Coincidence measurement
The term coincidence are medium-Latin ( co / con together '+ in , in' + cadere / cidere fall 'or together in a point) ( fall ), and generally means the coming together of events within a certain predetermined period of time. In measurement technology, especially in the fields of atomic, nuclear and particle physics, the principle of coincidence measurement with a circuit with at least two inputs is used, in which an output signal is only generated when pulses occur simultaneously at all inputs.
history
First application
Jean Jacques d'Ortous de Mairan (1678–1771) invented the coincidence method for pendulum measurements, which was later improved by Jean Charles Borda .
Recoil electron and scattered photons in the Compton effect
Walther Bothe and Hans Geiger applied a coincidence method in atomic physics to prove the simultaneous occurrence of the recoil electron and the scattered photon in the Compton effect . For this experiment, which they in 1924 by led / 25, they turned a narrow beam X-rays between two peak counter (forerunner of the Geiger-Müller counter tubes ), who were in a hydrogen atmosphere. Hydrogen absorbs the X-rays only weakly, but scatters them strongly. One counter tube, counter tube 1, remained open and thus filled with hydrogen. The other counter tube, counter tube 2, was covered with a platinum foil and the interior was filled with air. Since the platinum foil absorbs the recoil electrons, the air-filled counter tube 2 did not respond to electrons. The photons penetrated the film and released photoelectrons from the air, the walls of the counter tube and the film itself, which were registered by the counter tube 2. The open counter tube 1 registered almost no photons because they are only slightly absorbed by the hydrogen. The recoil electrons, however, are measured. Since the second counter tube does not register every incoming photon, a deflection of the second electrometer does not correspond to every response of the electrometer connected to the first counter tube. If, however, there is a coincidence of the recoil electron and photon, each deflection of the photon counter tube must correspond to a deflection of the electron counter tube. In this experiment, no complete agreement was measured, but the statistical evaluation showed a number of coincidences of 150,000 versus 1, which proves that these agreements are not random, but rather demonstrate the simultaneity of scattering and generation of a recoil electron.
The merit of the development of the coincidence measurement lies in the fact that Bothe and Geiger used an electronic registration of the response of the counter tubes, i.e. automatically registered the number of coincidence events and thus avoided the strenuous visual observation by the experimenters. This significantly increases the efficiency of the method. The coincidence measurement developed quite generally to an important investigation principle in the investigation of cosmic rays, in the investigation of elementary particle processes and the investigation of the Compton effect.
Cosmic rays
After Victor Hess discovered cosmic radiation in balloon flights in 1912 , Walther Bothe and Werner Kolhörster provided evidence in 1929 by means of coincidence measurements that this penetrating radiation originated outside the earth. They developed the technique of displaying the response of two or more Geiger-Müller counter tubes only if the response times followed each other within a predetermined short time interval . This happened when one and the same particle passed through all the counting tubes. The apparatus could only register radiation from certain directions. It was found that the particles fell preferentially perpendicular to the earth's surface; the intensity of incidence, however, decreased when the apparatus was tilted towards the horizon. This clearly points to the extraterrestrial origin, since in this case the perpendicularly incident particles have the shortest path through the earth's atmosphere and are thus least absorbed.
Further development
In 1954, Bothe received the Nobel Prize for Physics together with Max Born for his work on cosmic radiation with the coincidence method and on nuclear transformation .
Today the measurement of coincidence is an important means of investigating cosmic rays and all types of nuclear and elementary particle processes. The time resolution of the electronics from Bothe and Geiger was around 1 ms. Modern electronics allow resolutions below 50 ps, so that the detectors themselves are usually the limiting factor.
See also
literature
- Eduard W. Schpolski: Atomic Physics . 1993
- BM Jaworski, AA Detlaf: Dictionary of Physics.
- Emilio Segre: The great physicists and their discoveries. 1997