Radioactive Source Calibration of the CMS Hadron Endcap Calorimeters with Phase I Upgrade Electronics

The Phase I Upgrade of the CMS Hadron Endcap Calorimeters consists of new photodetectors and front-end electronics. The upgrade was installed successfully during the Year End Technical Stop of 2017-2018. As the first validation tests of the installation of the upgrade and in order to obtain the first calibration with the upgraded electronics and higher longitudinal segmentation, the responses of the Hadron Endcap Calorimeters were calibrated with a $^{60}$Co radioactive source. Here we describe the details of the source calibration operations.