一种小尺寸轫致辐射转换靶设计

以减小直线感应加速器X射线光源横向尺寸为目标,开展轫致辐射转换靶的设计。对聚焦打靶过程中电子束运动轨迹进行分析,指出同一个电子束轨迹分布,既可以描述为电子束在某纵向位置处具有一定的横向展宽,也可以描述为电子束保持较小横向尺寸时的轴向分布展宽,由此提出在束腰附近放置多个小靶片实现聚焦电子束有效阻挡的小尺寸多层靶概念设计。采用EGS4程序对X射线产额进行计算,发现靶厚度在一定范围内改变时X射线产额变化较小,基于这一规律完成了小尺寸多层靶的结构设计。进一步考察了一个设计应用实例,当聚焦电子束最小包络直径3 mm、会聚角100 mrad时,对比大尺寸靶,采用小尺寸多层靶可以获得等效直径减小约50%、产额减小约10%的X射线光源。该设计方法有望在相同的电子束品质和聚焦条件下,获得横向尺寸小于电子束最小束包络直径的X射线光源,具有一定的应用价值。

基于电子加速器的核共振荧光无损探测模拟计算

核共振荧光(nuclear resonance fluorescence,NRF)是一种新兴的无损探测技术,通过分析伽马能谱中的特征能量来识别特定同位素,在含未知爆炸物小型箱体的扫描检查中发挥重要作用。本研究利用电子加速器韧致辐射产生的X射线为射线源,通过蒙特卡罗模拟程序Geant4优化设计NRF背散射探测方案。结果表明,优化后的X射线泄漏率降低了386倍,束流不对称度低于2%,束流均匀度高于70%。基于HPGe探测器获得石墨样品、硝酸铵样品的NRF特征能谱验证了设计方案的可行性,利用重要性抽样法将石墨样品模拟计算效率提高了72.23倍。该设计方案和计算结果可为基于NRF的无损核探测系统研发提供技术支撑。

Measurement of the high energyγ-rays from heavy ion reactions usingČerenkov detector

The energetic bremsstrahlung photons up to 100 MeV produced in heavy ion collisions can be used as a sensitive probe for short-range correlation in atomic nuclei. The energy of the γ-rays can be measured by collecting the Čerenkov light in the medium induced by the fast electrons generated in the Compton scattering or electromagnetic shower of the incident γray. Two types of detectors based on pure water and lead glass as sensitive materials were designed for this purpose. The γresponse and optical photon propagation in the detectors were simulated based on electromagnetic and optical processes in Geant4. The inherent energy resolutions of 0.022(4) + 0.51(2)∕E^(1/2)_(γ) for water and 0.0026(3) + 0.446(3)∕E^(1/2)_(γ) for lead glass were obtained. The geometry sizes of the lead glass and water were optimized to 30 cm × 30 cm × 30 cm and 60 cm × 60 cm ×120 cm, respectively, to detect high-energy γ-rays at 160 MeV. The Hough transform method was applied to reconstruct the direction of the incident γ-rays, providing the ability to experimentally distinguish the high-energy γ-rays produced in the reactions on the target from random background cosmic-ray muons.

^(90)Sr同位素电池放射源的设计模拟与辐射优化

^(90)Sr具有较强的轫致辐射,在制备同位素放射源前,需要先模拟^(90)SrTiO_(3)陶瓷芯块的组分,设计热源盒的结构,并根据制造工艺对芯块、热源包壳进行轫致辐射评估。为了减少热电转换器件的受照剂量,本研究根据β射线的屏蔽原理,提出使用铝箔包裹芯块、优化芯块配料和增加氮化硼垫片的三种降低放射源轫致辐射的方法。MCNP模拟结果显示,三种方法都能有效降低轫致辐射。三种方法均会改变热源盒设计尺寸,且包裹铝箔的方法工程可行性较差。以上结果可为^(90)Sr温差热电池热源部分的设计与制造提供参考,并为电池的辐射优化提供理论方法。

Kinetics Characteristics and Bremsstrahlung of Argon DC Discharge Under Atmospheric Pressure

An improved self-consistent, multi-component, and one-dimensional plasma model for simulating atmospheric pressure argon glow discharge is presented. In the model, both the plasma hydrodynamics model and chemical model are considered. The numerical simulation is carried out for parallel-plate geometry with a separation of 0.06 cm. The results show that Ar＊ plays a major role in the discharge, which is mainly produced by ground state excitation reaction. The electron temperature reaches its maximum in the cathode sheath but maintains a low value （0.23 eV） in bulk plasma. Elastic collision is the dominant volumetric electron energy loss in atmosphere argon glow discharge, which is negligible in low pressure argon glow discharge. The metastable step-wise ionization is the main mechanism for electron production to sustain the discharge. However, the highest contribution to electron production rate is ground state ionization reaction. The bremsstrahlung power density is related to electric voltage. With the increase of the electric voltage, the bremsstrahlung power density increases, namely, the strength of ultraviolet radiation spectrum enhances in the cathode sheath.

