Research at the Tandem Accelerator

Transcription

1 SI Nuclear Energy in Central Europe '98 Terme Catez, September? to 10, 1998 Research at the Tandem Accelerator M.Budnar and P.Pelicon J.Stefan Institute, SI-1001 Ljubljana, Slovenia' ABSTRACT - In March 1997 a new Electrostatic Tandem Accelerator has been installed at J.Stefan Institute (JSI). The up to 2 MV voltage enables the acceleration oflight and heavy ions to final energies depending on the ion charge and the selected voltage. Two of five possible exit ports from the switching magnet have been already equipped with the beam lines and the measuring chambers. The radiations excited at the interaction of ions with matter, i.e., X-rays, visible light, electrons, scattered and recoiled particles, nuclear reaction products, all of them can be used for the investigation of samples by the so called Ion Beam Analytical (IBA) methods. The elemental concentrations, their profiles, and in some cases also the elemental speciations, can be determined for the studied samples. Therefore the accelerator offers an excellent research tool for the studies in experimental atomic physics as well as in several interdisciplinary fields. So far the most often uses of the IBA methods have been in environmental research, studies of materials and material surfaces, in archeometry, etc. With the use of nuclear microbeam having dimensions bellow 1 ^m the biomedical samples and the microelectronical devices can be efficiently studied. 1. Introduction The electrostatic accelerators which have been broadly used in the nuclear physics research in its early stage have received a new reputation nowadays with their application to atomic physics and to many interdisciplinary scientific fields. Besides that, when redesigned, they have been found as an excellent machines for several industrial application - modification of materials, sterilization of goods and equipment, treatment of wastes, etc. Many important international conferences deal with such accelerators and methods used there [1-3]. The installation of a new tandem accelerator at J.Stefan Institute has become realistic when an important financial support was allocated by the UN, IAEA, as well as the Ministry of Science and Technology of Slovenia for 1995/98 period. The decision supporting such an investment to Slovenia was based on the new conceptual orientation at the IAEA encouraging the spread of research to broader fields of peaceful use of nuclear energy and due to the support given by the IAEA to studies oriented towards the end-users. The group at Department of Low and Medium Energy Physics at JSI, first involved in the nuclear physics research- at the Van de Graaff accelerator (VDG), started with the atomic physics studies and the use of IBA methods approximately twenty years ago. With the VDG machine, designed and constructed in-house and planned for studies of nuclear structure, the users met several drawbacks and limitations when the atomic physics problems were tackled. This situation has been drastically improved recently with the installation of a new accelerator. Main research subjects planned to be investigated at the new machine are the studies in. experimental atomic physics, archeometry, environmental pollution, and materials characterization. Nuclear Energy in Central Europe '98 59

2 In the contribution the tandem accelerator and the methods used with them are described in short. The research subjects and plans for exploitation of the instrument are presented next. The advantages of having a low energy accelerator at home are discussed in the conclusions. 2. The Tandem Accelerator The 2 MV Electrostatic Tandem Accelerator - HVEE MC4117 TANDETRON - is an excellent choice when the accelerated ions with the energies in the MeV energy region are needed. The broad selection of ions, available ion currents, stable operation, computer control, and low radiation are the characteristics which offer a reliable tool for work in the basic and applied research fields. i i : K :.J i fi-ii-l-r ' \\ \^T= ION SOURCE j IOH SOURCE FOR HEAVY IOHS! FOR LIGHT IONS (Ci SpBtoiui Source) (Dtoptumilroi) I Figure 1 : A schematic view of the TANDETRON The accelerator, as schematically shown in Figure 1, is equipped with two ion sources. The duoplasmatron with the attached Li exchange channel serves for the production of light ions (H, He). The Cs sputtering source produces ions heavier than He. The selected negatively charged ions from the ion sources are transported through the ion optics and the bending magnet to the entrance of the accelerating stage. The TANDETRON has two accelerating tubes. The first one serves for the acceleration of the negatively charged ions. In the stripping channel filled with nitrogen at low 60 Nuclear Energy in Central Europe '98

