1:45 PM - 3:15 PM
[O11-P119] Measurement of Muon Attenuation by Water Using Cosmic Watch
[Motivation and Objective]
Muography has been widely used for imaging inner structures such as volcanoes and pyramids. In 2021, however, the Tokyo-bay Seafloor Hyper KiloMetric Submarine Deep Detector was installed in Tokyo Bay, successfully observing astronomical tides in real time for the first time. This breakthrough inspired my interest in applying muography to marine phenomena. The aim of this project is to observe the attenuation of muons by water in a simple way using Cosmic Watch. The goal of the research is to measure muon attenuation through water in a tank and examine the correlation between attenuation rate and water depth.
[Equipment]
This study used a cosmic ray detector called Cosmic Watch. Its mechanism is as follows (Figure A): when cosmic rays excite the scintillator, it emits light, which is then converted into an electrical signal by the SiPM (Silicon Photomultiplier). The signal is processed through an amplification/shaping circuit, and only signals exceeding a certain threshold are considered radiation signals. The peak voltages are converted into digital values ranging from 0 to 1023 and that data is recorded on an SD card. Environmental parameters such as temperature, humidity, and air pressure are also logged.
Also, to measure muons at an angle toward the water tank, I made a 30° inclined platform using cardboard based on a 1:√3:2 ratio.
[Method]
Measurements were conducted on February 5, 2025, at the Water's Blessing Tokyo Tank at Sumida Aquarium in Tokyo Skytree Town®. The Cosmic Watch detector was tilted 30° from the vertical, and measurements were taken at three different places, using two detectors stacked to ensure that only muons passing through both were recorded.
First, measurements were taken above the water tank (Figure B). Next, measurements were taken from the side of the tank. Only cosmic rays that had passed through the aquarium were detected (Figure C). Since the main tank is about 6 meters deep and the detector was tilted at 30°, muons passing through approximately 7 meters of water were detected. Lastly, measurements were taken from the side of the tank, but oriented to avoid muons passing through water (Figure D).
[Results and Discussion]
The data analyzed using Google Colaboratory are shown in Figures E, F, and G. The vertical axis shows the number of counts (detections), and the horizontal axis represents ADC values, which are integer values from 0 to 1024 digitally processed by the Arduino. Higher ADC values mean higher muon energy.
From the measurements above the tank, the detection frequency was about 0.1 counts per second. When measuring through the tank, the frequency dropped to about 0.05 counts per second. On the other hand, when measuring beside the tank (without passing through water), the frequency was around 0.1 counts per second. These results indicate that about 7 meters of water can reduce muon frequency by approximately half, while attenuation through air is trivial.
[Prospects]
Several problems arose through this experiment. First is the attenuation caused by the acrylic panel of the tank. The thickness of the tank’s acrylic wall is said to be about 280 mm, and it is likely that this also contributed significantly to the observed attenuation. This extraneous effect must be cut off to improve the accuracy of this experiment.
Second is about the power supply for the Cosmic Watch. A mobile battery was used to power the detector, but the device frequently shut down, leading to inconsistencies in each measurement duration. To prevent this, further trials to stabilize the power supply are necessary.
Finally, data from various water depths is needed. Since this experiment only considered a single water depth (about 7 meters), it merely confirmed that muon attenuation in water is significantly higher than in air. Collecting data at multiple depths would allow a more detailed analysis of how attenuation rate changes with water depth.
Muography has been widely used for imaging inner structures such as volcanoes and pyramids. In 2021, however, the Tokyo-bay Seafloor Hyper KiloMetric Submarine Deep Detector was installed in Tokyo Bay, successfully observing astronomical tides in real time for the first time. This breakthrough inspired my interest in applying muography to marine phenomena. The aim of this project is to observe the attenuation of muons by water in a simple way using Cosmic Watch. The goal of the research is to measure muon attenuation through water in a tank and examine the correlation between attenuation rate and water depth.
[Equipment]
This study used a cosmic ray detector called Cosmic Watch. Its mechanism is as follows (Figure A): when cosmic rays excite the scintillator, it emits light, which is then converted into an electrical signal by the SiPM (Silicon Photomultiplier). The signal is processed through an amplification/shaping circuit, and only signals exceeding a certain threshold are considered radiation signals. The peak voltages are converted into digital values ranging from 0 to 1023 and that data is recorded on an SD card. Environmental parameters such as temperature, humidity, and air pressure are also logged.
Also, to measure muons at an angle toward the water tank, I made a 30° inclined platform using cardboard based on a 1:√3:2 ratio.
[Method]
Measurements were conducted on February 5, 2025, at the Water's Blessing Tokyo Tank at Sumida Aquarium in Tokyo Skytree Town®. The Cosmic Watch detector was tilted 30° from the vertical, and measurements were taken at three different places, using two detectors stacked to ensure that only muons passing through both were recorded.
First, measurements were taken above the water tank (Figure B). Next, measurements were taken from the side of the tank. Only cosmic rays that had passed through the aquarium were detected (Figure C). Since the main tank is about 6 meters deep and the detector was tilted at 30°, muons passing through approximately 7 meters of water were detected. Lastly, measurements were taken from the side of the tank, but oriented to avoid muons passing through water (Figure D).
[Results and Discussion]
The data analyzed using Google Colaboratory are shown in Figures E, F, and G. The vertical axis shows the number of counts (detections), and the horizontal axis represents ADC values, which are integer values from 0 to 1024 digitally processed by the Arduino. Higher ADC values mean higher muon energy.
From the measurements above the tank, the detection frequency was about 0.1 counts per second. When measuring through the tank, the frequency dropped to about 0.05 counts per second. On the other hand, when measuring beside the tank (without passing through water), the frequency was around 0.1 counts per second. These results indicate that about 7 meters of water can reduce muon frequency by approximately half, while attenuation through air is trivial.
[Prospects]
Several problems arose through this experiment. First is the attenuation caused by the acrylic panel of the tank. The thickness of the tank’s acrylic wall is said to be about 280 mm, and it is likely that this also contributed significantly to the observed attenuation. This extraneous effect must be cut off to improve the accuracy of this experiment.
Second is about the power supply for the Cosmic Watch. A mobile battery was used to power the detector, but the device frequently shut down, leading to inconsistencies in each measurement duration. To prevent this, further trials to stabilize the power supply are necessary.
Finally, data from various water depths is needed. Since this experiment only considered a single water depth (about 7 meters), it merely confirmed that muon attenuation in water is significantly higher than in air. Collecting data at multiple depths would allow a more detailed analysis of how attenuation rate changes with water depth.