Knowing the spectrum near the output of the relatively new mini X-ray tube (MXRT) commercial models is fundamentally important in energy-dispersive X-ray fluorescence scan images, especially in the in vivo applications. This information is relevant for determining the absorbed dose during a measurement and for absolute quantification by a fundamental parameter method. However, it is not possible to measure it directly using a silicon drift detector (SDD) given the high saturation in the counts. In this work, an experimental methodology is developed for determining the kernel spectrum emitted by the MXRT, enabling the quantification of its energy flux density over short distances. Different distances were used: source-detector, solid emission angle (collimation), attenuation characteristics of the medium (air), and in a vacuum, within an energy range of 1-40 keV, to determine the X-ray tube spectrum. The spectrum is measured by an SDD, taking its efficiency and dead time into account. In order to verify the method, a spectrum that is rebuilt starting with the kernel is compared, under the same conditions, with a reference spectrum that is directly measured in air and with a theoretical spectrum obtained by the Ebel model. The results are consistent and validate the methodology employed in this work. Additionally, low-energy peaks were detected, corresponding to the tube material's L lines, which are not present in the original spectrum reported by the manufacturer.