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Volume 82, 1954-55
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Section A—Chairman's Address
The Present State of Cosmic Ray Research

Since the beginning of the century the physicist has been interested in cosmic rays—those ionizing particles and photons that Hess in 1912 showed increased intensity with altitude—what are they and where do they come from? There is as yet no final answer to these two questions; in trying to answer them the cosmic ray researcher has contributed to cosmology and posed difficult questions to the cosmologist and astronomer; in the examination of the nucleonic debris produced in the earth's atmosphere by the extremely energetic primaries discoveries have been made of the positron and several kinds of mesons; such phenomena as pair production and bremsstrahlung have been first observed; the first studies of nuclear reactions in the Bev range have been made However, the post-war construction and plans for accelerating machines to give controllable beams of heavy particles of several Bev energy offer more systematic and profitable methods of studying high energy nuclear reactions. Perhaps the cosmic ray researcher may make his best contribution in the next few years by answering his original questions on the character of cosmic rays and nuclear studies in the very high energy range of above 10 Bev.

Perhaps we might in a short summary of this type consider the present state of cosmic ray research under the headings:—

a

Nature of the primary rays.

b

Origin of the primary rays and how they get their energy.

c

Interaction of primary rays and their secondaries with the nuclei of the earth's atmosphere.

2.Constitution Of Primary Rays

Historically the work of Clay in 1927–29 on geomagnetic latitude variations showed that cosmic rays were charged particles; the work of Stormer and Vallarta on the observed assymetry of particles arriving from the west and from the east in relation to the earth's magnetic field defined the primary particles as being positively charged Vallarta's calculations (1948) also determined the minimum momenta of incoming particles in terms of geomagnetic latitude and zenith angle These results are best illustrated by Figures I. II and III.

2.1.Energy and Mass Spectrum

During the past few years, using chiefly the photographic plate technique, and to a lesser extent scintillation counters, interesting information has been obtained by balloons close to the top of the atmosphere on the mass energy distribution of the primary particles. This fruitful line of work, which is being ipursued vigorously at Bristol—Dainton et al (1952) and in Rochester-Kaplon et al (1952) is as yet statistically incomplete, but the general form of the results is shown in Figure IV.

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Fig. 1—The minimum energy of charged particles able to reach the earth.

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Fig. 2—Proton energies allowed by the earth's magnetic held in the direction 15° east, for various magnetic latitudes.
Fig. 3—Same for 45° west. From Vallarta (1939). Shaded portion represents penumbra.

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Fig. 4—Energy spectra. The limiting energies for vertical incidence corresponding to some geomagnetic latitudes are indicated by broken lines. The parts of the curves referring to energies below the limiting energy for 55° were extrapolated.

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2.2.Cosmic Ray Abundancies

It is interesting to compare the mass distribution of the particles arriving at the top of the atmosphere with cosmic abundancies:—

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Distribution of Primary Particles at the Top of the Atmosphere Relative to Cosmic Abundance.
Z Element Cosmic Abundance Relative to 10,000 atoms of H. Primary Cosmic Ray Abundance Relative to 10,000 protons
1 H 10,000 10,000
2 He 1,000 1,000
3 Li 160
4 Be
5 B 33
6 C 4.6
7 N 2.3 13
8 O 6.3 39
9 F 0.003
≥10 2.6 14

The cosmic abundance figures in the universe were obtained from Brown (1949)—the result of studies of spectra of stars and planetary nebulae, the absorption of light in interstellar space and on the chemical analysis of meteorites. The cosmic ray figures are a composite group from recent papers—viz, Kaplon, Peters et al (1952), Dainton et al (1952), Bradt and Peters (1950), Yagoda (1952), Hoang (1950), Freier et al., (1951) Biermann (1953). There is some disagreement over the important figures of cosmic ray abundances for elements 3, 4 and 5 and more information at an altitude free from suspicion of production as secondaries of heavier particles is required.

The general similarity of cosmic and cosmic ray relative abundances suggests that the problem of cosmic ray origin may be connected with more general cosmological problems, including the formation of the chemical elements in an element factory that was booming some 3–5 billion years ago.

2.3.Variation With Sidereal Time

If cosmic radiation were isotropic in the universe one could expect a small sidereal variation due to the rotation of our galaxy. Vallarta (1939) has estimated this to be 0.2% with a maximum at about 13½ hours sidereal time. The effect, is hard to distinguish from solar and other effects; Elliot and Dolbear (1950 and 1951) have attempted it by considering observations on both hemispheres of the earth.

