  The origin of the most energetic particles in nature, the ultra-high-energy
(UHE) cosmic rays, is still a mystery. Only the most energetic of these have
sufficiently small angular deflections to be used for directional studies, and
their flux is so low that even the 3,000 km^2 Pierre Auger detector registers
only about 30 cosmic rays per year of these energies. A method to provide an
even larger aperture is to use the lunar Askaryan technique, in which
ground-based radio telescopes search for the nanosecond radio flashes produced
when a cosmic ray interacts with the Moon's surface. The technique is also
sensitive to UHE neutrinos, which may be produced in the decays of topological
defects from the early universe.
  Observations with existing radio telescopes have shown that this technique is
technically feasible, and established the required procedure: the radio signal
should be searched for pulses in real time, compensating for ionospheric
dispersion and filtering out local radio interference, and candidate events
stored for later analysis. For the Square Kilometre Array (SKA), this requires
the formation of multiple tied-array beams, with high time resolution, covering
the Moon, with either SKA1-LOW or SKA1-MID. With its large collecting area and
broad bandwidth, the SKA will be able to detect the known flux of UHE cosmic
rays using the visible lunar surface - millions of square km - as the detector,
providing sufficient detections of these extremely rare particles to address
the mystery of their origin.
