Signal height in silicon pixel detectors irradiated with pions and protons

TL;DR

This study evaluates the radiation hardness of silicon pixel detectors irradiated with pions and protons, analyzing their signal response at high fluences relevant for LHC upgrades.

Contribution

It provides experimental data on the performance limits of CMS pixel sensors under extreme radiation conditions, extending knowledge beyond current LHC fluence levels.

Findings

Signal height decreases with increasing fluence.

Sensors maintain functionality up to high fluences.

Bias dependence of signal response characterized.

Abstract

Pixel detectors are used in the innermost part of multi purpose experiments at the Large Hadron Collider (LHC) and are therefore exposed to the highest fluences of ionising radiation, which in this part of the detectors consists mainly of charged pions. The radiation hardness of the detectors has thoroughly been tested up to the fluences expected at the LHC. In case of an LHC upgrade the fluence will be much higher and it is not yet clear up to which radii the present pixel technology can be used. In order to establish such a limit, pixel sensors of the size of one CMS pixel readout chip (PSI46V2.1) have been bump bonded and irradiated with positive pions up to 6E14 Neq/cm^2 at PSI and with protons up to 5E15 Neq/cm^2. The sensors were taken from production wafers of the CMS barrel pixel detector. They use n-type DOFZ material with a resistance of about 3.7kOhm cm and an n-side read out. As the performance of silicon sensors is limited by trapping, the response to a Sr-90 source was investigated. The highly energetic beta-particles represent a good approximation to minimum ionising particles. The bias dependence of the signal for a wide range of fluences will be presented.