Advanced pattern-matching trigger system design for the ARIANNA High Energy Neutrino Detector
A neutrino is one of the universe's essential ingredients. Neutrinos are very hard to detect because they have no electrical charges and interact little with other particles. Thus, extremely large and sensitive detectors are required to detect neutrinos. The Antarctic Ross Ice shelf ANtenna Neutrino Array (ARIANNA) is a proposed detector for Ultra High Energy (UHE) astrophysical neutrinos. It consists of a surface array of radio receivers and can observe 1 ns radio pulses generated by UHE neutrino interactions with oxygen and hydrogen nuclei in the ice of the Ross Ice Shelf.
Each ARIANNA station has four radio frequency antennas, four amplifiers, and a data acquisition system (DAQ). The DAQ of each station has four acquisition channels consisting of four daughter cards and a motherboard. Each daughter card has a custom CMOS digitization and real-time triggering circuitry (ATWD chip), and a field programmable gate array (FPGA) device. The Motherboard has four slots to connect with acquisition cards, another FGPA device for trigger control and data buffering, an embedded CPU with solid-state data storage, and interfaces to an Iridium satellite short burst data transceiver and a long-range wireless communications module.
Each acquisition card includes an Advanced Transient Waveform Digitizer (ATWD) chip; a high speed analog sampling, real time pattern matching triggering and digitizing integrated circuit. It has the ability to acquire the incoming waveforms at 2 GHz with over 11-bits of dynamic range. In each station, the acquisition cards receive detected amplified RF signals simultaneously and store them into 128 samples.
In addition, the ATWD has the ability to compensate for the fixed pattern noise (FPN) of the sampling and trigger circuitry, which are generated by variations in the gate to drain capacitance in the chip, or variations in the input offsets of the trigger comparators. If left uncorrected, FPN causes variations in trigger thresholds, effectively adding noise in the trigger. Calibration and cancellation of FPN is accomplished by programming per-comparator digital to analog converters to null the FPN at each comparator. After calibration, the RMS trigger noise is reduced by a factor of 3 to 4.
The data acquisition system is capable of accepting three types of triggers: external, forced, and thermal. An external trigger acts upon an external electrical input signal much like an oscilloscope's trigger and is used in the laboratory or in the field during experimental studies. A forced trigger is one that is caused by the acquisition system's CPU, and is typically used to force the periodic collection of data that is unbiased by the system's thermal trigger. These "thermal" triggers are the most interesting: they are generated by the signals that the data acquisition system is collecting. Noise - or the rare neutrino events ARIANNA is searching for - will at times cause input signals to exceed trigger thresholds. To allow for low thresholds while keeping trigger rates from being swamped by noise, the thermal trigger system is set up to accept only signal-like events rather than mere noise. This includes requiring bipolar triggers on a per-channel basis over a very brief (~4 ns) time period, plus a requirement that a majority of data acquisition channels (e.g., any 3 out of 4 channels) must all trigger within a brief time window (e.g., 64 ns). These more stringent requirements are expected to capture the vast majority of neutrino events while limiting the rate of "events" due solely to noise. After any triggering event, the sampling of incoming signal is halted, digitized data is read out from the acquisition cards and is stored locally in a solid-state memory card, and then it is transmitted to UC Irvine for further processing over Iridium satellite modem or long-distance wireless communication.
This dissertation focuses on the data acquisition system for ARIANNA, most particularly on the design and performance of its trigger system, including FPN calibration and correction and trigger efficiency.