This telescope in Ooty is over the muon

Udhagamandalam, in the scenic Nilgiris, is all decked up and bursting with tourists ahead of the 126th edition of the annual flower show in May at the Botanical Gardens. Enchanting as the sight is, floral beauty is farthest from the minds of the astroparticle physicists at the 70-year-old Cosmic Ray Laboratory (CRL), a segment of which lies just beyond the Gardens. Their sights are instead trained on what for them is just as beauteous: cosmic rays, or clusters of particles that move through space at very high energies. The ongoing experiment at the laboratory — to study cosmic rays at an ‘intermediate’ energy range — is its primary area of focus now. “It is a one-of-a-kind experiment,” asserts Fahim Varsi, the first author of a recent paper on the experiment in Physical Review Letters (bit.ly/muon-grapes). The lab itself is a tribute to scientific instrumentation, which typically does not get much favourable attention. 

The experiment goes by the fruity acronym GRAPES-3, which stands for Gamma Ray Astronomy PeV EnergieS – Phase-3. PeV, or petaelectronvolt, is a unit of energy used in particle physics (1 PeV is equivalent to the energy of a housefly moving at 20 kmph). As part of the experiment, the scientists have built up almost entirely in the lab a muon telescope — the most sensitive one on Earth — and an array of 400 scintillator detectors to study cosmic rays at 1-100 PeV. The experiment is a collaborative effort with Japanese researchers. Within a year, GRAPES-3 is expected to double the number of scintillation detectors and double the size of the muon telescope. 

A field station of the Tata Institute of Fundamental Research (TIFR) Mumbai, the lab operates from two sites: the primary one, proximate to the Gardens in Udhagamandalam or Ooty, and the other at Tamil Nadu’s Melkavatty village, some 8 km (and a bone-rattling drive) away. The Ooty centre houses the second phase of the experiment, currently in operation but being reworked, and the facility for casting and making the plastic scintillator detectors. 

Little is known about the subject of research at the CRL, namely, the cause, composition and origins of cosmic rays. These are showers of particles that rain down upon the Earth from the cosmos. They mostly consist of charged particles, about 90% of which are nuclei of hydrogen or protons. Nuclei of heavier atoms — ranging from helium to iron — make up the rest, along with uncharged particles such as neutrinos and gamma rays. Gamma rays are particles of short-wavelength, high-energy light. All of these carry clues to the question of the cosmic rays’ origins.

As the cosmic rays travel towards the Earth, they collide with particles in the atmosphere and break down into secondary showers of particles, including muons — heavier cousins of the electron — and more gamma rays. The first step towards unravelling the mysteries of the cosmic rays is to measure their energy and identify the type of particle that gave rise to the muon-electromagnetic shower. This is done in the CRL by studying the muons, electrons and gamma rays in the secondary showers, and from their number and energies, working back to the energies and constituents of the parent, or primary, cosmic rays. And helping them in this are scintillator detectors on the sprawling grounds of the Melkavatty centre.

At first sight, from a distance, the 400 white tetrahedral covers of the detectors appear like headstones in a war memorial or cemetery. To a bird’s eye, the detectors would show a hexagonal arrangement, with the control room at the centre and the muon telescope in a roofed enclosure on the side. In the control room, the health of the detectors is monitored continuously, supervised by Scientific Officer B. Rajesh. “Studying cosmic rays is like studying the machinery of the engine that drives the universe,” says Sunil Gupta, a Raja Ramanna Fellow at TIFR Mumbai and one of the architects of GRAPES-2 and GRAPES-3 telescopes. 

Science rubs shoulders with wildlife at this remote centre. Bison and bears often drop by at night, and a leopard has been caught on camera. A bear, possibly intrigued by the elaborate set-up, once pulled out the wires carrying the output. “It dragged the wires several hundred metres away,” says B. Hariharan, a scientist with TIFR Mumbai and a resident member at the CRL.

Amid this wilderness, the lab functions — facing and resolving bigger challenges. For instance, when the facility was being constructed, the administration had planned to buy scintillator detectors from abroad. But after the Pokhran nuclear tests of 1998 — the experiment was then in its second phase — the scientists found that countries were reluctant to supply the detectors as the equipment had a strategic value. “We therefore built up the detectors ourselves,” Gupta says. 

The resolve to indigenise was born of compulsion: the plastic detectors are crucial to the experiments. When the particles in the cosmic ray shower hit the detector slabs, which have been infused with two chemicals known as dopants, they produce ultraviolet scintillations, which are converted by the dopants into blue light. Lengths of wavelength-shifting fibre, strung through the plastic scintillator detector slabs, absorb the blue photons and convert them into green light. This is used to trigger a photomultiplier, and light is then converted to electric current using a photo-electric effect. This current is conveyed, with the appropriate detector number and time-stamp, to the control room, where the data is analysed or archived for later.

The square, one-metre scintillators, made of transparent and smooth sheets carrying a tinge of blue, were not always this perfect. At the time it was decided that the scintillators would be made in-house as part of GRAPES-3, Gupta was steering the project as the Principal Investigator. The initial slabs were yellowish and had air bubbles, which hindered the detection of particles. The scientists zeroed in on raw polystyrene material imported from Japan to make the plastic slabs. Initially, they were not aware of what dopants had to be used for a smooth and efficient conversion from ultraviolet to blue and then to green light. Gupta made over 70 attempts before they could figure out the casting technique and the right process.

