﻿ Time projection chamber

# Time projection chamber

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

## Construction

Typical TPC is composed of a cylindrical gas-filled volume that is centrally divided by a high voltage electrode in the two drift regions. Affixed to the end caps multiwire proportional chambers (multi -wire proportional chamber, MWPC ) form the anode. Since there the incoming electrons ( which are generated by the charged particle ) be amplified and detected, is also called the gain region. The sensitive volume is often very large. The largest current TPC is the ALICE TPC at the Large Hadron Collider at CERN. Your cylinder has a radius of 2.5 m and a length of about 5 m. The sensitive volume is 88 m3. Since in accelerator experiments almost the complete solid angle is covered, one speaks in such TPCs also of 4π detectors.

Often, a magnetic field parallel to the electric field generated, so that the particle tracks are curved due to the Lorentz force. From the radius of curvature of the pulse and the sign of charge of the particles can be determined. In addition, the magnetic field causes a reduction of the diffusion of the drift electrons and thus better resolution.

TPCs some use a different configuration in which the electric field has inside-out and the read-out chambers are located on the cylinder mantle. This brings advantages in particle tracks which run mainly parallel to the cylinder axis. However, this leads to a number of complications, since electric and magnetic fields are not parallel to each other, and thus to a lower resolution.

## Principle of operation

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

The MWPCs of lane drift chambers consist of several wire layers. The gating grid can be switched to transparent mode and is transparent to the electrons from the drift region, so that the TPC is "sharp" switched. By a suitable voltage, the gating grid but also be made ​​opaque, which is especially important in order to leave no ions from the gain in entering the drift region.

The incoming electrons are then amplified between the cathode and anode plane. This is done by a high electric field which accelerates the electrons to such an extent that they can ionize further gas molecules. The ion cloud produced in this avalanche-like process influenziert a mirror charge on the pad level ( a segmented metal plate as the bottom plane of the chamber ), which is registered by the readout electronics.

The signal is proportional to the initial ionization, ie to the energy loss of the charged particle. This energy loss depends on the Bethe -Bloch formula only on the particle velocity. Together with the pulse information from the track curvature in the magnetic field can thus determine the mass and identify the particle. Alternatively or additionally, other detectors can be used for particle.

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