Secondary Electron Bremsstrahlung Research Articles

Optimization of a YAP(Ce) prompt X-ray camera for imaging with spot scanning proton beams at clinical dose levels

Prompt secondary electron bremsstrahlung X-ray (prompt X-ray) imaging using a low-energy X-ray camera is a promising method for observing the beam shape from outside a subject. However, it has sometimes been necessary to conduct such imaging at a higher dose than the clinical level to acquire images with acceptable quality. To solve this problem, we optimized a prompt X-ray imaging system to use for spot scanning proton therapy system. The new camera had more than one order higher sensitivity to image several types of beams, including those at the clinical dose level. The optimized prompt X-ray imaging system uses a 4 mm diameter pinhole collimator to increase sensitivity, and it is combined with a larger YAP(Ce) scintillator to increase the magnification ratio and thus improve spatial resolution. We used a list-mode data-acquisition system with high count rate capability. Prompt X-ray images were acquired by irradiating a water phantom with proton beams from the spot scanning proton therapy system. Measurements were taken for pencil beams, spread-out Bragg peak (SOBP) beams, and a beam utilized in actual clinical therapy. For all of the beams, we could measure scanning spot images within a spill and evaluate the ranges for the accumulated images at the clinical dose level. From the list-mode data, we measured the temporarily altered positions of the scanning beams as well as the accumulations of the prompt X-ray images. The optimized prompt X-ray imaging system could improve sensitivity while maintaining better spatial resolution. The new system realized prompt X-ray imaging at the clinical dose level and holds promise for future clinical imaging of prompt X-rays.

Prompt X-ray imaging during irradiation with spread-out Bragg peak (SOBP) beams of carbon ions

Prompt secondary electron bremsstrahlung X-ray (prompt X-ray) imaging using a low-energy X-ray camera is a promising method for observing a beam shape from outside the subject. However, such imaging has so far been conducted only for pencil beams without a multi-leaf collimator (MLC). The use of spread-out Bragg peak (SOBP) with an MLC may increase the scattered prompt gamma photons and decrease the contrast of the images of prompt X-rays. Consequently, we performed prompt X-ray imaging of SOBP beams formed with an MLC. This imaging was carried out in list mode during irradiation of SOBP beams to a water phantom. An X-ray camera with a 1.5-mm diameter as well as 4-mm-diameter pinhole collimators was used for the imaging. List mode data were sorted to obtain the SOBP beam images as well as energy spectra and time count rate curves. Due to the high background counts from the scattered prompt gamma photons penetrating the tungsten shield of the X-ray camera, the SOBP beam shapes were difficult to observe with a 1.5-mm-diameter pinhole collimator. With the 4-mm-diameter pinhole collimators, images of SOBP beam shapes at clinical dose levels could be obtained with the X-ray camera. The use of a 4-mm-diameter pinhole collimator attached to the X-ray camera is effective for prompt X-ray imaging with high sensitivity and low background counts. This approach makes it possible to image SOBP beams with an MLC when the counts are low and the background levels are high.

Non-invasive range-shift measurement for therapeutic carbon-ion beams by detecting SEB with a CdTe imager

Non-invasive range measurement of particle beams is important to prevent deviation of the irradiated area in particle therapy. In this study, we made an experiment of imaging carbon-ion beams by setting the projection ranges into an acrylic target from 4.29 cm to 7.65 cm by a pitch of 0.21 cm. Secondary electron bremsstrahlung (SEB) generated on the beam trajectories were detected for the imaging by use of a pinhole camera system consisting of a pinhole collimator and a position-sensitive cadmium-telluride semiconductor detector (CdTe imager). Beam images were acquired, and their profiles along the longitudinal axis were numerically analyzed, to obtain a suitable parameter that was strongly correlated with the set range. Then we could propose a scheme to measure the particle beam range in the target during irradiation by use of the CdTe imager. As a result, the range shift can be measured in the case of the acrylic target with an accuracy of 0.1 cm.

