Gafchromic® EBT films for ion dosimetry

For dose delivery to patients, scanning ion beams are going to be increasingly used in the upcoming ion beam therapy facilities. Especially carbon ion beams are able to produce steep dose gradients. However, the currently used method for patient dose verification, employing ionization chamber arrays, provides a spatial resolution of 1 cm only. As continuous media, EBT films, widely used in photon therapy, are interesting candidates to be used for this purpose. The EBT film is the ancestor of the currently available EBT2 film. In our contribution two dimensional dosimetry and film response quenching in ion beams were investigated. For a real ¹²C patient plan a good qualitative agreement with the planned dose distribution including a high signal-to-noise ratio and a good resolution in the measured photon-equivalent dose was found. The depth-dose response of EBT films for a ¹²C ion beam shows response quenching, which rises towards the Bragg peak. It was quantified by the relative efficiency determined at different depths. Furthermore, the relative efficiency was measured in monoenergetic proton and carbon ion beams. All the measured efficiencies show no significant dependency on the dose up to the highest measured doses of 6 Gy. However, differences between proton and carbon ions as well as between carbon ion beams of different energies were observed. The measurements reveal, that the use of EBT films for absolute dose verification measurements requires to take the relative efficiency into account, dependent on the ion type and energy.     

气体放电空心阴极鞘层氩离子的蒙特-卡罗模拟研究

建立了气体放电空心圆筒阴极鞘层离子的自洽蒙特-卡罗模拟模型,对鞘层区内离子的输运过程进行了研究。考虑了离子与中性原子的电荷交换碰撞和弹性散射,用到了精确依赖于离子能量的电荷交换和动量输运截面。模拟了氩离子在空心阴极鞘层中的运动,得到了不同放电条件下自洽电场分布,离子的能量分布,角分布以及电子密度分布和离子密度分布。计算结果表明:离子在由鞘层边界向阴极运动过程中,离子能量分布的高能部分逐渐增大,角分布向小角度部分压缩,鞘层中的强电场对离子起加速和聚焦作用;在鞘层内离子密度分布比较均匀,只是在鞘层边界附近变化 关键词：     

Gafchromic HD-V2 investigations using MeV ion beams in vacuum

ABSTRACT

Gafchromic HD-V2 films are employed to study the response to light ions useful in radiotherapy, such as protons, helium and carbon beams. The effects induced by the ions at an energy within about 300 keV and 16?MeV were investigated in terms of optical absorbance measurements in the irradiated active region of the film. The employed ion doses range between 0.4 Gy and 12?kGy. The results show that the net optical density increases almost exponentially with the absorbed dose and that it becomes saturated after prolonged dose higher than 1?kGy. The optical density produced in the film was measured by light transmission measurements at 700?nm wavelength, at which is observed the highest gafchromic sensitivity. Calibration curves of optical density versus exposure dose and type of ion are given. The dependence on the ion stopping power was presented and discussed.     

Optimizing the Parameters of Zr Alloy Doping with Al,Fe, and Mo via the Irradiation of Respective Films on Zr Surfaces Using Ar Ions

The effect the sputtering of a film–substrate system has on the modification of near-surface layers of zirconium alloy E110 (Zr–1% Nb) in the ionic mixing regime upon irradiation by a beam of argon ions with a wide energy spectrum is considered. It is shown that to increase the atoms’ efficiency of penetration and achieve the optimum doping conditions via ionic mixing upon irradiation by a beam of ions with a broad energy spectrum, the ion energy in the beam must be increased while simultaneously lowering the reduced atomic beam mass by using a combined ion beam of different masses.     

The influence of argon ion bombardment on the electrical and optical properties of clean silicon surfaces

The effect of low energy noble gas ion bombardment on the electrical and optical properties of Si(211) surfaces has been investigated by surface conductivity and field effect measurements, ellipsometry and AES. With this combination of techniques, information is obtained concerning the electrical properties, the chemical composition and the damage of the surface layer. Upon ion bombardment in the energy range of 500–2000 eV, ellipsometry shows the formation of a damaged surface layer with optical properties close to those of an evaporated amorphous silicon film. In order to measure the conductivity changes as sensitive as possible, nearly intrinsic silicon crystals were used. For the clean, 5200 Ω cm Si(211) surface, bombarded only with a mass-analyzed argon ion beam, a small increase in conductivity is found to occur after a small ion dose (saturation after 5 × 10¹⁴ ions cm^?2 while after 5 × 10¹³ ions cm^?2 already half of the increase has occurred). The effect was found to be independent of ion energy between 500 and 2000 eV. As the field effect signal did not change after this treatment, it is concluded that the surface state density in the neighbourhood of the Fermi level shows a slight decrease.     

