Components of detector response function: Monte Carlo simulations and experiment

Components of the response function of an HPGe detector for 32 keV incident photons (Ba Kα x‐rays) were studied using a Monte Carlo program. Physical mechanisms and the role of incident photons, detector x‐rays, photoelectrons and Compton recoil and Auger electrons for each component were investigated. The position, intensity and shape of the components, particularly of the photoelectrons, were studied in detail. Two distinct components for photoelectron escape were identified by considering the fate of photoelectrons, Ge x‐rays and Auger electrons produced in the same interaction. In contrast to the often‐cited shelflike structure, it was found that both components have a slope. The contribution of recoil electrons to the spectrum for single, double and multiple Compton scattering followed by photoelectric absorption of the scattered photon was investigated. The results of the Monte Carlo simulations are presented along with the measured Ba x‐ray spectrum. Copyright © 2006 John Wiley & Sons, Ltd.     

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.     

Monte Carlo calculation of electron spectra produced in water phantoms by 20 keV-1 GeV photons

Abstract

A Monte Carlo calculation for the energy spectra of electrons and positrons produced in infinite and semiinfinite water phantoms by photons ranging in energies from 20 keV to 1 GeV are presented. The dominant processes considered are the photoelectric effect, Auger effect, Compton effect, pair, and triplet production. Bremsstrahlung produced by electrons and positrons with energies greater than 1 MeV is also included. The effect of multiple Compton scattering, not considered in the earlier calculation, for the infinite phantom for photon energies higher than 2 MeV has been incorporated. For a semi-infinite phantom, multiple Compton scattering and backscattering through the top are considered. The results are compared with the earlier calculations for the first-collision spectrum. It is found that the inclusion of multiple Compton scattering significantly increases the average number of electrons/cm³ per photon/cm² at all energies considered whereas bremsstrahlung reduces the number of high energy electrons and produces more low energy electrons in the spectrum.     

紫外激光烧蚀Cu产生等离子体辐射特性

通过时间分辨的光谱测量技术,测定了308 nm XeCl紫外激光烧蚀金属Cu在氮气环境中诱导产生等离子体的发射光谱及其强度随时间分布,实验结果表明等离子体辐射光谱线主要由原子光谱线、一价离子光谱线及连续辐射背景光组成,各种光谱线的数目、辐射强度、持续时间不同。结合实验结果对等离子辐射机理进行了探讨,认为电子通过逆韧致辐射获得较高的能量,连续辐射主要来自高能电子的韧致辐射,原子和一价离子的激发主要是通过电子与原子、离子的碰撞传能以及电子与离子的复合产生,并用其定性地解释了所观察的实验现象。     

Dose estimation and shielding calculation for X-ray hazard at high intensity laser facilities

An ionizing radiation hazard produced from the interaction between high intensity lasers and solid targets has been observed. Laser-plasma interactions create "hot" electrons, which generate bremsstrahlung X-rays when they interact with ions in the target. However, up to now only limited studies have been conducted on this laser-induced radiological protection issue. In this paper, the physical process and characteristics of the interaction between high intensity lasers and solid targets are analyzed. The parameters of the radiation sources are discussed, including the energy conversion efficiency from laser to hot electrons, hot electron energy spectrum and electron temperature, and the bremsstrahlung X-ray energy spectrum produced by hot electrons. Based on this information, the X-ray dose generated with high-Z targets for laser intensities between 10¹⁴ and 10²⁰ W/cm² is estimated. The shielding effects of common shielding items such as the glass view port, aluminum chamber wall and concrete wall are also studied using the FLUKA Monte Carlo code. This study provides a reference for the dose estimation and the shielding design of high intensity laser facilities.     

Measurement of the energy distribution of bremsstrahlung using Compton scattering

The GEANT 4.9.2 software package is applied for the simulation of primary and Compton scattered bremsstrahlung spectra. The possibility of restoring primary γ-radiation spectra with the energy 0.02–1200 MeV using Compton scattering on secondary scattering targets is studied. The advantages of light targets using are demonstrated. An additional bremsstrahlung contribution from e^±-pairs is calculated for high-energy γ quanta; the influence of CXR on the form of scattered X-ray quantum spectra is estimated. The spectra which have a multipeak structure are restored. For high-energy γ quanta there occurs significant spread of energies in the Compton scattered radiation spectrum, which imposes stringent conditions on collimation and detector resolution capability. The possibility of measuring the bremsstrahlung spectra from a primary target during high-angle Compton scattering is estimated.     

