IceTop Tank Calibration

Abstract This report outlines the preliminary method developed to calibrate IceTop tanks using through going single muon signals. The charge spectrum of each DOM running independently (without Local Coincidence) exhibits a clear signature of single muons. The Gaussian mean of the muon spectrum is reported as the value for the Vertical Equivalent Muon (VEM) for each DOM. The reconstruction of the raw DOM waveforms in terms of VEM is the scope of the IceTop Tank calibration. VEM is the fundamental unit in IceTop air shower analysis.

Introduction

When it is finished, the IceTop detector array will consist of 160 ice tanks,arranged in pairs on top of the 80 IceCube strings. First 8 tanks were deployed in 2005,followed by 24 tanks in 2006. Once closed, the tanks take about 6 months to fully freeze.In their first year, the tanks don’t reach thermal equilibrium with outside temperaturessince the Freeze Control Units are kept warm by the light bulbs inside the FCUs. The 2005tanks showed sensitivity to the light bulbs in their SPE rates, however, this is not observedin 2006 tanks.

The muon response of each tank has been monitored thoughout the freezing process, not a statistically significant change has been observed. The major uncertainties in the calculation of the muon response have been found to come from the DOM calibration and waveform reconstruction.

The design change between the tanks deployed in 2005 and 2006 lead to a 25% difference in the average muon response. The nominal design deployed in 2006 tanks yields 160 PE on average for single muons, while the yield is 220 PE in 2005 tanks. The variability from tankto tank is found to be about 8% for 2006 tanks, 10% for 2005 tanks.

Each DOM in the same tank agree in their muon response within 5-11%, which is within theuncertainty of DOM gain calibrations.

Description of IceTop Tanks

The tank shell is a 6mm thick black polyurethane of 1.1 in height and 1.9m in diameter.A second layer of 4mm, made out of zirconium fused polyurethane, is molded on the inner surface to act as the diffusely reflective liner. 90cm of clear ice is viewed by 2 DOMs.The top surface of the ice is filled with 20cm of perlite dust.

The DOMs register the Cherenkov photons emitted by the secondary particles of low energy air showers interacting in ice. Electrons, positrons and gamma particles convert in tanks yieldingsmall signals below 100 PE. The through going single muons leave a clear signature in spectrum between 150 and 300 PE.

perlite

ice

0.9 m clear ice

Diffusely reflecting liner

1.8 m

The Muon Calibration Data

The DOMs are configured in free running mode with no Local Coincidence and at a high threshold to enrich the muon sample. The nominal gain setting is 5E6 for all DOMs. 15min data is taken each week by TestDAQ, only 3min worth of data ( 20K hits per DOM) is transferred north. The data taking is furhter optimized by recording on the ATWD channel 1.

Analysis of the Data

An IceTop specific waveform processing module has been developed to reconstructthe raw DOM signals. The DOMCal calibration values are used for the DOM gains and the ATWDs. The residual baseline offset is further corrected by adjusting the waveform

Analysis of the Muon Telescope Runs taken at South Pole in January 2006

I this note I describe the data I took

using the UW-River Falls Muon

Telescope placed on IceTop Tank 39B.

This tank was deployed in 2005, and

had 2.1 feet snow build up on it.

The center of the tank was surveyed by

the South Pole station Survey Officer

on 2006/01/23. The Muon Telescope

(MT) was placed at the marked center

on 2006/01/30 for about 6 hrs running

on its batteries and its solar panel as

seen in picture.

( note: when the MT was picked up it

was still in working condition after 6

hrs.)

Description of the data

The TestDAQ run was started by Mark Krasberg about half an hour earlier than I started running the

MT. Five runs , each one hour long, were acquired. The data taking condition was optimized to

reduce the DAQ dead time and record primarily muon dominated data.• The DOMs, 3963 and 3964, were configured in free running

mode (no Local Coincidence) at 1E6 gain and at 100 PE threshold.• Only ATWD0 Channel 0 was recorded.

Under these conditions the TestDAQ recording rate

reached 320 Hz.

Figure 1 shows the usual exponential character of the time

difference between consecutive MT events. The MT trigger

rate is 2.7 Hz.

Figure 1: The time difference between consecutive MT events

The charge spectrum of the DOMs for all events are plotted in Figure 2 as the red histogram. The

threshold effect seems to have pushed the effective threshold to 150 PE, cutting into the valley.

For a full picture of the charge spectrum for these DOMs I use the Special Muon Run, run21080,

taken on 2005/11/05. Both runs were taken using domcal version 5.11, so they can be directly

compared without the domcal version uncertainties. Although the gains of the DOMs are

different ( in run21080 3963 is at 5E6 gain, 3964 at 5E5 gain) in these runs, the agreement in

charge spectrum is excellent. The Mean of the Muon Peak is at 204 PE for 3963, at 222 PE for

3964.

Figure 2. The charge spectrum of the DOMs 3963 and 3964. The red histogram is from the run describe here. For comparison, the spectrum from the muon run taken in 2005/11/05 is also plotted as the gray histogram.

DOM 3963 DOM 3964

The DOM signals

DOM 3963

DOM 3964

The coincident events were extracted

by matching the GPS times of the 2

independent data sets.

The MT GPS times were 1 sec ahead

of the DOM GPS times. After

correcting for this, the coincidence

search in [-2,2] μsec window yielded

9260 coincident events with DOM

3963 and 8900 for DOM 3964 in

4hrs 40min overlap time. Figure 3

shows the coincidence signal, as MT

being 70ns ahead of the DOMs.

The Coincident events

DOM 3963 DOM 3964

The waveforms of the coincident events were extracted from the DOM data. The charge spectrum

of the MT tagged events are shown in Figure 4. The mean of the vertical muon peak is at 199 PE,

1PE less than the all particle charge spectrum for 3963, and at 217 PE for 3964, 5 PE less than

all particle spectrum of this DOM.

Figure 3. The coincidence signal.

Figure 4. Vertical Muon Charge Spectrum. The high threshold condition in data cuts out the flat lower part of the spectrum.

In Figure 4, the charge spectrum of the coincident events (blue) is plotted against the allparticle spectrum. As expected, the vertical muons constitute the lower end of the Muon Peak,only a few PE less than the mean of the Muon Peak.

DOM 3963 DOM 3964

Figure 4. The Vertical Muon charge spectrum (blue) superimposed on the all particle spectrum (gray). The distributions are normalized to each other to emphasize the peak positions.

Simulations

Figure 5. All particle charge spectrum (black) for 2006 tanks (with zirko liner) yields 123 PE. All angle muons (blue) peak is at 126 PE. The vertical muon (muon with 0<theta<10 degrees) distribution (red) peak is at 114 PE.