Modelling of the Positive Column in an Argon-Hexamethyl-disiloxane dc Glow Discharge

A model is presented of the positive column of a dc glow discharge in argon with small admixtures of hexamethyldisiloxane (HMDS). The axial electric field, the ion production rates for direct-, stepwise-, pair-, and Penning ionization, the densities of metastable Ar atoms and of electrons, and the wall current of HMDS ions are calculated in dependence on HMDS admixture and discharge current density. For the calculations particle balance equations were used for a diffusion determined plasma in a mixture of two gaseous components. The reaction rates for the electron collision processes were determined applying the electron distribution function calculated for pure argon. Taking into account PENNING ionization of HMDS molecules by metastable argon atoms the decrease of electric field for increasing HMDS admixtures is according to the experimen-tally measured values. Also ion wall currents and electron densities are compared with experimen-tal values for thin film formation rate and results of probe measurements.     

The antiproton cell experiment—do antiprotons offer advantages over other particle beam modalities?

The use of heavy charged particles for cancer therapy has the potential for a significant improvement of the therapeutic window compared to standard X-ray treatments. This is due to the improved energy deposition profile, exhibiting a well-defined peak at a depth in target controllable by the initial energy of the beam. Particles heavier than protons in addition show an increase in biological effectiveness. Compared to protons or heavy ions, antiprotons deposit additional annihilation energy, mostly by low energy recoils, resulting in an increase of dose and also adding a component with high biological effectiveness in the target region. The relative magnitude of the physical energy deposition of antiprotons compared to protons was measured at Low Energy Antiproton Ring (LEAR) by A. Sullivan, but no study of the biological effect had been conducted prior to the Antiproton Cell Experiment (AD-4/ACE) experiment at CERN. The special conditions found at CERN present significant challenges, but also offer unique opportunities. 500?ns pulses of antiprotons are extracted from the Antiproton Decelerator (AD) at 500?MeV/c momentum. Biological cell samples are irradiated and clonogenic survival fractions are measured for various doses. To extract biological efficiency, the physical dose deposition is obtained by Monte-Carlo calculations in conjunction with shot-by-shot monitoring of the incoming beam intensity and profile using a silicon pixel detector. Also imaging of the pions resulting from antiproton annihilations in the target using silicon pixel detector technology to determine the actual range in more complex targets with strong variations in material densities was carried out. The feasibility of this technique using a novel arrangement of the detector was demonstrated. This paper describes the ACE experiment and focuses on the different detector activities within the AD-4/ACE collaboration, explaining the experimental set-up, physical and biological methods used, recent results, and future plans.     

短管螺旋波放电中等离子体参数测量和模式转化研究

对长度为45 cm的短放电管螺旋波放电等离子体进行了Langmuir探针、原子发射光谱以及集成电荷耦合检测器(ICCD)检测诊断,研究螺旋波等离子体的放电特性.Langmuir探针数据显示电子密度在射频功率增加过程中出现两次大幅增长,由此确认了放电模式的转换及螺旋波放电模式的出现.发射光谱测量结果与Langmuir探针测量的电子密度数据一致,发现Ar原子和Ar离子的谱线强度与放电模式变化有着密切相关性.而通过对不同放电模式的ICCD测量,获得射频功率吸收因放电模式转变而变化的方式,认为放电模式转换时电子行为和能量传递方式也发生着变化.     

用自洽的蒙特卡罗—流体结合模型模拟溅射过程

 采用自洽的蒙特卡罗－流体结合模型对溅射过程进行模拟,以了解等离子体粒子行为与溅射参数的关系。溅射过程包括气体放电和溅射原子传输。对于气体放电,蒙特卡罗部分模拟快电子和快气体原子,而流体部分则描述离子和慢电子。对于溅射原子传输,蒙特卡罗部分模拟溅射原子的碰撞过程,而流体部分则描述溅射原子的扩散和漂移。模拟的结果包括：等离子体粒子的密度和能量分布;不同电离机制对气体原子和溅射原子电离的贡献;不同等离子体粒子对阴极溅射碰撞的贡献;溅射原子的密度分布;溅射场和溅射粒子相对于入射离子能量和角度的分布;溅射原子经碰撞后在整个等离子体区的分布。     

High-intensity femtosecond laser absorption by rare-gas clusters

The energy absorption efficiency of high-intensity (～10^{16}W/cm^2) femtosecond laser pulses in a dense jet of large rare-gas clusters has been measured. Experimental results show that the energy absorption efficiency is strongly dependent on the cluster size and can be higher than 90%. The measurement of the ion energy indicates that the average ion energies of argon and xenon can be as high as 90 and 100keV, respectively. The dependence of the average energy of the ions on the cluster size is also measured. At comparatively low gas backing pressure, the average ion energies of argon and xenon increase with increasing gas backing pressure. The average ion energy of argon becomes saturated gradually with further increase of the gas backing pressure. For xenon, the average ion energy drops a little after the gas backing pressure exceeds 9 bar (3.2×10^5 atoms/cluster). The result showing the existence of a maximum average ion energy has been interpreted within the framework of the microplasma sphere model.     

