Ring-imaging Cherenkov detector

A ring imaging Cherenkov (RICH) detector is a type of detector for particle physics that uses Cherenkov radiation to measure the speed of charged particles. The radiation is emitted at a certain, speed-dependent angle to the direction of flight and then mapped onto a ring by the detector, which led to the name.

history

The principle of operation was first proposed by Jacques Séguinot and Thomas Ypsilantis , who worked at CERN in 1977 . Their research and development of very precise single photon detectors and suitable optics laid the foundation for the construction of the first large RICH detectors for the OMEGA spectrometer and the DELPHI experiment at the LEP .

Working principle

A RICH detector allows the identification of electrically charged particles by detecting the Cherenkov radiation ( emitted as photons ) of the particle that traverses a medium with a refractive index > 1 faster than the speed of light in this medium. The angle between the particle trajectory and the emitted radiation,, depends only on the known refractive index of the medium and the speed of the particle: ${\ displaystyle n}$ ${\ displaystyle \ theta _ {c}}$ ${\ displaystyle v}$

{\ displaystyle \ cos \ theta _ {c} = {\ frac {c} {nv}}}

Here is the speed of light in a vacuum. The speed of the particles can be measured by measuring this angle. ${\ displaystyle c}$

Parallel beams from different points on the trajectory through the medium are focused on a point of the detector by suitable optics, different directions on different points, a Cherenkov cone on a circular line. The circle is measured precisely with position-sensitive single photon detectors. The size of the circle then allows the Cherenkov angle to be determined.

A measurement of the momentum and the direction of flight of the particle, which is usually available from other parts of a detector, allows a prediction of the velocity for each type of particle. The comparison of the measured speed with the various predictions allows a determination of the particle type. Since the measurements are never exact, a relative probability is usually calculated for each type of particle. ${\ displaystyle v}$

Particles that are too slow do not generate Cherenkov radiation, this can also be used for identification. Particle identification is necessary for understanding the physics of the structures and interactions of elementary particles .

Measurement accuracy

All photons are emitted at the same angle, but the measurement inaccuracy of the detector results in a wider distribution of the measured angles. Since each particle emits many photons, the various measurements can be averaged, which makes the determination of the average angle more precise. This enables particle identification even at high particle energies, where the velocities of the different particle types differ only minimally.

The ability of a RICH detector to differentiate between different types of particles essentially depends on the following factors:

The effective angular resolution per photon
- Dispersion in the medium (the refractive index depends on the wavelength of the light)
- Image defects in the optical system
- Local resolution of the detector
Number of photons detected
- Length of material through which the particle flies
- Absorption of photons in the medium
- Absorption of photons in the optical system
- Quantum yield of the photon detectors

application

LHCb: Analysis of the decay of a B ⁰ meson into two pions (turquoise). The left graphic is without RICH data, the signal cannot be distinguished from the background (in particular, in red: B ⁰ → Kπ). Kaons identified by the RICH system have been removed in the right image, the signal can be clearly seen.

RICH detectors are of greatest importance in distinguishing between charged pions and kaons , since other types of detectors can hardly distinguish between these hadrons at high energies. Some analyzes look specifically for particle decays that produce kaons - RICH detectors allow the subsurface to be significantly reduced there. Important parameters are therefore "the probability of identifying a kaon as a kaon" and "the probability of not identifying a pion as a kaon".

Building types

Focusing and proximity imaging design

Various methods are used to depict Cherenkov light on a ring:

Focusing RICH

Scheme of the LHCb detector

A large mirror with a focal length is used in a focusing RICH . The photodetectors are located in the focal plane of the mirror. As a result, all photons are mapped onto a ring with a radius with the Cherenkov angle, regardless of their location along the particle path. Therefore long trajectories are possible in the medium. This type of construction is mainly used with gases, as the long trajectory is required there to generate enough photons. ${\ displaystyle f}$ ${\ displaystyle r = f \ theta _ {c}}$ ${\ displaystyle \ theta _ {c}}$

The LHCb experiment at the LHC uses two focusing RICH detectors. The first ( RICH1 ) is located directly after the vertex locator behind the collision point and is optimized for low-energy particles. The second ( RICH2 ) is located behind the magnet and the tracking system and is designed to differentiate between higher-energy particles.

AMS-02

AMS-02 on the ISS uses a RICH detector together with other types of detectors to analyze cosmic rays .

Proximity-Focusing RICH

In a proximity focusing RICH , the medium is thinner, so the photons are created within a very short distance and form a ring without additional optics. The ring has a radius of , where L is the distance between the active medium and the photodetectors. ${\ displaystyle r = L \ theta _ {c}}$

An example of this type of construction is the high momentum particle identification detector at the ALICE experiment at CERN.

DIRC

Scheme of a DIRC detector

In a DIRC detector ( Detection of Internally Reflected Cherenkov light ), the light is guided through total reflection to the detector, while its angle is retained due to the precise design of the elements and can be measured at one end of the detector. This type of detector was used in the BaBar experiment , for example .

