Astrophysics in polar ice

Published : Jun 03, 2005 00:00 IST

The Scott-Amundsen South Pole Research Station, a geodesic dome. -

The Scott-Amundsen South Pole Research Station, a geodesic dome. -

An account of a battle against the elements in the South Pole to set up a telescope that looks for neutrinos coming from distant objects in the northern hemisphere.

THE thermometer in the geodesic dome at the South Pole Research Station read -43oCelsius without the wind chill when we left in the morning. There was not much wind - both outside and within me - when we trekked the kilometre from the dome to the experiment site. It was bright and sunny as it has been for the past 10 days and nights that I have been here.

It is around 2-30 p.m. now. The sun is still shining fiercely - just looks but no substance, and the wind has punched in. The only warmth is within a tubular black structure that houses the paraphernalia we need to do this experiment - electronic equipment, computers, things we shall need for the next 10 hours or so. Just a little while ago, we were huddled in there, all 10 of us on the deployment team, going over our responsibilities, checking for last-minute snafus and fuelling up with sandwiches and other food. But, most of all, we were - at least I was - trying to pack all the heat I could, literally, because for the next 10 hours or so we were to experience the outdoors in the icy desert that is the South Pole.

It is time to get to work. Four layers of thermal wear, fleece and wind-resistant polypropylene surround each one of us. Hooded woollen caps cover all but the tips of our noses. Our footwear is over-sized winter shoes called "bunny boots" so big that my feet within had to take several more steps than the shoe to catch up. Several layers of gloves protect the hands, making it extremely difficult to grasp at anything. All in all, we are trussed like turkeys ready for battle.

We are here battling the elements, all for science - astrophysics. We are building the first stage of a new kind of telescope that looks down through the earth instead of up at the sky. The traditional telescope looks up into the sky collecting light from heavenly objects, whereas our telescope looks for elusive subatomic particles called neutrinos from distant objects in the northern hemisphere. Neutrinos are highly anti-social by nature. To make them stop and leave a signature of their presence, we need to throw a lot of matter in their path. So, we wait at the bottom of the earth looking for neutrinos coming from the top. The idea is to look for flashes of light given off by particles produced when a flying neutrino smashes into molecules of ice or of the earth. So, we melt holes in the South Pole ice and bury sensors in them, sensors that look for these light flashes.

Each of us has a station to work. Three people stand around the hole in the ice that is large enough to swallow a small person to depths of a kilometre within the ice. They handle the sensors, glass spheres that are the heart of this experiment, carefully clamping each sphere with carabiners (the kind used in mountain climbing) to the main line/cable that lowers them to their icy abodes. One hemisphere of these basketball-sized spheres is painted black to make it blind to the light flashes. The other hemisphere with its greenish tinge and smooth surface reflecting everything around looks like a giant glass eye. To shield these from the stinging wind that is blowing, a makeshift shelter using some metal sheets is built around them.

Each sphere has small metal appendages on its sides. Each time the glass eye sees light flashes in the ice, an electrical signal is sent to the surface through the appendages using specially designed cables that are part of the main line. Connecting the cable to the appendage is a tricky operation - one that is done best with bare hands. A much-needed electric heater rattles away near by.

Some five metres away, a Swedish collaborator, Perolof Hulth, mans the winch that controls the cable carrying the glass spheres. He sits on a spool, his hands on the controls, eyes on the folks at the hole and with a space heater near his feet. Icicles have started to form at the ends of his whiskers.

Diagonally across from him, about six metres away, is my station. My job is to control the spool carrying the fibre optical cable that goes down alongside the main cable into the hole. A laser feeds green light into the cable at the surface. This light is faithfully transported down the hole and fed to tiny plastic balls placed at 10-metre intervals along the cable. The green glow that these balls radiate is picked up by the light sensors - the ones closer to the glowing ball going off sooner than those farther away. Apart from being a means of testing our apparatus, it helps us check the positions of the sensors once they are buried in the ice.

