26th International Cosmic Ray Conference Salt Lake City, Utah August 17-25,1999The GRAAL Project (Gamma Ray Astronomy at ALmeria) makes use of the CESA-1 heliostat field at the
“Plataforma Solar de Almeria” (Spain) as a gamma-ray telescope with an energy threshold of about 100 GeV.
Cherenkov light generated by EAS is reflected by the heliostats and collected into photomultipliers (PMTs)
with nonimaging secondary optics. Each PMT collects the light reflected by 13 - 18 heliostats of 40 m2
using a Winston cone. After successful tests with two collecting cones, a more advanced setup on a dedicated
platform, using four collectors and 63 heliostats (total reflecting area of about 2500 m2
) is being installed.
A description of this setup together with Monte Carlo results about its excellent capabilities in the precise
determination of pulse arrival times are presented

Arqueros, Fernando

Ballestrín, Jesús

Berenguel Soria, Manuel

Borque, D.M.

Camacho, Eduardo F.

Díaz, M.

Gebauer, H.-J.

Plaga, R.

Portocarrero, C.E.

English

idUS. Depósito de Investigación Universidad de Sevilla

OG 4.3.07 1The GRAAL projectF.Arqueros1, J.Ballestrin2 , M.Berenguel3, D.M.Borque1, E.F.Camacho4,M.Diaz5, H.-J.Gebauer5, R.Plaga5, and C.E.Portocarrero11 Facultad de Ciencias Fisicas, Universidad Complutense, E-28040 Madrid, Spain2 CIEMAT-Departamento Energias Renovables, Plataforma Solar de Almeria, E-04080 Almeria, Spain3 Departamento de Lenguajes y Computacio´n, Universidad de Almeria, 04120 Almeria, Spain4 Escuela Superior de Ingenieros, Universidad de Sevilla, E-41012 Sevilla, Spain5 Max-Planck-Institut fu¨r Physik, 80805 Mu¨nchen, GermanyAbstractThe GRAAL Project (Gamma Ray Astronomy at ALmeria) makes use of the CESA-1 heliostat field at the“Plataforma Solar de Almeria” (Spain) as a gamma-ray telescope with an energy threshold of about 100 GeV.Cherenkov light generated by EAS is reflected by the heliostats and collected into photomultipliers (PMTs)with nonimaging secondary optics. Each PMT collects the light reflected by 13 - 18 heliostats of 40 m2using a Winston cone. After successful tests with two collecting cones, a more advanced setup on a dedicatedplatform, using four collectors and 63 heliostats (total reflecting area of about 2500 m2) is being installed.A description of this setup together with Monte Carlo results about its excellent capabilities in the precisedetermination of pulse arrival times are presented.1 IntroductionGround-based measurements of cosmic -radiation at energies below about 200 GeV promise to pro-vide crucial insights into the high-energy astrophysicsand cosmology. In order to reach low thresholds withtechniques based on ˇCerenkov light large mirror col-lection areas are needed. GRAAL, like two similarexperiments CELESTE and STACEE (see these pro-ceedings), employs the large mirror area of an exist-ing solar-power plant. The optical principle of Mini-GRAAL is different from these two experiments, how-ever.ˇCerenkov light from four groups of heliostats (with13,14,18,18 members, respectively) is directed ontofour single non-imaging “cone concentrators” eachcontaining a single large-area PMT. The light collect-ing cones have the form of truncated Winston coneswith an opening angle of 10o. Each cone is directedonto a point on the ground in the heliostat field andcollects the light from all heliostats which are locatedwithin the ellipse projected by the cone opening an-gle on the ground (see figure 1). The incoming lightfrom an air shower consists of a train of pulses fromFigure 1: Setup of the experiment. A dedicated plat-form on the 70 m level of the central tower of theCESA-1 field houses four Winston comes which re-ceive light form 13 - 18 heliostats in the field.the different heliostats ususally fully separated by pathlength differences. which are registered with four high-resolution flash ADCs.After successful experimental tests of the principle with two cones, using 27 heliostats, on a general pur-pose platform in the central tower (Arqueroset al. 1997) GRAAL has been funded fora 4 cone-setup in a newly constructed dedi-cated platform on the 70 m level of