Monte Carlo determinations of the beta dose rate to tooth enamel

The results of Monte Carlo simulations of the beta dose rate to enamel are presented. The dose rates are the most comprehensive to date, incorporating the beta spectrum from 21 different radionuclides, all internal conversion and Auger electrons, the majority of the bremsstrahlung radiation, effects due to radon loss in the uranium series, and variations in the moisture content of the sediment. Applications to a new dating technique in archaeology using electron spin resonance and beta-gradient isochrons are discussed.     

Simulation of methods for a rapid determination of the energy spectrum of bremsstrahlung from electron accelerators

Semiempirical computational methods for determining the energy spectrum of bremsstrahlung from pulsed electron accelerators are proposed. Input information for employing these methods includes the energy spectrum of electrons in a pulse, the effective angle of their incidence on the target, and the parameters of the converter target. It is shown that the methods in question can be used in a rapid determination of bremsstrahlung spectra for an arbitrary angle of the escape of bremsstrahlung, including its escape in the forward direction. Among other things, this provides a solution to the problem of dosimetry in radiation tests of electron systems.     

激光烧蚀金属Al诱导发光的动力学研究

利用时间分辨的光谱测量技术,测定了XeCl紫外激光在不同条件下烧蚀金属Al诱导产生光谱线及其强度随时间的分布.结果表明：等离子体辐射光谱线由原子光谱线、一价离子光谱线及连续辐射背景光组成.Al原子光谱线的辐射强度与持续时间为最大,一价离子光谱线次之,以连续辐射背景光为最小.对激光烧蚀金属诱导发光的机理进行了探讨.等离子体中连续辐射背景光来自高能电子的韧致辐射和电子与离子的复合,原子光谱线和一价离子光谱线主要来自等离子体中电子与离子的复合.用此机理定性地解释了所观察到的实验现象. 关键词： 激光烧蚀 时间分辨光谱 辐射机理 金属Al     

The penetration, diffusion and energy deposition of high—energy photon＊

This paper presents a new theory for calculating the transport of high-energy photons and their secondary charged particles. We call this new algorithm characteristic line method, which is completely analytic. Using this new method we cannot only accurately calculate the transport behaviour of energetic photons, but also precisely describes the transport behaviour and energy deposition of secondary electrons, photoelectrons, Compton recoil electrons and positron－electron pairs. Its calculation efficiency is much higher than that of the Monte Carlo method. The theory can be directly applied to layered media situation and obtain a pencil-beam-modelled solution. Therefore, it may be applied to clinical applications for radiation therapy.     

Features of the X-ray radiation of interacting multiply charged clusters

The features of coherent x-ray bremsstrahlung radiation arising from the interaction of nanoparticles, namely, a fast and an immobile cluster consisting of positively charged nuclei and electrons are analyzed. The polarized bremsstrahlung radiation (PBR) of fast charges on electrons of an immobile cluster and the conventional bremsstrahlung radiation (BR) of fast electrons on heavy charges of this cluster are analytically investigated.     

Observation of synchrotron radiation from electrons accelerated in a petawatt-laser-generated plasma cavity

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x rays extending to 50 keV is observed. The measured x rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the Maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy converted into x rays.     

Screening effect and possibility of diagnostics of atomic nuclei and electron distribution in structured material using bremsstrahlung beam of relativistic electrons

The possibility of diagnostics of atomic nuclei and electron distribution in a structured material using bremsstrahlung (BS) of relativistic electrons in the material and considering the electric field screening of nuclei by the electrons of the medium is discussed. The alternative calculation technique of the BS spectrum determining a distinctive relation between the frequency-angular BS properties and the relativistic factor of radiating electrons is considered. The dependence of the BS intensity on the screening electron distribution character is analyzed for light and heavy elements. It is shown that, in spite of a sharp decrease in the BS intensity as compared to the radiation for a bare nucleus, the screening does not completely suppress BS, which allows its use for structural analysis of a material.     

X-ray amplification by laser controlled coherent bremsstrahlung

A coherent x-ray generation scheme is proposed which involves characteristics of free electron lasers and atomic high harmonic generation schemes. A thin solid layer or any other periodic atomic structure with limited dimensions is exposed to a short, superintense laser pulse. The electrons are extracted from the layer due to the extreme force and penetrate periodically through the ionic structure. Consequently, thousands of harmonics of the laser radiation field are shown to coherently amplify along the interaction length. The small signal gain of the generated x-ray radiation exceeds that arising from the multiphoton Compton process in plasmas and is competitive with that in the leading x-ray free electron lasers.     

