[en] In proton therapy, highly conformal dose can be delivered to the target volume while minimizing the dose to adjacent normal tissues and critical organs as compared with the conventional radiotherapy using x-ray or electron beam. However, the proton dose distribution, especially the beam range, might deviate from the planned one due to dose calculation errors, organ motions during the treatment, anatomical changes, or patient setup errors. The uncertainty of the proton dose distribution necessitates an additional safety margin around the target volume, thus it restricts the degree of freedom of proton treatment. In order to fully utilize advantages of proton therapy and guarantee the patient safety, a means of monitoring in vivo proton dose in real-time is required. For this purpose, a prompt gamma imaging, which measures prompt gammas which have a close correlation with the proton beam range, was suggested, and the possibility for proton beam monitoring was experimentally demonstrated. For the clinical application, however, an optimized detection system for high energy gamma-ray imaging should be developed. The present dissertation suggests a new emission imaging method for high energy gammarays, gamma electron vertex imaging (GEVI), and experimentally demonstrates the feasibility for proton beam monitoring. To determine the emission position, in GEVI method, an incident gamma-ray is converted to an electron by Compton scattering, and then the trajectory and energy of the converted electron are measured by two hodoscopes and a calorimeter, respectively. To configure the imaging system, dedicated signal processing systems for component detectors were developed. To reduce data acquisition channels for a double-sided silicon strip detector (DSSD, hodoscope), a multiplexing based low-noise signal processing system was designed and constructed. For a plastic scintillation detector (calorimeter), an in-house amplifier module was used. Using these developed systems, characteristics of component detectors were estimated in terms of an energy resolution, a timing resolution, and a noise level. To experimentally demonstrate the proposed imaging principle, a proof-of-principle imaging system was constructed. From imaging experiments with gamma-ray sources and prompt gammas generated by proton nuclear interactions, it was confirmed that the GEVI method has a great potential for high energy gamma-ray imaging. Based on experimental results, a prototype GEVI imaging system was developed and tested for therapeutic proton beams to see the feasibility for proton beam monitoring. For the quantitative analysis, a 50% distal falloff position of the prompt gamma distribution was determined by using a methodology based on a 3^rd-order polynomial fitting. For 80, 120, and 150 MeV proton beams, imaging sensitivities (= effective events per each primary proton) were 6.91 × 10^-7, 9.04 × 10^-7, and 1.15 × 10^-6, and the falloff position having a linear relation with the beam range was determined with the uncertainty of 0.91, 0.86 and 0.72 mm for 6.24 × 10⁹ protons. Next, proton beams of 126 126, 132, 138, 144, 150, 156, 162, and 168 MeV were delivered to the solid water phantom, and prompt gammas were imaged by the imaging system which was fixed at 150 mm depth of the phantom. With experiment results, it was confirmed that the proton beam range can be verified with the distal falloff position of the measured distribution. In addition, prompt gamma distributions were imaged by changing the incident beam position, and we confirmed that the developed prototype system can monitor the proton beam with lateral (= depth) movement within 10 cm and vertical movement within ± 3 cm. In the present study, the GEVI imaging system was successfully developed, and the imaging performance was experimentally estimated. The developed imaging system can monitor the beam delivery position in the phantom and the beam range by using the correlation with the distal falloff position. We expects that in vivo proton beam monitoring can be performed using the developed imaging system. In addition, developed key techniques of a low noise signal processing, a multi-channel data acquisition, and an image reconstruction can be also utilized to develop various radiation imaging systems

