Tools of the TradeII. DETECTORS AND HOW THEY WORK

The Electroscope

• Invented by Pierre Curie

• In 1927 was a National Bureau of Standards instrument for measuring radium

QuickTime™ and a GIF decompressor are needed to see this picture.

In 1911, Victor Hess took a gold leaf electroscope on his balloon flight to detect radiation. When an electroscope is charged, its “leaves” repel. A radioactive source can ionize air molecules, which carry away charge, ending the repulsion. This is called leakage. But it’s difficult to quantify leakage - it even varies with temperature and humidity.

Cloud Chamber• In a cloud chamber, a

supersaturated atmosphere is created by allowing alcohol to evaporate, then cooling it. It’s ready to rain !

• A cosmic ray (or any radiation) can ionize air atoms inside the chamber by stripping electrons from them. These ionized air atoms serve as condensation points for the alcohol. The invisible alcohol vapor condenses to form alcohol liquid in tiny droplets, outlining the path of the energetic particle.

Many types of detectors are used within a strong magnetic field (B-field). This causes charged particles to curve. The amount of curvature depends on their charge, speed, and mass.

Bubble Chamber

http://teachers.web.cern.ch/teachers/archiv/HST2005/bubble_chambers/BCWebsite/index.htm

Another old but familiar type of detector also relies on the ionizing effect of radiation. The Geiger counter uses a sealed tube filled with easily ionizable gas. The charged gas ions contact the central wire and send a current. The counter amplifies the current into the familiar clicking of the geiger counter.

Geiger Counter

Spark Chambers

Pictures from Griffith Observatory. http://griffithobservatory.org/exhibits/edgeofspace_cosmicrays.html

Evolution of Technology• Early detectors relied on visual aids: in a cloud,

bubble or gas cloud• The use of high-speed photography gave time

information but still relied on the eyes• Eventually, electronic techniques led to

automation

Scintillators• There are two physical

phenomenon relied upon:

– Fluorescence : as an energetic particle travels near certain kinds of atoms, it gives those atoms energy, which they lose a short time later in the form of photons.

– Cerenkov light : when a very fast moving particle enters a medium in which the speed of light is slower, it loses energy by emitting photons in a kind of “shock wave”.

To detect the very faint light created either by fluorescence or cerenkov radiation, an instrument called a photomultiplier tube is used. These tubes, using the photoelectric effect, allow even a single photon to release a single electron, which is amplified into a signal of millions of electrons : electricity ! This can be counted by instruments.

Photomultiplier Tubes

BASIC SCINTILLATION

Cerenkov Radiation

You may know that the speed of light is the fastest speed possible. That speed is 300,000,000 meters per second. When we talk of the speed of light we mean the speed that light travels in a vacuum, such as the airless void of outer space. Physicists call this speed "c". When light travels through any material such as glass or air, it slows down slightly to a speed less than "c". Some high energy particles, such as the cosmic rays can travel faster than this. When that happens the particles emit faint flashes of blue light known as Cerenkov radiation. This light is vaguely analogous to the sonic boom produced by aircraft that travel faster than the speed of sound. It is important to remember that the cosmic rays are not traveling faster than "c". They are just traveling faster than light does after it has been slowed down by passing through a material.

Water Cerenkov Detectors

Water Cerenkov detectors, like scintillator detectors use photomultiplier tubes. But the dark box contains no scintillator. Instead, it’s filled with pure, clear water. When cosmic rays pass through the water they emit faint flashes of blue light known as Cerenkov radiation. The sides of the water tank are lined with reflective material and some of this light is reflected onto a photomultiplier which produces an electronic signal. The size of the signal can be used to find out how many cosmic rays passed through the detector.

Transistor technology is employed to make silicon strip detectors. Ionizing radiation creates loose electrons. That’s electric current, and can be detected by instruments. Although expensive, this technology offers excellent location resolution of the origin of the radiation.

A vertex detector, used at the point where particles collide in an accelerator.

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ATLAS DETECTOR AT LHC

• http://www.atlas.ch/etours_exper/index.html