The award of the Nobel Prize in Physics in 2015 to Takaaki Kajita of the University of Tokyo and Arthur B. McDonald, Chief Scientist at the Sudbury Neutrino Observatory (SNO) collaboration, Canada for proving that neutrinos change identities or ‘flavours’ from one type to another over time encouraged the world scientists to explore new avenues in the field of neutrino research. The India-based Neutrino Observatory (INO) project is a multi-institutional effort aimed at building a world-class underground laboratory with a rock cover of approximately 1,200 m for non-accelerator-based high-energy and nuclear physics research.

The country’s biggest basic science facility, to be built at a cost of Rs. 1,500 crore, is being funded by the Department of Atomic Energy (DAE) and Department of Science and Technology (DST) and will study atmospheric neutrinos produced by cosmic rays in the Earth’s atmosphere. It is the latest in a series of neutrino detectors and experiments being set up worldwide to promote research in particle physics.

India was a pioneer in neutrino experiments. Atmospheric neutrino research started in the Kolar Gold Fields (KGF) mine in India, which is one of the deepest mines in the world. It became the first laboratory for the detection of cosmic-ray-produced neutrinos in 1965. Neutrino research progressed further, especially in Japan, and led to two Nobel Prizes for Japanese physicists. Major underground Neutrino Observatories around the world are Sudbury in Canada, Soudan mines in the USA, Kamioka in Japan, under the Gran Sasso Mountains in Italy and two underwater neutrino observatories — Amundsen-Scott South Pole Station, Antarctica and ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch project) under the Mediterranean Sea off the coast of Toulon, France.

In particle physics, a lepton is an elementary particle of half-integer spin that does not undergo strong interactions. Leptons can either carry one unit of electric charge or be neutral. In addition to the electron (and its antiparticle, the positron), charged leptons include the muon (with a mass 200 times greater than that of the electron), the tau (with mass 3,700 times greater than that of the electron), and their anti-particles. Both the muon and the tau, like the electron, have accompanying neutrinos, which are called the muon-neutrino and tau-neutrino, respectively. In particle interactions, although charged leptons and their accompanying neutrinos can be created and destroyed, the sum of the number of charged leptons and corresponding neutrinos is conserved. This fact leads to dividing the leptons into three families, each with a charged lepton and its accompanying neutrino. They oscillate between ‘flavours’ as they propagate. A particle might start out as an electron neutrino, but as it moves, it morphs into a muon neutrino or a tau neutrino, changing flavours as it goes. Without neutrinos, the Sun would not shine and the air we breathe would not have formed.

The finite but tiny mass of the neutrino is important because neutrinos are by far the most numerous of all the particles in the universe (other than photons of light which are mass-less) and therefore even a tiny mass for the neutrinos can make them have an input on the evolution of the universe through their gravitational effects. Scientists don’t know how to subdivide any further at this point of time. However, specific capabilities of the INO detector may be of help to know the ordering of the three masses between the neutrinos. In a sense, neutrinos hold the key to several important and fundamental questions on the origin of the universe and the energy production in the stars.

Neutrinos are completely benign and are present in profusion in our atmosphere. Also known as ‘ghostly particles’ (but of a friendly kind), they are tiny, neutral, elementary particles naturally produced in the Sun, stars and in the atmosphere, which interact with matter via the weak force. There are many other natural sources of neutrinos including exploding stars (supernova), relic neutrinos (from the birth of the universe), natural radioactivity and cosmic ray interactions in the atmosphere of the Earth. They all produce billions of neutrinos which stream through our body every second; yet, only one or two of the higher-energy neutrinos get scattered from our body in a lifetime. They just go through us, unaware of our presence just as we are unaware of their presence.

Neutrinos have no effect on human beings but can help us understand how our universe has evolved as they have played an important role in the evolution of the universe. They are part of nature, but we cannot see or feel them because the probability of neutrinos interacting with matter is negligible and they simply pass through all matter. Hence it requires huge detectors and sophisticated instruments to study neutrinos. The INO is a kind of telescope that will allow scientists to look at and study neutrinos. It is placed underground so that the rocks above shield the detector from other energetic particles coming from space.

The INO underground laboratory

The plan of the INO includes setting up the flagship Iron Calorimeter (ICAL) detector in an underground laboratory in Pottipuram in Theni district of Tamil Nadu and construction of the InterInstitutional Centre for High Energy Physics (IICHEP) in Madurai.

Neutrinos, as mentioned before, are notoriously difficult to detect in laboratory because of their extremely weak interaction with matter. The background from cosmic rays (which interact much more readily than neutrinos) and natural radioactivity make it almost impossible to detect them on the surface of Earth. This is the reason neutrino observatories are located deep under the Earth’s surface. The key advantage of constructing a laboratory in a cavern in a mountain accessed by a 2-km tunnel is that it offers a low cosmic ray background environment and lowseismic zone (zone 2) which is necessary for specialised experiments.

