Expanded array for giant air shower observation at Akeno

Nuclear Instruments and Methods in Physics Research A247 (1986) 399-411 North-Holland, Amsterdam

399

EXPANDED ARRAY FOR GIANT AIR SHOWER OBSERVATION AT AKENO M . TESHIMA 2), H. OHOKA'l, Y. MATSUBARA 2), T. HARA' 1 , Y. HATANO 1), N. HAYASHIDA C.X . HE 3), M. HONDA'), F. ISHIKAWA 1), K. KAMATA 1), T. KIFUNE' 1, M. MORI 2), M. NAGANO'1, K. NISHIJIMA 4), Y. OHNO') and G. TANAHASHI' 1 ') Institute for

Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo, 188 Japan

2) Department of Physics, Kyoto University, Kyoto, 606 Japan 3) Institute for High Energy Physics, Academia Sinica, Beling, China 4) The Graduate School of Science and Technology, Kobe University, Kobe, 657 Japan

Received 16 September 1985 and in revised form 30 December 1985 As the first stage of a future huge air shower array, the Akeno array was expanded to about 20 km2 by adding 19 scintillation counters of 2.25 m2 area outside the present 1 km 2 array and installing a new data collection system . These detectors are connected successively by two optical fiber cables . The total number of electrons and the arrival direction of extensive air showers of 10'° GeV can be determined with accuracies of 25% and 3° respectively with this array. The present recording system is applicable to other experiments m which many sensors are connected in sequence. 1. Introduction It is important to study the origin and the propagation of the highest energy cosmic rays . The detailed study of the energy spectrum, the arrival direction distribution and the chemical composition are closely connected to these questions. The problem of propagation of cosmic rays is expected to become simpler, the higher the primary energy . This is because the gyro-radius of 10'° GeV cosmic protons is 3 kpc in our Galaxy, so that they cannot be confined in the Galaxy magnetic fields of 3 ,uG. Furthermore, the possible astronomical objects which are able to accelerate cosmic rays up to 10'° GeV are limited. Therefore anistoropy in their arrival direction is expected . They are also important in cosmology, in that the microwave background of 2.7 K radiation causes a steepening of the energy spectrum above 10 10 ' GeV for extragalactic protons [1][2] . So far, giant air showers produced by cosmic rays above 10'° GeV (GAS) have been observed at Volcano Ranch [3], Haverah Park [4], Narabrai [5], Yakutsuk [6] and Utah [7]. However, there are still discrepancies among the various groups on the energy spectrum and the arrival direction of cosmic rays above 10'° GeV . In order to clarify the present experimental ambiguities and to extend the observation to higher energies, a plan for a huge surface array of over 100 km 2 area is currently under consideration . For the present purpose it is required to deploy many detectors with a separation of about 1 km over the whole area and to measure the relative time difference of the arrival time 0168-9002/86/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

of shower particles among detectors with 50 ns resolution . Furthermore it is essential to not only acquire the data, but also to monitor and control each detector from the center in order to maintain long term stable operation. In order to keep the system reliable, we employ a hierarchic system configuration. The whole area is divided into four sections, each of which is called a "branch" . Each branch has an independent function of acquiring data and monitoring the detectors. In this report we describe the details of the "Akeno branch" which was just constructed at Akeno with the intention of being a part of the huge array. The response of this 20 km2 array (Akeno branch) for GAS is examined by Monte Carlo simulation. 2. "Akeno branch" array The detector arrangement of the "Akeno branch" is shown in fig. 1. The open circles designate scintillation counters of 2.25 m2 arranged with 1 km separation, connected to the new recording system . The closed circles are those of 1 m2 area connected to the existing "Akeno 1 km2 array" [8] recording system . The four large ones are connected to both recording systems. The open square is a muon detector of 20 m2 area in operation, while the closed ones will be operated within a few years. Each detector is connected to the next one by two optical fiber cables successively as shown by the solid lines in fig. 1 . One cable is used for the control of each module set at the detector, and the other for

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M. Teshima et al. / Giant air shower observation 200

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Fig. 3. The relation between the logarithm of the output signal and the photon intensity, which shows the linearity of the phototube and amplifier system of the scintillation counter.