气体放电阴极鞘层氩离子的自洽蒙特卡罗模拟

建立了气体放电阴极鞘层离子的自洽蒙特卡罗模拟模型,考虑了离子与中性原子的电荷交换碰撞和弹性散射,用到了精确依赖于离子能量的电荷交换和动量输运截面。模拟了氩离子在阴极鞘层中的运动,得到了不同气压下自治电场分布,离子的能量分布和角分布,发现:离子由鞘层边界向阴极运动过程中,离子能量分布的高能部分逐渐增大,角分布向小角度部分压缩,鞘层中的强电场加速和聚焦了离子;在鞘层边界附近的电场呈非线性。     

Resistance changes in thin metallic films under ion bombardment

Resistance changes in thin films of copper, aluminium and bismuth have been studied under the bombardment of nitrogen, carbon and argon ions. Variations in resistance with implantation dose have been observed upto doses of ∼ 3 × 10¹⁷ ions/cm² for ion energies in the range 40 to 120 keV. The results are discussed in terms of desorption of gases from the film and a composite action of sputter removal of the film and its structural changes upon ion bombardment. A simple theoretical model is discussed which can qualitatively explain the experimental observations.     

强流离子源光腔衰荡光谱应用研究

强流离子源是托卡马克中性束注入器的核心部件,为了满足未来对高能量离子束中性化效率的要求,负离子源成为中性束注入系统的首选。光腔衰荡光谱(cavity ring-down spectroscopy,CRDS)是一种超高灵敏探测吸收光谱技术。在强流负离子源中,利用氢负离子的光致剥离过程,CRDS可以用来测量氢负离子的绝对积分密度。与激光光致剥离法与光学发射光谱法相比,CRDS具有不受电磁干扰、不依赖等离子体参数、测量精度高等优点。强流离子源负离子密度测量所用CRDS系统由激光器、光学谐振腔、光电探测器和数据采集系统四部分组成。本文根据CRDS测量氢负离子密度的原理,详细推导了氢负离子密度的计算方法,给出了氢负离子密度测算表达式;然后,结合强流离子源实验室应用的具体情况,分析了各部分装置的选择原则与注意事项;最后,介绍了CRDS技术在德国马克斯-普朗克等离子体物理研究所、日本国立聚变科学研究所、意大利Consorzio RFX研究所强流负离子源研究中的应用情况。实验结果表明,源腔气压、源功率等源参数会影响氢负离子密度;铯的注入可以将氢负离子密度从1016 m-3量级提高到1017 m-3量级;同时,日本NIFS的实验结果证明氢负离子密度与引出电流呈线性关系。     

Influence of ion bombardment on residual stresses in diamond-like carbon films

The dependence of internal residual stresses in thin diamond-like carbon films grown by the PECVD technique on the most important growth parameters such as the power of the exciting RF discharge and the substrate bias potential is considered. The results have shown that the mechanical stresses in films reach the uppermost value of 1.9 GPa at the smallest values of power and potential. The stress decreases with the growth of both parameters and has only a slight dependence on the film thickness in the range 0.1–1 μm. The bombardment of the obtained films by argon ions with energy of 300 keV and phosphorus ions with energy of 200 keV has resulted in the reduction of compressive stress with the ion dose growth down to its inversion. AFM study of the bombarded films has revealed significant changes in their surface morphology.     

The Calculations of the Concentrations,of the Radial Distributions,and of the Radial Particle Currents of Excited and of Doubly Charged Ions in Low Pressure Discharges

A system of differential equations describing the radial profiles of the number densities and of the radial drift velocities of the ions and of the electrons in positive columns at low pressure containing several species of ions is derived. Excited ions and doubly charged ions, generated in two-step processes by electron impacts, the inertia of the ions and space charge effects are taken into account. For the excited ions de-excitation processes by electron collisions and by spontaneous emission are regarded. A set of nonlinear equations to determine the population densities and the initial values of the differential equations and corresponding boundary conditions are put up. Numerical solutions are given for discharges in argon under free-fall conditions similiar to argon ion lasers. One notices that without stepwise processes via excited ion levels the concentration of double charged ions remains small. In some cases the radial drift of the ions considerably reduces the population of the metastable ion levels. The radial density profiles of the double charged ions and of long-living excited ions considerably deviate from the squared radial profile of the electron density. In addition, for low degrees of ionization the theory of the free-fall column given by Tonks and Langmuir is extended to plasmas containing two species of ions.     

