Optimum thickness of carbon stripper foils in tandem accelerator in view of transmission and lifetime

Optimum thickness of charge stripper foils installed at the terminal of a tandem accelerator has been investigated from the view of (1) charge stripping effect, (2) transmission of ions through accelerator, (3) lifetime of foils for the irradiation of ions. For this purpose, measurements have been done for (a) transmission of H, Li, O, Br and Au ions, passing through 12 UD Pelletron tandem accelerator for carbon stripper foils of 1.8–19.5 μg/cm² thickness, at terminal voltages of 5 and 10 MV, and (b) lifetime of 2–15 μg/cm² thick Tanashi foils developed by Sugai by irradiating Au ions at the terminal voltage of 10 MV. The results obtained are as follows: (a) From the view of above items (1) and (2), the optimum thickness of foils is 10 μg/cm² for ions of Z=1, several μg/cm² for Z=8, and less than a few μg/cm² for heavier ions. (b) From the view of item (3), the lifetime of Tanashi foils by means of new arc-discharge method is demonstrated to be much longer than that of commercial foils for foils thicker than about 5 μg/cm² thick. This superiority rapidly decreases with decreasing foil thickness, and at around 2 μg/cm², the lifetime of Tanashi foils is at the most 2.4 times longer than that of commercial foils.     

Mounting stripper foils on forks for maximum lifetime

While research and development continue to produce forms of carbon for longer lasting stripper foils, relatively little attention has been paid to other factors that affect their survival in use. It becomes apparent that the form of carbon is only part of the issue. Specific mounting methods increase the lifetimes of carbon stripper foils. These methods are determined in part by the specific use and carbon type for a foil. With careful handling, appropriate adhesive, and slack mounting, premature breakage can be avoided. Foil lifetimes are then primarily affected by less easily controlled factors such as high-temperature expansion, shrinkage and evaporation.     

Multilayer carbon foils for cyclotron beam extraction

The TRIUMF Applied Technology Group operates high-power industrial cyclotrons for commercial radioisotope production. Two of these cyclotrons, TR30-1 and TR30-2, are capable of accelerating H⁻ ions to an energy of 30 MeV and beam currents in excess of 1000 μA. For many years, amorphous carbon foils of approximately 2.0 μm thickness have been utilized to extract proton beams from these accelerators.Novel multilayer foils consisting of layers of amorphous and diamond-like carbon (DLC) of 2.0±0.2 μm thickness were manufactured in-house by carbon arc and pulsed laser deposition, respectively. In the TR30 cyclotrons, the new composite foils with 25% DLC content show a three times longer lifetime than the purely amorphous foils, while maintaining their excellent physical and mechanical characteristics during irradiation.     

Carbon stripper foils for the Australian National University heavy ion accelerator

It is easy to say “Just float and mount the carbon foils". However learning the art can be more difficult than old masters have shown it to be. In this article we will share our experience of some difficulties we have had at Australian National University (ANU). We also present results from some in beam endurance testing of foils using carbon supplied by Vacuum Technology (Germany), Micromatter (Canada) and those made at the ANU (Australia).     

Measurement of lifetimes of thin carbon stripper foils produced by ion-beam sputtering

Thin carbon stripper foils used in high-intensity proton accelerators and heavy-ion accelerators must have long lifetimes. Thin carbon foils were fabricated by ion-beam sputtering using reactive and inert gas ions. The lifetime of the foils was measured using a KEK 650-keV high-intensity DC H⁻ (negative hydrogen ion) beam; changes in the foil thickness and surface deformations during irradiation were investigated. The lifetime of a typical stripper foil fabricated by heavy-ion-beam (Ar and Kr) sputtering was 60-70 times longer than that of the best commercially available foils. This paper reports a fabrication method for carbon stripper foils, along with an investigation of their lifetimes and changes in foil thickness during beam irradiation.     

