Distribution uniformity of laser-accelerated proton beams

Compared with conventional accelerators,laser plasma accelerators can generate high energy ions at a greatly reduced scale,due to their TV/m acceleration gradient.A compact laser plasma accelerator(CLAPA) has been built at the Institute of Heavy Ion Physics at Peking University.It will be used for applied research like biological irradiation,astrophysics simulations,etc.A beamline system with multiple quadrupoles and an analyzing magnet for laser-accelerated ions is proposed here.Since laser-accelerated ion beams have broad energy spectra and large angular divergence,the parameters(beam waist position in the Y direction,beam line layout,drift distance,magnet angles etc.) of the beamline system are carefully designed and optimised to obtain a radially symmetric proton distribution at the irradiation platform.Requirements of energy selection and differences in focusing or defocusing in application systems greatly influence the evolution of proton distributions.With optimal parameters,radially symmetric proton distributions can be achieved and protons with different energy spread within ±5% have similar transverse areas at the experiment target.     

Controlled transport and focusing of laser-accelerated protons with miniature magnetic devices

This Letter demonstrates the transporting and focusing of laser-accelerated 14 MeV protons by permanent magnet miniature quadrupole lenses providing field gradients of up to 500 T/m. The approach is highly reproducible and predictable, leading to a focal spot of (286 x 173) microm full width at half maximum 50 cm behind the source. It decouples the relativistic laser-proton acceleration from the beam transport, paving the way to optimize both separately. The collimation and the subsequent energy selection obtained are perfectly applicable for upcoming high-energy, high-repetition rate laser systems.     

Spectral enhancement in the double pulse regime of laser proton acceleration

The use of two separate ultraintense laser pulses in laser-proton acceleration was compared to the single pulse case employing the same total laser energy. A double pulse profile, with the temporal separation of the pulses varied between 0.75-2.5?ps, was shown to result in an increased maximum proton energy and an increase in conversion efficiency to fast protons by up to a factor of 3.3. Particle-in-cell simulations indicate the existence of a two stage acceleration process. The second phase, induced by the main pulse preferentially accelerates slower protons located deeper in the plasma, in contrast to conventional target normal sheath acceleration.     

Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams

Monte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.     

Proton therapy analysis using the Monte Carlo method

The range and straggling data obtained from the transport of ions in matter (TRIM) computer program were used to determine the trajectories of monoenergetic 60 MeV protons in muscle tissue by using the Monte Carlo technique. The appropriate profile for the shape of a proton pencil beam in proton therapy as well as the dose deposited in the tissue were computed. The good agreements between our results as compared with the corresponding experimental values are presented here to show the reliability of our Monte Carlo method.     

碳离子束射程快速验证方法的蒙特卡罗模拟研究

可通过楔形装置把离子束(质子和重离子)纵向上的的深度剂量分布转换成横向上的剂量分布,进行离子束射程的快速测量及验证。本工作通过基于GEANT4内核的GATE蒙特卡罗模拟平台,模拟计算了不同能量碳离子束在用于制作楔形装置的铜、铝、铁和有机玻璃等材料中的深度剂量分布,得到不同能量碳离子束在不同材料中Bragg峰位所处深度与能量之间的关系;模拟计算了不同能量碳离子束穿越单楔板、双楔板和大小组合楔板等楔形装置后横向上的剂量分布,得到了横向剂量峰值出现位置与碳离子束射程之间的关系。本文蒙特卡罗模拟研究为进一步开发重离子治疗当中的射程快速验证方法与设备奠定了坚实的基础。     

激光加速质子束对电磁孤立子的照相模拟研究

激光在次稠密等离子中传输, 由于频率下移而被俘获, 从而产生电磁孤立子. 根据先前理论及PIC 模拟给出的孤立子的演化过程, 对不同阶段孤立子的电磁场分布进行了建模. 使用Geant4蒙特卡罗程序, 模拟研究了激光加速产生的能量为几个MeV的质子束对后孤立子的照相. 分析了质子能量, 质子源尺寸等因素对照相结果的影响, 并利用了TNSA加速产生质子束的分幅特性, 开展了时间分辨的孤立子照相模拟研究. 模拟给出的质子照相结果验证了文献中给出的孤立子静电场模型, 为以后在实验上探测孤立子提供了理论依据. 关键词： 超短激光 质子照相 孤立子 蒙特卡罗方法     

