HAWC High Energy Upgrade with a Sparse Array

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by  Vikas Joshi

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(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2017
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HAWC High Energy Upgrade with a Sparse Array
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3 V.Joshi1,a fortheHAWCcollaboration2
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1MaxPlanckInstitutfürKernphysik,Heidelberg,Germany

2Foracompleteauthorlist,seewww.hawc-observatory.org/collaboration/
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I Abstract. The High Altitude Water Cherenkov (HAWC) gamma-ray observatory has
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h been fully operational since March 2015. To improve its sensitivity at the highest en-
p ergies,itisbeingupgradedwithanadditionalsparsearraycalledoutriggerarray.Wewill
- discussinthiscontribution,thedifferentoutriggerarraycomponents,andthesimulation
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resultstooptimizeit.
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1 HAWC and the Motivation for Outriggers
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HAWCissituatedincentralMexicoatanaltitudeof4100mabovethesealevel. Ithasawidefield
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7 of view of 2 sr and operational energy range of 0.1-100 TeV. It consists of 300 Water Cherenkov
3 Detectors (WCDs) in the main array encompassing a surface area of 20000 m2. The main array
6 WCDs comprised of cylindrical steel water tanks of diameter 7.3 m and height 4.5 m with 4 Photo
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MultiplierTubes(PMTs)(three8”andone10”)ineachoneofthem. HAWCdetectstheCherenkov
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1 lightproducedinthewaterbyparticlesgeneratedinanatmosphericairshower.
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WhentheenergyoftheprimaryparticleisoftheorderoftensofTeV,thefootprintoftheshower
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becomescomparabletothesizeofthemainarray. Therefore, mostoftherecordedshowersarenot
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containedinthearray,whichcauseschallengestoconstraintheshowerproperties. Toaddressthese
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challengestheconstructionoftheoutriggerarrayaroundthemainarrayhasstarted. Itwillincrease
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X the fraction of well-reconstructed showers above multi-TeV energies. The outrigger array will help
r in determining the position of the core of the shower falling outside the main array and it will also
a improvethedeterminationoftheprimaryparticle’sdirectionandenergy.
2 Outrigger Array
The outrigger array [1] consists of 350 cylindrical tanks of diameter 1.55 m and height 1.65 m (see
Figure1a). EachtankhasoneHamamatsuR59128"PMTatthebottomofthetank. Theoutrigger
array will be deployed in a circular symmetric way around the main HAWC array with a mutual
separationof12mto18m(seeFigure1b).
To trigger and readout, the system electronics developed for the FlashCAM [2] will be used.
FlashCAM is a readout electronics, which has been developed for the cameras of the medium-size
telescopesoftheCherenkovTelescopeArray.ThereasonforusingtheFlashCamreadoutforoutrigger
ae-mail:[email protected]
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Figure1. a. Outriggertankandmainarraytanks. b. OutriggerarraysurroundingthemainHAWCarray. The
redlinesshowsthedifferentsectionsoftheoutriggerarray.
array is that each PMT of the outrigger array is equivalent to a pixel of an Imaging Atmospheric
Cherenkov Telescopes (IACTs) camera. The outrigger array is divided into five sections with 70
outriggers in each of them. One such section will contain a readout and trigger electronics, which
wenamedasthe: FlashAdcelectronicsfortheCherenkovOutriggerNode(FALCON).Anodewill
contain 3 Flash-ADC boards, each of them can digitize 24 channels with a sampling speed of 250
MHzwitha12-bitaccuracy. Italsoallowsaflexibledigitalmultiplicitytriggeraswellasthereadout
offullwaveforms,withsettablelength(typically40samplesi.e.160ns)whichcanbeusedforcharge
extractionandsignaltiminginformation.
3 Simulations
We performed extensive simulations in order to optimize the outrigger array. This can be further
dividedintotwoparts:
1.SimulationstostudytheeffectofdifferentPMToptionsandtankcolors.
2.Simulationstodevelopalikelihoodfitmethodinordertofittheshowercoreandtoconstrainthe
showerenergyandthedepthoftheshowermaximum.
3.1 SimulationsforPMTOptionsandTankColors
InordertochoosethesizeofthePMT,differentPMTsizeshavebeensimulatedincombinationwith