the cen-tral tower. The data taking is planned to beginin June 1999 in every clear moonless night.The technical setup will eventually allow a re-mote control of the experiment, only underthe surveillance of a field operator. Besidesits technical aims of low energy threshold andvery high time resolution, this low-cost ex-periment is an important addition to the verysmall number of experiments which can takedata in this energy range. The fact that nonew VHE -ray sources were detected sincethe last ICRC might be connected to the factthat our poor understanding of high-energy as-trophysics in general has not allowed to se-lect the right targets. E.g. it has becomeclear recently that distance and brightness of-ray bursts (GRBs) are essentially uncorre-Figure 2: Location of light collecting cones with PMTs at theend (front diameter 1.06 m) in the platform seen from top. Thetower is the curved structure on top.lated, thus withdrawing the basis of the usual strategy to monitor only the brightest GRBs in the VHE becausethey were thought to be near and thus suffering relatively little absorption in the infrared background radiation.Our low-cost observing facility will be able “to take some risk” in the selection of targets.2 Hardware setupThe platform is an enclosure on the outside of the central tower housing the cones with the PMTs. The elec-tronics is located inside the tower electromagneticallyshielded from the cones (fig.2). In general GRAALcan achieve a very good time resolution because thereexist only four short cables that run exclusively withinthe platform enclosure (length several meters) from thephotomultipliers to the data acquisition. To take fulladvantage of this fact we register all four pulse train inonly one Digital Oscilloscope (Le Croy LC 564A) witha bandwidth of 1 GHz and a time bin of 500 psec. Thisensures that the FWHM of individual pulses (about 3.5nsec from our test, corrected for the larger bandwidthused in this experiment) is solely determined by mir-ror geometry and PMT properties. The digital scope isread out in sequence mode over a GPIB interface intoa PC, reaching a speed of about 260 “waveforms” (i.e.1000 time bins of 0.5 nsec width with 1 byte each)/sec,which is sufficient for a dead time below 10 % for ourmaster trigger rate which is expected to remain belowabout 10 Hz (each trigger containing four waveforms).Energy ( GeV )Effective Area ( m2  )Figure 3: Expected effective area as a function ofthe primary energy for the 4-cone setup.Our trigger identifies a pulse train of four pulses with NIM electronics in the two central cones and de-Amplitude ( mV )R.M.S. ( ns )Figure 4: Time resolution as a function of the pulseamplitude both for all identified peaks (solid line) andonly for non-overlapping peaks (dashed line).mands a certain total light level in all four cones in co-incidence with the sequence trigger. The time and am-plitude calibration of our setup is performed using blueLEDs (Nichia NSPB 500, maximal output at 450 nm)with a calibrator module that is fastened at the windowof the Winston cones. The LED in the module emits alarge part of its light in the forward direction towardsthe heliostat field. A very small fraction (adjustablewith a screw) is reflected back via total reflection intothe cone. The amount of light emitted in the backwarddirection is determined with a Quantacon RCA C31000that was previously calibrated by determining its singlep.e. peak and fluctuation behaviour. The LED oper-ating voltage is adjusted so that one LED pulse corre-sponds to about 100 p.e. These LED pulses are reg-ularly used in each run to verify time and amplitudecalibration. This calibration is cross checked by mea-suring the forward output both with the calibrated PMTand via the current of a calibrated photodiode when theLED is pulsed at 10 kHz. The forward output shines onthe heliostat field. When the heliostats are brought intoa “back reflection” position, the reflected LED pulses are used to verify the geometry and check the mirrorquality in regular intervals.3 Monte Carlo resultsA detailed MC simulation which uses the CORSIKA code (Capdevielle et al. 