Material and surface processing in J-PARC vacuum system

All systems comprising the Japan Proton Accelerator Research Complex (J-PARC), namely, the 400 MeV linac, 3-GeV rapid-cycling synchrotron, and 50-GeV synchrotron, were completed in 2009 and are now being used to supply high-power proton beams to secondary particle users. In order to minimize maintenance and shorten the pump-down and conditioning period, an ultra-high vacuum of 10⁻⁵ Pa or lower is required for the vacuum system. Here we present a review of the surface processing methods employed and the results of outgassing measurements for the components used in the vacuum system made from the following materials: electroformed copper, alumina ceramic, titanium, and stainless steel. The vacuum performance of these materials during accelerator operation is also reported.     

Method for stabilizing beam intensity and energy in the SPring-8 linac

In any electron accelerator facilities, radio frequencies (RF) for a linear accelerator and a circular accelerator that includes a booster synchrotron ring and a storage ring are completely different. There is not necessarily a sub-harmonic relation of two RFs between a linear accelerator and a circular accelerator. It is, however, indispensable to obtain a synchronous relation between both RFs and the timing of the gun trigger signal with the increasing beam current and shortening of beam time width from an electron gun in a linear accelerator. For a synchronous timing system in any electron accelerator facilities, there is no choice but to assemble a complicated system with the frequency dividers and multipliers in order to realize the synchronous relation between both RFs and the gun trigger signal. To simplify the complicated synchronous timing system, we have developed a new synchronization method for the RFs of both linear and circular accelerators. The new synchronization system has been installed into the synchrotron radiation facility, SPring-8 (Super Photon ring 8 GeV), which consists of a 1-GeV linac, an 8-GeV booster synchrotron and an 8-GeV storage ring. A 2856 MHz RF for the 1-GeV linac was generated by the 508.58 MHz RF of the 8-GeV storage ring with the new synchronous timing system, and the emission and acceleration under the condition of the shortening the beam time width in the linac was carried out. Since the synchronous relation between both the RFs and the gun trigger signal was realized by using the new synchronous timing system, the time jitter between the gun trigger signal and the RF phase of 2856 MHz was significantly reduced and resulted in beam energy stabilization. The new synchronous timing system has been used for usual beam operations at SPring-8. This timing system has achieved time jitters of 3.5 ps (rms) and beam energy stability of 0.009% (rms) under the condition of completely synchronized two RFs and the gun trigger signal.     

Energy response of LiF and Mg2SiO4 TLDs to 10-150 keV monoenergetic photons

Energy response of LiF:Mg,Ti, LiF:Mg,Cu,P and Mg2SiO4:Tb thermoluminescence dosemeters (TLDs) was measured in the range 10-150 keV for monoenergetic photons at SPring-8 of an 8-GeV synchrotron radiation facility. The photon beam was monitored by a parallel-plate free-air ionisation chamber calibrated with an uncertainty of 3%. Owing to the small dimension of the beam, a rotating holder was designed in order to irradiate TLDs uniformly. The measured responses of LiF to energy were approximately in agreement with the calculated dose absorption dependence in the soft tissue. However, two types of LiF TLDs presented the different luminescent responses to the photon energy. The response of LiF:Mg,Ti had a smooth curve, and that of LiF:Mg,Cu,P presented a local maximum at 30 keV and a local minimum at 100 keV. The Mg2SiO4:Tb response was nearly bone equivalent. Linearity of dose responses was also confirmed up to 2 Gy on each TL material.     

Production of the Hf isomer using a 4.5-GeV electron accelerator

High-productivity methods are required for the accumulation of long-lived isomers in amounts that are sufficient for the creation of experimental targets. A tantalum sample was activated with the Yerevan synchrotron using 4.5-GeV bremsstrahlung and the presence of ^178m2Hf was detected with good statistical accuracy by γ-activity measurements. The integrated and mean cross-section values were deduced from the experiment. The isomer-to-ground-state ratio was then estimated and compared with that known for the p+ Ta reaction studied at 660 MeV. In the present experiment, both converter and target were relatively thin for better definition of the experimental conditions. However, an assembly designed for high-productivity irradiations should be thick and then the converter can also serve as the target sample when irradiated with a high-energy electron beam. The optimization of the isomer production was solved analytically and the largest estimated yield was determined as calibrated to the experimental yield. The maximum yield of ^178m2Hf was found to be of about 3×10⁹ nuclei/s using an electron beam current of 100 μA. This is lower than the yield achieved with proton beams, although for a practical comparison the total cost and radiation safety conditions should be considered. The present results provide a basis for numerical estimations.     

