The Transition Radiation Array for
Cosmic Energetic Radiation (TRACER) is a new instrument for
direct, balloon borne measurements of heavy cosmic ray nuclei
(oxygen to iron) in the energy range from 1013 to
several 1014 eV per nucleus.
TRACER had a successful 14 day Antarctica flight in 2003,
traversing some 5,000 miles around the South Pole at an average
elevation of 125,000 ft.
Flying high above the atmosphere TRACER sampled over 50 Million
heavy Cosmic Rays (Carbon through Iron). TRACER has the ability
to identify each individual cosmic ray nucleus as it traverses
the detector. This ability was demonsatrated during its initial 30 hour
balloon flight from Ft. Sumner New Mexico in September 1999.
Using data from the Antarctic flight, TRACER will provide statistically
significant results on the energy spectra of cosmic ray nuclei beyond
1014 eV with good charge resolution, permitting the
separation of all individual element species from Oxygen to
Iron. Therefore, the investigation is expected to provide key
information on long-sought questions in cosmic ray astrophysics.
For instance, it is believed but still not proven by observations that
cosmic rays are accelerated in shock waves generated by supernova
explosions. The shape of their energy spectrum must exhibit this
origin, but it is also affected by energy dependent propagation of the
particles throught the galaxy. The figure indicates
two possible cosmic ray propagation models, one with a continously decreasing
pathlength proportional to
E-0.6, and a residual pathlength model with
a finite pathlength of 0.013 g/cm2.
The results expected with TRACER can well differentiate between such models.
Simulated data for oxygen and iron, indicating the statistical
quality of data from TRACER for a 12 day flight.
The statistical quality of data expected from a 12 day flight
corresponding to 60 m2 sr days is illustrated in the figure.
The cosmic ray energy spectrum is shown, as usual multiplied by
E2.75. The expected transition radiation detector results cover
an energy range from about 1012 to nearly 1015 eV.
The measurements will have significant overlap with present air shower
experiments allowing an improvement of the understanding of the interactions
within the atmosphere, and therefore of the interpretation of air shower measurements on the ground. For comparison, the figure includes results
from CRN on Spacelab 2.