Study of extensive air showers at Mount Norikura. II. Direct measurement of attenuation length

Abstract

Attenuation lengths for individual EAS passing through a 2 m layer of water have been measured by means of two layers of plastic scintillators. The attenuation lengths obtained vary over a wide range from about 70 g/cm2 to several hundreds of g/cm2 and, very rarely, the negative attenuation length of the growing stage has been obtained. The attenuation lengths observed are not sensitive to core structure (single or multiple core), but are sensitive to the age parameter of EAS determined from the lateral structure function. However, there are some discrepancies between age parameters estimated from longitudinal development and from lateral structure, the former yielding greater ages than the latter.The average value of attenuation length observed is significantly short compared with those for other methods so far used in a statistical way, especially for larger-sized EAS. Considering the calculated transition curve for EAS, and fluctuation in the level of the first interaction, the distribution of the attenuation length obtained is consistent with a model of EAS whose primary energy is 106 GeV, interaction mean free path 90 g/cm2, and inelasticity 0.7. A model of multiple production of baryons at an early stage of EAS is discussed.