The zenithal distribution of the cosmic ray neutrons in the high energy range

Abstract

The zenith angle distribution of cosmic ray neutrons in the energy range ≳ 10 GeV has been measured at an altitude of 3500 m above sea level (679 g cm−2) by means of a penetrating shower detector associated with a large area Geiger counter hodoscope. The penetrating showers were generated in thin layers of paraffin and graphite. The instrumental bias and the sistematic errors in the determination of the angles are discussed, and corrections applied whenever possible. The corrected distribution is well approximated with a law of the type exp [−m/cosϑ] withm = 7.1 ± 1.3, in agreement with the results of Walker. The discussion shows that this value has to be considered as a lower limit for the truem and is therefore difficult to reconcile with the assumption that the zenithal distribution is due solelyto the absorption of the particles of the nuclear cascade in the atmosphere, with a mean free pathλ a = 130 g cm−2 as determined by several authors. From the hodoscope information the angular distribution of ionizing secondaries around the direction of the primary neutron in also calculated. The distributions obtained for penetrating showers produced in carbon and in hydrogen are very similar. The analysis in terms of an isotropic distribution in the center of mass system of the incident neutron and a target nucleon yields an average energy of the observed eventsĒ = 24 GeV. The experimental results on the zenithal distribution are compared with the distributions calculated with the theory of the nuclear cascade in the atmosphere, as developed byBudini andMoliere. It is shown that the finite lifetime of some of the particles belonging to theN component can not account for the « anomalous » zenithal effect indicated by the measurements. It is suggested that the discrepancy may be removed with the choice of a value of the anelasticity parameter much greater than that assumed byBudini andMoliere.