高能工业CT探测器能谱响应影响校正研究

高能X射线工业CT探测器系统的能谱响应是一重要的特性参数。用实验方法测定探测器的能谱响应曲线存在很多困难。本工作利用Monte-Carlo模拟程序计算了CdWO₄探测器模块的能谱响应曲线,分析了9MeV驻波电子直线加速器X射线束的能量、剂量角分布以及X射线束能谱硬化对探测器模块探测效率的影响,为线阵探测器系统的一致性校正、非线性校正等提供了理论计算参数,并应用于工业CT图像校正,取得了满意的效果。     

Bayesian PIXE background subtraction

The subtraction of the X-ray background in a PIXE spectrum has been the subject of many investigations and different techniques have been developed. These techniques vary from filtering to fitting polynomial functions. The promising Bayesian Statistics technique has been used in this study to eliminate the background from the spectrum in a rigorous and self-consistent manner. We compare the results of the Bayesian background subtraction method to that obtained by stripping and the “rolling ball”; method.     

一个PIXE-RBS离子束分析系统的刻度

四川大学原子核科学技术研究所依托2.5 MeV范德格拉夫静电加速器搭建了质子诱发X射线荧光分析(Proton Induced X-ray Emission,PIXE)与卢瑟福背散射分析(Rutherford Backscattering Spectrometry,RBS)相结合的离子束分析系统,描述了该分析系统和刻度过程。通过10个金属单质的PIXE-RBS测量,刻度得到的仪器常数H值是一条随能量变化的曲线,然后采用最小二乘法拟合确定了X射线探测器前的Mylar膜有效厚度、选择性滤膜的有效厚度和中心小孔大小,从而得到H值。为了对刻度H值进行检验,在相同实验条件下测量了标准粘土样品元素成分,测量数据与证书数据符合得较好。刻度结果将用于以后的PIXE-RBS分析。     

超薄窗Si（Li）探测器在实验中的应用

用超薄窗Ｓｉ（Ｌｉ）探测器测量低能Ｘ射线，将传统的ＰＩＸＥ谱测量范围推广到如Ｓｉ、Ａｌ、Ｎａ等低原子序数元素。还对超薄窗Ｓｉ（Ｌｉ）探测器的具体应用作了描述。     

STIM evaluation in GeoPIXE to complement the quantitative dynamic analysis

The GeoPIXE software for quantitative PIXE trace element imaging and analysis is a well established package for evaluation of characteristic X-ray data for both PIXE and SXRF. For the case of microbeam applications on semi-thick samples knowledge of the local areal density distribution is important for precise quantification. A technique is reported to achieve this using the measurement of beam particle energy loss as it traverses the sample, as in scanning transmission ion microscopy (STIM). New functionality is added to the GeoPIXE code through integration of routines for STIM sorting of event-by-event data to create elemental maps of the mean energy after traversing the sample. Integration of stopping powers for a given sample matrix then permits the measured energy loss to be related to the local areal density. In a further step, this information is used for X-ray absorption corrections made directly to the PIXE analysis results.As a complement, user-written plugins operating on single STIM spectra have been used to compare the estimated areal density from chosen spots with the corresponding values calculated with the new GeoPIXE routines. The additions made to the code allow a more precise quantification to be done on inhomogeneous, semi-thick samples.     

External scanning micro-PIXE for the characterization of a polycapillary lens: Measurement of the collected X-ray intensity distribution

We developed a PIXE detection system for the analysis of medium-light elements which exploits a weakly focusing polycapillary lens for the transmission of the X-rays emitted from the target material to a Silicon Drift Detector. The polycapillary lens efficiently collects X-rays, while prevents back-scattered protons from impinging on the detector chip, thus avoiding electronics perturbation and consequent quality loss of PIXE spectra. The system is optimized for the detection of X-rays in the energy range 1–10 keV, when the emission from the target is induced by MeV proton beams with size of the order of a few hundreds of micrometers.This work reports the results of the lens characterization in terms of X-ray collection spot, i.e. the area of the sample actually “seen” by the lens, and its dependence on the X-ray energy. The lens properties have been measured using the external scanning microbeam facility of the Tandetron accelerator at LABEC-INFN in Florence. The detection system was used to detect X-rays from a set of pure elemental standards with an incident 3 MeV proton beam focused to a size of about 30 μm scanning an area of 1.9 × 1.6 mm². By measuring the spatial distribution of characteristic X-rays from each given material, the collection profile of the lens at the corresponding X-ray energy was obtained. Using several standards, the behaviour throughout the range 1–10 keV was examined. The sensitivity of the lens collection profile on the lens-sample out-of-focus distance was also investigated.     

Development of Soft X-Ray Sources with Electron Guns of Field Emission- and Thermal Cathode-Types and Their Application to Counter Experiments

Soft X-ray sources have been developed with electron guns of field emission- and thermal cathode-types. Those sources are used mainly to investigate the electron drift and avalanche properties in a gas counter. Thin films of aluminum, titanium, iron, copper and nickel are used as the X-ray targets. Beams of quasi-monochromatic X-rays are obtained with these targets. The emitted X-rays consist of characteristic and bremsstrahlung radiations, which are checked by using a high purity germanium detector. Generation of pulsed X-rays is also tried. As an example of the application of the X-ray source, results of measurements of the SQS (Self Quenching Streamer) phenomenon in a gas counter have been shown.     

