MADISON, WI and ARGENTINA--While detection of toxic chemicals in our immediate environment and monitoring atmospheric pollutants and greenhouse gases is obvious, the detection of cosmic rays--those high-energy particles (protons and nuclei) that originate in outer space--is much more esoteric in nature. “With our detector, we are looking for the most energetic particles in the known Universe,” says Stefan Westerhoff, associate professor at the University of Wisconsin (Madison, WI) and sub-system manager for atmospheric monitoring for the Pierre Auger Cosmic Ray Observatory (www.auger.org) in Argentina. The Auger Observatory is primarily concerned with detecting ultrahigh-energy (> 1018 eV) cosmic rays that excite air particles in the upper atmosphere and cause UV fluorescence (Reference: S. Y. BenZvi et al., Nuclear Instruments and Methods in Physics Research A, 574, 171–184, 2007).
Measurements of the amplitudes and time development of the fluorescence signals, recorded during clear and moonless nights, are used to determine the energies, chemical composition, and arrival directions of incoming particles. Because fluorescence from the air showers (at 300–400 nm) is affected by atmospheric particulates and weather conditions, UV measurements are accompanied by elastic backscatter light detection and ranging (LIDAR) measurements using a high-repetition UV laser (351 nm, 333 Hz, 0.1 mJ/pulse) source and a Hamamatsu (Bridgewater, NJ) R7400U PMT detector to provide local aerosol scattering and absorption properties of the atmosphere.
In addition to elastic backscatter LIDAR, Auger researchers are also exploring Raman LIDAR systems. “The Raman technique requires fewer assumptions on the aerosol optical properties than does elastic lidar and therefore, has smaller systematic errors,” says Westerhoff. “In addition, with Raman spectroscopy, it is possible to identify and quantify specific gaseous species or water.”
--Gail Overton