Linearity studies of HD-810 dosimeters by light ion beams

Radiochromic films (RCF), also called GafChromic? films, represent a performant material for accurate quantitative radiation dosimetry. Their compositions allow high dose sensitivity and fewer environmental dependence, giving a good feedback to the absorbed dose value and to the active media absorption, turning color upon being irradiated. The RCF take into account their reduced response near the Bragg peak due to a high linear energy transfer (LET). HD-810 GafChromic? films are tissue-equivalent, have easy optical readings and can be employed for ion dosimetry in radio diagnostic and therapy and for industrial applications. Such dosimeters were employed at Tandetron-Nuclear Physics Institute (?e?, Czech Republic) to study the responses of helium, proton and carbon beams, commonly employed in radiotherapy and microelectronics. The sensitivity of the detector is low enough to measure multiple-beam exposures. The induced effects by the ions in the energy range of 600?keV to 2.0?MeV were investigated in terms of optical absorbance, measured in the irradiated active region of the polymer. The employed ion dose range was between 40?Gy and 2.5?kGy. The experimental results show that the absorbance increases with the irradiation time (i.e. with the absorbed dose). The absorbance induced in the radio chromic film was measured at 673?nm, at which is observed the highest sensitivity of the films. Such data, together with the dose linearity and the dependence on the ion stopping power will be presented and discussed.     

Investigation of the initial stages of defect formationin carbon nanotubes under irradiation with argon ions

The formation of defects in carbon nanotubes under irradiation with argon ions is investigated. The π plasmons generated in single-walled and multiwalled carbon nanotubes are examined using electron energy-loss spectroscopy. In the course of experiments, the supramolecular structure of nanotubes is stepwise modified by an argon ion beam (the maximum irradiation dose is 360 μC/cm²). The content of argon ions implanted into a nanotube structure is controlled using Auger electron spectroscopy. The effect of ion irradiation on the π-plasmon energy E_π and on the half-width at half-maximum δE of the π-plasmon spectrum is determined experimentally. An expression relating the above quantities and the concentration of implanted argon is derived. It is shown that the formation of defects under ion irradiation is a discontinuous process occurring in a stepwise manner. A qualitative phenomenological interpretation is proposed for the experimentally revealed decrease in the π-plasmon energy E_π and for its attendant broadening of the π-plasmon spectrum. The assumption is made that the microscopic mechanism of the observed phenomena is associated with the narrowing of the energy π subbands in the electric field of charged defects generated by ions.     

利用CR39上的径迹鉴别激光加速离子产物

分别利用单能的质子束及碳离子束辐照CR39,并制定了规范的刻蚀条件及流程。通过对处理完成后的数据进行详细处理,得到了质子及碳离子的径迹直径-能量的响应曲线,可用于确定CR39上质子或碳离子的能量。以此为依据,得到了鉴别CR39上质子及碳离子的有效方法。在激光加速离子的实验中,通过测量CR39上的径迹大小及相对灰度,利用本文给出的标定数据,确认了质子径迹,得到了实验的质子能谱。     

Computer simulation of the two-stage ion polishing of a silicon surface

Comparative results of the computer simulation of surface sputtering by a focused ion beam with Gaussian density distribution and a uniform ion flux are presented. The experimental angular dependences of the yield of silicon sputtering by nitrogen and argon ions are used. The possibility of the optimization of ion polishing via methods of computer simulation is demonstrated.     

用离子束增强沉积从V2O5粉末制备高热电阻温度系数VO_2薄膜

采用了一种用离子束增强沉积从V2O5粉末直接制备VO2薄膜的新方法,将纯度为997%的V2O5粉末压成溅射靶,在用Ar离子束溅射的同时,用氩氢混合束对沉积膜作高剂量离子注入,使沉积膜中V2O5的V—O键断裂,进而被注入的氢还原,退火后获得热电阻温度系数(TCR)高达4%的VO2薄膜.高剂量的氩氢混合束注入对薄膜引入应力,使薄膜的转换温度降低、电阻温度曲线斜率变大,是薄膜TCR增大的原因 关键词： 离子束增强沉积 VO2薄膜 热电阻温度系数