Answer to Question #3781 Submitted to "Ask the Experts"

Can you please give me some information about a heavy electron (MezV) detector?

I am not aware of what your background is, so I will try to provide a summary answer that touches on a number of points relevant to your question. I hope my answer is not at too low or too high a level for your knowledge and understanding.

I am assuming that your question relates to "heavy electron" detectors. If I am misinterpreting your meaning, I apologize and please contact us again if this is the case.

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.

There is a wealth of information available on the Internet that relates to the topic of muon (heavy electron) detection. If the limited information I have provided is not sufficient, or if you are simply interested in more details, I would suggest you put key words such as "muon detector" into a search engine, such as Google.com, and browse to your satisfaction. Good luck!

George E. Chabot, PhD, CHP