IceCube

“The only true voyage of discovery … would not be to travel to new lands, but to possess other eyes.”
                                                                                                                 Marcel Proust

I was fortunate to have a particularly close involvement with one of the most extraordinary scientific instruments at the Pole, or for that matter on the planet, the IceCube Neutrino Observatory. 

Unlike the other astrophysical observatories at the Pole, all of IceCube's vital detection devices are invisible and inaccessible. In fact, they are buried up to 2.5kms deep in the ice of the Polar plateau. As I've mentioned, the Pole is quite high in altitude (9,300ft/2,800m), but the extraordinary thing is that below the surface, the ice extends all the way down to sea level, almost 3km down. The other extraordinary thing about the ice is that it's incredibly clear - clearer than crystal glass, clearer than distilled water, in fact, it's the most transparent solid known. Below 1.5kms down the pressure is so great that the trapped air bubbles that give ice its milky opacity dissolve into the crystal matrix of the ice and the transparency increases dramatically. This is the main reason the Pole was chosen for the Neutrino Observatory - but to explain that more fully I'll have to cover some basics.

Neutrinos 101

This is a blog, not a physics textbook, so I'll try to be brief. Neutrinos are elementary subatomic particles that interact with other matter only through the weak subatomic force which is responsible for radioactive decay, and which plays an essential role in nuclear fission. The weak force is one of the four forces of the "standard model" along with gravity, electromagnetism, and the strong force (which binds quarks together to form protons and neutrons). The weak force is so named because it is orders of magnitude weaker than either the electromagnetic or strong force and operates at very close range only.

Neutrinos have no charge (and so are not affected by the electromagnetic force) and have minuscule mass (and so are practically immune to gravity). Consequently, they have to essentially collide with another subatomic particle to have any effect on it. This results in the wonderful (and frustrating) property that they hardly ever interact with other matter at all. They are created through radioactive decay and other subatomic interactions (weak interactions) and are created in huge quantities in the sun, the atmosphere and core of the earth, and in high energy cosmic events such as novae, super-novae and gamma ray bursts. They are in fact the commonest particle in the universe (there are a billion for every atom of matter), but also one of the least interactive and so are surprisingly difficult to detect. Some 60 billion of them pass through each of your fingernails every second - none of them having a single interaction on the way. In all probability, in your lifetime only one of those billions and billions of neutrinos passing through your body will interact with an atomic nucleus - smashing it apart, but in all likelihood not causing any noticeable effect. 

Ray Jayawardhana in his fascinating book "Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe" says, "Neutrinos travel right through the Earth unhindered, like bullets cutting through fog. Besides, the Earth’s bowels generate neutrinos, as radioactive elements decay, and so do collisions of energetic particles from space in the upper levels of the atmosphere. Cataclysmic deaths of massive stars set off tremendous bursts of neutrinos, which escape these sites of mayhem unscathed and bring us news of awesome celestial events millions of light-years away. Moreover, our planet is immersed in a sea of cosmic neutrinos, which sprang forth when the infant universe was barely two seconds old."

The only way they can be detected is to have a huge detector so that the exceedingly rare probabilistic event that they will interact with a proton will happen often enough for us to be able to record them. This is where the South Polar ice cap comes in. The IceCube array occupies a cubic kilometer of ice at the Pole. Such a huge volume of ice ( 1 million cubic meters) captures several hundred neutrino interactions every day. 

The Array

I've already mentioned that the IceCube Neutrino array occupies a cubic kilometer of ice at the Pole. Specifically, the array consists of 5,160 photo-sensitive "Digital Optical Modules" or DOMs arrayed on 86 vertical "strings" that have been drilled into the ice. The strings are in a hexagonal grid spaced 125 meters apart and each string holds 60 DOMs spaced 17 meters apart. The logistics involved in creating the array are impressive. Between 2003 and 2011 close to 5 million pound of cargo were shipped from all over the world to the South Pole, requiring 181 LC-130 flights. That's a remarkable feat in itself but just add in the paperwork, the freight handling to get everything to Christchurch before the LC-130's are even loaded, and then the fuel and man-hours to make it all happen! Each bore hole required two days of constant drilling, and 4,800 gallons of gasoline to create, melting 200,000 gallons of ice in the process. At its completion in 2010, the Observatory had cost US$279 million.

 The IceCube Lab. My home base at the Pole.

The IceCube Lab. My home base at the Pole.

 The "Fern Drill" used to drill through the upper layers of compacted snow - hot water is pumped through the copper tube to melt through the ice as the drill is slowly lowered. It's a sculptural work all on it's own.

The "Fern Drill" used to drill through the upper layers of compacted snow - hot water is pumped through the copper tube to melt through the ice as the drill is slowly lowered. It's a sculptural work all on it's own.

 Checking on each DOM and its connection as it's lowered into the ice.

Checking on each DOM and its connection as it's lowered into the ice.

 DOM (063A - Golden) being lowered into the ice.

DOM (063A - Golden) being lowered into the ice.

 An artist's impression of the DOM array under the ice. If you could light it up, the ice really would be that clear.

An artist's impression of the DOM array under the ice. If you could light it up, the ice really would be that clear.

 Diagram of the complete IceCube array.

Diagram of the complete IceCube array.