Reconstruction of 12 MV bremsstrahlung spectra from measured transmission data

Transmission data for 12 MV bremsstrahlung beams produced by the process for continuous-pulsed 12 MV electrons coming from a linear accelerator hitting a 1.2mm thick plane tantalum target have been acquired with a combination of iron and Telfon attenuators. Two solid state dosimeters with LiF-TLD material surrounded Telfon were used as detectors. It has been checked that the experimental system achieves reasonably narrow-beam geometry by Monte Carlo simulations. From these transmission data, the original energy photon spectrum has been reconstructed using the iterative least-squares technique and compared with the spectrum calculated with Monte Carlo code system EGSnrc. The comparison shows that the numerical technique for analysis of transmission data can represent 12 MV bremsstrahlung spectrum acceptably well. The purpose of our work is to provide an effective way to reconstruct an unknown photon spectrum with high energy component and prove the correctness of this way.

The inverse Bremsstrahlung absorption in the presence of Maxwellian and non-Maxwellian electrons

Inverse Bremsstrahlung absorption(IBA) of an intense laser field in plasma containing Maxwellian and nonMaxwellian(with Kappa and q-nonextensive distribution functions) electrons is studied analytically. Our results show that IBA decreases with an increase in temperature at high intensities and a decrease in plasma density for all kinds of distribution functions. Another striking result is that IBA is independent of the laser intensity at low intensity but is dependent on it when the intensity is going to rise. Also, it could be find that the behavior of the absorption as the function of laser intensity for the Kappa distribution with κ= 10 at low intensity is close to that for the Maxwellian distribution, but at high intensity it is close to that in the presence of q-nonextensive electrons with q = 0.9. These results provide insights into the inverse Bremsstrahlung absorption in the laser-plasma interactions.

Monte Carlo simulation for bremsstrahlung and photoneutron yields in high-energy x-ray radiography

This paper reports on the results of calculations using a Monte Carlo code （MCNP5） to study the properties of photons, electrons and photoneutrons obtained in the converted target and their transportations in x-ray radiography. A comparison between measurements and calculations for bremsstrahlung and photoneutrons is presented. The radiographic rule and the effect of the collimator on the image are studied with the experimental model. The results provide exact parameters for the optimal design of radiographic layout and shielding systems.

Reconstruction of a 6-MeV bremsstrahlung spectrum by multi-layer absorption based on LiF:Mg, Cu, P

In this paper, TLD （LiF： Mg, Cu, P） is used as detector. A multi-layer absorption （MLA） model is designed. Combined with Monte-Carlo processes, a bremsstrahlung X-ray spectrum is reconstructed by an iterative method; the reconstructed results agree with the results of simulations by the MCNP process essentially, especially in middle energy region.

Characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids: The role of bremsstrahlung and radiation reactions

In this work, characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids are investigated by means of a newlydeveloped particle-in-cell (PIC) simulation code. The PIC code takes advantage of the recently developed ionization and collision dynamicsmodels, which make it possible to model different types of materials based on their intrinsic atomic properties. Within the simulations, bothbremsstrahlung and nonlinear Compton scatterings have been included. Different target materials and laser intensities are considered forstudying the parameter-dependent features of X/γ-ray radiations. The relative strength and angular distributions of X/γ ray productions frombremsstrahlung and nonlinear Compton scatterings are compared to each other. The threshold under which the nonlinear Compton scatteringsbecome dominant over bremsstrahlung is also outlined.

An algorithm for Monte Carlo simulation of bremsstrahlung emission by electrons

An algorithm for Monte Carlo simulation of bremsstrahlung emission by electrons based on the framework of Super MC is presented in this paper with efficient and accurate methods to sample the angular distribution and energy of bremsstrahlung photons. The photon energy is sampled according to scaled energy-loss differential cross sections tabulated by Seltzer and Berger. A novel hybrid model for photon angular distribution by low-and high-energy incident electrons is developed. The model uses Tsai's full form of angular distribution function with atomic form factors for high-energy incident electrons. For electrons o\500 ke V, a simple efficient and accurate analytical distribution function is developed, using adjustable parameters determined from the fitting of numerical values of the shape functions tabulated by Kissel et al. The efficiency of sampling photon energy is 80%. Our angular sampling algorithm for high-energy electron bremsstrahlung based on Tsai distribution function is very efficient(sampling efficiency*70%) in the useful photon energy range.

