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

– A0, 0 medium dependent

• The index can be viewed as a result of scattering.

– Scattering amplitude A(0)

vcn /

220

02

/11

An

)0()/(2

13

Ak

AZNn

Frequency Dependence

• The index varies with wavelength.

water glass

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

n

vncvv

/1cos

//cos

1

ˆˆ22

n

vnup

Emission Spectrum

222

2 1112

nx

N

• 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

HLnx

N

111

1222

137/1

Blue Light

• More photons are produced at short wavelengths.

• Enhanced visible light at the blue end of the spectrum.

– Characteristic glow from a reactor

222

1112

nL

N

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.