Performance Evaluation of Trigger Algorithm for MACE Gamma Ray Telescope

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Major Atmospheric Cherenkov Experiment: The state of the art Gamma Ray Telescope being built by BARC

Transcript of Performance Evaluation of Trigger Algorithm for MACE Gamma Ray Telescope

Performance evaluation of trigger algorithm for the MACE telescopeKuldeep YadavBARC Mumbai

February 20, 2013

(On behalf of: N. Bhatt, N. Chouhan, S.S. Sikder, A. Behere, C.K. Pithawa, A.K. Tickoo, R.C. Rannot, S. Bhattacharyya, A.K. Mitra, R. Koul )

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Detection technique for very high energy -raysE < 100 GeV direct observations on satellites E > 20 GeV indiret observations via extensive air showers (EAS) > 99% air showers produced in the atmophere are isotropic CR Typically -ray : CR : : 1 : 103 -104 Gamma-ray sources can be detected Identify a single photon event from the sea of background event (shower shape, muon contents) They emit so many photons that the number of particles from this direction stands out of the background (excess of events from certain sky positions) Cherenkov light from charged secondaries can reach the ground with tight time structure and maintaining direction of primaryKuldeep Yadav (BARC, Mumbai) ASI-2013 February 20, 2013 2 / 16

Detection of atmospheric Cherenkov radiationPurpose of detection of an EAS experimentally: Determine the type of particle (though difcult) and properties of the primary particle e.g. direction, energy and chemical composition in case of CR IACT: most successful

Characteristic features of Cherenkov pulse:facilitate its detection in the presence of NSB Narrow pulse width ( 5 ns) Limited angular size (< 1 ) on the ground Nature of its photon spectrum i.e. Cherenkov light peaks at short wavelength (blue/UV) whereas NSB peaks at longer wavelength Example A telescope with 1 eld of view and 10 ns trigger formation time would receive 5 photons/m2 from the NSB while a much higher expected value of 65 photons/m2 from a 1 TeV -ray shower. Thus, a 1 TeV shower should be detectable above NSB with a telescope having 1 m2 mirror area.

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Bias Curve

Bias curve obtained from a simple detector has two components Both components can be tted with power law Exponent of hard component is that of CR spectrumKuldeep Yadav (BARC, Mumbai) ASI-2013 February 20, 2013 4 / 16

Trigger threshold of the telescopeThe signal due to Cherenkov photons (pe) S = A R pmt = y E A R pmtwhere pmt : quantum efciency of the PMT

The noise level in terms of uctuations N = LONS A R pmt Note: in actual case the wavelength dependence of both Cherenkov and LONS production and collection should also we considered

The energy threshold: is the minimum -ray energy for which (S/N) is sufcient to adequately trigger the telescope. The smallest detectable light pulse is therefore inversely proportional to S/N, i.e.

Eth (1/y )Kuldeep Yadav (BARC, Mumbai)

LONS A R pmt


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MACE telescope

Location: Hanle, Ladakh (32.7 N, 79 E, 4200m asl) Number of Photometric Nights: 190/yr Diameter: 21 m Focal Distance: 25, m Light collector conguration: Paraboloid with graded FL Panel Size: 984 mm 984 mm Number of Panels: 352, Number of Facets: 1500 Total light Collector Area: 337 m2 PSF: R95 at 0 15 mm R95 at 1 43 mm Telescope weight: 150 T

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MACE camera

Integrated Camera (all signal processing instrumentation housed within the camera structure of 2mx2mx1.2m size) Temperature control of the camera during operation and standby condition. Conventional CDC/GHz SamplingKuldeep Yadav (BARC, Mumbai) ASI-2013 February 20, 2013 7 / 16

Trigger region of MACE camera

16 channel CIM module Total PMTs: 1088 (68 CIM) PMTs in the trigger region: 576 (36 CIM)Kuldeep Yadav (BARC, Mumbai) ASI-2013 February 20, 2013 8 / 16

Implementation of trigger algorithmSLT hardware

Two level trigger generation FLT: within the CIM SLT: sytem level trigger

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Trigger conguration 4NN tight cluster

Table: Number of combinationsTrigger Mode FULL N+N S+W & W+S N+2W 4W Desired 36 21 240 210 + 210 desired + undesired Due to implimentation 36 21 6588 5582 5612

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Estimates of chance trigger rates104(1) Chance 4NN --full alone ( 5ns ) (2) Chance 4NN -- (N+N) alone ( 5ns , 10ns) (3) Chance 4NN -- (S+W: W+S) alone ( 5ns , 10ns) (4) Chance 4NN -- total (1)+(2)+(3) (5) Rate at which trigger patterns need to be validated

Chance rate due to desired combinations1



Chance rate : 20 Hz




Full triggers: 3 36 21 4 R 4 t N+N triggers: 240 2 (2R 2 t )(2R 2 t )slt W+S triggers: 2 420 2 (3R 3 t )(R)slt

Rate ( Hz)


(3) (2) (1)

Full alone (49.05 % patterns)


Full +NN+SW+WS (91.88 % patterns)

Total chance rate due to implementation1


10-1 100

SCR ( kHz)

N+N triggers:6588 2 (2R 2 t )(2R 2 t )slt W+S triggers: 11194 2 2 (3R 3 t )(R)slt

TRIGGER INFORMATION IN TWO PHASES !Kuldeep Yadav (BARC, Mumbai) ASI-2013 February 20, 2013 11 / 16

Effective areaBiggest advantage of ground based Cherenkov telescopes Large effective collection area Satellite exp.: Geometrical area Cherenkov tel.: Cherekov pool (120 m radius) Large collextion area is essential due to power law nature of sources Effective area for point -ray sources R Aeff (E) = 2 0 Rp(R, E)dR Discretized form Aeff (E) = P 2 2 0 (Ri Ri1 )p(Ri , E) p(R,E): Probability of triggerEnergy (GeV) Effective Area (m )2


5pe-4NN-FullCascade 7pe-4NN-FullCascade 7pe-4NN-Full 10pe-4NN-FullCascade



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Trigger Efciency

Trigger Probability , p(R, E) =number of triggered showers Total number of showers generated

depends on Cherenkov photon density, trigger FOV, trigger multiplicity and single pixel threshold

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Trigger rates for Gamma-ray at various single pixel thresholdsDifferential rate:Number of particles (E and E+dE) trigger the telescope per unit time5pe-4NN-FullCascade 7pe-4NN-FullCascade 7pe-4NN-Full 10pe-4NN-FullCascade 1 Diff. Rate (Gamma-ray/sec)

D(E)dE = Aeff N(E)dE Peak of differential trigger rate determines the energy threshold E N (e) = 2.79 107 ( GeV )2.59 ph m2 s1 GeV 1

Trigger Mode

Threshold Energy (GeV)

Integral rate (Hz)














10 Energy (GeV)


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Simulation results: Effective area and trigger ratesEffective area Differential rates

Particle type

Threshold Energy (GeV) 18.78 27.27

Integral rate (Hz) 11.86 37.13

Particle tpye Protons Alpha Total rate

Gamma-rays Electrons

Threshold Energy (GeV) 127.1 660.8

Integral rate (Hz) 818.8 137.7 1000.49

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In the actual implementation of 4NN tight cluster trigger scheme provides an integral trigger rate for gamma-rays which is less than obtained from MC simulation of the MACE telescope. Which means the total trigger rate should not exceed the estimated data acquisition rate of cosmic-ray ( 1 kHz) Data acquisition of the MACE telescope is capable of handling a sustained trigger rate of 1 kHz.

Thank you

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