Using electron pulses, a sensitive prism and just the right amount of air, researchers at SLAC have revealed the true colors of fluorescent light triggered by ultra high energy cosmic rays. The results will shed light on an important discrepancy between two recent observations of this energetic phenomenon and provide a solid foundation for future generations of cosmic ray experiments.

Schematic drawing of an air shower. As the primary ultra high energy cosmic ray hits the Earth’s atmosphere, it rapidly decays into a cascade of thousands of lower energy particles, like electrons. The particles excite gas molecules, causing them to emit ultraviolet fluorescent light.

The High Resolution Fly’s Eye (HiRes) Experiment in the U.S. lead by the University of Utah and the Akeno Giant Air Shower Array (AGASA) in Japan exploit different techniques to detect and measure cosmic rays through what is known as an air shower—the multiplying cascade of decaying particles set off when an ultra high energy cosmic ray hits the earth’s atmosphere.

AGASA reconstructs the initial cosmic ray using detectors that collect air shower particles that fall to the ground. HiRes determines the energy of an event based on the total amount of ultraviolet light emitted by atmospheric gas molecules after they are excited by air shower particles, a technique called air fluorescence.

Now, in an international collaboration that includes members from SLAC, the Center for Cosmology and Particle Astrophysics (CosPA) in Taiwan, and HiRes (University of Utah, the University of Montana, Rutgers University), researchers are using the unique controlled laboratory environment at SLAC to investigate a potential source of the discrepancy on the UHECR spectrum at the very high energy regime. A precision measurement of a spectrally resolved air fluorescence yield, such as what they intend to do in E-165, will hopefully shed some light on this existing discrepancy between AGASA and HiRes results.

"There was a real need for independent calibration of air fluorescence. Laboratory experiments can have such importance to direct detection," said Pisin Chen (ARDA), who, together with Pierre Sokolsky of Utah, leads the SLAC experiment E-165, called FLASH (Fluorescence in Air from Showers).

To make complete and accurate measurements of the light spectrum in the first phase (Thin Target phase) of E-165 during their September 2003 run, the team shot pulses of 28.5 GeV electrons from the Final Focus Test Beam through a gas-filled chamber. While the air chamber is not long enough to trigger an air shower, the electrons induced the gas to emit fluorescent light.

"Even though we don’t let the particles shower in our Thin Target run, they still trigger fluorescence production and we are able to measure the precise number of photons produced per particle. By not letting it shower we know exactly what goes in and what comes out," said Kevin Reil (ARDA), a post-doctoral researcher on E-165.

Light production was tested under a variety of conditions. The chamber was filled alternately with pure dry air, pure nitrogen, different mixtures of oxygen and nitrogen, as well as ‘SLAC air’ (which includes various impurities like water vapor).

After shooting out from the gas molecules, the photons of light were then sent through a series of narrow band filters and amplified by a photo-multiplying tube before being measured. Each filter transmits only a narrow range of ultraviolet light, yielding a measurement of the total light produced along the spectrum.

To confirm the shape of the spectrum in a separate setup, light was sent through a spectrograph, which acts like a prism to separate the light into small wavelength bands, producing an almost continuous picture of fluorescence.

While Chen cautions that small deviations in the resolved fluorescent spectra will have some impact on energy calculations, the initial results support the measurements made by the air fluorescence method at HiRes.

The results could have vast importance to the 3,000 square kilometer Pierre Auger project in Argentina, which has already begun limited operation and is due for completion in 2005. This hybrid cosmic ray experiment combines both air fluorescence and ground array detectors.

"The precise measurement of this spectrum goes way beyond the HiRes, AGASA and Auger," Chen said. "Knowing the spectra of air shower fluorescence will have further implications for future generation of space based cosmic ray experiments."

Air fluorescence will be the only available technique for cosmic ray detectors placed on satellites, like NASA’s proposed OWL project and the joint US-European EUSO project. While air fluorescence is traditionally conducted in the desert, where humidity is extremely low, these projects will likely focus on air showers that fall over oceans in order to reduce contamination of background light. Until now, researchers didn’t know exactly how water vapor would impact the production and quality of cosmic fluorescent light.

"NASA is very interested in our fluorescence results under various levels of humidity," Chen added.