Measurement of carbon sputtering yield by N ion beam and lifetime dependence of resulting foils on thickness

Carbon stripper foils with a higher nitrogen content were made by ion beam sputtering with reactive nitrogen gas. Such foils seem to be very useful as strippers for high-intensity heavy ion accelerators. To know further characteristics of the lifetime of such carbon foils, we have measured the sputtering yield of the carbon source material at a sputtering voltage of 4–15 kV and the lifetime dependence of such foils on thickness. Lifetime measurement was performed with a 3.2 MeV Ne⁺ ion beam. The sputtering yield on average showed 0.75 atoms/ion at over 9 kV sputtering voltage. The lifetime of the foils noticeably depends on the foil thickness, and the thickness range as practical stripper foil is to be around 15 to 33 μg/cm². Two foils made at 13 kV showed extremely long lifetimes of 6800 and 6000 mC/cm² at maximum and the foils made above 10 kV lived longer than about 900 mC/cm², which correspond to about 270 and 40 times longer than commercially available best foils. We measured the thickness ratio of nitrogen to carbon in each foil made at the different sputtering voltages and at the different irradiation stages (mC/cm²) by RBS method. We also inspected the structure of a nitrided carbon foil by transmission electron microscopy.     

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.     

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.     

Carbon stripper foils held in place with carbon fibers

The Spallation Neutron Source (SNS) currently under construction at Oak Ridge National Laboratory, Oak Ridge, Tennessee, is planned to initially utilize carbon stripper foils having areal densities approximately 260 μg/cm². The projected design requires that each foil be supported by only one fixed edge. For stability of the foil, additional support is to be provided by carbon fibers. The feasibility of manufacturing and shipping such mounted carbon foils produced by arc evaporation was studied using two prototypes. Production of the foils is described. Fibers were chosen for satisfactory mechanical strength consistent with minimal interference with the SNS beam. Mounting of the fibers, and packaging of the assemblies for shipping are described. Ten completed assemblies were shipped to SNS for further testing. Preliminary evaluation of the survivability of the foils in the SNS foil changer is described.     

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.     

Polymer coating method developed for carbon stripper foils

The problem of handling the fragile carbon foils (mounting on the frame, placing in the stripper changer) that easily break when self-supporting has been solved by coating carbon foils with poly-monochloro-para-xylylene. It was found that the polymer-coating method could also be used to produce carbon foils thicker than 100 μg/cm² by alternated deposition of carbon and poly-monochloro-para-xylylene layers. Carbon foil of 500 μg/cm² thick and 10 cm in diameter was produced by this method and mounted to a foil holder. Results of lifetime measurement for singly coated foils are also presented.     

The preparation of 248CmF3 deposits on self-supported carbon foils

Another target preparative technique was recently added to the Isotope Research Materials Laboratory's (IRML) capabilities for custom target fabrication. In support of super-heavy-ion physics experiments, methods and equipment were developed for the preparation of ²⁴⁸CmF₃ deposits on carbon foils. The starting material was obtained as either a chloride or nitrate solution, converted to the flouride, and evaporated on carbon foil substrates. Deposits ranging from 40 to 570 μg/cm² were prepared as a 12-mm-diam spot on 45- to 60-μg/cm² self-supported carbon foils. The deposits were then overcoated with approximately 10 μg/cm² of carbon to minimize contamination problems during target handling. The high cost of ²⁴⁸Cm ($100/μg) and its limited availability were the key constraints in the development of preparative technology beyond the inherent radioactivity of ²⁴⁸Cm.     

Structure studies of carbon foils with the aim to improve the ability for heavy-ion stripping

Slackened carbon stripper foils of 3 to 5 μg/cm² produced by vacuum evaporation-condensation were graphitized by annealing at about 3300 K with a pulsed laser beam (1.06 μm). An average crystal size of 10 nm was measured by electron diffraction. The texture of the 002-plane is parallel to the surface of the foil as known from slightly annealed carbon foils. Radiation damages caused by high doses of heavy ions dramatically change the crystal structure. Electron diffraction patterns reveal newly formed graphite crystals with their 002-plane perpendicular to the texture.Compared to carbon foils of the same kind, but not graphitized, a lifetime prolongation of a factor of two was achieved for such foils tested in the terminal of the Munich MP tandem. The slope of the curve for heavy-ion transmission through carbon stripper foils varies for foils of the same kind and for different ion-beam conditions.     

