lepton, such as an electron, when they make a hit, and the electron,
being charged, is easy to detect. This is how we have learned a lot
about the neutrinos that stream down on us every second from
the Sun.
When light passes through material, such as water, it travels slower
than when in free space. So although nothing can travel faster than
light in a vacuum, it is possible to travel faster than light does
through a material. When a particle moves through a substance
faster than light does, it can create a kind of shock wave of visible
light, known as Cerenkov radiation. The Cerenkov radiation
emerges at an angle to the particle’s path, and the greater the
particle’s velocity, the larger this angle becomes. The
SuperKamiokande experiment detects neutrinos when they
interact in water to make either an electron or a muon, depending
on the neutrino’s type. These particles, unlike the neutrino, are
electrically charged and, moving faster than light through the water,
can emit Cerenkov radiation. By carefully analysing the patterns of
light, one can distinguish between muons and electrons created in
the detector, and hence between muon- and electron-neutrinos.
The Sudbury Neutrino Observatory (SNO) is 2070 metres below
ground in a nickel mine in Sudbury, Ontario. Its heart is an acrylic
vessel filled with 1,000 tonnes of ‘heavy water’, called deuterium, in
which a neutron joins the single proton of ordinary hydrogen. In
SNO, electron-neutrinos interact with the neutrons in the
deuterium to create protons and electrons, and the fast-moving
electrons emit cones of Cerenkov radiation as they travel through
the heavy water. The Cerenkov light forms patterns of rings on the
inner surface of the water tank, where it is picked up by thousands
of phototubes arrayed around the walls.
However, the key feature is that SNO can also detect all three types
of neutrino (see p. 100) through a reaction unique to deuterium.
A neutrino of any kind can split the deuterium nucleus, freeing the
neutron, which can be captured by another nucleus. The capture is
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Particle Physics