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Nuclear Instruments and Methods in Physics Research A 518 (2004) 180–182

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doi:10.1016

The fluorescene detector electronics of thePierre Auger Observatory

M. Kleifgesa,b

aForschungszentrum Karlsruhe, IPE, Postfach 3640, Karlsruhe 76021, GermanybObservatorio Pierre Auger, Av. San Martin Norte 304, 5613 Malarg .ue, Argentina

For the Auger Collaboration

Abstract

The Pierre Auger Observatory is a hybrid experiment to observe cosmic ray extensive air showers with energies above

1019 eV by a combination of a surface array and air fluorescence detectors (FD). Prototypes of both these detectors

have been successfully tested and operated. Currently the final design version of the electronics is undergoing

installation. Based on experience gained with the prototype we present the concept of the FD electronics and the

implemented trigger algorithms. The electronics continuously record the image of the 20� 22 pixel camera in a ring

buffer of 100 ms length with a time resolution of 100 ns: Digital trigger algorithms analyse these data to find straight

tracks originating from the fluorescence light. All individual detectors are synchronized with a 10 MHz signal generated

by a GPS clock module. Veto and dead time signals are centrally distributed to allow calibration with external light

sources during normal operation. A statistical method for the calibration was developed and tested.

r 2003 Elsevier B.V. All rights reserved.

PACS: 07.50.Qx; 07.05.Hd; 95.55.Cs

Keywords: Trigger electronics; GPS timing; UHE cosmic rays; Fluorescence telescope; Photomultiplier camera

1. Introduction

The four fluorescence detector (FD) stations area vital part of the Pierre Auger project, currentlyunder construction near Malarg .ue, Argentina.They are located around the perimeter of thesurface detector array and are intended to measureair fluorescence light emitted as secondary parti-cles cosmic rays pass through the atmosphere.Each station consists of 6 telescopes with aSchmidt optical system that cover a field of view

ddress: matthias.kleifges@ipe.fzk.de (M. Kleifges).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.10.054

of 30� � 30� each. The light is focused on a cameraof 440 photomultipliers (PMT) arranged in amatrix of 22 rows and 20 columns. A commercial,9U high, 1900 sub-rack beneath the camera holds 20First Level Trigger (FLT) and one Second LevelTrigger (SLT) board to read out and analyze thepixel information in 100 ns slices.Before starting the full production, the Auger

group has built and operated 2 prototype tele-scopes to evaluate the design and the performanceof each detector subsystem. They were operatedfrom May 2001 to April 2002, after which the finalhardware installation commenced. As the digital

d.

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M. Kleifges / Nuclear Instruments and Methods in Physics Research A 518 (2004) 180–182 181

electronics is based on very flexible re-program-mable FPGA logic it was easy to integrateadditional features and make small modificationsas the result of the prototype experience. Wedescribe in the following sections the final designof the analog and digital front-end electronics, thetrigger system and the DAQ.

2. Analog processing

The analog signal processing starts at the HeadElectronics (HE) which consists of 2 circular PCBsmounted directly behind the XP3062 hexagonalPMTs. The inner board holds an active voltagedivider which allows the biasing of the PMTdynodes with low power dissipation and highlinearity compared to a conventional design [1].The outer PCB contains a differential-input and abalanced output driver to transfer the PMT signalswith low noise and high dynamic range via twistedpair lines to the Analog Boards (AB) at the front-end sub-rack. The driver is located on a smallhybrid circuit, which has a symmetrical layout andlaser trimmed resistor pairs matched to 0.25% toreduce the common mode noise.The signals from the camera are processed by 20

AB, i.e. each board serves the 22 channels of asingle camera column. The AB holds a differentialline receiver, a programmable gain amplifier (tobalance the gain spread of the pixels) and a fourthorder anti-aliasing filter in front of the ADC. A 15-bit dynamic range is achieved by introducing anadditional low gain channel, which processes therare large pulses [1]. An onboard test-pulser allowsinjection of signals with programmable width andamplitude in each channel in order to check thefull system even though the camera is notconnected.

