Trace drift chamber

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The ALICE track drift chamber.

In physics, which is time projection chamber , also time projection chamber or according to its English name Time Projection Chamber (TPC) called a particle having a three-dimensional reconstruction of traces of electrically charged particles allows. It was invented in 1974 by David Nygren and has since been used in numerous experiments in particle and heavy ion physics .

construction

The typical TPC consists of a gas-filled cylindrical volume that is centrally (at half the length) divided into two drift regions by a high-voltage cathode . The multi-wire proportional chambers ( MWPC) attached to the end caps form the anode . There the arriving electrons released by the detected particle are multiplied. This is why one speaks of the reinforcement region . The sensitive volume is often very large. The currently largest TPC is the ALICE- TPC at the Large Hadron Collider at CERN . Your cylinder has a radius of approx. 2.5 m and a length of approx. 5 m. The sensitive volume is 88 m 3 . Since almost the entire solid angle is covered in accelerator experiments, such TPCs are also referred to as 4π detectors.

Often a magnetic field is applied parallel to the electric field , so that the particle tracks are bent due to the Lorentz force . The momentum and the sign of the charge of the particles can be determined from the radius of curvature . In addition, the magnetic field causes a reduction in the diffusion of the drift electrons and thus a better resolution.

Some TPCs use a different configuration in which the electric field points radially from the inside to the outside and the readout chambers are attached to the cylinder jacket. This has advantages in the case of particle tracks that predominantly run parallel to the cylinder axis. On the other hand, it leads to complications since the electric and magnetic fields are no longer parallel to each other and reduces the resolution.

Working principle

A charged particle traverses the gas volume of the TPC and ionizes the gas molecules along its track. The ionization electrons are accelerated in the direction of the end caps due to the high, homogeneous electric field that exists between the central electrode and the end caps (magnitude 400 V / cm). Due to collisions with other gas molecules, a constant drift speed is established. The multi-wire proportional chambers of the end caps register the two-dimensional projection of the particle track. The third dimension is obtained from the arrival time of the drift electrons on the end caps and the constant drift speed.

The MWPCs of trace drift chambers consist of several layers of wire. The gating grid can be switched to be transparent and is so permeable to electrons from the drift region that the TPC is "armed". Using a suitable voltage, the gating grid can also be made impermeable, which is particularly important in order not to allow ions from the amplification region to penetrate into the drift region.

The arriving electrons are then amplified between the cathode and anode level. This happens through a high electric field, which accelerates the electrons so strongly that they can ionize further gas molecules. The ion cloud generated in this avalanche-like process influences a mirror charge on the pad level (a segmented metal plate as the lowest level of the chamber), which is registered by the readout electronics.

The signal is proportional to the original ionization, i.e. H. the energy loss of the charged particle. Using the Bethe-Bloch formula, this energy loss depends only on the particle speed. Together with the pulse information from the track curvature in the magnetic field, the mass can be determined and the particle identified. Alternatively or in addition, further detectors can be used for particle identification.

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