analysis. Of equal
importance,asetofexperimentsattheStanfordLinearAcceleratorcenterhavenowclearlyconfirmedtheeffectand
exploreditinsignificantdetail[6,16,20,21]
A. FirstOrderEnergyThreshold&Sensitivity.
Toillustratethemethodology,weconsideraspecificexample. ThecoherentradioCherenkovemissioninanelec-
tromagnetice+e cascade arises from the 20% electron excess in the shower, which is itself producedprimarily
−
∼
3
by Comptonscattering and positron annihilationin flight. Consideringdeep-inelastic scattering charged-currentin-
teractions of a high energy neutrino n with a nucleon N, given generically by n +N ℓ +X, the chargedlepton
±
→
ℓ± escapeswhilethehadronicdebrisX leadstoahadroniccascade. IftheinitialneutrinohasenergyenergyEn , the
resultinghadroniccascadeenergywillEc=yEn ,whereyistheBjorkeninelasticity,withameanof y 0.22atvery
h i≃
highenergies,andaverybroaddistribution. TheaveragenumberofelectronsandpositronsN neartotalshower
e+e
maximumisoforderthecascadeenergyexpressedinGeV,or −
E
c
N . (1)
e+e
− ≃ 1GeV
Consider a case with En =1019 eV and a slightly positivefluctuationabovethe meangivingy=0.4. Thisleads
toE =4 1018 eV,givingN 4 109. TheradiatingchargeexcessisthenoforderN 0.2N . Single-
c e+e ex e+e
charged-p×articleCherenkovradia−tio∼ngi×vesatotalradiatedenergy,fortracklengthLoverafrequ≃encyband−fromn
min
ton ,of:
max
p h 1
W = a L 1 n 2 n 2 (2)
tot c −n2b 2 max− min
(cid:18) (cid:19) (cid:18) (cid:19)
(cid:0) (cid:1)
wherea 1/137isthefinestructureconstant,handcarePlanck’sconstantandthespeedoflight,andnandb arethe
≃
mediumdielectricconstant,andtheparticlevelocityrelativetoc,respectively.ForacollectionofN chargedparticles
radiatingcoherently(e.g.,withmeanspacingsmallcomparedtothemeanradiatedwavelength),thetotalenergywill
beoforderW =N2w. Insoliddielectricswithdensitycomparabletoiceorsilicasand,thecascadeparticlebunchis
tot
compact,withtransversedimensionsofseveralcm,andlongitudinaldimensionsoforder1cm. Thuscoherencewill
obtainuptoseveralGHzormore.
Fora4 1018eVcascade,N 8 108,andL 6minthevicinityofshowermaximuminamediumofdensity
ex
× ≃ × ≃
0.9withn 1.8asinAntarcticice. Takingthemeanradiofrequencytobe0.6GHzwithaneffectivebandwidth
o∼f600MHz,∼thenetradiatedenergyisW =10 7 J.Thisenergyisemittedintoarestrictedsolidangledefinedby
tot −
the Cherenkovcone at an angle q defined by cosq =(nb ) 1, and a width determined(primarily from diffraction
c c −
considerations)byDq csinq /(n¯L). TheimpliedtotalsolidangleofemittanceisW 2pDq sinq =0.36sr.
c c c c c
≃ ≃
ThepulseisproducedbycoherentsuperpositionoftheamplitudesoftheCherenkovradiationshockfront,which
yieldsextremelybroadbandspectralpoweroverthespecifiedfrequencyrange. Theintrinsicpulsewidthislessthan
100ps[20],andthispulsethusexcitesasingletemporalmodeofthereceiver,withcharacteristictimeD t=(Dn ) 1,
−
orabout1.6nsinourcasehere. Radiosourceintensityinradioastronomyistypicallyexpressedintermsoftheflux
density Jansky (Jy), where 1 Jy = 10 26 W m 2 Hz 1. The energy per unit solid angle derived above,W /W =
− − − tot c
2.7 10 7 J/sr in a 600 MHz bandwidth, produces an instantaneous peak flux density of S =1.4 107 Jy at the
− c
× ×
meangeometricdistanceD=480kmoftheiceinview,afteraccountingforthefactthatatthisdistancethegeometry
constrainstheFresnelcoefficientfortransmissionthroughtheicesurfaceto 0.12,sincetheradiationemergesfrom
∼
anglesclosetothetotal-internal-reflectance(TIR)angle.
