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A C.T.R. Wilson expansion-chamber was used to study the space-distribution of photoelectrons ejected from a gas by monochromatic x-rays. In agreement with Auger, and Watson and Van den Akker a more isotropic space-distribution was found for electrons ejected from the $L$ energy-levels than for those ejected from the $K$ energy-level. The distribution of the electrons from the $L$ energy-levels became less isotropic with an increase in frequency of the incident radiation. For a given radiation, the average forward momentum of the electrons from the $K$ energy-level was found to decrease with an increase in the binding energy of the parent atom. Within experimental error, however, for electrons from the $K$ energy level, even for different binding energies, the average forward momentum remained the same for a given velocity of ejection of the electron. The average forward momentum of electrons from the $L$ energy-level was greater than that for electrons from the $K$ energy-level for a given velocity of ejection. The space-distribution of electrons from the $K$ energy-level was in fair accord with the recent results of quantum mechanics.

Cloud chamber photographs at 4300 meters elevation show positive and negative electron tracks similar to those observed at sea-level, but positive-negative electron showers occur more frequently and, in general, consist of more numerous tracks. Showers of 2-4 tracks, 5-10 tracks, and 11-100 tracks occur respectively, 8.6, 21 and 29 times as frequently per unit time at 4300 meters as they do near sea-level. Further measurements on the energy loss in lead of electrons up to 400 MEV, are given. They show that in this range of energies the energy loss in lead is roughly proportional to the incident energy. About one percent of the exposures on Pike's Peak reveal the presence of strongly ionizing particles which in most cases seem to be protons. The proportion of such tracks is considerably greater than at Pasadena. These heavy tracks in general bear only little relation in direction to that of the incoming beam, and usually arise from a type of nuclear disintegration not heretofore observed. The energies of these heavily ionizing particles may rise to values so high as 150 MEV, thus indicating that the source of the particle energies is in the cosmic rays.

Energy values determined from curvature measurements of 75 cloud-chamber tracks of decay particles of cosmic-ray mesotrons at sea level, in a magnetic field of 7250 gauss, are here reported. The observed spectrum extends from 9 Mev to 55 Mev with an apparently continuous distribution of intermediate energy values and a mean energy of 34 Mev. The shape of the spectrum and the value of its upper limit are strong evidence that the mesotron disintegrates into an electron and two neutrinos. It is concluded that the mesotron has half-integral spin. The value of the observed upper limit of the energy spectrum corresponds to a mass value of the mesotron equal to 217±4 electron masses.

In a set of 23,000 cloud-chamber photographs taken in a study of penetrating showers, 134 examples of the decay of neutral $V$-particles, and 18 of the decay of charged $V$-particles, were observed. An analysis of the 152 examples leads to the following principal conclusions: (1) $V$-particles result from the impact of mesons and probably also of nucleons, upon nuclei. (2) $V$-particles are generally produced singly and not in pairs. (3) Two independent kinds of data, one based on measurements of angles and other purely spatial relationships, and the other based on measurements of momentum and specific ionization, lead to the conclusion that more than 80 percent of these neutral $V$-particles decayed by the production of a heavy positive and a light negative particle. The mass of the heavy positive particle in most instances was consistent with that of a proton, but in a few cases may have been somewhat less. The negative particle appeared most often to be a $\ensuremath{\pi}$-meson, although in a few cases a $\ensuremath{\mu}$-meson was indicated. In about 7 percent of the cases, the positive particle was light and had a mass consistent with that of a $\ensuremath{\pi}$- or $\ensuremath{\mu}$-meson. In these cases the mass of the negative particle was not well determined. So far, therefore, in these investigations, there is no direct evidence that a neutral $V$-particle decays into two $\ensuremath{\pi}$-mesons, or into a positive $\ensuremath{\pi}$-meson and a negatively charged proton, although a few cases may be so interpreted. (4) The data are consistent with the assumption of a two-body decay for a majority of the neutral $V$-particles, and therefore a rather extensive analysis is given based upon this assumption. (5) The energy release, or $Q$-value, of the decay of neutral $V$-particles was computed, on the assumption of a two-body decay, for those cases in which the production of a heavy positive particle was clearly indicated, and where momentum measurements were possible. On the assumption of a decay into a proton and a ${\ensuremath{\pi}}^{\ensuremath{-}}$ meson, the $Q$-values obtained ranged from 10 Mev to about 100 Mev. The great majority of cases however, may be described in terms of discrete $Q$-values at 35\ifmmode\pm\else\textpm\fi{}3 Mev, and 75\ifmmode\pm\else\textpm\fi{}5 Mev, although, of course, the data are consistent with distributions about these values. For the same set of cases, if the positive particle is assumed to be as light even as $1250{m}_{e}$, and/or the negative particle is assumed to be a $\ensuremath{\mu}$-meson, the distribution of $Q$-values is not greatly changed, and the apparent necessity for at least two different $Q$-values remains. In those few cases mentioned in (3) above, in which the positive particle was light, and in which the mass of the negative particle was undetermined, the energy release, computed on the basis of a two-body decay, depends upon the assumed identity of the decay products, and is about 100-130 Mev for the assumption that two $\ensuremath{\pi}$-mesons are produced, about 50-80 Mev for a positive $\ensuremath{\pi}$-meson and a negative $\ensuremath{\tau}$-meson, and about 30-80 Mev for a positive $\ensuremath{\pi}$-meson and a negative proton. (6) The rest mean lifetime of ${V}^{0}$ particles, which appear to yield protons and ${\ensuremath{\pi}}^{\ensuremath{-}}$-mesons as decay products, was found to be 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec for 26 cases having $Q$-values greater than 50 Mev, and 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec for 48 cases having $Q$-values less than 50 Mev, on the assumption of a two-body decay. In view of the rather poor statistics, the above two lifetimes are consistent with a single value, and lead to an average of 2.5\ifmmode\pm\else\textpm\fi{}0.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. The above lifetimes are not appreciably changed if a multibody decay of the neutral $V$-particle is assumed. (7) The data are not adequate to distinguish conclusively between a two-body and a multi-body decay, and, therefore, a discussion is also given in terms of a variety of possible decay schemes of the neutral $V$-particle.

