Papers Published

1. Goodman, J.A. and Ellsworth, R.W. and Ito, A.S. and MacFall, J.R. and Siohan, F. and Streitmatter, R.E. and Tonar, S.C. and Viswanath, P.R. and Yodh, G.B., Delayed hadrons in extensive air showers: evidence for the iron-group nuclei in primary cosmic-ray flux at energies ~10¹³-10¹⁵ eV, Phys. Rev. D, Part. Fields (USA), vol. 26 no. 5 (1), pp. 1043 - 60 [1043] .
   (last updated on 2007/04/15)

   Abstract:
   The distribution of arrival time of energetic hadrons in the near-core region of air showers of energies ~10⁴-10⁶ GeV relative to the shower front has been studied experimentally at mountain altitude. The observed rate of hadron events with (i) energy >50 GeV in the calorimeter, (ii) associated shower particle density >18 m^-2, and (iii) a signal ⩾5 equivalent particles in a plastic scintillator T₃ of area 0.54 m² placed under 220 g cm^-2 of absorber in the calorimeter is found to be 1.85×10^-3 m^-2 sr^-1 sec^-1. Of these events a fraction (0.55±0.05)% have shown the signal from T₃ to be delayed by 15 nsec or greater relative to shower particles. Monte Carlo simulations of experimental observations have shown that these requirements on energy and shower density enhance the sensitivity of the observed rate to the contributions due to showers initiated by heavy nuclei. Calculations also show that observed delayed hadrons are mostly associated with showers due to heavy nuclei. For interpretation of observed features two models for primary composition have been considered. In the first model the power-law spectra for protons and lighter nuclei are assumed to have spectral index γ_p and the heavy (iron group) nuclei the index γ_Fe. An agreement between the expectation and observation requires the values of γ_p and γ_Fe to be significantly different as -2.68 and -2.39 in the energy range ~10³-10⁶ GeV. In the second model the spectral index γ is assumed to be the same for all components and the spectra steepen by 0.5 at the same rigidity R_c. It is found that the values of γ and R_c should be -2.55 and 10⁵ GV/c, respectively, to match the observations. It is concluded that a successful understanding of experimental observations requires a relative change between the energy spectra of protons and heavy nuclei in the energy range ~10⁴-10⁶ GeV, which would make the proportion of iron-group nuclei about 40% of the primary flux at these energies

   Keywords:
   cosmic ray energy spectra;cosmic ray nuclei;cosmic ray protons;cosmic ray showers and bursts;