Thesensitivityofaradioantennaisdeterminedbyitscollectingapertureandthethermalnoisebackground,called
thesystemtemperatureT . TheRMSlevelofpowerfluctuationsinthisthermalnoise,expressedinWm 2Hz 1,is
sys − −
givenby
kT
D S = sys Wm 2Hz 1 (3)
A √D tDn − −
eff
wherekisBoltzmann’sconstantandA istheeffectiveareaoftheantenna.Notethatinourcase,becausethepulseis
eff
band-limited,theterm√D tDn 1. ForANITA,asingleon-boardantennahasafrequency-averagedeffectiveareaof
0.2m2. Forobservationsofice≃thesystemtemperatureisdominatedbytheicethermalemissivitywithT 320K,
sys
≤
assuming 140Kreceivernoisetemperature. TheimpliedRMSnoiselevelisthusD S=2 106Jy,givingasignal-
∼ ×
to-noiseratioof6.3inthiscase. Thesesimpleargumentsshowthattheexpectedthresholdforneutrinodetectionisof
order1019 eVeventotheedgesoftheobservedareaviewedbyANITA.Inpractice,eventsmaybedetectedatlower
energiesduetofluctuationsorinteractionsclosertothepayload,butmoredetailedsimulationsoftheenergy-dependent
acceptanceofANITAdonotdepartgreatlyfromthisfirst-orderexample.
4
III. INSTRUMENTDESIGN
A. OverviewofTechnicalApproach
Asindicatedbyitsacronym,ANITAisconceptuallyanantennaorantennaarrayoptimizedtodetectimpulsiveRF
eventswithacharacteristicsignatureestablishedbycarefulmodelingandexperimentalmeasurements. Thearrayof
antennasshouldviewmostoftheentireAntarcticicesheetbeneaththeballoon,outtothehorizon,toretainsensitivity
to most of the potential ice volume available for neutrino event production. It should have the ability to trigger
with high efficiency on events of interest, and should have the lowest feasible intrinsic noise levels in its receivers
tomaximizesensitivity. Itshouldhavebroadradiospectralcoverageanddual-polarizationcapabilitytoimproveits
abilitytoidentifythesignalsandrejectthebackgrounds.Itmusthaveimmunitytotransientorsteadyradio-frequency
interference. It must have enough spatial resolution of the source of measured pulses to determine if they match
expectedsignalsources,andtoallowforfirst-ordergeolocationandsubsequentskymappingiftheeventisfoundto
beconsistentwithaneutrino. Itmustmakeasmanydistinctandstatisticallyindependentmeasurementsaspossible
ofeachimpulsethattriggersthesystemcoveringallavailabledegreesoffreedom(spatial,temporal,polarization,and
spectral),becausethenumberofpotentialneutrinoeventsamongthesetriggersmaybeclosetozero,andthispotential
rarityofeventsdemandsthattheinformationcontentofeachmeasuredeventbemaximized.
Theseguidingprincipleshaveledtoatechnicalapproachthatcentersarounddual-polarization,broadbandantenna
clusterswithoverlappingfields-of-view,combinedwithatriggersystembasedonaheritageofRFimpulsedetection
instruments,bothspace-based(theFORTEsatellite[22,23])andground-based(theGLUEandRICEexperiments[4,
5]). Theneedfordirectiondetermination,combinedwiththeconstraintsonusableradiofrequencyrangedictatedby
iceparameters,leadstoanoverallgeometryforbothindividualantennasandtheentirearray,thatisgovernedbythe
requirementsforradiopulseinterferometryoverthespectralbandofinterest.
The key challenges for ANITA are in the area of background rejection and management of electromagnetic in-
terference (EMI). Impulsive interference events are likely to be primarily from anthropogenicsources, and in most
cases do not mimic real cascade Cherenkovradio impulses because they lack many of the requiredpropertiessuch
as polarization-, spectral-, and phase-coherence. A subset of impulsive anthropogenicinterference, primarily from
systemswheresparkgapsorrapidsolid-stateswitchingrelaysareemployed,canproduceeventswhicharedifficultto
distinguishfromeventsofinteresttoANITA,andthusthetaskofpinpointingtheoriginofanyimpulsiveeventisof
highimportancetothefinalselectionofneutrinocandidates.If,afterrejectionofallimpulsiveeventsassociatedwith
anyknowncurrentorpriorhumanactivity,thereremainsaclassofeventswhicharedistributedacrosstheintegrated
fieldofviewofthepayloadandintimeinawaythatisinconsistentwithhumanorigin,wemaythenbegintoconsider
thiseventclassascontainingneutrinocandidates. Whethertheysurvivewiththatdesignationwillultimatelydepend
onourabilitytoexcludeallotherknownpossibilities.