ON NOVEMBER 20, 1931, in a lecture before a large audience gathered at the Institut Poincaré in Paris, there were presented the first direct measurements taken by Carl D. Anderson of the energies of cosmic-ray tracks made with an apparatus capable of measuring, by the method of magnetic deflectibility in air, energies of the order of magnitude to be expected in cosmic-ray photon-encounters with electrons and nuclei, namely, from 27×10^(6) volts up to at least 500×10^(6) volts. These same photographs were also shown on November 23rd at a physical seminar at the Cavendish Laboratories. Cambridge, England. The eleven cosmic-ray-track photographs shown and discussed on these occasions1 brought to light a certain number of new and important facts presented essentially as listed below in both of these lectures, and these facts have now been checked by three times as many successful exposures.

Eighteen anomalous θ^0, (θ^0_(anom)), decay events observed in the California Institute of Technology magnet cloud chambers have been analyzed. Many of these decays are dynamically inconsistent with the τ^0→π^++π^−+π^0 scheme, but most are consistent with the decay processes: θ^0_(anom)→π^++π^−+γ, π^±+μ^∓+ν, and π^±+e^∓+ν. However, at least one event is inconsistent with each decay scheme. From the locations of the decays in the cloud chamber, the lifetime is found to be significantly longer than that of the normal θ^0 particle, called here the θ^0_(π2) particle. Other differences in the behavior of the θ0anom and θ^0_(π2) particles were also observed in the (a) momentum distributions, (b) origin locations, (c) relative numbers of θ^0_(anom) and θ^0_(π2) particles traveling upward, and (d) the types of V particles produced in association with the θ^0_(anom) and θ^0_(π2). It is concluded that not all the θ^0_(anom) decays can result from alternate decay modes of the θ^0_(π2). Moreover, many decays can be neither τ^0 decays nor alternate decays of the θ^0_(π2). The characteristics of the θ^0_2 particle proposed by Gell-Mann and Pais are consistent with those of the θ^0_(anom) particle, with the possible exception of the observed types of associations. An estimate was made of the relative number of θ^0_(anom) to θ^0_(π2) particles observed to decay in the cloud chamber. If all θ^0_(anom) decays are assumed to arise from decays of the θ^0_2 particle, then a lower limit for the θ^0_2 lifetime is found to be about 10^(−9) sec.

The first section of this report contains a brief summary of those experiments which have made substantial contributions to our knowledge of the nature and mode of absorption of cosmic-ray particles. Included is a discussion of the difficulties which inhere in the attempts to interpret the observed cosmic-ray phenomena in terms of the assumption that the cosmic-ray particles are practically all protons and positive and negative electrons. These difficulties are resolved by demonstrating that in the same momentum range there exist two types of particles, one of which is highly absorbed in a heavy material, mostly through radiation, while the other is relatively penetrating. Behavior of the first kind is typical of shower particles in general, and of all single negatron secondaries, which are presumably produced in elastic collisions with atomic electrons. This group is therefore to be identified with electrons. As it can be shown that particles of the penetrating group cannot be of protonic mass, it follows that they must be neither electrons nor protons. The simplest assumption is that their distinguishing property is a mass intermediate between the electron and proton. Confirmation of this view is found in the observation by other writers and by ourselves of particles whose range, ionization and curvature relations are such that they demand a mass in this region. The best range and ionization data seem to give mass determinations in the neighborhood of 200 electron masses. A discussion is given of certain difficulties which exist in the interpretation of cosmic-ray particles as electrons and mesotrons of a unique mass.

