11:30 AM - 11:45 AM
[SVC31-10] Sequential Radon measurement at Owakudani, Hakone Volcano(2nd Report)
Keywords:Sequential Measurement, Radon Concentration
Monitoring of radon concentration continues at Owakudani, Hakone, Japan, as a preliminary observation toward the realization of long-term helium gas in-situ monitoring. This is a part of a Grant-in-Aid for Scientific Research (KAKENHI) toward the understanding the signs of phreatic eruptions (Project No. 21K04602). There have been several phreatomagmatic eruptions in this century in which the preparatory process for an eruption was not recognized as a clear phenomenon, and the eruption suddenly struck, threatening human lives. This project directs to recognize any pre-eruption phenomenon in such phreatomagmatic eruptions. The final goal of this project is to continuously observe the helium concentration in the surrounding atmosphere, and we are developing prototypes of equipment that can separate helium gas from ambient atmosphere using the difference in gas permeability. In parallel with this, this radon observation is a test of the durability of the equipments under an environment filled with corrosive volcanic gases, as well as a preliminary observation of the timescale of volcanic gas concentration fluctuations. The Alpha GUARD2000, a portable radon meter manufactured by Saphymo GmbH of the Federal Republic of Germany, has an ionization chamber of which effective volume is 500mL. The chamber was filled with ambient air through a paper filter sending from a small air pumpand sealed in a plastic container. The seal of container is broken approximately once a month at the time of data collection, and except for the corrosion of the metal part of the serial cable converter in the 21st month, no particular corrosion has been observed, and the instrument has been operating well after two years of installation.
Radon concentration measurement started in January 2023, and continuous recording of data has continued from July 2023 to the time of this report (February 2025), and is still ongoing. The only interruption of more than one day during this period was from October 1 to 9, 2024. Radon concentration is measured as radioactivity in becquerels per second (Bq/s), and instrument temperature, relative humidity, and atmospheric pressure are recorded as well as radiation counts. Since the observation site is located at an altitude of more than 1,000 meters, the atmospheric pressure is around 900 hPa throughout the observation period, but the counts have not corrected for this to the standard atmospheric pressure. Because it is an outdoor areaway of a building, the radon concentration is lower than the national average (indoors) and averages about 6 Bq/m3 throughout the observation period. Here, the average indoor radon concentration in Japan is about 16 Bq/m3 (Suzuki et al., 2010; Ministry of the Environment, 2015). As in last year's report, the variation in radon concentration is generally about the square root of the counts.
The radon counting records after last year's report are shown in time series (Figure). A closer look reveals that pulse-like increases sometimes exceed the ranges of counting error, and in many cases, the value at 10 minutes after the next measurement decreases, but does not necessarily return to the pre-increase level. As in the previous reporting period, some periods of several to ten days characterized with the frequent increase in such pulsious increases were found. In addition, there are periods when the average counts appear to increase from the previous and following ones. The most obvious periods are (①) July 28-August 26, 2024 and (②) November 7-24, 2024, when radon concentrations averaged 5.6 Bq/m3 and 6.6 Bq/m3, respectively: however, since the respective 1 SD (sample standard deviation) values were around 4.7 Bq/m3 and 5.0 Bq/m3, these differences were not statistically significant. On the other hand, there was a significant increase in seismic activity on April 30, 2024 in the vicinity of Ohwakudani, Hakone, but no particular change in radon concentration was observed during that period (③). We will continue to monitor for any changes in these cycles and occasional increases of counts, and plan to continue observations at the same location until the helium observation equipment is completed, and after the helium observation equipment is installed, we will relocate to the same location as the helium observation equipment.
Radon concentration measurement started in January 2023, and continuous recording of data has continued from July 2023 to the time of this report (February 2025), and is still ongoing. The only interruption of more than one day during this period was from October 1 to 9, 2024. Radon concentration is measured as radioactivity in becquerels per second (Bq/s), and instrument temperature, relative humidity, and atmospheric pressure are recorded as well as radiation counts. Since the observation site is located at an altitude of more than 1,000 meters, the atmospheric pressure is around 900 hPa throughout the observation period, but the counts have not corrected for this to the standard atmospheric pressure. Because it is an outdoor areaway of a building, the radon concentration is lower than the national average (indoors) and averages about 6 Bq/m3 throughout the observation period. Here, the average indoor radon concentration in Japan is about 16 Bq/m3 (Suzuki et al., 2010; Ministry of the Environment, 2015). As in last year's report, the variation in radon concentration is generally about the square root of the counts.
The radon counting records after last year's report are shown in time series (Figure). A closer look reveals that pulse-like increases sometimes exceed the ranges of counting error, and in many cases, the value at 10 minutes after the next measurement decreases, but does not necessarily return to the pre-increase level. As in the previous reporting period, some periods of several to ten days characterized with the frequent increase in such pulsious increases were found. In addition, there are periods when the average counts appear to increase from the previous and following ones. The most obvious periods are (①) July 28-August 26, 2024 and (②) November 7-24, 2024, when radon concentrations averaged 5.6 Bq/m3 and 6.6 Bq/m3, respectively: however, since the respective 1 SD (sample standard deviation) values were around 4.7 Bq/m3 and 5.0 Bq/m3, these differences were not statistically significant. On the other hand, there was a significant increase in seismic activity on April 30, 2024 in the vicinity of Ohwakudani, Hakone, but no particular change in radon concentration was observed during that period (③). We will continue to monitor for any changes in these cycles and occasional increases of counts, and plan to continue observations at the same location until the helium observation equipment is completed, and after the helium observation equipment is installed, we will relocate to the same location as the helium observation equipment.