Counter Telescope

a device for the isolation and detection of high-energy particles moving in a certain direction. It contains two or more nuclear radiation detectors—for example, C₁, C₂, C₃, and C_A—that are positioned one after another along the particles’ path and are connected to coincidence or anticoincidence circuits (Figure 1). Counter telescopes may use detectors of a single type or of different types. Virtually any type of detector, including Geiger counters, scintillation counters, and Che-renkov counters, may be employed. Counter telescopes are used in the investigation of cosmic rays and in experiments carried out in particle accelerators.

Figure 1

The connection of the detectors to a coincidence circuit permits the electric pulses caused by particles passing through the telescope to be separated from the noise pulses of the detectors—that is, from the unavoidable background. The use of coincidence circuits also makes it possible to separate the pulses generated by the particles of interest from the pulses generated by other particles, which require a different amount of time to traverse the distance between individual detectors.

The use of an anticoincidence circuit permits the elimination of particles that pass through the telescope but differ from the particles of interest in some properties, for example, mean free path. (The detector C_A, which is connected to the anticoincidence circuit with the detectors C₁, C₂, and C₃, is located behind the detectors C₁, C₂, and C₃, which are connected to the coincidence circuit, and behind the filter, where the particles being detected are slowed and brought to a halt.)

The speed of operation of a counter telescope depends on the characteristics of the detectors and the resolving time of the coincidence and anticoincidence circuits. The angular resolution of the telescope—that is, its ability to discriminate particles traveling in a given direction—is determined by the dimensions of the detectors and the distance between them (the angle α). The solid angle β of the telescope depends on the size of the determining detector C₃ and its distance from the particle source. The other detectors are of such a size that they are traversed by all particles emerging from the source and passing through detector C₃.

The method of conjugate counter telescopes is used to single out two-particle nuclear reactions among concomitant processes. This method is based on the principle that for every two-particle reaction the angle of emission of the secondary particles, for a specified energy of the primary particle, is strictly defined by the laws of conservation of energy and momentum. Two counter telescopes positioned at such conjugate angles to the direction of the primary particle and connected to a coincidence circuit are said to be conjugate. The spatial resolution of a system of conjugate telescopes and, consequently, the system’s ability to single out the desired reaction among the accompanying reactions are characterized by the width of the peak on the curve for the number of coincidences as a function of the angle between the telescopes. The solid angles of conjugate counter telescopes are selected so that for each particle entering one of the counter telescopes (the determining telescope), the second particle, moving at the conjugate angle, enters the other counter telescope, regardless of the point of the target at which the reaction occurs.