29th International Cosmic Ray Conference Pune (2005) 00, 101–104

Data Digitization and Fluorescence Signal Recognition for Telescope
Array Experiment
A. Taketa , M. Fukushima , H. Ohoka , S. Ozawa, H. Sagawa, M. Takeda , 
H. Tokuno , S. Udo , T. Shibata , T. Matsuda , M. Tanaka , S. Ogio , Y. Tameda ,
J.D.Smith , S.B.Thomas , and TA group(a) ICRR, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8582 Japan(b) Institute of Particle and Nuclear Studies, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan(c) Osaka City University, 3-3-138 Sugimotocho, Sumiyoshi-ku, Osaka, 558-8585 Japan(p) Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550 Japan(e) University of Utah, 115 S 1400 E, Salt Lake City, UT 84112, USA

Presenter: A. Taketa (taketa@icrr.u-tokyo.ac.jp), jap-taketa-A-abs1-he15-poster
We have developed and made a test run of the Telescope Array (TA) Fluorescence Detector (FD) electronics.
In this paper, we introduce TAFD signal processing and the method of separation of fluorescence and nightsky back ground light, especially signal digitizing and one photomultiplier tube (PMT) level signal recognitionboard, we call as Signal Digitizer and Finder (SDF).

1.

IntroductionIn our Universe, Extraordinary High Energy Cosmic Rays (EHECRs), over 10 eV, exist. And some of themreach to the earth [1]. The main themes of TA experiment are, precise extrapolating the energy and the arrival
direction of each EHECR and obtaining the energy spectrum of them. Our experiment uses mainly two typesof detector, one is called as Surface Detector (SD) and the other is FD.
We will distribute 576 SDs in 1.2 km mesh and total covering area will be 760 km . Each SD is two layerof plastic scintillator of 3 m area and 1.2 cm thickness. The scintillation light is collected to PMT using
wavelength shifting fiber. SD detects air shower particle directly.

Figure 1. FD station (left), mirror (middle), PMT camera with pre-amplifier (right) for fluorescence measurement.

FD measures the UV fluorescence of molecular nitrogen generated by air shower particles. One FD station(See Fig 1 left) has 12 pairs of segment mirrors and PMT cameras. Total field of view of each FD station is3 to 34 in the elevation angle and 120  in the azimuth angle. The separation of each station is 30 to 40 km.
We have installed 1st FD stations and 2 pairs of mirror and camera on this summer. The segment mirror (SeeFig 1 middle) is 3.3 m diameter and composed of 18 hexagonal mirrors. The PMT camera (See Fig 1 right)
is composed of 16x16 2-inch hexagonal PMTs, each camera covers 18.0  x15.5 patch of the sky and each102

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PMT pixel covers 1.1 x1.0 . Blue glass filter is mounted on PMT’s photocathode for reduction of night sky
background light and acrylic window is placed in front of camera for protect from dust.

2.

Overview of FD electronics

Figure 2. dataflow diagram of FD (left) and diagram of the 1 PMT level fluorescence signal recognition process (right).

2.1 Analog block
Operation
gain of PMT will be set to 8  , and PMT is DC coupled. A load reactance of PMT charge is 500
and the voltage signal from the load reactanceis amplified by a factor 5 with a pre-amplifier attached at thePMT head. Pre-amplifier output is 100 semi-differential, and is sent to SDF though patch panel and 25 mcable.
SDF receives 16 semi-differential signals with receiver-amplifier and stretches the signal with waveformshaping-amplifier(WFSA), to reduce the digital aliasing error and to filter out the background analog noise.
WFSA
integration low-pass-filter and time constant ( ) is 50 ns, the response function of WFSAis!2nd#"%$'order&/.
")$(isand its gain is 1.  value is chosed to make the integral eliasing error less than * +,.
Output of WFSA is supplied into a 12-bit, 40 MHz Flash Analog Digital
Converter(FADC).
Maximumrange
 1of FADC is 2V, so least significant bit of FADC is equivalent to 2/2 00.5 mV. And all 16 outputs of WFSA is
also supplied into a summation amplifier, it works as a low gain channel. The gain of the summation amplifieris 1/16. Output of the summation amplifier is sent to FADC, too.
2.2 Digital block
The digitized signal, from FADC for the WFSA, is sent to a Field Programmable Gate Array (FPGA), wecall as the slave core. And The digitized signal, from FADC for the summation amplifier, is sent to another
FPGA, called as the master core. The slave core signalprocesses and stores the digitized waveform of WFSA,
recognizes the fluorescence signal, and send the result of signal recognition to another FPGA for board control,
we call the master core. The slave core, at first, sum up 4 adjacent time slice for saving memory. So basic
data become 14-bit, 10 MHz. We call this basic data as SUM4 count. The slave core has 8 event buffers

