Me taking measurements of accumulated snow above an IceTop tank.
Today I went out again with Sam, and also with Elisa who joined us for our afternoon outing. We had set ourselves an ambitious goal: to measure snow accumulation totals at all 162 IceTop tanks across an area of one square kilometer! We actually did about 154 out of 162, and we took all measurements in about three and a half hours, something that I still cannot believe myself. This number translates into 77 different locations that we drove to, as tanks were constructed in pairs.
Since it is thought that snow accumulation above IceTop's tanks can affect cosmic ray measurements with this experiment, the IceCube team is wanting to investigate this topic. But I will let the pictures speak for themselves. I am also posting a description of IceTop and of this season's research, written by Elisa, that will help clarify what are we doing here at the South Pole with IceTop.
Sam driving the snowmobile. We drove to 38 different tank locations!
Me taking measurements, with Elisa writing down the numbers from within the vehicle.
Boots sinking into the snow.
More measurements, with the IceCube Laboratory (ICL) in the background.
Back after three and a half hours. Me plugging in the snowmobile's engine block heater.
Elisa has kindly provided the following explanation concerning the work that is being done with IceTop:
Our project for the current summer season at the South Pole concerns studying the effects of snow accumulation on top of IceTop tanks, on the charge spectrum of cosmic ray particles.
IceTop is an array that uses the same physics principle of IceCube and the same hardware, but instead of being buried deep in the ice, IceTop detectors are deployed at the surface and aim to study mainly cosmic rays, more so than neutrinos. Snow is naturally blown up by winds in the Antarctic Plateau, which brings accumulation on top of IceTop tanks. One of the effects of snow accumulation has been already noticed by the IceTop collaboration: the electromagnetic component of the charge spectrum of cosmic ray particles has been significantly weakened in those tanks which have accumulated the greatest amount of snow. It is yet to be determined whether the muonic part of the charge spectrum is affected as well or not. So the main goal is to identify and understand all possible effects of snow accumulations over the tanks.
In order to achieve this we are using the so called 'muon taggers'. Muon taggers are simple, visible light detectors based on the scintillation principle. Two plastic scintillators have been coupled with photomultipliers (PMTs) and are later enclosed in wooden boxes to prevent interaction with external light. If a charged particle interacts with both the surface (IceTop) and the underground detector (IceCube) producing scintillation light at the same time, a coincident event happened. This charged particle is called a muon. Indeed, at this elevation above sea level (about 3,000 meters) cosmic ray particles impinging on the surface are mainly muons and electrons. Only muons possess an energy content high enough to interact with both scintillators. On the other hand, as electrons do not carry enough energy nor interact easily with matter, they will not be able to produce the same effect as muons do.
These coincident events are always recorded and get a time stamp from a GPS antenna. Further analysis will focus on comparing IceCube events with IceTop ones to see if the same event that lit both scintillators could indeed have enough energy to interact with the IceTop tank as well. By doing this, we have just tagged a muon!
We are currently using two types of muon taggers: small (with red boxes) and big (with flashy colorful boxes). These boxes are held by a wooden structure to keep the proper alignment, either vertical or with an inclination to the horizon, during the entire length of the data collection period. Data acquisition modules (DAQs) for these detectors are kept in heavy-duty plastic suitcases which are lined with a thick insulating layer to prevent them from getting too cold. These DAQs include a GPS antenna to do the time stamps, a high voltage battery for the PMTs, and an USB drive to store the data and provide a booting sequence for the electronics inside the DAQs.
Field work has consisted so far in deploying these detectors at the surface, on top of IceTop tanks, which is usually done in the morning. After connecting all cables and providing adequate power, data acquisition starts. Standard data runs last eight hours, so usually by the end of the day we will have retrieved all the DAQ suitcases and carried them inside the IceCube Laboratory (ICL) to allow warming up overnight. Afterwards, these timestamps will be used to grab the signals left by these particles in the IceTop tanks.
Cross-sectional view of an IceTop Tank. Credit: The IceCube Collaboration.
Photograph of a frozen IceTop tank ready to be closed. Credit: The IceCube Collaboration.
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