Time-of-flight based neutron background reduction in secondary-electron-bremsstrahlung imaging for in-vivo range verification of proton therapy: a Monte Carlo study

In proton therapy, range verification is important to monitor the location of the Bragg peak within the patient’s body. Secondary-electron-bremsstrahlung (SEB) imaging is a promising method of proton range verification. Unfortunately, the SEB images measured by the dedicated gamma camera contain neutron-induced backgrounds as well as the SEB counts. To improve the accuracy of range verification in SEB imaging, we propose to apply a time-of-flight (TOF) method to reduce neutron background in measured images of SEB. The SEB and neutron generation and transport were calculated by a Monte Carlo simulation. Proton pencil beams were irradiated to a water phantom and then the time spectra and profiles of SEB and neutron were obtained by scoring on a detector. The total count of neutrons was approximately 10 times higher than that of SEB and increased with higher proton energy. The TOF method, selecting a time window for the TOF spectrum of SEB, reduced the neutron background by more than 97%. We concluded that the TOF methods can dramatically reduce neutron background and improve the accuracy of proton range verification in SEB imaging.

Simulation study on carbon-ion beam imaging by measuring secondary electron bremsstrahlung using imaging plate

The purpose of this study is to evaluate the effect of a lead radiation shield on the ability of a beam imaging device consist of an imaging plate (IP) and a collimator by Monte Carlo simulations. Simulations were performed using PHITS. A carbon-ion beam was injected to an acrylic target. A tungsten collimator having a pinhole was placed at the distance of 31.2 cm from the beam. A lead radiation shield was placed on the tungsten collimator. An IP was placed under the collimator. Beam images were acquired by recording the position distribution of energy deposition on the IP. We confirmed that therapeutic carbon-ion beam images could be acquired using the imaging device combining the IP and collimator. It was found that removal of the lead shield had no effect on the imaging results.

Advantages of using larger-diameter pinhole collimator for prompt X-ray imaging during irradiation with carbon ions

Prompt secondary electron bremsstrahlung X-ray (prompt X-ray) imaging using a low-energy X-ray camera is a promising method for observing a beam shape from outside the subject. However, the images measured within short times suffer from statistical noise. Consequently, we performed prompt X-ray imaging with higher sensitivity using a larger-diameter pinhole collimator and compared the results with those of a conventional collimator. Prompt X-ray imaging was conducted during irradiation with pencil beams of 241.5-MeV/n carbon ions to a water phantom. A newly developed X-ray camera with a 4-mm diameter as well as conventional 1.5-mm-diameter pinhole collimators was used for the imaging in list mode, and we compared the prompt X-ray images, energy spectra, and time count rate curves between 1.5-mm-diameter and 4-mm-diameter pinhole collimators. The prompt X-ray images taken with the 4-mm-diameter pinhole collimators had ∼7 times higher sensitivity with 70 % lower offset fractions originating from the prompt gamma photons. Furthermore, the ranges were more precisely estimated with the 4-mm collimator than with the 1.5-mm collimator. The energy spectra showed less contamination by tungsten-characteristic X-rays for the 4-mm pinhole collimator. Even for images measured with 0.1-s intervals, the beam shapes and time count rate curves could be obtained with less statistical noise using the 4-mm-diameter pinhole collimators. The use of the 4-mm-diameter pinhole collimator attached to the X-ray camera had advantages for prompt X-ray imaging with high sensitivity and low background, enabling us to image the beams even with short-time measurements.

Deep learning-based in vivo dose verification from proton-induced secondary-electron-bremsstrahlung images with various count level

PurposeProton-induced secondary-electron-bremsstrahlung (SEB) imaging is a promising method for estimating the ranges of particle beam. However, SEB images do not directly represent dose distributions of particle beams. In addition, the ranges estimated from measured images were deviated because of limited spatial resolutions of the developed x-ray camera as well as statistical noise in the images. To solve these problems, we proposed a method for predicting high-resolution dose images from SEB images with various count level using a deep learning (DL) approach for range and width verification. MethodsIn this study, we adopted the double U-Net model, which is a previously proposed deep convolutional network model. The first U-Net model in the double U-Net model was used to denoise the SEB images with various count level. The first U-Net model for denoising was trained on 8000 pairs of SEB images with various count level and noise-free images which were created by a sophisticated in-house developed model function. The second U-Net model for dose prediction was trained using 8000 pairs of denoised SEB images from the first U-Net model and high-resolution dose images generated by Monte Carlo simulation. ResultsFor both simulation and measurement data, the trained DL model could successfully predict high-resolution dose images which showed a clear Bragg peak and no statistical noise. The difference of the range and width was less than 2.1 mm, even from the SEB images measured with a decrease in the number of irradiated protons to less than 11% of 3.2 × 1011 protons. ConclusionsHigh-resolution dose images from measured and simulated SEB images were successfully predicted by using the trained DL model for protons. Our proposed DL model was feasible to predict dose images accurately even with smaller number of irradiated protons.

