Background in the CMS Drift Tubes and detector ageing monitoring: Run 3 to Run 2 comparison

Continuous high levels of radiation in the CMS Drift Tubs can be the cause of early detector ageing. For this reason, in view of High Luminosity LHC, several measures have been taken to protect the detector and keep the background under control. The last and most effective intervention was the installation, during Long Shut down 2, of dedicated shields over the top external DT chambers, aiming at reducing the rates of thermal neutrons coming from the empty space above the detector. This note presents a comparison of rates measured before and after such intervention, along with results of monitoring detector efficiency to spot any evidence of ageing.