基于高能X射线散射的高原子序数物质探测方法

针对核材料的探测问题,利用其高原子序数（Z）的特性,提出了基于散射能谱解析识别物质原子序数的方法。该方法通过对X射线与物质相互作用所产生的散射光子的测量和分析来进行物质识别,这些光子包括正电子湮没光子、轫致辐射光子和康普顿散射光子,携带了物质原子序数的信息。蒙特卡罗模拟计算结果表明,该方法能够分析核素的原子序数,尤其对核材料等高Z物质的分析更为有效。采用LaBr₃(Ce)探测器测量了基于7MeV电子直线加速器的多个样品的散射能谱,结果表明,该方法能有效区分高原子序数物质。     

WDX-PIXE analysis of low energy X-rays using a microbeam

A high-energy resolution PIXE system developed at a heavy ion microbeam line was used to analyze low energy X-rays below 1 keV. The system is equipped with a plane crystal spectrometer with a gas flow position sensitive proportional counter (PSPC), which enables high-energy resolution PIXE analysis using a microbeam. In order to improve the detection efficiency for the low energy X-rays, the X-ray entrance window of the PSPC was replaced with a thin polymer film supported by a metal grid. As the result, the detectable energy range was extended to carbon K X-rays and chemical effect in Fe and Cu L X-rays could be detected. A preliminary result of high-energy resolution PIXE mapping of Cu mesh (#500) showed that it is possible to obtain the Cu L mapping image using a 2 MeV proton microbeam with the size of 20 × 20 μm.     

Differential bremsstrahlung cross sections for collisions of electrons with unscreened nuclei of low atomic numbers

Differential bremsstrahlung cross sections for unscreened nuclei are calculated using Sommerfeld-Maue eigenfunctions for the electronic continuum states. The photon energy spectrum and the shape function, defined as the ratio of the differential bremsstrahlung cross section to the photon energy spectrum, are tabulated for incident electron kinetic energies T₁ = 5, 10, 50, and 100 keV and for atomic numbers Z = 8, 18, 26, and 36. For sufficiently low Z and for incident electron kinetic energies and emitted photon energies greater than the K-shell binding energy of the target atom, the present results for unscreened nuclei are in reasonable agreement with and easier to compute than the atomic-field bremsstrahlung cross sections.     

Dual detector system for PIXE measurement covering a wide element range

Two different type of X-ray detectors were used for simultaneous PIXE measurement over a wide element range. One is a pure-Si detector with a thin Be-window, which is to keep high sensitivity for the detection of low Z element. The other is a CZT detector with a thick Be window of 250 μm. It has almost 100% detection efficiency for the X-rays with high energy of up to about 80 keV. By using the CZT detector that was arranged to have a large solid angle, the detection sensitivity of the high energy X-rays of around 30 keV was enhanced by about two orders of magnitude than that by the pure-Si detector. A pair of small permanent magnet was used to sweep away the back-scattered protons from the detector. With this magnet system, the background level in the spectrum of the pure-Si detector was significantly reduced.     

Beam charge integration in external beam PIXE–PIGE analysis utilizing proton backscattering with an extraction window

In this study,we present a new method for the indirect integration of beam charges in external beam proton-induced X-ray emission and proton-induced c-ray emission(PIXE–PIGE) analysis.We recorded proton spectra backscattered by a Kapton film extraction window in different sample situations and under different beam currents.We also simulated backscattering spectra using the simulation of backscattering spectra program(SIMNRA).We determined that in a specific geometrical arrangement,different sample situations did not significantly affect factor C_Q(the ratio between integral backscattering proton counts and integral beam charges).We also studied the reproducibility and beam current dependence of factor C_Q.The statistic factor of C_Q was28.95 ± 0.6 kilo counts/l C,with a relative standard deviation of 2.0 %.Significantly,in external beam PIXE–PIGE analysis,we were able to calculate beam charge integration from the integral backscattering proton counts in an energy region.     

Calculating the electron temperature in the lightning channel by continuous spectrum

Based on the theory of plasma continuous radiation,the relationship between the emission intensity of bremsstrahlung and recombination radiation and the plasma electron temperature is obtained.During the development process of a return stroke of ground flash,the intensity of continuous radiation spectrum is separated on the basis of the spectrums with obviously different luminous intensity at two moments.The electron temperature of the lightning discharge channel is obtained through the curve fitting of the continuous spectrum intensity.It is found that electron temperature increases with the increase of wavelength and begins to reduce after the peak.The peak temperature of the two spectra is close to 25 000 K.To be compared with the result of discrete spectrum,the electron temperature is fitted by the O I line and N II line of the spectrum respectively.The comparison shows that the high temperature value is in good agreement with the temperature of the lightning core current channel obtained from the ion line information,and the low temperature at the high band closes to the calculation result of the atomic line,at a low band is lower than the calculation of the atomic line,which reflects the temperature of the luminous channel of the outer corona.     