[en] Accurately determining the distal dose edge, where the dose decreases to 80% of its peak dose directly after the Bragg peak in the patient, is very important not only for successful treatment, but also for the safety of the patient in proton therapy. Recently it is proved that the proton dose distribution has clear relationship with the distribution of the prompt gammas, generated from the proton-induced nuclear reactions in the passage of the proton beam. A proof-of-principle measurement system, which could measure the prompt gammas distribution by scanning method with a single CsI(Tl) scintillation detector and a single-slit collimation system, showed that the location of the distal dose edge or the proton beam range in the water phantom can be accurately determined despite the high level background of neutrons and neutron capture gammas. However, the prototype measurement system developed only to prove the principle cannot be used in real clinical situations, principally due to the very large and heavy (160 cm x 100 cm x 82 cm; 500 kg) scanning system. Moreover, the scanning process used in the study is not suitable for 'spot scanning' or any other scanning techniques, because the spot of the proton beam continuously moves during treatment. For the clinical purpose, a small, array-type prompt gamma detection system incorporating a linear array of multiple CsI(Tl) scintillation detectors, a multi-slit collimation system, and multi-channel DAQ is under development to obviate the problematic scanning process. It is also fundament for the clinical application to assess the prompt gamma distribution according to the phantom size, material, shape, and location (considered the human characteristics) in proton therapy for the accurate determination of the distal dose edge. In the present study, the prompt gamma distributions according to the phantom-to-phantom variation were assessed based on the background fraction, indicating the distribution index as the ratio of the average background gamma counts to the peak gamma counts, by MCNPX code using the Monte Carlo method. The computational burden with the repeated calculation with the various phantoms was reduced with the employment of the parameterized source term and with the use of the 97-node computer cluster

[en] A beta coincidence spectroscopy has been studied for both objectives: (1) to measure beta spectrum of 238U in order to more accurately determine anti-neutrino spectrum emitted from 238U beta decay and (2) to more precisely measure positron production rate in order to develop positron annihilation spectroscopy. The important technique is to remove efficiently gamma-ray background interfering beta-ray signal. In this study the beta coincidence spectroscopy has been designed based on N. Haag, et. al.’s research which used beta-gamma coincidence counting method. Currently, the beta coincidence spectroscopy has been designed and installed to efficiently remove neutral particle backgrounds. The plastic scintillator and PMT detector was installed and the multi-wire chamber is in progress.

[en] This study was performed for suggesting a simulation method that can create accurate virtual models of objects with free curved surfaces and perform distortion-free MCNPX simulations. The virtual models acquired by using 3D scan equipment with an accuracy of approximately ±0.025 mm in length, compare with actual objects and are comprised of 11104 polygons. Generally, MCNPX simulations of objects with free curved surfaces are performed through voxelization. In this study, polygon model be tetrahedralized by TetGen for the construction of MCNPX geometry to distortion-free. Then, dose estimation was successfully performed after converting the virtual model into an MCNPX input. With this in mind, a voxelized model was constructed for comparison purposes. The dose estimation functions of the two models were found to be similar, showing a similar amount of computing time by using the mesh tally option with 2e7 histories: for the tetrahedralized model, 729.67 minutes; for the voxelized model, 720.11 minutes. (authors)

[en] The close relationship between the proton dose distribution and the distribution of prompt gammas generated by proton-induced nuclear interactions along the path of protons in a water phantom was demonstrated by means of both Monte Carlo simulations and limited experiments. In order to test the clinical applicability of the method for determining the distal dose edge in a human body, a human voxel model, constructed based on a body-composition-approximated physical phantom, was used, after which the MCNPX code was used to analyze the energy spectra and the prompt gamma yields from the major elements composing the human voxel model; finally, the prompt gamma distribution, generated from the voxel model and measured by using an array-type prompt gamma detection system, was calculated and compared with the proton dose distribution. According to the results, effective prompt gammas were produced mainly by oxygen, and the specific energy of the prompt gammas, allowing for selective measurement, was found to be 4.44 MeV. The results also show that the distal dose edge in the human phantom, despite the heterogeneous composition and the complicated shape, can be determined by measuring the prompt gamma distribution with an array-type detection system.

[en] In the present study, a new prompt-gamma activation imaging system was proposed for two-dimensional (2D) elemental distributions. The system, as based on coincidence measurement, consists of a high-purity germanium detector for the measurement of the prompt-gamma energy and a position-sensitive detector for the determination of the emission position. To estimate its feasibility, we performed the Monte Carlo simulations using the Geant4 toolkit. For an iron bulk sample with implanted nickel elements, we could recognize the shape of the nickel implant from the 2D image obtained by the imaging system using the energy information of characteristic prompt gammas emitted by neutron capture interactions.

