Arailability and Limitation of Multiplicity Measurements in the NM-64 Neutron Monitor at Syowa Station, Antarctica

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Multiplicity measurements using the NM-64 neutron monitor have been carried out continuously at Syowa Station, Antarctica, and concurrently once a year along a definite sea-level route between Japan and Syowa Station. The Syowa data obtained during the period from March 1967 to February 1969 are analyzed, together with those from the first two of the latitude surveys which are in progress since 1966. The barometric coefficient and the rigidity response function of the cosmic-ray neutron component are derived as a function of multiplicities from m=1 to m≥6. The multiplicity spectrum is investigated in the cases of the cosmic modulation phenomena such as solar proton event, Forbush decrease and diurnal variation. It is shown that the barometric coefficients and the magnitudes of intensity variations as observed in the solar proton and Forbush decrease events are decreasing with increasing multiplicity, while no significant multiplicity effect is recognized in the diurnal variation. A possibility of distinguishing the various modulation spectra of the primary cosmic radiation on the basis of the multiplicity measurements is examined quantitatively. By taking into account the behavior of higher multiplicities and the accuracy in low rigidity part of the differential response functions, the limitation of the NM-64 neutron monitor in the multiplicity work is discussed.
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