Operation “Pinpointing Tree Hollows” to Scan for Tree Health with Gamma Ray 

Chula nuclear engineers developed Thailand’s only gamma-ray tree scanner to pinpoint potential cavities in the trunks and roots that may cause the tree to fall and to inspect the quality of wooden structures while striving to increase the device’s accuracy. 

Cities need big trees to provide shade, purify the air, filter the dust, and fill the city with spectacular scenery and vibrancy, but often we hear of trees falling in storms and high winds, causing damage to properties and people’s lives. 

Taking down the trees to prevent and avoid such disasters should not be the best measure; instead, we should regularly monitor their health to keep the trees standing strong. Observing the health of the trees from the outside with the naked eye—watching the branches of the leaves—may not be enough, because “the outside may be bright, the inside may be hollow”. 

Dr. Manasavee Lohvithee, a member of a team of lecturers and students from the Department of Nuclear Engineering, Faculty of Engineering, Chulalongkorn University, has developed Thailand’s first and only device for scanning the inside of the trees using gamma rays to check their health, identify cavities, and measure the density of wood. Since 2021, the Chulalongkorn Nuclear Engineering team has developed the device to monitor the health of large trees in Chulalongkorn University under the Chula Big Tree Project: Phase 1. This includes giving tree health inspection services in various areas in Bangkok. For example, the inspection of big trees in Soi Somkid between Central Embassy and Central Chidlom and the inspection of wood quality for construction and architectural projects like the teakwood for the construction of the Community Hall at the Parliament complex (Kiakkai).  Lately, the Department was contacted by the Department of City Planning and Urban Development, Bangkok Metropolitan Administration (BMA) to discuss plans for the application of tree inspection techniques for the inspection of wooden architecture such as The Giant Swing (Sao Ching Cha). 

How do gamma rays point out the hollows inside tree trunks? 

Dr. Manasavee explains the choice of gamma ray in pinpointing the hollows inside tree trunks operation. 

“The characteristic of Gamma ray can be implied in a similar way as light: When obstructed by an object, the light cannot pass through, and when there is no obstruction, all light can pass through, and so can gamma ray.” 

Dr. Manasavee explains further that if there is no obstruction between the source and the radiation detector, a lot of radiation can be detected. But when there is an object to shield the radiation source, the amount of radiation detected could be more or less dependent on the properties, thickness, and ability to block radiation of the object. 

“Metal and materials with high atomic numbers, such as lead, have good gamma-ray attenuation properties. If it is placed between the radiation source and the radiation detector, it will almost completely block the radiation. This depends on the thickness of the material as well,” Dr. Manaswee said. “But if there is no object to shield the radiation, it means that the area is all air. The radiation detector can receive the total amount of radiation.” 

This difference in penetration abilities is why gamma radiation is used to locate cavities inside the trees. This is also known as the Gamma Transmission Technique. 

“A tree with high density and fully wooded will block almost all gamma rays. If a tree has a hollow inside, it will appear like there’s no object to block the ray, so the reading from the detector will show as such, and we will know the areas of the tree where there are hollows,” Dr. Manasavee explains. 

As for the source of gamma radiation used in the tree hollow examination, Dr. Manasavee said it comes from Cesium-137. Although natural in origin, it is not commonly found, as it is a controlled element for its potential radiation hazards. Cesium-137 must therefore only be purchased from the authorized authority of the regulator, the Office of Atoms for Peace. 

Tree Health Scanning Device 

Common tree health inspection uses a tool called the Resistograph, which requires drilling into the tree. Although effective in determining whether the tree has a hollow, it destroys the tree and causes negative long-term effects. However, the gamma-ray scan by the Chula nuclear engineering team is a non-destructive technique, aiming to inspect the internal of the tree without penetration into the wood at all. 

The gamma-ray scanner for tree health examination, designed by nuclear engineering master’s degree students, consists of three parts: 

1. Radioactive source housed in a cylindrical shield with a hole.
2. Radiation Detector – a cylindrical electronic device that measures radiation. 
3. Mechanical device to be installed around the tree. 

“We need to install and align the radiation source and radiation detector around the tree so that the radiation penetrates the tree to reach the radiation detector. We use the water level gauges to measure the alignment. After that, the device will be operated by mechanical parts so that the radiation detector and radiation source move from side to side through the tree simultaneously so that the changing amount of radiation can be detected,” Dr. Manasavee explains. 

The set of devices works automatically. At the touch of a button, the system works by irradiating radiation through the trees and collecting 1 plane of data. 

