How to find the smallest particles – Particle Detectors
Scientists have invented and built many types of devices to detect particles. Most experiments at Fermilab use huge detectors that consist of intricate configurations of different detection devices, designed to collect the most possible information about the different types of particles created in a particle collision.
Different types of particle detectors:
- Scintillators – When a high energy particle enters scintillating material, it causes atoms to emit light. In a domino effect, the light travels through the interior of the scintillator and produces a signal at the edges of the scintillator. The signal can be measured by light-sensitive devices called photomultiplier tubes. There are two kinds of scintillators: the inorganic kind, in which atoms are ionized; and the organic type, usually plastic, in which atoms are excited and emit photons when returning to the ground state.
- Wire Chambers – When a charged particle traverses a gas-filled chamber, it ionizes the gas atoms along its path. By installing planes of thin wires inside the chamber and applying high voltage to the wires, physicists 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. In 1992, Georges Charpak received the Nobel Prize for the invention of the multiwire proportional chamber.
- Cerenkov Detectors – Although light travels faster than anything else in vacuum, particles can travel faster than light in gases, liquids or solids. If charged particles travel faster than light in such a medium, they give up some of their energy by emitting blue light. In a transparent medium like water, ice, oil or gas, the blue light can escape and transmit the information that a fast charged particle has passed through. Using various media to optimize the size of the light cone around the particle's direction, physicists build Cerenkov detectors that contain light-sensitive devices called photomultipliers. (Video about Cerenkov light, 5 min.)
- Calorimeters – Most particles interact with matter when they travel through it. Depending on the type of interaction, a particle can lose a fraction or all of its energy. Calorimeters measure the energy lost and determine the total energy of the incoming particle. Electromagnetic calorimeters measure the energy of leptons (such as electrons) and photons (light particles) as they interact with the electrically charged particles inside matter. Hadronic calorimeters monitor the energy of particles containing quarks as they interact with atomic nuclei. To build calorimeters, physicists use a wide variety of materials including solid lead-glass, cesium iodide, liquid argon and silica aerogel.
- Silicon detectors – A semiconductor with a voltage applied across its junction constitutes a sensitive, high-precision tracking device. Physicists build silicon detectors that consist of layers of small pads with many tiny silicon strips a fraction of a millimeter 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, physicists obtain information on the directions the particles travel. In future detectors, instead of using strips, physicists hope to use silicon pixels with a much smaller area. This will increase the precision for determining particle tracks.
- Photography – Today, this technique is rarely used in high energy physics. Charged particles leave marks as they cross photographic plates. The famous physicist Roentgen, for example, discovered X-rays using a photographic plate. In 1950, C.F. Powell received the Nobel Prize for the photographic method of studying nuclear processes.
- Bubble Chamber – In the 1960s and 1970s, bubble chamber technology led to the discovery of many new elementary particles. Filled with a superheated liquid, the bubble chamber creates a track of small bubbles when a charged particle crosses the chamber, locally bringing the liquid to boil. In 1960, D.A. Glaser received a Nobel Prize for the invention of the bubble chamber, and in 1967, the Nobel Prize Committee honored L.W. Alvarez for the improved hydrogen bubble chamber. Since the data acquisition and analysis is rather slow, bubble chambers are no longer used for research purposes.
- Cloud Chamber – The tracks of charged particles can be made visible with vapor, which condenses as charged particles travel through it. Called the expansion method, this discovery earned C.T.R. Wilson a share of the 1927 Nobel Prize. Perfectioning that method, P.M.S. Blackett received the 1948 Nobel Prize for the development of the Wilson cloud chamber. Today, cloud chambers are used for class-room presentations. They are easy to build and can be used to watch cosmic ray particles at home.
Important aspects of detectors:
Electronics - The signals created in detection devices are usually weak and need amplification. Present-day technology allows to place tiny electronic chips close to the location at which the initial signal is produced. These chips are, to some extent, even capable of shaping the wave form of the event and comparing several signals to determine the most important ones. The miniaturization of electronic components has greatly improved the capabilities of particle detectors. Continuous research and development efforts aim at meeting the requirements of the next generation of experiments.
Computing & Data Acquisition - Modern particle detectors consisting of many different subsystems feature more than a million read-out channels. They are optimized to sift through millions of collisions per second to find the most promising collision events that could produce discoveries. To handle this wealth of information, scientists have designed sophisticated, intelligent data acquisition systems to weed out uninteresting events before writing the results to magnetic tapes for permanent storage. The subsequent analysis of the data requires an enormous computational power. The Fermilab Computing Division builds and maintains computer farms to accomplish this task.
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