|Last modified date：2024.02.08
Associate Professor / Quantum Radiation Sciences / Department of Health Sciences / Faculty of Medical Sciences
|Last modified date：2024.02.08
|Hideya Maeda, Akihiro Nohtomi, Naonori Hu, Ryo Kakino, Kazuhiko Akita, Koji Ono, Feasibility study of optical imaging of the boron-dose distribution by a liquid scintillator in a clinical boron neutron capture therapy field, Medical Physics, 1-13, 2023.08, 1Abstract
Background: Evaluation of the boron dose is essential for boron neutron capture therapy (BNCT).Nevertheless,a direct evaluation method for the boron- dose distribution has not yet been established in the clinical BNCT field. To date, even in quality assurance (QA) measurements, the boron dose has been indi- rectly evaluated from the thermal neutron flux measured using the activation method with gold foil or wire and an assumed boron concentration in the QA procedure. Recently, we successfully conducted optical imaging of the boron- dose distribution using a cooled charge-coupled device (CCD) camera and a boron-added liquid scintillator at the E-3 port facility of the Kyoto University Research Reactor (KUR), which supplies an almost pure thermal neutron beam with very low gamma-ray contamination.However,in a clinical accelerator-based BNCT facility, there is a concern that the boron-dose distribution may not be accurately extracted because the unwanted luminescence intensity, which is irrelevant to the boron dose is expected to increase owing to the contamination of fast neutrons and gamma rays.
Purpose: The purpose of this research was to study the validity of a newly pro- posed method using a boron-added liquid scintillator and a cooled CCD camera to directly observe the boron-dose distribution in a clinical accelerator-based BNCT field.
Method: A liquid scintillator phantom with 10B was prepared by filling a small quartz glass container with a commercial liquid scintillator and boron-containing material (trimethyl borate); its natural boron concentration was 1 wt%. Lumines- cence images of the boron-neutron capture reaction were obtained in a water tank at several different depths using a CCD camera. The contribution of back- ground luminescence, mainly due to gamma rays, was removed by subtracting the luminescence images obtained using another sole liquid scintillator phan- tom (natural boron concentration of 0 wt%) at each corresponding depth, and a depth profile of the boron dose with several discrete points was obtained. The obtained depth profile was compared with that of calculated boron dose, and those of thermal neutron flux which were experimentally measured or calculated using a Monte Carlo code.
Results: The depth profile evaluated from the subtracted images indicated reasonable agreement with the calculated boron-dose profile and thermal neutron flux profiles, except for the shallow region. This discrepancy is thought to be due to the contribution of light reflected from the tank wall. The simulation results also demonstrated that the thermal neutron flux would be severely perturbed by the 10B-containing phantom if a relatively larger container was used to evaluate a wide range of boron-dose distributions in a single shot. This indicates a trade-off between the luminescence intensity of the 10B-added phantom and its perturbation effect on the thermal neutron flux. Conclusions: Although a partial discrepancy was observed, the validity of the newly proposed boron-dose evaluation method using liquid-scintillator phan- toms with and without 10B was experimentally confirmed in the neutron field of an accelerator-based clinical BNCT facility. However, this study has some limi- tations, including the trade-off problem stated above. Therefore, further studies are required to address these limitations..
|Basic Knowledge of Neutron: Generation of Neutrons Accompanied with the High-Energy Photon Therapy
|Akihiro Nohtomi, Hideya Maeda, Naoya Sakamoto, Genichiro Wakabayashi, Takushi Takata, Yoshinori Sakurai, First optical observation of 10B-neutron capture reactions using a boron-added liquid scintillator for quality assurance in boron neutron capture therapy, Radiological Physics and Technology, 2022.03, 10B-neutron capture was observed optically using a boron-added liquid scintillator. Trimethyl borate was dissolved in a commercially available liquid scintillator at natural boron concentrations of approximately 1 wt% and 0.25 wt%. The boron-added liquid scintillator was placed in a phantom quartz bottle and irradiated by thermal neutrons (~105 n/[cm2 s]) for 150, 300, and 600 s. The luminescence of the liquid scintillator was clearly observed using a cooled charge-coupled device (CCD) camera during irradiation. The luminance value recorded by the CCD camera was proportional to the duration of irradiation by thermal neutrons. The luminescence distribution showed reasonable agreement with that of energy deposition by Li and alpha particles from 10B-neutron capture reactions calculated via Monte Carlo simulations. When trimethyl borate was not dissolved in the liquid scintillator (0 wt% natural boron), no visible luminescence was observed even after 600 s of irradiation. These findings demonstrate that the observed luminance originates from the Li and alpha particles generated by 10B-neutron capture reactions. Consequently, the luminescence distribution is directly related to the boron dose of the liquid scintillator. To the best of our knowledge, direct experimental optical observations of boron dose distribution have not yet been reported. This novel technique will be useful for quality assurance in boron neutron capture therapy (BNCT) because instantaneous neutron irradiation may be sufficient for the observing the intense neutron beam used in clinical BNCT (~109 n/[cm2 s]), and quick evaluation of the boron dose distribution is expected to be feasible..
