Exposure of humans to mixed fields of high- and low-linear energy transfer (LET) radiation occurs in many situations-for example, in urban areas with high levels of indoor radon as well as background gamma radiation, during airplane flights or certain forms of radiation therapy. From the perspective of health risk associated with exposure to mixed fields, it is important to understand the interactions between different radiation types. In most cellular investigations on mixed beams, two types of irradiations have been applied sequentially. Simultaneous irradiation is the desirable scenario but requires a dedicated irradiation facility. The authors have constructed a facility where cells can be simultaneously exposed to (241)Am alpha particles and 190-kV X-rays at 37°C. This study presents the technical details and the dosimetry of the setup, as well as validates the performance of the setup for clonogenic survival in AA8 Chinese hamster ovary cells. No significant synergistic effect was observed. The relative biological effectiveness of the alpha particles was 2.56 for 37 % and 1.90 for 10 % clonogenic survival.