Muon hodoscope URAGAN


In 2002, the development of a new recording system for the top coordinate detector began. The old system was designed for the study of rare events, and was not able to handle the total flux of atmospheric muons. This work was conducted with the support of the Department of Science, Industrial Policy and Entrepreneurship of Moscow and the Moscow Investment & Export Promotion Agency. In 2002, the structure of a new registration system was developed, and in 2003 together with specialists from the NGO "FORM" a special two-channel amplifier-shaper chip and a fast 16-channel reading board were designed. In 2004 – 2006, a multi-channel measurement system that converted the upper coordinate detector DECOR-V into the URAGAN was designed and manufactured. In this arrangement, the layout of streamer chambers in supermodules was restructured: foam sheet thickness between the planes of the chambers was reduced from 10 to 5 cm, and deviation of readout strips from mutually orthogonal arrangement was liquidated. As a result, unique muon hodoscope supermodules were sequentially put into operation in 2005, 2006 and 2007.



Figure 1. General view of the URAGAN setup.


The hodoscope (see Figs. 2, 3) consists of separate horizontal supermodules with the area of 11.5 square meters each located on the top of the Cherenkov Water Detector NEVOD (173 m above sea level). Each supermodule consists of eight layers of gas-discharge chambers, equipped with the system of external readout strips (2,560 X + 2,304 Y channels in one supermodule) with coordinates incremented by 1.0 cm and 1.2 cm in projections of X and Y, respectively. The layers of chambers are separated by foam plastic plates with thickness of 5 cm. The layer represents an assembly of 20 streamer tube chambers, each containing 16 square tubes with inner cross section 9x9 square millimeters and 3.5 m length, enclosed in a plastic case. The chambers are operated in a limited streamer mode which is provided by the choice of a special gas mixture (argon, carbon dioxide and n-pentane) and the respective operating voltage. Charged particles (muons) passing through the chambers cause the streamer discharge, which induces signals on X and Y strips. Each super-module is located on a separate movable platform that allows change the position of supermodules relative to each other and to the detection system of Cherenkov Water Detector NEVOD.


Figure 2, 3. Supermodule of the muon hodoscope URAGAN and its structure.

Triggering system and data acquisition system of the URAGAN setup are implemented as a set of identical, independent sub-systems of the supermodules with a clear common synchronization in calendar time. Thus, the rate of data acquisition from the setup is determined by the processing speed of a single supermodule. Extension of the setup is easily achieved by the addition of new supermodules, it increases the statistical reliability of the overall results, and the stability and identity of operation of the supermodules is controlled by the ratio of the recorded intensities.

Dataflow control of the supermodules and primary processing are carried out by the peripheral computers - industrial computers with ISA and PCI extension buses. Each peripheral computer of the supermodule can work both independently and as a part of the system. To acquire the data and generate test sequences, a digital in/out PCI 7200 ADLINK board is used. Selection of events and an accurate count of the time intervals are produced by a specially designed trigger unit.

The detection efficiency of a plane for a charged particle is mainly determined by the geometry of the tubes (wall thicknesses, etc) and is of about 90%. The presence of eight X,Y planes allows, on the one hand, organize a "soft" trigger, thereby increasing the efficiency of registration, and on the other hand, in the reconstruction of events, to select only straight tracks, really corresponding to the passage of muons. Thus, the coincidence of signals triggering at least four of eight planes with 99% efficiency identifies the passage of a charged particle through a supermodule.

The main element of the hodoscope data acquisition system is a specially designed fast readout board that provides amplification, discrimination, formation and storage of signals and serial data transmission from 16 strips. Each supermodule includes 160 X and 144 Y readout boards and 8 cross-cards to connect the signal lines and the power supply. The plane’s trigger signal is generated by the reading boards when any X-channel in a current plane is activated. The triggering condition for the supermodules’s measuring system is the arrival of signals from at least four different planes within 300 ns time gate. The average counting rate of one supermodule is about 1700 events per second. The supermodule response contains information about the hits in both X and Y strip projections. Track parameters (two projection angles) are reconstructed in real time and are accumulated in a two-dimensional array for one minute interval. Such an array of data (the matrix) is a "muon picture" of the upper hemisphere with a minute exposure. The reconstruction algorithm provides registration of muons with a high spatial and angular resolution (about 1 cm and 0.8°, respectively) in a wide range of zenith angles (from 0° to 84°). For reducing the reconstruction errors, only tracks which pass through the upper and the lower plane (i.e., tracks, crossing all the eight planes of the supermodule) are taken into consideration. This condition reduces the counting rate of supermodules by about 13%. The efficiency of reconstruction of the events is more than 90%; one-minute matrix contains about 8x104 reconstructed events.