Charge collection and charge sharing in heavily irradiated n-side read-out silicon microstrip detectors

Abstract

Hadron radiation damage of n-bulk silicon detectors leads to a change in the effective space charge from positive to negative. This effect is called type inversion. Type inversion occurs after exposure to a fluence of a few 10 13 proton cm −2 and is characterized by migration of the diode junction to the n + implanted side (n-side) of the detector. After inversion the charge collection efficiency (CCE) of segmented detectors at low voltage is higher when the n-side, rather than the p-side, is read out. A p-side read out (p–in–n) and an n-side read out (n–in–n) strip detector with identical strip geometry and a wafer thickness of 200 μm were simultaneously and inhomogenously irradiated to a maximum fluence of 7×10 14 proton cm −2 with 24 GeV c −1 protons. A comparison of the CCE at very high irradiation doses is shown with these two read out geometries. The inhomogeneous irradiation induces an inhomogenous distribution of the effective space charge with a transverse component of the electric field that could in principle affect the resolution properties of the microstrip detector. The inter-strip charge sharing properties, as a function of dose, for the n–in–n detector have been measured. No systematic distortion of the reconstructed cluster position was detected within the limits of the measurement accuracy. The detectors were manufactured using oxygen enriched silicon substrates to limit the degradation of the full depletion voltage under charged hadron irradiation. The measured CCE confirms that the use of oxygenated n–in–n detectors is viable up to fluences of 7×10 14 proton cm −2.