Measurements (1) of the energy loss of particles occurring in the cosmic-ray showers have shown that this loss is proportional to the incident energy and within the range of the measurements, up to about 400 Mev, is in approximate agreement with values calculated theoretically for electrons by Bethe and Heitler. These measurements were taken using a thin plate of lead (0.35 cm), and the observed individual losses were found to vary from an amount below experimental detection up to the whole initial energy of the particle, with a mean fractional loss of about 0.5.

I understand my task is to report on the part played by cosmic ray research in providing new information on the particles of physics, until such work was gradually driven into oblivion by the advent of the new accelerators. Because of my health, I have not had access to a library except under very trying and difficult circumstances, and hence this paper was prepared at my home, based wholly on a limited amount of material and my own recollections. This has inevitably tended to make for omissions in the paper, to make it less objective and more personal than it otherwise might or should have been. I regret the omission of the contributions provided by the extensive programs of research by Geiger-counters, ionization chambers, and the extremely important results from the photographic emulsion techniques originally devised by Cecil Powell, and later continued by many other investigators. In particular, I regret the omission of the innovative and important series of experiments carried out by Bruno Rossi beginning about 1930 based mainly on the adaptation of Geiger counter techniques to a study of the complex character of the cosmic radiation. His results as well as those of other investigators have had a strong influence on my thinking, and were very helpful in interpreting some of our own results.

A discussion is given of the data obtained in a recent series of flights of a B-29 airplane at altitudes up to 40,000 ft., in which a cloud chamber, actuated by Geiger counters, was operated in a magnetic field of 7500 gauss. The frequency of occurrence of single particles and electron showers at elevations of 800 ft., 14,100 ft., and 30,000 ft. is compared. Examples of heavily ionizing particles and of nuclear disintegrations are discussed. Direct measurements of the energies of cosmic-ray particles at 30,000 ft. are compared with similar measurements made at sea level. These measurements indicate that up to one-third of the particles which occur singly in the chamber, in the momentum range up to 107 gauss-cm, may consist of protons and the remainder of mesotrons, in contrast to sea level observations which show that protons are there present in only negligible numbers.

Out of a group of 1300 photographs of cosmic-ray tracks in a vertical Wilson chamber 15 tracks were of positive particles which could not have a mass as great as that of the proton. From an examination of the energy-loss and ionization produced it is concluded that the charge is less than twice, and is probably exactly equal to, that of the proton. If these particles carry unit positive charge the curvatures and ionizations produced require the mass to be less than twenty times the electron mass. These particles will be called positrons. Because they occur in groups associated with other tracks it is concluded that they must be secondary particles ejected from atomic nuclei.

A study of photographs taken in a very powerful magnetic field with the aid of a cloud chamber activated by counter responses shows (1) that such an arrangement has a strong selective action on showers; (2) that showers so selected have an exceptional multiplicity of tracks; (3) that the two counters show simultaneous responses when no single particle can pass through both of them; (4) that in general these showers consist of a mixture of positive and negative electrons rather than of electrons and protons, no effects certainly attributable to neutrons being observed; (5) that these electrons in getting out of the nucleus some times produce an intense photon spray of the nature of "brems strahlung"; (6) that these photon sprays are increasingly responsible for the simultaneous activation of the two counters the thicker the intervening lead; (7) that a light element like carbon has little effect in producing either showers or sprays; (8) that the total energy of a shower is not larger than that of single electrons (+ or -); (9) that the Dirac theory encounters certain difficulties in accounting for the observed effects.

Out of a group of 1300 photographs of cosmic-ray tracks in a vertical Wilson chamber 15 tracks were of positive particles which could not have a mass as great as that of the proton. From an examination of the energy-loss and ionization produced it is concluded that the charge is less than twice, and is probably exactly equal to, that of the proton. If these particles carry unit positive charge the curvatures and ionizations produced require the mass to be less than twenty times the electron mass. These particles will be called positrons. Because they occur in groups associated with other tracks it is concluded that they must be secondary particles ejected from atomic nuclei.

Publisher Summary This chapter describes the nature of cosmic-ray particles. Measurements of the energy loss of particles occurring in the cosmic ray showers have shown that this loss is proportional to the incident energy and is within the range of the measurements. It has been known for a long time that there exist particles of both penetrating and non-penetrating types. Crussard and Leprince–Ringuet have shown that either the absorption law changes with energy or that there is a difference in character among the particles. Independent evidence indicating the existence of particles of a new type has already been found, based on range, curvature, and ionization relations. Excellent experimental evidence showing the existence of particles less massive than protons, but more penetrating than electrons obeying the Bethe–Heitler theory, has just been reported by Street and Stevenson.

