- 第3日 6月24日（金） 15:00～15:20 D会場（413，414）
The helium isotope ratio (3He/4He ratio) varies among the geochemical reservoirs, such as the atmosphere, crust, and mantle. Since volcanic gases are derived from magma and released to the earth's surface through various processes, the 3He/4He ratio reflects the mixing ratios of components originating from each reservoir. The 3He/4He ratio of volcanic gases is expected to be a new tool of volcano monitoring, because the 3He/4He ratio may change before an eruption due to increase in magmatic activity. For the measurement of the 3He/4He ratio, a magnetic field mass spectrometer with a total weight of more than 1 ton has been used because the sensitivity to detect trace amounts of 3He and the mass resolution to distinguish 3He+ from HD+ are required. Therefore, it is necessary to bring the sample back to the laboratory to measure the 3He/4He ratio, and it is difficult to perform real-time monitoring, which is essential for volcanic observation. To make this possible, it was necessary to develop a method for on-site analysis using a portable mass spectrometer and a compact helium extraction system. We used a portable multi-turn time-of-flight mass spectrometer (MULTUM), which is small enough to be carried around and has high mass resolution enough to discriminate 3He+ from HD+. As a result of several improvements in sensitivity of MULTUM, it is possible to measure the 3He/4He ratio with an accuracy of about 4% per hour.