The investigation of the low-energy primary cosmic radiation has been extended to include the heavier elements of $Z\ensuremath{\ge}9$: Sec. 1; the light elements of Li, Be, and B: Sec. 2; and $\ensuremath{\alpha}$ particles: Sec. 3. The results of a previous paper on carbon, nitrogen, and oxygen have also been confirmed with better statistics. The energy spectra of all these components show a general similarity in shape. A possible deviation of the light-element spectrum from this similarity is discussed. The abundances of various elements in the low-energy region of 200 to 700 Mev per nucleon are essentially the same as observed in the higher energy region.

Eighty-four interactions of protons and neutrons were located in a 22 liter stack of nuclear emulsion by tracing back showers of minimum-ionizing particles to their origins. The distribution of the number of shower particles, and the number of heavily ionizing prongs are presented for 57 events with dip angles 17\ifmmode^\circ\else\textdegree\fi{}. The average energy of these events is 3.5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev. The average number of shower particles emitted in nucleon-nucleon collisions at this energy is 15\ifmmode\pm\else\textpm\fi{}5, as estimated from 8 interactions without heavy prongs. The angular distributions of the shower particles are presented for the 57 events. They can be transformed into a system in which the angular distribution is roughly symmetric. This is true even for the collisions with heavy target nuclei (${N}_{h}5$). The degree of anisotropy of the angular distributions is in disagreement with a hydrodynamical model of nucleon-nucleus collisions. A lower limit for the collision mean free path of the primary particles of 20 cm in emulsion was obtained. By scanning the forward cone of the primary interactions, 76 secondary interactions of charged and neutral shower particles were found. The distribution of the prong numbers, of the energy, and the characteristics of their angular distribution are presented. The best estimate of the ratio of secondary collisions produced by neutral particles, and the number produced by charged particles is: $\frac{{N}_{n}}{{N}_{\mathrm{ch}}}=0.40\ifmmode\pm\else\textpm\fi{}0.11$. Adding this result to other published data, it is concluded that 30\ifmmode\pm\else\textpm\fi{}6% of the particles produced in collisions having a primary energy of several Tev are not $\ensuremath{\pi}$ mesons. A collision mean free path of 41\ifmmode\pm\else\textpm\fi{}8 cm was found for the forwardcone shower particles.

Nuclear interactions of protons, neutrons, $\ensuremath{\alpha}$ particles, and heavier nuclei of average energy 250 Bev/nuc were studied in nuclear emulsion. The source of these particles were fragmentations of heavy primary nuclei of the cosmic radiation. Their energy was determined from multiple scattering measurements. The interaction mean free path for protons is 41\ifmmode\pm\else\textpm\fi{}10 cm, for $\ensuremath{\alpha}$ particles 27\ifmmode\pm\else\textpm\fi{}7 cm. The mean free path shows no significant change compared with measurements at lower energies. The mean number of shower particles $〈{n}_{s}〉$ depends appreciably on the mass of the target nucleus. Our best estimate for nucleon-nucleon collisions at 250 Bev is $〈{n}_{s}〉=8.8\ifmmode\pm\else\textpm\fi{}1.9$. A detailed comparison of the estimate of the primary energy obtained from the angular distribution of shower particles with the true primary energy is carried out. The angular distribution of the shower particles will, in an individual case, give a quite unreliable value for the primary energy. In the average, the angular distribution method will over-estimate the true primary energy by a factor of 1.3 for interactions with a number of heavy prongs ${N}_{h}\ensuremath{\le}5$. If ${N}_{h}>5$, the angular distribution will underestimate the true energy in the average by a factor of 1.8. The angular distributions can be transformed into a system in which they are symmetric. This is even true for collisions with heavy target nuclei (${N}_{h}>5$). The results for alpha particle and heavy nucleus collisions are quite similar.The inelasticity for the proton and neutron interactions shows large fluctuations for individual events. It depends weakly on the number of shower particles and on the mass of the target nucleus. Its mean value is 50%. The mean value for the alpha-particle collisions is 22%.

A nuclear interaction initiated by a proton of type 6 + l6/sub p/ was found in a nuclear emulsion flown above 110,000 fect. The angular distribution of the shower particles indicates that the primary energy was between 10 and l0/ sup 15/ ev. An unusually cnergetic electron-photon cascade was initiated which could be followed by 22.6 cm. This cascade appears to have been started by a single highenergy The effects of decaying into rays. Assuming a primary energyof 5 x 10/sup 14/ ev ( /sub c/ = 500), an energy of at least 15 Bev, in the center of mass systems would be obtained for the high-energy The effects of / sup 0/ meson. (A.C.)

A nuclear interaction initiated by a proton of type 6 + l6/sub p/ was found in a nuclear emulsion flown above 110,000 fect. The angular distribution of the shower particles indicates that the primary energy was between 10 and l0/ sup 15/ ev. An unusually cnergetic electron-photon cascade was initiated which could be followed by 22.6 cm. This cascade appears to have been started by a single highenergy The effects of decaying into rays. Assuming a primary energyof 5 x 10/sup 14/ ev ( /sub c/ = 500), an energy of at least 15 Bev, in the center of mass systems would be obtained for the high-energy The effects of / sup 0/ meson. (A.C.)

