We report on a sensitive survey for radio pulsar wind nebulae ( PWN ) towards 27 energetic and/or high velocity pulsars .
Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array , and utilised pulsar-gating to search for off-pulse emission .
These observing parameters resulted in a considerably more sensitive search than previous surveys , and could detect PWN over a much wider range of spatial scales ( and hence ambient densities and pulsar velocities ) .
However , no emission clearly corresponding to a PWN was discovered .
Based on these non-detections we argue that the young and energetic pulsars in our sample have winds typical of young pulsars , but produce unobservable PWN because they reside in low density ( n \sim 0.003 cm ^ { -3 } ) regions of the ISM .
However , non-detections of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 10 ^ { -5 } of their spin-down luminosity , implying an efficiency at least an order of magnitude smaller than that seen for young pulsars .