Geant4 simulation of fast-electron bremsstrahlung imaging at the HL-3 tokamak

To further research on high-parameter plasma,we plan to develop a two-dimensional hard X-ray(HXR)imaging system at the HL-3 tokamak to measure HXRs with energies ranging from 20 to 300 keV.The application of an array-structured detector ensures that this system can measure HXR-radiation spectra from the entire plasma cross section.Therefore,it is suitable for the study of fast-electron physics,such as radio-frequency wave current drives,fast electrons driving instabilities,and plasma disruptions in fusion research.In this study,we develop a simulation for calculating fast-electron bremsstrahlung in the HL-3 tokamak based on the Monte Carlo simulation code Geant4,in which the plasma geometry and forward scattering of fast-electron bremsstrahlung are considered.The preliminary calculation results indicate that the HXR energy deposi-tion on the detector is symmetrically distributed,even though the plasma distribution is asymmetric owing to the toroidal effect.These simulation results are helpful in constructing the relationship between the energy deposition on the detector and parameter distribution on the plasma cross section during HL-3 experiments.This is beneficial for the reconstruction of the fast-electron-distribution function and for optimizing the design of the HXR-imaging system.

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.

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.

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.

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.

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.

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.

Bremsstrahlung Radiation of Fast Electrons in Long Air Gaps

The results of an experiment on discharges in long atmospheric pressure air gaps at a pulsed voltage of amplitude up to 800 kV and risetime 150-200 ns are analyzed. In the experiment, a radiation pulse of photon energy 〉 5 keV and duration 10-20 ns has been detected. It has been shown that the x-ray pulse is due to the ＂runaway＂ of electrons from the head of an anode-directed streamer. The estimated maximum bremsstrahlung energy is about 5-10 keV. The presence of a maximum in the bremsstrahlung spectrum is due to that the photons emitted by electrons are absorbed by atoms of the gas in which the discharge operates.

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.

Photonuclear reactions on stable isotopes of selenium at bremsstrahlung end-point energies of 10-23 MeV

In this study,experiments were performed at bremsstrahlung end-point energies of 10-23 MeV with the beam from the MT-25 microtron using theγ-activation technique.The experimental values of relative yields were compared with theoretical results obtained on the basis of TALYS with the standard parameters and the combined model of photonucleon reactions.Including isospin splitting in the combined model of photonucleon reactions allows describing experimental data on reactions with proton escape in the energy range from 10 to 23 MeV.Therefore,taking into account isospin splitting is necessary for a correct description of the decay of the giant dipole resonance.

Isomeric pair 95m,g Nb in the photonuclear reactions on natMo at the bremsstrahlung end-point energy of 38–93 MeV

The ^(nat)Mo(γ,xnp)^(95m,g)Nb photonuclear reaction was studied using the electron beam from the NSC KIPT linear accelerator LUE-40.The experiment was performed using the activation and off-line γ-ray spectrometric technique.The experimental isomeric yield ratio(IR) was determined for the reaction products ^(95m,g)Nb at the bremsstrahlung end-point energy E_(γmax) range of 38-93 MeV.The obtained values of IR are in satisfactory agreement with the results of other studies and extend the range of previously known data.The theoretical values of the yields Y_(m,g)(E_(γmax)) and the IR for the isomeric pair ^(95m,g)Nb from the ^(nat)Mo(γ,xnp) reaction were calculated using the partial cross-sections σ(E) from the TALYS1.95 code for six different level density models.For the investigated range of E_(γmax),the theoretical dependence of IR on energy was confirmed-the IR smoothly increases with increasing energy.The comparison showed a noticeable difference(more than 3.85 times) in the experimental IR relative to all theoretical estimates.

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.

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。

Calculation of Gas Bremsstrahlung Power from Straight Section of Storage Ring at SSRF

The Shanghai Synchrotron Radiation Facility(SSRF)is a third-generation synchrotron radiation light source with 3.5GeV in energy,which is composed of the linear accelerator,the booster and the storage ring.The storage ring provides 16 standard straight sections of 6.5m and 4 long straight sections of 12 meters. Gas Bremsstrahlung(GB)produced by the interaction of the stored beam with the residual gas molecules in straight section,which is so intense and has a very small angular that the GB spectra,the GB power and the GB power distribution should be known.The characters of GB are studied by means of Fluka Monte Carlo code.Our result shows agreement with those obtained by the experiential formulae.

基于激光等离子体产生的轫致辐射源实现同核异能素^(152m)Eu的高效激发

同核异能素在宇宙元素合成中发挥着重要作用,并且在控制核能释放方面有潜在的应用。其中,铕(Eu)在现实中有重要的研究意义,例如152Eu被用来做放射性实验的标准源,并且其同核异能态^(152m1)Eu有73%的概率发生β-衰变产生天体p核素钆(^(152)Gd),因此^(152m1)Eu是产生天体p核素^(152)Gd过程中的重要核素。在本工作中,基于激光等离子体产生的轫致辐射源,我们在实验上实现了^(152m1)Eu(45.6 keV,T_(1/2)=9.31 h)的高效激发,其产额能达到8×10^(4)个粒子/发。此外,进一步通过Geant4-GENBOD程序数值模拟了^(152m1,m2)Eu的产额、产生时间以及峰值激发效率随电子温度的演化情况。研究发现,在入射电子电荷量固定为17.6 nC,且当电子温度达到15 MeV时,^(152m1)Eu和^(152m2)Eu的产额趋于饱和,分别为8×10^(6)和2×10^(5)个粒子/发;^(152m1)Eu和^(152m2)Eu的峰值激发效率分别有望达到约10^(17)和10^(16)个粒子/s,其中^(152m1)Eu和^(152m2)Eu的脉宽几乎不变,均为32 ps。超短超强激光技术能够极大提高同核异能素的激发效率,这将为研究宇宙元素合成问题以及控制核能释放应用研究提供一个重要的研究途径。

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。

^(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。