Counting muons amid the ‘shola’ forests
The detectors stand in the open meadow, protected by hard, dark-green shells. Their demeanour resemble Easter Island moai statues, silent and watching. Grazing cows occasionally nose up to them, but the detectors remain unperturbed.
We are a notch higher than the Botanical Gardens, nestled in the rolling blue hills of Ooty, where bisons roam at night. A wooden sign points to an outhouse with a sloping roof: Cosmic Ray Laboratory (CRL), Raj Bhavan, Tata Institute of Fundamental Research (TIFR), Udhagamandalam. There are more detectors in the woods next to the outhouse. Close to the building is a metal platform to mount a telescope and a narrow, square tunnel with tubes.
Outside, the wires jostle with shrubs, near hydrangeas—blue and white now, but they will take on other hues later in the year. It is past noon and the chirping of birds stirs the air.
The outhouse, over a century old, has several rooms. A capacious one at the back, now partitioned, houses a monitoring station stacked with electronics. Logbooks that record data from the detectors line a high shelf.
Happenstance memorabilia is strewn across the building. A photograph of Homi Bhabha, cosmic-ray physicist and father of the Indian nuclear programme, hangs in one. In another lies a photo from “An International Workshop on Very High Energy Gamma-Ray Astronomy”, a typical conference photo with the physicists sitting in pose, dated 1982. Next to the monitoring station are Ooty flower-show trophies.
In the front room, iterations of square, transparent slabs are placed on a table, covered with cloth. These plastic “scintillators” are an integral component of the detectors outside.
The front of the house is taken up by a workshop that forges these plastic slabs. In another time, the workshop maintained a cloud chamber, an early technology used to detect particles. Photos of horizontal lines with white streaks—particle cascades in this cloud chamber—are stuck on a noticeboard.
In the evening, it rains. The sodium lights are on. In that saturated air, the sky hangs low, seemingly close. What the laboratory is detecting are high-energy particles called cosmic rays.
The word “rays” is itself a historical artefact, harking back to a debate between two scientists, Arthur Compton and Robert Millikan. Millikan, who coined the term in the 1920s, thought cosmic rays constituted only photons, packets of light. In fact, their primary components are protons. These highly energetic charged particles bend due to Earth’s magnetic field just as they bend in magnetized cloud chambers.
Most lower energy particles are deflected to the Poles or back into space but the highest energy particles literally fall out of the sky. After the cosmic rays interact with the atmosphere, they shower the earth in a cascade of secondary particles. The slabs on the table are used to trigger tracking by detecting electrons in an air shower. Electrons fall on this surface, which absorbs their energy and re-emits it as light. Then photomultiplier tubes—the tubes in the square tunnel connected to the slabs—amplify the light signal. This light is reconverted into an electronic pulse, which can be analysed to estimate information like the energy of the primary particle.
At the base of the set-up is a muon detector. Muons—secondary particles produced as the primary particle strikes the atmosphere—do not decay into other particles. The primary could be extrasolar material, or even material from another galaxy. This reminds me of the expeditions of Kristian Birkeland, a Norwegian scientist who studied the Northern Lights in the early 1900s. He wanted to find out whether these lights could come right down to the tops of mountains.
The material Birkeland was studying came from the sun, ionizing the northern sky in ghostly hues of green, blue, red. In late 2016, a muon detector at the GRAPES-3 observatory—it grew out of the CRL and lies 7km from Ooty—found a way to correlate the two. That is, material from what could be galaxies outside our own and material coming from the sun as it hits Earth’s magnetic field. In doing so, it used metal pipes with inert gases in them, whose history goes back at least half a century.
In a corner office at the National Institute of Advanced Studies in Bengaluru, B.V. Sreekantan, who was involved in setting up the observatory in Ooty, sits across me in soft black sneakers. At almost 92, his recollections are sharp. As he tells it, Homi Bhabha initially wanted these cosmic-ray experiments to be carried out at the highest point in the Nilgiris, Doddabetta Peak—the word means big mountain in the local language. But the military wanted to base their surveillance operations there.