Stripping efficiency and lifetime of carbon foils

Charge-exchange injection by means of carbon foils is a widely used method in accelerators. This paper discusses two critical issues concerning the use of carbon foils: efficiency and lifetime. An energy scaling of stripping efficiency was suggested and compared with measurements. Several factors that determine the foil lifetime—energy deposition, heating, stress and buckling—were studied by using the simulation codes MARS and ANSYS.     

Lifetime improvement of slackened thin cluster carbon stripper foils by suppression of carbon buildup

We control the amount of carbon buildup on slackened thin cluster carbon stripper foils (less than 3.5 μg/cm²) by heating with a high-power infrared lamp during beam bombardment. Foil lifetime measurements were performed using 2.0±0.5 μA beams of 3.2 MeV Ne⁺ ions and quantified as the total charge/area before breakage. Lifetimes were obtained up to 1286 mC/cm² at maximum and 1139 mC/cm² on the average; these values are, respectively, approximately 51 times at maximum and 46 times on average greater than the best commercially available foils, when used unheated and unslackened.     

Comparison of carbon and corrugated diamond stripper foils under operational conditions at the Los Alamos PSR

To accumulate high-intensity proton pulses, the Los Alamos Proton Storage Ring (PSR) uses the charge-exchange injection method. H⁻ ions merge with already circulating protons in a bending magnet, and then are stripped off their two electrons in a carbon stripper foil. The circulating protons continue to interact with the foil. Despite efforts to minimize the number of these foil hits, like “painting” of the vertical phase space, they cannot totally be eliminated. As a result, foil heating and probably also radiation damage limit the lifetime of these foils. In recent years, LANL has collaborated with KEK to improve the carbon foils in use at PSR, and these foils now last typically for about 2 months. Recently, an alternative in the form of corrugated diamond foils has been proposed for use at SNS. These foils have now been tested in PSR production for a year, and have already shown to be at least as enduring as the LANL/KEK carbon foils. Advantages of the diamond foil concept, as well as some noteworthy differences that we observed with respect to the LANL carbon foils, will be discussed here.     

Mechanical properties and bonding structure of boron carbon nitride films synthesized by dual ion beam sputtering

The BCN films were synthesized on Si (110) wafers by using dual ion beam sputtering deposition from boron carbide target. The influences of ion assist source energy and N₂ relative flow rate on the surface roughness, mechanical properties and chemical bonding structure of BCN films were investigated systematically. The surface roughness was measured using non-contact optical surface profilometer and the mechanical properties of BCN films were evaluated with nano-indenter. The BCN films were characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that the BCN films' surface roughness varied in the range of 5–15 nm, and their hardness and reduced elastic modulus fluctuated in the scope of 18–29 GPa and 192–229 GPa, respectively. When the BCN films' surface roughness varied in the range of 8–12 nm, the values of hardness and reduced elastic modulus were fluctuated slightly. The BCN films with the smoothest surface (R_a = 5 nm) and the highest hardness of 28 GPa were obtained at the ion assist source energy of 200 eV and the N₂ relative flow rate of 50%. The BCN films were amorphous and contained several bonding states such as B–N, B–C and C–N with B–C–N hybridization.     

Fabrication of carbon nanoflakes by RF sputtering for field emission applications

In this paper, an ultra thin sheet-like carbon nanostructure, carbon nanoflake (CNF), has been effectively fabricated by RF sputtering on Si substrate without any catalyst or special substrate pre-treatment. The CNFs were chosen to be the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The effect of deposition parameters such as substrate temperature, gas flow ratio and RF power on the field emission properties is discussed in detail. The sheet-like structures with thickness of about 10 nm or less stand on edge on the substrate and have a defective graphite structure. The field emission properties of the sample deposited at the optimum deposition conditions are turn-on field of 5.5 V/μm and current density of 1.4 mA/cm² at 11 V/μm. Considering the inexpensive manufacturing cost, lower synthesis temperature and ease of large-area preparation, the CNFs with low turn-on field deposited by RF sputtering might have a potential application in field emission devices.     