Galaxies lacking dark matter produced by close encounters in a cosmological simulation

Electrons accelerated on Earth by a rich variety of wave scattering or stochastic processes generate hard non-thermal X-ray bremsstrahlung up to >~ 1 MeV and power Earth's various types of aurorae. Although Jupiter's magnetic field is an order of magnitude larger than Earth's, space-based telescopes have previously detected X-rays only up to ~7 keV. On the basis of theoretical models of the Jovian auroral X-ray production, X-ray emission in the ~2-7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell-Boltzmann distribution) bremsstrahlung. Here we report the observation of hard X-rays in the 8-20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope 0.60+/-0.22 - a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the keV to MeV energy range using coeval in situ measurements by the Juno spacecraft's JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted to arise in stochastic acceleration, rather than coherent acceleration by electric fields. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray auroras on Earth and Jupiter, which may be occurring on Saturn, too.

Observation and origin of non-thermal hard X-rays from Jupiter

Electrons accelerated on Earth by a rich variety of wave-scattering or stochastic processes1,2 generate hard, non-thermal X-ray bremsstrahlung up to ~1 MeV (refs. 3,4) and power Earth’s various types of aurorae. Although Jupiter’s magnetic field is an order of magnitude larger than Earth’s, space-based telescopes have previously detected X-rays only up to ~7 keV (ref. 5). On the basis of theoretical models of the Jovian auroral X-ray production6,7,8, X-ray emission in the ~2–7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell–Boltzmann distribution) bremsstrahlung5,9. Here we report the observation of hard X-rays in the 8–20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope of 0.60 ± 0.22—a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the kiloelectronvolt to megaelectronvolt energy range using coeval in situ measurements taken by the Juno spacecraft’s JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted as arising in stochastic acceleration, rather than coherent acceleration by electric fields10. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses11, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray aurorae on Earth and Jupiter, which may be occurring on Saturn, too.

Imaging and range estimations of prompt X-rays using YAP(Ce) camera during particle-ion irradiation to phantoms with air cavities

Low-energy X-ray imaging of prompt secondary electron bremsstrahlung X-rays (prompt X-rays) emitted during particle-ion irradiation is a promising method for range estimation. However, measurements have so far been conducted mainly for uniform phantoms of water or an acrylic block. Prompt X-ray imaging for phantoms with air cavities has not yet been extensively measured or evaluated with realistic conditions. Consequently, we conducted imaging of prompt X-rays using a pinhole YAP(Ce) camera during irradiation by protons as well as carbon ions to non-uniform acrylic phantoms with small cavities and then evaluated the images and estimated the ranges from the measured prompt X-ray images. The non-uniform acrylic phantom used for imaging had a cylindrical cavity with a 20-mm or 10-mm diameter in the phantom. During irradiation by protons or carbon ions, imaging of one of the phantoms was conducted using the pinhole YAP(Ce) camera with an air cavity as well as filling the cavity with an acrylic rod. For the phantom with a 20-mm-diameter cavity, the prompt X-ray images measured for both protons and carbon ions showed the shape of the cavity in the images, and the ranges could be estimated from the images. For the phantom with a 10-mm-diameter hole, although the shape of the hole could not be clearly observed, the ranges could also be estimated from the images. Furthermore, Monte Carlo simulated prompt X-ray images with different spatial resolution of the X-ray camera showed similar images to the measured images. We confirmed that prompt X-ray imaging of phantoms with air cavities using the pinhole YAP(Ce) cameras was possible and that prompt X-ray imaging is a promising approach for estimating the ranges for both protons and carbon ions, even for phantoms with air cavities.

Electromagnetic Signal of a Proton Beam in Biological Tissues for a Potential Range-Verification Approach in Proton Therapy

