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PMT DARK NOISE MONITORING SYSTEM FOR NEUTRINO DETECTOR BOREXINO BASED ON THE DEVICENET PROTOCOL AND WEB - ACCESS
A.S. Chepurnov1, D.I. Orekhov2, D.A. Maimistov2, A.A. Sabelnikov3, A.V. Etenko3 1 Scobelcyn Institute for Nuclear Physics of Moscow State University, Russia 2 Department of Physics, Moscow State University, Russia 3 RRC "Kurchatov Institute", Moscow, Russia E-mail: chas@rtm-cs.sinp.msu.ru;
Monitoring of PMT dark noise in neutrino detector BOREXINO is a procedure that indicates condition of the detector. Based on CAN industrial network, top level DeviceNet protocol and WEB visualization, the dark noise monitoring system having 256 channels for the internal detector and 256 channels for the external muon veto was created. The system is composed as set of controllers, converting the PMT signals to frequency and transmitting them over CAN network. The software is the stack of the DeviceNet protocols, providing the data collecting and transporting. Server-side scripts build web pages of user interface and graphical visualization of data.

INTRODUCTION TO BOREXINO
Borexino, a real-time device for low energy neutrino spectroscopy is nearing completion of construction in the underground laboratories at Gran Sasso, Italy (LNGS). The experiment's goal is the direct measurement of the flux of 7Be solar neutrinos of all flavors via neutrino-electron scattering in an ultra-pure scintillation liquid. The new technology developed for Borexino enables sub-MeV solar neutrino spectroscopy for the first time resolving the solar neutrino problem. Borexino will also address several other frontier questions in particle physics, astrophysics and geophysics.

in Fig. 2 is designed to achieve the timing properties needed for a variety of key tasks: reconstruction of the event position, pulse shape discrimination of - and types of events, and the identification of a variety of delayed coincidence tags with a wide range of time bases.

Fig. 2. Block diagram of the Borexino electronics layout. The DAQ software is entirely custom made, with extensive use of multi-tasking techniques. User interfaces are all based on WEB techniques.

CHALLENGE OF DARK NOISE
Fig. 1. Sketch of the Borexino detector. About 300 tons of liquid scintillator are shielded by 1040 tons of a transparent buffer liquid. The scintillation light is viewed by 2200 PMT's. Reconstruction of the position of point-like events allows the determination of a 100 ton fiducial inner mass - the solar neutrino target. Outward looking tubes on the steel sphere surface act as muon veto detector. They use the Cherenkov light produced by muons that intersect the outer water buffer. The basic observables for the identification of neutrino events in Borexino are the total energy released in the scintillator, as measured by the number of photons emitted. The electronic signal processing scheme shown The phenomenon of a one-photoelectric pulse in the photomultiplier lies in the fundamentals of event registration techniques used in Borexino. Only coincidence of many pulses from different devices during short period allows the conclusion about the presence of a substantial physical event in the detector volume. But the essential property of photomultipliers is to produce sporadic pulses; each one taken separately is almost indistinguishable from the one bearing the information on physical event. These spurious pulses, being mainly a result of photocathode thermoemission, followed by release of electrons with the subsequent amplification, are called photocathode dark noise. Other sources of


dark noise are: radioactive decay in the glass bulb of PMTs, cosmic rays, auto emission under electrical field supplement etc. Dark noise is known for its intensity of any single unit of photocathode area to depend mainly on the manufacturing technology and its operating temperature. Also, there is evidence that it can be influenced by other physical factors [1]. Under certain conditions, dark noise can garble a physical picture, and make normal operation of the detector impossible. This is especially important in view of scarcity of neutrino-like and supernovas events Borexino was intended to study. For this reason monitoring and analysis of dark noise is necessary. A continuous dark noise monitoring system is now being created to separately monitor the internal and external detectors. The measurement of medium frequency of pulses is made for the internal detector photomultipliers in bunches by 12, and for external detector where it was possible to perform an individual analysis for each photomultiplier. The monitoring system, formed by a set of modules of intellectual frequency meters (modules for external and internal detectors differ mechanically, but inherit the common architecture), integrated in an industrial computer network. Data accumulation and primary processing are done on the central computer of this network. An essential feature of the monitoring system is its independence from the main data acquisition system of the detector. This means that dark noise monitoring system will work continuously throughout the period of detector activity, overlapping whenever possible interspaces between main system runs. During the development of this system, we realized the great potential for data analysis. The dark noise analysis meets the following tasks: ·Continuous accumulation and storage of entire data array, visual current state mapping and histories by the request on the Web; ·Watching sporadic effects and irreversible failures in separate photomultipliers and groups of photomultipliers; ·Watching the correlated changes on groups; ·Estimation of the long-time tendencies; ·Estimation of periodic variations using the Fourier analysis; ·Research of response to cutoff of high voltage; ·Research of response to flashes of photographic cameras. ·Research of correlations with other physical characteristics (temperature, vibrations, acoustic noises etc.). ·Issue of the certificate of absence of dark noise anomalies at the supernova trigger release.

