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.
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
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
Breaking down the DOMs
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
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.
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.
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.
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.
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.
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.
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