IceCube Neutrino Observatory

Using the ice as a detector

Neutrinos pass through almost everything, so a detector needs a very large mass to catch a few. IceCube uses the Antarctic ice below the South Pole station as that mass. Construction began in 2005, during the Antarctic summers, when hot-water drills melted holes 2.5 kilometres deep, and strings of digital optical modules were lowered in before the water refroze; the detector was completed in December 2010. Francis Halzen of the University of Wisconsin–Madison is the lead scientist.

The detector holds 86 strings with 60 optical modules each, between 1.45 and 2.45 kilometres deep, in a volume of one cubic kilometre.

The first sources

In 2013 IceCube reported evidence of high-energy neutrinos from beyond the solar system, including events of petaelectronvolt energy. On 22 September 2017, the detector sent an automatic alert, IC-170922A, with a direction in the sky; telescopes around the world followed up, and found that a blazar, TXS 0506+056, was flaring in the same direction. Two papers in Science in 2018 gave the first evidence of a known object as a source of high-energy neutrinos.

In 2023 IceCube identified neutrino emission from the plane of our own galaxy.

An alert system

IceCube sends real-time alerts publicly within about a minute of a high-energy event, so that other observatories can look at the same place in the sky at once. The data are released in part to the public, and the collaboration has about 450 scientists in many countries. The detector is being extended with denser strings in the IceCube Upgrade and a larger array is planned.

Neutrino astronomy takes its time

IceCube took twenty years from proposal to first sources. Early arrays at Lake Baikal and in the Antarctic ice at AMANDA proved that the idea could work, and each step in size brought a step in sensitivity. The first source, TXS 0506+056, is a single object, and the diffuse flux of neutrinos from many sources is still being resolved. The research continues with more strings at greater density, and with a proposal for a much larger array. The history shows that a patient programme, with a clear technology path, can bring a new window on the universe.

Instruments and technologies

• Digital optical modules Glass spheres holding a photomultiplier and electronics that digitise the light signal and send it to the surface.
• Hot-water drill A drill using hot water at 90 degrees Celsius melted holes 60 centimetres wide to 2.5 kilometres in about two days.
• Surface array IceTop Tanks of ice on the surface that detect air showers and veto them.

The people

• Francis Halzen Lead scientist of IceCube.

Timeline

• 2005 Construction begins during Antarctic summers.
• 2010 Detector completed in December.
• 2013 Evidence for high-energy extraterrestrial neutrinos, including PeV events.
• 2017 Alert IC-170922A on 22 September points to the blazar TXS 0506+056.
• 2018 Two Science papers give the first evidence of a known blazar as a neutrino source.
• 2023 Neutrino emission from the Galactic plane identified.

Key lessons for observation today

• Use the environment as the instrument The ice is the detector. Natural media can be a cheaper sensor than a built one.
• Alert fast and openly Public alerts brought telescopes to the right place within hours. A fast open alert multiplies the value of a detection.
• Build in the season you have Drilling could take place only in a few summer months. Logistics windows set the pace of construction.

Sources

• IceCube, Facts
• Physics Today, IceCube pinpoints an extragalactic neutrino source
• Georgia Tech News, IceCube neutrinos point to long-sought cosmic ray accelerator (2018)
• Wikipedia, IceCube Neutrino Observatory (secondary)