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1600 m

Fig. 1. Map of the Akeno 20 km2 array. Open and closed circles designate scintillation counters of 2.25 m2 and 1 m2 area, respectively. Muon detectors are designated by squares. Closed ones are now in preparation. AB21 is the Akeno branch center.

sending data from the detector to the center . The advantages of using optical fibers are (1) the ability to avoid radio noise, (2) to avoid lightning which often causes serious damage to electronics and (3) to get a timing signal of better quality than can be obtained with a coaxial cable. The optical fiber cable, with its cross sectional structure shown in fig. 2, is hung on utility poles for electricity or telephone. The electrical power for each detector is supplied locally from a nearby electrical pole . Inside the Akeno 1 km2 array, unshielded detectors of 169 m2 total area, shielded detectors of 225 m2 (1 GeV for vertical muons) and 75 m2 (0 .5 GeV) area, and

SUSPENSION STRAND BINDING WIRE

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Fig. 2. The cross section of an optical fiber cable including two fibers which are hung on the utility poles along the road [10] .

53 fast timing channels with 10 ns resolution are available. These are especially effective for the investigation of the characteristics of GAS far from the core . 3. Electron detectors The particle density and the arrival time of electrons are measured by 2.25 m2 and 1 m2 scintillation counters (5 cm scintillator thickness). To extend the dynamic range of scintillation counters over 5 decades, only about 800 V is supplied to the phototube (R1512 Hamamatsu Photonics, 5 in . diameter). This is about half the recommended value. The amplifier gain is set to 1500. An example of the calibration curve of the output pulse width which corresponds to the logarithm of the pulse height is shown in fig. 3 as a function of input photon intensity of 10 ns duration . The preamplifier circuit is shown in fig. 4. The rise time of the pulses is about 100 ns . In case of large showers initiated by 10 10 GeV protons, more than 10 particles are expected to be observed by each detector within 1000 m from the core, hence a time resolution of less than 20 ns is obtained by setting the discrimination level corresponding to 0.5 particles. For the larger core distances where the density is low, the thickness of the shower disk is broader than 500 ns [9] and much greater than the rise time. In the arrival direction determination, each detector's data are weighted according to the thickness of the shower front at each core distance . Voltages supplied to the phototubes (hv), temperature and counting rate of detectors are monitored every hour at the center . The pulse width distribution for each detector is continuously accumulated at each detector and the data are collected and stored in the 10 MB Hard Disk at the center once a day. The discrimination level of the amplifier and the high voltage to the phototube can be controlled on request from the center. These functions enable us to maintain detector stability for long periods of time.

M. Teshima et al. / Giant air shower observation

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Fig. 4. The preamplifier used for scintillation counters . 4. Muon detector The muon detector of 20 mZ area in operation (AB23) consists of forty proportional counters under a concrete shielding of 1 m thickness. These counters are made of iron pipes (size 10 cm X 10 cm X 500 cm) enclosing P10 gas, which are of the type developed by Hayashida and Kifune [10] . The 1 m thickness of the concrete shielding corresponds to a threshold energy of 0.5 GeV for vertical muons. This thickness is sufficient to prevent leakage of the electromagnetic and hadronic components above 150 m from the core of showers of 10 9 GeV [11] . In the AB23 muon detector, the proportional counters are located outdoors, so that they are required to be kept in stable operation against humidity, changes in temperatures and mechanical shock for long periods of time . Therefore anticorrosive material is painted on their surface and waterproof treatment is applied to the hermetic seals. The preamplifier used is shown in fig . 5. This charge

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sensitive amplifier is employed to separate the amplifier more than 10 m from the counter. The dynamic range is about two decades. The 40 counters are divided into five sections, in each of which the total energy loss is recorded . The number of counters hit by muons is also recorded . In order to construct these detectors economically, some improvements are in progress. We are preparing counters to be buried directly under the ground . For this purpose, the new counter design features cylindrical tubes made of stainless steel. 5. Data collection system Fig. 6 illustrates the block diagram of the optical fiber network for the whole array. Solid and open circles indicate the optical-digital transmitter (E/O) and receiver (O/E) links. The large open circles containing the three small closed, open and cross symbols are bidirec-

