Introduction to Cosmic Ray Tracking

Introduction

The Cosmic Ray Tracking project is an attempt to establish drift chambers as a new and promising technology in ground-based cosmic ray physics.

The traditional method for measuring the particle cascades (the so-called Extensive Air Showers, EAS) from ultrahigh-energy cosmic rays, the scintillator array timing method, requires that many scintillators spread over a large area are hit by secondary particles of the air shower. The arrival times of the secondary particles are used to measure the arrival direction of the primary particle.

The tracking method, on the other hand, makes use of the good correlation between the arrival directions of primary and secondaries. Due to the more efficient use of the information which is available from the secondary particles, the tracking method needs less observed secondary particles for the same angular resolution as the scintillator method. Therefore, a lower energy threshold can be achieved with the tracking method. The disadvantage of the tracking method is that more sophisticated equipment is required than for the timing method.

The CRT detectors

The CRT detectors are made of two circular drift chambers of 1.8 m diameter, with a maximum drift length of 90 cm. A 10 cm thick iron plate between the drift chambers is used as a muon filter. Each drift chamber has 6 sense wires to detect the electrons from the ionisation of the argon-methane gas by relativistic particles. Two of the 6 wires have signal-readout on both sides (charge-division wires). Three or four wires, depending on the model, have additional readout of cathode strips (pads). The coordinates in drift direction are obtained from the arrival time of the electron clouds at the wires; the coordinates parallel to the wires are calculated from the charge-division and pad signals.

The CRT detector working principle looks like that:

different particle types in a CRT detector

Particle tracks are reconstructed from the recorded pulses in a drift chamber. Low energy electrons are absorbed in the iron plate. High energy electrons (above a few hundred MeV energy) may produce an electromagnetic shower in the iron plate with additional particles in the lower chamber. Such 'punch-through' electrons are usually identified. Muons usually pass the iron plate with little scattering and can be identified as a matching pair of a track in the upper chamber and one in the lower chamber. Only hadrons (mainly high-energy pions) have a good chance to be misidentified as a muon. Because these hadrons, like muons, originate from hadronic showers, they don't spoil the discrimination between the different types of extensive atmospheric showers caused by gamma-rays on one hand and cosmic rays (atomic nuclei) on the other hand.

Upper and lower chamber are rotated by 90 degrees. Therefore, the drift direction in one drift chamber is parallel to the wire direction in the other. This allows us to calibrate the drift velocity from the tracks of muons penetrating both chambers.

The detector itself is in a gas-tight container filled with argon-methan gas. Some of the existing detectors have stainless-steel containers while others have spherical aluminium containers. Ten of the detectors with steel containers are currently in operation at the site of the HEGRA experiment on La Palma. One of them is shown in the following picture:

Image of a CRT detector with open electronics box

Each detector has a separate electronics box with a local computer system for readout of the drift chambers via a 40 MHz FADC system and for reconstruction of the particle tracks before the processed data is transferred to a central computer system.

Scientific goals of the installation on La Palma

The CRT detectors on La Palma have been used to

check the angular resolution of these detectors,
investigate the angular distribution of particles in air showers,
evaluate shower reconstruction by the tracking method,
search for gamma rays above 10 TeV energy from gamma-ray bursts (GRBs),
and to examine the composition of cosmic rays above 100 TeV.

For some of these goals the shower data from the HEGRA array is required, e.g. for the composition study. For other goals the HEGRA data was helpfull too but not essential while for the GRB search the CRT detectors were operated without HEGRA to achieve a lower threshold energy.

For more information see our online papers or send e-mail to Konrad Bernlöhr

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