Laser Accelerated, High Quality Ion Beams

Intense beams of protons and heavy ions have been observed in ultra-intense laser-solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate very short pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions, we performed experiments at several high-power laser systems. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface. We will present recent experimental results demonstrating a transverse beam emittance far superior to accelerator-based ion beams. Finally, we will discuss the prospect of laser-accelerated ion beams as new diagnostics in laser-solid interaction experiements. Special fields of interest are proton radiography, electric field imaging, and relativistic electron transport inside the target.     

Application of a fast proton dose calculation algorithm to a thorax geometry

Treatment planning in proton therapy requires the calculation of absorbed dose distributions on beam shaping components and the patient anatomy. Analytical pencil-beam dose algorithms commonly used are not always accurate enough. The Monte Carlo approach is more accurate but extremely computationally intensive. The Fast Dose Calculator, a track-repeating algorithm, has been proposed as an alternative fast and accurate dose calculation. In this work FDC is applied to a proton therapy patient thoracic anatomy.     

Broad energy spectrum of laser-accelerated protons for spallation-related physics

A beam of MeV protons, accelerated by ultraintense laser-pulse interactions with a thin target foil, is used to investigate nuclear reactions of interest for spallation physics. The laser-generated proton beam is shown (protons were measured) to have a broad energy distribution, which closely resembles the expected energy spectrum of evaporative protons (below 50 MeV) produced in GeV-proton-induced spallation reactions. The protons are used to quantify the distribution of residual radioisotopes produced in a representative spallation target (Pb), and the results are compared with calculated predictions based on spectra modeled with nuclear Monte Carlo codes. Laser-plasma particle accelerators are shown to provide data relevant to the design and development of accelerator driven systems.     

Off-axis dose distribution for rectangle proton beam

This papcr modifies an analytical algorithm originally developed for electron dose calculations to evaluate the off-axis dose distribution of rectangle proton bcam. This spatial distribution could be described by Fermi-Eyges theory since a proton undergoes small-angle scattering when it passes through medium. Predictions of the algorithm for relative off-axis dose distribution by a 6 cm ＊ 6 cm initial monoenergetic proton beam are compared with the results from the published Monte Carlo simulations. The excellent level of agreement between the results of these two methods of dose calculation （〈 2%） demonstrates that the off-axis dose distribution from rectangle proton beam may be computed with high accuracy using this algorithm. The results also prompts the necessity to consider the off-axis distribution when the proton is applied to clinical radiotherapy since the penumbra is significant at the distal of its range （about 0.6 cm at the Bragg-peak depth）.     

Simulation study of indirect positron generation by an ultra-short laser

Positron generation by an ultra-short ultra-intense laser in an indirect manner has been studied with two-dimensional particle-in-cell (PIC) and Monte Carlo (MC) simulations. In this generation scheme, positrons are produced with energetic electrons accelerated by an ultra-shot laser pulse propagating through an underdense plasma. The dependence of the positron beam properties on the plasma length and secondary target (converter) thickness was investigated in detail. The simulation results reveal that the positron yield is strongly correlated with the total energy of laser-accelerated electrons; both the temperature and divergence of the positron beam are sensitive to the plasma length; and the positron beam has a pulse duration comparable to the incident electron beam. In addition, it is indicated that even with the optimal converter thickness, only a small fraction (11.4%) of positrons can escape out and most of the detected positrons originate from the back edge of the converter.     

Preliminary shielding analysis for the CSNS target station monolith

The construction of the China Spallation Neutron Source (CSNS) has been initiated at Dongguan,Guangdong,China.In spallation neutron sources the target station monolith is contaminated by a large number of fast neutrons whose energies can be as large as those of the protons of the proton beam directed towards the tungsten target.A detailed radiation transport analysis of the target station monolith is important for the construction of the CSNS.The analysis is performed using the coupled Monte Carlo and multi-dimensional discrete ordinates method.Successful elimination of the primary ray effects via the two-dimensional uncollided flux and first collision source methodology is also illustrated.The dose at the edge of the monolith is calculated.The results demonstrate that the doses received by the hall staff members are below the required standard limit.     