利用轫致辐射谱回推超热电子温度

超短超强激光打靶产生的超热电子与固体靶相互作用时会产生轫致辐射X射线。利用蒙特卡罗方法,对电子在固体靶中传输产生的轫致辐射X射线进行了模拟。1 MeV电子束与固体靶作用产生的轫致辐射谱模拟结果表明,轫致辐射谱高能段斜率受靶厚度及靶材料的影响不明显。麦克斯韦分布的电子束及单能电子束与30 m铜靶作用的模拟结果显示,两种电子源产生的轫致辐射谱在电子束能量或温度较高时基本一致。给出了一种利用轫致辐射谱斜率反推超热电子温度的定标方法。模拟了不同温度下超热电子产生的轫致辐射光子的能量角分布及光子数角分布,结果显示辐射光子能量通量和光子数随着电子温度的提高越来越向前倾,并给出了另外一种由轫致辐射能量角分布反推超热电子温度的定标关系。     

非圆截面环流器硬X射线辐射强度的绝对测量

本文给出了逃逸电子在孔拦上轰击出的硬X射线能谱及其辐射强度,描述了硬X射线闪烁谱还原成能谱的分析处理法,并对NaI(T1)探测器的绝对光峰效率进行了测量。     

Transmutation of nuclear wastes using photonuclear reactions triggered by Compton backscattering photons at the Shanghai laser electron gamma source

Based on the facility of the Shanghai Laser Electron Gamma Source (SLEGS), the transmutation for nuclear wastes such as ¹³⁷Cs and ¹²⁹I is investigated. It is found that nuclear waste can be transmuted efficiently via photonuclear reaction triggered by gamma photons generated from Compton backscattering between CO₂ laser photons and 3.5 GeV electrons. The nuclear activities of ¹³⁷Cs and ¹²⁹I are evaluated and compared with the results of transmutation triggered by bremsstrahlung gamma photons driven by ultra intense laser. Due to the better character of gamma photon spectrum as well as the high brightness of gamma photons, the transmutation rate of Compton backscattering method is much higher than that of the bremsstrahlung method.     

Transmutation of nuclear wastes using photonuclear reactions triggered by Compton backscattering photons at the Shanghai laser electrongamma source

Based on the facility of the Shanghai Laser Electron Gamma Source (SLEGS),the transmutation for nuclear wastes such as 137Cs and 129I is investigated.It is found that nuclear waste can be transmuted efficiently via photonuclear reaction triggered by gamma photons generated from Compton backscattering between CO2 laser photons and 3.5 GeV electrons.The nuclear activities of 137Cs and 129I are evaluated and compared with the results of transmutation triggered by bremsstrahlung gamma photons driven by ultra intense laser.Due to the better character of gamma photon spectrum as well as the high brightness of gamma photons,the transmutation rate of Compton backscattering method is much higher than that of the bremsstrahlung method.     

Radio waves and γ-ray emission from astrophysical sources

A mechanism of X- and γ-ray emission by relativistic electrons at interaction with astrophysical objects (maser clouds) is considered and the contribution of this mechanism to the formation of a continuous spectrum and its correlation with radio-wave radiation are investigated. The radiation yield upon deexcitation of a quantum system by an electron exceeds that of “conventional” (i.e., without a change in the quantum-system state) bremsstrahlung at small nuclear charges (Z < 10) and may be about 8% of the total γ-ray intensity. It is shown that in the direction in which the γ-ray intensity increases, one should observe radio waves corresponding to a particular maser source.     

Radiation from silver films bombarded by low-energy electrons

Emission spectra from Ag films irradiated by low energy electrons (20–1500 eV) have been measured, and the results compared with theory. For relatively smooth films, two peaks in the spectra are resolved. One at 3.73 eV, the volume plasmon energy, is attributed to transition radiation and/or bremsstrahlung. The second, at about 3.60 eV, is very sensitive to surface roughness in both position and magnitude and is produced by roughness-coupled radiation from surface plasmons. For rough films, the roughness-coupled radiation dominates the emission. In addition to spectral shapes, the polarization of the radiation and its intensity as a function of electron energy were measured. The experimental results are compared with new calculations of roughness-coupled emission which account for most of our observations. They indicate that high wavevector roughness components play the dominant role in the emission process.