General instrumentation

Abstract

In many of the experiments in nuclear physics which use beams of charged particles it is necessary accurately to detect, to measure and to position these beams in some target chamber. The target chambers themselves are discussed more fully in section 2.2 and so the discussion in this section will be confined to the general problem of bringing the beam from the accelerator to the chamber. This chamber is often positioned in a well-shielded vault at some distance from the accelerator and, since the charged-particle beams usually have a finite emittance, some form of focusing of the beam must be provided to reduce the otherwise large loss in intensity. The beam will normally be transported in a high vacuum maintained in a metal beam tube. It is often necessary to measure the beam at various points along this tube to optimise the efficiency of the beam transport and focusing system. For beams which are d.c. this entails interrupting the beam andcatching it on insulated, conducting, flags which can be introdûced at various points in the beam path. At high energies a measurement of the intensity can be obtained from the secondary electron emission following its passage through a very thin foil. The same technique can be used to detect very-low-intensity beams or individual particles.1,2 When the beam is pulsed alternative methods can be used which do not require interruption of the beam. These methods rely either on purely electrostatic pick-up or on purely electromagnetic pick-up from the pulsed beam. In the electrostatic case the measurement is made from the charge induced on a set of wires forming a parallel conductor condenser, or on a pair of cylindrical cavities forming part of a tuned circuit, or on a pick-up loop with diode rectification.3–6