A shower containing ∼360 charged and neutral shower particles produced by a magnesium nucleus of 2×10¹⁴e.v. provides a source of mesons whose interactions have been studied over an aggregate track length of about 3 meters in emulsion. It is first shown that in very large showers not only the incident energy can be determined but also the energy of the shower particles. The angular distribution of the shower particles agrees closely and their multiplicity is consistent with Fermi’s theory.

It is shown that the interaction cross-section of charged shower particles is geometric and that at an energy ∈ ∼ 1000µc² they produce on the average 5 to 6 mesons. The mesons are preferentially emitted in the backward direction in the reference system in which the momentum of the incident meson balances that of a nucleon in the target. The excitation produced in the target nucleus is much lower than in nucleon-produced showers of comparable multiplicity.

The charged mesons arenot emitted in pairs by a heavier meson of short lifetime.

Charged shower particles as well as electrons produce electron-positron pairs with a cross-section of the order of that predicted by theory.

The number of neutral mesons decaying intoγ-rays in a time ≤10⁻¹⁴ sec. is nearly equal to the number of charged shower particles.

There exists evidence for the emission of neutral particles, different from neutrons, capable of producing meson showers.

The energy balance of the event suggests that a few shower particles carry an appreciable fraction of the total energy.

A method is described for constructing out of individual sheets of nuclear emulsion a detector of large volume and stopping power which can be used for observing entire chains of successive nuclear interaction and decay processes initiated by high energy particles.

Such a detector which had been exposed for several hours in the stratosphere has proved particularly valuable for detecting K-mesons and Hypersons* and for studying their modes of production and decay.

In an emulsion block detector¹ exposed in the stratosphere three cases of the production ofτ-mesons in nuclear disintegrations have been observed and 5 cases of negativeK-mesons which when brought to rest are captured by nuclei. In one case the production of aτ-meson is associated with the production of a charged hyperon in the same nuclear event. In another case the production of a negativeK-meson is associated with the production of a slow positiveK-meson. The production ratio of slowτ-mesons to slow positiveπ-mesonsτ/π⁺=1·1⁺0·8% and the ratio of slow negativeK-mesons to slow negativeπ-mesonsK⁻/π⁻=0·95±0·5%. The mass of theτ-meson is found to be$$M_\tau = 975 \cdot 9 \pm 2 \cdot 2 m_e $$ and that ofK⁻-mesons$$M_{K^ - } = 927 \pm 75 m_e $$ Proof is given that all three of the decay products of theτ-mesons areπ-mesons. It is suggested that allτ-mesons observed to decay in emulsions are positively charged and that theK⁻-mesons observed by us may be the negative counterpart of this particle. Possible relations between the various phenomenologically distinguished types of chargedK-mesons are discussed.

A new method is developed for determining the mass of particles coming to rest in nuclear emulsions. Multiple coulomb scattering is measured with cells whose lengths vary along the trajectory in such a manner as to compensate for the momentum loss of the particle and keep the mean deviation between adjacent cells constant over the entire track. It is shown that this procedure is more convenient and inherently more accurate than that based on scattering measurements with constant cellsize. The method has been applied toK-mesons which at the end of their range either decay into a single charged relativistic particle (K⁺-mesons) or into 3π-mesons (τ-mesons) or give rise to capture stars (K⁻-mesons). The results are within experimental error consistent with the assumption that the mass of these three classes of particles are identical and equal to the well established mass of theτ-mesons. The average mass of a group of 9 longK-mesons determined with our scattering procedure is M_K=974±42m_e.

The method of determining the primary flux of nuclei with atomic number Z⩾3 developed by Bradt and Peters, has been extended to include all nuclei of charge Z⩾2. It is shown that the method permits the identification of primary helium nuclei with an efficiencyν⩾90%. The primary α-particle flux obtained in this way is in very good agreement with that obtained by other methods and the relative flux values for nuclei of atomic number 3⩽Z⩽5 and 6⩽Z⩽9 agree with previous determinations.

It is estimated that about 1,000 nuclei of radioactive Be¹⁰ (2·7 million years half-life) are produced per square meter per second by cosmic ray induced nuclear disintegrations in the atmosphere. The conditions for observing the resulting activity in rain water and in various regions on the earth are favourable and may be useful for measuring sedimentation rates and other geological surface changes during the Tertiary.

Small local dislocations in processed emulsions give rise to spurious scattering whose effect on tracks is indistinguishable from the multiple scattering produced by coulomb interaction. Because of this effect the atomic number of fast primary cosmic ray nuclei will be underestimated in more than 50% of the cases in experiments where multiple scattering of tracks and ionization measurements are employed for charge determination. The spurious scattering was investigated by measuring the track contours of 100 very long tracks due to energetic primary particles; the effect was present in comparable strength in all plates and all types of emulsions which have been investigated. It is too small to affect measurements on medium energy particle tracks (protons with energy below 600 MeV and heavy nuclei with energy below 300 MeV/ nucleon). It dominates, however, other sources or error and noise for tracks of energetic particles although it does not preclude occasional observations of very low scattering values. New methods for measuring various forms of noise have also been developed in the course of this work and the noise level for scattering measurements has been reduced below previously accepted values.

Spurious scattering is presumably largely responsible for the discrepancies which appear when one compares the primary charge and energy spectra derived from experiments involving scattering measurements with the corresponding spectra derived from numerous other experiments which employ different techniques.

If the experiments based on scattering measurements are omitted, the remaining evidence strongly favours a spectrum in which the energy per nucleon is nearly independent of atomic number for all primaries. It also favours a charge distribution which has a pronounced minimum for charges 3≤ Z≤ 5 and, therefore, yields a fairly low upper limit for the amount of interstellar matter traversed by primary cosmic ray nuclei.