That was Ooty in the 1950s and 1960s, with a population of maybe 10,000, and known for its tennis matches. It was a British collector in Coimbatore, John Sullivan, who was credited with founding the Ooty colony in the early 1800s. The British had monopolized the area since the original inhabitants, the Todas, a hill tribe.
But the British had left. The governor of Tamil Nadu told Bhabha, why not set up your laboratory in Raj Bhavan, where the basic infrastructure already exists? Right behind what would later become the tourist spot of the Botanical Gardens, a scientific laboratory was set up in 1955.
Sreekantan, who was born near Ooty, connects two separate strands in the history of Indian physics. He was responsible not just for work done at high altitudes like Ooty, but also in the depths of the Kolar Gold Fields (KGF), where the realization that muons could be screened at those depths led to the establishment of a neutrino observatory. When the KGF was flooded in the early 1990s, the 8,000-odd proportional counters—the metal pipes with inert gases—which had been used to detect neutrinos, were transported to Ooty.
At its new location, the GRAPES-3 (short for Gamma Ray Astronomy at Peta Electron Volt (PeV) EnergieS Phase-3) experiment checks these proportional counters for rust, refills them with a mixture of argon and methane gas, rewelds them, passes a current through them and puts them back to work after extensive testing. When a particle passes through these counters, it ionizes the gas and a pulse is triggered.
Later, as I read about the history of Ooty in a tourism pamphlet, I find a parallel. The oldest church in Ooty is St Stephen’s, where Sullivan’s wife and daughters were laid to rest. The teak wood used in the church came from Tipu Sultan’s Lalbagh Palace—demolished by the British sometime in the 19th century—and the massive beams for its construction were hauled up by elephants. There was reuse then as there is with the GRAPES-3 experiment. This was also true of previous experiments in Ooty. Sreekantan, for instance, remembers buying surplus defence equipment wholesale from Chor Bazaar in Mumbai for the CRL.
The equipment would have been used for precision timekeeping for a broad range of experiments. At the time, there wasn’t much light pollution in Ooty, and experiments based on the bluish Cherenkov radiation were carried out at the CRL. These experiments—which could be done only on moonless nights—would require parabolic mirrors. Searchlights that had been used in World War II served the purpose well.
This captures my attention and I ask Sunil Gupta, who heads GRAPES-3, about these experiments.
Cherenkov is very weak radiation, says Gupta. In Delhi or in other cities, the sky will look blue or grey; it is pitch dark in more secluded locations. Yet light is still streaming in from the dark parts of the sky. That is why sophisticated cameras work under the night sky without a flash. It was estimated by the researchers at one point that even on these moonless, cloudless nights, about 100 million photons stream through every second in an area the size of a human fingernail.
Of these, about 10,000 photons may be Cherenkov radiation from cosmic rays. This type of radiation also comes at very short intervals, making precision timekeeping crucial. If the data is recorded to the order of 10 nanoseconds (a nanosecond is a billionth of a second), its signal is enhanced enormously.
When he was carrying out these experiments, Gupta kept his schedule locked to a different day-night phase. He would get up at 4pm and be ready to carry out observations throughout the night. Peering through a telescope, he would see the night sky reflected in its focal plane. Since specific objects were being tracked, the mirrors had to be moved at the same rate at which Earth rotates. This was done by locking on to known stars. By this regular calibration, he came to recognize the night sky in Ooty and could even tell the time by it.
But it turns out that Ooty wasn’t especially well placed for such observations. There were only a few months when it was suitable, factoring in the cloudless, moonless nights. As light pollution increased, the experiments moved away from Ooty and, by the mid-1980s, they reached the current GRAPES-3 site. Eventually, Pachmarhi in Madhya Pradesh became the site for such experiments and, then, the cold desert of Hanle in Ladakh.
As we near the GRAPES-3 site, an expanse dotted with metallic pyramidical structures comes into view. The symmetry of these pointy metallic thingamajigs, arranged on a rolling landscape in a hexagonal geometry, makes them look like mini UFOs.
