Heavy Nuclei in the Primary Cosmic Rays over Minnesota

An analysis of the heavy primary cosmic rays ($Z\ge 2$) has been made by using a stack of emulsions which was exposed at a high altitude on September 18, 1956, over Minnesota. In order to examine the low rigidity portion of the heavy primary cosmic rays, we have studied only the stopped particles in this stack. In this work, two methods to identify the charges of various particles have been employed: (1) track width measurement and (2) $\delta$-ray density vs residual range. The energy spectra of $\alpha$, $L$, and $M$ groups in the primary cosmic radiation have been obtained. Their energy spectra show the existence of a maximum in each group, and the energy regions of the maxima increase with increasing charge of heavy element. Some possible interpretations of these spectra are discussed.