Inapreviousballoonexperiment[24],wefoundAntarcticatoberelativelyradioquietatballoonaltitudesoncethe
payloadleavesthevicinityofthelargestbases. Whatcontinuouswave(CW)interferenceispresentcanbemanaged
withcarefultriggerconfigurationandthresholdadjustmenttoservo-adjustforthetemporaryincreasesinnarrow-band
powerthatoccasionallyare seen. With regardto impulsiveinterference, we foundtriggersdueto itto be relatively
infrequentawayfromthemainbasesthougheventhesmallerbasesdidoccasionallyproduceburstsoftriggers.
Alesswell-understoodbackgroundmayarisefromultra-highenergyairshowerswhichcanproduceatailofradio
emissionouttoANITAfrequencies,buttheseevents,thoughtheymayproducetriggers,areeliminatedonthebasis
oftheirdirection,arisingfromabovethehorizon,andtheirlossofcoherenceatVHFandUHFfrequencies.However,
inallcasesabove,ANITAmaybepresentedwithunexpectedchallenges.
B. BackgroundInterferenceIssues.
BecauseANITAoperateswithextremelyhighradiobandwidthoverfrequenciesthatarenotreservedforscientific
use,theproblemofradiobackgrounds,bothanthropogenicandnatural,iscrucialtothedevelopmentofarobustmis-
siondesign.Wehavenotedpreviouslythatthethermalnoisepower[61]providestheultimatebackgroundlimitation,
for both impulsive and time-averaged measurements, in much the same way that photon noise providesone of the
ultimatelimitstoopticalimagingsystems.
Electromagneticinterferencemaytakedifferentforms:near-sinusoidal”CarrierWave”(CW)interferencecanhave
veryhigh narrow-bandpowerand saturate the system, or it can appear at a low level, sometimesas a composite of
contributionsfrommanybands, andeffectivelyact to raise the aggregatesystem noise. ImpulsiveEMI oftenarises
fromelectronicswitchingphenomena,andmaytriggerthesystemevenifitcannotbemistakenforsignalsofinterest,
5
sincethetriggershouldbeasinclusiveaspossible. ANITAhasonlyonechancepertrueneutrinoeventtodetectand
characterizethe radio wavefrontas it passes by the payload; thusit mustbe as efficientas possible at triggeringon
anythingsimilar to theeventsofinterest. Inthe end, itisthe informationcontentofa giventriggeredmeasurement
thatwilldeterminethe confidencewithwhichwe canascribeittoa neutrinoorigin. Thisconclusionistheprimary
missiondesigndriverforthetypeofpayloadandthenumberofantennas.
Thedesign ofthe mission, payload,ballooncraft,andallancillaryinstrumentationmustthereforebe evaluatedin
thelightofwhetheritproducesEMI,mitigatesit,respondsappropriatelytoit,orfacilitatesrejectionofit. Intheend,
whenallbackgroundinterferencehasbeenrejected,whatisleftbecomesthesubstanceforANITAscience.
1. AnthropogenicBackgrounds.
Backgroundsfromman-madesourcesdo notin generalposea risk of beingmistakenforthe signalsofscientific
interest,unlesstheyarisefromlocationswherenohumanactivityispreviouslyknown. Aswewillshowlaterinthis
report,ANITA’sangularreconstructionabilityforterrestrialinterferenceeventsgivesaccuraciesoforder1degreeor
better,enablinggroundlocationofeventsourcestoalevelmorethanadequatetoremoveeventsthatoriginatefrom
knowncampsoranthropogenicsources.HumanactivityinAntarcticaishighlycontrolledandpositionsandlocations
for all such activity are logged with high reliability during a season. However, man-made sources can still pose a
significantriskofinterferingwiththe operationoftheinstrument. Interferencefromman-madeterrestrialororbital
sourcesisaubiquitousprobleminallofradioastronomy.InthisrespectANITAfacesavarietyofpotentialinterfering
signalswithvariouspossibleimpactsonthedataacquisitionandanalysis.
a. Satellitesignals. Orbitingsatellitetransmitterpowerisgenerallylowinthebandsofinterest.Forexample,the
GPSconstellationsatellitesatanaltitudeof21000km,havetransmitpowersoforder50Winthe1227MHzand1575
MHzbands,withantennagainsof11-13dBi.Theimpliedpowerattheearth’ssurfaceis-127dBWm 2maximumin
−
the1227MHzband. TheimpliedRMSnoisevoltageforANITA,giventheantenna’seffectiveareaatthisfrequency,
is of order0.7 µV, far below the RMS thermalnoise voltage ( 10 15µVRMS) referencedto the receiverinputs.