2.4.Sudden Increases

Since reliable readings of cosmic ray intensity commenced in 1936 there have been four major increases in activity:

  • 1942, January 31.

  • 1942, February 7.

  • 1946, July 25.

  • 1949, November 19.

and in each case these bursts were proceeded by a radio fade out caused by a violent chromospheric eruption near an active sunspot. In 1946 and 1949 the newly developed techniques of measuring radio noise showed a tenfold increase from the sun.

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The 1949 increase of neutron intensity as reported by Adams (1950) strongly suggested a smaller than average energy per particle than possessed by normal cosmic radiation.

3.Origin op Cosmic Rays

3.1.Experimental Data

We might usefully reiterate the experimental facts we know about the primary cosmic rays before attempting interpretation.

i

The particles arising are positively charged with an atomic number distribution comparable with that of cosmic abundances. There is some evidence for the presence of B, Be and Li which do not seem to occur in stellar and instellar matter. There is no evidence for the presence of fast electrons.

ii

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Energies range as high as 1016 ev and have a distribution given by N (> U) = k (1 + U)−n and this has been evaluated by Winckler et al. (1950) to give numerical values of (N > U)=0 4(1 + U)107

iii

While the average intensity of the order of 1010 ev is very high, the flux of the order of 104 particles per square meter per second is low, resulting in an average power of about 10-5 watt per metre2—about the same order as starlight and 10-8 of that of solar radiation. The energy density of about 10-13 joules m3 is, approximately that of thermal radiation in interstellar space.

iv

Present evidence suggests spatial isotrophy to within 0.2%.

v

The direct contribution of the sun to cosmic rays is less than 1%, and on the neutron evidence of Adams and Braddick (1950) a sudden solar burst contributes particles mainly in the low end of the momentum spectrum.

3.2.Cosmological Aspects of the Problem

It is on this basic data coupled with other astronomical and cosmological knowledge that theories have been propounded as to the origin of cosmic rays and how they get their energy.

A discussion of current theories of cosmology is beyond the scope of this paper, but it might be useful if we reviewed those aspects applicable to cosmic ray production.

There are several theories put forward to explain the origin and relative abundance of the elements, but none so far presented are without serious difficulties. On a recent survey Alpher and Herman (1953) divide the theories into two main classes—equilibrium theories and non-equilibrium theories At present the theory of element formation principally by neutron capture reactions in an expanding universe seems to meet with the fewest difficulties. According to McCrea (1951) we have the spontaneous creation of neutrons and protons at the rate of 10-41 kg m-3 sec-1 to maintain a constant density of 10-24 kg m-3 at the present expansion rate. McCrea explains the matter created as the mass equivalent of the work done by stress in the energy momentum tensor in

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the universal expansion. Dirac (1937) prefers to think in terms of a time variation of the constant of gravitation.

Many of the cosmic ray particles are assumed (Fermi — 1949) to be injected into an accelerating system from stellar explosions where in some cases energy equivalent to the mass of the sun is radiated in a few weeks—1042 particles with energies up to 1 Bev. The number of novae occurring in our galaxy is about one per 100 years; the intermittent nature of the production process however presents no difficulties as the life-time of the primary cosmic ray protons has been estimated to be of the order of 107 years. Sunspots, stellar spots and other disturbances undoubtedly make some contribution—in fact a large number of sources contributing in a random manner to the total flux fits in well with the experimental data.

3.3. The Accelerating Process

In older to accelerate the particles to the observed energies it is necessary to assume that interstellar magnetic fields are present—this can be justified on a theoretical treatment of ionized fluid matter in motion by Biermann (1953) who calculated fields of the order of 10-10 weber m-2 from the turbulence resulting from the rotation of our galaxy We have other evidence of magnetic fields in interstellar space from astronomical evidence—e.g., that of Jentsch and Unsold (1948) obtained by studies of the polarization of light from stars and the “reddening” or stars due to scattering by interstellar dust These effects indicate that there is some orientation, by magnetic fields, of the dust particles which must be assumed to be magnetic—the latter supported in itself by recent observations of showers of small magnetic needles in the upper atmosphere in association with meteors.

