A detection system having a receiver for detecting a material having a magnetic resonance response to illumination by pulses of ultra-wideband (UWB) electromagnetic radiation is disclosed. The receiver comprises a detector for detecting the pulses after they have interacted with the material, and a discriminator arranged to identify in the detected pulses the magnetic resonance response of the material. By scanning an item tagged with a tag having a material having a magnetic resonant response, by illuminating the item with UWB pulses and identifying in detected pulses the magnetic resonance response of the material, items can be located, imaged, or activated. The magnetic resonance response of the tag can cause activation of the tag. The tag can have a magnetic resonance response arranged to provide an identifiable magnetic resonance signature such that different tags can be identified and distinguished by their signatures.