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
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
"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
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
"NASA is very interested in our fluorescence results under various
levels of humidity," Chen added.
Air Shower Models
In the next phase of the SLAC experiment, Chen and his team will shoot
the same electron pulses through a ceramic material called alumina. Using
different thicknesses of this dense material, they will recreate the
progress of full air showers at various depths through the atmosphere.
This experiment will test models of air shower development and give
scientists an astonishingly intimate look at the cascade of particles and
the fluorescent light they trigger.