GAMMA-HADRON SEPARATION BASED ON CERENKOV PHOTON
ARRIVAL TIME STUDIES
Chitnis, V. R.
P. N. Bhat
Atmospheric Cerenkov technique largely depends on the ability of the
experiment to detect and discriminate the Cerenkov light produced by
gamma rays from astronomical objects in the presence of an abundant
background due to cosmic rays. Here we have carried out systematic Monte
Carlo simulation studies of the timing information of Cerenkov photons
produced by VHE gamma rays and hadronic primaries. We have
investigated several measurable parameters such as radius of the shower
front, pulse shape parameters (rise time, decay time and FWHM) and arrival
time jitter of Cerenkov photons. The efficiency of these parameters to
discriminate between gamma ray and hadronic showers has been studied in
terms of quality factor. This study has been carried out for vertical
as well as inclined showers and for various altitudes of observation.
Among the parameters considered here, Cerenkov pulse decay time and
relative arrival time jitter of Cerenkov photons are found to be
particularly useful. Decay times of pulses from hadron generated showers
are in general longer and hence yield a quality factor of ~ 1.3 - 1.6,
resulting in a rejection of ~ 88% - 93% of hadronic showers, over
the primary energy range of 250 GeV - 1 TeV. Similarly, hadronic showers
exhibit a larger jitter yielding a quality factor of ~ 2, resulting in
a rejection of ~ 94.4% - 98.8% of hadronic showers. Better
discrimination with decay time is possible if we choose detectors only
within the hump region. This result holds for inclined showers as well as
different observation levels. Radius of the shower front on the other hand,
has limited use in rejecting protonic showers. It however, provides good
rejection efficiency for heavy primaries, e.g., for iron nuclei the
quality factor of ~ 2 is attainable, resulting in the rejection
of ~ 83% of iron showers.
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On 23 Aug 1999, 11:46.