First Year Performance Paper - Section 5.1
5.1 Tank calibration and performance
Two types of data were taken with IceTop in 2005: calibration files and event triggers. For IceTop, the calibration files consist of tank events with no coincidence requirement, as illustrated in Fig. 22. The figure shows the charge spectrum for the high-gain DOM in one IceTop tank. The spectrum for the low-gain DOM has a similar shape. The characteristic spectrum is a combination of a steeply falling spectrum of electrons and -rays converting in the tank with a peak due to muons traversing the tank. There is also a contribution from small air-showers, which becomes increasingly important in the highenergy tail of the distribution. A muon penetrating 90 cm of ice (the vertical height of the ice layer) typically deposits approximately 190 MeV of energy in the detector (more or less depending on its zenith angle). The muon peak is broadened by the angular distribution of the muons, by corner-clippers, and by fluctuations in energy deposition, as well as by statistics of photon collection. The steady flux of muons with its characteristic peak provides an ideal means of calibrating and monitoring a water (or ice) Cherenkov detector [31,32].
The single tank rate corresponding to the threshold of 100 photo-electrons in Fig. 22 is approximately 1.5 kHz. Approximately 1 kHz of this is from muons. This rate is consistent with a previous measurement using a muon telescope . We use the peak of the inclusive muon distribution at approximately 240 PE for calibration. Each PE corresponds to a nominal energy deposition of
We can assess the uniformity of response of a tank to a given deposition of energy by comparing the response of the two DOMs in the tank to the same events. Differences in response can occur for several reasons, including small deposits of energy farther from one DOM than the other, direct hits on the photocathode, etc. The in-tank fluctuations in response are measured using one high-gain and one low-gain DOM, so this study is limited to the range of common linear response. The result is shown in Fig. 23a for a sample of air-showers. (Both panels show difference in charge divided by average charge, so the results are confined by definition to lie between -2 and +2.)
To decide whether the tank response is sufficiently uniform, we compare the response of two tanks at the same station to the same set of air-shower events in Fig. 23b using the low-gain DOM in each tank. To the extent that fluctuations in single tank response are small compared to differences between tanks, this is a measure of fluctuations in the air-shower front. This result shows that uniformity of tank response is satisfactory because fluctuations in response of a tank to a particular deposition of energy are small compared to intrinsic fluctuations in the shower front.