A design for a 280-kV ion accelerator

The innovative design features for a 280-kV ion accelerator are described. These features include a versatile in-line ion injector which was modelled after a commercial ion gun and an acceleration tube and power-transfer system that were made, economically, in-house. The accelerator is capable of generating a mass-filtered ion beam of solid or gaseous species. Maximum beam current observed during initial trials with N₂⁺ was 0.6 μA at an acceleration potential of 50 kV, rising as acceleration potential was increased to 1.5 μA at 280 kV. Spot size remained invariant at approximately 3 mm diameter.     

Development of flexible long lived carbon stripper foils

We have developed flexible long-lived carbon stripper foils of about 10 μg/cm² in thickness with an enhanced carbon arc-discharge method. The foils are not only mechanically strong, but also long-lived against the bombardment of a 3.2-Mev Ne ion beam of 1 μA in intensity with a beam spot of 3.5 mm in diameter. The average lifetime is 15 h. This value is about 30 times longer than those of commercially available foils. We have found the areas of the foils under the heavy ion bombardment to change their crystal structure. An inspection by means of an electron microscope shows that the nature of the area has transformed to a crystallographic characteristic from amorphous crystals.     

Ion beam reactively sputtered silicon nitride coatings

An ion beam source was used to deposit silicon nitride films by reactive sputtering a silicon target with an Ar+N₂ beam. The nitrogen fraction in the sputtering gas was 0.05 to 0.80 at a total pressure of 6 to 20×10^?5 torr. The ion beam current was 50 mA at 500 V. A rate of deposition of about 2 nm min^?1 (0.12 μm h^?1) was found, and the spectra indicated that Si₃N₄ was obtained for a fraction of nitrogen higher than 0.50. However, the AES spectra also indicated that the sputtered silicon nitride films were contaminated with oxygen and carbon and contained significant amounts of iron, nickel, and chromium, most probably sputtered from the holder of the substrate and target.     

Charge-stripping foil changer with energy adjuster function

A charge-stripping foil changer that has an energy adjuster function was constructed. One of the 30 foils in the changer is placed at the beam position without opening the vacuum chamber. The energy of the beam behind the charge stripper is adjusted by changing the effective thickness of the foil, which is changed by tilting the foil at an angle from 0° to 60°. A uranium beam was accelerated to 345 MeV/nucleon using two charge-stripping foil changers with an energy adjuster function.     

Preparation of multilayer targets for g-factor measurement of rotational levels

The three-layer-sandwich targets of ^58,60Ni–Fe–Cu, ¹¹B–Fe–Cu and ^67,68,70Zn–Fe–Au have been prepared for g-factor measurement of rotational levels. The ferromagnetic Fe middle layers of 1.2–1.5 mg/cm² Fe have been made by rolling interrupted by annealing against hardening. Thick recoil stopper layers (5–12 mg/cm²) of Cu and Au on one side of Fe foils have been fabricated by vacuum evaporation. Isotopic Zn, B and Ni layers of 150–400 μg/cm² have been deposited on the opposite side of the Fe foil by electron beam evaporation and focused ion beam sputtering, respectively. For better homogeneity the substrates were rotated.     

Precision measurement of target position during irradiation

We have developed a technique to measure the position of thin target foils (≤ 10 μm/cm²) during irradiation with an accuracy of ∼ 7 μm. The experiment makes use of a kinematic coincidence technique, recording the scattered as well as the recoil particles in a precisely known geometrical setup. For 3 μm/cm² carbon foils bombarded by 4 MeV ¹²C²⁺ ions, target foil movements of about 4 μm/h were observed.     

Excitation functions for 197Au(α, xn) reactions in the 10–40 MeV energy range

The stacked foil technique has been used to measure the excitation functions for the reactions ¹⁹⁷Au(α, xn), x = 1 to 3. A beam of alpha-particles of energy ≈ 40 MeV has been made to bombard a stack of ten gold foils. The thickness of the gold foils was selected so as to obtain an energy degradation of the order of 2 to 4 MeV. The induced activities in each foil were followed using a HPGe system. Theoretical excitation functions for these reactions based on the equilibrium decay model do not agree, in general, with the measured ones towards the high energy tails. Excitation functions have also been calculated simulating the pre-equilibrium emission by a hybrid model. The experimental excitation functions are best reproduced by taking a mixture of both the equilibrium and pre-equilibrium emissions.     

