1. Occupancy tolerances for the TPC
and background issues
Paul Colas

2. The background in the TPC and its consequences
The beam background and the physics events produce ionization in the TPC.
Two consequences:
Occupancy
Space charge -> distortions (ExB). (Charge of 110 BX integrated)
Occupancy up to, say, 5% can be dealt with (to be refined, quantified)
Note that the anti-DID, which helps lowering the background, might also introduce ExB distortions in the TPC.
P. Colas

3. Occupancy in the TPC
3 108 voxels From phys. processes : dominated by gg → minijets (0.3% on the lowest radius padrow) But beam backgrounds are of the same order (pairs from beamstrahlung, photons and neutrons backscatters from collimators and quadrupoles, muons from the beam halo. The hit distribution is peaked at low radius (T. Krontscheid PhD, Bonn 2015)
P. Colas

4. OCCUPANCY at CLIC
CLIC LCD simulation
(A. Sailer, 2011)
P. Colas

5. Guinea Pig (pair background) Geant 4
Simulations
Guinea Pig (pair background) Geant 4
P. Colas

6. Other backgrounds
Neutrons hit nuclei (mainly protons) in the TPC gas, which ionize Photons > 1 MeV make low-mass e+e- pairs which curl along B (horizontal lines in the detector) Lower energy photons ionize and make Compton (‘snow’ in the detector)
P. Colas

7. DID field enters the simulation
P. Colas

8. TO BE DONE
All these ‘Background in the TPC’ calculations date back to 2008 (Adrian Vogel’s PhD thesis, DESY, Thorsten Krautscheid’s Diploma thesis, Bonn). This was LCD time. Should be repeated with the latest baseline layout (new L*, ILD,…) and the (anti-)DID field optimized for luminosity vx BG. Also neutron absorbers, collimator geometry, etc… need to be re-optimized.
P. Colas