IceCube telescope: Extreme science meets extreme electronics

Why the South Pole?

Under the IceCube project, scientists are using large volumes of ice below the South Pole to catch glimpses of rare neutrinos that crash into atoms in the ice. Unlike photons or charged particles, “neutrinos are the only particles that can travel across the universe without interference,” said Karle.

However, their ability to pass through the Earth unhindered makes cosmic neutrinos extremely difficult to detect. It requires immense instruments to find and record them in sufficient numbers to trace their origin.

The team located the telescope at the South Pole because the device requires “the clearest and purest ice in as large a quantity as possible,” according to the University of Wisconsin. While looking through the earth, the IceCube telescope uses the Antarctic ice cap as a shield against cosmic rays, while using the planet as a filter to select neutrinos.

Breaking down the DOMs

It’s hard to picture a telescope with a volume of one cubic kilometer. The IceCube telescope consists of more than 5,000 DOMs on a total of 86 strings. Each string, with 60 DOMs attached, is carefully lowered into a drilled hole, with DOMs installed at depths between 1,450 and 2,450 meters.

When neutrinos pass through the ice sheet and collide with atoms, their impact creates a blue light. IceCube’s DOMs are designed to detect that “very faint lighting,” said Karle. With more than 5,000 DOMs, the IceCube telescope can track and record the elusive patterns of energy deposition in the ice.

Each DOM functions as a complete data acquisition system. Key building blocks include: a photomultiplier, a high voltage generator, a set of LEDs and a main board.

The photomultiplier inside each DOM first locally obtains and records the high quality data in the waveform. The digitized waveform is then digitally transmitted to the surface.

A lot of things need to happen in that process. For example, the digitized waveform needs to be time-stamped using a clock, like a local oscillator, calibrated against a master oscillator on the surface. A custom ASIC waveform digitizer is there to accomplish the task in concert with a very stable quartz crystal oscillator.

The data, then, needs to be enhanced over ambient noise.

The DOM also contains a "flasher board" consisting of a set of LEDs. The flashers, using an intense pulse, help determine the optical properties of the ice while measuring the precision of the timing.

“All sensors need to run on the same time scale, but the ice is not completely uniform,” said Karle. Given the different optical properties in the ice, it is critical to run computer simulations to calibrate absolute time and relative time, he explained.

The DOM also includes a main board integrated with Altera’s FPGA and embedded 32-bit ARM CPU. The board runs software to perform several different calibration tasks. In conjunction with ADCs and DACs, the main board performs state control, message management, analog calibration, time calibration, monitoring and other housekeeping functions.