Radiocarbon dating of microgram samples: Accelerator mass spectrometry and electromagnetic isotope separation

An exploratory experiment was performed to investigate methods for extending the direct atom counting technique to natural radiocarbon samples containing only micrograms of carbon. A threefold approach was adopted for the study: direct measurement of a microgram-size sample, in the form of elemental carbon; dilution of a few micrograms of an environmental (atmospheric particulate) reference sample with inert (“dead”) carbon; and implantation of carbon ions (as CO⁺) into copper foil prior to placement in the tandem accelerator ion source. The last experiment, which linked work with the NBS variable-geometry electromagnetic isotope separator and the NSRL tandem Van de Graaff, automatically led to isotopic enrichment during the implantation step. All three phases of the experiment were unique (not previously attempted) and gave consistent results, indicating great promise for the future application of direct atom counting to individual trace chemical species of environmental or archaeological importance. The directly measured sample of elemental carbon was prepared from the new ¹⁴C dating standard, and it yielded a result having an imprecision of 3% where ions were counted for about 15 min using less than 40 μg carbon in the tandem ion source.     

Inelastic processes in ion scattering spectroscopy of solid surfaces

In ion scattering spectroscopy of solid surfaces, spectral peaks corresponding to single scattering do not always appear at a kinetic energy given by a well-known formula that is calculated for the elastic collision of a projectile ion with a free target atom. This is because (a) the target atom is actually not free but bound to surrounding atoms at the surfaces, (b) the target atom may be excited electronically, and (c) inelastic electron exchange between the projectile ion and the surface may occur. These three inelastic effects are discussed on the basis of experimental results in the low energy region. Auger and resonance electron exchange processes between the projectile ion (or atom) and the surface are also discussed.     

Long range implantation by MEVVA metal ion source

Metal vapor vacuum are(MEVVA)source ion implantation is a new technology used for achieving long range ion impantation.It is very improtant for research and application of the ion beam modification of materials.The results show that the implanted atom diffusion coefficient increases in Mo implanted Al with high ion flux and high dose.The implanted depth is 3-11.6 times greater than that of the corresponding ion range.The ion species.doses and ion fluxes play an importnat part in the long-range implantation.Especially,thermal atom chemistry have specific effect on the long-range implatation during high ion flux implantation at transient high target temperature.     

New method of evaluation for interatomic interaction potential in LEIS with large-angle scattering using the two-atom scattering model

The interaction potential between an incident ion and a target atom in impact-collision ion scattering spectroscopy (ICISS), which is a specialization of low energy ion scattering (LEIS) and its variants, i.e. ICISS with detection of neutrals (NICISS), coaxial ICISS (CAICISS) and impact-collision atom scattering spectroscopy with detection of neutrals (NICASS), has been evaluated by the new method using the dependence of the total scattering angle on the impact parameter for the first collision in the numerical calculations based on the two-atom scattering model (TWASM). From the comparison of determined values of scaling factor for the Firsov screening length by three-dimensional computer simulations with calculated ones by TWASM, it became obviously that the interatomic potentials for the various combinations of an incident ion and a target atom in LEIS are suitably given by the Moliere potential with the reduced Firsov screening length employing the scaling factor obtained in TWASM calculations.     

N~+和Bi~+离子注入InP反冲原子P和In深度分布偏离的研究

本文给出了用背散射和沟道技术所分析的N~+和Bi~+注入InP中In和P原子位移的面密度分布。发现移位原子In和P面密度分布峰的深度随注入剂量的增大而加深。在相同的剂量下磷原子分布峰与表面的距离比In原子峰的大。从移位原子面密度分布峰的移动和分布的展宽与注入剂量的关系出发,研究了离子注入过程中所存在的反冲效应。并讨论了移位原子的反冲机理。     

A method for very low fluence implantations using Rutherford scattering

A simple method for extremely low fluence ion implantation is described. It is based on implanting ions which are Rutherford backscattered (RBS) from a thin gold layer into the desired target. This method enables ion implantations to be carried out, as are needed to realize quantum centers on the atomic scale to serve as qubits and other nano sized devices. The required implantation fluences are orders of magnitude below the commonly-used current integration capabilities; hence control on the implanted fluence is usually complicated. The described method enables control on the implanted fluence even when extremely low. The dependence of the energy and fluence of scattered ions on the angle and scattering target thickness is analyzed by using SRIM simulations. These are verified for the case of N scattering implantation by direct counting in a surface barrier detector and for the case of Xe by counting the tracks that scattered and implanted Xe ions leaved in HOPG as viewed by scanning probe microscopy.     