∼ −
Currentsatellite systemsdonottypicallyoperateinANITA’sband,howevertherearesomelegacysystemsthatcan
producedetectablepowerwithinANITA’sband. Aswewilldiscussinalatersection,ANITAhasencounteredsome
satelliteinterferenceinthe200-300MHzrange,butithasnotcausedsignificantperformancedegradationtodate.
Satellitesdonotingeneralintentionallyproducenanosecond-scaleimpulsivesignals;however,suchsignalsmaybe
producedbysolid-staterelayoractuatoractivityonasatellitethatischangingitsconfiguration. Suchsignalswould
appearto comefromabovethe horizon, butmightalso show up in reflectionoff the ice surface. In this latter case,
the Fresnel coefficient for such a reflection will in general signficantly boost the horizontal polarization of such a
reflection,andthischaracteristicprovidesastrongdiscriminator,iftheinitialabove-the-horizonimpulsewasforsome
reasonnotdetected.
b. Terrestrial signals. The primary risk for terrestrial signals is not that they trigger the system. Terrestrial
sources often do producesignificant impulsiveinterference, and will trigger our system at significantrates anytime
thepayloadiswithinviewofsuchanthropogenicsources. However,suchtriggersareeasilyselectedagainstinpost-
analysis since their directions can be precisely associated with known sources in Antarctica. The greater issue for
ANITAoccursifthereisastrongtransmitterinthefieldofviewwhichsaturatestheLNA,causingitsgaintodecrease
so that the sensitivity in that antenna is lost. The present LNA design tolerates up to about 1 dBm output before
saturation,withaninputstagegainof36dB.Thusasignalof0.25µWcoupledintotheantennawouldposeariskof
saturationandtemporarylossofsensitivity.
Sincetheantennaeffectiveareaisoforder0.6m2atthelowendoftheband,ANITAthereforetoleratesuptoa0.2
MWin-bandtransmitteratornearthehorizon,oraseveralkWin-bandtransmitternearthenadir,accountingforthe
off-axisresponseoftheANITAantennas. MostofthehigherpowerradarandothertransmittersinuseinAntarctica
areprimarilyattheSouthPoleandMcMurdostations. Suchsystemsdidreduceoursensitivitywhenthepayloadwas
incloseproximitytoMcMurdostation,andtoalesserdegree,wheninviewoftheSouthPolestation.
2. Otherpossiblebackgrounds.
a. Lightning. Lightningisknowntoproduceintenseburstsofelectromagneticenergy,butthesehaveaspectrum
that falls steeply with frequency, with very little power extendinginto the UHF and microwave regimes. Although
lightningdoesoccuroverthe SouthernOcean[25, 26], itis unknownon theAntarctic continent. We donotexpect
lightningtocompriseasignificantbackgroundtoANITA.
6
b. CosmicRayAirShowerbackgrounds. Cosmicrayextensiveairshowers(EAS)atEeVenergiesalsoproduce
an electromagnetic pulse, known from observations since the late 1960’s. The dominant RF emission comes from
synchrotron radiation in the geomagnetic field. This emission is coherent below about 100 MHz, transitioning to
partialcoherenceaboveabout200MHzintheANITAband. Althoughtherehasbeenarecentincreaseinactivityto
measuretheradiocharacteristicsofEASeventsinthecoherentregimebelow100MHz[27],thereisstilllittlereliable
informationregardingthepartiallycoherentregimewhereANITAissensitivetosuchevents,althoughinfactseveral
oftheearlydetectionsofsucheventswereat500MHz[28]. TheradioemissionfromEASishighlybeamed,sothe
acceptanceforsucheventsisnaturallysuppressedbygeometry.Theyarealsoexpectedtohaveasteeplyfallingradio
spectralsignature, and thusan invertedspectrum comparedto eventsoriginatingfromthe Askaryanprocess, which
hasanintrinsicrisingspectrumoverthefrequencyregionthatcoherenceobtains,andaslowplateauanddeclineabove
thosefrequencies.
