A high resolution method for measuring cosmic ray composition beyond 10 TeV

Abstract

The accurate determination of the elemental composition of cosmic rays at high energies is expected to provide crucial clues on the origin of these particles. Previous direct measurements of composition have been limited by experiment collecting power, resulting in marginal statistics above 10 14 eV, precisely the region where the "knee" of the cosmic-ray energy spectrum is starting to develop. In contrast, indirect measurements using extensive air showers can produce sufficient statistics in this region but generate elemental measurements which have relatively large uncertainties. Here we discuss a technique which has become possible through the use of modern ground-basedČerenkov imaging detectors. We combine a measurement of theČerenkov light produced by the incoming cosmic-ray nucleus in the upper atmosphere with an estimate of the total nucleus energy produced by the extensive air shower initiated when the particle interacts deeper in the atmosphere. The emission regions prior to and after the first hadronic interaction can be separated by an imagingČerenkov system with sufficient angular and temporal resolution. Monte Carlo simulations indicate an expected charge resolution of ∆Z/Z < 5% for incident iron nuclei in the region of the "knee" of the cosmic-ray energy spectrum. This technique also has the intriguing possibility to unambiguously discover nuclei heavier than iron at energies above 10 14 eV. The identification and rejection of background produced by charged particles in ground based gamma-ray telescopes is also discussed.

First ground-based measurement of atmospheric Cherenkov light from cosmic rays

“…The charge bands cover a Z range from 1 to 28. The contribution of elements with Z > 28 is likely negligible: although no flux measurements for these ultra heavy cosmic rays exist in the energy region of importance here, measurements of energy spectra for elements with Z > 28 at lower energies [1,17] and flux predictions [18] in the TeV energy region are more than 3 orders of magnitude below the flux of the iron band. The contribution of each charge band to the total simulated flux and the energy distribution of the events inside each band are weighted according to the measured fluxes given in [1,16] (referred to in the following as reference composition).…”

“…In 2001, Kieda et al [9] proposed a new method for the measurement of cosmic rays with Imaging Atmospheric Cherenkov Telescopes (IACTs). The central idea of this method is to detect the Cherenkov light emitted by primary cosmic-ray particles (so-called Direct Cherenkov light) from the ground.…”

“…Because the DC-light is emitted higher in the atmosphere, it is emitted at a smaller angle than the EAS-light, and is therefore imaged closer to the shower direction in the camera plane. A typical emission angle for DC-light is 0.15 to 0.3 , whereas most of the EAS-light is emitted at angles greater 0.4 from the direction of the primary particle (for a more detailed discussion see [9]). Cherenkov cameras, with pixel sizes of 0:1 are therefore able to resolve the DC-emission as a single bright pixel between the FIG.…”

“…The energy range to which this technique can be applied depends on the charge of the primary particle [9]. At lower energies the limiting factor is that the primary particle momentum must exceed the Cherenkov threshold.…”

“…Protons dominate the cosmic-ray flux in the energy region of interest (see Table II). Additionally, protons emit only a negligible amount of DC-light compared to their EAS-light yield at these energies [9], therefore any detection of DC-light in proton simulations can be considered a fluctuation of the EAS-light yield and therefore a false detection. As shown in Fig.…”