Kildea, J

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29th International Cosmic Ray Conference Pune (2005) 5, 135138Shower Reconstruction Techniques for STACEEJ. Kildea , J. Ball, D.A. Bramel, J. Carson, C.E. Covault, D.D. DriscollP. Fortin, D.M. Gingrich , D.S. Hanna , A. Jarvis, T. Lindner , R. Mukherjee,C. Mueller , R.A. Ong, K. Ragan , R.A. Scalzo  , D. A. Williams	and J. Zweerink(a) Department of Physics, McGill University, Montreal, QC H3A 2T8, Canada(b) Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, CA, USA, 90095(c) Dept. of Physics & Astronomy, Barnard College, Columbia University, New York, NY, 10027(d) Department of Physics, Case Western Reserve University, Cleveland, OH, USA, 44106(e) Centre for Subatomic Research, University of Alberta, Edmonton, AB T6G 2N5, Canada(f) TRIUMF, Vancouver, BC V6T 2A3, Canada(g) Lawrence Berkeley National Laboratory, Berkeley, CA, USA, 94720(h) Santa Cruz Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA, USA, 95064Presenter: J. Kildea (kildea@physics.mcgill.ca), can-kildea-J-abs2-og27-posterThe Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is an atmospheric Cherenkov detec-tor that detects astrophysical gamma rays using the shower-front-sampling technique. STACEE is a fullyoperational detector utilizing 1 GS/s Flash ADCs on all channels, providing important pulse height and tim-ing information for discriminating between  -ray and hadron events. We discuss shower-front reconstructionmethodologies and gamma/hadron separation techniques that utilize the nanosecond timing and pulse heightinformation provided by STACEE’s Flash ADCs.1. IntroductionGround-based atmospheric Cherenkov telescopes are well suited to the task of Very High Energy (VHE)  -ray astronomy (in the energy range from 50 GeV to 50 TeV). Given the non-thermal power-law spectra of -ray emitting sources, a large collection area is vital at very high energies, where the flux of photons dimin-ishes rapidly. By incorporating the atmosphere as part of the detector, ground-based  -ray telescopes offer adistinct collection area advantage when compared to their lower-energy (below 50 GeV) space-based counter-parts. Unlike space-based detectors, however, ground-based instruments suffer the lack of an anti-coincidenceshield to guard against background cosmic rays. To overcome this disadvantage, sensitive detection techniqueshave been developed that distinguish  -ray air showers from the large background of air showers induced byhadronic cosmic rays. We describe here the main shower reconstruction methodologies currently employedand/or under investigation by the STACEE collaboration for gamma/hadron separation.2. The STACEE DetectorSTACEE is a shower-front-sampling atmospheric Cherenkov telescope that uses the facilities of the NationalSolar Thermal Test Facility (NSTTF) in Albuquerque, New Mexico [1]. The NSTTF is a solar power researchfacility incorporating a 200 ft central receiver tower and an array of heliostats (solar mirrors). STACEE usessecondary mirrors, in the central receiver tower, to focus Cherenkov light reflected by the heliostats ontocameras having a total of 64 photomultiplier tubes. A one-to-one mapping between heliostats and PMTsallows the Cherenkov shower-front to be sampled independently at 64 different locations in the heliostat field.STACEE uses a custom-built trigger system to select Cherenkov events from amongst the background of night-sky light fluctuations. In the event of a Cherenkov trigger, amplified and AC-coupled signals from the PMTs are136 J. Kildea et al.recorded, together with a GPS timestamp, using 8-bit FADCs (one per PMT). The FADCs provide importanttemporal and intensity information, at a sampling rate of 1 GS/s, which is fully utilized in the offline analysisprocedure.A careful investigation of candidate gamma/hadron separation parameters requires extensive use of air showerand detector simulations. For  -ray and hadronic air shower simulations, STACEE uses the CORSIKA package[2]. Custom ray-tracing and Monte-Carlo algorithms are used to simulate data generation by the telescope.Measured optical and electronic throughputs are incorporated into the simulations package.3. Shower ReconstructionIn order to reconstruct the properties of the primary photon, an accurate estimate of the shower core locationis essential. The shower core position on the heliostat field is the point at which