Proton Elastic Scattering (PESA)
Physical Principles: The technique is similar to RBS but instead of looking at backward angles like in RBS, the scattered incident ions are measured in forward angles. As the mass separation is rather low in forward angles, only low Z elements like hydrogen can be measured. In this way PESA is ideally suited for measurement of hydrogen concentration in thin samples like for example aerosols. An example of PESA spectrum of a thin aerosol sample deposited on Teflon filter is shown in Figure 1.
Figure 1 PESA spectrum of elastically scattered protons in the forward direction (scattering angle 30o) from the thin Teflon filter loaded with ambient aerosol. The broad peak is due to forward scattered protons from hydrogen in the aerosol sample and the right-hand peak due to scattering from all other elements, primarily carbon and fluorine.
In IBA laboratory PESA spectra are acquired at 30o which provides well resolved H peak. Calibration is performed by using thin Mylar sheets (35 micrograms/cm2) which has well known hydrogen concentration. The background subtracted peak area was found to be proportional to the hydrogen concentration over the concentration range 0-200 micrograms/cm2. Typically minimum detection limits of the order of 0.2 micrograms/cm2 were obtained for 3 mC runs corresponding to approximately 1 ng/m3 of hydrogen in the air.