From six cloud-chamber photographs of unusual V0 decay events, the following conclusions are drawn: (1) there is a neutral V particle that decays into two particles lighter than κ mesons with a Q value too small to be consistent with a θ0(π, π, 214 Mev) particle; (2) some of these events cannot be explained in terms of the decay of a τ0(π0, π-, π+, Q∼80 Mev) particle; (3) these events can be explained by any one of a number of three-body decay schemes, but two different types of V particles must be postulated if two-body decays are assumed.

Cloud chamber photographs of cosmic-ray tracks in a magnetic field up to 17,000 gauss are shown. On the assumption that the particles producing the tracks are traveling downward through the chamber rather than upward, particles of positive charge appear as well as electrons. From the specific ionization along the track it is concluded that the positives are protons, and are not nuclei of charge greater than unity. No evidence is uncovered demanding the introduction of a neutron for cosmic-ray phenomena. Eight examples of associated tracks are shown. Energies range from below 10 6 electron-volts to values in a few cases of the order of 10 9 electron-volts. Energy values for 70 tracks are listed. The scattering of cosmic particles in traversing a 6.0 mm lead plate is measured.

A theory of cosmic-ray bursts is advanced with which the energy released in them is assumed to come not from the cosmic-rays themselves but from the battery which charges the electroscope. These bursts then represent, according to this theory, instrumental accidents which must be eliminated before the remaining fluctuations can be interpreted. The discovery that the remainder of the ionization observed in a cosmic-ray electroscope is due wholly to positrons and negatrons shooting through the chamber makes it possible to compute the fluctuations to be expected from a random distribution of these electron shots. The observed fluctuations, after the elimination of bursts, are found to be somewhat larger than the fluctuations thus computed. From the amount of this excess the percentage of cosmic-ray "showers" (two or more associated tracks) can be computed and is found by Evans and Neher to be in general agreement with the number directly observed in cloud-chamber experiments.

Direct measurements of the momenta of cosmic-ray particles at an altitude of 30,000 feet have recently been reported.

Thirty-four "forked tracks" similar to two previously reported by Rochester and Butler have been observed in 11,000 cloud-chamber photographs of cosmic-ray penetrating showers, confirming their conclusion that new unstable neutral and charged particles exist. The lifetime of the neutral particles is found to be about 3×10^-10 sec., and some information as to the nature of the decay products is given.

Curie and Joliot (1) have reported that under the bombardment by α-particles from Po, several of the light elements, viz., B, Mg, Al undergo transmutations which result in the production of radioactive isotopes which have half-lives of the order of several minutes and disintegrate by the ejection of positrons.

We have reported(1) approximate energy distributions of the positrons emitted by various substances activated by proton or deuton bombardment. A typical photograph is reproduced in Fig. 1. To find whether a relation exists between the maximum energy of the bombarding particles and the energies of the disintegration positrons we have studied numerous samples of carbon bombarded by protons at peak voltages of 900,000 and 700,000, supplied us by Dr. Lauritsen and Mr. Crane.

In cloud-chamber experiments the frequent occurrence of associated tracks has been observed.(1) It has been pointed out that a simple binary collision cannot explain all the associated tracks.(2)

Measurements of the energy release or Q value in Λ0 decay have previously been reported [1]. From the photographs obtained in the 48-in. magnet cloud chambers now operating in Pasadena, a set of the most accurately measurable Λ0 decays have been selected for calculations of Q values.

A report of the direct observation of positrons ejected from lead by hard gamma-rays has been published.(1) A source of radiothorium with its disintegration products was used. To obtain a well-defined beam the gamma-rays were collimated by allowing them to pass through a half-inch hole in a series of lead blocks totaling 18 inches in length. That the beam actually was well defined is shown by the fact that most of the observed tracks appeared to originate within a rather narrow region, about 3 cm wide. In this series of photographs the beam passed vertically downward through the side wall of a Wilson chamber across which were placed a 2 mm lead sheet, and below this a 0.5 mm sheet of aluminum. A magnetic field of 430 gauss normal to the face of the chamber and uniform to within 10 percent was used.

THE existence of particles intermediate in mass between protons and electrons has been shown in experiments on the cosmic radiation1. Since at present so little is known concerning the properties of these particles, for example, the exact value of the mass, the laws governing their production, their stability against disintegration, etc., it may yet be too early to assign to them a name. But inasmuch as several names have already been suggested, namely, dynatron, penetron, barytron, heavy electron, yukon and x-particle, it may be wise to consider the matter at this time.