Effect of impurities in charge-exchange carbon foil on foil thickness reduction

Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending on the original foil thickness, changes in thickness during beam irradiation vary: thin foils (∼10 μg/cm²) thicken by build-up, medium thickness foils (∼15 μg/cm²) remain unchanged, and thick foils (∼20 μg/cm²) become thinner. The thickness reduction differs even under identical manufacturing processes and conditions.The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can be said that impurities contained in the foil cause foil thinning.Carbon foils contain impurities such as water. These impurities dissociate and combine with carbon and then evaporate. Taking this into consideration, we examined the gas composition during beam irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen contained in the foil plays a role in foil thinning.     

Angular dependence of sputtering effects by ethanol cluster ion irradiation on solid surfaces

In order to clarify the interactions of ethanol cluster ions with solid surfaces such as Si(100), SiO₂ and Au surfaces, sputtering effects were investigated by changing the acceleration voltage and incident angle (θ) of the cluster ions. The sputtered depth at a normal incidence of θ = 0° increased with an increase of the acceleration voltage. The sputtering ratio of Si to SiO₂ was approximately 10, which suggested that chemical reactions between Si and ethanol produced silicon hydride as a dominant etching material. Furthermore, the sputtered depth of Si surfaces by ethanol cluster ion irradiation had a maximum value at an incident angle between 10° and 60°, and the angle corresponding to the maximum peak increased with an increase of the acceleration voltage. On the other hand, for the physical sputtering of Au surfaces by ethanol cluster ion irradiation, the sputtered depth decreased with the increase of the incident angle, and the change was in accordance with cos θ.With regards to the angular distribution of sputtered particles, Si surface atoms were ejected by ethanol cluster ion irradiation according to a cosine law distribution. This indicated that the Si surfaces were chemically sputtered by ethanol cluster ion irradiation. On the other hand, for the case of Au surfaces, the ejection of the sputtered particles changed to the under-cosine law. This was ascribed to the lateral sputtering effect of ethanol cluster ion irradiation.     

Material dependence of electronic sputtering induced by MeV-energy heavy ions

Masses, yields and emission energies of secondary ions have been systematically measured for solid targets bombarded by Si ions over an energy range between 0.5 and 5 MeV for studying roles of the electronic collision in the formation process of secondary atomic ions. The targets used were insulators (Al₂O₃, SiO₂), semiconductors (Si, GaP, GaAs, GaSb and InSb) and a metal (Al). The obtained feature of the secondary ion emission depends characteristically on the target species. Singly and multiply charged positive ions are produced for Al, Si and P, respectively, from the Al, Al₂O₃, Si, SiO₂ and GaP targets. In the cases of GaAs, GaSb and InSb, however, the production rates of positive As and Sb ions are strongly depressed. The obtained emission energy distributions of atomic ions from the conductor and the semiconductors are very broad and have gradually decaying tails at the high-energy side in contradiction to those of singly charged atomic and cluster ions from the insulators. The most probable and mean energies of atomic ions are proportional to their electric charge irrespective of the target conductivity. Large cluster ions are produced from the SiO₂, Al₂O₃, GaSb and InSb targets. The yields of clusters from SiO₂, GaSb and InSb show power-law dependences on their sizes and the yield of clusters from Al₂O₃ decreases exponentially with increasing size. These results show that the clusters from SiO₂, GaSb and InSb are emitted directly from the surface. The clusters from Al₂O₃ may be indirectly produced by coagulation of molecules in a selvage region near the surface.     

Microstructural study of iron nitride thin films deposited by ion beam sputtering

An amorphous iron nitride thin film was deposited using reactive ion beam sputtering of iron by a beam of argon and nitrogen ions. Nitrogen content in the film as determined from conversion electron Mössbauer spectroscopy (CEMS) and X-ray photoelectron spectroscopy (XPS) was FeN_0.7. The mass density of the film was calculated using energy-dispersive X-ray reflectivity (EDXRR) measurements and is found to be 6.0 gm/cm³. CEMS shows that the film is nonmagnetic in nature. Morphology of the film is obtained from atomic force microscopy (AFM). The surface roughness of the film does not increase appreciably beyond that of the substrate even after a deposition of 131 nm of material with these qualities the film is a good candidate for the multilayer superstructure of a nuclear Bragg monochromator of the type ⁵⁶FeN_0.7/⁵⁷FeN_0.7.     

