Elastic backscattering cross sections of protons on oxygen

Absolute values of elastic scattering cross section of protons in the energy region 600–2,000 keV have been measured on oxygen for scattering angles between 110 and 160°. The method applied in the present work is advantageous in regard to determine absolute cross sections, since quantities such as proton particle fluence and detection geometry can be neglected. The results show that the measured cross sections are considerable higher than Rutherford cross sections for proton energies close to 2,000 keV. No resonances are observed in the energy region studied.     

Electron capture from atomic hydrogen by slow fully stripped lithium ions

Cross sections for electron capture by Li³⁺ ions from H atoms have been measured in the energy range 9–42 keV. Above 20 keV the data are in good agreement with the theory of Ryufuku and Watanabe while there is a discrepancy at the lower energies.     

Observation of high-energy electrons from a metal target irradiated by protons with a mean energy of 25 keV

Electrons with abnormally high energies of up to 16 keV are detected from an iron target irradiated by ions (H⁺, Fe⁺, Fe²⁺, Fe³⁺) with energies ranging from 20 to 100 keV from the plasma of a high-power femtosecond laser pulse with an intensity of 10¹⁶ J/(s cm²). These electrons indicate that the energy of an incident ion is almost completely transferred to an electron knocked out of the target. In a range of 6–16 keV, the spectrum of electrons knocked out of the K shell of iron atoms by protons with an energy of 22 ± 2 keV is quasi-exponential with an exponent of 4 keV. For 8-keV electrons, the double differential cross section for ionization by such protons is estimated as 10^?7 b/(eV sr).     

Photoelectron branching ratios and partial ionization cross-sections for CO and N2 in the energy range 18–50 eV

Branching ratios for photoionization to various ion states of CO and N₂ have been measured in the energy range 18–50 eV using an electron—electron coincidence technique at an ejected electron angle of 90°. The branching ratios, which show a marked variation with energy are shown to be in good agreement with the results of conventional photoelectron spectroscopy at those energies where quantitative PES measurements have been made. The results indicate that the effect of the assymmetry parameter, β, is relatively insignificant in the case of CO and N₂. Partial oscillator strengths have been determined from the branching ratios using recent total photoabsorption data. In addition to the expected one electron ion states, others are observed arising from two electron processes and the variation of these cross-sections with energy loss is also reported.     

Angular and energy distribution of electrons produced by 200–500 keV protons in gases

The cross beam method was used to measure double differential ionization cross sections for electron production by 200–500 keV protons incident on a molecular beam. The ejected secondary electrons were detected by an electron multiplier and an electrostatic 90 °-spectrometer which was movable in the scattering chamber from 18 °–155 ° with respect to the incident proton direction. After the elimination of disturbing electric and magnetic fields, electrons were measured at energies down to 1 eV. The absorption of the very slow electrons along their path through the spectrometer could be minimized by using a relatively low gas pressure in the scattering chamber of a few 10^?5 Torr. The efficiency of the electron detector was determined with an electron source whose emission rate per solid angle was known. Absolute cross sections and their angular dependence were obtained by measuring the slow electrons—ejected with a relatively high rate—without the molecular beam; these data being used to normalize the electron spectra acquired with the molecular beam. Ionization cross sections with 300 keV protons incident on helium are discussed and except for the very slow electrons, a good agreement is found with results of Ruddet al.     

飞秒激光与等离子体相互作用过程中超热电子能谱的测量

 报道了在3TW飞秒激光器上完成的激光 等离子体相互作用过程中产生的超热电子的能谱测量结果。能谱测量显示：在较低的能段，超热电子能谱先是呈现一个局部的平台，然后迅速衰减，呈现非类麦克斯韦分布，这是由于几种加热机制共同作用，其中占主导地位的是反射激光对电子的加速；在较高的能段，超热电子能谱呈类麦克斯韦分布，拟合的温度远远高于已知的温度定标律给出的温度，其原因在于超热电子分布的高能尾部本身的抬高和激光的自聚焦及成道。     

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H⁺ and H⁰) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.     

Electron capture by fast protons from helium-like ions

Four-body formalism of boundary corrected continuum intermediate state (BCCIS-4B) approximation have been applied to calculate the single-electron capture cross sections by fast protons through some helium-like ions in a large energy range from 30–1000 keV. In this model, distortion has been taken into account in the entrance channel. In the final channel, the passive electron plays the role of screening of the target ion. However, continuum states of the projectile and the electron in the field of the residual target ion are included. The comparison of the results is made with those of other theoretical investigations and experimental findings. The present calculated results are found to be in good agreement with the available experimental findings.     

质子轰击钨靶表面时的电子发射产额与靶温度的相关性研究

在中国科学院近代物理研究所兰州重离子加速器国家实验室测量了能量为50~250 keV 的质子入射不同温度下钨靶表面的电子发射产额。实验结果发现,不同能量的质子引起的电子发射产额均随着靶温度的升高而降低;利用功函数对温度的依赖性定性地解释了该结果。在不同靶温度下,总电子发射产额与电子能损的比值随着质子能量的增加而逐渐变小;利用靶原子不同壳层中电子之间的电离竞争机制来解释实验结果。     

Proton-proton scattering from 0.35 to 1.0 MeV

Differential cross sections for the scattering of protons by protons have been measured at energies from 0.35 to 1.0 MeV over an angular range from 24° to 110° in the c.m. system with an absolute accuracy of 0.2 %. Special care was taken at energies around the interference minimum where, due to the extremely small values of the cross section, the accuracy has been 0.8 % at θ_{c.m. = 90°}. The resulting value for the minimum energy is E_IM = 382.48 ± 0.10 keV. The high accuracy could be achieved mainly by using a new double compensated calorimeter for the current integration.     

