Ben Still describes new plans to upgrade a huge tank of water surrounded by light detectors, so that it can detect antineutrinos

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Heavy metal is being added to one of the worlds largest particle physics experiments to allow it to see antimatter for the first time¹. For years the Super Kamiokande neutrino observatory has been a world leader in the field of neutrino particle physics. Last week the international collaboration of scientists who run the experiment announced that in 2016/2017, for the first time in over a decade, the experiments ultra sensitive detector will be shut down for an upgrade.
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A common view among physicists is that a key piece of our Universe’s Big Bang creation story is locked up in our understanding of the tiny differences in behaviour of neutrinos and their antimatter version - antineutrinos. The upgraded Super Kamiokande detector will be able to distinguish between the interactions of these two particles inside the detector - something it has been incapable of until now. Because the experiment has been the largest and most successful neutrino experiment to date it is expected that we will soon be close to filling in the missing piece of the creation story puzzle.

Super-Kamiokande (Super-K)

Since construction completed in 1998 1996 Super-K has opened its doors just twice for upgrades and repairs, last time in 2006. It is one of the most beautiful man made structures on this planet.

Super-K detects neutrino particles via their interaction with water. Too small to interact directly with the water molecules, neutrinos interact with neutrons in the nucleus of the Hydrogen and Oxygen atoms from which water is made. The interaction of neutrino and neutron produces a proton and a second charged particle. The second charged particle that is produced depends upon the type of neutrino interacting: an electron-neutrino produces and electron; a muon-neutrino produces a Muon (which is simply a heavier version of an electron).

νe + n → e- + p

(electron-neutrino + neutron → electron + proton)

νμ + n → μ- + p

(muon-neutrino + neutron → muon + proton)

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The charged particle produced alongside the proton has enough energy that it is born travelling faster than the speed of light in water. You may have heard that nothing can travel faster than light, and this is true for light in empty space. But when light travels through water, glass, or indeed anything other than empty space, then the electrons in surrounding atoms slow the light down. Light travels through water at roughly 75% of the speed with which it travels through empty space. This means it is not against the laws of physics for a charged particle to travel faster than light within water. If this happens a blue light known as Cherenkov radiation is emitted. This Cherenkov radiation is picked up by almost 12,000 light sensitive detectors surrounding the water, which turn it into electrical signal to be interpreted by computers.
Electron
Cherenkov radiation from an electron in Super-K Photograph: Super-K/Super-K

If an antineutrino interacts it interacts with a proton in the nucleus of Hydrogen or Oxygen atoms. In this interaction a neutron and charged antiparticle are produced. The antiparticle that is produced again depends upon the antineutrino interacting: an electron-antineutrino produces an anti-electron (positron); a muon-antineutrino produces a anti-muon (which is simply a heavier version of a positron). While these antiparticles have a different sign electric charge to their mirror particle cousins, they still create Cherenkov radiation in exactly the same way because the size of the charge (and their mass) is the same. Super-K can therefore not distinguish if it is particles or antiparticles producing the Cherenkov radiation. This leads then to Super-K scientists not being able to tell if it was a neutrino or antineutrino interaction they witnessed.
Muon
Cherenkov radiation from a muon seen by Super-K Photograph: Super-K/Super-K

anti-νe + p → e+ + n

(electron-antineutrino + proton → positron + neutron)

anti-νμ + p → μ+ + n

(muon-antineutrino + proton → antimuon + neutron)

Gadolinium

The addition of Gadolinium, by dissolving small amounts of Gadolinium salts, changes the game plan. Gadolinium is great at capturing neutrons, sucking them right into its nucleus. Just as a ball rolling to the bottom of a hill loses gravitational energy, a neutron falling into, and being captured by, a nucleus also loses energy. The ball transfers gravitational energy into movement (kinetic energy); a captured neutron gives all of its energy to the nucleus it is captured by. The now excited nucleus need to lose energy and does this by emitting light.

The speed of a ball at the bottom of a hill depends on the height of the hill. The amount of energy given to the nucleus by the neutron and then emitted as light when captured depends upon the atom it is captured by. Some atoms require neutrons to lose more energy than others; each atom has a unique energy of light emitted during neutron capture. If neutrons are captured only by Gadolinium atoms then the light they emit will be at a definite and singular energy.

more with loads of photos:

http://www.theguardian.com/science/life-and-physics/2015/jul/21/heavy-metal-upgrade-to-detect-antimatter