The ARGO-YBJ experiment in Tibet( ) A.Aloisio( 1 ),C.Ba i( 2 ),K. Z. Bao( 3 ),F. Barone( 4 ),B. Bartoli( 5 ) P.Bernardini( 6 ), C.Bleve( 6 ), P.Bran hini( 2 ),S. Bussino( 2 ),E.Calloni( 5 ) P.Camarri( 7 ), B. Y.Cao( 8 ),R. Cardarelli( 7 ),M. Casolino( 7 ) S.Catalanotti( 5 ),A.Cavaliere( 7 ),S.Cavaliere( 5 ),F.Cesaroni( 6 ),M.Cha( 9 ) P.Creti( 6 ),G. Cusimano( 10 ), G.D'Al i Staiti( 10 ), Danzengluobu( 11 ) V.D'Elia( 7 ),I.De Mitri( 6 ),B. D'EttorrePiazzoli( 5 ), M.DeVin enzi( 2 ) T.Di Girolamo( 5 ),G.Di S ias io( 5 ),Z.Y. Feng( 12 ),Y. Fu( 8 ),X. Y. Gao( 13 ) Q. X.Geng( 13 ),H.W. Guo( 11 ), H.H. He( 9 ),Q. Huang( 12 ),M.Ia ova i( 5 ) N.Iu i( 2 ),H. Y. Jai( 12 ), F. M.Kong( 8 ),H. H.Kuang( 9 ), Laba iren( 11 ) B. Li( 3 ),J. Y. Li( 8 ),B. Liberti( 7 ),G.Liguori( 14 ),Z. Q. Liu( 13 ),H. Lu( 9 ) X.H.Ma( 9 ),G.Man arella( 6 ),S.M.Mari( 2 ),G.Marsella( 6 ),D.Martello( 6 ) X.R. Meng( 11 ),L. Milano( 5 ),T. Mineo( 10 ), J. Mu( 13 ),L. Ni astro( 10 ) D. Orlando( 6 ),M.Panareo( 6 ),A.Paoloni( 7 ),Z. R.Peng( 9 ),P.Pistilli( 2 ) G.Raso( 10 ),B. Sa o( 10 ),L. Saggese( 5 ),R.Santoni o( 7 ),L.S arsi( 10 ) P.R. Shen( 9 ),C.Stanes u( 2 ), M.Storini( 2 ),L. R.Sun( 3 ),S. C.Sun( 3 ) A.Surdo( 6 ),Y.H.Tan( 9 ),P.Vallania( 15 )( ),S.Vernetto( 15 ),C.R. Wang( 8 ) H.Wang( 9 ),H.Y. Wang( 9 ),Y. N. Wei( 3 ), Q. K. Yao( 3 ),G.C.Yu( 12 ) X.D. Yue( 3 ), A.F. Yuan( 11 ), H.M.Zhang( 9 ),J. L. Zhang( 9 ), N.J. Zhang( 8 ) T.J. Zhang( 13 ), X.Y. Zhang( 8 ), Zhaxisangzhu( 11 ),Zhaxi iren( 11 ) andQ. Q. Zhu( 9 ) ( 1
) INFNSezionediNapoliandUniversitadelSannio-Benevento, Italy
( 2
) INFNandDipartimento diFisi adell'Universita\RomaTre"-Roma,Italy
( 3
) Zhenghou University-Henan, China
( 4
) INFNSezionediNapoliandDipartimentodi S ienzeFarma euti hedell'Universita
Salerno, Italy
( 5
) INFNandDipartimento diFisi adell'Universita-Napoli,Italy
( 6
) INFNandDipartimento diFisi adell'Universita-Le e,Italy
( 7
) INFNandDipartimento diFisi adell'Universita\TorVergata" -Roma,Italy
( 8
) ShandongUniversity-Jinan,China
( 9
) IHEP -Beijing,China
( 10
) INFNSezione diCataniaandIstituto diFisi aCosmi aIFCAI/CNR-Palermo,Italy
( 11
) TibetUniversity-Lhasa,China
( 12
) SouthWestJiaotongUniversity-Chengdu,China
( 13
) YunnanUniversity-Kunming,China
( 14
) INFNSezione diPavia-Italy
( 15
) IstitutodiCosmogeo si adelCNRandINFN-Torino,Italy
(ri evutoil4Di embre2000;approvatoil12Febbraio2001)
(
) Paper presented at the Cha altaya Meeting on Cosmi Ray Physi s, La Paz, Bolivia,
July23-27 ,2000.
(
1. {Introdu tion
Thestudyof radiation in theenergy range100GeV{20 TeVis mainlydevotedto
-astronomy,forbothpointanddi usesour es,andgamma-raybursts(GRBs)physi s.
