  An exclusive measurement of the dd -> 3He n \pi 0 reaction was carried at a
beam momentum of p = 1.2 GeV/c using the WASA-at-COSY facility. For the first
time data on the total cross section as well as differential distributions were
obtained. The data are described with a phenomenological approach based on a
combination of a quasi-free model and a partial wave expansion for three-body
reaction. The total cross section is found to be \sigma(tot) = (2.89 +-
0.01(stat) +- 0.06(sys) +- 0.29(norm)) \mu b. The contribution of the
quasi-free processes (with the neutron being target or beam spectator) accounts
for 38% of the total cross section and dominates the differential distributions
in specific regions of the phase space. The remaining part of the cross section
can be described within a partial wave decomposition indicating the
significance of p-wave contributions in the final state.