3 pressure the ion charge changes. The positive ions are then accelerated again in the second accelerating tube. The final energy of the ions depends on the selected voltage at the high voltage terminal and the charge produced in the stripping channel. At the exit of the accelerator the electrostatic quadrupole triplet lens enables the transport of the ions along the beamline. The switching magnet with five exit ports serves for the selection of the beamline as well as for choosing the ions with the proper energy and charge. The accelerating voltage of up to 2 MV is obtained through the Cocroft-Walton type of rectifier which smooths the 40 KHz voltage from the RF driver through the semiconductor diode stacks. Such a solution gives much better stability of the machine (bellow 200 V at 2 MV) regarding to the previous ones where high voltage at the terminal is achieved by the transport of charges with the charging belts. At two of five possible exit ports the beamlines have been installed already. One of them serves for measurements of samples in vacuum what means that at the end the vacuum measuring chamber equipped with necessary detectors is installed. The second beamline is intended for measurements in-air. For this purpose the line is equipped with a thin window allowing the passage of the ions out from the vacuum system. Therefore the samples are irradiated in-air what gives an additional possibility of measuring large or delicate samples. The plans for the installation of two additional beamlines are underway where one of them will be equipped with the magnetic qudrupole triplet lenses for focusing the ion beam to dimensions bellow one micron. The construction of these lines is scheduled for the years 1999/ The EBA Methods When accelerated ions with energies in the MeV region interact with matter several radiations are produced. Due to the ionization and excitation of atoms X-rays, visible light, and electrons are liberated. The projectiles are scattered on the nuclei which can be recoiled out from the sample. The nuclear reactions taking place at these energies give several reaction products from gamma rays to light particles. The detection of these radiations can serve for the studies of the samples (see Figure 2). The use of ions at studies of samples led to an acronym Ion Beam Analysis (IB A) for methods mentioned. The detection of characteristic X-ray radiation produced by ions is a basis for the PIXE (Paticle Induced X-ray Emission) method. The achieved resolution of modern energy dispersive semiconductor detectors is good enough for resolving X-rays corresponding to nearly all elements of the periodic system from Ne to U. For lighter elements the X-ray radiation is too soft to be detected as it absorbs on the way from the sample to the detector. The sensitivity of the PIXE method depends on the atomic number and is the best for elements with Z= :: when detecting K X-rays, and for Z=60-70 when measuring L X-rays. The minimum detection limits for these elements are bellow ppm (parts per million) and small amounts of material (nig) are sufficient for the analysis. Still much lower concentrations of the elements in the sample can be measured (bellow 1 ppb) if the original sample is preconcentrated by the chemical procedures before the analysis. The PIXE method is therefore appropriate for sensitive, multielemental analysis of the elemental concentrations in the sample. Nuclear Energy in Central Europe '98 61

4 When scattered projectiles or recoiled nuclei from the sample are detected the concentration profiles of the elements in the sample can be measured. The method based on the detection of the scattered projectiles is called RBS (Rutherford Backscatiering Spectrometry) for detection of particles in the backward direction, or RFS (Rutherford Forwardscatiering Spectrometry) if the forward direction is used. From the spectra of the scattered particles the information about the Figure 2 : The Ion Beam Analysis (IBA) methods. sample composition and the concentration profiles can be extracted. The methods are therefore excellent for the concentration profiles determination up to the depths comparable with the range of the projectiles in the sample. The RBS is appropriate for elements heavier than projectiles, and the RFS for lighter ones. Comparable to RFS, but much more sensitive for light element concentration profiles determination is the ERDA (Elastic Recoil Detection Analysis) method where the recoiled nuclei from the sample are detected. This offers an excellent support for the determination of hydrogen concentration profiles in the samples, and also others if heavier projectiles are used. The scattering IB A methods have found a broad use in the studies of surfaces, thin iayers, and semiconductor materials. ; The detection of the nuclear reaction products excited at the nuclear reactions between the projectiles and the nuclei from the sample is the basis for the NRA (Nuclear Reaction Analysis) method. In the case when gamma rays from the reaction are used the method is called PIGE (Particle Induced Gamma-ray Emission) spectrometry. These methods are very sensitive for the selected projectile-target combinations when the nuclear reaction cross-sections are big, or when the reactions are of resonance character. For example, in the case of 15 N projectiles the l H( 15 N, oy) I2 C reaction serves for sensitive profiling of hydrogen in the samples. 62 Nuclear Energy in Central Europe '98

