The term "heavy electron," sometimes used by individuals interested in the physics of high-energy particles, refers to a particular type of charged particle called a mu meson, or simply muon. As do electrons, muons have a charge of 1 (can be either negative or positive) and have an average lifetime of somewhat more than a microsecond. They have a mass that is about 200 times that of a conventional electron and behave similarly to electrons in many respects. When protons and other primary cosmic particles from various stars, including our own sun, interact in the atmosphere, they produce some secondary particles called pi mesons, or pions. The pions have very short lifetimes, on the order of 0.01 microseconds, and decay to produce muons. These muons, many with extremely high energies, some extending into the TeV (trillions of electron volts) range, have long enough lifetimes to reach the surface of the earth and represent a high-energy (hard) component of secondary cosmic radiation at the earth's surface. Muons can also be produced following high-energy collisions of charged particles with materials in high-energy particle accelerators. The muons decay to electrons and particles called neutrinos. The muons can interact by ionization and excitation processes with materials, which they contact, in a fashion similar to the interactions of electrons. Consequently, many of the detector types that are often used for muon detection are similar to detectors used for electron and other charged-particle measurements.
A common type of detector that has been used for muon detection is a plastic scintillation detector. This detector, which is often made of polymethyl methacrylate (trade name Lucite™ or Plexiglas™), frequently with organic scintillator incorporated into the plastic, produces light flashes as a result of a muon passing through and depositing energy in the detector. The light is detected usually with a light-sensitive electronic tube, called a photomultiplier tube (PMT). The PMT converts the light to an electronic pulse that can be processed and recorded using appropriate electronics. A simple muon cosmic radiation detector may consist of two such scintillation detectors, each with a rather large area and a thickness often small compared to the area dimensions and each with its own PMT; the detectors would be separated from each other by several inches. The electronic analysis system is such that only electronic pulses that occur nearly simultaneously in both detector systems are recorded; this type of counting is referred to as coincidence counting. The reason for using two detectors and performing coincidence counting is because a muon traveling downward from the atmosphere will most likely be energetic enough to pass through both detectors (especially if they are positioned with their large areas parallel to the earth), depositing some energy in each detector and producing measurable light flashes; radiation of other types, such as gamma rays or electrons, coming from other sources will very likely have directions and/or energies that prevent them from interacting simultaneously in both detectors. Thus, if only those pulses are counted that occur simultaneously in both detectors, there is a high probability that they are associated with muon interactions. If you are interested in more details about this kind of detection system, please go to the Lawrence Berkeley National Laboratory Web site where they describe how to construct and use such a system
There have been other much-more complex and sophisticated muon detectors constructed for use, especially at high-energy accelerator facilities. These have employed a number of detection systems in addition to scintillation systems, for muon detection and measurement. Some of these systems have been designed to track muon position and direction as the particle passes through the detectors, often arranged in multiple planes. Detectors have also been designed for making energy (usually particle momentum is measured) measurements. Detectors such as gas proportional detectors that use a system of multiple anode wires and what are called cathode strips (that allow very short ion collection times) have been applied in a special design that allows determination of particle position as it produces ionization in the gas. Systems that employ magnetic fields and allow measurement of bending angles of the muons as they traverse the field provide information about muon momentum and energy. Devices called calorimeters have also been used to obtain information about particle energy by measuring the heat generated by interactions in selected materials.
3. A Simple Muon Detector
Many teaching laboratories have exercises in which students record cosmic ray muons with a simple geiger counter and use the resulting counts as an exercise (often tedious) to examine the properties of the poisson distribution. Geiger counters have too small a collecting area for astrophysical studies in which we need a sufficient count rate to be able to study intensity variations at the level of a fraction of a per cent above the statistical noise due to the poisson process. However, the principle of a laboratory muon detector for astrophysics is still very simple and uses some of the counting concepts of the geiger counter experiment in which each pulse is treated equally and is assumed to result from the passage of a muon.
