In the IceCube baseline design, 80 strings are regularly spaced by 125 m over an area of approximately one square kilometer, with OMs at depths of 1.4 to 2.4 km below the surface, as shown in fig. 59. In sections 5 and 6 the performance criteria and experimental requirements for this device have been distilled from the scientific goals. In addition to explicit technical requirements, however, the instrumental design concept must incorporate many additional factors such as system cost, software/hardware integration issues, engineering risks, lowest practical power dissipation, noise rejection, noise generation, thermal and physical shock, data flow, Mean Time Before Failure (MTBF) of components, production Quality Assurance (QA), deployment, constraints on commissioning personnel, remote intervention capabilities, calibration, cost of operations, as well as disparate factors such as safety and educational values. The process of optimization involving all these disparate issues has led to an innovative approach, with many attractive features. Nevertheless, no exotic, high speed, brute force, or unproven elements are needed in this concept.
There are three fundamental elements in this architecture, common to both IceCube and IceTop:
The instrument design is based on a decentralized digital architecture [157]. The motivation for this approach arises primarily from the need to acquire, from a km-scale instrument, quite complex information in the presence of substantial backgrounds.
Roughly 1.5 kHz of downgoing cosmic ray muons penetrate the geometric volume of IceCube, about six orders of magnitude more than muons induced by neutrinos. The Cerenkov radiation cones generate complex optical signals that are further complicated by scattering in the ice before detection. Parent directionality is not uniquely determined by simple time differences – a horizontal muon generates equal quantities of upgoing and downgoing light. In most cases, neutrino-induced upgoing muons are distinguishable from backgrounds only with sophisticated fits using global space-time correlations.
Given the high background/signal ratio and event complexity, it would be imprudent to filter out the atmospheric muons with tight trigger logic at an early stage. In addition, the downgoing muons offer an ever-present and copious calibration source, as well as a potential arena for the detection of subtle astrophysical processes, or possibly even signatures of extreme energy release events in the cosmos.
As noted in section 5, data and simulations show that, for both neutrino-induced and background events, the information content at the photomultiplier (PMT) ranges from single photons arriving over a span of about 1 μs to many hundreds (even thousands) of photons arriving within 15 ns. Extremely high-energy events may generate many thousands of photoelectrons in a continuum of complex patterns lasting several microseconds.
Figure 60 shows a fairly typical raw waveform obtained from a DOM operated in coincidence mode to capture signals induced by a down-going muon. In this figure, the first photon to be detected appears in the ATWD record at about sample 20. Somewhat later, a larger number of detected photons appears, possibly the result of a shower process at some distance from the direct path of Cerenkov light, or possibly the result of multiple muons passing nearby. From a study of these waveforms it is clear that the arriving signals are frequently complex, in the range of 15% for muon-induced signals. Because of the optical scattering, all photons must be included in the analysis to determine the nature of the event.
A measure of the parent muon neutrino energy comes primarily from the capability to record very large energy losses producing electromagnetic showers along a muon trajectory. For electron neutrinos, the showers occur in isolation, requiring for identification that no muon trajectory is present. Tau neutrinos at extreme energies can produce two showers separated by a short distance. The earlier parts of the shower-induced waveforms vary most rapidly and reach the highest amplitudes. The most useful information about shower direction is found here, imposing challenging technical requirements.
Time measurement of these signals over the km-scale volume must be established and maintained with a resolution and accuracy of ≤5 ns. The capture of this highly variable complex information requires high-speed (≥200 Msps), high dynamic range (≥14 bits) waveform recording. These technically challenging desiderata provide the impetus for an innovative instrument design.
The architecture makes use of modern ideas that rely on time-stamps applied very early to each datum. This approach reduces the amount of real-time circuitry to a remarkably small level relative to traditional DAQ concepts. Correspondingly, data flow is built on conventional networking techniques using messages that may contain either data or control content. Advantages of the architecture based on the DOM include:
The discussion of the Instrument Design elaborated below will benefit from a brief description of the performance obtained with a string of 41 Digital Optical Module (DOM) prototypes deployed in January 2000 by the AMANDA Collaboration. These DOMs were intended to serve two purposes:
Both of these purposes were achieved, despite challenging schedule and financial constraints. The DOM prototypes of AMANDA string 18 were designed, built, and deployed in about 14 months, and strong support throughout the collaboration during this period was essential for the ultimate success of the effort. Because schedule pressures were so severe, little attention could be given to the issues of reliability and QA. Nevertheless, most of the DOMs of string 18 have delivered extremely stable performance throughout the nearly two year period of operation. One DOM board seems to have developed an internal problem; PMT HV can still be set so that this module continues to contribute to AMANDA data. Two PMT HV bases appear to have failed, indicating that re-engineering to enhance QA for this subsystem may be needed. There appears to be an intermittent electrical connection in the main cable for another DOM. The remainder of the 41 DOMs have operated continuously with no glitches, crashes, or other indications of instability.
During CY 2000, the DOMs were equipped with a simple surface DAQ that could control PMT HV, set other operating parameters, and return housekeeping data and occasional waveforms to the surface. In January 2001, a subset of four DOMs was equipped with a more complete surface DAQ based on single-channel Test-Boards, designed initially for commissioning of DOM boards. With the four-channel Test-Board DAQ, time synchronization is possible, permitting data from different DOMs to be correlated to a few nanoseconds, and even demonstrating the detection of downgoing muons. Most of the technical results described in this section come from an analysis of data obtained with the Test-Board DAQ.
The successful operation of AMANDA string 18 has proven not only the feasibility of the digital system architecture, it has also shown that the associated software is both straightforward to design and stable in operation. Much of the software framework that has enabled these technical results will serve as a springboard for subsequent IceCube efforts.
The block diagram of the Test-Board DAQ is shown in figure 61. For reference, the various approaches used by AMANDA are described in section 7.5.