“When we see that the radiation reading is inconsistent and the tree is assumed to be hollow, we will confirm by turning the device to a perpendicular 90-degree angle and running the measurement again on a vertical plane to confirm the location, direction, and size of the cavity.” 

Detecting the cavity deep to the root 

In addition to scanning the trunk for cavities in the trees, in 2024,, the Nuclear Engineering Department has improved the gamma-ray scanner to inspect underground around the root area. 

“Roots are very important as they serve as an indicator if the tree can still stand,” Dr. Manasavee explained and shared that this tree root scanning project was an initiative of Associate Professor Nares Chankow, who assigned his fourth-year students to experiment with Gamma Radiation Transmission to analyze the characteristics under the soil in the tree root area.

“Trunk scanning uses one radioactive source and one detector, but the root scanner uses one radioactive source, with a tube leading underground, to release radiation through the root to the 8 radioaction detectors above ground to cover all the required angles,” explains Dr. Manasavee detailing the more challenging root scanning operation. 

“Scanning the roots is challenging because the roots are small. Even though the tree is large, the roots are only no more than 5-10 cm in diameter. They also spread extensively, so detection of the radiation is inconsistent. The soil is also an important variable, such as types (clay, sandy soil, etc.), porosity, and soil moisture, which can affect the attenuation of the radiation, making it difficult to measure.” 

“The students are still working on fine-tuning the instrument for better underground measurement. Using multiple probes and changing the probe position will help solve the current problems. They also study the way to measure the radiation more accurately by including both penetrating and scattering radiation from various angles.” 

Taking Care of the Trees’ Health 

One factor to determine the health of the trees is based on the location and size of the cavity. “If the cavity is larger than half the size of the trunk, it could mean that the tree may not be healthy,” said Dr. Manasavee. 

Although the tree hollow is an indicator of tree health, there are other factors and surrounding elements that arborists use in assessing the tree’s well-being, such as growth tissue (Cambium). 

“Growth tissue (Cambium) is located between the wood and the outer bark. Even when the tree has a large cavity in the middle of the trunk, but if the growth tissue is strong enough, it can support the structure of the tree. However, if the cavity is close to the growth tissue, it means that the ability of the growth tissue to support the tree is reduced.” 

There are many ways to treat a tree with a cavity, such as treating it with medication, injecting substitutes into the cavity, and using tree support. 

 “In principle, an expert must inform you that a tree has signs of risks. After that, we will scan the tree for hollows to confirm. Arborists will look at the data and decide if the tree would need support. The civil engineering department will install tree support based on the location of the cavity specified in the data. The support has to be there permanently.” 

Developing a device that can read at precise angles 

The Nuclear Engineering Department continues to improve the device so that it can accurately measure the radiation output to precisely pinpoint the cavity location within the trees to prolong their lives. 

“In the past, the mechanical component of the system was a half-plastic, half-rubber belt, causing it to sometimes not move at the same time. There is no stability. One push of the button will gain 30 seconds of data collection, then the button must be pushed again to move data collection to the next spot.” 

“For the current equipment, we have used the data gathered on locations to stabilize the device further. We stopped using the plastic and rubber belt and have changed to steel belts, aiding the movement to be accurate in a short distance of about 2 cm. So we get more detailed data. Also, the device can move automatically. With one push of a button, the whole system can operate in one go. We can also control the device with our mobile phone using the wifi signal, giving the staff more distance from the radiation measurement and more safety.” 

In the future, Dr. Manasavee and the gamma-ray scanner development team intend to make the device more detailed and easier to install. 

“We want to make the device rotate at smaller angles like 0, 10, 20, and 30 degrees. The more detailed angles will help us know the true size of the cavity. We want to design a mechanical system as a rotating disk that can change the angle and collect data without having to constantly uninstall and reinstall the device for each different angle. We want the data to be cross-sectional, similar to entering a CT scan tunnel, giving us a more detailed description of the cavity’s direction with a single scan.” 

Radiation transmission techniques are principles utilized in industries such as neutron radiation, X-rays, and gamma radiation. The department is working on coming up with a more appropriate design and choosing the right radioactive source in relation to the object to be tested to help further develop human resources and benefit society. 

Agencies interested in scanning trees with gamma radiation can contact the Department of Nuclear Engineering, Faculty of Engineering, Chulalongkorn University, Tel. 0-2218-6781 

Email: nutech.chula@gmail.com
Facebook: https://m.facebook.com/NuclearChulaEngineering/ 

Watch an interview on using gamma rays for tree scanning

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