Selection of μ-mesons by a Čerenkov counter system and an investigation of the interactions produced by them in lead plates in a Wilson cloud chamber

Abstract

In this chapter, the conclusions drawn from the experiments will be stated, and the future uses of the counter system will be discussed.

It will to obvious from the results that a .,as Čerenkov counter system of the type described, is an extremely useful tool for the detection of muons in the high energy range. The most useful property of the counter is of course, its velocity dependence. This has certain advantages over other selecting arrangements, which select on a momentum basis, e.g. elimination of contamination in beam experiments.

It had been shown that at 10 atmospheres pressure, the large Čerenkov counter can select particles which are 0.0009c above the threshold velocity. This threshold is set by the least number of photons which can be .elected by the photomultiplier (90). If Figure (6.1) is considered the properties and limitations of the Čerenkov counters can b e seen over a wide range of momenta. The table shown below has been calculated from Figure (4.1), which snows the increase in photon number with increasing momentum. It is calculated on the basis of a 100 cm. long sensitive length. As has been stated before, the practical threshold momentum is displaced further from the theoretical threshold with increasing momentum. In fact, with such a counter, it is possible that even if the particle is travelling with the velocity of light, insufficient photons will be produced to give a count. The only thing which can be done to rectify this situation is to increase the length of the counters. If, for example, a particle of momentum as high as 20 Gev./c was to be selected with an atmospheric pressure counter, it would have to be 4.8 metres long, and of such design to collect all the photons produced in the sensitive region. Of course, such a counter should theoretically select all particles above 4.5 Gev./c and thus the spread is becoming very large. It is this effect which puts a practical high momentum limit on the counters, and 20 Gev./c would certainly be the upper limit to any type of counter which we would consider possible to design.

Pulse height discrimination at these higher momentum values is also impossible, as can be seen from Figure (4.1), as the curves flatten out after a sharp initial rise. At the higher momentum values the increase in photon number with increasing momentum is extremely slow.

Tne experiments using cosmic radiation are at a great disadvantage compared to experiments using machine intensities. There fore, the fundamental interest is in being able to select particles in an energy range above the limits of the present accelerators. It does not therefore, appear possible to do this with a Čerenkov counter system alone. A combination of Čerenkov counters and ionisation counters using the relativistic rise in the latter, is being considered for future use. With such an arrangement, it might be found possible to select muons with an energy greater than those produced by accelerators.