Geologically, mountains in southern parts of India have compact, dense rock, mostly gneiss whereas the Himalayan region predominantly consists of metamorphic sedimentary rock with pockets of gneiss. A considerable area of mountains in the Tamil Nadu region has mainly the hard rock Charnockite (named after Job Charnock, traditionally regarded as the founder of the city of Kolkata).

Facilities in INO

To detect neutrinos, very large and very sensitive detectors are required. For the INO, it is proposed to construct a large Iron Calorimeter (ICAL) – 132-m long, 26-m wide and 20-m high – consisting of 50,000 tons of magnetised iron plates arranged in stacks with gaps in between, where Resistive Plate Chambers (RPCs) would be inserted as active detectors, the total number of 2m × 2m RPCs being around 29,000. Two smaller caverns will also be used for setting up experiments for neutrino double detector and dark matter. The aim is to make precision measurements of the parameters related to neutrino oscillations. The ICAL at INO will detect and measure atmospheric neutrinos to study their properties, including the mass ordering of the three tiny neutrino masses using matter-enhanced neutrino oscillations.

The ICAL detector can also be used to search for evidence of longrange interactions between neutrinos and matter-dark matter annihilation occurring in the Sun, primordial magnetic monopoles and evidence for or against the anomalous events found by the proton decay detector in Kolar Gold Fields. The INO will also aid the development of detector technology and its varied applications, especially in the areas of medical imaging. The location of the detector in the INO will make it possible to push down to almost 8º N latitude in South India, within proximity to the Equator as compared to other detectors that are at latitudes of 35º N and above. This will provide an added advantage to cover the whole sky and study solar neutrinos passing through the Earth’s core.

The aim of the INO is to make precision measurements of the parameters related to neutrino oscillations. Because of its ability to distinguish between the positive and negative muons, this detector can determine the ordering of the neutrino masses, which is not very well known at present. No other detector either existing or planned may be able to provide an answer in the next 10 years.

The second phase of the proposed plan is to use this detector as the fardetector of a long-base-line (6,000 to 11,500 km) neutrino experiment using the neutrino beam from a neutrino factory in Japan, Europe or the USA. These are neutrinos that will be produced in a future accelerator facility which would be beamed towards the detectors situated in a different part of the Earth. This is feasible because the proposed detector at the INO will be capable of charge identification, which is crucial for this mode of operation.

Participating institutions

As a result of the support from various research institutes, universities and scientific community, a Neutrino Collaboration Group (NCG) has been established for the India-based Neutrino Observatory. A memorandum of understanding (MoU) was signed by the directors of the seven primary participating institutes on 30 August 2002 to enable smooth functioning of the NCG; the institutes are: Tata Institute of Fundamental Research (TIFR), Mumbai; Bhabha Atomic Research Centre (BARC), Mumbai; Institute of Mathematical Sciences (IMSc), Chennai; Saha Institute of Nuclear Physics (SINP), Kolkata; Variable Energy Cyclotron Centre (VECC), Kolkata; HarishChandra Research Institute (HRI), Allahabad and Institute of Physics (IOP), Bhubaneswar. There are thirteen other project participants.

Perceived hazards of the Neutrino Observatory

There are some perceived hazards of the proposed underground observatory claimed by the local community. They fear that the excavation and blasts needed to bore the tunnel in the mountains will endanger the biodiversity of the Western Ghats. Other concerns voiced range from radiation and structural damage to the mountain to the emission of hazardous chemicals. But the fact is that the experiments to be performed in the INO when it comes up will neither produce any radioactivity nor will it emit any radiation. Experts involved in its planning have refuted all these claims as baseless and unfounded. The proposed laboratory is not expected to cause any adverse effect on the environment and biodiversity. There may be a small impact during the initial construction phase which will be minimised after the laboratory is ready. The entire construction will be under the supervision of a specialised team of scientists, engineering crews, geologists, environmentalists and so on.

The INO will have a great impact on the emerging high-energy physics scenario in the country. The NCG has the goal of creating an underground neutrino laboratory with the long-term goal of conducting decisive experiments in neutrino physics as also other experiments that require such a unique underground facility. The INO will serve as a Centre for Excellent Education to train a large number of youngsters in the field of cutting-edge science and technology. People trained at INO will not only participate in Indian projects but will also have the expertise to contribute to other high-energy and nuclear physics projects around the world. Over the long term, the INO is expected to develop into a world-class underground science laboratory straddling many fields like physics, biology, geology and allied engineering fields. So, the INO will provide an extraordinary opportunity to bring state-of-the-art scientific technology and computing to local area without any adverse environmental impact. Apart from educational training, there will be tremendous economic benefits by stimulating growth of the local industries on a broad set of fronts – engineering, electronics, computer sciences, etc. The INO is expected to recover the loss of scientific achievements in the field of neutrino research in India which occurred due to closure of the KGF mines in 1995.