The muons are detected by detecting the brief flash of light which they produce when passing through a piece of plastic scintillator. Such scintillator is commonly used in large quantities in high energy physics laboratories and is cheap compared with sodium iodide which is required for gamma-ray studies. We have purchased scintillator from common high energy physics suppliers but we have also been given surplus material from such laboratories. For the purpose of this experiment, the scintillator can be rather old and somewhat yellowed and still perform perfectly well even though it may be regarded as unsuitable for high energy physics. The light from the scintillator is detected using a photomultiplier tube. Again, most conventional tube types are adequate for this purpose.
The only serious limitation on the apparatus is that a sufficient rate of muons must be detectable. This means that, with a sea level flux of about one muon per square centimeter per minute, a scintillator area of the order of one square meter is required. This is in order that poisson statistical uncertainties (the square root of the number of counts) permit intensity variations of a fraction of a per cent to be resolvable in a few minutes of recording. Such a size of scintillator is best viewed by a photomultiplier with a diameter of at least 75mm (we use a common laboratory size of 120mm) from a distance of at least 600mm. The whole detector must be made light-tight (and photomultipliers are sensitive to the smallest of light leaks) so we enclose the scintillator/photomultiplier combination in a sealed wooden or galvanised steel box which is either roughly cubic or pyramidal with the photomultiplier at the apex.
If the enclosure is light-tight, the photomultiplier output will consist of a series of pulses, each one representing the passage of a muon through the scintillator. These pulses can be detected (after preamplification or not as one prefers) using a discriminator. This is a circuit, often found in university nuclear physics laboratories, which produces a standard logic pulse when an input signal, such as the pulse from the photomultiplier, exceeds a preset voltage (or current) level. We vary the photomultiplier gain with its high voltage adjustment to set the count rate from the discriminator to correspond to the expected muon rate i.e. about 150 counts per second for a one square metre scintillator. The experiment consists of counting these logic pulses over predetermined periods of time such as a quarter of an hour and observing how those numbers vary over periods of days or longer.
Muon Particle - Myon
Electrically charged unstable elementary particle with a rest energy of 105.658 MeV corresponding to 206.786 times the rest energy of an electron (0.511 MeV). The muon has an average half-life of 2.2 • 10-6 s. The muon belongs to the elementary particle group of the leptons.
Most cosmic rays are protons, which are abundant in the universe.
Primary cosmic rays are particles such as a single proton (nuclei of hydrogen; about 90% of all cosmic rays) up to an iron nucleus and beyond, but being typically protons and alpha particles (identical to helium nucleii; majority of the remaining 10%) traveling through the interstellar medium. Most of these originate outside of our solar system (galactic cosmic rays GCR's - i.e. from Supernovae), but some of them come from the sun.
When these primary cosmic rays hit Earth's atmosphere at around 30,000m above the surface, the impacts cause nuclear reactions which produce pions. These pions decay into a muon and muon neutrino (= antineutrino) at about 9000 m altitude, which rain down upon the surface of the earth, traveling at about 0.998c. Many muons decay on the way down into Neutrinos and an electron while others reach the surface, but there are still enough to be detected fairly easily. Actually, about 200 rain down on each square meter of Earth every second. Details about particle decay (pion-muon-neutrino-antineutrino).
The Project - Simple DIY (do-it-yourself) Cosmic Ray Detector
Muon Particle Experiment - Cosmic Ray TelescopeDetecting Cosmic Rays is easy! This website provides schematics for the whole muon experiment (electronics schematic, variable high voltage power supply schematic and a drawing for the detector assembly). Check the new cloud chamber page for a non-electronic way for particle detection and visualization.
We detect the muons by utilizing a homebrew Geiger-Müller detector. The Geiger counters are supplied by high voltage, which creates a very high electric field near the anode of the detectors. When a cosmic particle enters one detector, it strips off some electrons of some atoms. These electrons move towards the positively charged wires, are accelerated by the huge electric field and have enough energy to strip more electrons from other gas molecules. These electrons are accelerated too in order to strip more and more electrons. This electric avalanche consisting of more than a billion negative charges rains down on the positively charged wire, causing a current which flows into the simple detection circuit.
Since other particles are stimulating the detector aswell, we will use 2 detectors to avoid false detection. Other particles originating from i.e. terrestrial radiation will also cause stimulation, but those particles have too less energy to penetrate both detectors. They will end up either in the first detector or shortly after it. So we simply have to look for almost instant detections in both detectors and consider this as successful detection.