IceCube
IceCube Neutrino Observatory

PDD - IceTop

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5 Expected IceCube Performance

5.10 IceTop

The solid surface above IceCube allows the possibility of a surface air shower array that can be used for calibration (by providing a set of tagged muon bundles), for veto of certain backgrounds generated by large air showers and for cosmic-ray studies. A square kilometer array of suitably designed detectors on a 125 m grid has full efficiency for air showers from 1015 eV to 1018 eV. Thus it covers an important energy range from below the knee of the primary spectrum to an energy where we may expect at least the beginning of a transition from galactic cosmic-rays to cosmic-rays of extragalactic origin.

5.10.1 Tagged Muon Bundles

Figure 56 summarizes the characteristics of IceTop events in IceCube. Bundles range from ∼ 10 muons at the top of IceCube from 1015 eV protons to ∼ 104 muons in the core of a shower generated by an iron nucleus of 1018 eV. Most of the muons are concentrated in a core with typical size less than the spacing between IceCube strings, and about half the muons in the bundle range out inside IceCube. The high energy events contain muons with sufficient energy to produce bursts of radiation in IceCube. There will be about 100 tagged air showers per day with multi-TeV muons in IceCube. Since a muon that goes through IceCube deposits about 200 GeV or more of energy, one sees from the diagram that there will be of order ten thousand tagged events per year in IceCube in which more than 100 TeV of energy is deposited by the muons. The ability to identify and measure such coincident events readily is a unique feature of a deep neutrino detector under a solid overburden.

The surface detectors are modelled on the water Cerenkov tanks pioneered by the Haverah Park air shower experiment [89] and currently used in the Auger detector now under construction in Argentina [155]. At the South Pole the water freezes, but the Cerenkov light generated by relativistic charged particles is essentially the same as in water. This has been verified by a small test tank deployed inside the current South Pole Air Shower Experiment (SPASE) [156] during the 00/01 Antarctic season. Design and layout of the tanks is described in 6.9.

We have adapted Auger tank simulation tools and used them to simulate IceTop tanks in order to simulate waveforms expected in showers with energies in the PeV–EeV energy range. Figure 57 shows a sample IceTop waveform generated in this manner.

Most of the showers that trigger the detector will be near the threshold energy of approximately 200 TeV. Air showers with this energy typically contain 1 to 10 muons with sufficient energy to reach the deep detectors within a radius of 20-30 m of the shower trajectory. A fraction of these events will be used for calibration of the angular response and reconstruction algorithms of IceCube, as mentioned above.

5.10.2 IceTop as a Veto

In addition to providing a sample of events for calibration and for study of air-shower-induced backgrounds in IceCube, the surface array will act as a partial veto. All events generated by showers with E > 1015 eV can be vetoed when the shower passes through the surface array. In addition, higher energy events, which are a potential source of background for neutrino-induced cascades, can be vetoed even by showers passing a long distance outside the array. In particular, showers with E > 1017 eV and cores a km outside the array will produce a minimal trigger near the boundary of the array.

Figure 56: Diagram showing the energy spectra of muon bundles in IceCube associated with air showers (proton primaries) that can be measured with IceTop. Each curve is labelled at the left by the primary energy of the associated shower and on the right by the number of events per logarithmic energy interval that strike the surface array within the solid angle determined by IceTop and IceCube. The surface array is fully efficient for showers above 1015eV. The arrows indicate muon energy at production in the atmosphere needed to reach the top and the bottom of IceCube. Muons above ∼ 3 TeV will tend to produce bursts of radiation in IceCube.
Figure 57: The upper panel shows the waveform of the signal at 300 m from the shower core for a proton-initiated shower of 100 PeV. The simulation is for an 8 in PMT at the center of the tank viewing photons reflected from the bottom and sides of the ice tank. The lining of the cover is black. The marks at the top of the figure indicate the arrival time at the top of the ice of muons (top set of marks), electrons (middle set of marks) and γ-rays (bottom set of marks). The lower panel shows the the muon contribution to the waveform. In both panels the amplitude is plotted as a function of time at the output of the base of the PMT. Most of the delay is due to the drift time of electrons in the PMT.

5.10.3 Cosmic-ray Physics

The IceCube-IceTop coincidence data will cover the energy range from below the knee of the cosmic-ray spectrum to > 1018 eV. Each event will contain a measure of the shower size at the surface and a signal from the deep detector produced by muons with Eμ > 300 GeV at production. At the high elevation of the South Pole, showers will be observed near maximum so that shower size will give a relatively good measure of primary energy. The ratio of the muon-induced signal in IceCube as a function of primary energy (as determined by shower size at the surface) will give a new measure of primary composition over three orders of magnitude in energy. In particular, if the knee is due to a steepening of the rigidity spectrum, a steepening of the spectrum of protons around 3 × 1015 eV should be followed by a break in the spectrum of iron at 8 × 1016 eV.