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tional optical-digital links. Solid and open squares indicate the electric connectors, transmitter and receiver . The optical fiber used is a graded index multimode fiber (Fujikura Ltd) with a transmission loss of less than 3.0 dB/km for light of 0.85 tint wavelength . The bit rate of the optical-digital link is 0-10 7 bps (FFL-D100TC and FFL-D100RC made by Fujikura Ltd) . Each detector has a module called "detector control unit (DCU)" . Several DCUs are connected to a common string (two optical fibers ; command line and data line) successively and are connected to a module called line control unit (LCU). The LCUs are managed by a branch control unit (BCU). The DCUs are connected to the BCU through LCUs with HDLG protocol whose communication data rate is 625 kbps. The BCU controls all the detectors in the branch . The branch center accommodating the LCUs and BCU (AB21), and the Akeno data center located at the center of the Akeno 1 km2 array are connected by two optical lines with RS232C protocol. Usually, the amplitude and arrival time of every signal from the detector are being accumulated in a cyclic memory of DCU. Each DCU receives timer signals from the center so that its local clock is in phase with the center clock. In order to attain time precision within 10 ns we adopt a vernier system using delayed clocks . The vernier scale is obtained by coding the state of the five clocks, each of which is delayed by 20 ns successively, in every 10 ns interval . The time chart and data codes are illustrated in fig. 7. By decoding the clock data which are coincident with the signal, we can know its incident time. We have checked the synchronization precision of each DCU timer by supplying simulated detector pulses to two different DCUs spaced 2.3

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M. Teshrma et aJ. / Grant air shower observation

km apart. The derived distribution of the relative difference between incident times recorded by the two DCUs is shown in fig. 8. Errors in the synchronization of DCU timers are found to be less than 20 ns . At the center the coincidence condition is judged from the status information from all detectors (on/off) as is described in the next section. When the condition is fulfilled, a data acquisition command is fed to all DCUs through the command line . Then each DCU searches for the corresponding data within ±100 p.s of the coincident time and sends the data (signals' amplitude and arrival time) to the branch center . The stored GAS data at the branch center are sent to the Akeno data center and stored on a 10 MB hard disk. Since the detectors are deployed over a large area, it is essential to have facilities for monitoring and controlD1

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ling each detector at the center to guarantee long term stable operation as described in sect . 3. Monitor data are stored in the disk periodically and can be displayed on the color graphics terminal at the data center. At most, 16 DCUs per LCU, and 8 LCUs per BCU can be provided . Furthermore, the BCU is designed to be connected to a higher level controller (SCU) for future expansion. Eight BCUs can be managed by a SCU at maximum. In this system we can expand the array by just extending the two optical fiber cables and adding units such as DCU, LCU and BCU. The details of the data processing system are described in the appendix and in ref. [12] .

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M. Teshima et aL / Giant air shower observation

differences in optical fiber lengths and the distances of the detectors. With the usual requirement of any 6-fold coincidence among 23 detectors, we expect the rate of accidental coincidences to be about 0.5 Hz . In order to reduce the rate of accidental coincidences, a diode matrix coincidence circuit is adopted. The circuit is shown schematically in fig. 9a, with the constituent "AND" circuit shown in fig. 9b . We set the coincidence condition as 6-folds of neighboring detectors according to a diode matrix pattern. In this way the

rate of accidental coincidences can be reduced by a factor of 10 3 as compared with the usual system which accepts any 6-fold coincidence of detectors, so that the actual triggering rate is about 1 .5 per hour.

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7. Array response The response of the present array to GAS, the detection efficiency and the sensitivity for electron/muon size and arrival direction were examined by analyzing

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M. Teshima et at. / Giant air shower observation

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artificial showers which were simulated by the Monte Carlo method . Properties such as the electron lateral distribution, the muon lateral distribution and the shower front structure at large core distances between 500 and 3000 m for 10 9-101° GeV EAS are studied with the 4 km2/20 kmz array in conjunction with the data of the dense 1 km2 array [9,13] . Simulation was carried out with these results. The effective area is estimated by analyzing 100000 artificial showers which are distributed uniformly over a wide area following the size spectrum with exponent of - 3 . The expected frequency of air showers and the effective area are shown in fig. 10 . The histograms show the response to showers whose cores hit inside the array. The broken line shows the area for all events including showers outside the array. The threshold energy for 5 detectable air showers is found to be about 108 GeV and the recording efficiency reaches about 100% for 10 95 GeV. Inside the array, the real effective area is about 13 km2, but for showers of energy above 10 1°

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M. Teshima et al. / Giant air shower observation

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array of 100 km 2 are now being planned. The "Akeno branch" array described in this paper has proved reliable and effective for further expansion. This recording system is applicable to any other experiment in which many sensors are connected to strings in sequence, for example such as the DUMAND project array [141 . Acknowledgements

