Measurement of the neutron-neutron scattering length ann with the reaction pi -d --> nn gamma in complete kinematics.

The neutron-neutron scattering length a/sub nn/ is determined from the low energy part of the neutron spectrum of the reaction ..pi../sup -/d..-->..nn..gamma.. with pions at rest. Since neutrons are detected in coincidence with the photon, the kinematics is completely determined. We obtain a/sub nn/ = -18.7 +- 0.6 fm, in agreement with a result of a previous experiment based on the analysis the photon spectrum.