IceCube

Our project, ARA is designed to detect the radio signals from neutrino interactions at the South Pole. This year we were there to install a testbed array which would allow us to get a feel for the engineering challenges as well as get the some characteristics of the area where we would start installing the actual array. We were unofficially piggybacking off of the IceCube experiment, which just completed the installation of the last string while I was down there. The IceCube experiment looks for flashes of light the result from neutrino interactions in the ice. The entire experiment costed about $270 million and took ten years to complete. Basically they melt 2 foot diameter holes in the ice that are 2.5km deep and then lower down strings of sensors into the holes before the holes freeze in. The entire array consists of 86 strings with 60 sensors on each. A diagram of the array can be found here, the image is too big for the blog. IceCube has a good website for the experiment that you can browse here.



In this photo, which I found online, you can see the firn drill. This drill is used to get through the first 50 meters or so of snow. They pump hot water through the coils and I believe some is sprayed out to help lubricate the drill. It takes about 20 hours to drill the first 50 meters because they want a real straight hole. The firn then makes a sharp transition into ice, and the drillers know they have reached this level when they can see water starting to pool over the top of the drill down the hole.




















































I took this photo of them lowering down the last string. In the background you can see the ice drill. After the firn drill has made it into the ice, they lower down the ice drill with a weight stack below it. The weight stack has a nozzle on the end that sprays hot water. The drill camp can produce 200 gallons of 80 degree C water at 1000psi each minute. To do this it takes a lot of fuel, I believe 4,000-7,000 gallons per hole which translates to about two LC-130 flights just to get the fuel in. It takes another flight to get all the hardware down for each hole. The holes end up being about 2500 meters deep and it takes the ice drill 20 hours to go the 2500 meters.



















Here you can see the hose reel in the foreground with the drill tower in the background. The drillers label each hole by the number of hours they have to lower the string in before the hole diameter becomes too narrow. I believe this last hole was a 24 hour hole, but it generally takes 10 hours or so to lower the string down if they don't run into any problems.
















Here you can see the signal and power cable fro the hole being fed off of the reel into the tower.















Here is a photo of the last DOM (Digital Optical Module) with the last string still being lowered down the hole. The DOM is made of a glass pressure housing with some electronics and a photomultiplier tube (PMT) which detects the flashes of light. The PMT is so sensitive that it can detect the trace amounts of radioactive decay that comes from radioactive isotopes in the sand used to make the glass.



















This is shot taken by a coworker looking down the last borehole. Once the last DOM is attached the entire string is lowered over a kilometer. Basically the entire array looks at a 1 cubic kilometer section of ice that is over a kilometer below the surface. This ice is extraordinarily clear however there are layers of dust in the ice that complicate the analysis. The drillers do have a life preserver available to throw to somebody if they did happen to fall down the hole. However the person probably wouldn't be in good shape because it's about a 50 meter fall and the water in the hole is at 80 degrees or so. The hole takes weeks to entirely freeze in. It freezes from the top down, and the temperature at the bottom is only slightly below freezing while at the top the temperature may be 60 below.