Victor Hess took a gold leaf electroscope on his balloon flight to detect radiation. When an electroscope is charged, its “leaves” repel. A radioactive

QuickTime™ and aGIF decompressorare needed to see this picture.

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.Electroscopes are easy to build using tape, and easy to charge using a balloon rubbed on your hair. Overnight, the charge will leak away.

How do we detect tiny invisible fast moving particles from the cosmos?

Almost all methods are indirect.

We rely on the energetic particle interacting with more normal “stuff”, and hope the result of that interaction will be detectable.

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Another older type of detector is the 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.

QuickTime™ and aYUV420 codec decompressorare needed to see this picture.

Both techniques rely on photography to record the state of the chamber at any particular moment. But then, analyzing the photographs is a huge amount of work !

So, they’re not used for research much anymore.

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 chambers are very like cloud chambers, except the medium is a liquid on the verge of boiling.


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.

This technology can be scaled up to enormous detectors used in particle accelerators. But it’s not used so much in astrophysics.

There’s no fundamental difference between energetic particles created by scientists in particle accelerators, and cosmic radiation produced in the cosmos. Physicists use the same instruments to study both.


A similar technology, the spark chamber, is used to create electric signals. Ionizing radiation allows a current to briefly flow between the negatively charged wire gauze and the positively charged wire.

Often, we try to detect light.

This can be produced in 2 ways :

a) Fluorescence : as our 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.

b) 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 tube

QuickTime™ and aGraphics decompressorare needed to see this picture.

Scintillator DetectorsThe scintillator detector is made up of a special piece of plastic called a `scintillator'. When fast moving, charged particles, such as cosmic rays pass through the scintillator they excite the atoms in the plastic by giving them some energy (the cosmic ray then slows down a little). The excited atoms then fluoresce, losing this energy by emitting some photons of light. The light is detected by a "photomultiplier".

The detectors are usually arranged in a grid formation (or array), allowing measurements of each shower to be made at several points. Information from the detectors tell us how many particles struck the detector and the time that they hit. By adding up the number of particles recorded by each of the detectors we can estimate how many particles were in the shower and from that we can make a good guess as to the energy of the cosmic ray that started the shower. We can use the time that each detector was struck to measure the direction the cosmic ray was traveling when it hit the Earth's atmosphere.


The photomultiplier, as its name suggests, multiplies the small flash of light into a large electrical signal that can be measured. From the size of the electronic signal we can tell how many particles passed through the scintillator. The scintillator and the photomultiplier are housed in a dark box so that the only light detected is caused by cosmic rays. This kind of detector is illustrated in the diagram.


This is actually 6 scintillator detectors, arranged in pairs. Because pmt’s spontaneously generate some signals (this is called noise), they are typically run in coincidence. Confidence increases that a real particle has passed through when both detectors register within a small window of time.

These detectors are arranged to count particles from above, from an angle, and from horizontal directions. The difference in the counting rates suggests that most of the counts are from cosmic rays.

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.





Waves created by a stationary object.

Waves created by by an object traveling at sub-sonic speed.

Waves created by an object traveling at the speed of sound.

Bow wave created by object traveling at supersonic speed.

QuickTime™ and aYUV420 codec decompressorare needed to see this picture.

This jet is supersonic, and although the bow wave is very high pressure (lots of air molecules piled up), the area right behind the bow wave is very low pressure, and condensation results.

This is only an analogy for cerenkov radiation.

But it’s picturesque…

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. This kind of detector is used in the Auger array under construction, and, using ice, at the South Pole.

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.

Documents

Victor Hess took a gold leaf electroscope on his balloon flight to detect radiation. When an electroscope is charged, its “leaves” repel. A radioactive