[en] In proton therapy, accurate monitoring of the in-vivo proton dose distribution is essential in order to deliver the planned dose to the tumor volume within a minimal safety margin. Recently, a strong correlation between the distributions of the proton dose and the prompt gammas was found, and various prompt-gamma distribution-measurement systems, including collimation-based systems, Compton cameras, knife-edge imaging systems, and ion vertex imaging systems, have been proposed. In the present study, the feasibility of proton dose distribution monitoring was tested using a two-dimensional measurement system for prompt gammas. The measurement system, developed in the present study, incorporates a vertically-aligned one-dimensional array of gamma sensors, a parallel multi-hole collimator, a precision movement system, and a digitizer- and LabVIEW-based automatic data acquisition system. A 45-MeV proton beam of 0.5 nA was delivered to a polymethyl methacrylate (PMMA) phantom, and the two-dimensional prompt-gamma distribution was measured using the developed system. The proton beam range could be quantitatively determined to within a 1.6-mm error by sigmoidal curve-fitting with the Boltzmann equation. A comparison of the prompt-gamma distribution as measured by our detection system with the proton dose distribution as measured independently by using Gafchromic EBT films positioned inside the PMMA phantom showed good agreement. Both results imply that it is, indeed, possible to confirm the patient’s proton dose distribution by using two-dimensional prompt-gamma measurements.

[en] Nowadays, evaluation of amounts and distributions of radioactive waste is an important preparatory step in the process of nuclear reactor decommissioning. For tentative estimation of radioactive waste, a cell-based rigorous 2 step (R2S) method usually is used; however, a poor resolution caused by the averaged flux and spectrum in a cell is still a great challenge because of leading to underestimated or overestimated results. To overcome the poor resolution, several systems were introduced. Neither system, however, provides any function for evaluation of radioactive waste amount and distribution. Thus, it is additionally required to classify radioactive waste based on the results of activation calculation. In this study, the advanced R2S (AR2S) system was developed. To verify the performance of the system, its results for a verification problem were compared with those of the cell-based R2S method. The results showed good agreement, which is to say, within 2.0% relative error. Also, several characteristics of fine/coarse mesh were analyzed. To demonstrate the performance of the AR2S system, the radioactive waste from the Japan Power Demonstration Reactor (JPDR) was estimated, and the result indicated a high-resolution distribution. Therefore, it is expected that the AR2S system will prove useful for precise evaluation of radioactive waste

[en] To verify in-vivo proton dose distribution, a 2-dimensional (2D) prompt-gamma measurement system, comprised of a multi-hole collimation system, a 2D array of CsI(Tl) scintillators, and a position-sensitive photomultiplier tube (PS-PMT), is under development. In the present study, to determine the optimal dimension of the measurement system, we employed a series of Monte Carlo simulations with the MCNPX code. To effectively measure the high-energy prompt gammas while minimizing background gammas, we determined the collimator hole size, collimator thickness, and scintillator length to be 0.4 x 0.4 cm², 15 cm, and 5 cm, respectively. Thereafter, the performance of the optimized measurement system was estimated for monoenergetic proton pencil beams. The peak locations of the prompt-gamma distributions for 80- and 150-MeV proton beams were clearly distinguished, and the correlation between the beam range and the peak location was confirmed by using the measurement system. For a 200-MeV proton beam, however, the peak location could not be determined due to the dominance of background gammas and the lateral dispersion of the proton beam at the end of the beam range. Based on these simulation results, a prototype 2D prompt-gamma measurement system currently is under construction and, upon completion, will be tested with therapeutic proton beams.

[en] The large-area Compton camera was developed and its performance was evaluated with various experiments. A novel feature of this work is the use of large monolithic scintillation detectors for enlargement of the Compton camera. In doing so, the efficiency of the Compton camera has been dramatically improved without degradation of image resolution. Moreover, the experiment in this work demonstrated that the largearea Compton camera could define the three-three dimensional location of the sources for the distance of nearly 50 cm. Overall, the results of this study serve to demonstrate the great potential of the large-area Compton camera based on large monolithic scintillation detector. In the near future, we will change the cylindrical-PMT-based detector module into the square-shaped-PMT based module to improve the performance. With a new detector module and a system optimization study, the performance of the large-area Compton camera could be improved.