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Volume 82, 1954-55
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4. Interaction of Cosmic Rays with the Earth's Atmosphere

The majority of cosmic ray research has been in this field; it has been profitable in discovering several kinds of mesons and their relations to nuclear reactions. It has been only moderately successful in aiding a better understanding of the nature of primary cosmic rays.

In a short paper I must of necessity omit some major aspects of the subject, particularly those obvious cosmic ray characteristics necessary to interpret other cosmic ray information—e.g., latitude, longitude and altitude effects, atmospheric pressure influences, etc. These effects are established and future work is more likely to be along the lines of improved accuracy and coverage rather than novel in approach.

4.1. High Energy Nuclear Reactions

The primary particles have energies ranging as high as 1015 ev—average about 1010 ev—and when they encounter other nuclei in the atmosphere may cause evaporation of that nucleus into lighter nuclei and various types of mesons. The reactions have been observed with the aid of photographic plates and cloud chambers and are known as “evaporation” stars. The subject has been well reviewed by Rochester and Rosser (1951) who classify into two types of stars, (a) low energy stars whose tracks consist mainly of protons of a few Mev energy, (b) high energy stars consisting of evaporation particles, knock on nucleons and mesons and requiring Bev particles for initiation. As examples a heavy primary particle U = 1010 ev, Z = 16 has produced 34 heavily ionizing particles and 17 relativistic particles—Le Prince-Ringuet (1949) Kaplon et al. (1949) show a primary alpha particle of 1013 ev colliding with a silver or bromine nucleus and producing a very narrow cone shower of 23 relativistic singly charged particles (mostly mesons) and a wider core of 33 relativistic particles plus 18 heavy ionizing particles.

4.1.1. Penetrating Showers

The particles of the stars may themselves produce further nuclear disruptions Others reach the earth's surface without further nuclear collisions and thus account for penetrating showers—predominantly downward directed narrow cones of very energetic nucleonic debris of energy ranges up to 1,000 Mev and with about an average of 100 Mev. The nucleonic debris mesons (mostly π), protons, deuterons and tritons can penetrate several inches of lead (Camerini et al. 1950).

4.1.2. Mesons

A list of mesons so far positively identified (Powell, 1953; U.S.A.E.C, 1953) is shown in the following table:—

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Table of Elementary Particles
Particle. Symbol. Mass Me. Lifetime Seconds. Mode of Decay. Remorks.
Electron e- l Stable — 1897
e+ 1 Stable — Annihilation with e-
Neutron ν° < 002 — — Never directly observed
μ meson μ+ 210 22 ×10-6 e++αν0 1936–38
μ- 210 22 ×10-6 e-+αν0 1936–38
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πmeson π+ 276 2 × 10-8 μ++ν° 1947
π- 276 2 × 10-8 μ-+ν° 1947
π° 262 10-14 αhν 1950
Neutron ν2 ν°2 800 10-9 π++π- 1947
τ Meson τ+ 969 10-9 π++π++π- 1949
τ- 969 10-9 π-+π++π- 1949
χMeson χ+- 1100? 10-9 π+-+neutral 1952 Charge Unknown
κMeson κ+ 1300? 10-9 μ++2neutral 1911
Proton p+ 1836 Stable — 1911
Neutron n 1836 720 p+e-+ν° 1932
Neutral ν1 ν1° 2200 10-9 p++ν- Produced with cosmotion 1951

There have been reports of other particles u±, S, V± but the evidence for then existence is not as convincing as for the above particles. (See Le Prince-Renguet, L., and Rossi, B., 1953).

The μ meson is not produced directly from the nucleus and has only a weak reaction with it. It arises from the decay of other mesons—directly from the π and κ and indirectly from the others.

The π, κ, χ and τ mesons interact strongly with the nucleus with approximately geometrical cross sections; they may be regarded as types of quanta of the nuclear field. The Compton wavelength of the π meson h/mu has a value (10-13 m) of the same order as the range of the nuclear field. If we regard the heavier mesons as “heavy quanta” it would be reasonable to assume that the field is made up of a number of components with different smaller ranges defined by the Compton wavelength of the different particles. Certainly these particles play an important part in nuclear reactions of protons in the 5 Bev range. The immediate problem is to establish accurate values for masses and lifetimes, critical energies for production and modes of decay. The Bev accelerating machines are already beginning to provide this information and with their controlled beams offer a more systematic attack than cosmic ray studies except at the highest energies.

4.2. The Soft Component

The soft component of cosmic rays has been known and studied for many years—such characteristics as time, altitude, latitude and absorption variations. It has been established that the soft radiation is partly continuous and partly in the form of showers of varying size. It is this latter topic and that of neutron distribution that is attracting most attention, so we might usefully restrict our discussion on the soft component to these topics.

4.2.1. Auger or Extensive Air Showers

These showers consist mainly of low energy electrons and gamma rays which are initiated by a single high energy particle and are formed by successive cascade processes involving pair production and brensstrahlung. Important properties under investigation are:

a

Size—frequency relations particularly at the larger distances (and energies).
b

Variation with sidereal time in relation to the origin of the primaries.
c

core structure of showers and their directions of arrival.
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4.2.2. Neutrons

Neutrons in cosmic rays are interesting in that they arise from two causes.

a

The majority from star evaporations of atmospheric nuclei and representing about 50 neutrons produced per primary particle.
b

The observations of Adams (1950) who detected a fivefold increase in neutron flux during a solar flare and of Simpson (1952) who claims a 27 day period associated with the solar activity suggest that increases in neutron activity may represent an increase in primary cosmic particles in the low (> 10 Bev) energy range or even some neutrons from the sun.

More experimental evidence in both these fields is required.

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