Neutrons have the power to penetrate metals or heavy elements such as calcium, silicon, and iron. Neutrons also have high sensitivity, so they can be used to detect elements such as boron and chlorine. An accelerator-driven, compact neutron system has been developed in RIKEN for practical use on job sites.In this paper, a pixel imaging detector for fast neutrons with energy levels above 1 MeV is developed and used to produce images of an iron rod and air pockets through 30 cm of concrete. Also, the salt concentrations of 4 cm- and 5 cm-thick mortar blocks are measured, and a correlation diagram is obtained for up to 1 kg/m3.

We are studying the non-destructive detection of salt gradients in concrete using a neutron-captured promptgamma-ray analysis at the RIKEN Accelerator-driven compact Neutron Source (RANS). The neutrons at RANS are generated via a 7-MeV proton and 300-µm-thick beryllium target and are moderated via a polyethylene moderator. Samples, e.g., mortar samples with varying salt contents and part of an existing salt-damaged bridge, were irradiated by neutrons; then, the gamma rays following the neutron capture reaction were detected by germanium detectors placed near the samples. From the results, we obtained the detection sensitivity of the chloride content in a medium.

As a nondestructive measurement of chloride ion distribution in concrete is important from the viewpoint of preventive maintenance against chloride attack causing deterioration of many concrete structures, a diagnostic technique of a nondestructive measurement method using a neutron-captured prompt gamma-ray analysis (PGA) is being developed. As the first step of development, the γ-ray sensitivities of mortar samples with different chloride ion concentrations were determined experimentally by PGA using the RIKEN accelerator-driven compact neutron source. The results showed that the present detection system was sensitive to a chloride ion concentration of 1 kg/m 3 , which is lower than the marginal chloride ion concentration of 1.2 -2.5 kg/m 3 to incur corrosion. The time of flight measurement technique with pulsed neutrons was applied concerning the depth profile of chloride ion distribution in concrete.

To meet strong demand for realizing an effective tool to diagnose salt distribution in concrete infrastructures, we have started a development of a new technique using a prompt gamma neutron activation analysis (PGNAA) at the RIKEN accelerator-driven compact neutron source (RANS). So far, by applying PGNAA we have experimentally confirmed that neutrons from RANS can detect small enough amounts of chlorine within the marginal concentration of around 1.2 kg/m3 to involve steel corrosion. In this study, we have proposed two methods to derive the salt depth profile which is critical information of steel corrosion start. The first one utilizes a difference in the intensity ratio of two different γ-ray energies of interest, which is depending on the depth where the neutron capture reaction arises inside the concrete. The second is called the collimator method that measure γ-rays coming through a collimator around detector. Detection of γ-ray associated with 35Cl coming from the assembly of concrete has been also simulated with conditions of neutrons from RANS and a collimator. The feasibility of the method was discussed.