|Akihiro Nohtomi, Yui Kanzaki, Naoya Sakamoto, Hideya Maeda, Observation of water luminescence for diagnostic 120-kV X-rays by using PMT and CCD camera, Nuclear Instruments and Methods in Physics Research A, 988, 164935, 2021.01, Luminescence emitted from water was recently observed by Yamamoto et al. for X-rays with lower energy than the Cerenkov-light threshold. This phenomenon contradicts the conventional consensus that such low-energy photons cannot generate luminescence. Because properties and mechanism of this phenomenon have not been revealed yet, we carried out a series of observations of water luminescence during irradiation of diagnostic 120-kV X-rays by using a photomultiplier tube (PMT) and a charge-coupled device (CCD) camera. As a result of PMT measurement, the light yields owing to luminescence per unit dose deposition to water decreased almost linearly with the increase in the pulse duration time; approximately 25 % reduction of light yield was observed as the duration time increased from 0.025 s to 0.5 s. Therefore, the luminance intensity was not simply proportional to the dose deposition to water. In addition to water, sulfuric acid was used for luminescence imaging by a CCD camera for comparison. The profile curve of luminance along the beam center axis for sulfuric acid showed a somewhat different tendency in comparison to that for water; a notable bump appeared in the entry region of the X-rays. Based on these observations, the possibility of the contribution of sonoluminescence to the water luminescence can be discussed..
|Yumika Hanada, Akihiro Nohtomi, Junichi Fukunaga, Yoshiyuki Shioyama, DEVELOPMENT OF A NEUTRON DOSIMETRY SYSTEM BASED ON DOUBLE SELF-ACTIVATED CsI DETECTORS FOR MEDICAL LINAC ENVIRONMENTS, Radiation Protection Dosimetry, doi:10.1093/rpd/ncaa218, 1-9, 2020.11, In the present study, by using double self-activated CsI detectors, the development of a neutron dosemeter system whose response indicates better agreement with the International Commission on Radiological Protection-74 rem-response was carried out to simply evaluate the neutron dose with high accuracy. The present double neutron dosemeter system, using a slow-neutron dosemeter (thermal to 10 keV) and a fast-neutron dosemeter (above 10 keV), consists of CsI scintillators wrapped with two types of neutron energy filtering materials: polyethylene and B4C silicon rubber. After optimization of each filter thickness, to confirm the validity of our method, the neutron ambient dose equivalents under several operating conditions of medical linear accelerators (Linacs) were evaluated using a Monte Carlo simulation and an experiment with the present dosemeter. From these results, the present dosimetry system has enabled a more accurate neutron dose evaluation than our conventional dosemeter, and the present dosemeter was suitable for the neutron dosimetry for 10 MV Linac environments..