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6.7 6.3 7.1 7.5 7.9 Fig. 15 . The distribution of determined muon sizes (N,n out) for a simulated shower a constant muon size (Nit inpute) as inner shower . GeV the effective area becomes 20 km2. The distribution of calculated shower sizes, arrival directions and core positions are shown in figs . lla, 12a and 13a respectively for showers falling inside the array (inner showers) . Distributions for showers outside the array (outer showers) are shown in figs . llb, 12b, 13b. For inner showers of 10 1° GeV we can determine the electron size with 25% accuracy, the arrival direction within 3° and the core position within 50 m. However, for outer showers, the corresponding values are 150%, 9° and 150 m, respectively . Therefore care should be taken when the outer showers are used for the determination of primary energy spectra and arrival directions . The size dependence of errors for each parameter is shown in fig. 14. The distnbution of errors in muon size determination with our planned arrangement is shown in fig. 15 . As seen in this figure the muon size will be determined with 30-40% accuracy. 8. Conclusion The observation of ultrahigh energy cosmic rays with a 20 km 2 array has started at Akeno. The threshold energy for detectable air showers is 10 85 GeV and the recording efficiency reaches 100% for 10 9'5 GeV. The electron size and arrival direction are determined with accuracies of 25% and 3° respectively. The muon size will be determined with 30% or 40% accuracy with our planned arrangement . In order to clarify the origin of the highest energy cosmic rays, it is important to increase the statistics with data of good quality. The measurements of two components, electrons and muons, with an extended

The authors wish to thank Prof . Suga and other attendants of the Workshops on the giant air shower held between 1980 and 1985 for fruitful discussions . They also acknowledge Prof. M. Crouch and Prof. 1. Kondo for their kind advice on the manuscript. The simulation was carried out with the FACOM M380 at the Computer Room of the Institute for Nuclear Study, University of Tokyo. Appendix 1 . Data transfer 1 .1 . The line mode and communication format of strings

Signals in the fiber cable are carried by light of 0.85 p.m with a PW(pulse width)-modulated subcarrier of 625 kHz (10/16 MHz). Each DCU regenerates 625 kHz clock pulses and data signals from received signals. Clock pulses are used to generate a 10 MHz system clock in the DCU through PLL logic. In order to get time synchronization of the DCU timer to that of the BCU, the following process is used . As shown in fig. 16, the time frame denoted by the cross hatched field in the data signals on the C-line is being sent periodically every 409.6 ps to DCUs . It clears the lowest significant 12 bits of the timers and also sets the two most significant bits LO and LI . The timers of all DCUs are cleared and set simultaneously, so they are synchronized to that of the BCU with a constant delay for the signal propagation . The two most significant bits, LO and Ll, are used to label in which cycle between timer frames the data were recorded . Commands can be issued by the LCU to the DCUs at any time between the timer frames . Communication data rates are 625 kbps . The command format is based on HDLC (high level data link control) protocol . The value of the address field shows the location of the DCU to which the LCU is sending a command, and is given by a hexadecimal number from "00" to "FF" . Therefore, altogether 256 independent addresses can be achieved by the LCU. Among them, 255 are used for designating any one of the DCUs, but the final one, given by "FF", is used to send a command to all DCUs (broadcast addressing). However, the maximum number

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M. Teshima et al. / Giant air shower observation

of stations which can be registered as trigger detectors is 16 per string due to the restriction of the time slot . Commands carried by the information field consist of a command (2 bytes) and an operand (6 bytes) . About 40 commands are available in our system. CRC (cyclic redundancy check character) fields are used to check for transmission errors, i.e. to determine whether the calculated CRC value in the receiver is the same as the value sent from the transmitter . The calculation is executed automatically in the LSIs (Z80A-SIO) of both sides. In case of an error, the SIO of the receiver generates an error interrupt and the CPU executes an error routine for the recovery . The transmission on the D-line is performed in two alternative modes. One is the trigger mode and the other is the communication mode, as shown in fig. 17 . Usually the D-line is in trigger mode and the trigger status (on/off) of up to 16 detectors on the line is sent to the LCU. As shown in fig. 17a a time slot is assigned to each DCU. Each DCU puts the status as a voltage pulse in its time slot of 400 ns (Nd _< 8) or 200 ns (8 < Nd < 16) width showing whether there was a particle incidence in each cycle of 3.2 I.s . Nd represents the number of detectors in a string. The maximum available number of time slots is 16 . In the LCU, these serial data are demultiplexed to parallel format and sent to the coincidence circuit which examines whether or not coincidence conditions are fulfilled within the gate of 40 t.s . When coincidence conditions are satisfied, the BCU issues a command to the DCUs through the LCU to stop data acquisition, using the coincidence bit "COIN" in the timer frame signal in the C-line . The DCU changes the D-line from trigger mode to communication mode to send data in 16 byte blocks as shown in fig. *- 409 .6psec~

PARITY

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Fig . 16 . Data structure diagram of the C-line .

