1. Excess Radiation Damage of CMS Endcap Hadron Calorimeter and “Finger” Structure of Tiles as a Possible Method of Radiation Hardness Increasing I.Emeliantchik, CMS RDMS Collaboration National Center of Particle and High Energy Physics Belorussian State University International School-Seminar The Actual Problems of Microworld Physics Belarus, Gomel July 26 - August 7, 2015

2. HCAL Radiation Hardness, Predicted and Real CMS HCAL TDR prediction CMS experimental result 2012 (laser data) Attempts to reproduce plastic behavior at different irradiation conditions

3. Goals of Our Radiation Research Two goals were set: to explain the contradiction between expected and real radiation damage of plastic scintillator; to propose a method to improve radiation hardness In current state the calorimeter will not even withstand the expected term of operation with current luminosity, not speaking about High Luminosity

4. Importance of Dose Rate E. Biagtan, E. Goldberg et al., "Effect of gamma radiation dose rate on the light output of commercial polymer scintillators", NIM B93 (1994) p. 296-301 Dose rate influence was researched earlier, but that wasn’t taken into consideration during HCAL developing

5. Production of Tiles Tiles of SCSN-81 Milling cutters for shifter grooves Most radiation loaded tiles (sigma-tile #29) were produced using the same design documentation as in current calorimeter

6. Irradiation of tiles Tiles irradiation was done at gamma-installation UGU-240 of the Institute of Nuclear Energy Research – Sosny of National Academy of Science of Belarus. The installation is a radioactive source 60 Co with activity 110 kCi, which can be submerged in water for the time of sample setting. The operative zone is a room 4  4 m 2, in which one can walk freely (while the source is submerged), selecting a position with proper dose rate. We have chosen positions with dose rates close to real ones at working calorimeter. The light output was measured at JINR.

7. Explanation of excess degradation of plastic scintillator Extrapolation of the obtained results to the dose rate of the first stage of calorimeter work (30 fbarn -1 ) gives light output decreasing comparable with observed one. Graph drawing in semi-logarithmic frame of reference allows linear fitting

8. Radiation Hardness Improvement with Help of Additional Segmentation of Radiation Loaded Tiles Existing segmentationProposed segmentation Radiation hardness improves due to decrease of the distance of light passing through the dim plastic Number of photosensors is increasing, but SiPM (which will be implemented anyway) are very miniature, so it is not a problem

9. Production and Irradiation of Tile Segments Tile segments were made and irradiated with doses up to 25 Mrad. Measurements had shown that the calorimeter in this configuration will withstand the whole term of operation in the mode of High Luminocity (3000 fbarn -1 )

10. Current Version of Endcap Calorimeter for High Luminosity Silicon ECAL Endcap Silicon Forward HCAL EndcapPlastic Backward HCAL Endcap

11. Conclusion Excess plastic scintillator degradation is explained New method of calorimeter radiation hardness improvement is proposed