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Cosmic Rays 31435-1436Cosmic Rays at Sea-Level - Extensive Air Showers and the detection of cosmic rays.5 Cosmic Rays at Sea-Level Secondary cosmic rays as measured at sea-level have a different distribution of particles from the primary rays. 1.Very few primary ray protons reach sea-level, where the penetrating or hard cosmic rays consist mainly of charged muons. It is sufficient to say here that upper atmospheric cosmic rays contain largely the so-called p-mesons or pions (mass 273me) and m-mesons or muons (mass 207me) of both signs. 2.There are further secondaries, positrons, electrons and photons, occurring in showers of innumerable particles. These make up the soft component , being absorbed by 100-200 mm of lead.3. At sea-level muons and electrons of both signs predominate. When p0 -mesons, created by fast proton collisions with oxygen, nitrogen and other nuclei in the atmosphere (see Fig. 25.1), decay into g-rays of over 100 MeV energy, the latter produce electron-positron pairs of almost the same energy.

These then generate new and very energetic photons by Bremsstrahlung, production having a continuous spectrum with a maximum energy given by hnmax=E - meC2 ,

where E is the energy of the incident electron of mass me The new Bremsstrahlung g-rays create further electron-positron pairs and they in turn produce Bremsstrahlung and so the process continues until the whole of the initial p0 - decay energy is dissipated. 4.This multiplication process is called an electron-photon 'shower According to this theory the number of positrons and electrons in cosmic rays should increase as the earth is approached. This is actually true to within about 15 km of the earth's surface, below which height the intensity decreases again, as originally found by Hess. These electron-photon cascade shower lengths are short enough in metals to be observed experimentally. In the air the electron shower path length is about 30 km and in lead about 5 mm, so that they can easily be observed in a cloud chamber as shown in Fig. 25.5.

.6 Extensive Air Showers1. In addition to the narrow electron-photon showers just described, there are 2.extensive air showers containing hundreds of millions of particles reaching the earth together and covering many thousands of square meters . These large air showers are due to the ease with which the low energy electrons and photons are deviated from the main path of the shower by multiple collisions with atmospheric nuclei. Since the total energy of a shower should be about equal to the energy of the primary particle (proton) causing it, we can get some idea of the energy of the latter by measuring the total energy of the shower particles. By this means a figure of 1020 eV for the maximum energy of the cosmic ray primary component is obtained. If a 1020 eV particle collides with an air particle one can imagine the next generation of particles having sufficient energy to give many further energetic collisions. 3. Many mesons and nucleons are so produced, giving rise to a penetrating shower which we could call a nucleonic cascade in contrast to the electronic cascade described in the previous section. The main components of these nucleonic showers are p-mesons and nucleons which are the penetrating component at sea-level. Most of the components of the nucleonic showers are radioactive.

.7 The Detection of Cosmic Ray ParticlesMost methods described in last semester can be used in the detection of the charged particles contained in cosmic rays. The oldest method is the 1.Wilson cloud chamber in which much of the early research was done and in which physicists were able to recognize the tracks of (a-rays, b- rays, protons, etc., very readily). Cloud chambers were used extensively until 1947 when the 2. nuclear emulsion method was developed as a complementary techniqueNuclear emulsions are still used on a large scale where cheapness is an important item in a research budget The bubble chamber cannot be used for cosmic rays because the lifetimes of the events are too short, and the cosmic rays arrive at random. This method of detection is ideal when used in conjunction with the pulsed beams from, say the Bevatron, which are also of this order of duration. The bubble chamber can therefore be used to investigate artificially produced strange particles rather than those produced in cosmic ray bursts The direction of a cosmic ray burst can be determined with the 3. Geiger-counter 'telescope'. Three or more Geiger tubes are arranged parallel to each other like the rungs of a ladder so that when a particle passes down the 'ladder' it discharges the whole set of counters simultaneously. When such a coincidence takes place the electronic amplifiers record a 'count'. Particles incident obliquely to the ladder cannot trigger-off all the tubes and no count is recorded. Thus a direction can be selected and the cosmic ray angular intensity determined by scanning. Unfortunately the identification of individual particles is impossible with this arrangement. In the case of the nuclear emulsion plate each individual particle leaves a characteristic track which can be identified by the skilled worker. Features which are used for identification are track length, grain density and track 'wobble', and plates are now put together in stacks so that details of the whole event can be followed. A careful measurement of track characteristics gives an estimate of the mass of the particle, but the sign is not so easy to find as in a cloud chamber, which can easily be operated in a deflecting magnetic field The use of counters arranged in coincidence, anticoincidence and in delayed coincidence, together with the use of counter-controlled expansion chambers and various emulsion techniques, forms the basis of nearly all cosmic ray measurements of direction, and intensity. This is particularly true for atmospheric and sea-level investigations of very energetic multiparticle events. It is only at great heights, where the unwanted background is low, that the single counter can be used successfully. All events recorded on photographic plates in cosmic ray work or in high-energy particle collision physics can now be computer scanned and analyzed.