Ion Accelerator, Beam Lines, and End Stations
- Two ion sources
- 3.0-MV electrostatic tandem ion accelerator
- Injector and analyzing magnets
- Beam lines
- End station for materials modification and analysis
EMSL's ion accelerator system (Figure 1) is equipped with two ion sources, a 3.0-MV electrostatic tandem ion accelerator, injector and analyzing magnets, beam lines, and end stations that are used for materials modification and analysis.
The resources at the Ion Beam Materials Analysis Laboratory are most effectively used for collaborative research, where access to unique combinations of characterization tools, computational facilities, and ion beam capabilities, as well as interaction with experienced staff members to develop customized research strategies, can be advantageously used to address scientific and technological challenges. Some of the research areas to which these capabilities are applied include thin film analysis, buried interface analysis, radiation effects in solids, ion beam synthesis of nanostructures, atmospheric aerosol characterization, and theoretical modeling using atomistic simulations.
Ion Sources and Injector Optics
EMSL's radio frequency (Alphatross) plasma source is dedicated to producing ions from gases, especially helium. Ions from most other elements are produced from solid sources using Cs+ sputtering in the SNICS II (Source of Negative Ions by Cesium Sputtering) ion source. Negative ions from either source can be injected into the accelerator by an injector magnet at the system's low-energy beam line. The low-energy beam line is equipped with several other components that include an electrostatic x-y steerer to steer the beam, a beam profile monitor to measure the profile of the ion beam, a Faraday cup for current measurements, and an einzel lens for focusing.
The National Electrostatics Corporation (NEC) model 9SDH-2, 3.0-MV tandem electrostatic accelerator (Figure 2) is equipped with two Pelletron charging chains capable of carrying 300 mA of charging current to the terminal. Since the tandem accelerator provides two stages of acceleration (i.e., negative ion acceleration from the source end to the terminal in the middle and positive ion acceleration from the middle to the high-energy end), the final energy depends on the charge state of the ion. The accelerated ions can be focused through the high-energy beam line using a magnetic quadrupole and a y-axis electrostatic steerer, which are attached to the high-energy beam line. The analyzing (switching) magnet is equipped with seven ports at ±45°, ±30°, ±15°, and 0° with respect to the accelerator. Three beam lines, which are currently located at the +30°, +15°, and -15° ports of the analyzing magnet, along with corresponding end stations, have been established for materials modification and analysis.
The following routine operating envelope defines limitations for routine operations. Experiments outside this envelope must be reviewed and approved.
|Beam||Faraday Cup Beam Current (Particle μA)||Terminal Voltage (MV)||Acceptable Target Materials|
|Deuteron||0.15||0.75||All except deuterium, lithium, or beryllium|
|Alpha||0.05||3.0||Only atomic number ≤ 13|
|Alpha||1.0||3.0||Only atomic number > 13|
|6Lithium||0.10||3.0||All except beryllium|
|Carbon - Fluorine||1.0||3.0||Any|
|Any nonradioactive element with atomic number ≥ 10||10.0||3.0||Any|
The +30° beam line is equipped with a single slit followed by a matched set of two slits to control the divergence of the ion beam, a quadrupole magnet at the focal position of the switching magnet to enhance the focusing capability of the beam line, x and y electromagnetic steerers, a Faraday cup for current measurement, and a beam profile monitor. Differential pumping is included in the beam line and apertures so that the end station can be kept in the low 10-10 Torr pressure range. The +15° beam line is dedicated to performing ion implantation; it is equipped with an NEC raster scanner unit. The -15° beam line is dedicated to carrying out routine analytical work.
Turbo pumps are attached to all beam lines and ion sources. Typical base pressure in the low-energy beam line is 1×10-8 Torr to 2×10-8 Torr, and the mid-to-high 10-9 Torr range in the high-energy beam lines. When the ion accelerator is in operation, typical pressures in the high-energy beam lines are in the low-to-mid 10-8 Torr range.