Pointsour esaregala ti andextragala ti obje ts;di usesour esin lude theGala ti
Plane,mole ular loudsandSupernovaRemnants. Atthese energiesthepau ityofthe
uxeshampersthedete tionfromspa e;thehighestenergydete tedtillnowisfewtenth
ofGeVfrom theEGRETexperimentonboardtheCGROsatellite. Sin ethedete tion
oftheCrabNebulain1989bytheWhippleCollaboration[1℄,ground-based -astronomy
has olle tedbrilliantresultsbymeansof
Cerenkovteles opes;uptonow5gala ti and
3 extragala ti obje ts have been dis overed in the TeV energy region. During these
years this te hnique has been well developed, rea hing a low energy threshold ( 300
GeV,andevenlessforsolarpowerfa ilitiesusedaslight olle tors)andahighsensitivity
forpoint sour es(.1/10 oftheCrab ux,the \standard andle"for TeVastronomy).
Thelimits are the low duty- y le ( 5%), sin e observations an be doneonly during
lear moonless nights, and the small eld of view (# < 4 Æ
). This means that only
one andidate, sele ted sour e at atime an be observed, with no dis overypotential
forunknownsour es. A large eldofviewisevenmore ru ial forGRBsdete tion,the
probabilityof o urren eof theseunpredi tableeventsby han ein the eld ofviewof
a
Cerenkovteles opebeingnegligible(< 10 4
).
Adi erentapproa hhasbeendoneusingEASarrays,thatsample ontinuouslyand
withalarge eldofview( sr)thelateraldistributionofextensiveairshowers. But
due to thepoor or la king osmi rays ba kgroundreje tion, and to thehigher energy
threshold(E>10TeV),nohighlyreliabledete tionhasbeendonesofar; alowenergy
threshold, parti ularly for extragala ti sour es (AGNs and GRBs), is requireddue to
the -rayabsorptionintheintergala ti spa ebyintera tionwithstarlightphotons.
Fromthese onsiderationswe andedu ethene essityofadete torwithalarge eld
Fig. 1. {Comparison between the longitudinal development of -indu edshowers of primary
energy100GeV{1000TeVand theshowerre onstru tionthresholdfordi erentte hniquesat
variousheights.
theserequirements,imagingtheele tromagneti omponentofsmallairshowersathigh
altitudewith: highspa e-timeresolution,togetahighgranularitypi tureoftheshower;
full overage,to de rease theenergy threshold withoutlosing showerdetails and large
olle ting area to in rease the sensitivity and the energy range. The ombined e e t
of the full overageand high altitude lo ation on the threshold energy an be seenin
g.1, where the longitudinaldevelopmentof -indu edshowersof di erentenergies is
ompared with the typi al sensitivities of two di erent te hniques lo ated at di erent
altitudes. The threshold energy of a traditional sampling array operating at 2000 m
a.s.l. anbeloweredfrom50 TeV to1TeV usingafull overage arpet andto
200GeVmovingfrom2000to4000m a.s.l.,withatotalgainfa toras highas250.
WiththesefeaturestheGeV(satellites)andTeV(
Cerenkovteles opes)energyregions
anbebridgedandtheenergyrangebetween20GeVand300GeV,thatis,forthetime
being, ompletelyunexplored, an beatleastpartlya essed.
2. {The dete tor
The ARGO-YBJ dete tor [2℄ is lo ated in Tibet, China, at the Yangbajing High
Altitude Cosmi Ray Laboratory (30 Æ .11 N, 90 Æ .53 E, 4300 m a.s.l., 606 g/ m 2 ). It
onsistsofasinglelayerofResistivePlateChambers(RPCs)ofdimensions7874m 2
with full overage (a tivesurfa e 92%). Thearea surrounding the entral arpet is
partially( 20%)instrumentedwithRPCsupto11199m 2
,thetotala tivesurfa e
being6700m 2
. As hemati planofthedete torisshownin g.2.
TheRPCsperforman e,in termsoftemporalandspatialresolution, atareasonable
ost makes this devi e an optimum dete tor for large area overing. A layer of 0.5
Fig.2.{S hemati viewoftheARGO-YBJbuildinganddete tor.
onstru tion,triggeringand dataa quisition,thebasi modulebeinga\ luster". Ea h
luster hasasurfa eof44 m 2
and isbuilt by12RPCs; ea hRPC isread bymeans of
10\logi alpads"6056 m 2
, orrespondingto 8stripsORed. Onepad 0.34m 2
area
orresponds to one pixel, givingthegranularityof thedete tor for timing and parti le
densitymeasurements. Firedstrips oordinates anbere ordedtogiveabetterparti le
lo ation ifneeded. For ea h event thetrigger multipli ity (numberof red pads), the
oordinatesof redpads, theirtiming andthenumberof red strips( orrespondingto
thenumberofdete tedparti les)arere orded. Thetriggerthresholdissettoaminimum
numberof red pads; theexpe ted triggerrate forat least15 pads red (based onthe
measuredpadrateatYBJ400Hz)is 20KHz. Thehugeamountofdatathat has
to be olle ted,analyzed and re ordedrepresentsone of thegreatest hallenge forthis
experiment.