In the present work we investigate the fundamental properties of a proton beam's electromagnetic signal as a candidate for range verification in proton therapy. We compute the electric and magnetic fields from a pulsed proton beam in biological tissues in a cylindrical geometry via a dedicated analytical solution of the Maxwell equations. The tissues are specified through their dielectric response and stopping power, where we consider a straight trajectory with a range that corresponds to protons with an initial energy of 150 MeV. We analyze the characteristics of the pulse and the impact of the pulse shape. The effect of the tissue's conductivity on the deposited charges is specifically taken into account. In addition, we calculate and analyze the radiative pulse through the Li\'enard-Wiechert approach. We find that the conductivity charge relaxation has a big influence on the electric field strength, even in the presence of boundaries. It strongly depends on its environment, i.e., the permittivity, conductivity, and boundaries of the surrounding tissues. The electric field that originates from the primary protons vanishes in a time scale of ns. Conversely, the magnetic field is independent of its environment and constant in strength during a rectangular pulse ($10\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$). For $\ensuremath{\mu}\mathrm{A}$ peak beam currents it lies in the pT range, 10 cm away from the beam axis, and is therefore detectable through, e.g., optical magnetometry. Its spatial profile, however, does not exhibit a distinct feature with respect to the range, but decreases rather smoothly and characteristically along the beam axis. The typical velocity dependence of the magnetic field strength is suppressed, due to the increasing charge density downstream. Finally, the primary protons give rise to an electromagnetic pulse in the radio spectrum (it peaks at approximately 9 GHz). It originates spherically around the range and consists partially of bremsstrahlung and Cherenkov radiation. Yet, a broader comparison to other radiation sources, e.g., secondary electron bremsstrahlung, is required. Based on our findings, we conclude that the magnetic field signal has favorable properties that motivate its detection experimentally for range-verification purposes.

Calculation of secondary electron bremsstrahlung in the binary encounter approximation using Dirac–Hartree–Fock–Slater velocity distributions

One of the main factors that determine the detection limit in particle-induced X-ray emission (PIXE) spectrometry is the continuous X-ray background. In the case of impinging protons, a complete description requires five radiative processes. Among them, secondary electron bremsstrahlung (SEB) is one of the largest contributions and describes the bremsstrahlung radiated by the electrons ejected from the target atoms by proton impact. The calculation of SEB cross sections in the binary encounter approximation (BEA) entails knowledge of the velocity distributions (VDs) of the atomic electrons. For this purpose, analytical hydrogenic VDs are currently used for all the (sub)shells. In the present work, numerical VDs calculated from Dirac–Hartree–Fock–Slater (DHFS) wave functions are employed. Additionally, a compact equation for the SEB differential cross section is derived. Results for the ionisation and SEB cross sections of Al, Ag, and Au calculated from the DHFS and hydrogenic VDs are compared in detail.

Dose image prediction for range and width verifications from carbon ion-induced secondary electron bremsstrahlung x-rays using deep learning workflow.

Imaging of the secondary electron bremsstrahlung (SEB) x rays emitted during particle-ion irradiation is a promising method for beam range estimation. However, the SEB x-ray images are not directly correlated to the dose images. In addition, limited spatial resolution of the x-ray camera and low-count situation may impede correctly estimating the beam range and width in SEB x-ray images. To overcome these limitations of the SEB x-ray images measured by the x-ray camera, a deep learning (DL) approach was proposed in this work to predict the dose images for estimating the range and width of the carbon ion beam on the measured SEB x-ray images. To prepare enough data for the DL training efficiently, 10,000 simulated SEB x-ray and dose image pairs were generated by our in-house developed model function for different carbon ion beam energies and doses. The proposed DL neural network consists of two U-nets for SEB x ray to dose image conversion and super resolution. After the network being trained with these simulated x-ray and dose image pairs, the dose images were predicted from simulated and measured SEB x-ray testing images for performance evaluation. For the 500 simulated testing images, the average mean squared error (MSE) was 2.5×10-5 and average structural similarity index (SSIM) was 0.997 while the error of both beam range and width was within 1mm FWHM. For the three measured SEB x-ray images, the MSE was no worse than 5.5×10-3 and SSIM was no worse than 0.980 while the error of the beam range and width was 2mm and 5mm FWHM, respectively. We have demonstrated the advantages of predicting dose images from not only simulated data but also measured data using our deep learning approach.

