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.     

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.     

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.     

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.     

SNS stripper foil development program

When the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) becomes fully operational it will be the world's highest intensity neutron spallation source. The charge-exchange injection section in the accumulator ring, which strips the injected H⁻ beam to H⁺, requires a stripper foil 260 μg/cm² thick, 12 mm wide, a height of at least 20 mm, and support from just the top edge. The foil will get very hot due to the 1.4 MW, 1 GeV, 60 Hz H⁻ beam that passes through the foil, in addition to the 7–10 foil traversals each circulating proton makes through the foil. The planned upgrade to 3 MW beam power presents an even greater challenge. To meet this challenge a diamond foil development program has been underway at ORNL since 2001. Both microcrystalline and nanocrystalline foils have been developed and tested. In this paper we will discuss the SNS injection process, stripper foil requirements, and results from the diamond foil development and testing program.     

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.     

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.     

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.     

Molecular plating of actinides on thin backings

Actinide targets on thick and thin backings are needed for experiments at heavy-ion accelerators. One of the efficient ways to prepare such targets is by molecular plating. Although many laboratories have successfully prepared targets on thick backings by this technique, it is quite difficult to make targets on thin backings (100 μg/cm² up to 1 mg/cm²). In recent years, we have plated targets on thin Ni and carbon backings, for example ²³⁴U targets on a 200 μg/cm² Ni backing. The Ni foils, evaporated on a copper substrate, are available commercially. We used these foils to plate ²³⁴U and afterwards we removed the copper by dissolving it in a mixture of ammoniacal trichloroacetic acid. In this way 400 μg/cm^{2 234}U targets were prepared on a 200 μg/cm² Ni backing. A 100 μg/cm^{2 243}Am target was prepared by plating onto a 75 μg/cm² carbon film left on its glass substrate for later floating. We found that a plating cell made from Teflon was difficult to use because it scratched the C film producing a liquid leak at the joint of the column and the C film. This sealing surface needs to be extremely smooth to avoid leakage. A column made of Delrin™ was then tried and did not produce any scratch on the carbon film surface. This column was used to prepare 100 μg/cm^{2 243}Am targets. Details of the technique will be presented.     

Preparation of isotopic Zn and Cd targets

^nat,68Zn and ^natCd have been reduced from their oxides with high yields (about 90%) using carbon as reductant. A water-cooled copper pin collector was used resulting in higher yield and better reproducibility. ^natZn and ^{nat, 113, 116}Cd targets of 20–3000 μg/cm² on carbon backings of 20 μg/cm² have been deposited by focused ion beam sputtering. ^{67, 68, 70}Zn targets of 200–400 μg/cm² on iron backings of about 1.3 mg/cm² have been prepared using a rotating substrate setup to improve target homogeneity. Using a special rolling technique, ^{64, 66, 68, 70}Zn and ^{110, 114, 116}Cd have also been rolled to thin foils.     

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.     

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.     

Boron foils for RDDS experiment

Application of the deposition method based on the vibrational motion of micro particles in an electrostatic field [I. Sugai, Nucl. Instr. and Meth. A 397 (1997) 81] is described for the production of isotopic ¹¹B foils. The method proved suitable for target production of this typically brittle material when a very flat target surface was required. The goal to produce ¹¹B targets of 160–350 μg/cm² was achieved by depositing the boron on a thin foil substrate, such as Nb and Sn. The coated foil was stretched flat before it was mounted on a frame.     

Preparation of isotopically enriched mercury sulphide targets

Production of HgS targets, based on HgO, is described. The HgS is precipitated from HgO dissolved in diluted HNO₃ by gaseous H₂S. Subsequently the target is manufactured by evaporation–condensation of HgS in vacuum. This way the ²⁰⁴HgS layers of 500 μg/cm² on a backing carbon foil of 26 μg/cm² were produced.     

Fabrication of W target on carbon backing

¹⁸⁴W enriched isotopic target of 210 μg/cm² thickness on carbon backing of 100 μg/cm² thickness has been made in ultra-high vacuum environment by evaporation method using 6 kW electron beam at Inter University Accelerator Centre (IUAC). Hundred and thirty milligrams of enriched ¹⁸⁴W powder was used in this target preparation process. This target has been successfully used in two nuclear reaction experiments performed at IUAC. Methods adopted to convert the tungsten powder to a special pellet form in order to minimize the consumption of the expensive material, preparation of stress relieved carbon-backing foil, steps taken to make the carbon withstand the heat generated during the tungsten evaporation, the method of tungsten fabrication and details of ultra-high vacuum evaporator facility of IUAC are discussed.     

Gas permeability of thin polyimide foils prepared by in-situ polymerisation

The entrance windows to the gas detector chambers as well as to the target containers used in high-energy and high-intensity accelerators must be as thin as possible to minimise energy losses of the particles used in astrophysics and nuclear physics studies. Because of their good physical properties, polyimide foils are often considered as suitable material for such windows, but commercially available foils, having a thickness greater than 7–8 μm (>1 mg/cm²), would cause energy losses of particles significant for some nuclear reactions studied. Foils prepared by in-situ polymerisation can, however, be as thin as 0.07 μm (10 μg/cm²). The permeability of 4 μm foils produced by in-situ polymerisation has been measured at room temperature for He and Ar. For He measurements were performed in the pressure range of 4–70 mbar and for Ar in the range of 20–140 mbar and the permeability was found to be in good agreement with the values published for the thicker commercial foils.     

Nucleation and initial growth processes of diamond thin films for stripper foils

Carbon ion beam stripper foils were fabricated from diamond films synthesized on silicon via chemical vapor deposition. Fine-grained polycrystal diamond foils with decent surface flatness were obtained using a nucleation enhancement pretreatment process. Freestanding diamond foils were formed by etching a portion of the silicon substrate on which the diamond films well-adhered. In preliminary lifetime evaluations, the 1–3 μm-thick diamond foils lasted between 20 and 420 min for 3.2 MeV Ne⁺ion-beam charge stripping.     

Carbon micro-ribbon and strip polarimeter targets for the AGS and RHIC

Current techniques used to produce carbon micro-ribbon targets 5 μm wide×3.7–4.5 μg/cm²×25-mm long will be described. Developmental emphasis was to provide nearly identical micro-ribbons with the minimum number of atoms per unit of length, and to position them within ±0.5 mm of the desired location on C-shaped frames.The foil strip targets to be described were 200–600 μm wide×3.7–4.5 μg/cm²×51 mm long. These were produced from 25-mm-wide carbon film deposits that were scribed using a jig prior to dissolving the betaine/sucrose release agent under ethanol.Both types of targets required methods and devices that differed significantly from those reported previously for substrate texturing, masking, vacuum deposition, releasing from the substrate, and mounting. Sets of 12–24 of the targets have been made for the 2006 run period at BNL.     

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.     

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.