Cosmic rays are the only sample of matter from
distant regions of the Galaxy, which can be directly observed by experiments
near the top of the Earth’s atmosphere. In particular, the ATIC investigation,
and the results have implications for particle acceleration in supernova
remnants or other exotic objects, transport of particles through the interstellar
medium and the nature of the cosmic ray sources.
ATIC is a tracking, ionization calorimeter instrument
that has been built by researchers from the US (SU, LSU, UMD, NRL), Russia
(MSU), Korea(SNU) and Germany(Max-Plank Institute). It consists, from
the top, of scintillator strip hodoscopes within an inert carbon matrix,
built by SU and LSU, and a Bismuth Germinate scintillating crystal calorimeter,
built by LSU. An incident particle has its charge measured by the Si-matrix,
its trajectory determined by the scintillator hodoscopes and its energy
measured by the calorimeter from the cascade developed in the detector.
The charge, energy and direction provide the raw data needed for this investigation.
The explosions of massive--Suprnovae--leaves violent,
fast expanding, remnants which are believed to be the site of the “cosmic
accelerator” which works via the action of magnetic fields at the expanding
shock front. Such and acceleration model predicts a change in the energy
spectrum of the elements, starting with Hydrogen (charge = 1), then Helium
(charge = 2) and progressing to heavier nuclei. ATIC is designed specifically
to look for such changes in the hydrogen, helium and heavy element spectra.
The ATIC payload weighs about 1360 kg, is contained
within a pressurized gondola shell and consumes about 400 watts of power,
provided by two panels of solar cells. To make the needed measurements
of these rare, ultra-high energy particles, the ATIC balloon payload must
be flown on at least four long-duration flights of 10 days each, i.e. around-the-world
either in the northern or southern hemisphere. ATIC is scheduled for a
test flight in spring 2000 to be followed by long-duration flights in each
of the following years, as permitted by the ballooning schedule. From these
flights will emerge a new understanding of particle acceleration in Supernova
remnants and lead to new techniques for other investigations.