Ag－与He原子碰撞的单电子脱附截面的实验研究

 对Ag^-与He原子碰撞的单电子脱附过程进行了实验研究,使用增长率方法在5~10keV能量范围内对脱附截面进行了测量,得到在20keV时的典型值为6.6×10^-6 cm²。实验结果的不确定度约为8%。     

Das elektronische Bremsvermögen von Stickstoff und Sauerstoff für niederenergetische Protonen

The electronic stopping power of molecular oxygen and nitrogen for protons with energies between 1 keV and 30 keV has been measured using a differentially pumped stopping cell. Our results give a surprisingly good confirmation of the Lindhard-Scharff statistical theory which predicts a linear velocity dependence of the electronic stopping power at low projectile energies. Moreover our data are in fair agreement with earlier high energy (E ≧ 20 keV) measurements in other laboratories. The combination of the present measurements with theoretically calculated nuclear stopping powers yields an estimate of the atomic stopping power. This estimate leads to substantially lower atomic stopping powers at low energies compared to values derived from range measurements. This result may have interesting implications on auroral hydrogen emissions.     

光子与相对论麦克斯韦分布电子散射截面的蒙特卡罗计算方法

高温全电离等离子体的辐射输运问题中,光子与电子的Compton散射与逆Compton散射是其中重要的特性,光子与相对论麦克斯韦电子散射的描述及截面的计算非常复杂且费时.本文提出了一种用于模拟计算光子与相对论麦克斯韦速度分布电子散射截面的蒙特卡罗计算方法.给出了各步骤的具体实现办法,推导了对应的计算公式,研究了相对论电子速率抽样方法,编写了光子与相对论电子散射的微观截面的蒙特卡罗计算程序.开展了高温全电离等离子体中,不同能量光子与不同温度电子散射的微观散射截面计算和分析.模拟计算结果显示,在电子温度低于25 keV情况下,本文方法与多重数值积分方法的计算结果非常接近;但随着电子温度继续升高,二者差异逐渐增大并较明显,经分析,可能是本文方法目前的电子速率抽样偏差所致,希望将来能够找到更好的相对论电子速率抽样方法以克服此缺陷.     

Energy dependence of site‐specific radiation damage in protein crystals

It is important to consider radiation damage to crystals caused by data collection when solving structures and critical when determining protein function, which can often depend on very subtle structural characteristics. In this study the rate of damage to specific sites in protein crystals cooled at 100 K is found to depend on the energy of the incident X‐ray beam. Several lysozyme crystals were each subjected to 3–26 MGy of cumulative X‐ray exposure by collecting multiple data sets from each crystal at either 9 keV or 14 keV. The integrated electron density surrounding each S atom in the structure was calculated for each data set and the change in electron density was evaluated as a function of dose at the two energies. The rate of electron density decrease per cubic Å per MGy was determined to be greater at 14 keV than at 9 keV for cysteine sulfurs involved in disulphide bridges; no statistically significant differences in the decay rates were found for methionine sulfurs. These preliminary results imply that it might be possible to minimize certain types of specific radiation damage by an appropriate choice of energy. Further experiments studying a variety of photolabile sites over a wider range of energies are needed to confirm this conclusion.     

Electron emission induced by keV protons from tungsten surface at different temperatures

The electron emission yield is measured from the tungsten surface bombarded by the protons in an energy range of 50 keV-250 keV at different temperatures. In our experimental results, the total electron emission yield, which contains mainly the kinetic electron emission yield, has a very similar change trend to the electronic stopping power. At the same time, it is found that the ratio of total electron emission yield to electronic stopping power becomes smaller as the incident ion energy increases. The experimental result is explained by the ionization competition mechanism between electrons in different shells of the target atom. The explanation is verified by the opposite trends to the incident energy between the ionization cross section of M and outer shells.     

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract

Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.     

Energy loss straggling measurements for helium ions in Mylar and polypropylene polymeric foils

In the present contribution, we report results on energy straggling of He ions penetrating Mylar and polypropylene thin polymeric foils. The measurements were performed in the 900–3000 keV incident particle energy range by using the indirect transmission technique developed previously. The experimental straggling data are corrected to consider the roughness effects due to target thickness inhomogeneity. As expected, the roughness contribution to straggling is more important for helium than for proton ions and decreases as the ion energy increases. At low velocities, (<500 keV/amu), the variation of the experimental energy straggling results differs strongly from predictions based on Bohr’s formalism, and with increasing energies, the experimental results approach gradually the Bohr values.     

Absolute efficiency calibration of high-purity germanium detector in the range 120–8500 keV

This article reports the efficiency response curve of the high-purity germanium detector over the wide energy range, covering from 120 to 8500 keV. The efficiencies were measured for different counting geometries by using point radionuclide standards (mono-energetic as well as multi-gamma emitters) supplied by IAEA and the capture gamma-ray facility installed at PINSTECH nuclear reactor PARR-1. The measured efficiencies were required to fit with a suitable fitting function for interpolation within the energy range of interest. Several fitting functions were proposed in the literature covering different energy ranges. The functions giving the best fit to experimental data are presented. The work has successfully extended the response curve beyond 1500–8500 keV, which is the region where the standard calibration radionuclides are not available. The thermal neutron capture gamma-ray facility provided the collimated neutron beam, extracted from the core of the reactor and made to react with ammonium chloride target to produce the capture gamma rays for determining the efficiencies in the extended region. It was found that the capture gamma-ray provides a satisfactory solution to extend the absolute efficiency calibration in the MeV range. It was also found that the fitting function that is linear in its parameter was highly satisfactory up to 1500 keV but proved insufficient upto 8500 keV. The exponential function giving the good fit over the range has been presented. Good agreement has been found between the experimentally measured absolute efficiencies and the predicted result.