5 The RBS, RFS, ERDA, and NRA are standard-free depth profiling methods for depth ranges up to approximately a few microns with a depth resolutions nm. They serve for several applicative studies in electronics, microelectronics, deposition of tribological, anti-corrosive, optical and superconducting thin films, for controlling the ion beam and plasma surface modification of materials, and in the fusion research. They are often used for dynamic in-situ diagnostics. When visible light (IL - Ion Luminescence) or electrons (SES - Secondary Electron Spectrometry) and Auger electrons (AES'- Auger Electron Spectrometry) excited in the sample are detected, not only the elemental composition of the sample are deduced, but also the chemical speciations of the particular elements can be obtained. The use of these methods for the analysis of materials and material surfaces is permanently increasing. With the nuclear microbeam the palette of the IBA methods broaden still further. Namely, all the IBA methods applied with a macro beam ( a few mm) can work equally well also at the micron dimensions. A new information is obtained when the microbeam is scanning across the sample giving the concentration maps of the samples with the precision close or bellow 1 um. If the sample is thin enough (~ mg/cm2) the beam passes through it but its energy changes due to the interaction with matter. This so called STIM (Scanning Transmission Ion Microscopy) is therefore used for determination of sample structure and composition. For studying the microelectronic devices the IBIC (Ion Beam Induced Charge) method can be used. Here, the device is wired and biased as in the normal operation. When the microbeam with dimension of 1 um or less (and of low current density - bellow 0.1 fa/um 2 ) scans across the device the particular contacts are triggered. A precise information about the device characteristics can be obtained this way. 4: Studies at J.Stefan Institute TANDETRON Atomic Physics Ions with energies of up to a few MeV are very useful for studies of processes in atomic inner shells. The excitation and ionization processes in the atom are in general very complicated, and it is similarly true for the decay of the excited states. Despite the prevalence of the processes wich are conditioned with an overwhelming central force of the atom, there are many interesting phenomena of the multiparticle correlations. The system of the projectile and atom is in general a complicated multiparticle conglomerate which allow many combinations leading to the collective processes. These are of big interest for modern studies in experimental atomic physics giving an additional information about the quantum behavior of the multiparticle systems. Many sophisticated experimental tools have been developed recently offering necessary conditions for detection of such faint phenomena with good enough resolution, sensitivity, and precision. In our case the AES (Auger electron spectrometry) has been developed for the measurements of spectra of Auger electrons from gas target. The construction of a new HRXRS (High resolution X-ray spectrometer) is under way. The spectrometer will employ a curved crystal in Johann geometry and CCD position sensitive detector what altogether will enable the spectrometry with resolution close to the natural width of the X-ray-lines. Nuclear Energy in Central Europe '98 63