Validation of Monte Carlo simulation of 6 MV photon beam produced by Varian Clinac 2100 linear accelerator using BEAMnrc code and DOSXYZnrc code

The Monte Carlo model for the photon-beam output from the Varian Clinac 2100 linear accelerator was validated to compare the calculated to measured PDD and beam dose profiles The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible. The Monte Carlo codes used in this work were the BEAMnrc code to simulate the photons beam and the DOSXYZnrc code to examinate the absorbed dose in the water phantom. We have calculated percentage depth dose (PDD) and beam profiles of the 6 MV photon beam for the 6 × 6 cm², 10 × 10 cm² and 15 × 15 cm² field sizes. We have used the gamma index technique for the quantitative evaluation to compare the measured and calculated distributions. Good agreement was found between calculated PDD and beam profile compared to measured data. The comparison was evaluated using the gamma index method and the criterions were 3% for dose difference and 3 mm for distance to agreement. The gamma index acceptance rate was more than 97% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. The Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc and DOSXYZnrc codes package.     

Microdosimetric Evaluation on the Metallic Nanoparticle-Mediated Dose Enhancement in Radiotherapeutic Proton Irradiation

Monte Carlo simulations are performed on the dosimetric effect of metallic nanoparticles in a clinical proton irradiation.With an in-water hitting model of a single nanoparticle,the secondar.y electrons dose,deposited around the particle surface,is calculated for the proton irradiations in a typical spread-out Bragg peak.The dose enhancement,as the ratio of electron doses from the target particle and background water,is evaluated for the dependence on the depth of hitting,particle size,elements,coating material and thickness.The results indicate a significant dose enhancement on the particle surface within-200 nm,but a fast decay in further distance.The dose enhancement presents a consistency along the spread-out Bragg peak,a positive dependence on both the particle size and electron density,but a strong attenuation by surface coating.Particle cluster may increase the incdividual dose enhajncement by electron crossfire,but is only noticeable in a compact case.The dose enhancement potentiates a radiosensitization use of metallic nanoparticles in clinical proton therapy,but challenqging meanwhile with the narrow ranqge of enhancement effect.     

发散型准直器限束孔应用于被动散射质子放疗可行性研究

被动散射质子放疗（Passive Scattering Proton Therapy,PSPT）是质子治疗的主要技术之一,束流通过准直器限束孔（Aperture）时因边缘散射效应导致患者体内剂量分布偏离理想状态。使用蒙特卡洛软件TOPAS（TOol for PArticle Simulation）对质子束流经过发散型与传统型准直器限束孔后进入水模体中的过程建模,分析两种准直器的边缘散射效应对剂量及中子能谱分布的影响,分别测试70,110,160,200,230,250 MeV能量下的质子束流,发现传统Aperture在70 MeV的质子束下边缘散射效应最明显,在距水箱表面0.5 cm深度处横向剂量曲线平坦度、均匀度分别达到4.63%,108.05%,随着深度增加边缘散射效应逐渐减弱,在布拉格峰位置处接近消失。使用发散型准直器限束孔后,在70 MeV下平坦度、均匀度分别降至1.28%,101.31%,对于100,160,200 MeV质子束均有不同程度改善。对于能量接近250 MeV的质子束,发散型准直器限束孔设置下横向剂量曲线并无优势。边缘散射效应导致的剂量不均随水深增加而减弱,对于各个能量质子,使用发散型准直器限束孔后次级中子减少。研究结果表明,发散型准直器限束孔应用于PSPT效果显著,为进一步应用于临床提供数据支撑。Passive Scattering Proton Therapy (PSPT) is one of the main technologies for proton radiation therapy. The dose distribution in the patient deviates from the ideal state due to the edge scattering effect when the beam passes through the aperture. In this paper, TOPAS, a Monte Carlo software, was used to simulate the passive scattering treatment head. The influence of the edge scattering effect of the two aperture sets on the dose distribution was compared. The proton beam at 70, 110, 160, 200, 230 and 250 MeV was tested respectively. We found that the scattering effect of the conventional aperture is most obvious at 70 MeV, and the flatness and hetergeneity of the lateral dose curve at the inlet of 0.5 cm of the tank reach 4.63%, 108.05%, respectively. The dose shift caused by the edge scattering effect decreases with increasing water depth and disappears at the Bragg peak. After using the divergent aperture, the flatness and uniformity at 70 MeV are reduced to 1.28% and 101.31%, respectively, and the 100, 160, and 200 MeV proton beams are improved in different extents. For a proton beam with an energy close to 250 MeV, there is no advantage in the lateral dose curve of the divergent aperture. For all energy protons, the secondary neutrons are reduced with divergent aperture. The results show that the divergent aperture is effective for PSPT and this study provides data support for further application in clinical practice.     