Data Digitization and Fluorescence Signal Recognition for Telescope Array Experiment103

thus causing no dead time to the DAQ system. The master core mainly stores the digitized waveform of thesummation apmlifier, generates test pulse for debugging and transfers the result of signal recognition to thefluorescence track pattern recognition board, we call as Track Finder (TF). TF does the 1 camera level signal
recognition, and sends the result to FD station control unit, we call as Central Trigger Distributor (CTD). CTD
judges the result from TF, and distributes trigger to all modules in the FD station. Especially, the inter-mirror
fluorescence track pattern recognition is done by CTD. CTD also distributes clock and reset signal, and stores
the time stamp information from a global positioning system unit on itself.
SDF, TF, CTD are all VME 9U modules, and have two types extended VME-bus. We call, one as KEKJ0, andthe others as J3. KEKJ0 is used for 2 3.3V , 3 5V DC power supply and the transfer clock, reset, trigger, andcontrol signal from TF to SDF. J3 is used for the transfer the result of the 1 PMT level signal recognition fromSDF to TF.
All of fluorescence signal recognizing processes are made in successive pipeline every 12.8 4 s. And all processes are done in the digital logic circuit. We will introduce the detail of 1 PMT level fluorescence signal
recognizing process in the next section. The detail of TF and CTD will be reported by our colleague in thisconference.

3.

Detail of SDF

3.1 Process of 1 PMT level signal recognition
The 1 PMT level fluorescence signal recognition in the slave core is as follows (See Figure 2):
1. Moving average in each time window of 0.8, 1.6, 3.2, 6.4, 12.8 4 s are calculated. These time window
widthes are optimized for maximazing S/N ratio.
2. Average and standard deviation are calculated from past 1.6 ms ,they are updated each 0.4 ms.
3. Moving average counts are normalized by them.
4. If the normalized counts are larger than threshold constants, the slave core interpret it as the fluorescencesignal. Threshold constants are variable for each time window.
5. If any one of FADC was saturate, the slave core also interpret it as the fluorescence signal.
3.2 Main components of the SDF
One SDF processes 16 PMT imput and main components are follows (See Figure 3):
1. 16 pairs of the receiver amplifier and WFSA.
2. 1 summation amplifier.
3. 16+1 FADCs, 1 is for the summation amplifier, others are for WFSAs.
4. 8+1 FPGAs, 1 is for the master core. Eace one of other FPGAs has a couple of slave core.
5. 1 complex programmable logic device for VME interface.
6. 2 digital-to-analog converter for generating test pulse.
7. 2 NIM outputs and 4 NIM inputs, they can be used for any digital signals.

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Tipical waveform of a Xeon lamp flush
7000
6000
sum4 count

5000
4000
3000
2000
1000
00

5

10

1520
micro second

2530

336

338

pedestal of SDF
100000standard deviation:1.4 countnumber of entry

10000
1000
100
101324

Figure 3. a picture of 2nd prototype SDF

326

328

330
332
sum4 count334

Figure 4. Tipical waveform (top) and pedestal ofanalog back gound noise (bottom) of SDF.

3.3 Basic performance of the SDF
1. The standard deviation of the analog back gound noise is typically 2 SUM4 counts, this value is equevalent
to 0.25 mV,%.
 
2. The linearity error of DC input is less than * 56, and that of 25 ns width pulse is less than 7* 58.

4.

Acknowledgements

We appreciate all TA collaborator and technicians of MEISEI inst. They made great efforts to support us about
design, construction and debugging of the SDF.

References[1] M. Takeda et al., Phys. Rev. Lett. 81, 1163 (1998).
[2] F. Kakimoto et al., 28th ICRC, 1029.
[3] S. Kawakami et al., 28th ICRC, 1033.