TIME AXIS

17b. In communication mode, signals in the D-line are transmitted by PW-modulated 625 kHz subcarrier, similar to that used in the C-line . This data format is the same as the command on the C-line except for the data length. In collecting monitor data from the DCUs, the D-line is also changed to communication mode . 1 .2. Data transfer between the LCU and BCU Data transfer between the LCU and BCU is executed in 8-bit parallel handshake mode by Z80A-PIO on each side . Data rates are about 20 kbps . The data length is variable over a range of 256 bytes. Data are transferred in 7-bit ASCII code with parity. In order to maintain synchronization of transferred data between the LCU and BCU, an opening and closing code is added to the data. Parity checks and data summing checks are carried out to prevent bit error and loss of bytes. 2. DCU 2.1 . A general description The DCU is equipped with a microprocessor as shown schematically in fig. 18 . DCU consists of three major parts; (1) communication section, (2) detector data processing section and (3) detector monitor and control section. The Z80A and its family ICs (Zilog Inc.) are used in the DCU. Z80A-SIO (Serial Interface) and DMA (8257, Intel Corp .) play significant roles in the high rate communication and data acquisition . (1) Communication section. Through SI01, HDLC communication with the central LCU is carried out in DMA mode . At each station one can also issue commands to the DCU via SI02 by using the Hand Held Computer (NEC PC-8201) with RS232C protocol, in l

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M. Teshima et at. / Grant air shower ohseroation TIME SLOT MODE (TRIGGER MODE)

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Fig. 17 . Upper part : Format of the D-line in trigger mode . Middle part : Format of the D-line m communication mode. Lower part : Signals on the D-line in raw data mode . order to check and control the scintillation detector locally. (2) Data processing section. The data processing section consists of a timer and a counter. A double buffer is provided so that new data will not be lost even if one buffer is occupied with the data reading DMA cycle (10 ,us). The timer and counter are used to record the arrival time and densities of shower particles. (3) Monitor and control section. The components designated CTC, DAC and ADC belong to the monitor and control section. The CTC (counter/timer controller) is used to accumulate the counting rate of cosmic ray particles and their amplitude distribution (in logarithmic scale PHA) . The DAC (digital to analog converter) is used to control the high voltage supplied to the phototube. This high voltage (hv) output can be PWA CNT. RATE

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varied from 0 to -1100 V by controlling the primary voltage from 0 to 5 V. The value of by and the temperature around the circuit board and scintillation detector are monitored by using double slope integrating ADCs. PH data are continuously accumulated in the DCU and collected and stored on disk at the center once a day. Monitor data consisting of hv, temperature and detector counting rate are also collected and recorded at the center every hour . By monitoring these data stored on disk, we can check variations in gain and linearity of detectors and adjust to predetermined values through commands from the central terminal . We can control the discrimination level for signal amplitude as well as the supplied hv . The detector gain is controlled by combining these two. H.V . MONI. RS232C TEMP . MONI. COMMUNICATION

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M. Teshima et al / Giant air shower observation

2.2. DCU string interface and timer

The block diagram of the DCU string interface is shown in fig. 19. The optical signal on the C-line from LCU is converted to an electrical signal by link module O/E (optical to electric) and is sent to PLL and to SIO-RX through the timer frame stripper (TFS). The original signal is also sent out to the next DCU after conversion back to an optical signal by E/O. On the C-line from the LCU, the data are carried by modulating the width of 625 kHz subcarrier pulses (pulse width modulation) . PLL generates the system clock of 10, 10/2, 10/4, 10/8 and 10/16 MHz (625 kHz) from the reproduced subcarrier . This 10 MHz clock is used for the timer of the DCU and is synchronized with the BCU timer . When the TFS detects the timer frame (every 409.6 ~Ls), the 12 LS13 bits of the DCU timer (TIME1 and TIME2) are cleared. In addition the 2 MSB bits are set equal to the LO and Ll bits in the timer frame restarted to synchronize the clock with the central BCU timer (c .f. fig. 16). The TFS also picks up one of two direct commands from BCU. One is "COIN" which is used to stop the data accumulation into the cyclic memory to prevent overrunning, and the other is "RST" used to reset the DCU when it is in a fatal state as a result of some accident . The TFS also deletes the received data timer frame and lets only the command frame pass to SIO. Serial data are converted into paral-

