Dependence of the silicon detector response to heavy ions on the direction of incidence: Computer simulations versus experimental data

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The pulse height spectra generated by the irradiation of a silicon detector with heavy ions show a striking dependence on the orientation of incident ions with respect to the crystal axes of the detector. If the direction of incidence is parallel to low-index axial channels, pulse height spectra with very sharp peaks are obtained. In this case, the energy resolution of the detector can be further improved if the thickness of the dead layer is minimized by introducing a high-field region. These effects are demonstrated for 2.813 MeV oxygen ions at [1 1 1] axial channeling, {1 1 0} planar channeling and `random' incidence. If the beam is parallel to the [1 1 1] axial channel, an energy resolution of better than 1% is obtained. The observed phenomena are explained by the dependence of the ratio between electronic and nuclear energy loss mechanisms on the incidence direction. Binary collision computer simulations by the program Crystal-TRIM yield a quantitative understanding of the orientation dependence and the influence of the dead layer. The calculated pulse height spectra show a good agreement with the measurements.