ANITAmaydetectsucheventseitherbydirectsignalsorreflectedsignalsofftheicesurface,inamannersimilar
to that mentioned above for posible impulses from satellites. The EAS signals are known to be linearly polarized,
withtheplaneofpolarizationdeterminedbythelocalgeomagneticfielddirection. Sincethefieldis largelyvertical
inthepolarregions,thereisatendencyfortheEASradioemissiontobehorizontallypolarizedforairshowerswith
largezenithangles. ANITA’sfield-of-view,whichhasmaximumsensitivitynearthehorizon,thusfavorsEASevents
with these largezenithangles. Such eventswhenobserveddirectlyarrivefromanglesabovethehorizon,butunder
therightcircumstancestheymayalsobeseeninreflection,thusappearingtooriginatefrombelowthehorizon. They
mightthusbeconfusedwithneutrino-likeeventsoriginatingfromundertheice,iftheirradio-spectralandpolarization
signaturewas notconsidered. In an appendixwe will addressthis possible physicsbackgroundand show whyit is
straightforwardtoseparateitfromtheeventsofinterest.
C. CSBFSupportInstrumentationPackage.
UNIVERSAL
SCIENCE FLIGHT COMPUTER SSCTIEANCCKE TEPR(AUMCTIKPNA)AGTEION
TDRSS HI−RATE COMM1
SCIENCE LOW−RATE
INTERFACE SCIENCE "PARTY LINE"
INTERFACE
LOS
TRANSMITTER
COMM2 LOW−RATE
SCIENCE
INTERFACE
SCIENCE LOS
DOWDANTLAINK COMM1 ISOLATOR COMM2
TDRSS SIDE IRIDIUM SIDE
IRIDIUM CMD
CROSSOVER
GPS GPS
RECEIVER BACKUP CMD RECEIVER
DECODER
BACKUP NAV SYSTEM
MIP−BASED
TDRSS UHF CMD LOS PCM IRIDIUM IRIDIUM 9601
TRANSEIVER TRANSCEIVERS TRANSMITTER ENCODER MODEM W/ ad INTERFACE
DCAMTADT DDUROPSWLSLNINLKINK LORS(OE CTTMRHDA ON UNSPLMYLII)TNK LDOOSW DNALTINAK CDMAITRDAI DU DIPUOLMWINNKLINK CDROIWTINCLAILN KPARAMETERS
FIG.1:BlockdiagramoftheNSBFSIP.
Supportfor NASA long-durationballoon payloadlaunchesand in-flightservices is providedthroughthe staff of
the Columbia Scientific Balloon Facility (CSBF), based in Palestine, Texas, USA. CSBF has developed a balloon-
craftSupportInstrumentPackage(SIP),anintegratedsuiteofcomputers,sensors,actuators,relays,transmitters,and
antennas,forusewithallLDBscienceinstruments. TheCSBFSIPiscontrolledbyapairofindependentflightcom-
puters which handle science telemetry, balloon operations, navigation, ballast control and the final termination and
descentofthepayload. AsystemdiagramoftheSIPisprovidedinFigure1. AScienceStack,aconfigurablesetof
blockmodules,isalsoavailableasanoptiontotheSIPprovidingsuchfunctionsasasimplescienceflightcomputer,
analog-to-digitalconversion,andopen-collectorcommandoutputsforadditionalinstrumentcommandandcontrol.
7
The SIP also provides the telemetry link between the ANITA flight computer and data acquisition system and
groundbasedoperations. DatafromtheANITAcomputerissentoverseriallinestotheSIPpackagewhichhandles
routingandtransmissionoverline-of-sight(LOS),TrackingandDataRelaySatelliteSystem(TDRSS),andIRIDIUM
communicationpathways. ANITAutilizestheNSBFSIPScienceStacktoprovidetheabilitytocommandtheflight
CPUsystemoffandonandrebootthecomputerduringflight.
WithregardtocomputationalresourcesoftheSIP,thesearedesignedtofulfillexistingLDBrequirements,including
preserving a full archive of all telemetered data that is passed through the SIP from the science instrument. This
functionthusprovidesanadditionalredundantcopyofthetelemetereddatathatcanbeusedifthereistelemetryloss
orcorruption.
OneimportantcharacteristicoftheSIPrelevanttoANITAisthatitisnothighlyshieldedfromproducinglocalEMI,
atleastattheextremelylowlevelrequiredforcompatibilitywithANITAsciencegoals.OfnecessitytheSIPwasthus
enclosedinanexternalFaradayhousing,withconnectors,andpenetratorsdesignedinamannersimilartowhatwas
donefortheANITAprimaryelectronicsinstrumentation.
