# Cherenkov Detectors. Index of Refraction When light passes through matter its velocity decreases. –Index of refraction n. The index depends on the medium.

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Cherenkov Detectors

Index of Refraction When light passes through matter its velocity decreases. –Index of refraction n. The index depends on the medium. –Wavelength dependence –A 0, 0 medium dependent The index can be viewed as a result of scattering. –Scattering amplitude A(0)

Frequency Dependence The index varies with wavelength. waterglass

Faster than Light A charged particles passing through matter will polarize some atomic electrons. If the particle exceeds the speed of light c/n then an electromagnetic shock wave will be formed. First observed by Pavel Cherenkov in 1934.  

Cherenkov Radiation The Cherenkov radiation has a characteristic angle compared to the particle. –No radiation below  = 1/n The Cherenkov light is linearly polarized in the plane of the particle. –u, v unit vectors along photon and particle directions

Emission Spectrum The number of photons from Cherenkov radiation is fixed for a given wavelength by the angle of the radiation. This can be integrated within a range of wavelengths. –Detector sensitivity

Blue Light More photons are produced at short wavelengths. Enhanced visible light at the blue end of the spectrum. –Characteristic glow from a reactor

Threshold Detector If any light is emitted, then the particle  exceeds 1/n. Varying the pressure of a gas in a detector can allow the identification of particles that exceed a desired speed. Mazziotta, GLAST (2005)

Particle ID Momentum and speed differ based on the mass of the particle. Beam magnets can select a fixed momentum. Cherenkov counters can identify particles by mass. Mazziotta, GLAST (2005) Pions Electrons

Ring Imaging A particle with velocity v creates light at a fixed angle. A spherical mirror will focus the light into a ring of fixed radius. –Center sets the particle position –Radius sets the speed These are called RICH detectors. LHCb

Neutrino ID Cherenkov imaging is used in neutrino detectors. –Underground observatories Muons from  -neutrinos make a clean ring. Electrons from e-neutrinos make a diffuse ring. –Electrons interact and shower Super Kamiokande

Lead Glass Calorimeter An electron will form a shower in a material. The path length of all electrons is proportional to the incident energy. Lead glass make an ideal calorimeter. –Dense, transparent PHENIX @ BNL

Electromagnetic Shower The OPAL detector used over 12,000 lead glass blocks to measure electron energy. Electrons came from the decay of Z bosons. –Reconstructed the Z mass.

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