A detector supplement device for integration in a spectroscopy setup with the spectroscopy setup including a vacuum chamber, a light source, a sample irradiating a reflected photon beam and a charged particle beam in the same direction of propagation into a radiation detector which is able to detect ultrafast electric currents originating from charged particles. The detector supplement device includes a Rogowski coil placeable inside the vacuum chamber between the sample and radiation detector. The charged particle beam is guided through the hollow core of the Rogowski coil allowing synchronized measurements of electrical currents due to the charged particle beam correlated to the reflected photon beam, while irradiation of the reflected photon beam and the charged particle beam takes place in the same direction of propagation.