2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

CHAPTER 6
predictions from the Grand Unified Theories of
decay of protons with a decay-time of about
10 28 yr. Though this experiment did not observe
an unambiguous positive signal, it was able to place
an upper limit of about 10 31 yr on the decay-time
for some of the proposed decay modes.
Phenomenological studies
In addition to these experimental results, there
were many phenomenological studies which
established several important results. For example,
it was shown that the charge composition of
nuclei in cosmic rays at low energies could be
consistently understood if and only if one
explicitly considered the wide range of ages of
these particles. Based on the observed energy
spectrum of ultra high energy cosmic rays, a limit
was placed on the density of a possible
background of low energy neutrinos. The
calculated flux of high energy neutrinos produced
in the atmosphere and the observed rate of
neutrino interactions in the underground
experiments, was used to draw inference on crosssection
for neutrino interactions in 10–100 GeV
range.
Another stream of investigations concentrated on
the propagation of cosmic rays in the galactic disk
and their interactions with interstellar matter to
calculate the elemental/isotopic ratios at low
energies, including the expected flux of
antiprotons. A particularly important result was
that spallation of heavy nuclei during the diffusive
transport of cosmic rays in the galaxy play an
essential role in explaining the abundance of the
light elements.
These studies contributed to the development and
refinement of the popular leaky-box model for
cosmic ray propagation.
Cosmic Ray Research at PRL
When Bhabha initiated cosmic ray experiments at
the IISc in Bangalore, one of his associates was
Vikram Sarabhai. C.V. Raman, the Head of the
department, strongly encouraged Sarabhai to
pursue research in cosmic rays, which he saw as
the new frontier in physics. This factor, plus his
close interaction with Bhabha, a pioneer in this
field, led Sarabhai to select cosmic rays as his main
research interest. Based on his research at
Bangalore and Apharwat in Kashmir, Sarabhai had
established the small diurnal and semi-diurnal
variations in cosmic-ray intensity at ground level.
And when he founded the Physical Research
Laboratory in 1947, understanding the details of
these variations and their causes became the
major programme of cosmic-ray research at PRL.
This involved the development of sophisticated
instrumentation, data gathering and modeling. As
the nature of cosmic-ray anisotropy was not
known during the exploratory phase, innovation
was made in the form of cosmic-ray telescopes
with various opening angles in the east–west
plane, the narrowest semi-opening angle being
2.5 0 . Atmospheric effects due to air mass and its
distribution were corrected for by measuring the
meteorological parameters. Another substantial
improvisation was the pair of cosmic-ray
telescopes symmetrically inclined with respect to
the vertical. In this set-up the atmospheric
processes are equal but the primary anisotropy is
different for the two telescopes.
These studies at PRL, and the analysis of the data
from ionization chamber network of Carnegie
Institution, resulted in the following significant
contributions during this period:
• Establishment of cosmic-ray anisotropy as
something originating beyond the atmosphere
and geomagnetic field.
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