Nitriding of titanium by NH3 RF plasma: a study of the corrosion resistance and the mechanical properties of the protective films formed at the solid surface

The exposure of a titanium sample to an NH₃ low pressure plasma leads to the formation of a nitriding layer. The products formed at the titanium surface were identified by XRD spectroscopy. The modification of the corrosion resistance characteristics of titanium due to the NH₃ plasma treatment were investigated by electrochemical tests. The recorded polarization curves of the treated titanium samples were used to determine the values of the corrosion potential E_corr. This study confirms the increasing of the corrosion resistance as a function of the time exposure and the injected electric power in the silica reactor. The plasma treatment also induces drastic changes of the titanium target in hardness.     

Vacuum system for the 3-GeV RCS in J-PARC

The 3-GeV RCS (rapid cycling synchrotron) in the J-PARC (Japan Accelerator Research Complex) project accelerates a proton beam of 0.333 mA up to 3 GeV. At 25-Hz repetition rate, the RCS generates a high-power beam of 1 MW. In such an accelerated proton beam, neutrons as well as gamma rays are generated. The cumulative energy dose will be of the order of 100 MGy over 30 years of operation. So as to minimize the radiation exposure during maintenance, it is necessary to construct a vacuum system with reliable components which have a long life in such a high level of radiation. In addition, it is necessary to keep the operating pressure of the beam in ultra high vacuum to suppress pressure instability. Thus we should think not only of the outgassing mainly due to ion-induced desorption but also of the pumping efficiency. Based on the above, the vacuum system was designed as follows: The ring is divided by isolation valves into 6 sections (3 straight and 3 arc sections), which can be pumped down independently. To avoid any eddy current loss, ceramic ducts are used in the bending and focusing magnets. These ducts are connected to titanium ducts placing the Ti bellows between. Here, we adopt pure Ti as a material for the ducts and bellows because of its small residual radioactivity. The ring is evacuated with 20 ion pumps (0.7 m³/s) and 24 turbomolecular pumps (TMPs) (1.3 m³/s), which are attached to the Ti ducts. The TMPs are used not only for rough pumping but also for evacuation during the beam operation. Especially a collimator system for localizing beam losses in a restricted area is evacuated with the TMPs, because the outgassing from this region will probably be the greatest. In addition, the straight sections for beam injection and extraction are pumped down mainly by the TMPs. Each arc section is pumped by 4 ion pumps and 2 TMPs. To realize the above system, we developed components such as large aperture ceramic ducts and TMPs with high radioactive resistance, as well as several kinds of heat treatment to reduce the outgassing. Finally, we realized the UHV without baking in the RCS and the beam operation has been successful to date.     

Calculation of the pressure in the unit-cell vacuum system of Taiwan Photon Source

Taiwan Photon Source is a third-generation 3-GeV synchrotron light source under construction. The electron storage ring of circumference 518.4 m contains 24 unit cells and 24 long straight sections. Each cell of length about 14 m contains two bending chambers and two short straight chambers. To estimate the pressure distribution in the cell vacuum system, a program combining an iterative method and a Monte-Carlo method was used to calculate the pressure. In the modeling, the rate of thermal outgassing of the chamber surfaces and the yield of photon-stimulated desorption of the absorbers are obtained from the experimental results. To provide the effective pumping speed of various pumps that depends on the gas, the composition of residual gases from photon-stimulated desorption was assumed to be 80% H₂ and 20% CO. The pressure calculation for the vacuum cell compares the beam cleaning efficiency during the early commissioning stage when the accumulated beam dose attained 1 A h and 100 A h. The effects of the confined pumping design are evaluated through the modeling to assess whether the commissioning can be accelerated. The result of the pressure calculation obtained with the Monte-Carlo method shows that a mean pressure rise per beam current at 1.9 × 10⁻¹⁰ Pa mA⁻¹ after beam cleaning to a beam dose 100 A h is reasonable and within typical specifications of the synchrotron light source.     

Mechanical and ellipsometry measurements of thin TiN layer prepared by PIII

Titanium nitrides have good mechanical, biomedical and optical properties, therefore they are used to harden and protect cutting and sliding surfaces and as a non-toxic exterior for bio-medical applications. Nitrogen plasma immersion implantation (PIII), in which the diffusion of nitrogen from low pressure r.f. plasma is combined with the implantation of nitrogen ions at energies up to 30 kV, is an effective tool for nitriding titanium and titanium alloys. In this work, samples of pure titanium were nitrided by PIII at different negative high voltage pulses. The properties and the characteristics of the processed samples were evaluated using X-ray diffraction (XRD), Auger electron spectroscopy (AES), ball-on-disk type tribometer, surface profilemeter, and ellipsometry measurements. The results show that, the wear resistance of the untreated sample in comparison to the PIII treated samples is extremely poor and the friction coefficient for the PIII treated samples is decreased to the half value in comparison to the untreated titanium, this attributed to the formation of the solid solution titanium α-Ti(N) and the cubic TiN phases. Ellipsometric measurements were carried out on the PIII treated samples at different negative high voltage pulses. A three layers model was used to fit the calculated data to the experimental ellipsometric spectra. The thickness, surface roughness and refractive index increase with increasing the negative high voltage pulses. The refractive index at 550 nm increases from 1.83 to 2.09 as the negative high voltage pulses increases from 10 to 30 kV.     

