Use of thin ionization calorimeters for measurements of cosmic ray energy spectra

Abstract

The reliability of performing measurements of cosmic ray energy spectra with a thin ionization calorimeter has been investigated. Two Monte Carlo simulations were used to determine whether energy response fluctuations would cause measured spectra to be different from the primary spectra. First, Gaussian distributions were assumed for the calorimeter energy resolutions which (a) did not change with energy, (b) increased with energy, and (c) decreased with energy. The second method employed a detailed Monte Carlo simulation of cascades from an isotropic flux of protons, with incident energies representing(a) a simple power law, (b) a power law with a bend, and (c) an energy dependent spectral exponent. The results show that as long as the energy resolution does not change significantly with energy, the spectral indices can be reliably determined even for σ E / E=50%. However, if the energy resolution is strongly energy dependent, the measured spectra do not reproduce the true spectra. Energy resolutions greatly improving with energy result in measured spectra that are too steep, while resolutions getting much worse with energy cause the measured spectra to be too flat. Since thin calorimeters have energy resolution approximately constant with energy, they offer a viable means for measuring power law spectra. They are also suitable for detecting spectral bends or energy dependent spectral exponents, provided sufficient exposure is available.