The plastic balls are flimsy and the smallest of tugs on the fibre optical cable could snap them. My responsibility was to keep this from happening - a simple job made trickier given the loss of dexterity of my hands, thanks to the oversized gloves and mitts. A kind colleague has provided me with a makeshift shelter - a wooden packaging crate about two metres long and more than half a metre wide, placed on its side and with packaging foam at the bottom to keep the feet warm. Yet, it was very hard to keep from questioning the sanity of it all.

Yes, the science was sound.

We made this trip to the South Pole to do a unique kind of astronomy. To study exotic locales in the universe such as exploding stars and black holes using something other than light - neutrinos.

Light, or photons to be technically correct, has been used by astronomers for years, according to Robert Morse, a scientist at the University of Wisconsin, Madison. "One of the reasons photons are so valuable is that they are neutral. They can travel in straight lines and don't get bent by the magnetic fields in the intervening space. And the other thing is that they are stable in that they do not decay on the way," Morse says. These characteristics are important in messenger particles that can bring us information from very distant objects.

Charged particles would be swayed by intergalactic magnetic fields. So, if and when they do reach us, they could not point back to their source. Another subatomic particle - neutron - would have a better sense of direction since it has no electric charge, but it is unstable and decays spontaneously into other particles.

"When you think of available messenger particles, then, the list is incredibly short," says Morse, "it is really only the photon and the neutrino."

"The neutrino is a tiny little subatomic particle that is charge-less and has very little mass," explains Rellen Hardtke, a graduate student in physics at the University of Wisconsin, Madison. "It is the accountant's particle because it was created out of a need," adds Morse.

Back in the 1930s, when they were studying the decay of a neutron, the energy of the initial neutron did not add up to the energy of the final constituents - the proton and the electron. This violated one of the most basic tenets of physical science, the law of conservation of energy. The energy you put in should be equal to the energy you get out. When famous scientists such as Niels Bohr were ready to abandon this law, Wolfgang Pauli came up with an alternative. "Basically what he said is: This is what we think we should have. This is what we've got. He subtracted the two and everything that was missing was just glommed on to this particle called the neutrino," says Morse, "and I guess it got its name, rather whimsically, from Enrico Fermi. Neutrino means the little neutral one."

The quality of the neutrino that makes it valuable as a probe into the far reaches of space is not just its electrical neutrality but its extreme anti-social nature. "It is the most anti-social particle that we know of," says Morse, "and that's its strength and also its curse." "Because of this nature of neutrinos, not only they can reach us from the edge of the universe, but also they can reach us from near black holes," says Francis Halzen. He is a Professor of Physics at the University of Wisconsin, Madison and the Director of the University's Institute for Elementary Particle Physics Research.

Though the photon is used extensively as a messenger in astronomy, it is not the most versatile. "One of the bad things about the photon is that it is so gregarious and it likes to talk to everybody along the way," says Morse. Especially high-energy photons - from exotic sources such as black holes, pulsars and neutron stars - get absorbed by other radiation. This is not a limitation for reclusive neutrinos. They are so small and slippery that they go right through almost everything. Billions of millions of neutrinos pass through our bodies every second with very little chance of stopping.

And so here we are in one of the most inhospitable places on earth looking for one of the most anti-social particles in the universe. Looking around, all I see is this expanse of white. Reflecting sunlight makes it bright, unyielding and blinding white. Blinded we would very well be if not for glasses specially coated to prevent all forms of ultraviolet light from entering the eye. There seem to be just two colours here - the white of the snow cover and our red "purkas" as we work at our stations. The fifth sensor is making its way slowly down the hole. As I watch and wait, there are two thoughts in my head - "Boy! Is it cold?" and "Does this make sense? Will neutrinos stop to chat with our sensors?"

