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Measurement of ultra-high energy cosmic rays with CHICOS


Brobeck, Elina (2009) Measurement of ultra-high energy cosmic rays with CHICOS. Dissertation (Ph.D.), California Institute of Technology.


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 10^16 - 10^19 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 chi-squared 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 10^17 eV and 10^19 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 10^20 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 > 10^20 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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:cosmic rays; UHECRs
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • McKeown, Robert D.
Thesis Committee:
  • Stone, Edward C. (chair)
  • Harrison, Fiona A.
  • Kamionkowski, Marc P.
  • McKeown, Robert D.
  • Readhead, Anthony C. S.
Defense Date:18 September 2008
Non-Caltech Author Email:elina (AT)
Funding AgencyGrant Number
Weingart FoundationUNSPECIFIED
Record Number:CaltechETD:etd-10192008-143041
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4187
Deposited By: Imported from ETD-db
Deposited On:27 Apr 2009
Last Modified:26 Dec 2012 03:06

Thesis Files

PDF (elina_brobeck_thesis.pdf) - Final Version
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