Reducing the occupancies in the calorimeter endcaps of the CLIC detector Suzanne van DamSupervisor: André Sailer

CERN, 6 March 2014

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Introduction• Beam-beam interactions• Background incoherent pairs• Scatter in forward region of CLIC detector • High occupancy in HCal

Suzanne van Dam, 6 March 2014

CERN-THESIS-2012-223

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Occupancy reduction

Suzanne van Dam, 6 March 2014

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

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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

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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

Suzanne van Dam, 6 March 2014

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Contributions to occupancy • 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

Suzanne van Dam, 6 March 2014

Energy deposits in HCal endcap

Hits in scoring plane

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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 HCal endcap (N);– Number of hits in the scoring plane (H).

Suzanne van Dam, 6 March 2014

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Energy• Energy spectrum for hits in the scoring plane• HCal endcap threshold is 300 keV

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Support tube material• Iron

Suzanne van Dam, 6 March 2014

photons

neutrons

• Iron based:– Iron– Stainless steel– Cast iron– Borated steel

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Support tube materialphotons

neutrons

• Iron based:– Iron– Stainless steel– Cast iron– Borated steel

• Neutron moderating and absorbing:– Pure polyethylene (PE)– PE + Li2CO3– PE + H3BO3

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Support tube materialphotons

neutrons

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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

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.

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Combine materials• Polyethylene & stainless steel

Suzanne van Dam, 6 March 2014 11Total thickness of support tube 100 mm

Combine materials• Polyethylene & stainless steel

• Tungsten & stainless steel

Suzanne van Dam, 6 March 2014 11Total thickness of support tube 100 mm

Summary and conclusions• The high occupancy due to incoherent pairs in the HCal Endcap 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

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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

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