A nuclear interaction of type $0+20p$ observed in nuclear emulsion was analyzed by measuring the energies and angles of the secondary particles. The primary energy, as determined from the angular distribution of the tracks, is 2.7\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev. This value is in agreement with an independent estimate obtained from the total energy dissipated. A secondary collision of type $0+20n$ was also analyzed in the same way. Its energy is 1.4\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev, which is comparable to the primary energy. The inelasticity of the primary event is ${0.54}_{\ensuremath{-}0.19}^{+0.16}$.The energy and angular distributions of the shower particles in the center-of-mass system (c.m. system) are given for both events. The shower particles show a correlation in the sense that those with the highest energies are emitted in the c.m. system under small angles with the shower axis.The energy distribution of the mesons in the c.m. system is peaked toward low energies and shows a remarkably long tail at high energies extending up to 10 Bev. One of these particles is a ${\ensuremath{\pi}}^{0}$ meson, which carries off about 23% of the total energy.The average value of the transverse momentum of the shower particles is 0.3\ifmmode\pm\else\textpm\fi{}0.05 Bev/c.

In order to obtain a production spectrum of mesons in the center-of-mass system, three high-energy nuclear collisions were selected which satisfy very stringent criteria for nucleon-nucleon collisions and for which the momenta of all charged particles could be measured in the laboratory system. The results show that more than one-half of the mesons have energies less than 1 Bev in the center-of-mass system. Only a few particles emitted under very small angles in the forward and backward direction have higher energies, extending up to 10 Bev. The low-energy end of the spectrum is compared with results obtained in experiments with the Berkeley Bevatron.

From a large stack of G-5 emulsions flown at Texas, 41° N geomagnetic latitude, the charge spectrum of the cosmic radiation at 104 000 feet was obtained. Special emphasis is given to the observation of the detailed shape of the spectrum in the regionZ > 9. The gap-counting is extensively used even at these highZ values after very careful calibration with break-up events and δ-ray countings. The charge spectrum thus obtained was extrapolated to the top of the atmosphere by making use of the fragmentation probabilities in air which were obtained from the analysis of a total of 209 interactions in the same stack. This extrapolation gives for the ratio of the light nuclei, L, (Z = 3, 4 and 5) and the heavy nuclei, H, (Z ⩾ 9), to the medium ones, M, (Z = 6, 7 and 8) the values 0.32 ± 0.07 and 0.48 ± 0.10, respectively, at the top of the atmosphere. The fragmentation probabilities in hydrogen, the main constituent of interstellar matter, were also obtained from the careful study of the interactions in the stack and allowed to make a further extrapolation of the charge spectrum from the top of the atmosphere to the one at the source region of the cosmic radiation. The ratio of the heavy nuclei to the medium ones was found to be 0.66 ± 1.6 at the source. This charge spectrum at the source region is compared with the average chemical abundances of the elements in the universe as well as with those in certain types of stars. The results seem to indicate a close similarity of the chemical abundance curve of the cosmic radiation with that of certain types of young stars.

A microscope, with coordinate motions of 21×15 cm which may be used in the analysis of large nuclear plates is described with particular attention given to constructional details. The design includes a magnetic clutch for fixing plate position, a rotating super-stage and dial indicators to observe both horizontal and vertical movement. A stage noise D̄noise≦0.029 μ for cell lengths up to 1 mm was obtained.

The production of K-mesons by 3 GeV π−-mesons has been studied in an emulsion stack by area scanning for stopped K-mesons and tracing to the originating interactions. Two examples of K-meson production are described which can be analyzed as due to pion-nucleon collisions. In one event a K-meson is produced as the only charged particle. In the other event, a K+-meson and a minimum ionization track are the only secondaries. This event can best be interpreted according to the scheme π− + P → K+ + Σ0 + π− assuming the collision to be with a free proton. Both of the events can be interpreted consistently with the Gell-Mann-Pais scheme.

A summer Institute for Cooperative Emulsion Research was jointly sponsored by the National Science Foundation and the University of Chicago during the months of July and August, 1956. The work of the Institute was made possible through the great encouragement which we have received from the National Science Foundation and the authorities of our own University. It was of particular importance in this connection that the Nuclear Physics Branch of the Office of Naval Research, Washington, D.C., gave full support to the program of the Summer Institute and that we received permission to use microscopes and other equipment owned by the US Navy.

Stacks of 600 μm pellicles were exposed to the 4.6 and 3.0 GeV pion beam of the Berkeley Bevatron. A number of heavy unstable particles and hyperfragments were obtained. The ratio of the frequency of occurrence of K+ and K--mesons has been found to be approximately unity in contrast to the large positive excess in the proton beam. As a possible explanation, it is suggested that the K--meson is produced in conjunction with a K+-meson according to the following reaction: π-+p → K++K-+N. Two hyperfragment decays of hydrogen have been fo nd in which the π--meson stops. One of the hyperfragments was identified as a 1 4 H* produced by a ⌆--hyperon; the other one is a 1 3 H* produced directly by the high energy pion. The range of all the charged particles was accurately determined which yielded a reliable value of the binding energy. The value of the binding energy (B.E.) for 1 3 H* was found to be 1.2 ±0.6 and for 1 4 H* B.E.(∧0) = 3.3 ±1. The mean free path for nuclear collisions was determined for 5.7 GeV protons and 3 GeV π-mesons. The results yielded for protons 37.6 ±5.3 cm and for π--mesons 35.5 ±5.0 cm. Both mean free paths are longer than the geometrical mean free path for nuclear collisions in the emulsion, which is 25 cm.