Resistance plate chamber
Resistive plate chambers (short- RPC , of Engl. Resistive Plate Chamber ) are special gas-based particle detectors that help in particle physics -Experimenten charged identify particles on their air time. Compared to other time-of-flight detectors, RPCs have an improved time and spatial resolution.
construction
RPCs consist of two high-resistance glass or Bakelite plates ( resistive plates ), which enclose a volume of gas and are coated with copper on the outside . The copper layers represent electrodes between which a high voltage difference is applied in order to generate a homogeneous electrical drift field in the gas volume . Alternatively, the electrodes can be segmented into strips, the cathode and anode strips usually being rotated through 90 ° relative to one another. This makes it possible, by combining the signals from both sides, to obtain the point of passage of the charged particle through the detector as a spatial coordinate.
Working principle
An electrically charged particle passing through the RPC detector ionizes gas molecules along its trajectory. In the homogeneous field, the electrons drift to the anode are accelerated toward it creates a charge carrier avalanche (so-called. Gas amplification ), which an electrical signal in the electrodes influenziert . Only charge carriers that are generated near the cathode by primary ionization drift in the electric field long enough to obtain sufficient energy to influence a measurable signal. Primary ionization, which takes place too far from the cathode, therefore does not contribute to the measurement of a signal.
The good time resolution of these detectors in the picosecond range results from the high field strength of the homogeneous drift field. RPCs are usually operated in proportional or streamer mode. In the latter operating mode, the field strength is so high that photons are emitted parallel to the ionization in the gas , which in turn ionize gas molecules. A gas discharge occurs that extends through the entire gas volume. These so-called streamers are finally neutralized by the high-resistance plates, so that continuous operation of the detector is possible.
use
RPCs are used as time-of-flight detectors which, in combination with the known flight path (which is determined with other detectors, e.g. drift chambers , track drift chambers, etc.) enable the speed of charged particles to be measured. Furthermore, particles can be identified (i.e. their mass ) by additionally measuring their momentum . The accuracy with which the different types of particles (e.g. protons and pions ) can be separated from one another depends directly on the time resolution of the time-of-flight detector used.
Many high energy physics experiments use RPCs as time-of-flight detectors and the like. a. CMS and ATLAS .
MMRPCs
MMRPCs ( Multi-strip Multi-gap Resistive Plate Chambers ) are a further development of RPCs with an improved time resolution. In these detectors the gas volume is divided into several small sub-volumes by resistance plates. This results in a reduction in the area in which primary ionization can take place and in the drift time. Signals are thus influenced more quickly in the electrodes, the response time of the detector decreases.
Time resolutions of up to 100 ps can be achieved with MMRPCs.
Web links
- The CMS Resistive Plate Chamber (English)