Attenuation of neutron monitor radiation in the atmosphere

Abstract

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.