Forward Recoil Elastic Spectrometry (FRES)

FRES is a technique used to non-destructively measure depth concentrations of hydrogen or deuterium in solids.

An energetic beam of alpha particles impinges on a sample whose normal is tilted 75 degrees away from the incident ion beam. Hydrogen or deuterium is scattered in the forward direction from a depth ranging from 0 to 5000 Å. Both the incident ion and the recoiled particle will experience an energy loss which will yield the depth of the scattering event. A detector is placed at 150 degrees relative to the incident ion beam. Particals scattered in the forward direction will include hydrogen (and/or deuterium) but will generally be dominated in quantity by the scattered incident ions. An appropriately chosen stopper foil is placed in front of the detector to stop the scattered incident particles but allow the recoiled sample particles to pass through to be detected. Slits are placed in front of the detector to accurately define the scattering angle.

FRES will provide useful results in the top 5000 Å of a sample with a limit of detection of about 0.1 atomic percent. The best depth resolution that one can expect is considered to be about 100Å.

In the example below, we used FRES to determine the diffusion coefficients of deuterated polybutadiene in polystyrene at elevated temperatures. Sections of the sample illustrated in the schematic were held at specific temeratures for 5 minutes. Differences in diffusion rates can be see in the spectra below by taking note of the trailing edge of the deuterium peak. As expected the sample held at the higher temperature shows an increase in the rate of diffusion. The experiment was performed by professor P.F Nealey of the University of Wisconson while he was a graduate student in the chemical engineering department at MIT. (See Nealey, P.F., Cohen, R.E., and Argon, A.S. Macromolecules 1993, 26, 1287-1292)

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