  The S=1/2 spin chain material SrCuO2 doped with 1% S=1 Ni-impurities is
studied by inelastic neutron scattering. At low temperatures, the spectrum
shows a pseudogap \Delta ~ 8 meV, absent in the parent compound, and not
related to any structural phase transition. The pseudogap is shown to be a
generic feature of quantum spin chains with dilute defects. A simple model
based on this idea quantitatively accounts for the exprimental data measured in
the temperature range 2-300 K, and allows to represent the momentum-integrated
dynamic structure factor in a universal scaling form.