|Sota Tanaka, Tadatoshi Kinouchi, Tsuguru Fujii, Tetsuji Imanaka, Tomoyuki Takahashi, Satoshi Fukutani, Daisuke Maki, Akihiro Nohtomi, Sentaro Takahashi, Observation of morphological abnormalities in silkworm pupae after feeding 、137CsCl-supplemented diet to evaluate the effects of low dose-rate exposure, SCIENTIFIC REPORTS 10 (2020) 16055, https://doi.org/10.1038/s41598-020-72882-y, 10, 10:16055, 2020.10, Since the fukushima Dai‐ichi nuclear power plant (fDnpp) accident, morphological abnormalities in
lepidopteran insects, such as shrinkage and/or aberration of wings, have been reported. Butterflies
experimentally exposed to radiocesium also show such abnormalities. However, because of a lack
of data on absorbed dose and dose–effect relationship, it is unclear whether these abnormalities
are caused directly by radiation. We conducted a low dose‐rate exposure experiment in silkworms reared from egg to fully developed larvae on a 137CsCl-supplemented artificial diet and estimated
the absorbed dose to evaluate morphological abnormalities in pupal wings. We used 137cscl at
1.3 × 103 Bq/g fresh weight to simulate 137cs contamination around the fDnpp. Absorbed doses were estimated using a glass rod dosimeter and Monte carlo particle transport simulation code pHitS. Average external absorbed doses were approximately 0.24 (on diet) and 0.016 mGy/day (near diet); the average internal absorbed dose was approximately 0.82 mGy/day. Pupal wing structure is sensitive to radiation exposure. However, no significant differences were observed in the wing-to-whole
body ratio of pupae between the 137cscl‐exposure and control groups. these results suggest that silkworms are insensitive to low dose‐rate exposure due to chronic ingestion of high 137cs at a high concentration..
|Miyu Shimazu, Genichiro Wakabayashi, Akihiro Nohtomi, Yusuke Koba, Kiyomitsu Shinsho, Thermal Neutron Flux Measurement by Counting Conversion Electrons from 134mCs Generated in a CsI Scintillator, 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference, NSS/MIC 2018 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference, NSS/MIC 2018 - Proceedings, 10.1109/NSSMIC.2018.8824515, 2019.11, A new method to measure low intensity neutrons such as secondary neutrons generated in radiotherapy has been developed. In the proposed method, thermal neutron flux is determined by measuring the conversion electrons from 134mCs generated in a CsI(Tl) scintillator irradiated by neutrons. Because the generated 134mCs is contained in the scintillator as internal sources, the emitted conversion electrons fully stop in the scintillator and counted with high efficiency. The thermal neutron flux in the neutron radiography facility of the Kindai University's research reactor, UTR-KINKI, was measured by the proposed method. The cadmium difference method was used to separate the contributions of thermal and epi-thermal neutrons. The result was in good agreement with the one obtained by the conventional gold foil activation method. It was concluded that the proposed method is useful for the measurement of thermal neutron flux..
|Taishi Ueki, Akihiro Nohtomi, Genichiro Wakabayashi, Junichi Fukunaga, Toyoyuki Kato, S. Ohga, A design study of application of the CsI self-activation method to the neutron rem-counter technique, Radiation Measurements, 10.1016/j.radmeas.2019.106181, 128, 2019.09, A design study of application of the CsI self-activation method to the “neutron rem-counter technique” was investigated. A CsI crystal served as both the main target material and 4π counter of neutron activation method in our proposed CsI self-activation method. A commercially available CsI gamma-ray dosimeter was selected as the neutron detector and surrounded with several neutron filters made of polyethylene and silicon rubber containing B4C. The geometric structure of these neutron filters was optimized by a Monte Carlo calculation to make the neutron response similar to the ICRP-74 rem-response. Optimization of this method, the residual sum of squares (RSS) of the calculated neutron response and the ICRP-74 rem-response were minimized by changing the thickness of each filter sequentially. In addition, experimental verification using a Pu–Be source and photoneutrons from a 10 MV-X medical linac has been conducted. From these results, it is concluded that it may be practical for evaluating neutron ambient dose-equivalent simply around medical linacs..
