GRAPES 3 Moun Telescope

  • Category
    Science & Technology
  • Published
    28th Mar, 2019


For the first time in the world, researchers at the GRAPES-3 muon telescope facility in Ooty have measured the electrical potential, size and height of a thundercloud that passed overhead on December 1, 2014.


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  • At 1.3 gigavolts (GV), this cloud had 10 times higher potential than the previous record in a cloud.
  • This is not because clouds with such high potentials are a rarity, but rather, because the methods of detection have not been successful so far.
  • Clouds have negative charges along their lower side and positive charges on top and can be several kilometres thick.
  • Using a computer simulation and the observed muon intensity variations, the group worked out the relationship with the electric potential of the cloud.
  • Scientists calculated that the potential of the cloud they were studying was approximately 1.3 GV.
  • Until now, no one has ever measured potential, size and height of a thundercloud simultaneously. That is the reason for all the excitement.

What is GRAPES 3 Experiment?

  • The GRAPES-3 experiment located at Ooty in India started as a collaboration of the Tata Institute of Fundamental Research, Mumbai, India and the Osaka City University, Osaka, Japan.
  • At present many institutions from India and Japan are in collaboration.
  • GRAPES-3 (Gamma Ray Astronomy PeV EnergieS phase-3) is designed to study cosmic rays with an array of air shower detectors and a large area muon detector.
  • It aims to probe acceleration of cosmic rays in the following four astrophysical settings. These include acceleration of particles to, (i) ~100 MeV in atmospheric electric fields through muons, (ii) ~10 GeV in Solar system through muons, (iii) ~1 PeV in our galaxy through nuclear composition of cosmic rays, (iv) ~100 EeV in nearby universe through measurement of diffuse γ-ray flux.
  • The GRAPES-3 is located at N11.4o, E76.7o, and 2200m above mean sea level. The observations began with 217 plastic scintillators and a 560 m² area muon detector in 2000.
  • The scintillators detect charged particles contained in extensive air showers produced by interaction of high energy cosmic rays in the atmosphere.
  • At present the array is operating with ~400 scintillators that are spread over an area of 25,000 m². The energy threshold of muon detectors is 1 GeV.


    • Muons and other particles are produced when cosmic rays bombard air particles surrounding the earth.
    • The muons produced can have positive or negative charge. When a positively charged muon falls through a cloud, it loses energy.
    • If its energy falls below 1 giga electron volt (GeV), which is the threshold of detection of the GRAPES-3 muon telescope, it goes undetected.
    • On the contrary, a negatively charged muon gains energy when falling through the cloud and gets detected.
    • Since there are more positive than negative muons produced in nature, the two effects don’t cancel out, and a net change in intensity is detected.

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