# X-ray-Induced Heating in the Vicinity of the X-ray Interaction Point
## 4 Discussion  
Herein, we investigated X-ray-induced heating in the vicinity of the X-ray interaction point. When a solution or metal plate was exposed to X-rays, the temperature increased around the X-ray-exposed site. No appreciable radiation damage, such as gas bubbles, was observed in the water in the capillary tube. However, the generation of gas bubbles was observed in crystallization solutions and crystal suspensions. This large difference is considered to be due to the generation of gases after the ionization of a crystallization solution or protein crystals. Despite the distinct compositional differences of the water, crystallization solution, and crystal suspension in the capillary, they all showed similar trends of temperature increase. Similar to the solution inside the quartz capillary, the temperature increase showed a similar trend for all metals exposed to X-rays. However, the increase in temperature around the X-ray-exposed area in the metal was lower than that in the solution in capillary, while the temperature increase and decrease for X-ray exposure and non-exposure showed similar trends for Cu, Al, and Zn metal plates, the temperature increase was different. In particular, the temperature increase of Cu was lower than that of Al or Zn. Factors that may be involved in the measurement of temperature include X-ray transmittance, thermal conductivity (368, 239, and 113 W/(m\(\cdot\)K) for Cu, Al, and Zn, respectively, at 20 \({}^{\circ}\)C and 1 bar [46]), and X-ray absorptivity. We conclude that X-rays can cause a temperature increase in the surroundings of a material, with differences in temperature depending on the type of material. Although, in our experiment, we observed a similar trend in temperature change between the capillary solutions and metal plates, the precise temperature difference between the solutions in capillaries or on the Al and Zn plate could not be distinguished experimentally. To measure the temperature more precisely in future, it will be necessary to conduct measurements in a controlled environment, such as a thermal enclosure. Meanwhile, we were unable to measure the temperature at the X-ray interaction point in this experiment because the RTD could not be directly exposed to the X-rays. Instead, based on the result that the temperature increased as the temperature sensor was closer to the X-ray exposed area, we estimate that the temperature at the X-ray exposed location is higher than the temperature we measured. Based on our results, the structures previously determined at room temperature using capillaries in MX have the potential to be determined at temperatures higher than the actual room temperature [47]. Similarly, in previous SSX studies, diffraction data were collected at room temperature by delivering crystals at a low flow rate via a capillary [48] or through a viscous medium [49; 50; 51; 52]. During SSX data collection at room temperature, if X-ray exposure raises the temperature around the X-ray beam point on the capillary or viscous material, the crystal itself will increase in temperature before the X-ray interaction. Accordingly, as the crystal at the X-ray interaction point reached elevated temperatures, the actual structure is likely to have an even higher temperature. The possibility of such a temperature increase during these SSX experiments needs to be investigated experimentally in the future. Furthermore, temperature is an important factor for the stability of macromolecule crystals [53; 54]. In previous SSX experiments using capillaries, a lysozyme solution was used as a model sample, due to its high crystal stability and excellent diffraction intensity [48; 49; 50; 51; 52]. Accordingly, it is presumed that the crystal stabilizes even when the temperature increases. If a temperature-sensitive sample is used, the diffraction quality of the crystal is affected by an increase in temperature. Here, we conducted an experiment to reduce the extent of the temperature increase during exposure and non-exposure to X-rays by using an X-ray shutter.