Management of surface contamination for items and floors is crucial during decommissioning operations at the TEPCO's Fukushima Daiichi Nuclear Power Station and in nuclear fuel-handling facilities. In particular, understanding the distribution of alpha contamination is vital for internal exposure control. However, in environments with extremely high background radiation, such as gamma rays and neutrons, conventional alpha survey meters suffer from spurious counts generated by these radiation types, making accurate measurement difficult.

Therefore, we have developed a handheld survey meter that integrates the detector, signal processing unit, 2D display, and battery into a unit compact enough to be held in one hand. It is capable of accurately detecting and "visualizing" only alpha rays, even in the presence of other radiation types (Fig. 1). The instrument employs a structure in which a scintillator (a material that luminesces upon radiation exposure) is directly coupled to a multi-pixel photosensor. By optimizing both the opacity and thickness of the scintillator, the device ensures that the highly localized light produced by an alpha ray reaches only a small subset of photosensor pixels directly, enabling clear discrimination between alpha rays and other types of radiation.

We demonstrated that the device exhibits negligible false counts in high-gamma environments exceeding 1 Sv/h and in neutron-field environments of 300 µSv/h. It was shown to visualize contamination levels ranging from very low values (approximately 1/100th of the legal surface limit for alpha, 4 Bq/cm²) to levels exceeding the upper measurement range of conventional instruments. This technology is expected to contribute significantly to radiation-safety management in decommissioning and dismantlement operations.

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