D. GondolaStructure.
FIG.2: ANITApayloadinflight-readyconfigurationwithlaunchvehicle.
Thegondolastructureconsistsprimarilyofanaircraft-gradealuminumalloyframe.Amatrixoftubularcomponents
is pinnedtogether via a combinationof socket joints, tongueand clevis joints, and quarter-turncam-lockfasteners.
ViewsjustpriortothelaunchofthepayloadshowthestructuralelementsofthegondolainFig.2. Theframeisbased
onoctagonalsymmetrywhereeightverticalmembers,pluscrossbracing,provideaninternalbackbonethatallowsfor
theattachmentofspoke-liketrussestowhichthehornantennasfasten. Threering-shapedclustersofquad-ridgedhorn
antennasconstitutethe primaryANITAsensors. Thetoptwo antennaclustershaveeightantennaseach. Positioned
aroundtheperimeterofthe baseis a sixteenhorncluster. All ofthe eighthornclustershavea 45 azimuthaloffset
◦
anglebetweenadjacentantennas,witha22.5 azimuthaloffsetbetweenthetoptworings.Theantennasinthesixteen
◦
hornringareoffsetfromeachotherby22.5 . Alloftheantennasintheuppertwoeighthornringsandtheonesin
◦
thesixteenhornringarecanteddown10 belowhorizontaltooptimizetheirsensitivity,basedonMonteCarlostudies
◦
of the effects of the tapering of the antenna beam when convolved with the neutrino arrival directions and energy
spectrum.
The nearlycircular planethat is established by the sixteen antennaring, nearthe base of the gondola, providesa
largedeckareaformostoftheotherpayloadcomponents.Thisregioniscoveredbylightweightpanelsmadeofdacron
sailclothonthe topsideanda reflectivelayerontheundersideto maintainthermalbalance. TheANITAelectronics
housing, the NASA/CSBF SIP, and the battery packs are mountedon the structuralribs of the deck. Most external
metallicstructureispaintedwhitetoavoidoverheatinginthecontinuoussunlight,andcriticalcomponentssuchasthe
instrumenthousingsandreceiversarecoveredwithsilver-backedteflon-coatedtapetoprovidehighreflectiverejection
ofsolarradiationandhighemissivityforinternalheatdissipation.
8
E. Powersubsystem.
TheANITApowersystemiscomposedofaphotovoltaic(PV)array,achargecontroller,batteries,relays,andDC-
to-DCconverters.ThePVarrayisanomni-directionalarrayconsistingofeightpanelsconfiguredinanoctagon,with
thepanelshangingvertically(seeinstrumentfigure).AlthoughPVpanelsflownonhigh-altitudeballoonsaretypically
orientedat 23 tothehorizontal,inAntarctica,thelargesolaralbedofromtheiceresultsinmoreirradianceincident
◦
∼
onthepanels(formostconditions)iftheyhangvertically.Eachpanelconsistsof84solarcellselectricallyconnected
in series. They were mounted on frames made of aircraft-grade spruce wood with a coarse webbing (Shearweave
style1000-P02)stretchedontheframes.
The PV arrays were designed and fabricated by SunCat Solar. The solar cells used were Sunpower A-300 cells
witharatedefficiencyof21.5%anddimensions12.5cmsquareandthickness260um.Bypassdiodeswereplacedin
parallelwithsuccessivegroupsof12cellswithinapanel(7-diodes/panel)tomitigatetheeffectofapossiblesinglecell
opencircuitfailureduringflight.Additionally,ablockingdiodewasplacedbetweeneachpaneloutputandthecharge
controllertopreventcross-chargingofpanelswithdifferentoutputvoltagesresultingfromdifferentilluminationsand
temperatures.ToreduceFresnelreflectionlossesforhigh-refractiveindexsilicon(n=3.46at700nm),thesiliconcells
hadtwoanti-reflective(AR)coatingsapplied. AnARcoatingwithrefractiveindexn=1.92wasappliedbythesolar
cellmanufacturers.Additionally,duringfabricationofthepanelsbySunCatSolar,asecondARcoatingwithrefractive
index1.47wasapplied.ThisresultsincalculatedFresnellossesof13-14%forincidenceanglesfrom0to40 .
