On the weighting field and admittance of irradiated Si-sensors

Abstract

In this paper the weighting field $E_W$ and the frequency dependence of the admittance Y of n${}^{+}$p pad sensors irradiated by 24 GeV/c protons to equivalent fluences in the range $\hat{I}{¦}_{\mathrm{eq}}=3$ to $13\times 10^{15}$ cm${}^{-2}$ are investigated. 1-D TCad simulations are used to calculate $E_W$. For $\hat{I}{¦}_{\mathrm{eq}}<10^{13}$ cm${}^{-2}$ $E_W$ depends on position and time. However, for higher $\hat{I}{¦}_{\mathrm{eq}}$ the time constant $\ddot{I}„$ is much longer than the typical electronics readout time and $E_W=1/d$ ($d$ = sensor thickness). It is demonstrated that the increase of the resistivity of the Si bulk with irradiation is responsible for the increase of $\ddot{I}„$. The admittance Y of irradiated pad sensors has been measured for frequencies between f = 100 Hz and 1 MHz and voltages between 1 and 1000 V at -20${}^{\circ }$C and -30${}^{\circ }$C. For f < 1 kHz the parallel capacitance C${}_p$ shows a f dependence. A model with a position-dependent resistivity is able to describe the data. It is concluded: 1. The weighting field of a highly irradiated sensor is the same as the weighting field of a fully depleted sensor before irradiation. 2. Models with a position-dependent resistivity describe the frequency dependence of C${}_p$ for irradiated sensors.

6 pages, 10 figures, IEEE Nuclear Science Symposium 2019