the primary would impact,were it to travel unobstructed through the atmosphere. Three methodologies used by STACEE for shower corelocation are described below.Template Fitting Method The template fitting method, detailed in [3], involves the generation of a largenumber of Monte-Carlo simulated charge templates that represent the charge from each PMT under variousconditions. Templates are compiled using showers simulated over a large range of zenith angles, azimuthangles, and core locations. By finding the template which best matches a particular event, an approximate corelocation for that event is obtained. The mean core resolution for  -rays obtained using this method is about22 m, when applied to simulated  -rays with energies between 20 GeV and 5 TeV generated on a differentialspectrum of   .Early Shower Method The early shower method exploits the temporal profile of a  -ray shower’s Cherenkovfront. The front of a  -ray air shower comprises light originating at different heights along the particle cascade[4]. Light from shower maximum forms the bulk of photons in the shower-front, resulting in a spherical shapeat ground level. However, since particles in the shower travel faster than the Cherenkov light that they produce,light that is emitted lower down along the shower’s trajectory is detected first, before it has had time to movefar from the core. This results in an apparent distortion of the sphere into a more conical profile. By calculatingthe center-of-gravity of the light in the first few nanoseconds of the shower-front, the shower core position canbe accurately determined. Furthermore, given that the light pool at the beginning of the Cherenkov front ismuch smaller than that of the complete front, truncation effects, due to the finite sampling area, that otherwisewould impact on the center-of-gravity calculation, are diminished.By accounting for time-of-flight delays from the heliostats to the PMTs, and by applying a type of softwaretrigger condition across each nanosecond sample of the FADC traces, the first nanosecond sample of the showercan be determined. With the beginning of the shower identified, the centre-of-gravity of the first few nanosec-onds is calculable. The mean core resolution obtained using this method is  26 m.Grid Alignment Method The grid alignment method was recently developed by the CELESTE collabora-tion, [5, 6], and was shown to work well when applied to their data. Since the Cherenkov front originating atshower maximum is roughly spherical, Cherenkov photons arrive at different heliostats at different times, de-pending on the heliostat positions relative to the front. A simple sum of the FADC traces provides, as a result,a pulse that is short and wide. By realigning the traces, however, such that the different propagation times fromshower maximum to the heliostats are fully accounted for, the summed FADC trace will be narrower and havelarger amplitude. Hence, by iterating over a grid of potential shower maximum positions, and calculating theresultant height to width ratio (  ) of the realigned FADC traces for each grid point, the shower maximumposition can be determined. The grid point with the narrowest and tallest resultant pulse (  ﬁﬀﬃﬂ "! ) liesShower Reconstruction Techniques for STACEE 137Figure 1. (Left) Distribution of #%$'& values for each point on a grid constructed at shower maximum altitude (12.5 kma.s.l.), for a simulated ( -ray air shower. The grid point with the tallest and narrowest summed FADC trace ) #*$'&,+.-0/"1corresponds to the best estimate of the shower maximum location. Grid point coordinates correspond to their projectiononto the heliostat eld. (Right) Distribution of #%$'& values for a simulated proton air shower. The atness of the protondistribution compared to the peaked ( -ray distribution can be used as a gamma/hadron discriminant [5, 6].closest to the true shower maximum, figure 1 left. A straightforward projection of the shower maximum pointto ground level provides the position of the shower core.The core-finding procedure, as developed by CELESTE, is a two step process, whereby a coarse grid is usedto find the approximate shower maximum position, followed by a fine grid around this point to improve theaccuracy. Application of the method in STACEE’s analysis code requires just one step—the fine grid—sincean approximate shower core location is already provided by the template