|Akihiro NOHTOMI , Masaaki TOKUNAGA, Genichiro WAKABAYASHI, Kiyomitsu SHINSHO, Neutron Distribution Measurement by the Self-activation of a CsI Plate with CCD Camera using a Decaying Self-activation Imaging Technique, JPS Conf. Proc. 24, 1, 011041, 2019.01, Neutron intensity distribution on a CsI scintillator plate has been observed by a CCD camera and analyzed by using a “decaying self-activation imaging technique”. The decaying self-activation imaging, which has been proposed recently by our group, is based on the analysis of time variation of specific radio-activities generated and remained inside the CsI plate after the termination of neutron irradiation. The luminance distributions of a CsI plate are recorded every one minute as a series of images by a cooling type CCD camera with a telescope lens in a black box. Then the time variations of luminance (mean pixel values) of the images are fitted on a “pixel-by-pixel basis” with a multi-exponential function. By this, two components of 128I (half life : 25 min) and 134mCs (half life : 174 min) are extracted as the fitting curves. The initial luminance values of individual component of radioactivity, which correspond to the values at the termination of neutron irradiation, are plotted as separate images. A conversion factor between the generated activity in a CsI plate and the observed luminance value is evaluated as “light yield”..
|Ryosuke Kurihara, Akihiro Nohtomi, Genichiro Wakabayashi, Yoshinori Sakurai, Hiroki Tanaka, Preliminary design study of a simple neutron energy spectrometer using a CsI self-activation method for daily QA of accelerator-based BNCT, journal of nuclear science and technology, 10.1080/00223131.2018.1523757, 56, 1, 70-77, 2019.01, For recent boron neutron capture therapy (BNCT), accelerator-based neutron sources have been actively developed in place of reactor-based neutron sources. In this study, a novel neutron energy spectrometer for the daily quality assurance (QA) of BNCT was designed on the basis of a CsI self-activation method for accelerator-based neutron sources. The spectrometer design was optimized in terms of its energy resolution. To verify its applicability to high-intensity BNCT neutron fields, some practical simulations were performed. It was shown that the designed spectrometer was able to evaluate a neutron energy spectrum in approximately 900 s after an instantaneous neutron irradiation. In addition, its energy resolution was sufficient for detecting an unexpected distortion in the spectrum. The results confirm that the designed spectrometer can be employed for the daily QA of BNCT to check that the expected spectrum remains unchanged..
|Ryo Kakino, Akihiro Nohtomi, Genichiro Wakabayashi, Improvement of neutron spectrum unfolding based on three-group approximation using CsI self-activation method for evaluation of neutron dose around medical linacs, Radiation Measurements, 10.1016/j.radmeas.2018.06.021, 116, 40-45, 2018.09, We previously evaluated ambient neutron dose equivalent by using the self-activation of a CsI scintillator around a high-energy medical linear accelerator (linac) 128I saturated activities were successfully converted to neutron spectrum and ambient neutron dose equivalent by neutron spectrum unfolding with the “three-group approximation.” The principle of the three-group approximation is based on the assumption of fixed shapes of neutron energy spectra for each of the three energy regions to evaluate the neutron spectrum effectively. However, such a neutron dose evaluation with the unfolding method might be affected by the difference between the actual fast neutron energy spectrum and the assumed spectrum. In the present work, we modified the unfolding method by taking into account the differences in the shapes of fast neutron energy spectra for various medical linacs. We verified the unfolding method using Monte Carlo simulation with several neutron spectra obtained from published research articles. The modified three-group approximation evaluates the neutron doses more accurately than the conventional unfolding method..
|Soichiro Honda, Akihiro Nohtomi, Keita Machidori, Genichiro Wakabayashi, Shape distortion of 128I ß- spectrum observed by a self-activated CsI(Tl) scintillator for high-sensitivity neutron measurements, Nuclear Instruments and Method in Physics Research A, 2017.04, The factors causing the distortion of the 128I ß- spectrum detected by a self-activated CsI(Tl) scintillator were studied to verify the correctness of the spectral shape and the appropriateness of the discrimination setting for ß-particle counting by the scintillator. These criteria are essential for the correct evaluation of radioactivity generated in a scintillator volume by the self-activation method, which was recently proposed by our group.
A pulse height defect caused by the partial escape of ß! particles from the surface of the scintillator crystal shifts the ß! spectrum toward the lower-energy region when smaller CsI(Tl) scintillators are used (the ß-escape effect). For larger CsI(Tl) scintillators, an increase in pulse height caused by the summing of 0.443 MeV prompt γ-rays from the excited state of the 128I daughter nuclide (128Xe) affects the shape of the ß! spectrum considerably, resulting in a shift toward the higher-energy region (the γ-summing effect). The extent of the contributions of these two effects was examined by a Monte Carlo simulation of various cubical CsI(Tl) crystals of different sizes. It was found that the distortions caused by those two effects effectively cancel each other out for a medium-size cubical CsI(Tl) crystal with a side length of approximately 3 cm. This finding is very useful for the practical applications of the self-activation method.