1992) and which takes into∆β( deg )∆Zenith ( deg )Figure 5: Angular resolution for gamma-rays in therange 200 - 400 GeV incoming from South-West witha zenith angle of 20 deg.account all experimental uncertainties and night skybackground (LONS) was performed (see Arqueroset al. 1997 for a more detailed description). Thetrigger parameters have been adjusted in order to geta background detection rate of about 10 Hz includ-ing accidental coincidences ( 2 Hz). The effec-tive area of the telescope has been calculated as afunction of the primary energy both for gamma-raysand protons. As an example, figure 3 shows the re-sult for a zenith angle of 20 degrees. From this fig-ure, an effective area of about 9000 m2 for gamma-rays of 200 GeV is expected while the effective areafor protons of the same energy is lower by a factorof about 100. A method to identify the Cherenkovpulses out of the noise and the corresponding he-liostats has been developed, with very good results(about 95% of success). Figure 4 shows the r.m.s. ofthe time error distribution as a function of the fittedpulse amplitude. In this figure the amplitude val-ues of 10, 20, 50 and 150 mV correspond to 4, 14,30 and 63 photoelectrons respectively. As expected,the time resolution is worse for those incident direc-tions in which strong peak overlapping (F.Arqueros et al. 1997) takes place (compare solid and dashed linesin figure 4). This figure shows a very good time-resolution with an uncertainty well below 1 ns for not verysmall peaks. The very good time resolution achieved with our set-up allows an accurate reconstruction of theCherenkov-light front and thus the measurement of the incident direction of the primary particle. For eachevent, the Cherenkov-light front was fitted to a plane, its director vector being the most probable incomingdirection of the EAS. For those input directions showing peak overlapping we have chosen the peak identifica-tion which gives the lowest 2 value in the front fitting. Figure 5 is a 2-dimensional histogram of the error inthe reconstructed direction for gamma-rays in the energy range between 200-400 GeV. The strong asymmetryshown in this angle distribution is correlated with the spatial asymmetry of the mirrored area at ground. Fromthis data we have inferred an angular resolution of about 0.2 deg; i.e. 63% of the reconstructed events areincluded within 0.2o of the real direction.From the MC results we can infer some prediction on the sensitivity of the telescope. Assuming a cos-Core distance ( m )Σ(t fit-t mea)2 /N (ns2 )Figure 6: Reduced 2 of the fit to the shower front in time as functionof shower-core distance from the centre of the field.mic background dominated by protonswith a differential energy spectrum fol-lowing a power law with index 2.7, adetection of the Crab nebula (using theflux and spectrum data determined bythe Whipple collaboration (Hillas et al.1998)) at a 5  level is expected af-ter about 20 hours of on-source datagathering without any cuts. The appro-priate cut in reconstructed angle (fig-ure 5) reduces this time down to about2 hours. A further increase in sensi-tivity will be achieved by hadron re-jection. A method for gamma/hadron(g/h) separation based solely on thetime properties of the shower front isunder development. From this workwe show in figure 6 the 2 value of thefront fitting as a function of the coredistance, for gamma-ray showers. Thisfigure shows an average time deviationsquared on the order of 1 ns for show-ers hitting the centre of the field in-creasing up about 3 ns for showers atabout 120 m core distance. For protonshowers the front fluctuations are stronger and thus higher 2 values are expected. The small 2 near theshower core also allows an estimation of the shower-core position.ReferencesF.C.Andres et al. (GRAAL coll.) 1998, Proc. 16th ECRS, Alcala de Henares p.401F.Arqueros et al. (GRAAL coll.) 1997, Proc. “Towards a Major Atmospheric Cerenkov Detector - V”,KrugerPark (ed. O.C. de Jager) p.240J. N. Capdevielle et al. 1992, KfK report 4998A.M. Hillas et al.(Whipple coll.) 1998, ApJ 503, 744

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