3. {Sensitivity to pointsour es and GRBs
Figure3showstheexpe tedsensitivityoftheARGO-YBJdete torfor1yearof
data-takingand5standarddeviationsstatisti alsigni an e, omparedwiththeCrabNebula
ux. It an beseenthat the expe ted sensitivity isabout1/10the Crab ux overthe
wholeenergyrange100GeV{20TeV.
Theexpe tedsensitivityofARGO-YBJtoGRBsisshownin g.4forSingleParti le
(SP)andLowMultipli ity(LM)te hniques. WithSingleParti lete hniquethe ounting
rateof the dete tor is re ordedat xed time intervals; aburst is dete tedif it givesa
ountingratesigni antlyhigherthantheba kground,withnoinformationonthearrival
dire tion. The Low Multipli ity te hnique onsists in the dete tion of very small air
showersthatareanalyzedandtheirarrivaldire tionisre onstru ted;asigni antex ess
Fig.3.{Minimum uxfrompointsour esdete tablebyARGO-YBJduring1yearofdatataking
and5s.d. statisti alsigni an e, omparedwiththeCrabNebula ux. Reportedsensitivities
ofotherexperimentsareonlyindi ative.
haveto bedone: apower-law -ray ux dN dE =KE phm 2 upto E max is assumed,
with spe tral slope = 2.0 (the average spe tral slope measured by EGRET in the
30MeV{10GeVregionbeing =1.950.25),timedurationt=1sandstatisti al
signi an e=4s.d.. Moreoveritis onvenienttoexpresstheGRBssensitivityinterms
ofF
min
, i.e. theminimumdete table uen eforaGRB emittingintheenergyrange1
GeV{E
max
withspe tralslope . In g.4the uen eoftheGRBsdete tedbyEGRETis
alsoreported,extrapolatingtheirspe trumupto100GeVwiththeirindividualspe tral
index. It anbeseenthat iftheirspe traextendat leastupto100GeV,mostofthem
ouldbedete tedbyARGO-YBJ.
4. {The ARGO-test atYangbajing
Thetest[5,6℄hasbeen arriedoutin theperiod February-May1998. FifteenRPCs
( orrespondingto about one luster prototype) havebeenput in operation, for atotal
area of 51 m 2
and an a tive area of 46.2 m 2
(91% overage). Di erent gas mixtures
have been tested to hoose the best working onditions due to the low atmospheri
pressure. Data have been olle ted with and without a 0.5 m lead onverter. The
time jitter distribution of pad signals is uniform for all pads with an average s.d. of
1.3 ns; the resulting intrinsi RPC time resolution is 1 ns. The angular resolution,
obtainedbydividingthepadarrayintotwoindependentsub-arrays( hess-boardmethod)
and omparing the twore onstru ted showerdire tions is
# 2
Æ
for eventswith pad
multipli ity100. Thee e t oftheleadlayerismoreimportantatlowmultipli ity,as
expe ted, loweringtheangular resolutionfrom 8 Æ
to5 Æ
forpad multipli ity 35.
Forthe naldete tor,duetoitslargerdimensions,theexpe tedangularresolutionis
Fig. 4.{Minimum uen efromGRBsobservablebyARGO-YBJas afun tion ofthe ut-o
energyEmax for Single Parti le (SP) and Low Multipli ity (LM) te hniques (see text). The
pointsaretheextrapolationupto100GeVof14EGRETspe tra.
The overall results of the test look well promising for future operation of the full
ARGO-YBJdete tor.
5. {Con lusions
TheARGO-YBJdete torhastheuniquepossibilitytobridgetheGeVandTeVenergy
regions,providingfull omplementarityto severalsatelliteprograms. For the rst time
a 50 m 2
RPCs arpet has been su essfully operated at high altitude, on rming
that RPCs an beused eÆ iently( 95%) to dete t airshowerswith ex ellent time
resolution(1ns).
Theresults of the analysis of data olle tedon-site on rm theeÆ ien y and time
resolutionassumedinthesensitivity omputationforthefulldete tor. Thebuildinghas
been nishedand theinstallationof theRPCsstarted in November2000. Data taking
willstartduring2001witha800m 2
arpet,addressedtoGRBsphysi s.
REFERENCES
[1℄ WeekesT.C.etal.,Astrophys.J.,342(1989) 379.
[2℄ Abbres iaM.etal.,Astroparti lePhysi swithARGO,(1996),andBa i C.etal.,The
ARGO-YBJProje t,(1998).Theseunpublisheddo uments anbedownloadedattheURL:
http://www1.na.infn.it/wsubnu l/ os m/ar go/a rgo.h tml.
[3℄ VernettoS.etal.,Pro .26thICRC(Salt LakeCity),vol.4(1999)p.28.
[4℄ VernettoS.,Astropart.Phys.,13(2000)75.
