  Ultra-high energy (UHE) neutrinos and cosmic rays initiate particle cascades
underneath the Moon's surface. These cascades have a negative charge excess and
radiate Cherenkov radio emission in a process known as the Askaryan effect. The
optimal frequency window for observation of these pulses with radio telescopes
on the Earth is around 150 MHz. By observing the Moon with the Westerbork
Synthesis Radio Telescope array we are able to set a new limit on the UHE
neutrino flux. The PuMa II backend is used to monitor the Moon in 4 frequency
bands between 113 and 175 MHz with a sampling frequency of 40 MHz. The
narrowband radio interference is digitally filtered out and the dispersive
effect of the Earth's ionosphere is compensated for. A trigger system is
implemented to search for short pulses. By inserting simulated pulses in the
raw data, the detection efficiency for pulses of various strength is
calculated. With 47.6 hours of observation time, we are able to set a limit on
the UHE neutrino flux. This new limit is an order of magnitude lower than
existing limits. In the near future, the digital radio array LOFAR will be used
to achieve an even lower limit.