In addition to the factors mentioned above, the efficiency of scintillation light collection by the photodetectors also affects the shape distortion of the ß! spectrum slightly through spectral line broadening due to the degradation of the energy resolution. This effect was estimated using a simple model with different discrimination settings for ß pulse counting.
|Akihiro Nohtomi, Genichiro Wakabayashi, Hiroyuki Kinoshita, Soichiro Honda, Ryosuke Kurihara, Junichi Fukunaga, Yoshiyuki Umezu, Yasuhiko Nakamura, Saiji Ohga, Katsumasa Nakamura, High Sensitive Neutron-detection by using a Self-activation of Iodine-containing Scintillators for the Photo-neutron Monitoring around X-ray Radiotherapy Machines, JPS Conf. Proc. 11, 050002 (2016), 2016.11, A novel method for evaluating the neutron dose-equivalent as well as neutron fluence around high-energy X-ray radiotherapy machines has been proposed and examined by using the self-activation of a CsI scintillator. Several filtering conditions were used to extract energy information of the neutron field. The shapes of neutron energy spectra were assumed to be practically unchanged at each three energy regions (thermal, epi-thermal and fast regions) for different irradiations around an X-ray linac whose acceleration potential was fixed to be a certain value. In order to know the actual neutron energy spectrum, an unfolding process was carried out for saturated activities of 128I generated inside the CsI scintillator under different filtering conditions; the response function matrix for each filtering condition was calculated by a Monte Carlo simulation. As the result, neutron dose-equivalent was estimated to be 0.14 [mSv/Gy] at 30 cm from the isocenter of linac. It has been revealed that fast neutron component dominated the total dose-equivalent..
|Akihiro Nohtomi, Ryousuke Kurihara, Hiroyuki Kinoshita, Soichiro Honda, Masaaki Tokunaga, Heita Uno, Kiyomitsu Shinsho, Genichiro Wakabayashi, Yusuke Koba, Junichi Fukunaga, Yoshiyuki Umezu, Yasuhiko Nakamura, Saiji Ohga, An application of CCD read-out technique to neutron distribution measurement using the self-activation method with a CsI scintillator plate, Nuclear Instruments and Method in Physics Research A, A832, 21-23, 2016.06, In our previous paper, the self-activation of an NaI scintillator had been successfully utilized for detecting photo-neutrons around a high-energy X-ray radiotherapy machine; individual optical pulses from the self-activated scintillator are read-out by photo sensors such as a photomultiplier tube (PMT). In the present work, preliminary observations have been performed in order to apply a direct CCD read-out technique to the self-activation method with a CsI scintillator plate using a Pu-Be source and a 10-MV linac. In conclusion, it has been revealed that the CCD read-out technique is applicable to neutron measurement around a high-energy X-ray radiotherapy machine with the self-activation of a CsI plate. Such application may provide a possibility of novel method for simple neutron dose-distribution mea- surement..
|Akihiro Nohtomi, Yoko Ariyoshi, Momoko Yamauchi, Hiroyuki Kinoshita, Soichiro Honda, Genichiro Wakabayashi, Junichi Fukunaga, H. Akamine, Yoshiyuki Umezu, Yasuhiko Nakamura, Study on high-sensitive neutron-detection by the self-activation method with a CsI(Tl) scintillator, Radiation Detectors and Their Uses, Proceedings of the 29th Workshop on Radiation Detectors and Their Uses (KEK Proceedings 2015-8), 50-53, 2015.12.
|Kento Terasaki, Fujibuchi Toshioh, Takatoshi Toyoda, Yutaka Yoshida, Akasaka Tsutomu, Akihiro Nohtomi, Junji Morishita, An evaluation of the basic characteristics of a plastic scintillating fibre detectors in CT radiation fields, Radiation Protection Dosimetry, 2015.10.
