Development of an alpha- and beta-imaging detector using a thin-stilbene plate for radon-222 progeny measurements

Morishita, Yuki

no journal, , 

Radon-222 (Rn) is a naturally-occurring radioactive gas, and the measurement of this isotope and its progeny is of interest from the viewpoint of protection against internal exposure. The author has developed a new alpha/beta-imaging detector combined with a waveform digitizer capable of imaging alpha- and beta-particle locations simultaneously. A 0.9-mm-thick stilbene plate was optically coupled to a position-sensitive photomultiplier tube (PSPMT), and the analog signals from the PSPMT were transferred in parallel to the waveform digitizer. For 5.5-MeV alpha particles, the detection efficiency was 97.2% for 2 steradians, and the energy resolution of the 5.5 MeV alpha peak was 21.6% full width at half maximum (FWHM). The imaging detector was able to discriminate between alpha and beta particles via the pulse-shape-discrimination (PSD) technique, as well as being capable of alpha- and beta-particle imaging. The imaging detector was used for Rn-progeny measurements. When measuring the Rn progeny, both Bi and Po are imaged simultaneously, and a correlation is found between the positions of Bi and Po. The developed detector will be useful as a Rn detector and in alpha-continuous-air monitoring for nuclear facilities.

An In-situ alpha air monitor for the retrieval of fuel debris at the Fukushima Daiichi Nuclear Power Station

Honda, Fumiya; Tsubota, Yoichi; Tamakuma, Yuki*; Tokonami, Shinji*; Ikeda, Atsushi; Nakagawa, Takahiro

no journal, , 

A large number of radioactive aerosols, especially alpha particles, are expected to be generated during the fuel debris retrieval on the Fukushima Daiichi Nuclear Power Station. From the viewpoint of preventing contamination of the surrounding environment and the vicinity of the human-access area, it is important to measure the activity concentration of airborne radioactive substances inside the primary containment vessels (PCVs). In particular, it is necessary to monitor the concentration of particulates containing -nuclides (-aerosols), which have high effective dose coefficients upon inhalation. This presentation reports the development of an in-situ alpha air monitor (IAAM) for direct measurement of -aerosols by combining a flat-type flow path (FFP), an air heater, a ZnS scintillator, and a multi anode photomultiplier tube. The monitor should operate under high humidity with the maximum counting rate of approx. 2.110 cpm. To achieve the two requirements, the monitor was designed to keep the air sufficiently dry without overheating the detector, and to reduce the detection of coarse particles. This study also conducted a basic performance test using the developed IAAM with a modified FFP. As a result, we could keep the humidity of the air less than 30%RH by heating the inlet of the FFP to 80 C. In addition, by placing the FFP in a vertical position and installing a bend at the air intake port, coarse particles were reduced approx. 1/2-1/3. These achievements enable the monitor to measure -aerosols more precisely in the viewpoint of internal exposure assessment.

Development of projection autoradiography technique using magnetic fields

Kurita, Keisuke; Sakai, Takuro; Suzui, Nobuo*; Yin, Y.-G.*; Iikura, Hiroshi; Kawachi, Naoki*

no journal, , 

We have developed a novel technique named projection autoradiography. Charged particles in a magnetic field move spirally following the Larmor precession. By utilizing this phenomenon, it is possible to control the trajectory of beta particles emitted from the sample and project the RI distribution onto the IP. Specifically, a uniform magnetic field is applied between the sample and IP, then the beta particles move spirally toward the IP without the severe deterioration of the sensitivity and resolution. In order to prove the new technique, an autoradiography experiment was performed. This result proved that projection autoradiography technique is promising for various applications, such as direct taking a radiograph of cultured samples in a petri dish, etc.

Developing delayed gamma-ray spectroscopy for reprocessing plant nuclear safeguards; Neutron detection system development

Lee, H.-J.; Rodriguez, D.; Rossi, F.; Koizumi, Mitsuo; Takahashi, Tone

no journal, ,