Development of a fast electron bremsstrahlung diagnostic system based on LYSO and silicon photomultipliers during lower hybrid current drive for tokamak

A novel real time fast electron bremsstrahlung (FEB) diagnostic system based on the lutetium yttrium oxyorthosilicate scintillators (LYSO) and silicon photomultipliers (SiPM) has been developed for tokamak.The diagnostic system is dedicated to study the FEB emission in the hard x-ray (HXR) energy range between 10 and 200 keV during the lower hybrid current drive.The system consists of a detection module and three data acquisition and processing (DAP)boards.The detection module consists of annulus LYSO-SiPM detector array and a 12-channel preamplifier module.The DAP boards upload the data to the host computer for displaying and storing through PXI bus.The time and space resolutions of the system are 10 ms and 4 cm,respectively.The experimental results can show the evolution over time and the spatial distribution of FEB.This paper presents the system performance and typical discharge results.

金属屏蔽设置对极端电子辐射下滑环绝缘构件深层充电影响因素分析

在太空极端电子辐射环境中,高能电子穿透金属屏蔽会引发滑环绝缘构件的深层充放电现象从而诱发功率传输部件(solar array drive assembly,SADA)的绝缘故障,甚至导致整星的失效。屏蔽外壳的材料、结构及配置方案等会对滑环绝缘构件的深层充电产生直接影响,因此有必要深入研究外壳的屏蔽作用并提升其防护效用,从而降低放电事故的可能性。为此建立三维滑环绝缘构件深层充电模型,得到了滑环绝缘构件电场和电势的三维分布。研究了金属屏蔽材料、屏蔽结构以及双层屏蔽对电子辐照下滑环绝缘构件最大电场的影响。结果表明,相同面密度下高原子序数金属材料比低原子序数金属材料能更好地屏蔽高能电子,但高原子序数金属材料因韧致辐射具有更高的光子透射率,容易对绝缘材料造成损伤;局部加强的屏蔽结构在质量一定的情况下能进一步降低滑环绝缘构件内部的电场畸变,在1 mm铝屏蔽质量的基础上,最大降低程度约25%;采用双层金属屏蔽时,高原子序数的金属材料在滑环绝缘构件内侧更有利于高能电子的屏蔽,其原因在于低原子序数金属材料在最外侧时会先降低高能电子的能量,从而更好利用高原子序数金属材料具有高背散射系数的特点。

The Role of Forced Oscillators of Coupled Circuits in Radiation Physics: New Linear Accelerator Design Improving Tomo-Scanning Technology (Radiotherapy and CT), Heisenberg-Euler Scatter, and Extension to Bremsstrahlung with GeV Electrons

The quantization of circuits has received to be rather attractive in domains of solid state—molecular—and biophysics, since the quanta referred to as Q-bits play a significant role in the design of the quantum computer and entangled structures. Quantized circuits cannot be applied without modifications, since the energy differences are not equidistant and the polarization of the excited states has to be accounted for having particular importance for the creation of virtual states. Applications of the presented theory are scanning methods in radiotherapy without multi-leaf collimators, which may be realized in tomo-scanning radiotherapy and in the keV domain, which provides a new design of CT. The problem of lateral scatter in the target and energy storage by heat production is significantly reduced by a multilayer system with focusing the impinging electrons at the walls and by a magnetic field. The verification of the Heisenberg-Euler scatter of crossing beams of 9 MV is a central problem of photon physics and can be solved by the new bremsstrahlung technique. A comparison with GEANT 4 Monte-Carlo data indicates that the presented method also works in the GeV domain, and a multi-target can improve the bremsstrahlung yield. GEANT 4 provides the spatial distribution, whereas the virtual oscillator states only show the created energy spectrum. In every case, the exploitation yield can be drastically improved by the superiority of the focused multitarget system compared to a single standard target, and the door to new technologies is opened.