6 The orientation into fostering basic research in the laboratory is conditioned not only because of physics being one of our important interests but is attractive for the Ph.D. students and postdocs also. Interdisciplinary studies The IBA methods work effectively in many interdisciplinary research fields due to their multielemental character, undestructiveness, relatively low price, fastness, and sensitive analysis. In our laboratory we are involved in the following subjects mostly : Studies of archeometrical problems. Here, an exceptional interest of museums scientists, restaurateurs, and archivists have been expressed for analysis of several archeological artefacts, ancient documents, pottery, etc., to understand better the origins and the technology used at the time. Analysis of environmental samples, and especially aerosols, where the information about their elemental contents serve for understanding global transport, environmental pollution, and influences on urbane and working population. - Analysis of thin layers, hard coatings for tools, and of special technological materials (Ti for ion getter pumps). Here, the concentration profiles and their changes during the technological procedures are of broad interest. Composition of materials with the special emphasis on the trace element contents which influence the material physical and chemical characteristics. Common work with the researchers from other fields boundary to physics on several interdisciplinary subjects is necessary to solve interesting questions stemming from the challenges of modern life. Working on solution of such items is a good basis for broader general education of public also...,.- -* Future plans for the interdisciplinary research Further research orientations of the laboratory are manly conditioned by the financial possibilities. The full exploitation of the accelerator will be achieved when all the exit ports will be equipped with the beamlines and when they will become completely operational. The plans for installation of microbeamline are prepared and its construction is hoped to start soon. The interdisciplinary research fields where we are continuing our activities and are trying to become still more involved are : - archeometry, - environment, - study of materials, thin layers and material surfaces, - study of micro electronical materials and components, - analysis of biomedical samples. The study of materials and their surfaces by the IBA methods have already found large interest at the nuclear society community For illustration a few examples are mentioned here in short. In the fuel cycle the production of iodine at uranium fission is high. On the other hand the iodine is one of the most fed fission products from the point of view of stress-corrosion-cracking of Nuclear Energy in Central Europe '98

7 the zirconium cladding tubes. Besides it performs a problem for the waste storage ( 129 I, x I/2 ^ yr). Therefore the implantation profiles of iodine into Zr were studied ad well as its diffusion in Zr during annealing. The formation of ZrO 2 were studied also. The RBS method with 3 Mev He has been found as very useful for the analysis [4]. At the JET development the plasma/wall interactions and plasma boundary phenomena are very important. For the study of impurities in plasma the surface/collector probes are introduced in the plasma and exposed to discharges. The probes are subsequently analyzed in-situ by various surface analytical techniques : the IBA ones as RBS, ERDA, NRA, and PIXE, and others, as Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS), respectively. The IBA analysis are performed with the 1.6 MV NEC tandem accelerator installed at the tokamak and dedicated just for the analysis of the collector probes. The concentration profiles of H, D, T, C, O, Be, and several metals are determined this way [5], The ions from the tandem accelerator are very practical for the collaboration of the track detectors also. Such a work was initiated recently by the group from the JSI Reactor Centre. 5. Conclusions The review of the possibilities offered by the methods at the tandem accelerator is presented. Besides the basic research in atomic physics many interdisciplinary problems can be studied successfully. The Ion Beam Analysis (IBA) methods are very suitable for elemental concentrations and their concentration profiles determination. The use of microbeam broadens the possibilities still further. The opportunity to have a modern accelerator in-house represent an excellent basis for education of undergraduate&graduate students. Also it can serve for more general public information about the usefulness of the low energy accelerators for several aspects connected with modern life. References [1] Proceedings of the 13 th International Conference on Ion Beam Analysis, Lisbon, Portugal, 1997, eds. M.F. da Silva, J.C. Soares, M.Breese, NIM B (1998) [2] Proceedings of the 15 lh International Conference on Nuclear Microprobe Technology and Applications, Santa Fe, NM, USA, 1996, eds. B.L.Doyle, C.IMaggiore, G.Bench, NTM B130 (1997) [3] Proceedings of the 14 th International Conference on Application of Accelerators in Research and Industry, Denton, TX, USA, 1996, eds. J.L.Duggan, I.L.Morgan, ATP Conference Proceedings 392, Woodbury, New York [4] N.Chevarier, et.al., NIM B (1998) 784 [5] J.C.B.Simpson, et.al., NIM B40-41 (1989) 842 h'uclear Energy in Centred Europe '98 65