whole system provides 196 independent data streams, each of them has meaning of the average count rate of PMT group of 12. The data rate is 1 value per second, which is in 0 to 500 KHz range. Apart from the frequency data, the system returns 14 values of power supply temperatures and actual voltages +5V and -5V, used for precision analog circuits. These parameters are very important for the FLASH ADC subsystem operation. Absence of one of the voltages or overheating may fail or even damage the analog adder; hence the PMT signals will be lost for analysis. Muon veto scaler system consists of 256 information channels; each has meaning of single PMT dark noise frequency. The data rate is 1 value per second, which is in 0 to 500 KHz range. Each channel data width is 4 bytes. The estimate value of internal detector is roughly 10 times more than of external one. Thus the total amount of data, coming from the system, is estimated as 196+256=452 frequency channels by 4 bytes gives 1808 bytes per second or approx. 1,8 KB/s, equal ~ 155,5 MB per day or ~57 GB per year. As the Borexino data taking period is scheduled for 10 years continuously, it gives estimated value of 570 GB. This estimate shows that the PMT dark noise rate data can be stored in data base located on one modern storage media assembled of a few hard drives. Storing the technological data is inexpedient The module diagram of the software components and their interaction with other data acquisition facilities is shown on Fig. 3. Different processes and data modules can be placed on different CPUs and communicate via network file system.
Inner detector
Slave 1

Outer detector
Slave 13 CAN Network
Slave 14

Slave 2

...

Slave 1

Slave 2

Slave 3
CAN Network

Slave 4

Inner detector station

Outer detector station Subsystem of Data acquisition

Subsystem of D ata acquisition

R ow data server

R ow data server

Ether net

Station

Row data client

DataServer
Web data server

Storage (files or database)

Ether net

Web station

Web station

Web d ata client
Borexino configuration D B
System of visualization (CG I scripts)

Web d ata client
Offline analyzer (CGI scripts)

CAN-BUS BASED MONITORING SYSTEM
The software of the Borexino dark noise monitoring is designed to collect, store, present and process information about the frequency of PMT dark noise. The data is supplied by the two independent systems of CAN-based scalers ­ the main system (internal detector) and muon veto (external one). The main system contains 14 groups of 16 channels each, and only 14 channels in group are used. Thus the

O n-the-fly reports

. Fig. 3. Software chart for Borexino dark noise monitoring system At present time the monitoring system for internal detector is completely assembled and in operation, the external system is under final tuning.


As an example of the monitoring system operation results, the Fig. 4 presents a fragment of real data, acquired from the detector during the test run in December 2003. The time dependency of average dark pulses frequency for 200 channels within few minutes with sampling rate 1 Hz. On the diagram one can see dark noise spikes, disabled channels, different average values in different channels, also the detailed analysis finds the cross-channel correlations. All this supports the major assumptions on the dark noise properties stated in [6].

framework useful for development of distributed realtime control systems for big experimental installations.

REFERENCES
1. . . , . . , . . , . . , . . , . . , . . . . , 2001, 46, . 5, . 786-789 Ultra-low background measurements in a large volume underground detector. Astroparticle Physics, 8 (1998) 141. A large scale low background liquid scintillator detector: the counting test facility at Gran Sasso. Nuclear Instr and Methods A 406 (1998) 411. Science and Technology of BOREXINO: a real-time Detector for Low Energy Solar Neutrinos. Astroparticle Physics 16 (2002) 205. Supernova neutrino detection in Borexino. L. Cadonati, F.P. Calaprice, M.C. Chen Astropartcle Physics 16 (2002) 361-372. ., .. . «» (2003)

2. 3.

4. 5. Fig.4. Plot of Borexino single electron noises

CONCLUSIONS
Positive experience of the application allows one to consider the components of CAN-bus technology as a

6.

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