Individual evidence

↑ J. Seguinot, T. Ypsilantis: Photo-ionization and Cherenkov ring imaging . In: Nuclear Instruments and Methods . tape 142 , no. 3 , May 1977, pp. 377-391 , doi : 10.1016 / 0029-554X (77) 90671-1 .
↑ SH Williams, DWGS Leith, M. Poppe, T. Ypsilanti: An Evaluation of Detectors for a Cerenkov Ring Imaging Chamber . In: IEEE Transactions on Nuclear Science . tape 27 , no. 1 , February 1980, p. 91-95 , doi : 10.1109 / TNS.1980.4330809 .
↑ T. Ekelöf, J. Séguinot, J. Tocqueville, T. Ypsilantis: The Cerenkov Ring-Imaging Detector: Recent Progress and Future Development . In: Physica Scripta . tape 23 , 4B, April 1981, pp. 718 , doi : 10.1088 / 0031-8949 / 23 / 4B / 023 .
^ In 1972, the OMEGA spectrometer was commissioned in the West Area and more than a million collisions were recorded that very first year. CERN-EX-7203328, March 1972.
↑ RJ ApSimon include: The recent operational performance of the CERN omega ring imaging Cherenkov detector . In: IEEE Transactions on Nuclear Science . tape 33 , no. 1 , February 1986, p. 122-131 , doi : 10.1109 / TNS.1986.4337063 .
^ R. Arnold et al .: A ring imaging Cherenkov detector, the DELPHI Barrel RICH Prototype: Part A: Experimental studies of the detection efficiency and the spatial resolution . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 270 , no. 2-3 , July 15, 1988, pp. 255-288 , doi : 10.1016 / 0168-9002 (88) 90695-X .
↑ Guy Wilkinson: In search of the rings: Approaches to Cherenkov ring finding and reconstruction in high energy physics . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 595 , no. 1 , September 21, 2008, p. 228–232 , doi : 10.1016 / j.nima.2008.07.066 .
↑ ^a ^b LHCb collaboration: Performance of the LHCb RICH detector at the LHC . In: European Physical Journal C . 73, No. 5, 2013, ISSN 1434-6044 , pp. 1-17. doi : 10.1140 / epjc / s10052-013-2431-9 .
↑ A.Augusto Alves Jr. et al: The LHCb detector at the LHC . In: JINST 3 S08005 . 2008.
^ ALICE collaboration: The High Momentum Particle Identification Detector. Retrieved April 18, 2014 .

[1] J. Seguinot, T. Ypsilantis: Photo-ionization and Cherenkov ring imaging . In: Nuclear Instruments and Methods . tape 142 , no. 3 , May 1977, pp. 377-391 , doi : 10.1016 / 0029-554X (77) 90671-1 .

[2] SH Williams, DWGS Leith, M. Poppe, T. Ypsilanti: An Evaluation of Detectors for a Cerenkov Ring Imaging Chamber . In: IEEE Transactions on Nuclear Science . tape 27 , no. 1 , February 1980, p. 91-95 , doi : 10.1109 / TNS.1980.4330809 .

[3] T. Ekelöf, J. Séguinot, J. Tocqueville, T. Ypsilantis: The Cerenkov Ring-Imaging Detector: Recent Progress and Future Development . In: Physica Scripta . tape 23 , 4B, April 1981, pp. 718 , doi : 10.1088 / 0031-8949 / 23 / 4B / 023 .

[4] In 1972, the OMEGA spectrometer was commissioned in the West Area and more than a million collisions were recorded that very first year. CERN-EX-7203328, March 1972.

[5] RJ ApSimon include: The recent operational performance of the CERN omega ring imaging Cherenkov detector . In: IEEE Transactions on Nuclear Science . tape 33 , no. 1 , February 1986, p. 122-131 , doi : 10.1109 / TNS.1986.4337063 .

[6] R. Arnold et al .: A ring imaging Cherenkov detector, the DELPHI Barrel RICH Prototype: Part A: Experimental studies of the detection efficiency and the spatial resolution . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 270 , no. 2-3 , July 15, 1988, pp. 255-288 , doi : 10.1016 / 0168-9002 (88) 90695-X .

[7] Guy Wilkinson: In search of the rings: Approaches to Cherenkov ring finding and reconstruction in high energy physics . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 595 , no. 1 , September 21, 2008, p. 228–232 , doi : 10.1016 / j.nima.2008.07.066 .

[LHCbRICH-8] LHCb collaboration: Performance of the LHCb RICH detector at the LHC . In: European Physical Journal C . 73, No. 5, 2013, ISSN 1434-6044 , pp. 1-17. doi : 10.1140 / epjc / s10052-013-2431-9 .

[9] A.Augusto Alves Jr. et al: The LHCb detector at the LHC . In: JINST 3 S08005 . 2008.

[10] ALICE collaboration: The High Momentum Particle Identification Detector. Retrieved April 18, 2014 .