Sensitivity improvement of YAP(Ce) cameras for imaging of secondary electron bremsstrahlung x-rays emitted during carbon-ion irradiation: problem and solution

Low-energy x-ray imaging of secondary electron bremsstrahlung x-rays emitted during carbon-ion irradiation is a promising method for range estimation and could be used for imaging with almost clinical dose levels of carbon ion. However, the number of counts in images with clinical dose levels is relatively small, making it difficult to obtain precise range estimations. Since improving the sensitivity of the x-ray camera may solve this issue, we developed two new types of x-ray cameras. One uses a 1 mm thick, 40 mm × 40 mm cerium-doped yttrium aluminum perovskite (YA1O3: YAP(Ce)) scintillator plate combined with a 2 inch square flat panel photomultiplier tube (FP-PMT) contained in a 2 cm thick tungsten shield with a pinhole collimator positioned 50 mm from the scintillator; the other uses a 0.5 mm thick, 20 mm × 20 mm YAP(Ce) scintillator plate combined with a 1 inch square position sensitive photomultiplier tube (PSPMT) contained in the same tungsten shield with a pinhole collimator, but with the scintillator positioned closer (30 mm) to the pinhole collimator to obtain a similar field of view. For both cameras, we used a wider angle (∼55°) pinhole collimator to measure the phantom closer to improve sensitivity. Although the 40 mm × 40 mm YAP(Ce) camera had high system spatial resolution, the background count fractions were high and produced a high count area at the center of the images due to the pulse pileup of the signals. With the 20 mm × 20 mm YAP(Ce) camera, we obtained x-ray images with low background counts without a high count area at the image center. By smoothing the measured images, we were able to estimate the ranges even for clinical dose levels. We therefore confirmed that one of our newly developed YAP(Ce) cameras had high sensitivity and is promising for the imaging of secondary electron bremsstrahlung x-rays during irradiation of carbon ions in clinical conditions.

A Simulation Study on Estimation of Bragg-Peak Shifts via Machine Learning Using Proton-Beam Images Obtained by Measurement of Secondary Electron Bremsstrahlung

We investigated an estimation method of Bragg-peak shifts via machine learning using proton-beam images obtained by measurement of secondary electron bremsstrahlung (SEB) by Monte Carlo simulation. Proton beams having energy of 139 MeV were incident on a water phantom with randomly placed air spheres inside, and 6400 pairs of “proton-beam images” and “a Bragg-peak shift” were prepared and then multiple linear regression analysis was carried out. A good agreement was found between the actual Bragg-peak shifts and predicted values in both the training and test sets. The coefficients of determination of the obtained prediction model were 0.899 for the training set and 0.894 for the test set. Consequently, we found that a prediction model with small variance and high prediction performance could be obtained using the SEB data.

Development of a YAP(Ce) camera for the imaging of secondary electron bremsstrahlung x-ray emitted during carbon-ion irradiation toward the use of clinical conditions

Low-energy x-ray imaging of the secondary electron bremsstrahlung (SEB) x-ray emitted during carbon-ion irradiation is a promising method for range estimation. However, it remains unclear whether the method can be used for imaging with the clinical dose levels of carbon-ion and whether the bremsstrahlung x-ray can be detected from the deeper part of the body. To clarify these points, we developed a new high resolution low-energy x-ray camera and conducted imaging of the SEB x-ray during the irradiation of carbon-ions of different energies and intensities. Imaging was also tried with an x-ray camera using a human-head-sized, 17 cm diameter cylindrical phantom. To develop a high resolution imaging detector for a low-energy x-ray, we used a 20 × 20 × 0.5 mm thick cerium-doped yttrium aluminum perovskite, YA1O3 (YAP(Ce)) scintillator plate, which was optically coupled to a 25 mm square high quantum efficiency (HQE) type position sensitive photomultiplier tube (PSPMT). The imaging detector was encased in a 2 cm thick tungsten container and a pinhole collimator was attached to its camera head. After evaluating the camera’s performance, SEB x-ray imaging was tried during irradiation of the carbon-ion and compared the results with a Monte Carlo simulation. We imaged the beam tracks by the SEB x-ray in real-time during irradiation of the carbon-ion and imaging and range estimation were possible even with near clinical dose level of 7.5 × 108 particles of carbon-ion. Clear images of a SEB x-ray were also obtained for a 17 cm diameter cylindrical phantom. The measured images were good agreement with the Monte Carlo simulation. We confirmed that our developed YAP(Ce) camera is promising for imaging SEB x-rays during irradiation of carbon-ions even near clinical conditions.

A simulation study on reduction of the background component using veto counters for imaging of therapeutic proton beams by measuring secondary electron bremsstrahlung using a parallel-hole collimator

A simulation study on reduction of a background (BG) component for an imaging method of therapeutic proton beams by measuring secondary electron bremsstrahlung (SEB) component was performed using the Monte Carlo method. We simulated the images obtained by measuring the SEB and BG components using a parallel-hole-type X-ray camera placed close to an irradiation target. We found that reduction of the BG component was possible by utilizing veto counters placed close to the imaging detector of the X-ray camera. The ratio of the BG component to the sum of the SEB and BG components was able to be reduced from 73% to approximately 20%.

