Reducing the occupancies in the calorimeter endcaps of the CLIC detector

1. Reducing the occupancies in the calorimeter endcaps of the CLIC detector Suzanne van Dam Supervisor: André Sailer CERN, 6 March 2014

2. Introduction • Beam-beam interactions • Background incoherent pairs • Scatter in forward region of CLIC detector • High occupancy in HCal CERN-THESIS-2012-223

3. Occupancy reduction • The high occupancy has to be reduced • Support tube can provide shielding • Optimize support tube material and thickness

4. Simulation of occupancy • Simulate background for each geometry • Estimate the occupancy • Need data from a few bunch trains (312 BX/train) • Find number of particles passing through support tube • Correlated to occupancy • Need data from ~10 BXs • Geometrical adaptations to the detector model: • Introduce a scoring plane around support tube • Make support tube geometry variable

5. Simulation of occupancy • Simulate background for each geometry • Estimate the occupancy • Need data from a few bunch trains (312 BX/train) • Find number of particles passing through support tube • Correlated to occupancy • Need data from ~10 BXs • Geometrical adaptations to the detector model: • Introduced a scoring plane around support tube • Made support tube geometry variable through text file

6. Contributions to occupancy Energy deposits in HCalendcap • Occupancy per particle type: • Photons and neutrons contribute • Compare to number of hits from different particles in the scoring plane: • Photons have a relatively large impact • Reflect this in the relation between hits in the scoring plane and the occupancy Hits in scoring plane

7. Figure of merit • To minimize the occupancy, minimize neutron (n) and photon (γ) hits (H) with a relative weight (w) • Assume linear dependence on each particle type • This can be expressed in a figure of merit (FOM): • Weights follow from the ratio of: • Number of energy deposits above threshold and within timing cut in the HCalendcap (N); • Number of hits in the scoring plane (H).

8. Energy • Energy spectrum for hits in the scoring plane • HCalendcap threshold is 300 keV

9. Support tube material • Iron photons neutrons

10. Support tube material • Iron based: • Iron • Stainless steel • Cast iron • Borated steel photons neutrons

11. Support tube material • Iron based: • Iron • Stainless steel • Cast iron • Borated steel • Neutron moderating and absorbing: • Pure polyethylene (PE) • PE + Li2CO3 • PE + H3BO3 photons neutrons

12. Support tube material • Iron based: • Iron • Stainless steel • Cast iron • Borated steel • Neutron moderating and absorbing: • Pure polyethylene (PE) • PE + Li2CO3 • PE + H3BO3 • Short radiation length: • Tungsten • Lead photons neutrons

13. Combine materials • Polyethylene for neutron shielding • Iron-based materials for photon shielding • Tungsten for further photon shielding • To shield both photons and neutrons, materials should be combined.

14. Combine materials • Polyethylene & stainless steel Total thickness of support tube 100 mm

15. Combine materials • Polyethylene & stainless steel • Tungsten & stainless steel Total thickness of support tube 100 mm

16. Summary and conclusions • The high occupancy due to incoherent pairs in the HCalEndcap is caused by neutrons and photons • Photons have relatively more impact on the occupancy • Minimization of the occupancy is based on minimizing the number of particles passing the support tube • Therefore a figure of merit is defined that reflects the higher impact of photons: • Simulations show that • Tungsten is suitable for photon shielding • Polyethylene is suitable for neutron shielding • To shield both neutrons and photons materials should be combined • A high contribution from photon shielding materials is needed

17. Outlook • Maximize shielding by reducing inner radius of support tube • Use as much tungsten as structural strength allows • For neutron shielding add polyethylene to a structure of tungsten and stainless steel