A comprehensive experimental study has been made of the NM-64 neutron monitor. The counting rate has been measured as a function of the thickness of the reflector, of the separation of the counters, of the thickness of the inner moderator, and of the gas pressure in the counters. It is shown that the figures given in the internationally approved specification represent a satisfactory optimization of the design. The counting rate of the NM-64 monitor is found to be 3.3 times that of an IGY monitor of the same horizontal area. The origin and movement of both fast and thermal neutrons within the monitor have been investigated using cadmium sheets and by studying events producing counts in more than one counter.

Measurements obtained at Deep River, Canada, from two large neutron monitors, and at Cambridge, Massachusetts, from a high counting rate meson monitor, during the solar cosmic-ray injections of November 12 and 15, 1960, are reported. In addition, rate-meter pen traces of the neutron increases and a magnetometer trace of H, obtained at Deep River, are reproduced. The solar cosmic-ray increase of November 12 appears to be unique in that while it was in progress a sharp Forbush decrease happened to occur as shown by the MIT meson intensity. Half an hour before the onset of the Forbush decrease, and coincident with a conspicuous increase of H, the intensity of the solar cosmic radiation doubled and then exhibited strong rapid oscillations. We advance arguments that the changes of intensity of the solar cosmic ray• observed at high latitudes at the time of the magnetic disturbance and Forbush decrease are due to the earth sampling solar cosmic rays trapped in the gas cloud responsible for these latter effects. The events of November 12 and 15 are both shown to be in agreement with a recent model for the magnetic fields in the inner solar system. At the time of the solar cosmic-ray increase of November 15, the earth was already inside a trapping region, and periodic oscillations of the solar cosmic-ray intensity were observed lasting for about 2 hours. It is suggested that these oscillations may be closely connected with the storage mechanism. 1363 1364 STELJES, CARMICHAEL, AND McCRACKEN over an hour to reach the same fraction of its initial maximum intensity. A r•markable succession of fluctuations of the solar cosmic rays of periodicity about 20 minutes, lasting for slightly more than 1 hour after the beginning of the November 15 increase, was observed at Deep River. In view of these interesting features it seems worth while to describe and discuss the observations without waiting for data from other cosmicray stations. It has recently been shown [McCracken and Palmetra, 1960; Obayashi and Hakura, 1960] that previously observed solar flare cosmic-ray effects are in agreement with a simple model for the magnetic fields in the inner solar system. We shall present the experimental data from both Deep River and MIT and show that the same model provides a good explanation of the November events as observed at our laboratories. We shall not, in this paper, try to apply any further tests of the model requiring detailed data from elsewhere in the world, but we shall make some general predictions of what may have been observed elsewhere. We shall also describe the newly observed fast fluctuations in detail and discuss their possible significance. The model is the one originally suggested by Cocconi, Gretsch, Morrison, Gold, and Hayakawa [1958] at Varenna in 1957. This model has since then been strongly advocated by Gold [1959], for example, at the 1959 'Symposium on the Exploration of Space.' INSTRUMENTS Deep River Laboratory. The Deep River Laboratory is 145 m above sea level at 46.10øN. Lat. and 77.50øW. Long. The geomagnet...

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An NM-64 neutron monitor latitude survey made by road transport in the summer of 1965 in Canada, the United States, and Mexico was extended, in the summer of 1966, to the western seaboard of the United States and Hawaii. In 1966 in the vicinity of Mt. Hood (2.43 GV), Palomar Mt. (5.71 GV), and Mt. Haleakela (13.3 GV) advantage was taken of the possibility of changing altitude without significant change of geomagnetic cutoff. At each of these places also a smaller lead polyethylene neutron monitor was flown at seven different pressure altitudes between 530 mm Hg and 140 mm Hg. The airborne monitor was calibrated in terms of the NM-64 when the transport van was at 10 000 ft on the summit of Haleakela. It was found that graphical smoothing of vertical trajectory cutoffs in latitude and longitude eliminated an unnatural kink in the latitude curve near Mexico City. The 1965 and 1966 rates were adjusted to May 12–13, 1965, the date of the IQSY maximum, using data from a fixed monitor and allowing for the dependence of the secular fluctuations on cutoff and altitude. The attenuation coefficient throughout the atmosphere was determined from the data and also a new specific attenuation coefficient which is a function only of rigidity of the primary flux and depth in the atmosphere. The specific attenuation coefficient has a comparatively high value consistent with that observed for solar proton events.

In this, the last of a set of five papers reporting latitude surveys carried out in 1965 and 1966 at the time of and soon after the IQSY cosmic-ray maximum, the observations are reduced to a common atmospheric depth and at the same time the attenuation coefficients in the atmosphere for both the neutron monitor and the muon monitor are determined as functions of altitude and latitude. The latitude variation of the neutron monitor at sea level is compared with observations reported for the previous cosmic-ray maximum in 1954–55 and found to be similar. The altitude variation of the neutron-monitor attenuation coefficient is discussed in detail with reference to the maximum near 600 mm Hg currently attributed to neutrons produced in the monitor by stopping muons. It is shown that the stopping-muon effect is insufficient to account for the maximum. It is shown, using the Gross transformation, that a geometrical effect associated with the omnidirectional nature of the incident cosmic radiation may be the main factor producing the maximum. A specific absorption coefficient representing the differential effect in the neutron monitor of vertically-incident primaries of given magnetic rigidity is deduced. Good agreement is obtained with the observed attenuation in the atmosphere of solar protons of mean energy 2 GV and also with the reported results of a Monte Carlo calculation of the attenuation of the nucleonic cascade in air.