  The CMS drift tubes (DT) muon detector, built for withstanding the LHC
expected integrated and instantaneous luminosities, will be used also in the
High Luminosity LHC (HL-LHC) at a 5 times larger instantaneous luminosity and,
consequently, much higher levels of radiation, reaching about 10 times the LHC
integrated luminosity. Initial irradiation tests of a spare DT chamber at the
CERN gamma irradiation facility (GIF++), at large ($\sim$O(100)) acceleration
factor, showed ageing effects resulting in a degradation of the DT cell
performance. However, full CMS simulations have shown almost no impact in the
muon reconstruction efficiency over the full barrel acceptance and for the full
integrated luminosity. A second spare DT chamber was moved inside the GIF++
bunker in October 2017. The chamber was being irradiated at lower acceleration
factors, and only 2 out of the 12 layers of the chamber were switched at
working voltage when the radioactive source was active, being the other layers
in standby. In this way the other non-aged layers are used as reference and as
a precise and unbiased telescope of muon tracks for the efficiency computation
of the aged layers of the chamber, when set at working voltage for
measurements. An integrated dose equivalent to two times the expected
integrated luminosity of the HL-LHC run has been absorbed by this second spare
DT chamber and the final impact on the muon reconstruction efficiency is under
study. Direct inspection of some extracted aged anode wires presented a melted
resistive deposition of materials. Investigation on the outgassing of cell
materials and of the gas components used at the GIF++ are underway. Strategies
to mitigate the ageing effects are also being developed. From the long
irradiation measurements of the second spare DT chamber, the effects of
radiation in the performance of the DTs expected during the HL-LHC run will be
presented.