Viscous flow during sintering of attrition milled nanocrystalline titanium powders

Nanocrystalline powders are expected to exhibit viscous flow during sintering due to presence of substantial amount of quasi-amorphous layers at the grain boundaries. The present investigation attempts to determine the contribution of viscous flow during sintering of nanocrystalline titanium powders. Model equations originally suggested by Frenkel are applied on the shrinkage data to determine the activation energy for viscous flow, frequency factor and the coefficient of viscosity. Nanocrystalline titanium powders were found to exhibit lower activation energy for viscous flow, higher frequency factor and higher coefficient of viscosity as compared to micron sized titanium powders.     

Depth-profiling of phase composition and preferred orientation in a graded alumina/mullite/aluminium-titanate hybrid using X-ray and synchrotron radiation diffraction

X-ray diffraction (XRD), grazing-incidence synchrotron radiation diffraction (GISRD) and multi-wavelength synchrotron radiation diffraction (MWSRD) have been successfully used for near-surface depth profiling of phase composition and texture in a functionally-graded alumina/mullite/aluminium-titanate hybrid prepared by an infiltration process. Depth profiling of near-surface information both in the nanometer and micrometer scale has been done by (a) varying the X-ray wavelength, (b) varying the grazing-incidence angle, and (c) gradual polishing of the sample surface with diamond lap. Results show a distinct gradation in the phase abundance near the surface of the hybrid sample. The distribution of mullite near the surface is highly textured and shows a distinct depth-dependent gradation in preferred grain-orientation. The unique but powerful capability of XRD, GISRD and MWSRD as complementary tools for depth profiling the near-surface region of graded materials has been demonstrated in this work.     

Combustion of TiO2-Mg and TiO2-Mg-C systems in the presence of NaCl to synthesize nanocrystalline Ti and TiC powders

The combustion process of TiO₂-Mg and TiO₂-Mg-C systems with sodium chloride as an inert diluent was investigated. The values of combustion parameters and temperature distribution on a high-temperature wave according to the amount of sodium chloride were obtained by the thermocoupling technique. The leading stages of combustion processes are found and the sizes of reactionary zones were estimated. It is shown that the introduction of NaCl in an initial mixture promotes the formation of a nanocrystalline structure of the final products. As a result, nanosized titanium, and titanium carbide powders have been successfully obtained.     

Preparation of homogeneous titania coating on the surface of MWNT

Preparation of homogeneous and stable inorganic coatings on the surface of multi-wall carbon nanotubes (CNTs) was studied. Precursor compounds such as titanium (IV) bromide and titanium (IV) chloride were used to cover the surface of CNTs under either solvent-free or solution conditions. As-prepared titania layers were characterized by transmission, scanning electron microscopy and X-ray diffraction techniques. Results revealed that homogenous coverage can be achieved in a controllable way.     

Lattice strain evolution in IMI 834 under applied stress

The effect of elastic and plastic anisotropy on the evolution of lattice strains in the titanium alloy IMI834 has been examined during a uniaxial tensile test, by in situ monitoring on the Engin instrument at the ISIS pulsed neutron source. Measurements were made at load during an incremental loading test. The data is analysed in the light of the requirements for engineering residual stress scanning measurements performed at polychromatic neutron and synchrotron diffraction sources. Comparisons between the measured strains from different lattice families and the predictions from an elasto-plastic self-consistent model are made. Agreement is good in the elastic regime and for most diffraction planes in the plastic regime.     

A simple H2O2-assisted route to hollow TiO2 structures with different crystal structures and morphologies

Hollow TiO₂ structures have been synthesized on a large scale in H₂O₂ aqueous solution by a simple hydrothermal method. The morphologies and crystal structures of hollow TiO₂ structures can be controlled by titanium sources. Hollow anatase TiO₂ microspheres composed of nanoparticles are prepared using titanium powder as titanium source. With the pH value increased, TiO₂ nanoparticles and cuboids with cylindrical hollow interiors are formed, respectively. However, hollow rutile TiO₂ microspheres comprising radially aligned nanorods are fabricated using TiCl₃ as titanium source.     

