Changes in the low-rigidity primary cosmic radiation during the large Forbush decrease of May 12, 1959

One of the largest and most striking Forbush decreases on record occurred on May 12, 1959. The Deep River, Canada, neutron intensity plotted in Figure 1 for the period from April 10 to June 30 shows clearly this large decrease and subsequent exponential recovery (H. Carmichael, private communication, 1959). Of particular interest are data obtained relating to the rigidity and composition of primary cosmic-ray particles on two balloon flights during this period. The detector used in these experiments consisted of a lucite Cerenkov detector and a NaI scintillation counter. The simultaneous measurement of the Cerenkov counter and the scintillation counter outputs allows a direct determination of the charge and velocity of the incident particles [McDonald and Webber, 1959].