and early shower methods. The meancore resolution obtained using the combined grid-alignment and early shower methods is  21 m.4. Gamma/hadron SeparationGamma/hadron separation for atmospheric Cherenkov telescopes typically involves selection of  -ray-likeevents, based on fitted parameters with measurable differences for  rays and hadrons. Selection cuts for eachparameter are determined using simulations and/or optimization on real data from a known source. At presenttwo main gamma/hadron separation parameters are employed by STACEE; the shower direction and the gridratio (referred to as 2 by the CELESTE collaboration). Both require an estimate of the shower core position.The shower direction is determined by fitting the measured Cherenkov front for a conical time profile, withthe point of shower maximum as a free parameter. A line from the estimated core position on the ground tothe shower maximum, as determined by the fit, provides the reconstructed direction of the shower. Since rays are anticipated from the source direction (center of the field-of-view), an excess of ON-source events isexpected with reconstructed directions close to the center, see figure 2 left. Using the reconstructed directionalone, STACEE can detect the Crab Nebula with good statistical significance [7].The grid ratio parameter was developed by the CELESTE collaboration [5, 6], and is closely related to thegrid alignment method for finding the shower core, described above. When correctly realigned for the showermaximum position, the parameter 3 of the summed FADC traces is at its greatest value. As shown in figure1, 3 for  rays falls off rapidly from the shower maximum location, since the FADC pulses quickly fall outof alignment. For hadrons, however, where the Cherenkov front is not spherical [4] and the pulses are poorlyaligned to begin with, the  distribution is much flatter. The CELESTE group parameterize the  fall-138 J. Kildea et al.Figure 2. (Left) Distribution of 4 for ( rays and protons. 4 is the angle between the source direction and the reconstructedshower direction. Gamma rays, from the source at the centre of the eld-of-view, have small values of 4 . Bins have equalarea on the sky. (Right) Distribution of the grid ratio parameter for ( rays and hadrons.off using the ratio of the average 3 calculated at a distance 200 m away from shower maximum, to at shower maximum, 50687:9";=<?>A@A@ﬃB687:9";=<BDCFEG. This ratio, referred to here as the grid ratio, is a powerful gamma/hadrondiscriminant for the shower-front-sampling technique. Its power in STACEE simulations is demonstrated infigure 2, right. Application of a grid ratio cut, determined from simulations, to STACEE Crab Nebula data hasproven quite successful [7].5. ConclusionUsing FADC data STACEE can locate the core of  -ray showers with an accuracy of  21 m, according tosimulations. Two gamma/hadron discriminators, the reconstructed shower direction and the grid ratio param-eters, show great potential in STACEE simulations and have been used to improve STACEE’s detection of theCrab Nebula [7]. While work on other potential gamma/hadron separation parameters is ongoing, a plannedoptimization of the shower direction and grid ratio selection cuts, using Crab Nebula data, is expected to yieldan improvement in STACEE’s sensitivity for other  -ray sources.This work was supported in part by the National Science Foundation, NSERC (the Natural Sciences and En-gineering Research Council of Canada), FQRNT (Fonds Que´be´cois de la Recherche sur la Nature et les Tech-nologies), the Research Corporation, and the University of California at Los Angeles.References[1] Gingrich, D. M., et al., Proc. of the 2004 IEEE Nuclear Science Symposium (astro-ph/0506613).[2] Heck, D., Knapp, J., Capdevielle, J. N., Schatz, G., & Thou, T. 1999, Rep. FZKA 6019, Forschungszen-trum Karlsruhe[3] Scalzo, R. A., et al., 2003, Proceedings of the 28th International Cosmic Ray Conference, Tsukuba, Japan[4] Sembroski, G., et al., 1997, Proceedings of the 4th Compton Symposium on Gamma Ray Astrophysics[5] Bruel, P., et al., 2004, Proceedings of Frontier Science 2004, Physics & Astrophysics in Space[6] Manseri, H., et al., 2005, Proceedings of Gamma 2004. AIP Conference Proceedings, Vol 745 645-650[7] Kildea, J., et al., 2005, these proceedings

Shower Reconstruction Techniques for STACEE

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