"So, what makes you think the neutrino is going to stop in your detector," asked a lady in the audience when Morse was giving a public lecture once. "The matter of fact is that most of them don't," says Morse. To be stopped, the neutrino has to make a direct hit on a proton (another subatomic particle present in all material) and this is a highly unlikely event. "Roughly, probably one in a billion," adds Morse. This elusiveness did not escape Pauli when he postulated the existence of the neutrino. He is known to have said: "I have done a terrible thing. I have postulated a particle that cannot be detected." Leon Lederman, a Nobel Prize winner has said: "A particle that reacted with nothing could never be detected. It would be fiction. The neutrino is just barely a fact."

A big help is the fact that as their energy increases, neutrinos become less reticent. Relatively speaking, of course. This inability to communicate their presence very well makes it necessary to cast a wider net, so to speak, to catch neutrinos. "Because they rarely interact with matter, you need a very big detector," explains Hardtke. To build a detector of such a size would be prohibitively expensive and, therefore, "neutrino astronomers" use naturally occurring ones such as deep oceans or the Antarctic ice.

The entire earth is offered as a target to the incoming neutrino to better its chances of a direct collision. What we are looking for is neutrinos coming from the northern hemisphere and flying out of the South Pole.

On the rare occasion that they score a direct hit neutrinos smash into the protons of the earth or the ice and produce a daughter particle called the muon. "The collision is like a billiard-ball collision - a head-on collision where the neutrino stops, passes out of existence and the muon zooms off in the same direction. This is just like when a cue ball hits a billiard ball and the billiard ball moves in exactly the same direction taking away all the energy," explains Morse. The muon, which is nothing but an over-weight electron, is a charged particle that is easy to detect.

When the muon races through ice, water or some other transparent medium at speeds greater than the speed of light in that medium, it generates a blue light called "Cerenkov light". This is the same eerie glow seen in the water covering a nuclear reactor. "This Cerenkov light produced by the muon is exactly like the sonic boom shock wave produced by a supersonic aircraft or, for that matter, like the bow wave of a fast boat. When the boat is travelling faster than water waves, we see the characteristic bow wave," explains Morse. This bow wave of light created by the muon can be detected by light-sensing detectors in the medium.

The idea, therefore, is to create a grid of light-sensing detectors in water or ice. "The blue Cerenkov light produced by a passing muon sets each of these detectors off - flip, flip, flip... and the order and time in which detectors are hit in this three-dimensional array of sensors creates a path through the ice which we can reconstruct from our data," says Hardtke.

"Imagine a smooth lake in the morning, like Lake Mendota (in Madison, Wisconsin). I've got a bunch of buoys over the water. They are sitting there motionless and a boat passes, though kicking up this bow wave and when the bow wave hits the buoys, they start to move. If I had a little stopwatch in every one of these buoys, I knew exactly when they started to move, I would be able to reconstruct the bow wave and, therefore, the direction of the boat. This is exactly how the experiment works," explains Morse.

The idea sure is sound and I must have bought into it pretty thoroughly. All the way from southern India (Chennai), where the seasons are described as hot, hotter, and hottest, to Madison, Wisconsin, to the South Pole - I must have believed in the science. But standing inside this crate, shuffling from one foot to the other to keep from losing all feeling in my feet, trying to decide between keeping my nose uncovered or my sun glasses glazed with condensation, it sure feels like the deep ocean would have been a better choice for this experiment. That would have meant going to Hawaii or to Greece.

Neutrino telescopes actually started in the deep ocean. The Deep Underwater Muon and Neutrino Detector (DUMAND) was designed to detect neutrinos in four-kilometre-deep ocean water off the coast of Hawaii. Twenty years after that, the idea of doing neutrino astronomy in ice came up. Quite accidentally, according to Halzen. He was speaking at a colloquium in Kansas and a glaciologist in the audience, Ben Zeller, asked him: "Why don't you use ice?" To Halzen's immediate reply that ice was "all bubbly and dirty", Zeller said: "No, it's not. If you go deep enough, it's clear."

