Charge collection properties of prototype sensors for the LHCb VELO upgrade

TL;DR

This paper evaluates the charge collection performance of prototype sensors for the LHCb VELO upgrade, examining effects of irradiation and bias voltage, and demonstrating their ability to meet charge requirements post-irradiation.

Contribution

It provides a comprehensive testing campaign of sensor prototypes under irradiation, revealing charge multiplication phenomena and differences in guard ring regions, advancing sensor development for high-radiation environments.

Findings

Sensors collected sufficient charge after high fluence exposure.

Charge multiplication observed in some irradiated sensors.

Significant differences found in guard ring charge collection.

Abstract

An extensive sensor testing campaign is presented, dedicated to measuring the charge collection properties of prototype candidates for the Vertex Locator (VELO) detector for the upgraded LHCb experiment. The charge collection is measured with sensors exposed to fluences of up to $8 \times 10^{15}~1~\mathrm{\,Me\kern -0.1em V}~ \mathrm{ \,n_{eq}}~{\mathrm{ \,cm}}^{-2}$, as well as with nonirradiated prototypes. The results are discussed, including the influence of different levels of irradiation and bias voltage on the charge collection properties. Charge multiplication is observed on some sensors that were nonuniformly irradiated with 24 GeV protons, to the highest fluence levels. An analysis of the charge collection near the guard ring region is also presented, revealing significant differences between the sensor prototypes. All tested sensor variants succeed in collecting the minimum required charge of 6000 electrons after the exposure to the maximum fluence.