Charge collection efficiency and resolution of an irradiated double-sided silicon microstrip detector operated at cryogenic temperatures

Abstract

This paper presents results on the measurement of the cluster shapes, resolution and charge collection efficiency of a double sided silicon microstrip detector after irradiation with 24 GeV protons to a fluence of 3.5 × 10 14 p/cm 2 and operated at cryogenic temperatures. An empirical model is presented which describes the expected cluster shapes as a function of depletion depth, and is shown to agree with the data. It is observed that the clusters on the p-side broaden if the detector is under-depleted, leading to a degradation of resolution and efficiency. The model is used to make predictions for detector types envisaged for the LHC experiments. The results also show that at cryogenic temperature the charge collection efficiency varies depending on the operating conditions of the detector and can reach values of 100% at unexpectedly low bias voltage. By analysing the cluster shapes it is shown that these variations are due to changes in depletion depth. This phenomenon, known as the "Lazarus effect", can be related to similar recent observations on diode behaviour.2

Status of the LHCb VErtex LOcator

“…The highly non-uniform nature of the irradiation also poses a challenge, as the detector has to withstand the high bias voltages needed for the irradiated part in the region of the detector which is not yet inverted. Prototyping for the VELO has led to the choice of n-on-n technology [8,9,7]. Future areas of research within the VELO group include Czochralski silicon, for which test beam data from a high resistivity strip detector have been taken and are under analysis, p-bulk silicon [10], and thin detectors.…”

Evaluation of fine pitch p-onp and n-on-n irradiated silicon microstrip sensors for the LHCb experiment

“…If the detector is underdepleted, as in the lower schematic of Figure 5, there is an underdepleted layer next to the p + implants which acts as an insulator when AC-coupled fast electronics are used [8]. Charge from a traversing particle will only drift in the depleted region, and a mirror charge will be induced in the aluminium readout lines of several nearby strips, defocusing the cluster [9]. Furthermore, in the outer region of the p-on-n prototype detectors charge may also be induced in the routing lines in the second metal layer.…”

“…Furthermore, in the outer region of the p-on-n prototype detectors charge may also be induced in the routing lines in the second metal layer. These problems are not expected to occur for an underdepleted n-on-n sensor [9], where the underdepleted region is on the opposite side of the detector to the segmented n + implants.…”