This set Halzen thinking. Ice seemed to have distinct advantages over water. Though the South Pole is hard to get to, it is cheaper than a ship. The ice is also a stable platform all through the year. The infrastructure already existed. There was the Scott-Amundsen South Pole Station, a geodesic dome within which are slightly raised, orange, heated buildings housing people, computers, communications facilities, a hospital, and a gymnasium and a galley.

Cargo planes on skis flew in supplies and people. Heated "Jamesways" or black, cylindrical tent-like structures provided shelter for people to live in and work. (Blue, heat-efficient buildings on stilts, affectionately called "beaker boxes", have now replaced these). They already had a galley that could feed 100 people at a time with supplies flown in from New Zealand. Heavy machinery and crew for any construction work were available too. Satellite communication made it easy to send data and bridge the distance. A call from the South Pole would be as simple and clear as placing a call from Florida. Yes, the South Pole seemed to be a viable option.

The initial reaction of folks at the National Science Foundation whose purse they were hoping to tap was not very encouraging. But a glaciologist working on the Greenland Ice Coring Project offered a hole in the ice to test sensors in. Morse and a fellow researcher, Tim Miller, were flown off to Greenland to put light sensors in these borrowed holes to test the feasibility of using ice to detect muons. The holes were 250 m deep and the sensors were scrounged from another experiment. But, lo and behold - the idea worked. Despite the not-very-clear, shallow ice, the sensors could detect muons. However, this was not the only positive from the trip to Greenland.

This was when Morse met Bruce Koci, the future driller for AMANDA, the Antarctic Muon and Neutrino Detector Array. Koci, an aerospace engineer who designed the air speed systems for Boeing 747s and 737s, had switched careers in the 1970s to drill holes in ice. He had designed and worked on thermal drills, solar-powered drills and hot-water drills in his career as a driller.

The specs were straightforward - deep holes in the ice (at least 1,000 m deep), as straight as possible, about 50 - 60 cm wide. It had to be hot water drilling, according to Koci. "It is the only way to drill straight holes. If we drill a hole two feet (60 cm) in diameter and 8,000 feet (2,400 m) deep and drop a marble from above, it will fall straight down and never touch the sides of the hole," says Koci. "With a mechanical drill, we could probably get within 0.25 degrees or so - that's tens of metres off," he adds. Hot water drilling, moreover, would be the fastest.

The hole we are working on today was drilled by a jet of hot water, about an eighth of an inch wide and gushing at about 60 miles (96 km) an hour. Since the water just follows gravity, the hole should be plumb straight. It took slightly more than three days to drill and here we are - deploying the sensors. Working for over 10 hours or so, it is a relief to see the last of the 20 sensors go down the hole.

Now, we wait for the water to re-freeze around the sensors.

As the water changes into ice, it expands and so the pressures in the hole, if you are under a kilometre of ice, can increase to about 8,000 pounds per square inch (562.5 kg/cm2). For an analogy - If a woman wearing high heels were to step on your toes, the pressure would probably be about a 1,000 pounds per square inch (70.3 kg/cm2). The light sensors are essentially glass spheres and they could crack under these pressures. So, they are housed in special vessels called benthos spheres or pressure spheres that are 1.5-inch thick (3.8 cm) glass and tested to 10,000 pounds per square inch (703.1 kg/cm2).

It takes about three days for the watery grave to turn into ice. Through this time, the pressures and temperatures at several points in the hole are monitored using special sensors that have been sent down the hole. We wait anxiously to turn the light sensors on. It is a test of technology, science and theory.

Power on. Everything is silent. The sensors seem to be alive and well. Koci and his team start to work on the next hole. Since the station closes in the middle of February, four holes are all we get this season. Four holes, at depths between 800 m and 1 km in the ice, with 20 light sensors in each hole, 10 m apart.

