9:30 AM - 9:45 AM
[SVC29-03] Continuous Radon measurement at Owakudani, Hakone Volcano
Keywords:Radon measurement, Hakone Volcano, Phreatic eruption, Owakudani, Volcanic gas
There have still been several cases of sudden eruptions threatening human lives in this century even in Japan. This may be attributed to the fact that the eruption process is difficult to identify in the absence of significant signals related to magma ascent. Phreatic eruptions fall into this category, which are also important for volcanic disaster mitigation. Thus, we have carried out observation research at Owakudani, Hakone Volcano, with the aid of KAKENHI (21K04602) aiming at understanding any signs of such phreatic eruptions. The ultimate goal of the project is a continuous measurement of helium gas concentrations, which may immediately reflect any change in underground conditions. Our prototype instrument is being designed as for equipment utilizing the high permeability of helium gas. In parallel with this development, continuous radon observations are also being carried out to test the durability of the equipment in an atmosphere with corrosive volcanic gases and to make preliminary observations of the timescale of changes in volcanic gas concentrations, to evaluate the possibility of long-term in-situ observations.
The radon measuring device is a portable radon meter Alpha GUARD2000 (manufactured in 2010) of Saphymo GmbH, Germany. It has an ionisation chamber with an effective volume of 500 mL inside. Environmental air is introduced into the chamber with high voltage applied, and radiation of radon decay is measured. The sensitivity to gamma radiation is not high, as the electromagnetic pulses of charged particles emitted by the decay of radionuclides in the chamber are measured. Because ambient air is introduced into the chamber through a paper filter, it is expected to measure virtually all radon decays without distinction of nuclides. To reduce corrosion, the instrument is housed in a plastic container box and the air outside the box is pumped through the paper filter by a small air pump (Alpha Pump), which is a genuine accessory of the radon meter. The through-holes are sealed with caulking material, and packing with EPDM-rubber gap tape is inserted between the box lid and body of the container for effective sealing. Data is stored in the instrument itself and regularly imported into a Windows PC on which a genuine database (DataExpert) for data storage is installed.
The radon concentration is lower than Japan’s national average in indoor environments because this observation is conducted in an open outdoor dry area, and the average over the observation period is around 6 Bq/m3. The average indoor radon concentration in Japan is around 16 Bq/m3 (Suzuki et al., 2010; Ministry of the Environment, 2015). Although the fluctuations are generally about the square root of counting, they can also increase in pulses beyond the degree of counting error. Such sudden increase mostly returns to the level before the increase within 10-20 minutes, which is the next or the second next measurement. As the observation site is at an elevation of more than 1,000 metres, the atmospheric pressure is around 900 hPa throughout the observation period; the Radon concentration is not corrected to the standard atmospheric pressure. In addition to the 10-15 days and 3-5 days cycles due to frequent increases in pulse-like instantaneous values. Further, the radon concentration has been increasing since 24 September. This change in radon concentration can be regarded not only as an increase in the maximum value, but also as an increase in the baseline; the data collection in September was carried out at 13:00 on 13 September, so it was not due to changes in the measurement condition or interruptions caused by unpacking the equipment. We will further watch whether there are any changes in these two cycles or increased Radon concentration since last September. This radon observation will continue until the helium monitoring equipment is rolled out.
The radon measuring device is a portable radon meter Alpha GUARD2000 (manufactured in 2010) of Saphymo GmbH, Germany. It has an ionisation chamber with an effective volume of 500 mL inside. Environmental air is introduced into the chamber with high voltage applied, and radiation of radon decay is measured. The sensitivity to gamma radiation is not high, as the electromagnetic pulses of charged particles emitted by the decay of radionuclides in the chamber are measured. Because ambient air is introduced into the chamber through a paper filter, it is expected to measure virtually all radon decays without distinction of nuclides. To reduce corrosion, the instrument is housed in a plastic container box and the air outside the box is pumped through the paper filter by a small air pump (Alpha Pump), which is a genuine accessory of the radon meter. The through-holes are sealed with caulking material, and packing with EPDM-rubber gap tape is inserted between the box lid and body of the container for effective sealing. Data is stored in the instrument itself and regularly imported into a Windows PC on which a genuine database (DataExpert) for data storage is installed.
The radon concentration is lower than Japan’s national average in indoor environments because this observation is conducted in an open outdoor dry area, and the average over the observation period is around 6 Bq/m3. The average indoor radon concentration in Japan is around 16 Bq/m3 (Suzuki et al., 2010; Ministry of the Environment, 2015). Although the fluctuations are generally about the square root of counting, they can also increase in pulses beyond the degree of counting error. Such sudden increase mostly returns to the level before the increase within 10-20 minutes, which is the next or the second next measurement. As the observation site is at an elevation of more than 1,000 metres, the atmospheric pressure is around 900 hPa throughout the observation period; the Radon concentration is not corrected to the standard atmospheric pressure. In addition to the 10-15 days and 3-5 days cycles due to frequent increases in pulse-like instantaneous values. Further, the radon concentration has been increasing since 24 September. This change in radon concentration can be regarded not only as an increase in the maximum value, but also as an increase in the baseline; the data collection in September was carried out at 13:00 on 13 September, so it was not due to changes in the measurement condition or interruptions caused by unpacking the equipment. We will further watch whether there are any changes in these two cycles or increased Radon concentration since last September. This radon observation will continue until the helium monitoring equipment is rolled out.