Distributed detector of air showers RUSALKA
Distributed detector of air showers RUSALKA consists of set of elementary detection units, so called base stations. The schematic layout of a base station is shown in Fig. 1. Each base station contains:
- two scintillation counters
- DAQ box with
- QNet readout card to acquire data from the scintillation counters, digitize them and assign a timestamp using GPS timing information;
- HV power supply for scintillation counter PMTs;
- Low voltage power supply for GPS receiver and QNet readout card;
- single-board PC.
The purpose of the scintillation counter is to produce an electrical signal when a particle goes through it. Thanks to its simplicity and reliability scintillation counters are widely used in particle and nuclear physics to register particles. In the RUSALKA setup each scintillation counter produces a signal every time when it is hit by a charged particle from the extensive air shower (EAS). Of course, any EAS contains lots of neutral particles. They also may produce a signal in the scintillator counter, but the probability of this is less than 100%. The signal in the scintillation counter is formed in two steps. First, energy deposited by an ionizing particle in the scintillation is converted into the flash of visible light. Since the light is very faint, it is registered, amplified and converted into electric pulse by a photomultiplier (PMT).
Construction of the scintillation counter.
Typical RUSALKA scintillation counter consists of two plates of plastic scintillator. Thickness of each plate is 2 cm and dimensions 30 cm x 80 cm, so the total sensitive area of the counter is 60 cm x 80 cm. Light is collected by a set of wavelength shifting optical fibers. Each fiber with diameter of 1 mm is glued into the 1.2 mm trench along the scintillator plate, as shown in Fig 2. Optical fibers guarantee transfer of light to PMT with minimum losses. Pitch of fibers is 30 mm so light from the detector is collected by means of 20 fibers. All fibers are bundled and glued into a polished optical connector attached to a PMT. The opposite end of every fiber as well as sides of scintillator plates are covered by aluminium foil to improve the light collection efficiency. The diameter of PMT is 25 mm. In the RUSALKA counters PMTs FEU-85 (USSR) or Hamamatsu (Japan) are used. All parts of the counter are placed into a metal container (dimensions 300х700х50 mm) shown in Fig.3.
GPS-receiver is a key point of the whole RUSALKA setup. Usually GPS receivers are used to determine the position, however they are equally good for precision timing. Let's explain it in more detail. GPS operation is based on signals from 24 satellites located at 6 orbits around Earth. Each satellite transmits data about its position timing information. All satellites are synchronized with a good precision. GPS receiver uses this information collected from several satellites to compute its location and absolute time. The time is determined with a precision up to 50 nanoseconds. Considering absolute time mlivniured by GPS receivers at every RUSALKA counter one can combine the detected signals and search for coincidence between them. Moreover, the coincidence like this can be searched between any counters, for instance between RUSALKA counters and similar systems in Prague, Bucharest or Warsaw!
The most sophisticated part of a base station is the DAQ system. Its purpose is to collect signals from the scintillation counters, amplify and digitize them, add timing information from the GPS receiver and transfer to remote server for storage and processing. RUSALKA DAQ consists of the readout card Qnet developed at Fermilab for the analogous educational project Quarknet and managed by a single-board PC. Qnet readout card, the PC and power supplies (HV and low voltage) are placed in a metal box shown in Fig. 5.