溅射辅助微波等离子体化学气相沉积SiCN晶体

在微波等离子体化学气相沉积系统中,利用脉冲氮离子束溅射二氰二氨靶产生的碳氮粒子作为合成前驱物,在石英玻璃基片上研究了SiCN晶体的合成。用扫描电子显微镜(SEM)、X射线能谱(EDX)、X射线衍射(XRD)和X射线光电子能谱(XPS)研究了基片温度对薄膜的形貌、成分和结构的影响。结果表明:随着基片温度的降低,沉积物由截面为六方形的结晶良好的SiCN晶体(800℃)变成发育不完全的聚片状晶体(700℃),直到变成颗粒细小的无定形碳氮薄膜(550℃)。衍射峰的强度以及晶胞参数a和c的值随温度的降低而减小。薄膜为C原子部分取代Si3N4中的Si原子位置而形成的SiCN晶体,其中N原子主要与Si原子结合,C原子以sp3C—N、sp2CN和sp2CC键的形式存在。降低基片温度有利于提高薄膜中的C含量和sp3C—N键的含量。     

Proposals for producing novel periodic structures on silicon by ion bombardment sputtering

It is proposed, and confirmed analytically, that if multiple ion fluxes are incident simultaneously, at different oblique polar angles but at the same Azimuthal angle, on to a target, then the individual ripple patterns generated by sputtering are superimposed to produce novel surface structures. It is also shown that multiple focused ion fluxes incident on to rotating targets can produce circular novel patterns.     

离子束溅射生长Ge纳米薄膜的表面形貌观察

采用离子束溅射技术并按正交试验方案生长了不同厚度以及在不同条件下退火的Ge纳米薄膜,用AFM图谱对薄膜的表面形貌进行了表征.结果表明厚度为2.8nm的Ge膜在600℃下退火10min,出现了高4nm、直径50nm左右的Ge岛,而10nm厚的Ge膜在720℃下退火120min,岛的数量较多且分布比较均匀.通过离子束溅射机理和沉积原子之间的扩散运动,对这些现象进行了较为合理的解释.     

Catalyst-free synthesis of carbon and boron nitride nanoflakes using RF-magnetron sputtering

Catalyst-free boron nitride (BN) and carbon (C) nanoflakes have been produced by direct radio frequency (RF)-magnetron sputtering on molybdenum and tungsten substrates at or above temperatures of 1000 °C and 800 °C, respectively. Selected-area electron diffraction (SAED) shows that the films are polycrystalline and contain disordered graphite and hexagonal BN. Transmission electron microscopy (TEM) reveals curved or twisted flakes up to several hundred nanometres in length. High resolution transmission electron microscopy (HRTEM) confirms the nanoflake structure to be turbostratic, which is intermediate between an amorphous phase and the ordered layered phases of hexagonal BN or graphite.     

Ion sputtering rates of C, CrxCy, and Cr at different Ar ion incidence angles

To study the ion sputtering of a layered structure with different layer densities and ion sputtering yields a trilayer structure of C-graphite(46 nm)/Cr_xC_y(60 nm)/Cr(69 nm) was sputter deposited onto smooth silicon substrates. The ion sputtering rates of amorphous carbon, amorphous Cr_xC_y and polycrystalline Cr were determined by means of Auger electron spectroscopy depth profiling as a function of the angle of incidence of two symmetrically inclined 1 keV Ar⁺ ion beams in the range between 22° and 82°. The sputtering rates were calculated from the known thicknesses of the layers and the sputtering times necessary to remove the individual layers. It was found that the sputtering rates of C-graphite, Cr_xC_y carbide and Cr were strongly angle dependent. The experimental sputtering yields were in agreement with the theoretical results obtained by calculation of the transport of ions in solids, but the sputtering yields of C-graphite measured at ion incidence angles larger than 29° were smaller than the simulated ones.