Properties of amorphous carbon films deposited by ion beam methods

Amorphous carbon films were prepared by the ion beam sputtering of graphite, by the ion beam sputtering of graphite with simultaneous ion bombardment of the growing film, and by primary ion beam deposition using a beam from a methane- argon plasma. The films are semiconducting in nature, having band gaps of the order of 1 eV. A nuclear reaction involving an energetic (2 MeV) beam of ¹¹B⁺ was used to obtain hydrogen profiles of the films. It was found that the electrical, optical and mechanical properties of the films could be correlated with the hydrogen content. The observed properties are explained qualitatively in terms of amorphous semiconductor theory.     

A study of very thin DLC foils as a gas barrier

Measurements of vacuum tightness and mechanical strength of diamond-like carbon (DLC) foils in the thickness range of 1–7 μg cm⁻² have been performed with a purpose to evaluate suitability of foils as a gas barrier. Hydrogen and argon at pressures from 10⁻² Pa to 20 kPa were used as test gases. The permeation rate specified as conductance density was found for the best sample of self-supporting foil to be around 1.5×10⁻³ l and 3.3×10⁻⁴ l s⁻¹ cm⁻² for H₂ and Ar, respectively. Conductance density of the same foils mounted on the frames with a mesh along the apertures as support was about twice higher than that for the self-supporting ones, likely due to the mechanical imperfections of the foil assemblies of the first ones. On the other hand, mesh-supported foils as thin as 3 μg cm⁻² and of 5 mm in diameter were withstanding the pressure of up to 18 kPa, while self-supporting foils of the same thickness ruptured at around 1.2 kPa. There was no observed relation between thickness of the foil and its mechanical properties and permeation rate. This suggests that rather tears and pinholes present in foils are the limiting factors of the foil–vacuum tightness and strength. Results obtained in the studies, presented in this work, demonstrate the ability of very thin DLC to isolate a high vacuum beam line from a gas cell in a variety of applications and ability to withstand the gas pressure relevant, in particular, to some gas-filled ionization chambers.     

Exchange and observation systems for charge stripper foils at the J-PARC 3GeV-RCS

The Japan Proton Accelerator Research Complex (J-PARC) has been under construction in Tokai-mura, Ibaraki, Japan. Three independent charge stripper devices are set up at the injection line of the 3 GeV Rapid Cycling Synchrotron (RCS). The H⁻ beam accelerated by a 181 MeV Linac is charge-exchanged to a H⁺ beam by the first stripper foil, and then injected into the RCS. The H⁰ and H⁻ fractions of the beam, which are not stripped by the first stripper foil, are converted into a H⁺ beam by the second and the third stripper foils.We have designed the charge exchange devices by adopting the transfer-rod system for moving the foils in a vacuum. We have fabricated a new type of transfer-rod, which can move over a distance of 1500 mm.We have also developed a new telescope system to observe possible wrinkles and pinholes of the foil. The system withstands more than 1 MGy of radiation dose and has a resolution of 250 μm at a distance of 10 m from an object.     

Determination of thicknesses and impurities of targets by measuring backscattered particles from 2 MeV 4He+-bombardment

The measurement of target thicknesses and impurity contents using backscattering, as well as the principle of this technique are briefly described. The targets are irradiated in a beam of 2 MeV ⁴He⁺ ions. The backscattered ions are detected using a Si(Au) solid-state detector. Gold and calcium fluoride on thin carbon backing and selfsupporting silicon foils were determined from peak width measurements. Oxygen and tungsten impurities in a silicon target being prepared by vacuum evaporation from a tungsten boat were determined using the same method. The thickness of a thin aluminium deposit on thick tantalum backing was obtained from the energy shift of the tantalum peak between two spectra recorded with respectively the aluminium and the tantalum facing the ion beam. Copper contamination on the surface on an iron layer electroplated on copper foil was determined by comparison of the copper peak area with the iron peak height in the backscattering spectrum.     

An investigation of the Ar+ ion-enhanced reaction of CCl4 on Si(100) by secondary ion mass spectrometry

The Ar⁺ ion-enhanced reaction of carbon tetrachloride (CCl₄) on Si(100) at room temperature is investigated at primary ion energies of 2 and 9 keV using the secondary ion mass spectrometry (SIMS) technique. Static SIMS shows that CCl₄ reacts with Si at room temperature. This surface reaction is enhanced by simultaneous sputtering with an Ar⁺ ion beam, the reaction rate being higher at 9 keV than at 2 keV. Possible products of surface reaction are discussed.