NEC Pelletron Accelerator
The NEC Pelletron accelerator can accelerate H and He ions to 2 MeV for He+ and 3 MeV for He2+. There is a single beam line for RBS and ERDA experiments.
For Rutherford Backscattering Spectrometry (RBS):
Rutherford backscattering spectrometry can provide information about a material’s composition as a function of depth, as well as its crystallinity and degree of disorder. Different information can be obtained depending on the gas species used to form the ion beam as well as the incident energy of those ions.
The most common usage of RBS at MRL employs a He+ beam to determine the composition of a material. A majority of the incident He+ ions implant into the target sample, but a small fraction undergo a direct collision with a nucleus of an atom inside of the sample. These ions backscatter with a given energy, based on the element of the atom that was collided with as well as its depth in the sample. The He+ ions also lose energy traveling through the solid target, and therefore useful information can only be gathered up to a depth of about one micron below the surface of the sample. For this reason, this technique is most useful for thin films and membrane specimens. Software is available that allows the user to fit the collected data so that thin film/membrane thicknesses as well as compositions can be determined.
The RBS chamber at MRL is equipped with apertures 1, 2, and 3 mm in diameter. We can only load samples that are 4 inches or smaller in diameter. Once inside, we have about 3 inches of travel on the holder. There is a goniometer for channeling studies as well as sample rotation to prevent channeling.
For Elastic Recoil Detection Analysis (ERDA):
Elastic recoil detection analysis is a second technique that can be performed on the same beam line as RBS. This technique is similar to RBS, but instead of analyzing the backscattering of particles from the ion beam, ionized atoms from the sample itself are recoiled toward a detector. Only elements with a lower Z than that of the incident ion beam can be analyzed, so with a He+ beam, only H can be detected. As with RBS, the accessible depth range is about one micron.