Fabrication of homogeneous titania/MWNT composite materials

MWNT/titania nanocomposites were prepared by an impregnation method and subsequent heat treatment at 400 °C. Precursor compounds such as titanium (IV) propoxide and titanium (IV) ethoxide were used to cover the surface of CNTs under solution conditions. Electron microscopy and X-ray diffraction techniques were carried out to characterize the as-prepared titania layers.     

Influence of nano-reinforcements on the mechanical properties and microstructure of titanium matrix composites

The goal of this work is the evaluation of nanoscaled reinforcements; in particular nanodiamonds (NDs) and carbon nanotubes (CNTs) on properties of titanium matrix composites (TiMMCs). By using nano sized materials as reinforcement in TiMMCs, superior mechanical and physical properties can be expected. Additionally, titanium powder metallurgy (P/M) offers the possibility of changing the reinforcement content in the matrix within a very wide range. In this work, TiMMCs have been produced from titanium powder (Grade 4). The manufacturing of the composites was done by hot pressing, followed by the characterisation of the TiMMCs. The Archimedes density, hardness and oxygen content of the specimens in addition to the mechanical properties were compared and reported in this work. Moreover, XRD analysis and SEM observations revealed in situ formed titanium carbide (TiC) phase after hot pressing in TiMMCs reinforced with NDs and CNTs, at 900 °C and 1100 °C respectively. The strengthening effect of NDs was more significant since its distribution was more homogeneous in the matrix.     

An oxygen nonstoichiometry study of barium polytitanates with hollandite structure

A series of barium titanium oxides with various Ba/Ti ratios having a hollandite structure were obtained by firing chemical synthesized oxide precursors under reducing atmosphere conditions (H₂/N₂ gas mixture flow) at temperatures of 1100°C and above. An oxygen nonstoichiometry was determined by a thermogravimetric analysis through oxidizing in air. It was shown that barium titanium hollandites may adopt both oxygen excess and oxygen-deficient stoichiometries within the structure. As a limiting case the barium titanium hollandites with only tetravalent titanium ions were obtained. The influence of oxygen nonstoichiometry on the hollandite monoclinic-tetragonal phase transition and a barium superstructure ordering was investigated and discussed. Distortion scaling rules are considered to compare this work with earlier investigations on hollandites.     

The influence of structure changes in the properties of TiCxOy decorative thin films

The main purpose of this work consists in the preparation of titanium oxycarbide, TiC_xO_y, thin films, in which the presence of oxygen changed the film properties between those of titanium carbide and those of titanium oxide. Varying the oxide/carbide ratio allowed to tune the structure of the films between titanium oxide and carbide and consequently electronic, mechanical and optical properties of the films. The depositions were carried out from a TiC target by direct current, dc, reactive magnetron sputtering, varying the oxygen flow rate. The obtained results showed that the film's properties can be divided into 3 different regimes — i) carbide, ii) a transition zone and iii) an oxide one. X-ray diffraction results revealed the occurrence of a face-centered cubic phase (TiC-type) for low oxygen content, also obtained in the TiC_1.6(O) film, with a clear tendency towards amorphization with the increase of the oxygen flow rate. For the highest oxygen contents, the results revealed the development of a mixture of poorly crystallized TiO₂ phases. The colour results indicated a strong dependence on the O/Ti ratio. A progressive reduction of hardness and residual stresses with the increase of the O/Ti ratio was also observed. The residual stresses, as well as the film structure, seem to play an important role on the adhesion of the coatings. The static friction coefficient revealed also some correlation with the mechanical properties, but mainly with the surface roughness.     

Synthesis of titanium dioxide with oxygen vacancy and its visible-light sensitive photocatalytic activity

A novel active carbon reducing process was developed for the synthesis of titanium dioxide with oxygen vacancy. In this process a nanocomposite of hydrolyzed titanium(IV) tetra-isopropoxide and the active carbon was annealed in air. The formation reaction, visible-light absorption, and visible-light sensitive photocatalytic activity of the titanium dioxide with oxygen vacancy samples were investigated using XRD, TG-DTA analyses, FE-SEM, EDS, and measurements of electric conductivity, BET specific surface area and photocatalytic activity. The nonstoichiometric titanium dioxide with oxygen vacancy sample has a rutile structure and its chemical formula can be written as Ti(IV)_1−xTi(III)_xO_(2−x/2)□_x/2, where □ is oxygen vacancy. The oxygen vacancy was introduced into the rutile structure by reducing reaction of the active carbon in a phase transformation process from anatase to rutile. The samples showed visible-light absorption with an absorption edge around 570 nm and high surface visible-light sensitive photocatalytic activity.