ENERGY RESOLUTION OF SILICON DETECTORS - APPROACHING THE PHYSICAL LIMIT

This work gives a review about the response of silicon detectors to electrons, protons, deuterons and alpha particles. The detector nonlinearity (pulse height as a function of particle energy) and the shape of the response function (pulse height distribution for monoenergetic particles) are discussed in detail. For electrons, the measured nonlinearity is found to be negligible within the experimental uncertainties. The response function of a particle implanted and passivated silicon detector (PIPS) for electrons (200 to 1000 keV) is measured with thin radionuclide sources of conversion electrons. From these measurements an accurate value of the Fano factor is derived. For light ions (protons, deuterons and helium ions), measurements are compared to calculations using a detector model, which assumes a thin dead layer at the front contact, followed by a fully sensitive region. This model takes into account electronic energy loss straggling in the dead layer, thickness variations of the dead layer, multiple scattering and nonelectronic losses in the sensitive region, electron-hole pair statistics, a particle dependence of the energy per electron-hole pair and electronic resolution. For H and He ions, at low energies (40 to 700 keV) the nonlinearity is mainly due to the finite thickness of the dead layer, and in the MeV range the particle dependence of the energy per electron-hole pair contributes considerably, in accordance with the model of Lennard. Measurements of the response function for monoenergetic He ions yield a detector resolution of 8 keV (FWHM) in the energy range 3 to 6 MeV. The well known asymmetric shape of the response function is found for both, H and He ions, at all energies. Using our detector model, calculated spectra agree quantitatively with the corresponding measurements both in the width and in the shape, without any adjustable parameter in the calculations. Based on this detector model, the ultimate resolution which can be obtained with silicon detectors is discussed.