In parallel to the scintillator array, the muon telescope does its work — registering the number of muons per second incident on the site. Located in square, four-roofed enclosures spread over 560 square metres, this telescope has been made of 3,712 proportional counters. These were inherited from the Kolar Gold Fields neutrino experiment (conducted to detect atmospheric neutrinos in the 1980s). For the new muon telescope, these were manufactured in-house. The entire front-end electronics system of this massive structure was also built in-house over a decade by a team led by engineer Kaviti Ramesh of CRL (TIFR, Mumbai).

While the gamma ray and charged particle counts give an estimate of the strength of the incident cosmic ray shower, the muon readings reveal the composition of the particles hitting the proportional counters in the telescope. "It is a complex process, but at the end of it you can measure the energy of the primary cosmic rays and the mass and composition of the particles that make it up," says Pravata K. Mohanty, one of the Principal Investigators of the project.

Maintaining and calibrating the scintillators is no mean task. Though everyone chips in, it is the primary responsibility of D.B. Arjunan, who joined the team 33 years ago. Every unit takes an hour to calibrate, and he checks about ten scintillators a day. Engineer Kaviti Ramesh of CRL (TIFR, Mumbai) reckons that he may have calibrated 2-3 lakh detectors so far. Arjunan is due to retire soon, as are many of the senior leaders of the lab. 

One of the reasons why the researchers call the muon telescope the most sensitive one on Earth is that its size and the number of proportional counters enable it to detect a huge number of muons in unit time. The Japanese team collaborating with CRL pointed out to the Indian scientists that in an hour the telescope detects about 100 million muons, Gupta says. "This gives us a resolution of about 0.01%. No device on Earth can match this," he says.

The telescope has several uses and applications. For instance, changes in the density of the atmosphere due to thermal tides, which are correlated with the Sun, may be accurately measured. The magnetic field induced in the interplanetary space by solar wind can cause a change in the muon stream flowing into the detector. According to Gupta, this change in the solar magnetic field can be measured to a high resolution of about 0.1 nanotesla or even higher. The sungazer spacecraft Aditya-L1, launched by the Indian Space Research Organisation in 2023 (Sungazer Aditya-L1 set for takeoff), also has a magnetometer that can measure the magnetic field of the solar wind with a resolution of 0.1 nanotesla. "Aditya-L1 of course measures it locally (in space), whereas we measure it from the ground with an accuracy of 0.1 nanotesla or even better," says Gupta. He adds that their Japanese colleagues helped measure the characteristics of the interplanetary medium, the solar wind and space weather.

"It is a very versatile detector," Gupta says.

In the paper published in Physical Review Letters in January 2024, Varsi and the other researchers at GRAPES-3 described their observation of a small blip in the proton spectrum of cosmic rays, which had not been seen earlier. After ensuring that this blip was not an artefact of the experiment, the researchers reckoned it could portend the existence of supernovae close to the Earth, spewing hydrogen nuclei.

The experiments have drawn the attention of scientists elsewhere. Hongbo Hu, a senior scientist at the Chinese LHAASO cosmic ray experiment, describes the recent result from CRL as "very important". For one, he says, it connects the results between space- and ground-based experiments with a high precision, which would shed light on future high-precision spectra measurements from ground-based cosmic ray experiments. Second, the discovery of the kink of the proton spectrum provides useful information about the origin, acceleration and propagation of cosmic rays in the Milky Way. "To understand the major sources of the proton below and above the kink will be an important work for future experiments," he says. 

The LHAASO experiment observes gamma rays in the 100 GeV to TeV range (1 GeV equals a billion electronvolts and one TeV, a trillion) and now observes cosmic rays in the TeV-EeV range, which includes the GRAPES-3 range. That observatory, in Sichuan province in China, is higher up, at 4,400 metres above sea level. It is one of the newer facilities that could rival GRAPES-3’s sensitive observations and also verify the results.

Hu’s sentiment is reflected by Pratik Majumdar, who is with the Kolkata-based Saha Institute of Nuclear Physics and is associated with the cosmic ray experiment MAGIC in La Palma, Canary Islands, Spain. "This is still an open question but the Ooty results are very important as they throw up a new area of investigation, and they pose more questions… which I am sure will be crucial in understanding the origin of cosmic rays."

GRAPES-3 is working on doubling the number of scintillation detectors and the size of the muon telescope. The detectors and counters are in place and, when launched within a year, the web they cast on the cosmic rays will almost double in size. Ramesh and Mohamed Rameez, Principal Investigator of the facility, are also planning to make and supply detectors to the Akeno muon telescope in Japan. 

Rameez also seeks to develop the scintillation detectors further and make them sensitive to fast neutrons. These are emitted by fissile waste material. Having sensitive scintillation detectors may help assess the nature and quality of nuclear waste. These can also be used in screening cargo on aeroplanes and shipments for even the tiniest quantities of fissile material being transported. And the credit for having made these sensitive instruments will go to scientists who have borne the baton for decades, and then passed it on.

Decades of effort — GRAPES-1 was from 1965 to 1985; GRAPES-2 from 1986 onwards; and GRAPES-3 from 2001 — have shaped the facility into a powerful tool. The recent observations about the proton spectrum could lead to revamping old models of how these particles are produced. When the reinforcements — namely, the doubling of scintillation detectors and the proportional counters making up the muon telescope — are operative, scientists will have an even sharper eye on the cosmic rays, those nomads from elsewhere in the cosmos.