And all of this is now frozen into the ice. Once the DOMs are lowered on their cables, the holes are filled with water and refrozen. All that appears above the surface is the thick braid of cables which are routed across the ice surface and then into the main building of the IceCube Lab.

Here the cables get divided up and distributed to banks of processors that do the initial processing, event reconstruction and filtering of the data to see if any recorded event is worth looking at in greater detail. These events are then uploaded to the IceCube servers and then transmitted during the short daily period of satellite connection to the scores of scientists across the world who are taking part in the collaboration at any one time. Hard Drives recording all of the events and more detailed data are shipped back to the US every few months.

 So the red cable goes......

So the red cable goes......

The Signal

So what do the scientists 'see'? Or more directly, what do the DOM's capture? 

When neutrinos or other subatomic particles collide with protons in the hydrogen or oxygen nuclei in the ice, they scatter and result in the emission of a charged lepton (an electron, or a muon (a sort of heavy electron) or a tau). If this collision is high energy, the charged particle can fly off at close to the speed of light, in fact it may move faster than a photon can travel in ice and in so doing creates a tiny blue flash of Cherenkov radiation which has been likened to the light equivalent of a sonic boom. The DOMs can detect a single photon of light and so when this tiny flash occurs multiple DOMs register its passing and so can map its intensity and direction. 

Unfortunately neutrinos aren't the only source of these charged particles. They can also be generated by cosmic ray (subatomic particle) cascades in the atmosphere or ice and through radioactive decay in the earth. IceCube is located under 1.5km of ice in part to filter the incoming cosmic rays. The genius of the IceCube Observatory though is that its DOMs actually look down, not up. If any particle manages to get through the earth it must be a neutrino - anything else would have been absorbed along the way. The IceCube observatory is a gigantic neutrino telescope which uses a cubic kilometer of ice as its primary mirror and the entire planet as a filter.

Any charged particle created by a neutrino follows the same path as the original neutrino. So any charged particle path revealed through Cherenkov radiation that shoots upward through the ice as seen by the DOMs must have been generated by a neutrino following the same path.  The neutrino may have arisen though cosmic ray interactions on the other side of the planet, but if it is very high energy it probably  has an astronomical source. These high energy neutrinos are what IceCube was primarily designed to detect.

The processors in the IceCube Lab receive huge amounts of data from charged particles coming from other directions too. This data gives scientists insights into the solar wind and other sources of cosmic rays. IceCube registers more than 100 billion muons per year, produced by the interaction of cosmic rays in the Earth’s atmosphere. There have to date only been a few high energy neutrino detections - the first three were observed in 2003 and were named Bert, Ernie and Big Bird!

  IceCube's traces of the highest energy neutrinos ever recorded. From left to right, Bert, Ernie and Big Bird.

IceCube's traces of the highest energy neutrinos ever recorded. From left to right, Bert, Ernie and Big Bird.

This image shows the graphical representation that is generated for each event. Below is another info-graphic from the IceCube website which explains the representation in detail. 

how_does_icecube_work.png

The IceCube Collaboration website lists highlights of the scientific findings to date. Clearly this has been an incredibly successful instrument and its useful life has only just begun.

 

Signal achievements

Why would an artist be interested in working with IceCube? There are many things that I find fascinating about IceCube.

• The engineering required to create the instrument and the physics underlying the project are awe inspiring. As a builder of unique instruments myself I'm absolutely overwhelmed by the gizmo itself and the huge amount of research, collaboration, testing, unique engineering and shear hard work required to bring it to fruition. 

• The mastermind behind the IceCube project Prof. Francis Halzen said this about the project - “To have your career on the line half a world away is hard enough. But to know that you have embroiled so many others in the same improbable adventure, that your funders and colleagues expect results, and that you are totally powerless to affect the outcome, is a form of exquisite torture.” 
The ability of Prof. Halzen to garner the support of the NSF and to develop the complex international collaboration that IceCube entails today is impressive. I'm often disheartened by the lack of collaboration in the visual arts. The myth of the solo genius struggling in the atelier is still dominant in my field and I'm inspired by IceCube to see what a different model of art practice, one involving complex international collaboration, might look like. 

• A key focus of my current research is the relationship between noise and signal. IceCube (and the other astrophysical instruments at the Pole) are striving at the very edge of perceptible signal. The huge amount of 'noise' that must be sorted through to find the 'signal' of neutrinos is almost unbelievable. Placing the array under more than a mile of ice in one of the remotest places on earth cuts some of the noise. Pointing the DOMs downward to use the earth as a cosmic ray filter cuts a lot more of the noise. Even then, high energy events coming from astronomical sources are a tiny fraction of the total number of recorded events. With this scant data scientists then strive to understand what these rare events might tell us about the physics of the formation of the universe or the demise of massive stars. The capacity to sort through layer, after layer, after layer of noise to discern the faintest of signals and then extrapolate from that signal to understand the nature and history of the universe is the signature achievement of contemporary astrophysics. For me, this search for understanding is the most significant cultural act of contemporary humans. As an artist I wonder what I can contribute to this endeavor?

• Some of the noise that IceCube is trying to filter is proving to be really interesting to other scientists studying glaciology and the solar wind. One man's noise is another's signal. All noise is essentially signal - it depends on how you look at it.