  The MEGAPIE project is a key experiment on the road to Accelerator Driven
Systems and it provides the scientific community with unique data on the
behavior of a liquid lead-bismuth spallation target under realistic and long
term irradiation conditions. The neutronic of such target is of course of prime
importance when considering its final destination as an intense neutron source.
This is the motivation to characterize the inside neutron flux of the target in
operation. A complex detector, made of 8 micro fission-chambers, has been built
and installed in the core of the target, few tens of centimeters from the
proton/Pb-Bi interaction zone. This detector is designed to measure the
absolute neutron flux inside the target, to give its spatial distribution and
to correlate its temporal variations with the beam intensity. Moreover,
integral information on the neutron energy distribution as a function of the
position along the beam axis could be extracted, giving integral constraints on
the neutron production models implemented in transport codes such as MCNPX.
