  The MEGAPIE project is one of the key experiments towards the feasibility of
Accelerator Driven Systems. On-line operation and post-irradiation analysis
will provide the scientific community with unique data on the behavior of a
liquid spallation target under realistic irradiation conditions. A good
neutronics performance of such a target is of primary importance towards an
intense neutron source, where an extended liquid metal loop requires some
dedicated verifications related to the delayed neutron activity of the
irradiated PbBi. In this paper we report on the experimental characterization
of the MEGAPIE neutronics in terms of the prompt neutron (PN) flux inside the
target and the delayed neutron (DN) flux on the top of it. For the PN
measurements, a complex detector, made of 8 microscopic fission chambers, has
been built and installed in the central part of the target to measure the
absolute neutron flux and its spatial distribution. Moreover, integral
information on the neutron energy distribution as a function of the position
along the beam axis could be extracted, providing integral constraints on the
neutron production models implemented in transport codes such as MCNPX. For the
DN measurement, we used a standard 3He counter and we acquired data during the
start-up phase of the target irradiation in order to take sufficient statistics
at variable beam power. Experimental results obtained on the PN flux
characteristics and their comparison with MCNPX simulations are presented,
together with a preliminary analysis of the DN decay time spectrum.
