tainly artificial, but we may pick up several to ten percent of all showers with a strong bias towards the heavy-primary-initiated showers. We plotted on the celestial sphere the arrival directions of those showers, whose n" to N ratios are between the maximum and eight-tenths of the maximum value. Table I and Fig. 2 show the distribution in right ascension of the arrival directions for the showers of various sizes and for different series of experiments. The apparent features of this distribution are a "bright" region between 3 and 15 hr right ascension and a "hole" in the region between 15 and 18 hr, along the band around 35°N declination. On the other hand, no indication of anisotropy can be seen if we plot all the showers without respect to the above criterion. This bright region, incidentally, corresponds to the general direction which is perpendicular to the Orion arm of the galaxy where the solar system is located. 7 The probability of a statistical fluctuation producing the observed anisotropy is estimated to be less than one percent.This apparent anisotropy leads us to the tentative conclusions that, first, at least several percent of cosmic rays in the energy range of the extensive air shower (10 16 -10 18 ev) are produced by heavy primaries, and second, that the maximum intensity is at right angles to the axis of the galactic arm or, probably, to the direction of more or less regular magnetic field of the galaxy. 8At a recent conference 1 several groups, including this one, reported indications of a possible anisotropy in the arrival directions of very energetic cosmic rays. 2 " 4 Our report was based mainly on data obtained at the MIT Volcano Ranch station in 1959-60, but also included data obtained by the Cornell group and by the MIT group at Agassiz. The apparent anisotropy, (*niax~^min)/ ( / max + / min^ was about (25±7)% for primary energies of the order of 5xl0 17 ev, and about (70± 30)% for energies of the order of 4xl0 18 ev. The minima were located in sidereal time between 12 hr and 18 hr, and between 18 hr and 24Although the conclusion is still of a temporary nature, it appears to encourage further discussions and investigations. We wish to emphasize the importance of investigations of this type at different latitudes in order to survey the part of the sky which is out of sight at our latitude. This kind of information would be crucial for the confirmation and the interpretation of the anisotropy. Summarized in a report by A. M, Conforto, Proceedings of the International Conference on Cosmic Rays and the Earth Storm, Kyoto, Japan, September, 1961 [Suppl* J. Phys. Soco Japan (to be published)], Part HI.We might recall the work by Y. Sekido, S. Masuda, S. Yoshida, and M. Wada [Phys. Rev. 83, 658 (1951)], whose studies of the possible anisotropy of M-meson flux very deep underground have also shown a similar feature. 8 S 0 Hayakawa (private communication).hr, respectively. Another series of measurements was carried out at Volcano Ranch in 1960-61. However, events recorded in the second run we...
The variation in the composition of the penetrating component of extensive air showers at sea-level has been investigated as a function of shower density. The proportion of N-particles in the penetrating component was observed to decrease as the shower density decreased. It is suggested that this behaviour is due to the filtering action of the atmosphere on the more absorbable particles in the nuclear cascade which has passed maximum development.
SummaryThe arrival times of the penetrating particles of extensive air showers relative to that of the electrons have been studied by means of short reaction time Geiger counters. A 50 channel hodoscope has been used to identify the penetrating particles and measure their residual ranges where these lay between 15 and 30 cm of lead. From observations on 782 showers of median density 70 particles m-' in which a Penetrating component was detected, it was deduced that between 3 and 9 particles in 104 shower particles from showers of median density 28 particles m-', have delays <5 X 10-8 sec and are able to penetrate 15 em of lead. It follows that 81-94 per cent. of the delayed particles found by Jelley and Whitehouse (1953) without the use of absorbers, must be stopped by 15 cm of lead. This indicates a height of production below 0·8 km for at least half of these particles.Two of the penetrating delayed !-I.-mesons were stopped in the lead, and their heights of production calculated to be l·O~g:rkm and 4'7~~:~km on the assumption that the delays were due to velocity differences. Thirty' other !-I.-mesons for which the individual time lags were not significant were also stopped in the lead and gave a mean delay indicating production below an altitude of 250 m. The remaining 208 !-I.-mesons which did not stop could not be assigned a height of production.One delayed proton was found in the total of 29 delayed events observed, and nine events could have been oblique particles lagging on the electrons by virtue of path differences.
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