UHV End Station
The ultrahigh (UHV) end station (Figure 3), attached to the +30° beam line, is equipped with several UHV surface science capabilities in addition to ion beam techniques at two levels. The first level is equipped with a low-energy electron diffraction spectrometer, a cylindrical mirror analyzer for Auger electron spectroscopy, a hemispherical analyzer and X-ray source for X-ray photoelectron spectroscopy, an oxygen plasma source, sputter cleaning capabilities, and thin film deposition capabilities and ports for sample transfer to and from externally mounted vacuum chambers. Ion-scattering measurements are performed in the second level using one of four surface barrier detectors:
- A fixed-position detector at a scattering angle of 150° for routine Rutherford back scattering (RBS) spectrometry and channeling measurements
- A fixed-position surface barrier detector at a 30° scattering angle for elastic recoil detection analysis (ERDA) measurements
- A fixed-position surface barrier detector at 107° for grazing channeling measurements
- A fixed-position detector at 135° with foil to measure protons and alpha particles during nuclear reaction analysis (NRA) measurements.
In addition, the second level is equipped with a bismuth germanate detector to measure gamma rays during NRA experiments. The sample manipulator has three axes of rotations (polar, azimuth, and tilt), three axes of translation (x, y, and z), and is interfaced with EMSL's sample transfer capability. The manipulator can heat the samples to 1300 K or cool them to 130 K. Upon request, sample cooling of 60 K can be obtained by liquid helium cooling. Several leak valves, connected to many different gas bottles, are attached to the chamber. The UHV end station is primarily used for buried interface analysis, damage/defects quantification, and crystalline quality investigation using RBS/channeling techniques, and light element (including hydrogen) quantification using ERDA and NRA in the materials.
Microbeam End Station
The +30° beam line extends through the channeling end station to the microbeam end station where experiments can be performed with a beam size of 20 microns or better. The microbeam end station is also equipped with capabilities for conventional ion beam techniques including RBS, NRA, and particle induced X-ray emission (PIXE). The sample manipulator capabilities are similar to those in the channeling end station manipulator (discussed below), except for the absence of azimuth and tilt rotations. The microbeam end station is primarily used for characterizing micro clusters under high-vacuum and UHV conditions.
Implantation and Channeling End Station
This end station (Figure 4), located at the +15° beam line, is mainly dedicated to ion implantation experiments. Ion-scattering measurements are performed in the second level using one of four surface barrier detectors:
- A fixed-position detector at a scattering angle of 150° for RBS spectrometry and channeling measurements
- A fixed-position surface barrier detector at a 30° scattering angle for ERDA measurements
- A movable surface barrier detector for RBS measurements
- A movable detector to measure protons and alpha particles during NRA measurements.
In addition, the second level is also equipped with a bismuth germanate detector to measure gamma rays during NRA experiments. The sample manipulator has three axes of rotations (polar, azimuth, and tilt), three axes of translation (x, y, and z), and is interfaced with EMSL's sample transfer capability. The manipulator has the capability for heating samples to 1300 K or cooling them to 130 K. Several leak valves, connected to many different gas bottles, are attached to the chamber. This end station is primarily used for buried interface analysis, damage/defects quantification, crystalline quality investigation using RBS/channeling techniques, high-energy ion implantations, and light element (including hydrogen) quantification using ERDA and NRA in the materials. Although ion implantation is carried out under high-vacuum conditions, UHV conditions can routinely be achieved by baking the chamber.
Analytical NEC End Station
Routine, rapid analytical work can be performed efficiently in EMSL's commercial (NEC) RC 43 end station (Figure 5), which is attached to the -15° beam line. This end station is equipped with most of the standard ion beam analytical capabilities, including RBS, NRA, PIXE, and ERDA. One fixed-position surface barrier detector with thin aluminum foil at 170° scattering angle (used for NRA measurements) and another surface barrier detector are mounted on a rotating platform inside the chamber. An X-ray detector with windows to reduce the background for PIXE measurements and a sodium-iodide detector for NRA measurements are attached to the chamber. A computer-controlled five-axis sample manipulator is attached through the bottom port of the chamber. A large number of samples of any size can be mounted on this sample holder. This end station is also configured to characterize atmospheric aerosols using PIXE, proton elastic scattering analysis (PESA), and scanning transmission ion microscopy (STIM). Aerosol samples collected on a 170-mm-long teflon strip can be mounted on this special sample holder for PIXE, PESA, and STIM measurements.
Individuals may use the end stations independently for their research, following the necessary training. The accelerator may also be used, but only following extensive training.
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