3. Digital processing and trigger

20 FLT boards digitize the signals from theattached AB with 12-bit resolution and 10 MHzsampling rate. The digital data are continuouslystored in a 16-bit � 64 K RAM memory, which isorganized in 64 ring-buffers of 1000 words each. If

the trigger system finds a fluorescence light track inthe camera image, the data recording continueswith the next available ring-buffer. Otherwise, thering-buffer data is overwritten after 100 ms; but thelast 1000 words are always present.The ADC data are also used to calculate the

statistical data ðSx; Sx2Þ of the latest 65 536 ADCvalues. These data are read out to calculate themean value and the variance of the signal [2].The ADC values are also passed through a

programmable digital filter which smoothes therandom fluctuations of the sky background. Thenoise is reduced by more than a factor 2 bycalculating the sliding sum of the last 4–16 ADCsamples [3]. In each channel, sliding sum valuesabove an adjustable threshold generate a 20 mslong pixel trigger. The rate of these triggers ismeasured and kept constant between 100 and200 Hz by regulating the trigger threshold for eachchannel individually. With this robust regulationscheme we prevent increasing random trigger rateswith changing background light intensities.The SLT board scans the FLT pixel trigger data

for small track segments of 5 adjacent pixels incertain patterns by using highly parallel, pipelinedpattern recognition logic [4]. In total, 37 163different combinations of pixels are checkedduring the 1 ms long pixel trigger read out periodwhich corresponds to a computing power of morethan 1011 operations=s:We found that about 80% of the prototype

events are caused by the cosmics hitting the PMTsor by Cherenkov light. In contrast to the realshower events—where the pixel trigger timesfollow a space–time linearity—these events areseen by the trigger all at the same time within7150 ns: Thus, the final SLT logic provides amultiplicity signal with 100 ns resolution to rejectthese undesirable event classes.

4. Recent progress and outlook

In order to reach an accuracy of o120 ns wehave built a self-made GPS clock based on theMotorola Oncore UT+ receiver, which is alsoinstalled in the surface detector electronics. Thenew unit generates programmable veto, deadtime

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M. Kleifges / Nuclear Instruments and Methods in Physics Research A 518 (2004) 180–182182

and trigger signals which are distributed alongwith clocks (10 MHz and 1 pps) to each front-endsub-rack and the various Auger calibration sys-tems. The Eye PC sets up the timing of the signalsthrough an onboard RS232 interface.We also developed with the prototype, a fast

method for the calibration of the PMTs andelectronics. Light from a blue LED is distributedthrough optical fibers to each telescope andilluminates the full camera with a rectangularlight pulse of about 60 ms length. The gain isdetermined by the analysis of fluctuations of thesignal taking into account the limiting band-width of the filter stages and effects due to thedigitization [5].The prototype phase of the Pierre Auger

experiment validated the conceptual design of theFD electronics. A pre-production array of 100

surface detectors and 2 FD detectors with 6telescopes each is currently installed. The comple-tion of the observatory is expected in 2005.

References

[1] S. Argiro, et al., Nucl. Instr. and Meth. A 461 (2001) 440.

[2] M. Kleifges, et al., IEEE Trans. Nucl. Sci. NS-50 (4) (2003)

1204.

[3] H. Gemmeke, et al., First measurements with the AUGER

fluorescence detector data acquisition system, in: K.-H.

Kampert, G. Hainzelmann, C. Spiering (Eds.), Proceedings

of the ICRC Conference, 2001, Hamburg, p. 769.

[4] H. Gemmeke, et al., The Auger fluorescence detector

electronics, in: K.-H. Kampert, G. Hainzelmann, C.

Spiering (Eds.), Proceedings of the ICRC conference,

2001, Hamburg, p. 737.

[5] A. Menshikov, et al., IEEE Trans. Nucl. Sci. NS-50 (4)

(2003) 1208.