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High-Resolution Alpha-Autoradiography with Contact Microscopy Technique

We have been developing a novel method for high-resolution neutron-induced alpha-autoradiography (NIAR) using CR-39 plastic track detectors and an atomic force microscope (AFM) with contact microscopy technique. In this technique,sliced samples such as tissues including boron compounds are mounted on CR-39 plates, and then irradiated by thermal neutrons. The irradiated samples are exposed to soft X-rays, and then etched in NaOH solution for short time.Etch pits for alpha/lithium particle tracks and relief for transmission X-ray image of the specimen can be observed on the CR-39 surface with an AFM at about 100 nm resolution. In the NIAR, discrimination of proton background is required for quantification of boron concentration, and incident angle measurement is significant for accurate positioning of the alpha/lithium tracks on the specimen image. These properties were tested for the measurement of small etch pits observed with an AFM. It was confirmed that alpha/lithium particles and protons could be distinguished by etch pit size, and that incident angle of those charged particles could be measured at the accuracy of several degrees. These informations approve the reliability of the NIAR, especially for the subcellular measurement of boron compound distribution inside a cell in boron neutron capture therapy (BNCT).