Creation of High Energy/Intensity Bremsstrahlung by a Multi-Target and Focusing of the Scattered Electrons by Small-Angle Backscatter at a Cone Wall and a Magnetic Field—Enhancement of the Outcome of Linear Accelerators in Radiotherapy

The yield of bremsstrahlung (BS) from collisions of fast electrons (energy at least 6 MeV) with a Tungsten target can be significantly improved by exploitation of Tungsten wall scatter in a multi-layered target. A simplified version of a previously developed principle is also able to focus on small angle scattered electrons by a Tungsten wall. It is necessary that the thickness of each Tungsten layer does not exceed 0.04 mm—a thickness of 0.03 mm is suitable for accelerators in medical physics. Further focusing of electrons results from suitable magnetic fields with field strength between 0.5 Tesla and 1.2 Tesla (if the cone with multi-layered targets is rather narrow). Linear accelerators in radiation therapy only need to be focused by wall scatter without further magnetic fields (a standard case: 31 plates with 0.03 mm thickness and 1 mm distance between the plates). We considered three cases with importance in medical physics: A very small cone with an additional magnetic field for focusing (the field diameter at 90 cm depth: 6 cm), a medium cone with an optional magnetic field (field diameter at 90 cm depth: 13 cm) and a broad cone without a magnetic field (the field diameter at 90 cm depth: 30 cm). All these cases can be positioned in a carousel. Measurements have been performed in the existing carousel positioned in the plane of the flattening filter and scatter foils for electrons.

不同复合阳极靶轫致辐射场参数的数值模拟

电子束辐照材料后发生轫致辐射可以产生脉冲X射线,轫致辐射产生X射线的能谱、转化效率等参数与电子束能量,靶材料及其结构相关。利用二极管的电压、电流波形计算了电子束参数,建立了复合靶蒙特卡罗粒子输运计算模型,模拟了电子和光子在不同材料中的输运规律,研究了钽和有机玻璃组成的复合阳极靶对X射线辐射场的影响,结果对于产生低能、高注量、高转化效率、低透射电子能量用复合阳极靶的设计具有指导意义。计算结果表明:复合阳极靶中有机玻璃具有软化电子能谱、衰减透射电子的作用;采用慢化电子靶比相同厚度薄靶产生的X射线能谱相对要硬;减小钽的厚度有利于减小平均光子能量,而增加钽的厚度有助于提高能量转化效率;选用有机玻璃2 cm、钽10μm的薄靶时,X射线平均能量为133.22 keV,光子能量转化效率为0.055%。

10~50 keV的X射线管轫致辐射能谱的解析计算

X射线管便于产生X射线,广泛应用于工业、考古及医学等领域,其能谱分布是应用基础。本文通过蒙特卡罗模拟软件Geant4模拟入射能量为10、20、30、40和50 keV的电子与铜靶的相互作用,得到了电子在不同穿透深度下的计数、能量分布及角度分布,对模拟数据拟合,得到了10~50 keV的电子在铜靶中的电子深度分布及路径长度纠正系数的经验公式。将经验公式和相应轫致辐射截面数据结合,建立了入射能量为10~50 keV的电子在铜靶中产生轫致辐射能谱的解析计算模型。通过和模拟的X射线管轫致辐射谱比较,验证了该解析计算模型的准确性。

等离子体控制优化激光烧结Ni_(30)Cr_(25)Al_(15)Co_(15)Mo_(5)Ti_(5)Y_(5)高熵合金涂层的显微组织与性能

提出一种激光诱导等离子体的主动抑制方法,以优化Ni_(30)Cr_(25)Al_(15)Co_(15)Mo_(5)Ti_(5)Y_(5)高熵合金(HEA)涂层表面激光烧结(SLS)的晶体结构和摩擦学性能,该过程采用脉冲激光和不同保护气体。结果表明,无保护气体或He气氛下的电子温度(T_(e))和等离子体射流体积均高于Ar-He气氛下的。SLS过程中发生明显的相变和金属原子扩散,α-AlFe_(3)、α-AlNi和γ-AlFe_(2)Ni化合物沉淀形成冶金结合。等离子体通过逆韧致辐射(IB)吸收较低能量加强激光与HEA之间的相互作用,使涂层显微硬度增加,动态摩擦因数降低,这说明等离子体控制对激光增材制造存在重要影响。

高能电子加速器屏蔽体轻量化模拟设计

利用FLUKA软件,对10 MeV电子加速器轻量化屏蔽进行模拟优化设计。从屏蔽源项剂量率优化与屏蔽层材料筛选两方面开展研究。结果显示,在束流扫描窗下布放6 cm厚度水的束流接收槽,可使屏蔽源项剂量率降低约33.17%;筛选出的聚乙烯+铅组合屏蔽材料较混凝土屏蔽体能够大幅度节省空间,减小屏蔽厚度,并使屏蔽体重量降低约81.74%。研究成果可为10 MeV工业辐照电子加速器实现可移动化提供技术支持。