We have investigated the effect of alloying Cu to the Kondo-semimetal
CeNiSn on its specific heat, magnetic susceptibility and mu SR relaxat
ion rate. We find a strongly non-monotonic behaviour, in that the elec
tronic specific heat first increases very strongly (at 5% Cu), before
decreasing again when magnetic order appears (at 10% and 20% Cu). The
mu SR data show that no spin freezing to a spin glass state occurs for
concentrations less than similar to 6% Cu and that the Ce3+ spins rem
ain in a fluctuating state below this concentration. We interpret this
behavior as a consequence of the destruction of the high coherence of
the antiferromagnetic spin fluctuations in pure CeNiSn, when Cu is al
loyed to the system.