◦
Themaximumpowerpoint(MPP)voltageandcurrentgeneratedbythesecellsunderstandardconditions(STC)are
0.560Vand5.54Arespectively.However,theactualVandIvaryconsiderablydependingupontheirradianceandcell
temperature. Thesingle-celltemperaturecoefficientforthevoltageis-1.9mV/C.PVpaneltemperaturesvariedover
therangeof-10Cto+95C,dependingupontheirradianceincidentuponthecells. Thetemperaturesweremeasuredby
semiconductortempsensors(AD590)gluedtothebackofcells. PVarraycircuitcomponents(diodes)alsointroduce
lossesintheoutputvoltageandpower.TheactualmeasuredPVvoltageinputtothechargecontrollerduringtheflight
rangedfrom42.5to47V(ingoodagreementwithestimatesusingthecelltemperatureandtemperaturecoefficient)
andthecurrentwasabout9Agivingatotalpowerof400W.
The omni-directionalarrayis inherentlyan unbalancedsystem; i.e. the irradianceincidenton each paneldiffers.
Foragivenorientationofthegondola,somepanelsaredirectlyirradiatedbysunlightplussolaralbedofromtheice
and others are irradiated only indirectly from solar albedo. Additionally, for those that are directly irradiated, the
solar incidenceangleisdifferent. Thisresultsin individualpanelsthatgenerateverydifferentcurrentsatanygiven
time. Becauseofthedifferingtemperatures,theindividualpanelsalsohavesignificantlydifferentoutputvoltages(the
voltagedifferencesaresmallcomparedtothecurrentdifferences)thatfeedintothechargecontroller. Asmentioned
above,theblockingdiodespreventcross-chargingofpanelsgeneratingdifferentvoltages.
Whenusinganunbalancedarray,toachievethemaximumpoweroutput,itisimportanttouseachargecontroller
that senses and operates at the actual MPP as opposed to one that operates at a constant offset voltage from the
arrayopen-circuitvoltage. WeusedanOutbackMX-60chargecontrollertosupplypowertotheANITAinstrument.
ConductiveheatsinkswereinstalledonthepowerFETsandtransistorsandtheheatwasconductedtotheinstrument
radiatorplate. We operatedinthe24V modeandflewninepairsof12VPanasonicLC-X1220P(20AH)leadacid
batteriesthatwerechargedbythechargecontrollerandwouldhaveprovided12hoursofpowerincaseofPVarray
failure.
The InstrumentPower boxconsistedof the MX-60chargecontroller, solid-statepowerrelays, and VicorDC/DC
convertersfor the externalradio-frequencyconditionalmodule (RFCM) amplifiers. The main power relays for the
cPCI crate were controlled by discrete commands from the SIP. All other solid-state relays were controlled by the
CPU, either under software controlor by commandsfrom the ground. The DC/DC box consisted of Vicor DC/DC
converterswhichprovidedthe+5,+12,-12,+3.3,+1.5,and5voltagesrequiredbythecPCIcrateandperipherals.All
voltagesandcurrentswerereadbythehousekeepingsystem.
F. RadioFrequencysubsystem
1. Antennas.
Figure2showstheANITApayloadconfigurationjustpriortolaunchinlate2006atWilliamsField,Antarctica.The
individualhornsareacustomdesignproducedforANITAbySeaveyEngineering,Inc.,nowasubsidiaryofAntenna
ResearchAssociates,Inc.ThesehornsaretheprimaryANITAantennas,andmaybethoughtofasaflaredquad-ridged
waveguidesection;thebackofthehorndoesinfactterminateinashortsectionofwaveguide. Thedimensionofthe
9
3 dB bandwidth
200−1280 MHz
Seavey Quad−ridge
horn, ANITA model
FIG.3: Left: AphotographofanANITAquad-ridgeddual-polarizationhorn. Right: Typicaltransmissioncoefficientforsignals
intothequad-ridgedhornasafunctionofradiofrequency.
mouthisoforder0.8macross,andthehornscanbeclose-packedwithminimaldisturbanceofthebeamresponsesince
thefringingfieldsoutsidethemouthofthehornaresmall. Figure3showsanindividualantennapriortopainting,and
acorrespondingtypicaltransmissioncurveindicatingtheefficiencyforcouplingpowerintotheantenna,asafunction
ofradiofrequency.