Investigation of metal matrix systems for cosmogenic Al analysis by accelerator mass spectrometry

We report experiments designed to help optimize accelerator mass spectrometry (AMS) of ²⁶Al (in the form of Al₂O₃) for geochronologic and geomorphologic applications. Analysis times are long and the precision of AMS are restricted by counting statistics for ²⁶Al, which are in turn limited by the intensity of Al⁻ beam currents. We show that ion beam currents are affected by the metal matrix in which Al₂O₃ is dispersed, by the matrix-to-Al₂O₃ mixing ratio, and for at least some matrices, such as Ag, by the depth to which the sample is packed in the AMS cathode. Typical instantaneous Al⁺⁷ currents (μA) produced by the LLNL CAMS Cs sputter ion source and measured in a Faraday cup after the accelerator are 2.26 for samples in Ag, 2.17 in Re, 2.00 in Nb, 1.92 in V and 1.73 in Mo. The AMS counting efficiency (Al⁻ ions detected per Al atom loaded in the target) for a constant analysis time (900 s) and for equimolar mixtures of Al₂O₃ and matrix is in the range of 6 × 10⁻⁵–9 × 10⁻⁵ in the order Ag > Re > Nb > V > Mo. Additionally, we observed a correlation between the ion detection efficiency (Al ions detected per Al atoms loaded) and the matrix work function and inverse vaporization enthalpy of the matrix and beam current. Typical currents (μA) obtained with elemental Al are 13.3 for samples in no matrix, 3.23 in V, 3.14 in Nb, 3.07 in Re, 2.85 in Mo, 1.46 in Ag. The ion detection efficiency for elemental Al correlates strongly with matrix electron affinity. Thus, our data indicate that the current practice of mixing Al₂O₃ with Ag is reasonable until a means is found to produce cathodes of elemental Al.     

Post collision stark-interaction after excitation of fast ions by a solid surface at grazing incidence

The anisotropic distribution of the angular momenta of atoms or ions excited by the ion beam surface interaction at grazing incidence is modified by post collision Stark-interaction due to the electric field at the surface. By studying the effect of the Stark-mixing on the anisotropy in dependence of the time the atom spent in the electric field conclusions can be drawn with respect to the nature of the field and the excitation process.     

Experimental study on cold fusion using deuterium gas and deuterium ion beam with palladium

A series of experiments using deuterium gas and low energy deuteron beam with palladium has been designed at Mississippi State University to allow for the observation, if it exists, of cold fusion. Three experiments were performed. One involved the diffusion transient of deuterium gas into palladium. The gas was cooled by liquid nitrogen, and its temperature was permitted to rise to room temperature, changing from near –34°C to 19°C in 75 minutes. A spherical lithium neutron detector, 21 cm from the palladium, gave an audible indication of neutron levels approximately twice the background. A second experiment used a deuterium ion beam (1 kev) which bombarded a palladium target. An average counting rate of 36±6 counts for 2 minutes was measured by a BF₃ tube with a paraffin moderator, 50 cm from the target. The background varied from 1–7 counts for each 2 minutes of counting period and averaged 4±2 counts in 2 minutes. A nitrogen ion beam impinging on the same palladium target produced 6 counts for a 2-minute counting period. A third experiment used a hydrogen ion beam first, then a nitrogen ion beam, finally a deuterium ion beam to bombard the same palladium target. These ion beams had energy less than 1 kev, and created neutron counts in the range of background. The palladium specimens were a piece of foil and a tube which used to be the palladium leak in a neutron generator. These preliminary experiments will be repeated, improved, and extended later.     

涂硼正比计数管涂硼厚度对探测效率影响的研究

涂硼正比计数管是可用于反应堆压力容器外对中子注量率进行监测的关键设备,涂硼正比计数管的涂硼厚度对自身的本征探测效率有影响。仿真不同厚度硼层中¹⁰B(n,α)⁷Li生成的⁷Li和α粒子的输运过程,计算硼层界面位置离子整体射出率并给出涂硼正比计数管本征探测效率的计算方法。仿真与计算结果表明：单位核反应率下硼层界面处离子整体射出率在涂硼厚度小于1.5μm时与硼层厚度近似呈正线性相关,在涂硼厚度大于1.5μm后随涂硼厚度增大而增速变缓,在涂硼厚度为3.6μm时达最大为1.3×10^-4 (cm²·s)^-1。基于离子整体射出率的结果进一步计算得到涂硼正比计数管本征探测效率与涂硼厚度之间关系曲线,该关系曲线可以为涂硼正比计数管研制中选择合适涂硼厚度、确定最佳的探测效率提供参考。     

