Experimental energy straggling of protons inSiO2

Abstract

The energy straggling of proton beams in SiO2 has been measured in the energy range from 30 to 1500 keV using the transmission, nuclear reaction analysis and Rutherford backscattering techniques. The experimental results are compared with theoretical models. We observe that at energies around 200 keV the values obtained are larger than theoretical estimations. The straggling effect produced by the electron bunching in molecular media was calculated and it was found to be a possible cause of these differences at intermediate energies. The slowing down of fast light ions in matter is mainly due to interactions with the target material electrons. Due to the different energy transfers involved, and the dispersion in the number of interactions, the energy loss experienced by an ion-beam traversing matter is a statistical process which leads to an energy straggling in the case of pure elements as well as compound materials. The energy broadening of the ion beams arising from this phenomenon has technological implications such as the spatial dispersion of implantation profiles. Usually the energy straggling is characterized by the mean square deviation V 2 of the energy distributions. At low energies only the valence electrons participate in the energy loss and straggling while at higher ion velocities the atomic core electrons also contribute @1,2#. This means that when considering compound materials the energy strag- gling is affected by chemical bounds at low energies. Due to the fact that the straggling V 2 is essentially determined by the processes of larger energy transfer, at higher energies the chemical bound effect plays a minor role due to the increas- ing participation of atomic core electrons. In this case a first- order approximation to V 2 can be calculated by adding the V j 2 values of the composing elements considering the sto-