First Year Performance Paper - Section 2.3.3
2.3.3 IceTop Tanks
About 25 m from the top of each string there will be an IceTop station. There are two tanks per station separated by 10 m. The 80 IceTop stations will form an air-shower array with a nominal grid spacing matching the 125 meter string spacing. The spacing between tanks at a station is chosen to maximize the probability that single-station hits are caused by small cosmic-ray showers that contain only one muon capable of penetrating to the deep detector. (A single-station event is defined as one in which there is a coincidence between the two tanks at one station with no hits in adjacent stations.) Such cosmic-ray induced muons are the main background for a neutrino telescope.
Tanks are deployed in shallow trenches so that, after backfilling, the tank tops are initially at the surface (to minimize buildup of drifting snow). At the bottom of the trench between each pair of tanks is a surface junction box to which the associated IceTop DOMs and hole cable are connected before backfilling. Tanks are cylindrical with 1.8 m diameter. The depth of ice in the tank is 90 cm. Standard survey techniques are used to determine the tanks position to within 5 cm.
Each tank is viewed by two downward facing DOMs with their lower hemispheres embedded in the ice. There are two DOMs per tank for flexibility and redundancy. Operating the two PMTs in a tank at different gains allows an increase in dynamic range beyond what is available for a single gain setting. Alternatively, the DOMs may be adjusted to the same gain to give a more uniform tank response to incident particles. In the data described here, one DOM was operated at a gain of approximately 5 × 106 and the other at approximately 5 × 105. With this setting, the signals generated by single through-going muons lie within the range of linear response for both the high-gain and low-gain DOMs.