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Dan Peterson, Cornell University
Presented at the Workshop on Detector R&D 20101007
Development of a Low-Material TPC Endplate
for the ILD Experiment at the ILC
Background
Motivation
Physics goals of the International Large Detector (ILD) at the International Linear Collider (ILC) require
that the design of the TPC endplate simultaneously achieves rigidity and stability while minimizing the
material contribution upstream of the endcap calorimeter.
The requirement for rigidity and stability is driven by the momentum resolution. The unprecedented
requirement for charged particle momentum resolution at the ILC, σ(1/p) ~ 2x10-5/GeV, will require
spatial resolution in the ILD TPC on the order of 100μm. Obtaining this resolution will require precision
calibrations for positions of detector elements, the pad response, and the drift path in the electric and
magnetic fields. Inexact calibrations of some of the above quantities can result in similar track
distortions, compounding the task of unambiguously resolving the separate effects.
Further track distortions result from positive ion buildup in the drift volume and on the face of the
detector elements. These distortions are likely to vary on a time scale that is shorter than that required
for accurate calibration of the detector and drift, necessitating the resolution of the other effects.
Calibrations typically are initialized with physical measurements of the detector and fields then refined
with precision track-based measurements. However, because of the ambiguity of the cause of track
distortions, the calibration will require some precision information that is independent of track-based
calibrations. The ambiguity can be resolved if readout elements are measured to an accuracy of
Δ(x or y) < 50μm. Maintaining the position accuracy will require a structure that is rigid at the level of
the required accuracy, especially if power pulsing is used to reduce the heat source at the endplate.
The competing requirement for low scattering material in the TPC endplate is driven by the
requirements of the particle flow analysis. Recent simulations of the effect on reconstruction of jets in
the ILD endcaps show that there is negligible degradation in the observed jet energy resolution if the
endplate material is limited to about 25%X0. Current estimates of materials in the ILD endplate are:
detector elements and amplifiers, 5%; cooling, 2%; signal and power cables, 10%. Thus, there remains
about 8%X0 in the material budget for the rigid mechanical structure of the endplate.
Development of an advanced endplate that meets both the precision and low-material requirements is
in progress. Studies include CAD modeling and prototyping and include endplate structures at three
levels of development: the ILD endplate, a replacement endplate for the LCTPC LP1 prototype, and
small structures to study the mechanical details of construction techniques. A low-material construction
endplate will be constructed in the next year. Experience gained from the deployment of the advanced
LP1 endplate will provide input to the design of the ILD endplate, which will be included in the ILD
Detailed Baseline Design (to be completed in 2012).
A TPC, which offers a combination of good single point resolution and a continuous measurement
volume, has been selected by the ILD experiment (left) as the central tracking device. The ILC TPC
(right) being developed by the LCTPC collaboration has 3.4m diameter and 4.3m length. To achieve the
point resolution goal, the readout will use Micro-Pattern-Gas-Detector gas amplification. The endplate is
being designed to support the readout modules, and meet the precision and low-material goals.
Previous to the current work, the Cornell group designed and built the endplate system (main support
frame and mating module frames) for the LCTPC collaboration 0.77m diameter prototype TPC (“LP1”).
This prototype is operated at DESY to study MPGD readouts and calibration and alignment issues.
Designed as a small section of the full ILD endplate, the endplate supports seven modules, ~270mm x
170mm, arranged in arcs. Modules are supported by a framework in the main support frame and
intermediate brackets, providing simple access to maintain the modules. This endplate meets the
alignment requirements (Δ(x or y) < 50μm) employing a framework cross-section that improves rigidity
and iterative machining, with intermediate stress relief, applicable to a Mg-Si hardened aluminum alloy.
Short-term measurements of stability are difficult. However, the rigidity under load can set a scale for the
stability. Under a 2 millibar load (typical overpressure) the maximum deflection is ~30μm.
Investigating Possible Designs for the LP1 and ILD Endplates
1
2
3
4
While the current LP1 endplate (above) meets the precision position requirement and is expected to meet the
stability requirements for the ILD, it does not meet the material requirement of 8%X0. The material contribution is
16.9%X0. (This is the contribution from the main structure of the LP1 prototype endplate, averaging all material
including uninstrumented areas and the outer ring.) In preparation for the Detailed Baseline Design, Cornell and
LCTPC are developing an ILD endplate design that will meet precision, stability, and material requirements. A new
endplate for the LP1 prototype will be used as a testing device for advanced ILD designs.
Computer models of four designs of the LP1 endplate have been studied for material and strength. These are
shown at left and include (1) the current LP1 endplate, (2) a version with lightened outer ring and uninstrumented
areas, (3) a version with structural ribs replaced with carbon or Kevlar fiber, and (4) a spaceframe.
Each of these were studied with finite-element-analysis to measure the deflection under the gas overpressure (2
millibar). The figures (right) show details of the spaceframe model and the deflection.
As shown in the table, the material goal can be achieved with either lightened design or the spaceframe. However,
the spaceframe is predicted to provide significant stiffening. Furthermore, the spaceframe design can be more
readily scaled up to the ILD size because depth of the spaceframe can be increased without a significant increase
in material.
Injecting Some Reality
mass
kg
the current LP1
material
%X0
18.9
16.9
33
1.5
Lightened (all aluminum)
8.9
8.0
68
3.2
Lightened (Al-C hybrid)
7.4 (Al)
1.3 (C)
7.2
<168*
< 4.8*
8.4
7.5
Space-Frame
(* upper limits based on the Al frame without reinforcement. )
Outlook
Thus, this study will progress through three
levels of endplate development:
Before proceeding with designing the ILD endplate, or even building an advanced LCTPC-LP1 endplate, the
strength of the assembled interfaces and complicated structures must be understood. For example, calculations
based on the solid model of the spaceframe (above-left) are not expected to duplicate measurements on a real
assembly (above-right). The complex interfaces of the struts with the mounts and the mounts with the beams
are not modeled. In the case of the Al-fiber hybrid design, the strength added by the reinforcing depends on the
details of the fiber geometry and is not modeled.
Therefore, before proceeding with the design of the low-mass LP1 endplate, the strength of complicated
structures will be derived by comparing the strength of constructed simple beams with the model for each of the
designs as shown below. The construction of the solid models will be tuned to agree with the measurements.
(2 millibar applied load)
deflection
stress
microns
Mpa (yield = 241)
simple beams (now),
where we will study details of the interfaces
needed for realistic FEA predictions of the
strength of the endplates,
the LP1 advanced endplate (1 year),
which will be installed in the LP1prototype and
monitored over a year of operation, and
the Detailed Baseline Design ILD endplate
(2012).
23
4.2