Quantitative analysis of ionization quenching in CaF:Ce using PHITS track-structure simulations

Hirata, Yuho; Kai, Takeshi; Ogawa, Tatsuhiko; Matsuya, Yusuke; Sato, Tatsuhiko; Watanabe, Kenichi*; Kato, Takumi*; Kawaguchi, Noriaki*; Yanagida, Takayuki*

Radiation Measurements, 193, p.107651_1 - 107651_8, 2026/04

https://doi.org/10.1016/j.radmeas.2026.107651

CaF:Ce has a high potential to be used as a dosimeter due to its high optically stimulated luminescence (OSL) intensity. However, when phosphors such as CaF:Ce are irradiated with swift ions, the OSL intensity per dose decreases due to quenching effects. Traditionally, quenching effects in phosphors have been evaluated based on energy deposition density, using linear energy transfer (LET) as a metric. However, the relationship between quenching effects and LET varies with ion type, complicating unified evaluations. The track structure in PHITS can precisely simulate the radiation interactions. In this study, we simulated the detector response of CaF:Ce irradiated with swift-ions and compared these results with experimental data. The comparison suggests that the quantum yield of OSL is a critical parameter influencing the quenching effect in CaF:Ce. These findings are expected to contribute to the development of improved phosphor detectors.

Development of a phoswich detector for low-energy gamma rays emitted from alpha emitters

Morishita, Yuki; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*

Radiation Measurements, 188, p.107502_1 - 107502_7, 2025/11

https://doi.org/10.1016/j.radmeas.2025.107502

The decommissioning of the Fukushima Daiichi Nuclear Power Station requires thorough inspection of piping for contamination, including alpha nuclides. Since external alpha particle measurements are impractical, detection relies on gamma-rays emitted by the alpha nuclides. Therefore, a phoswich detector for detecting low-energy gamma-rays was developed and experimentally validated. The detector was designed with consideration of energy deposition characteristics and consists of YAP:Ce or HR-GAGG scintillators in combination with BGO scintillators, employing a photomultiplier tube for signal amplification. Validation procedures included Monte Carlo simulations and measurements using actual radiation sources. Both measurement and simulation results demonstrate a correlation in scintillator energy depositions across different gamma-ray energies. Pulse Shape Discrimination (PSD) plots effectively differentiate between low-energy and high-energy gamma-rays, thereby confirming the predictions from simulations. These results suggest promising potential for developing a sensitive low-energy gamma-ray detector utilizing various scintillator combinations. The phoswich detector shows promise for effectively detecting low-energy gamma-rays emitted by alpha nuclides in piping.

Development of a compact detector for measurement of alpha contamination in piping

Morishita, Yuki; Peschet, L.; Yamada, Tsutomu*; Nakasone, Takamasa*; Kanno, Marina*; Sasaki, Miyuki; Sanada, Yukihisa; Torii, Tatsuo*

Radiation Measurements, 183, p.107414_1 - 107414_6, 2025/04

https://doi.org/10.1016/j.radmeas.2025.107414

In the decommissioning nuclear facilities, it is crucial to inspect piping for contamination to prevent worker exposure to alpha-emitting nuclides. Traditional methods using gamma rays and neutrons are inadequate for detecting small amounts of alpha nuclides due to the short range (approximately 4 cm) of alpha particles in air. To address this, we developed a compact detector capable of distinguishing between alpha particles for direct measurement within pipes. This detector, comprising a ZnS(Ag) scintillator for alpha particles and a plastic scintillator for beta particles (gamma rays), was coupled to a small photomultiplier tube. The system demonstrated high accuracy in differentiating between alpha and beta radiation through pulse shape discrimination (PSD). Monte Carlo simulations and empirical measurements confirmed the detector's effectiveness, achieving a 51.3% detection efficiency for alpha particles with negligible sensitivity to beta and gamma radiation. This innovation presents a significant advancement for direct alpha contamination measurement in environments with high beta and gamma backgrounds, such as the Fukushima Daiichi Nuclear Power Plant decommissioning site.

An Alpha imaging detector for on-site measurement of Plutonium and Neptunium

Morishita, Yuki; DiPrete, D. P.*; Deason, T.*; Nagaishi, Ryuji

Radiation Measurements, 181, p.107366_1 - 107366_8, 2025/02

https://doi.org/10.1016/j.radmeas.2024.107366

Nuclear fuel materials such as Plutonium (Pu) and Neptunium (Np) are produced as by-products of reprocessing operations, necessitating precise understanding of their contamination distribution within controlled areas for radiation protection. This study presents the development and application of an alpha particle imaging detector for on-site detection of Pu and Np contamination. The detector's performance was evaluated using various alpha sources, demonstrating promising energy resolution and spatial resolution. Subsequently, Pu and Np oxide samples were measured at the Savannah River National Laboratory, showing the detector's effectiveness in on-site applications. The detector enabled simultaneous measurement of radioactivity and energy spectrum of individual particles, facilitating rapid discrimination between Pu and Np. The imaging detector has potential for enhancing on-site detection of alpha nuclides in nuclear facilities, aiding in decontamination efforts and environmental monitoring.

