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Abstract: The aim of this research is to observe phenomena inside of a H2 18O water target during the proton beam irradiation. We made a target unit which enabled to visualize and tested it by using a 30MeV proton beam from a cyclotron at KIRAMS. In the case of a closed target system, we found that the bubbles inside of the target become smaller and dispersed over the volume as the beam intensity increases and the pressure on the water becomes higher. As a result of this study, it turned out that the target can absorb the initial proton energy completely and there arises no need to have an additional energy degradation length. It has been also found that the target has a saturation point of the irradiation current to the liquid phase due to bulging of the target windows. Consequently, RI production yield in the target depends on this value. For designing a high yield water target, suppression of the target windows and improvement of the cooling efficiency are required. [Copyright &y& Elsevier]