|Akihiro Nohtomi, Genichiro Wakabayashi, Accuracy of neutron self-activation method with iodine-containing scintillators for quantifying 128I generation using decay-fitting technique, Nuclear Instruments and Method in Physics Research A, A800, 2015.08, Decay curves of 128I activity were numerically simulated by a computer program for various conditions including different initial count rates (R0) and background rates (RB), as well as counting statistical fluctuations. .
|Fujibuchi Toshioh, Akihiro Nohtomi, Shingo Baba, Masayuki Sasaki, Isao Komiya, Yoshiyuki Umedzu, Hiroshi Honda, Distribution of residual long-lived radioactivity in the inner concrete walls of a compact medical cyclotron vault room, ANNALS OF NUCLEAR MEDICINE, 10.1007/s12149-014-0918-6, 29, 1, 84-90, 2015.01.
|Genichiro Wakabayashi, Akihiro Nohtomi, Eriko Yahiro, Fujibuchi Toshioh, Junichi Fukunaga, Yoshiyuki Umezu, Yasuhiko Nakamura, Katsumasa Nakamura, Makoto Hosono, Tetsuo Itoh, Applicability of self-activation of an NaI scintillator for measurement of photo-neutrons around a high-energy X-ray radiotherapy machine, Radiological Physics and Technology, 8, 1, 125-134, 2015.01, NaIシンチレータに含まれるヨウ素に自己放射化に基づく高感度中性子検出方法を提案し、研究用原子炉ならびにがん治療用X線発生装置で原理検証実験を行った。その結果、〜10^2[n/cm2/s]程度までの熱中性子束を測定可能であることが判明した。この装置を用いて、高エネルギーX線治療装置から発生する光中性子の量を、オンラインにてサブリアルタイムで測定出来るシステムが実現できることが示唆された。.
|Daiki Nakanishi, Akihiro Nohtomi, Ryoji Tanaka, Genichro Wakabayashi, A method of neutron-energy evaluation based on the position distribution of recoil protons, Progress in Nuclear Science and Technology, 4, 653-656, 2014.03, Cone-like acryl converters have been used for transforming the energy-distribution information of incident fast neutrons into the spatial-distribution information of recoil protons. The characteristics of neutron–proton conversion have been studied up to around 10 MeV by using an imaging plate (IP). A notable and interesting signal enhancement due to recoil protons generated in an acryl converter was observed on IP images for irradiation with a 252Cf source. Similar experiments were also performed in the radiation field of a research nuclear reactor and an accelerator-based neutron generator. A Monte Carlo calculation was carried out in order to understand the spatial distributions of the signal enhancement by recoil protons; these distributions promisingly involve the energy information of incident neutrons in principle. Consequently, it has been revealed that the neutron energy evaluation is surely possible by analyzing the spatial distributions of signal enhancement that is caused by recoil protons..
|Akihiro Nohtomi, Fukai Toyofuku, Ayaka Sato, Syuji Nagamine, Genichiro Wakabayashi, An observation of the pulse-shape property of a polyethylene-lined proportional counter for fast neutrons and gamma-rays, Radiation Detectors and Their Uses, Proceedings of the 27th Workshop on Radiation Detectors and Their Uses (KEK Proceedings 2013-9), 51-62, 2014.01.
|Akihiro Nohtomi, , A method of neutron energy evaluation by using an imaging plate and cone-like acryl converters with a geometrical modulation concept, Nuclear Instruments and Method in Physics Research A , A633, 36, 2011.03, Cone-like acryl converters have been used for transforming the energy-distribution information of incident fast neutrons into the spatial-distribution information of recoil protons. The characteristics of neutron–proton conversion have been studied up to around 10 MeV by using an imaging plate (IP). A notable and interesting signal enhancement due to recoil protons generated in an acryl converter was observed on IP images for irradiation with a 252Cf source. Similar experiments were also performed in the radiation field of a research nuclear reactor and an accelerator-based neutron generator. A Monte Carlo calculation was carried out in order to understand the spatial distributions of the signal enhancement by recoil protons; these distributions promisingly involve the energy information of incident neutrons in principle. Consequently, it has been revealed that the neutron energy evaluation is surely possible by analyzing the spatial distributions of signal enhancement that is caused by recoil protons..