It is also proposed that the magnetic lines of force will be bent in a way more or less analogous to the streamlines of the turbulent interstellar space (Unsold, 1951). The cosmic ray particles follow very closely the magnetic lines of force and leave the galaxy only when the lines of force pass into inter-galactic space—and this loss can be shown to be small.

A magnetic field alone will not produce particle acceleration—an electric field is also necessary. The high conductivity of interstellar space precludes a lightning type of electric field and the electric fields are therefore assumed to arise by induction from local changes in the magnetic fields strength—see Riddiford and Butler (1952). Certainly the changes in magnetic intensity observed in sunspots would give electric fields sufficient to accelerate charged particles to cosmic ray energies. Fermi (1949) suggests that the particles gain energy from the irregular swirling motion of the interstellar clouds and magnetic fields; a minimum “injection energy” of about 200 Mev is necessary for protons to gain energy.

Confirmation of the presence of Li, B and Be nuclei in the primary cosmic rays is important; these elements are not present in ordinary cosmic abundances and any theory of production in cosmic rays by collision evaporation of heavier elements meets with the objection of an already too high a ratio of heavy nuclei to protons in cosmic rays.

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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+° 1947
π- 276 2 × 10-8 μ-° 1947
π° 262 10-14 αhν 1950
Neutron ν2 ν°2 800 10-9 π+- 1947
τ Meson τ+ 969 10-9 π++- 1949
τ- 969 10-9 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-° 1932
Neutral ν1 ν1° 2200 10-9 p+- 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.

5. Conclusion

This has been of necessity a brief summary of the cosmic ray research field, and many topics have of necessity been neglected. The field as a whole is a fertile one for research, and in view of the comparatively low cost of equipment and her favourable geographic position for certain investigations one in which New Zealand can make a contribution.

List of References

The list given below is far from complete, and is intended to direct attention to the more recent papers referred to in the text.

Adams, N. Phil Mag. 41, 503 (1950).

Alpher, E. A. and Herman, C. H. Ann. Rev of Nuclear Scienre 2. 1, (1953)

Biermann, L. Ann. Rev of Nuclear Science 2. 339 (1953).

Bradt, H. and Peters, B. Phys. Rev. 80, 943 (1950).

Brown, H. Revs. Modern Physics 21, 625 (1949).

Camebini, V, Fowler, P. H., Lock, W.O, and Muirhead, H. Phil. Mag. 41. 701 (1950)

Dainton, A. D. and Kent, T. W. Phil. Mag. 43, 729 (1952).

Dirac, P. A.M. Nature 139, 323 (1937).

Elliot, H. and Dolbear D. W. N Proc. Phys. Soc. A63, 137 (1950).

—— J. Atmos. Terr. Physics 1, 205 (1951).

Fermi, E. Phys. Rev. 75. 1169 (1949).

Freier, P. S. Anderson, G. W. Naugle, J. E and Net, E. P. Phys. Rev. 84, 322 (1951).

Hoang, T. F. Compt. Rend. 230, 292 (1950).

Jentsch, C. and Unsold, A. Z. Phys. 125, 370 (1948).

Kaplon, M. F., Peters, B. and Bradt, H. L. Phys. Rev 76, 1735 (1949).

Kaplon, M. F. Peters, B. Reynold, H. and Ritson, D. Phys. Rev. 85, 295 (1952)

Le Prince-Ringuet, L. Phys. Rev. 76, 1273 (1949).

Le Prince-Ringuet, L. and Rossi, B. Phys Rev 92, 722 (1953).

McCrea, W. H. Proc. Roy. Soc. A206, 562 (1951).

Powell, C. P. Proc. Roy. Soc. 1278, 221 (1954).

Riddiford, L. and Butler, T. Phil. Mag. 43, 447 (1952).

Rochester, G. D. and Rosser, W. G. V. Reports on Progress in Physics 14, 228 (1951).

Simpson, J. A., Fonger, W. and Wilcox, L. Phys. Rev. 92, 722 (1953).

Unsold, A. Phys Rev. 82, 857 (1951).

Vallarta, M. S. Phys. Rev. 55, 1 (1939).

—— Phys. Rev. 74, 1837 (1948).

Winckler, J. J. Stroud, W. G. and Shanley, T. B. Phys Rev 79, 650 (1950).

Yogoda, H. Phys. Rev. 85, 720A (1952).