Continuous backgrounds in heavy-ion induced X-ray emission

We report here continuous X-rays emitted from targets bombarded with Si³⁺ ion beams of 1.435 MeV/amu over the range of target-atomic number Z_T = 6-29. It is shown that the production cross sections of continuous X-rays can be well scaled with a function of the product of the K-shell radius of the united atom and the minimum transfer momentum of the projectile while MO X-ray spectra are scaled with the K X-ray energy of the united atom. These continuous X-rays may be interpreted as bremsstrahlung produced from a united atom which is formed by a projectile ion and a target atom during collision; we call this radiation united-atomic bremsstrahlung. In consideration of this continuous background, we discuss utility of a trace element analysis with high energy heavy-ion beams especially for analysis of chemical states by using a crystal spectrometer.