These are keepers of the same scintillators and pipes as the detectors at the old CRL. The 400 scintillators are spread across an area of 25,000 sq. m, while the proportional counters to detect muons have four stacks across over 500 sq. m.
There is real-time monitoring going on at GRAPES-3. The scintillator array detects air showers, around 35 a second by the electrons coming from them, then a decision is taken whether to record it. With these particles coming close to the speed of light, computers are too slow for the decision, so the experiment relies on rather sophisticated timekeeping to record a shower event.
Atul Jain is the main electronics expert at the site. He has spent close to 20 years there, through the experiment’s different iterations. The fact that Ooty is close to three state borders and has indigenous tribal inhabitants means that there is a natural mix of population and a confluence of people. With GRAPES-3, of course, scientists from across India and other countries are coming to the observatory.
Jain tells me about the work ethic that evolved at the lab where people are trained beyond their specialization and about the need for precision instrumentation—for instance, they build many of the detector components themselves, thereby avoiding dependence on the manufacturers of costly components. Scintillators, for instance, can cost Rs1 lakh per metre.
Jain asks me to imagine looking at the hills from a distance. As the resolution of our viewing field grows, we see that the hills are not green at all, he says, it is the trees. Then we realize that the trees are not fully green, it is the leaves that are green. It is a successive resolution of structure that technology makes possible.
This resolution could be of time too. In 2012, the group at GRAPES-3 published a paper about a microelectronics system built around a chip from the European Organization for Nuclear Research (CERN) in Geneva that could keep time to the order of picoseconds. At that time interval, even light travels only a few millimetres, allowing the direction of the shower to be found as it hits the counters.
In the waning daylight outside the cafeteria lies the workshop where proportional counters from the KGF have been put back into use, and new ones are assembled. A control room in another building is right in the middle of the detectors. In a corner with a low-hanging roof that requires a hard helmet, proportional counters have been silently recording data for 17 years.
B. Srinivasa Rao, who heads the civil and mechanical engineering teams, shows me around. Given that he needs to be outdoors, he wears what looks like a cowboy hat as a shield against the incoming ultraviolet radiation. I tell him that the offices all have rather nice views. He responds wryly that without scenery there can be no science.
All around there is farming—potatoes and tea are grown, and there are eucalyptus trees. The vegetation is called shola locally, deriving probably from Tamil. There is an Indian Council of Agricultural Research (ICAR) institute nearby. Large squirrels, wild rabbits, rats and the occasional wild cat inhabit the area. I learn from Jain that the cameras recorded a wild cat on the move last year—the dim light ensured that it wasn’t clear which animal it was. He shows me a clip of the animal prowling through the detectors.
In June 2015, the muon detector at GRAPES-3 detected a breach in Earth’s magnetic field, which extends tens of thousands of kilometres into space and protects us from ionizing radiation. It then correlated it to what is called a coronal mass ejection (CME).
A CME is essentially a plasma cloud from the sun ejected into space. These ejections can drive severe solar storms, affecting space weather. It turns out that as the CME travelled through the interplanetary magnetic field, the magnetic field of the sun extending across the solar system, it was in a direction opposite to Earth’s. This, in turn, caused Earth’s magnetic field to weaken.
The 2-hour opening allowed a burst of cosmic rays through that was recorded at GRAPES-3 as a surge in muon flux. With more charged particles coming through, the breach was associated with Aurora Borealis sightings and disturbances like radio blackouts.
I recall reading about what is referred to as a Carrington event. In 1859, a solar storm of even greater intensity had knocked out telegraph lines for several hours, with the Aurora visible across the world. Named after a scientist who initially studied the phenomena, it serves as a reminder of what the occurrence of a similar event could mean today—frying VLSI circuits on Earth and in space, and shorting high-power transmission lines as the air becomes ionized.
In the pleasant weather of Ooty it is hard to think of such catastrophic events, of violent solar storms and worldwide power disruptions. Leaving on a bus at night, I can see only the mist and fractal branches of tall trees silhouetted in the moonlight.