2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

CHAPTER 6
research at PRL. In this phase, detectors
responding to much higher energy were desirable.
A thrust towards large counting rate was
necessary. Several cosmic ray telescopes, vertical
and inclined, were constructed with large area
plastic scintillators at Ahmedabad. Other high
counting rate monitors that operated during this
time were the Super Neutron Monitor at
Ahmedabad and the IGY type at Gulmarg.
Research activity was tapered off after the sad
demise of Sarabhai at the end of 1971 and the
collection of data discontinued after 1974.
There were several significant results from this
period. In addition to the much awaited effect of
the reversal of solar polar field on the diurnal
anisotropy, the totally unexpected reversal of
cosmic ray gradients perpendicular to the ecliptic
was seen for the first time.
The observations made with Pioneer and Explorer
spacecrafts by the PRL group at the University of
Texas at Dallas provided proof for the acceleration
of cosmic-ray particles in shock fronts and turbulent
sector boundaries. The dominant role played by
convection mechanism and field-aligned diffusion
in producing anisotropies was discovered.
Current efforts : GRAPES
With the advent of X-ray and γ-ray astronomy, and
as a result of diversification of interest of the
cosmic ray group at PRL, high energy cosmic ray
research is currently being pusued mainly by the
TIFR group. Prompted by the success of the 24-
detector array at Ooty (GRAPES-1), the group is
now attempting to make a more accurate
measurement of the nuclear composition of the
primary flux around the “knee” in the energy
spectrum in an attempt to obtain new information
on the nature of the processes responsible for
accelerating particles to PeV energies and their
propagation in the galactic disk. The GRAPES
(Gamma Ray Astronomy at PeV EnergieS) series
of experiments at Ooty aims at distinguishing
between acceleration and propagation as the
cause of the steepening of the energy spectrum
at the “knee” through a measurement of the
The GRAPES array at Ooty
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