  Hybrid silicon pixel detectors are currently used in the innermost tracking
system of the Compact Muon Solenoid (CMS) experiment. Radiation tolerance up to
fluences expected for a few years of running of the Large Hadron Collider (LHC)
has already been proved, although some degradation of the part of the silicon
detector closer to the interaction point is expected. During the LHC upgrade
phases, the level of dose foreseen for the silicon pixel detector will be much
higher. To face this aspect, dedicated irradiation tests with fluences above
$\mathcal{O}(10^{15})$ n$_{\textrm{eq}}$/cm$^2$ have been performed on the
silicon sensor and readout chip. Changes in the operation of the sensor and
readout chip as a function of the fluence are presented. The charge collection
efficiency has been studied: partial recovery of the detector efficiency can be
achieved by operating the detectors in a controlled environment and at higher
bias voltage.