Imaging of monochromatic beams by measuring secondary electron bremsstrahlung for carbon-ion therapy using a pinhole x-ray camera

A feasibility study on the imaging of monochromatic carbon-ion beams for carbon-ion therapy was performed. The evaluation was based on Monte Carlo simulations and beam-irradiation experiments, using a pinhole x-ray camera, which measured secondary electron bremsstrahlung (SEB). The simulation results indicated that the trajectories of the carbon-ion beams with injection energies of 278, 249 and 218 MeV/u in a water phantom, were clearly imaged by measuring the SEB with energies from 30 to 60 keV, using a pinhole camera. The Bragg-peak positions for these three injection energies were located at the positions where the ratios of the counts of SEB acquisitions to the maximum counts were approximately 0.23, 0.26 and 0.29, respectively. Moreover, we experimentally demonstrated that it was possible to identify the Bragg-peak positons, at the positions where the ratios coincided with the simulation results. However, the estimated Bragg-peak positions for the injection energies of 278 and 249 MeV/u were slightly deeper than the expected positions. In conclusion, for both the simulations and experiments, we found that the 25 mm shifts in the Bragg-peak positions can be observed by this method.

Theoretical detection limit of PIXE analysis using 20 MeV proton beams

Abstract Particle-induced X-ray emission (PIXE) analysis is usually performed using proton beams with energies in the range 2∼3 MeV because at these energies, the detection limit is low. The detection limit of PIXE analysis depends on the X-ray production cross-section, the continuous background of the PIXE spectrum and the experimental parameters such as the beam currents and the solid angle and detector efficiency of X-ray detector. Though the continuous background increases as the projectile energy increases, the cross-section of the X-ray increases as well. Therefore, the detection limit of high energy proton PIXE is not expected to increase significantly. We calculated the cross sections of continuous X-rays produced in several bremsstrahlung processes and estimated the detection limit of a 20 MeV proton PIXE analysis by modelling the Compton tail of the γ-rays produced in the nuclear reactions, and the escape effect on the secondary electron bremsstrahlung. We found that the Compton tail does not affect the detection limit when a thin X-ray detector is used, but the secondary electron bremsstrahlung escape effect does have an impact. We also confirmed that the detection limit of the PIXE analysis, when used with 4 μm polyethylene backing film and an integrated beam current of 1 μC, is 0.4∼2.0 ppm for proton energies in the range 10∼30 MeV and elements with Z = 16–90. This result demonstrates the usefulness of several 10 MeV cyclotrons for performing PIXE analysis. Cyclotrons with these properties are currently installed in positron emission tomography (PET) centers.

Development of a low-energy x-ray camera for the imaging of secondary electron bremsstrahlung x-ray emitted during proton irradiation for range estimation

Imaging of secondary electron bremsstrahlung x-ray emitted during proton irradiation is a possible method for measurement of the proton beam distribution in phantom. However, it is not clear that the method is used for range estimation of protons. For this purpose, we developed a low-energy x-ray camera and conducted imaging of the bremsstrahlung x-ray produced during irradiation of proton beams. We used a 20 mm × 20 mm × 1 mm finely grooved GAGG scintillator that was optically coupled to a one-inch square high quantum efficiency (HQE)-type position-sensitive photomultiplier tube to form an imaging detector. The imaging detector was encased in a 2 cm-thick tungsten container, and a pinhole collimator was attached to its camera head. After performance of the camera was evaluated, secondary electron bremsstrahlung x-ray imaging was conducted during irradiation of the proton beams for three different proton energies, and the results were compared with Monte Carlo simulation as well as calculated value. The system spatial resolution and sensitivity of the developed x-ray camera with 1.5 mm-diameter pinhole collimator were estimated to be 32 mm FWHM and 5.2 × 10−7 for ~35 keV x-ray photons at 100 cm from the collimator surface, respectively. We could image the proton beam tracks by measuring the secondary electron bremsstrahlung x-ray during irradiation of the proton beams, and the ranges for different proton energies could be estimated from the images. The measured ranges from the images were well matched with the Monte Carlo simulation, and slightly smaller than the calculated values. We confirmed that the imaging of the secondary electron bremsstrahlung x-ray emitted during proton irradiation with the developed x-ray camera has the potential to be a new tool for proton range estimations.