University of Wisconsin-Madison

First Year Performance Paper - Section 1

1 Introduction

Concepts for building a detector large enough to study neutrinos from astrophysical sources have evolved since the early days of experimental neutrino physics nearly 50 years ago [1,2]. First successes occurred with the observations of neutrinos from SN1987A [3,4,5] and of solar neutrinos [6,7,8,9,10,11]. Efforts to build a detector large enough to identify the less abundant but higher energy neutrinos produced by hadronic interactions in cosmic-ray sources began with DUMAND [12]. Neutrino telescopes currently in operation are NT200+ (Baikal) [13] and AMANDA [14]. These detectors work by observing a large volume of clear water or ice to detect Cherenkov light from relativistic charged particles produced in neutrino interactions in or near the target volume.

Several arguments lead to the conclusion that a fiducial volume of at least a cubic kilometer is needed to observe neutrinos from high-energy astrophysical sources [15]. Several groups are exploring use of optical Cherenkov radiation in water or ice to detect neutrinos [16,17,18,19]. Neutrino telescopes deep in the ice can be calibrated using a surface air-shower array, that can improve the rejection of the background associated with cosmic-rays, and allow the study of cosmic-rays. Projects based on other techniques (horizontal air-showers, radio and acoustic signals from neutrino-induced events) are also being developed [20]. The latter generally have energy thresholds several orders of magnitude higher than the 50-100 GeV range of ice and water Cherenkov detectors.

IceCube builds on the successful deployment and operation since 1996 of the AMANDA neutrino telescope [21]. AMANDA consists of 677 optical sensors distributed on 19 strings instrumenting a volume of more than 107 m3 at a depth between 1500 and 2000 meters in the ice at the South Pole. In addition to its larger volume, IceCube differs from AMANDA in two significant ways. First, in the IceCube sensors, the signals are digitized in the optical sensors to minimize loss of information from degradation of analog signals sent over long distances. Second, a free-running 20 MHz oscillator in each IceCube sensor serves as a local clock and provides time stamps (at 20 MHz and 40 MHz) for internal operations in the sensor, including timing the arrival of photons. The clock drift is less than 2 ns per second. This local clock is calibrated automatically relative to a master clock on the surface. All time-stamps are converted to Universal time (UTC) at the central counting station.

The IceCube neutrino observatory will consist of 4800 optical sensors or Digital Optical Modules (DOMs) installed on 80 strings between 1450 m and 2450 m below the surface [22]. The In-Ice array is complemented by a surface array, called IceTop. IceTop will consist of 160 ice-tanks, in pairs, near the top of each string. Each tank has two DOMs for redundancy and extended dynamic range.

This paper describes the performance of the first string and eight surface tanks that were installed between November 2004 and January 2005. Section 2 introduces the design, production, deployment and configuration of the detector. Detailed descriptions of the digital optical module and its electronics, the timing method, the surface detectors and data acquisition will be given in separate papers. After the overview, we describe the calibration of gain and timing (section 3). Then follow three sections that deal in turn with reconstruction of muons in the deep string, reconstruction of air-showers with IceTop and analysis of coincidences with externally triggered detectors, including AMANDA. We summarize results of the first season in a concluding section and describe the plan for completion of the IceCube construction project.