Physics Colloquium explores Ultra High-Energy Particles

The latest Physics Colloquium delved into the explanation and observations of ultra high-energy particles. Professor Lawrence Wiencke, who also led the talk, led a team at Mines that has developed a way to more effectively record these cosmic rays of extremely rare occurrence using UV pulse lasers at the Pierre Auger Observatory in Argentina. These ultra high-energy cosmic rays are defined as possessing a kinetic energy greater than 10^18 electronvolts and are exceedingly rare with an estimated few hundred or so impacting Earth daily. Even more rare still are extreme-energy cosmic rays that have been observed to have macroscopic energies of over 10^20 electronvolts. This is equivalent to the energy of a baseball travelling at 60 miles per hour and 7 orders of magnitude greater than the energies of particles accelerated using the Large Hadron Collider at CERN. The astroparticles are in the form of protons, neutrons, or atomic nuclei and travel at very nearly the speed of light, or around 1 femtosecond slower than light would in a year.

An object requires exponentially more energy to accelerate as it further approaches the speed of light, so the seemingly minute extra velocity of high-energy cosmic rays over other astroparticles with mass equates to particles with exponentially higher energies. At these speeds, due to time dilation, the particles are able to reach Earth from a greater distance before decaying, possibly from the center of our own galaxy or originating from elsewhere in our local supercluster of galaxies. With the average neutron existing for a brief fifteen minutes at rest before beta decay, less energetic particles originating from these distances would never reach Earth’s atmosphere.

The flux of these particles is astoundingly low at one per square kilometer per century. Even without the protective aid of the atmosphere, one would be much more likely to be exposed to a lethal dose of common gamma rays than to be struck by an ultra high-energy particle. Due to the particles’ rarity it is impossible to detect an individual particle directly. As the ultra high-energy particles encounter the Earth’s atmosphere, the energy is dissipated into an “air shower” of subatomic electrons, photons and muons that act perpendicular to the direction of the original particle. This phenomenon has historically been measured with ground-based receivers that use the Earth’s atmosphere as a calorimeter to measure the “air showers” that are produced.

With help from a pulsed UV laser system designed by a team from the Colorado School of Mines, the Pierre Auger Telescope located in the Argentinian Andes uses specialized fluorescence detecting telescopes that photograph the resulting plane of UV light from the “air shower”. The dispersed subatomic particles are more densely located at the center of the plane, indicating the original position of the high-energy particle. A series of water tanks spread out over the 3000 square km observation area detect when the subatomic particles impact the Earth’s surface. Using the density and time differences at each station, a direction of impact can be calculated. Using the fluorescence detection method, more accurate measurements can be attained. However even with a receiving area of 3000 square km, scientists at the observatory can still only measure a particle impact every few weeks, indicating their extremely rare occurrence.

The fluorescent streaks occurring from high-energy cosmic rays could be observed using a much larger area, as well as minimizing the effects of light pollution from the ground with a similar space based detector. This is the theory behind the European Space Agency’s Extreme Universe Space Observatory, which is scheduled for a balloon test launch in 2014. In 2017 the system consisting of three large Fresnel lenses will attach to the Japanese JEM module on the International Space Station. The now named JEM-EUSO will use techniques similar to those developed at the Colorado School of Mines by Dr. Lawrence Wiencke and team to detect the brief flashes of UV light originating from the extensive air shower that occurs when the high-energy particle collides with the nucleus of an atom of air. The resultant fluorescent UV photons are tracked every few microseconds to create a time segmented image of the “air shower”. This new experiment, which has been hampered recently by programming cuts, will provide a plethora of information on the mysterious high-energy particles. It could provide an answer to their origin and further the knowledge of our own solar system’s formation.

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