A new mapping acquisition and processing system for simultaneous PIXE-RBS analysis with external beam

The combination of ion beam analysis techniques is particularly fruitful for the study of cultural heritage objects. For several years, the AGLAE facility of the Louvre laboratory has been implementing these techniques with an external beam. The recent set-up permits to carry out PIXE, PIGE and RBS simultaneously on the same analyzed spot with a particle beam of approximately 20 μm diameter.A new mapping system has been developed in order to provide elemental concentration maps from the PIXE and RBS spectra. This system combines the Genie2000 spectroscopy software with a homemade software that creates maps by handling acquisition with the object position. Each pixel of each PIXE and RBS maps contains the spectrum normalised by the dose. After analysing each pixel of the PIXE maps (low and high energy X-ray spectra) with the Gupixwin peak-fitting software, quantitative elemental concentrations are obtained for the major and trace elements. This paper presents the quantitative elemental maps extracted from the PIXE spectra and the development of RBS data processing for light element distribution and thin layer characterization. Examples on rock painting and lustrous ceramics will be presented.     

Improved detection limits in PIXE analysis employing wavelength dispersive X-ray spectroscopy

A wavelength dispersive X-ray spectroscopy was employed to measure the proton induced Lα X-ray emission spectra of Ag, Pd and Cd targets in order to lower detection limits for trace amounts of Pd and Cd in silver matrix. The measurements were performed with a Johansson-type crystal spectrometer having an energy resolution below the natural linewidths of the measured L X-ray lines. As a direct consequence of such ultrahigh experimental energy resolution, detection limits of only few tens of ppm were reached. The method presented in this work can be used in general to improve substantially PIXE detection limits compared to standard energy dispersive spectroscopy for the measurement of trace elements with atomic number in the close vicinity of the atomic number of the target matrix element.     

Surface analysis with high energy time-of-flight secondary ion mass spectrometry measured in parallel with PIXE and RBS

The Surrey Ion Beam Centre (IBC) is routinely using focused MeV primary ions to generate two-dimensional molecular maps using time-of-flight secondary ion mass spectrometry (ToF-SIMS) collected simultaneously with particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) spectra. Measurements made with the ToF-SIMS, PIXE, and RBS device with a focused and scanned MeV primary ion beam provide a more complete elemental and molecular evaluation of the target sample’s surface. In this paper, we explore the use of high electronic energy loss by MeV primary ions in the surface region of the target as a method for generating molecular images of the surface. We provide analyses of the relative secondary ion yield of leucine molecules as a function of primary ion velocity using MeV primary ions. We also demonstrate our ability to collect PIXE, RBS, and ToF-SIMS images generated using the same MeV primary ion.     

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 SuperMC 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 of ＜500 keV,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.     

Analysis of X-ray spectra in 14.5-GHz ECR ion source for optimizing operation conditions

An electron cyclotron resonance (ECR) ion source operating at 14.5 GHz was developed for the generation of charged ions at the Korea Atomic Energy Research Institute (KAERI). Experiments were carried out to study the plasma inside the ECR ion source by analyzing the X-ray spectra generated by it. The X-ray energy distribution and electron energy inside the plasma chamber are influenced by the status of the heated plasma. That status depends on various operation parameters such as microwave power, injected gas-pressure, and solenoid and trim coil currents. X-ray spectra were recorded to find the correlation between the plasma and the X-rays for variations in the operation parameters. A standard NaI(Tl) detector was used for that purpose. The X-ray energy distribution was studied in the range of 100–500 W for radiofrequency power. The influence of the injected gas pressure and the mirror ratio in the emission of X-rays were analyzed.     

脉冲硬X射线能谱软化方法数值分析

基于轫致辐射原理,提出了通过轫致辐射靶优化设计软化脉冲硬X射线能谱的方法。采用MCNP程序模拟了复合薄靶和反射靶的输出参数,分析了复合薄靶中转化靶和电子吸收材料厚度对脉冲硬X射线能谱、转换效率以及透射电子份额的影响;给出了反射靶透射和反射X射线能谱、转换效率的差异及其随电子入射角度的变化规律。根据模拟结果分析了两种方法的可行性,并进行了实验验证,为轫致辐射靶的优化设计提供参考。     

Bremsstrahlung energy spectra from electrons of kinetic energy 1 keV ≤ T1 ≤ 2000 keV incident on neutral atoms 2 ≤ Z ≤ 92

A tabulation is presented of theoretical predictions for the electron bremsstrahlung energy spectrum from neutral atoms as a function of atomic number Z, incident electron kinetic energy T₁, and fraction of energy radiated $\text{k}\text{T}{}_1$. Tabulated values were obtained by interpolation from a smaller set of calculated data points. These calculated points were obtained by describing bremsstrahlung as a single-electron transition in a relativistic self-consistent screened potential and, after expansions in partial waves, by performing a numerical evaluation for radial wavefunctions and matrix elements.