Measurement of Ultra-High Energy Cosmic Rays with CHICOS

Citation

Brobeck, Elina (2009) Measurement of Ultra-High Energy Cosmic Rays with CHICOS. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9YH6-N064. https://resolver.caltech.edu/CaltechETD:etd-10192008-143041

Abstract

The California HIgh school Cosmic ray ObServatory (CHICOS) is a ground-based scintillator array designed to measure the extended air showers of ultra-high energy cosmic rays. The goal of the project is to gain insight into the origin of ultra-high energy cosmic rays by measuring the energy spectrum and the distribution of arrival directions.

The CHICOS array has been in operation since 2003. It consists of 77 pairs of scintillator dectectors deployed at schools in the San Fernando and San Gabriel valleys near Los Angeles, and is designed to observe cosmic ray air showers at energies of 10^18 eV and above. In addition, the Chiquita subarray is designed to observe smaller showers in the energy range of 10¹⁶ - 10¹⁹ eV.

We present new descriptions of the air shower lateral distribution function and time distribution function, which have been derived from AIRES-generated simulated air showers. The new functions are specific to the CHICOS altitude and allow for a maximum likelihood shower reconstruction method, which is more appropriate to the CHICOS data than the χ² minimization method. We present several analyses of the accuracy of the reconstruction software in the energy ranges available to the Chiquita and CHICOS arrays.

The energy spectrum between 10¹⁷ eV and 10¹⁹ eV has been measured by the Chiquita subarray. At the lowest energy range, it is found to agree with previous measurements, while the measured flux falls below previous experiments for energies greater than approximately 10^17.5 eV. The CHICOS energy spectrum above 10^18.4 eV is found to agree with previous results published by AGASA. However, we do not observe the cutoff in the spectrum at 10²⁰ eV reported more recently by the Auger and HiRes Collaborations.

A correlation analysis between CHICOS data and nearby active galactic nuclei (AGN) was performed. No excess of cosmic rays was observed in the vicinity of nearby AGN. The maximum correlation was observed for cosmic ray events with E > 10²⁰ eV and for AGN with z < 0.009, with P_chance = 21%. This is consistent with random correlations from an isotropic distribution, a result also found by HiRes, but in disagreement with Auger.

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