While the discovery of cosmic rays fetched physicist Victor Hess a Nobel in 1936, it was noted scientist Homi Bhabha who realised the importance of cosmic ray research in India. Homi Bhabha initiated the study of high-energy particles by starting a high-altitude laboratory at Ootacamund, writes P R Vishwanath.
In August, the world celebrated the centenary of an important discovery, that of cosmic rays. A daring young Austrian went up in a balloon and discovered that the earth was getting continuously bathed in a torrent of charged particles from the sky.
This discovery fetched Victor Hess a Nobel Prize in 1936. What he had discovered were mostly protons coming from all directions. They were found to be isotropic and also very energetic. These were called cosmic rays which became a very important field of research in physics for the next 50 years. Many fundamental particles like positrons, pi mesons, muons, etc were discovered first in experiments connected with cosmic rays.
In India, it was another young man who realised the potential for cosmic ray research. Particle accelerators were about to be introduced in the West and he realised that such research cost a lot of money.
It was the early 1940s and he had just then returned from England after conducting theoretical studies in this field. Cosmic rays were ideal since the particle beam was a gift from the heavens. The name of the young man was Homi Bhabha.
Cosmic rays contain various particles like protons, pi mesons, muons, electrons, etc of very high energy. The energies obtained in cosmic rays are much higher than those available in manmade accelerators including LHC, the largest one.
Protons and pi mesons are better studied at high altitudes whereas deep underground is the place to study muons. Homi Bhabha initiated the research of high-energy particles by starting a high-altitude laboratory at Ootacamund. He also realised the advantage of having deep mines in the country and thus muon studies were initiated in Kolar Gold Fields in the 1950s. Bhabha also started scientific ballooning at Hyderabad in India, near the geomagnetic equator.
Among the Indian cosmic ray experiments of those times, the ones which had the maximum impact were those conducted in KGF. The neutrino was postulated by the great physicist, Wolfgang Pauli, to explain the possible breakdown of law of conservation of energy in a phenomenon called beta decay.
It is a difficult particle to detect since it is both neutral and interacts very little with other particles. Reines and Cowan detected the first neutrino near a nuclear reactor in the US almost a quarter of a century after its prediction. But those were artificial neutrinos.
Research at KGF
The TIFR group, which had started experiments in KGF mines in the early 50s, had found that the number of muons, the only particles capable of penetrating to great depths, was extremely small at great depths and thus almost a background-less environment for conducting studies on neutrinos.
The TIFR group, along with the scientists from University of Durham, UK and Osaka City University, Japan started the neutrino experiment in 1964 and the first neutrino event was detected in 1965. This was the first natural neutrino event ever discovered.
This experiment was a precursor to bigger ones 30 years later which fetched Nobel Prizes to scientists of Japan and USA. The Indian group at KGF consisted of M G K Menon, V S Narasimham, P V Ramanamurthy and B V Sreekantan.
Later the same venue saw many interesting experiments including the famous proton decay experiments of the mid-1980s. Indian expertise in cosmic rays was instrumental in the beginning of several new fields of research like high-energy physics, X-ray astronomy, radio carbon dating etc.
In high-energy physics, it led to Indian groups participating in experiments in CERN and Fermilab in the past and the LHC experiments at present. In X-ray astronomy, several satellite based studies were conducted and a multi-wavelength study satellite called Astrosat is expected to be launched soon.
The field of high-energy gamma ray astronomy to study the origin of cosmic rays was started in the 1970s at Ootacamund and Gulmarg. This research is being continued today with a low threshold gamma ray telescope called HAGAR at Hanle in Ladakh at an altitude of 4,300 meters. Extensive air shower experiments are being carried out at Ootacamund for studies of very high energy cosmic ray primaries. The Indian neutrino observatory, a very ambitious programme, is expected to start near Madurai in the next few years.
The field of cosmic ray research has come a long way since its discovery by Hess exactly 100 years ago. It was a witness to the great discoveries of growth of particle physics in the first 50 years. The energies of these particles are so high that they can never be achieved with man-made accelerators. Further, today, it is a tool to study the processes in exotic astronomical sources like pulsars and quasars.