# Presentation: 428413

## The Design of a GEM Based TPC with Interpolating Pad Readout

**The Design of a GEM Based ****TPC with Interpolating ****Pad Readout**

- B. Yu
*Brookhaven National Laboratory*
*October. 20, 2003*

## The LEGS Experiment with TPC (Spin ASYmmetry Detector)

## Performance Simulation of TPC with MWPC

## Pros and Cons of GEM vs MWPC

|  | GEM | MWPC |
| --- | --- | --- |
| Dead area | minimal | significant |
| Parallax | no | yes |
| E×B effect | no | yes |
| Signal formation | Fast (<0.1μs) | Slow (~μs) |
| Spatial spread of charge | Small (diffusion limited) | Medium (wire to cathode spacing) |
| Complexity in construction | “Simple” (self supporting, modular) | Wire frame needs to be integrated to the pad plane for support |

- These are some of the reasons for choosing GEM against MWPC

## Performance Limitations of a Wire Chamber

- Other considerations:
- Support of wire tension (strong frame)
- Dead space associated with wire frame and support

## Test Chamber with Pad Readout

- Collaboration with the PHENIX Upgrade Program
- Standard 10cmx10cm GEM foils supplied by F. Sauli from CERN
- A drift stack can be attached to the chamber

## TPC Gas Properties

- TPC Drift Field

## Gas Gain vs. Voltage

- Ar+20% CO2, 5.4 keV x-rays (~1mm2, 2kHz), Ed=1kV/cm, Et=4kV/cm, Ei=5kV/cm

## GEM Discharge Probability

- Multiple structures provide equal gain at lower voltages. The discharge probability on exposure to a particles is strongly reduced.
- The LEGS TPC is designed to operate with an effective gas gain less than 1000, with a double GEM structure.

*S. Bachmann et al, Nucl. Instr. and Meth. A479 (2002) 294*

## Interpolating Pad Readout in a Wire Chamber

- The spread of the induced charge on the cathode pad plane is mostly determined by the wire to cathode distance, typically several mm.
- Electronic noise contribution to the position resolution ~ 2×*ENC*/*Q*total.
- To achieve 200μm rms resolution using pads with 2mm width, the electronic noise from each channel must to be less then 1/20th of the total anode charge.

## Interpolating Pad Readout with GEMs

- The spread of the electron cloud at the anode pad plane is mostly determined by the diffusion process.  It is typically well under 1mm

## Measure the Charge Spread on the Anode Plane

- Most probable pulse height from a set of 4 anode strips at 0.4mm pitch as a function of x-ray position @ 0.1mm steps
- 3mm drift gap, 2mm transfer gap, 2mm induction gap, Ar+20% CO2

## Intermediate Strip Patterns

- Single Intermediate Zigzag

- Two Intermediate Strips

- Other interpolating pad designs and their x-ray uniform irradiation responses

## “Line Response” of a Fine Zigzag Pattern

- 5.4 keV x-ray beam (0.1mmx3mm) stepped at 100μm intervals, center of gravity algorithm

- Overall rms position error: 93μm
- Including ~ 100μm fwhm x-ray photoelectron range,
- 100μm beam width, and alignment errors.

## Ion Feedback in a Double GEM Chamber

## Space Charge Buildup in the TPC Volume

- Calculation on the space charge buildup in the drift volume shows the distortion on the track position is negligible. (r<1mm)

- The calculation assumes:
- Uniform space charge density
- 40kHz track rate
- 30% ion feedback @ 1kV/cm drift field
- Effective gas gain 5000

- Distortion field strength due to ion buildup

## Double GEM Gas Gain Uniformity

- Collimated 5.4keV x-ray (~1mm2), scanned the detector with a 1mmx1mm grid, over 9cmx9cm area.

- Relative Amplitude

- Pulse height histogram of all entries on the map

## Schematic View of the TPC

- HV Cathode Plane

- Double GEM planes

- Interpolating anode pad plane with front end ASICs

- Digital readout board

## Layout of the Anode Pad Plane

- Pad size: ~2mmx5mm, 26 rowsTotal # of Channels: ~9000

## Prototype TPC with Large Anode Pad Plane

## GEM Gas Gain Uniformity

## Close-Up of a section of the GEM Foil Artwork

## GEM Foils Mounted on Frames

## High Voltage Distribution

- Drift field: 650V/cm

- VGEM: 400V

- Transfer Field: 2kV/cm  (3mm)

- Induction Field: 4kV/cm  (3mm)

- VGEM: 400V

## High Voltage Connection to the GEMs

## Poor Man’s Plated Through Holes

- To apply the high voltage to both sides of the GEM foil and each sectors, plated through holes have to be made on the voltage contacts.  CERN workshop declined to implement the plated though holes on the Kapton foils, so we have to improvise with drilled holes and conductive epoxy.

## Gas Seal

## The Evolution of the Pad Plane Design	

- The optimistic early stage: Summer 2003
- 26 pad rows, 9360 channels
- GEM foil design sent to CERN for fabrication

- In December 2003, a PC board was built to test the parameters in making the blind vias.  At depth/diameter aspect ratio of ~1.5:1, our PC Shop was able to make the connections.
- In January 2004, design of an 8 chip prototype board began.  However, the plating of the blind vias were not successful (our shop was in the process of replacing the plating tank and as a result, the plating of the board was done offsite at a commercial facility). About half of the connections through the blind vias were highly resistive.

## The Evolution of the Pad Plane Design	

- The Evolution of the Pad Plane Design

- We-have-a-problem stage: Summer 2004
- Pad Rows down to 24, 8256 channels.
- Using larger drill size, the 8 chipprototype board was successfullybuilt.
- Layout of the final board began.
- The reduction of 2 row allowed amore favorable blind via aspect ratio to be used.

## The Evolution of the Pad Plane Design	

- We-are-in-trouble stage: Dec. 2004
- Pad Rows down to 22, pad plane divided into 4 identical quadrants. 7296 channels
- By Dec. 04, only 1/8 of the board layout was complete.  No commercial vendor has agreed to build the board based onthe current layout.  No hope of auto-routing: the rest of the 7/8 will take a long time to finish.
- Further reduction in channel count is necessaryModularity is needed to get the board done in time
- Exploring a different board construction usingsolder ball bonding to by pass the blind vias

## Options for Divide the Pad Plane into Quadrants

- The 57 chip design was chosen based on the argument that the detector performance increase as a square root of the number of pad rows and degrade linearly with the average pad width.

## Key Features and Parameters

- Designed for low rate (~kHz), low multiplicity environment: single sample per channel per trigger
- TPC Active Volume:Inner diameter ~9cm; Outer diameter ~35cm; Drift Length: 50cm
- Double GEM amplification, gas gain <1000
- Drift field ~ 600V/cm (30kV high voltage), total drift time ~ 5μs.
- Interpolating zigzag anode pad plane, 200μm position resolution for stiff tracks
- Readout channel count >7000
- Customized ASICs, 32 channels per chip, 40mW per chip
- Electronic noise <250e, 500ns peaking time, single peak time and amplitude measurement, timing resolution ~ 20ns
- 8 sets of ADCs digitize the sparsified and serialized data streams, worst case event processing time <0.5ms