different tank wall colors. Here we present the results for the 3" and 8" PMT with tank wall colors
blackandwhite. Wehavefocusedonthefollowingfiguresofmerit:
1.AveragenumberofPhoto-Electrons(PEs)observedatagivendistancefromtheshowercore.
2.RMSofthedistributionofthetimedifferencebetweenneighboringtankpairsforthearrivaltime
ofthefirstPE.
ItcanbeseenfromtheFigure2thatonegets10timesmorePEswiththe8"PMTincomparison
tothe3"PMTandtheeffectofthewhitewallcolorinthecontrastofblackwallcoloris20%increase
in the number of PEs observed. Furthermore, the white wall color is more diffusive than the black
Givetheexacttitleoftheconference
wall color, and the loss of the timing information by using the white wall color it can be more than
20%(seeFigure3)incomparisontotheblackwallcolor.Itcanbeconcludedthatwedon’tgainmuch
intheaveragenumberofPEsobservedbyusingthewhitewallcolorandweloseconsiderablyinour
timinginformation. Wedecidedthatblackwallcolortanks(lessdiffusive)with8"PMTseemstobe
theappropriatechoice.
pe Black, Energy = 3 TeV pe Black, Energy = 3 TeV
White, Energy = 3 TeV White, Energy = 3 TeV
m103 Black, Energy = 10 TeV m104 Black, Energy = 10 TeV
White, Energy = 10 TeV White, Energy = 10 TeV
Black, Energy = 30 TeV Black, Energy = 30 TeV
White, Energy = 30 TeV White, Energy = 30 TeV
102 Black, Energy = 50 TeV 103 Black, Energy = 50 TeV
White, Energy = 50 TeV White, Energy = 50 TeV
10 102
0 20 40 60 80 100 0 20 40 60 80 100
Distance from the shower core [m] Distance from the shower core [m]
Figure2. AveragenumberofPE(µ )observedfor3”PMT(left)and8"PMT(right)withblackandwhite
pe
tanksasafunctionofdistancefromtheshowercorefordifferentenergies.
k)
n2.2 RMS ratio for E=3TeV
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k t 2 RMS ratio for E=10TeV
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a RMS ratio for E=30TeV
Bl1.8
nk/ 1.6 RMS ratio for E=50TeV
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Whit1.2
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0 20 40 60 80 100
Distance from the shower core [m]
Figure3. RatiooftheRMSs(seeSection3.1)forwhite/blacktanksfor8”PMTasafunctionofdistancefrom
theshowercorefordifferentenergies(E).
3.2 SimulationsforLikelihoodCoreFitMethod
Toconstrainthecorelocationofthemulti-TeVγ-rayshowersfallingoutsidethemainHAWCarray
alikelihoodcorefitterisbeingdeveloped. InFigure4wecanseethatacoreresolutionof<10mis
achievedbyjustusingtheoutriggersforenergies>10TeVandforzenithangleupto30◦.Inaddition,
thislikelihoodmethodalsoconstrainstheshowerenergyanddepthoftheshowermaximum. Inthe
nextstep,thislikelihoodfitmethodfortheoutriggerswillbemergedwiththeoneforthemainarray
toultimatelyimprovethecoreresolutionformulti-TeVshowers.
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m Center of Gravity, Zang = 0 deg
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[ Likelihood Fit, Zang = 0 deg
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o 40 Center of Gravity, Zang = 15 deg
ti Likelihood Fit, Zang = 15 deg
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l Center of Gravity, Zang = 30 deg
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s 30 Likelihood Fit, Zang = 30 deg
Re Center of Gravity, Zang = 45 deg
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Likelihood Fit, Zang = 45 deg
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or 20 PRELIMINARY
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Energy [TeV]
Figure4.Thecoreresolutionobtainedwithalikelihoodfitincomparisonwiththecenterofgravityofthesignal
fordifferentzenithangles(Zang).Theverticaldashedlinesrepresentthebinningintheenergyrange.Thepoints
ineachoftheseenergybinscorrespondtothe68%containmentofthecoreresolutiondistribution.
4 Current Status of the Outrigger Array
The deployment of the outrigger array has already started. FALCON electronics is being used to
takethedatafromthefirstsetofoutriggersinstalledattheHAWCsite. IntegrationoftheFALCON
readoutwiththecentralDAQisongoingandwillbefinishedsoon. Acompleteoutriggerarraywill
befullyoperationalbytheendofthenextyear.
Acknowledgements
Weacknowledgethesupportfrom:theUSNationalScienceFoundation(NSF);theUSDepartmentof
EnergyOfficeofHigh-EnergyPhysics;theLaboratoryDirectedResearchandDevelopment(LDRD)
programofLosAlamosNationalLaboratory;ConsejoNacionaldeCienciayTecnologa(CONACyT),
Mexico (grants 260378, 55155, 105666, 122331, 132197, 167281); Red de Fsica de Altas Energas,
Mexico;DGAPA-UNAM(grantsIG100414-3,IN108713,IN121309,IN115409,IN113612);VIEP-
BUAP(grant161-EXC-2011);theUniversityofWisconsinAlumniResearchFoundation;theInstitute
ofGeophysics,PlanetaryPhysics,andSignaturesatLosAlamosNationalLaboratory;theLucBinette
FoundationUNAMPostdoctoralFellowshipprogram.
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