Tel 8-bit data in SIO and transferred to RAM (random access memory) by DMA. The mode of the D-line from DCU to LCU is selected by MPX (multiplexer) as either O/E or E/O (cf. fig. 19). In trigger mode MPX selects the output of the OR gate . The DCU puts the detector's status (ON/OFF) in its assigned time slot through the phase controller (PHASE CONTLR), and mixes it with those of other detectors in the OR gate . In communication mode, in which the DCU sends data to the LCU, MPX selects the modulated output of SIO-TX . During the interval in which other DCUs are sending data to the LCU in communication mode, MPX must select the output of the OR gate, and the output of the phase controller is required to be disabled . Transmitted data from other DCUs pass through O/E, the OR gate, MPX and E/O to the LCU . 3 . Central unit, LCU and BCU

The LCU consists of a one-board microcomputer, a string interface with the DCUs, a trigger demultiplexer and an interface to the bus line of the BCU. The BCU consists of a one-board microcomputer, a common bus, a clock and a timer frame generator, and a communication section (RS232C and HDLC port). At the branch center, each LCU controls one DCU string .

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M Teshima er al / Grant air shower observation

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TO DCU

TX D E M P X

ELECTRIC

0/E

OPTICAL

FROM DCU

rn m U U

ONE SHOT

TO TRIGGER CIRCUIT Fig. 20 . Block diagram of string interfaces of LCU.

Fig. 20 shows a block diagram of the string interface of the LCU. In the C-line from LCU to DCU, the timer frames and the commands from SIO-TX of LCU are multiplexed at MPX. The CPU sends these commands through SIO between timer frames . These data are modulated on a 625 kHz subcarrier, and after conversion to optical signals by E/O they are sent to the DCU.

In the D-line coming from DCU, optical signals are converted to electrical pulses and demultiplexed to communication data and trigger pulses . Communication data are then demodulated and sent to SIO-RX . Serial data are converted to parallel format in SIO and sent to RAM by DMA. The time multiplexed detectors' status data (ON/OFF) are converted from serial to parallel

and sent to coincidence circuits through the programmable one shot whose pulse width is controlled by the

LCU. The detectors' status is also sent to a phase comparator which is used to tune the phase position of the time slot of each DCU. In the system initialization

process, the LCU issues commands to each DCU one by one to adjust the phase value of the phase controller shown in fig. 19 until the phase of a particular DCU's status reaches the appointed position with an accuracy of 50 ns .

References

[3] [4] [5] [6] [7] [gl

K. Greisen, Phys . Rev. Lett . 16 (1966) 748. G.T . Zatsepin and V.A . Kuzmin, Sov. Phys . JETP Lett . 4 (1966) 78 . J. Linsley. Proc . 8th ICRC, Jaipur, vol. 4 (1963) p. 295. D.M . Edge et al ., J. Phys . A6 (1973) 1612 . C.J. Bell et al ., J. Phys . A7 (1974) 990. O.S . Diminstein et al., Proc . 15th ICRC, Plovdiv, vol. 8 (1977) p. 154. R.M . BaltrusaiUS et al ., Phys. Rev. Lett . 54 (1985) 1875 . T. Hara et al ., Proc . 16th ICRC, Kyoto, vol. 8 (1979) p.

135 . [9] M. Teshima et al ., Proc . 19th ICRC, La Jolla 7 (1985) p. 320. [10] N . Hayashrda and T. Kifune, Nucl. Insu. and Meth. 173 (1980) 431 . [I I] Y. Matsubara et al ., Proc . 19th ICRC, La Jolla 7 (1985) p. 119. [12] H. Ohoka and M. Teshima, ICR-Report-135-85-16 (Institute for Cosmic Ray Research, University of Tokyo, 1985) [13] T. Hara et al., Proc. 18th ICRC, Bangalore, vol. 11 (1983) p. 276. [14] DUMAND Proposal University of Hawaii .

(1982) Hawaii

Dumand Center,