  We report on the extension of the experiments (P. M. Walmsley et al., Phys.
Rev. Lett. 99, 265302 (2007)) on the decay of quasiclassical turbulence
generated by an impulsive spin-down from angular velocity Omega to rest of
superfluid 4He in a cubic container at temperatures 0.15 K - 1.6 K. The density
of quantized vortex lines L is measured by scattering negative ions. Following
the spin-down, the maximal density of vortices is observed after time t ~ 10
Omega^-1. By observing the propagation of ions along the axis of the initial
rotation, the transient dynamics of the turbulence spreading from the perimeter
of the container into its central region is investigated. Nearly homogeneous
turbulence develops after time t ~ 100 Omega^-1 and decays as L proportional to
t^(-3/2). The effective kinematic viscosity in T=0 limit is nu = 0.003 kappa,
where kappa=10^-3 cm^2 / s is the circulation quantum.