0o QR−192175 0o QR−192175
5 dB 5 dB
−30o 30o QR−192176 −30o 30o QR−192176
0 QR−192179 0 QR−192179
−5 QR−192180 −5 QR−192180
QR−192181 QR−192181
−60o −10 60o QR−192182 −60o −10 60o QR−192182
−15 QR−193184 −15 QR−193184
QR−193185 QR−193185
−20 QR−187825 −20 QR−187825
−25 −25
−90o 90o −90o 90o
Vertical H−plane mean relative gain Horizontal E−plane mean relative gain
0o 0o
5 dB 5 dB
−30o 30o −30o 30o
0 0
−5 −5
−60o −10 60o −60o −10 60o
−15 −15
−20 −20
−25 −25
−90o 90o −90o 90o
Vertical E−plane mean relative gain Horizontal H−plane mean relative gain
FIG.4: Left: Antennavertical-polarizationrelativedirectivityindBrelativetothepeakgainforbothEandH-planes. Right: the
samequantitiesforthehorizontalpolarization. Ninedifferentantennasareshown. Gainisfrequency-averagedforaflat-spectrum
impulseacrossthebandfordifferentanglesin
10
The averagefull-width-at-half-maximum(FWHM) beamwidthof the antennasis about45 with a corresponding
◦
directivitygain(theratioof4p tothemainbeamsolidangle)ofapproximately10dBiaverageacrosstheband.Fig.4
illustratesthisforninedifferentANITAantennas,showingthefrequency-averagedresponserelativetopeakresponse
alongtheprincipalantennaplanes(E-planeandH-planeforbothpolarizations)asafunctionofangle. Thechoiceof
beampatternforthese antennasalso determinedthe 22.5 angularoffsetsin azimuth,asthis waschosento provide
◦
goodoverlapbetweentheresponseofadjacentantennas,butstillmaintainingreasonabledirectivityfordetermination
ofsourcelocations.
Byarranginganazimuthallysymmetricarrayof2clustergroupsof8+8(upper)and16(lower)antennas,eachwitha
downwardcantofabout10 ,weachievecompletecoverageofthehorizondowntowithin40 ofthenadir,virtuallyall
◦ ◦
oftheobservableicearea. Theantennabeamsinthisconfigurationoverlapwithintheir3dBpoints,givingredundant
coverageinthehorizontalplane. The 3mseparationbetweentheupperandlowerclustersof16antennasprovides
∼
a verticalbaselineforestablishingpulse directionin elevationangle. Becausethepulsefromacascadeis knownto
be highly linearly-polarized, we convert the two linear polarizations of the antenna into dual circular polarizations
usingstandard90 hybridphase-shiftingcombiners.Thisisdonefortworeasons:first,alinearlypolarizedpulsewill
◦
produceequalamplitudesin bothcircularpolarizations,andthussome backgroundrejectionisgainedbyaccepting
only linearly-polarizedsignals; and second, the use of circular polarizationsremovesany bias in the triggertoward
horizontalorverticallypolarizedimpulses.
el0az0 horizontal
1
QR−192175
QR−192176
effective height, m/ns−00..055 QQQQQQQRRRRRRR−−−−−−−111111199999982222337111111878888829012455
−1
5 10 15 20 25 30 35
time, ns
el0az0 vertical
1
QR−192175
effective height, m/ns−00..055 QQQQQQQQRRRRRRRR−−−−−−−−111111119999999822222337111111187788888269012455
−1
5 10 15 20 25 30 35
time, ns
FIG. 5: Left: Antenna impulse response as measured for nine ANITA antennas, here inunits that also give the instantaneous
effectiveheight[20].Right:ANITAimpulseresponseasitappearsvariousstagesofthesignalchain.
BecauseANITA’ssensitivitytoneutrinoeventsdependscruciallyonitsabilitytotriggeronimpulsesthatriseabove
the intrinsically impulsive thermal noise floor, ANITA’s antennas and receiving system must preserve the narrow
impulsivenatureofanysignalthatarrivesattheantenna. Fig.5showsthemeasuredbehaviorofthesystemimpulse
response at various stages. On the left, we show details of the measured impulse response of nine of the flight
V
antennas, in unitsthat give the instantaneouseffective heighth (t). The actualvoltage time response (t) at the
eff
antennaterminals,assumingtheyareattachedtoamatchedload,isthenjusttheconvolutionofthisfunctionwiththe
E
incidentfield (t):
1
V E
(t) = (t) h (t)
eff
2 ⊗
where the convolutionoperatoris indicatedby the symbol . This equationcan also be expressedas an equivalent
⊗
frequencydomainform,thoughinthatcasethequantitiesareingeneralcomplex.
On the right, we show the evolution of an Askaryan impulse through the ANITA system. The initial Askaryan
impulse(a)iscompletelyunresolvedbytheANITAsystem,sinceitsintrinsicwidthisoforder100ps(reference[20]