B. V. Sreekantan

Use of plexiglas sheets coated with wavelength-shifter dyes for detection of atmospheric Cherenkov radiation: A new technique for gamma ray astronomy at energies of ∼ 1013–1014 eV

Abstract A 900 cm2 area plexiglas sheet coated with a thin film of a combination of wavelength shifter dyes is used for detection of atmospheric Cherenkov ultraviolet photons in extensive air showers at Cosmic Ray Laboratory, Ooty in southern India. The response of the coated sheet is compared with an otherwise identical but uncoated plexiglas sheet in showers of different primary energies and at different distances from the axes. It is shown that the coated sheet is an efficient detector of Cherenkov UV photons in showers of energies > 1014 eV. Based on these observations it is suggested that an array of detectors made from plexiglas sheets, coated with wavelength-shifter dyes, with interseparations of about 50 m can be effectively used as a large area detector for studies of cosmic sources of gamma rays of energies > 1013 eV. Using the relative timing technique such an array can provide a large collection area, good angular resolution and wide aperture. The advantages of this new technique, relative to conventional detectors such as large area reflectors for Cherenkov photons and arrays of particle detectors for air showers, are discussed.

Detection of pulsed y-rays at energies above 300 GeV from pulsars — T.I.F.R. experiments

A brief account is given of the search for y-rays from pulsars made by the T.I.F.R. group in India. It is concluded that for energies above 300 GeV the flux of pulsed y-rays from the Crab and Vela pulsars is much less than would be expected by extrapolation of the spectra measured in the gigaelectronvolt range. It is still not certain whether the pulsars have been detected above 300 GeV because of variability in the apparent signals from year to year.