Novel Method for Using Neutron Beam to Inspect Concrete Structures for Salt Content

Scientists at the RIKEN Center for Advanced Photonics (RAP) have employed a technique, with the help of the RANS compact neutron source, to non-destructively quantify the salt content of structures like tunnels, bridges, and elevated roadways, which can be degraded due to exposure to salt from seawater and other sources.

The deaths of 37 people due to the collapse of a bridge in Genoa, Italy, in August 2018 has highlighted the risk posed by aging infrastructure. Similar to many countries, Japan is faced with crucial problems since a majority of its tunnels and bridges were built at the time of the high economic growth in the 1960s and 1970s and are now facing degradation. However, inspecting these structures is a time-intensive task. For instance, evaluating the salt content of cement structures is usually carried out by boring out a core—an action that is not only time-consuming but can also damage the structure to a certain extent.

The researchers chose to look for a better method for carrying out inspections, using a neutron beam—a device in which high-energy neutrons are emitted in a beam—emitted by a compact neutron source that they had created. Neutrons are an impressive and innovative means for imaging structures since they can penetrate very far into metallic materials, which is feasible due to the fact that they do not interact through the electromagnetic force, and thus are not influenced by electric charge. Occasionally, they tend to interact with nuclei in the materials through which they penetrate, resulting in the release of gamma rays that can be detected.

For this experiment, the researchers employed their compact neutron source, which produces neutrons by bombarding a beryllium target with protons. The beam was used to irradiate a series of concrete blocks with salt squeezed between them, where “prompt” gamma rays—gamma rays emitted instantaneously following irradiation by neutrons—were measured by high-resolution germanium detectors. Different elements can be detected by evaluating the energy of the prompt gamma rays emitted from the atoms in the concrete blocks. For instance, the energy peaks from the prompt gamma rays emitted from chlorine—one of the components of salt—are 517, 786, 788, 1165 keV, and so on.

Through this experiment, the team was able to show the presence of salt even when it was surrounded by between 12 and 18 cm of concrete. The time taken for each measurement was around 10 minutes.

This is very exciting, because Japan is suffering from serious infrastructure degradation, and it is impossible to predict when a major accident will happen. Our feasibility study has shown that neutron beams can indeed be used to measure whether the salt content of a concrete structure is within the legal limits set by the government. Our next challenge is to build a compact neutron source that is small enough to be readily transported to various infrastructures to conduct measurements.

Yoshie Otake, Study Lead Author, RIKEN Center for Advanced Photonics

The outcomes of the study were presented in October at the 18th JSMS Symposium on Concrete Structure Scenarios, held by the Society of Materials Science, Japan.

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