Particle Detectors

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Particle Detectors

• By Cigdem Ozkan
• December 2004

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Why?

• Matter (smallest scale) made of elementary
  particles. Over the past century studies on the
  atom have revealed the smallest things human
  beings have ever found. How were these particles
  found?

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Three Steps

• Accelerators Collision of particles at high
  energies to look at what's inside these
  particles, but also to use the energy of their
  collisions to create different particles of
  matter.
• DetectorsUnveiling the tiniest constituents of
  matter with accelerators is only half the battle.
  An extraordinary particle detector to observe
  what happens in high-energy collisions.
• Computing Data Acquisition Tonnes of data -gt
  computer farms to sift thru and analyse data.

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Second Step Particle Detectors

• Instruments to count particles, visualise tracks,
  measure particle energies, record time-of-flight
  and identify different particles
• Depending on the type of accelerator and the
  particles and forces to be studied, various
  detection devices are arranged in intricate
  configurations.

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Types of Detectors

• Cloud Chambers The tracks of charged particles
  can be made visible with vapour, which condenses
  as charged particles travel through it

Wilsons Cloud Chamber (c. 1912)
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Types of Detectors

• Photography Not used at all. Charged particles
  leave marks as they cross photographic plates.
  Roentgen gt X-rays
• Geiger Counter Gas-filled tube with a central
  wire at high voltage to collect the ionisation
  produced by incident radiation. Detects alpha,
  beta, and gamma radiation although it cannot
  distinguish between them. Very limited.

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Types of Detectors

• Bubble Chamber Filled with a superheated liquid,
  creates a track of small bubbles when a charged
  particle crosses the chamber, locally bringing
  the liquid to boil. Data acquisition and analysis
  is rather slow, bubble chambers are no longer
  used for research purposes.

Alan and the bubble chamber
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Types of Detectors

• Wire Chambers Charged particle traverses a
  gas-filled chamber, ionises the gas atoms along
  its path. Installing planes of thin wires inside
  the chamber and applying high voltage to the
  wires, can record the signals caused by gas ions
  attracted to the wires. The data reveal the
  arrival time of a particle as well as its track.

A Wire Chamber for the GLAST (Gamma Ray Large
Area Telescope) project
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Types of Detectors

• Transition Radiation Detectors (TRD) When a
  charged particle crosses the boundary between two
  media with different dielectric properties (that
  is to mean, with different index of refraction),
  it can emit an electromagnetic radiation, known
  as transition radiation. Radiation intensity is
  approximately linear with the Lorentz ? factor
  (E/m0c2) of the particle. TRD can be employed as
  a mean of identifying particles measuring their
  ?, or as a threshold detector, to distinguish
  between particles which emit or not transition
  radiation.

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Types of Detectors

• Calorimeters - Most particles interact with
  matter when they travel through it and can lose a
  fraction or all of its energy. Calorimeters
  measure the energy lost and determine the total
  energy of the incoming particle. Electromagnetic
  gt the energy of leptons and photons as they
  interact with the electrically charged particles
  inside matter. Hadronic gt monitor the energy of
  particles containing quarks as they interact with
  atomic nuclei.To build calorimeters, a wide
  variety of materials including solid lead glass,
  cesium iodide, liquid argon and silica aero gel
  are used.

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Types of Detectors

• Scintillators Based on electronic detection of
  the scintillation light - the flash of light
  produced when a fast charged particle passes
  through certain fluorescent materials. Energetic
  neutral particles such as gamma rays and neutrons
  can also be counted by detecting the charged
  secondary particles which result from their
  interaction with matter. The first scintillation
  counting was done by eye (Rutherford, early
  1900's) gt Geiger counter gt photomultiplier tube.
  Since that time, a number of highly efficient
  scintillator materials (organic and inorganic)
  and special photomultipliers have been developed
  for this purpose.

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Types of Detectors

• Photomultiplier Tubes detect photons by first
  producing electrons in a photo-cathode (by the
  photoelectric effect). These electrons are
  collected by a shaped electric field and then
  multiplied.

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Types of Detectors

• Silicon Detectors A semiconductor with a voltage
  applied across its junction constitutes a
  sensitive, high-precision tracking device.
  Consist of layers of small pads with many tiny
  silicon strips a fraction of a millimetre wide.
  Charged particles passing through the silicon
  create electric signals that exactly indicate
  which strips the particles have crossed. As the
  particles pass through many layers of silicon,
  can obtain information on the directions the
  particles travel.

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Types of Detectors

• Cerenkov Detectors The resultant polarization of
  local atoms as the charged particles travel
  through the atmosphere results in the emission of
  a faint, bluish light known as "Cerenkov
  radiation. In a transparent medium, blue light
  can escape and transmit the information that a
  fast charged particle has passed through. Various
  media to optimise the size of the light cone
  around the particle's direction. Also contain
  light-sensitive devices called photomultipliers.
  Cosmic Ray detection

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Types of Detectors (Cerenkov)
Whipple Air Cerenkov Detector
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After all is said an done

• Unfortunately all does not end with being able to
  detect particles. A lot of effort goes into the
  up keep of the detectors, data storage and of
  course data analysis
• New detectors mean new problems but also new
  horizons