高分辨快电子能量损失谱仪的改进

新研制的成功的电子能量损失符合谱仪是由电子能量损失谱仪、正离子飞行时间质谱仪、负离子习行时间质谱仪及相应的电子学符合测量系统构成。该系统可以同时获得电子与原子分子碰撞后散射电子、电离或解离正离子、解离负离子或电离电子的信息。利用该装置测量了一氧化碳分子的光电离效率曲线及其离子碎片的部分光学振子强度密度。     

Molecular dynamics simulations of low-energy argon ion sputtering of copper clusters on polyethylene surfaces

The normal bombardment of the targets consisted of single 13-, 27- or 39-atom copper cluster on a surface of polyethylene by Ar ions with energies of 100, 200 and 400 eV is examined using molecular dynamics simulation incorporating long-range many-body covalent bonding potential for hydrocarbons and a potential based on a embedded atom model for copper. Sputtering yield and its dependence on the energy of bombarding ion and size of the pre-deposited copper cluster are discussed.     

用于溅射负离子源的透射表面电离器的研制

介绍了１种应用于表面电离型溅负离子源的球面形透射表面的电离器，并阐述了原理。这种电离器可使铯蒸气直接通过，避免了铯蒸气绕射造成电离表面铯原子通量低的缺点，增大了铯离子的产额也使离子源的流强较采用非透射型电离器时提高了５０－８７％。‘     

Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide’s measurements of the Cu⁻ population sputtered from a clean Cu-surface, at emission energies below 100 eV.     

Liquid secondary ion mass spectrometry: I. Molecular ion intensities as a function of primary ion pulse frequency

A liquid matrix and high primary ion currents are employed on a time-of-flight mass spectrometer to produce molecular ions in a manner similar to that employed for fast atom bombardment (FAB) techniques on sector instruments. The primary ion beam is pulsed and the molecular yield is found to depend upon sample concentration, instantaneous primary ion current, and pulse repetition rate. The latter, in particular, indicates that a finite recovery time is required to repair radiation damage from high flux particle beams.     

Electronic stopping power of atoms for ions

Within the first-order Born approximation the impact parameter dependent energy transfer and random stopping power for ions accompanied by electrons were determined. The ion and atom were described by many electron ground states. The excitation and ionization of both ion and atom were taken into account. With the Bethe sum rule and closure relation, the random stopping was shown to have the Bethe form. For the Molière form factors, the analytical results were obtained. The effective charge was discussed in the random and channeling conditions. Comparison with previous calculations was carried out.     

Study of axial double layer in helicon plasma by optical emission spectroscopy and simple probe

In this work we used a passive measurement method based on a high-impedance electrostatic probe and an optical emission spectroscope (OES) to investigate the characteristics of the double layer (DL) in an argon helicon plasma.The DL can be confirmed by a rapid change in the plasma potential along the axis.The axial potential variation of the passive measurement shows that the DL forms near a region of strong magnetic field gradient when the plasma is operated in wavecoupled mode,and the DL strength increases at higher powers in this experiment.The emission intensity of the argon atom line,which is strongly dependent on the metastable atom concentration,shows a similar spatial distribution to the plasma potential along the axis.The emission intensity of the argon atom line and the argon ion line in the DL suggests the existence of an energetic electron population upstream of the DL.The electron density upstream is much higher than that downstream,which is mainly caused by these energetic electrons.     

Homogeneity study of an Fe impurity in unalloyed Zn using PIXE

We have studied the distribution of an Fe impurity in unalloyed Zn material along line scans in spots of 1 mm diameter each using PIXE. In order to achieve the necessary precision (1%) in the determination of this low Fe content (about 100 mg/kg), a sufficient counting rate of Fe X-rays is needed without overloading the counting electronics with Zn X-rays at the same time. Therefore, the development of our PIXE analysis system had to take into account: (a) optimising the thickness of a Ni absorber foil in order to selectively reduce the counting rate of Zn without adding an unproportionally high Ni counting rate, (b) implementing a triggered ion beam deflection system capable of handling very high counting rates (up to 8 kcps), (c) developing the spectrum evaluation code to take account of the selective absorption by the Ni foil. The results of our PIXE measurements showed an inhomogeneity in Fe of up to 10%, considered too high to accept this zinc material as a new reference material.     

Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.     

Progress on Photomultipliers and Image Intensifiers at EMI Electronics Ltd.

This paper reviews recent work at E.M.I. on photomultiplier tubes. Dark current measurements by various techniques are presented, including data on tubes with high quantum efficiency and minimum thermionic emission suitable for low energy scintillation counting. Mention is made of a new fast tube, a small tube, and modified ion detectors. Two four-stage image intensifiers are described.