Development of contamination detection system combined with various remote devices

Morishita, Yuki; Miyamura, Hiroko; Sato, Yuki; Oura, Masatoshi*

Radiation Measurements, 171, p.107064_1 - 107064_7, 2024/02

https://doi.org/10.1016/j.radmeas.2024.107064

The detection of alpha and beta contamination locations is important for decontaminating nuclear facilities. In the high radiation dose rate environment at the decommissioning sites, the contamination measurement by the workers is not effective. Thus, we developed a remote automatic contamination measurement system using a new scintillator-based detector. A 50 mm 100-m-thick YAlO(Ce)(YAP:Ce) scintillator was coupled with a flat panel-type multianode photomultiplier tube. The detector was installed downwards at the bottom of a robot. It has an energy measurement capability, and the energy measurement could discriminate the alpha particles from the beta and Rn alpha particles. With the energy information, alpha and beta particles could be identified and mapped simultaneously. In addition, a slow-moving robot could be used to obtain statistically sufficient counts in a single run measurement, allowing the evaluation of surface contamination density using only alpha particles. The remote automatic contamination measurement system will be useful in visualizing the contamination distribution in environments that are inaccessible to workers.

A Phoswich alpha/beta detector for monitoring in the site of Fukushima Daiichi Nuclear Power Station

Morishita, Yuki; Takasaki, Koji; Kitayama, Yoshiharu; Tagawa, Akihiro; Shibata, Takuya; Hoshi, Katsuya; Kaneko, Junichi*; Higuchi, Mikio*; Oura, Masatoshi*

Radiation Measurements, 160, p.106896_1 - 106896_10, 2023/01

https://doi.org/10.1016/j.radmeas.2022.106896

This study proposes a newly developed phoswich alpha/beta detector that can discriminate alpha and beta particles emitted from the alpha and beta contaminations in the FDNPS site. The phoswich alpha/beta detector is made up of two layers of scintillators that detect alpha and beta particles. The pulse shape discrimination (PSD) method was used to detect alpha particles in high beta particle and high gamma-ray (comparable to gamma-ray dose rate near the FDNPS reactor building) environments. Due to a 23.3% full width at half maximum (FWHM) energy resolution for alpha particles, the detector can be used to distinguish between nuclear fuel materials such as plutonium and its radon progeny (Po-214). Moreover, the detector could distinguish alpha particles from Cs gamma rays with a dose rate background up to 9.0 mSv/h. It is the first to demonstrate that the developed phoswich detector can be used to discriminate and measure alpha and beta particles of the actual contaminated FDNPS samples.

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

Morishita, Yuki

Radiation Measurements, 140, p.106511_1 - 106511_7, 2021/01

https://doi.org/10.1016/j.radmeas.2020.106511

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. We have developed a new alpha beta-imaging detector combined with a waveform digitizer capable of imaging alpha- and beta-particle locations simultaneously. This detector is used for Rn -progeny measurements. The thin-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 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. 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-fuel facilities.

Attempt to estimate the background pulse height spectrum of the CeBr scintillation spectrometer due to terrestrial gamma ray components; Application in environmental radiation monitoring

Kowatari, Munehiko; Tanimura, Yoshihiko; Kessler, P.*; Rttger, A.*

Radiation Measurements, 138, p.106431_1 - 106431_6, 2020/11

https://doi.org/10.1016/j.radmeas.2020.106431

Radon measurements with a compact, organic-scintillator-based alpha/beta spectrometer

Morishita, Yuki; Ye, Y.*; Mata, L.*; Pozzi, S. A.*; Kearfott, K. J.*

Radiation Measurements, 137, p.106428_1 - 106428_7, 2020/09

https://doi.org/10.1016/j.radmeas.2020.106428

We have developed a compact, organic-scintillator-based alpha/beta spectrometer for radon measurements and have characterized it using a unique, small radon chamber. The spectrometer is composed of a through-silicon via (TSV) silicon photomultiplier (or SiPM) and a 6 mm 6 mm 6 mm stilbene crystal cube. Analog signals from the SiPM are sent to a digitizer. The detector is housed in a light-tight box, with a stacked air filter installed in one side of the box to enable Rn gas to diffuse to the inside. We conducted one experiment with the spectrometer and an AlphaGUARD detector placed in a basement at the University of Michigan, and we conducted other experiments with both detectors placed in a small radon chamber together with Ra sources. By applying a pulse-shape-discrimination technique, we were able to separate the alpha and beta spectra simultaneously and clearly and to measure them quantitatively. We found two peaks in the measured alpha spectrum: a lower-energy peak due to Po and a higher-energy peak due to Po. We found a linear relation between the radon concentration y from AlphaGUARD and the counting rates from the stilbene-SiPM detector. The alpha/beta spectrometer is less than 10 mm thick, and we expect that it will be easy to increase the sensitivity with future device construction. Thus, this compact, organic-scintillator-based alpha/beta spectrometer shows promise for use in novel radon-detection systems.

Prototype test of a portable thyroid dose monitoring system using gamma-ray spectrometers

Nishino, Sho; Tanimura, Yoshihiko; Yoshitomi, Hiroshi; Takahashi, Masa

Radiation Measurements, 134, p.106292_1 - 106292_5, 2020/06

https://doi.org/10.1016/j.radmeas.2020.106292

In the situation of a severe nuclear accident, radioiodine monitoring in thyroid should be performed for a large number of people immediately after accident. The portable thyroid dose monitoring system which can be used in a high dose rate condition is in development. In this presentation, the result of performance test using prototype model will be described.