It is time to get the laser going. It will tell us if the sensors are working the way they should and if the ice is behaving as the glaciologists said it would. We are in for a surprise.

We are seeing two things we did not expect. Laser light from the plastic balls we put into the ice is being seen by the optical modules much later than expected. Also, the light seems to last much longer than expected.

"A light source 100 nanoseconds (a nanosecond is a billionth of a second) away was not showing `first light' till 600 or 700 nanoseconds later. The next thing, which was even more surprising, was that late light was two to three microseconds late (a microsecond is a millionth of a second), which meant that Jesus Christ! That light lasted a long time. That meant the ice was loaded with bubbles, but there was no absorption (of light)," explains Morse.

Within 12 hours of data taking, we knew we were in trouble with AMANDA - AMANDA-A was smack in the middle of bubbly ice. The compacting of snow that fell several thousand years ago formed this ice. Air pockets trapped by the falling snow made the bubbles we were sitting in and the glaciologists were wrong about several things. There were more bubbles than they had predicted and they were bigger in size. Also, the Antarctic ice was more transparent than anyone expected.

Sitting at the South Pole, preparing for station closing, all this was a little too hard to digest. What this meant and how we could work around it were things left for warmer weather and lower altitudes. I was leaving in a couple of days and the others were to follow a few days later on the last flight. After thawing for a few days in New Zealand, we were headed for home.

This was the 1993 - 1994 season.

We spent the year following this expedition on analysing the data from the four strings we buried in the ice. We seemed to be doing more glaciology than astrophysics - but finally, we had our answers. The bubbles were five times larger than glaciologists had said they would be, but the ice could transmit a streak of blue light as far as 100 m, not just the 8 m that they had predicted.

AMANDA had to go deeper to avoid the bubbles but our telescope could really see far.

By the 1999 - 2000 season AMANDA II was completed and the telescope had grown. Apart from the 80 sensors we deployed on four strings, there are now over 700 light sensors on 19 strings at depths over 1,500m in the South Pole ice. The sensors are arranged in a cylinder 1,000 m tall and 200 m in diameter.

Scanning through the data obtained so far, Hardtke and other members of the collaboration have identified 3,000 bona fide neutrino events. However, they do not seem to be coming from any single hot-spot in the sky. The search is on for hot-spots in the neutrino sky - point sources like black holes, neutron stars and so on.

"There is no doubt that our main result (with AMANDA II) is that we have discovered (discovered is the word, we did not quite anticipate the way it would work - we were lucky) a way to build the ultimate kilometre-scale neutrino telescope that is required to do the science," says Halzen.

The future?

"Large as it is, AMANDA II is still a prototype capable of seeing only the brighter and closer sources of neutrinos. Just as in astronomy, bigger helps you see farther," writes Halzen.

Work is on to instrument a cubic kilometre of the South Pole ice with over 5,000 optical sensors. This, aptly, is called the "IceCube" detector.

"We should reach the kilometre scale in 2006-07 and the detector will be completed by 2010," says Halzen. He is convinced that "IceCube" will do everything neutrino astronomy promises. Morse is still optimistic about results yet to come from AMANDA II.

"Oh! We will see point sources with AMANDA. I hope so. There is no substitute for IceCube. But, time is on our side and if we should see something, it could be the next major detection. That could be AMANDA's legacy - the first non-zero result. IceCube might just increase the accuracy," says Morse.

According to Halzen, whenever astronomers set their sights on newer wavelengths of light, lucky discoveries were made. "Nature was more imaginative than the people who probed it. If after 20 years of working on AMANDA we only discover what we set out to discover, it will be, in many ways, the most disappointing result," he writes.

Serendipity apart, an icy graveyard deep down under might well be the site where nature reveals its next big secret.

Dr. Vijaya Swaminath, an astrophysicist by training, was part of the research being done at the South Pole. This article is based on her doctoral dissertation on neutrino astrophysics.

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