Francis Halzen
What did Francis Halzen win the Nobel Prize for?
What is the advantage of using Antarctic ice for the IceCube Neutrino Observatory?
What significant observations has IceCube made?
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Francis Halzen (born March 23, 1944, Tienen, Belgium) is a Belgian physicist who was awarded the 2026 Nobel Prize for Physics for his work on the IceCube Neutrino Observatory in Antarctica and the discovery of high-energy neutrinos from astrophysical objects.
- Born: March 23, 1944, Tienen, Belgium
- Education: Catholic University of Leuven (master’s degree, 1966; Ph.D., 1969)
- Affiliation: University of Wisconsin–Madison
Halzen received a master’s degree and a Ph.D. from the Catholic University of Leuven in Belgium in 1966 and 1969, respectively. Since 1971 he has worked in the physics department at the University of Wisconsin–Madison.
Neutrinos are uncharged subatomic particles that are very difficult to observe because they rarely interact with matter. In 1987 Halzen heard about Soviet plans to detect radio waves from neutrinos interacting with particles in Antarctic ice. He soon determined that detecting Cherenkov radiation, light given off by neutrino interactions with matter, would be a better method.
Halzen queried his friend, physicist John Learned, about the feasibility of an Antarctic neutrino observatory. Learned was working on an experiment, Deep Underwater Muon and Neutrino Detector (DUMAND), to detect Cherenkov radiation in the deep ocean near Hawaii. As Halzen later wrote, “Learned immediately appreciated the advantages of an Antarctic neutrino telescope.”
The advantages were many. Putting the photomultiplier tubes that would detect the Cherenkov radiation in ice would allow the experiment to be easily operated from the surface. The deep Antarctic ice is in total darkness, whereas DUMAND had needed to contend with interference from storms, bioluminescent marine life, and natural radioactivity from potassium-40 in the water. The National Science Foundation already operated a large research facility at the South Pole. The Antarctic neutrino observatory, Halzen realized, “would be much cheaper and easier to build than DUMAND.”
In 1991 Halzen and collaborators published results from a successful pilot experiment in which Cherenkov radiation from cosmic-ray muons was detected in Greenland ice. Construction began on the Antarctic Muon and Neutrino Detector Array (AMANDA) in 1993. Strings of photomultiplier tubes were lowered into holes in the ice drilled with hot water. The first tubes were at a depth of between 800 and 1,000 meters (2,600 and 3,300 feet), but tiny bubbles in the ice at that depth scattered the Cherenkov radiation. Only below 1,400 meters (4,600 feet) did the ice become clear enough. AMANDA showed that Cherenkov detectors could work in deep Antarctic ice, but a much larger observatory was needed to detect astrophysical neutrinos.
In 2001 Halzen became principal investigator of AMANDA’s successor, IceCube. Construction on IceCube began in 2004, and it was finished in 2011. IceCube consists of 86 strings, each made up of 60 photomultiplier tubes placed 17 meters (56 feet) apart. The strings are spaced 125 meters (410 feet) apart in a triangular grid. The photomultiplier tubes are at depths between 1,450 and 2,450 meters (4,760 and 8,040 feet) and cover a volume of 1 cubic kilometer (0.24 cubic mile) of ice.
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Shortly after completion IceCube began observing neutrinos with petaelectron volt (PeV; 1 PeV = 1015 eV) energies. IceCube’s neutrino observations suggest that they are produced by the same sources that produce ultra-high-energy cosmic rays. IceCube has even been able to identify neutrinos from specific active galactic nuclei (galaxies that emit large amounts of radiation because of material falling on a supermassive black hole at their centers).