100 MeV质子束流品质诊断及剂量测量准确性评估方法研究

质子辐射生物学效应是太空放射生物学和质子束放疗研究的重要基础,可为空间环境下人员的危险性评估以及质子治疗优化设计提供科学依据。依托加速器建立相应的生物样本辐照技术是开展此类研究的前提条件。中国原子能科学研究院最近建立的100 MeV强流质子回旋加速器提供的中能质子束流为目前国内能量最高,特别适合用于太空放射生物学和质子治疗相关研究。本研究中,利用在束和离线等多种手段建立了中能质子束流诊断和剂量测量方法,对加速器引出的100 MeV质子照射野大小、均匀性等束流品质以及剂量测量系统准确性进行了分析和评估。结果表明,对光子剂量响应好的LiF(Mg, Ti)热释光探测器,对90 MeV质子同样具有良好的剂量响应关系,可作为中能质子剂量准确性评估的手段之一。在5.0 cm×5.0 cm照射野范围内,加速器引出的100 MeV质子束流的均匀性好于90%,在线剂量测量系统准确性好于93%,束流品质和剂量测量条件基本满足辐射生物学的要求,可为质子辐射生物学效应研究的开展提供可靠保障。     

Brunel-dominated proton acceleration with a few-cycle laser pulse

Experimental measurements of backward accelerated protons are presented. The beam is produced when an ultrashort (5 fs) laser pulse, delivered by a kHz laser system, with a high temporal contrast (10(8)), interacts with a thick solid target. Under these conditions, proton cutoff energy dependence with laser parameters, such as pulse energy, polarization (from p to s), and pulse duration (from 5 to 500 fs), is studied. Theoretical model and two-dimensional particle-in-cell simulations, in good agreement with a large set of experimental results, indicate that proton acceleration is directly driven by Brunel electrons, in contrast to conventional target normal sheath acceleration that relies on electron thermal pressure.     

用于激光驱动质子束测量的腔式束流位置探测器模拟研究

根据激光驱动质子束流低发射度、短脉冲、单束团低电量的性质,研究腔式束流位置探测器(BPM)测量激光加速器产生的质子束团横向位置的可行性问题。针对质子束团的大横向分布和发散角问题,推导了其通过腔式BPM的输出信号,结果表明该信号与集中从束团对称中心、倾斜一定角度通过的束流产生的输出信号相同。依据上述原理,使用CST软件进行了腔式BPM的设计和仿真,确定了矩形谐振腔波导耦合的方案。讨论了该方案的腔式BPM对于激光加速束流的适用性和不同激光驱动质子束流参数的分辨率,并针对PW级激光加速系统进行了分辨率估算。     

激光加速质子束空间电荷力研究

激光加速器可以输出具有独特品质的质子束,例如μm尺寸、ps脉冲长度和高峰值电流。强流粒子束的空间电荷力效应较强,对面向应用的束流传输提出了挑战。通过二维PIC模拟研究了激光加速后与质子速度接近的电子的影响。采用椭球模型估算空间电荷力的影响,比较不同电荷分布的差异。结果表明每束团质子数超过1010后空间电荷力显著影响质子束传输,甚至严重破坏束流品质。空间电荷力的影响在20 ps后显著减弱,离开靶约1.2 mm。