1. TPCs for EIC: Developments and Challenges
Thomas K. Hemmick

2. EIC Tracking Requirements…
JLEIC Detector
BeAST
ePHENIX

3. A meeting of the minds @ Temple Univ.
The earliest EIC R&D tracking efforts (eRD3, eRD6, …) rapidly closing out initial programs.
Groups with gas-tracking interests held a meeting at Temple in May 2017:
New proposal guided specifically by updated EIC vision.
Physics
Luminosity
Background
BeAST
Jlab-EIC
ePHENIX
Link to EMCAL
Link to SILICON
Link to PID
Hand-drawn consensus EIC detector design

4. Next Generation TPC Concepts
Traditionally TPCs are considered as slow devices:
Long time to drift the primary electrons to the gain stage.
LONGER time to dump the positive ions down the drain.
Operation cycle:
“Gate” is closed preventing positive ion back flow and electron drift to avalanche stage.
Trigger causes gate to open for period necessary to collect electrons.
Gate closes for period necessary to reject ions.
Device ready for next event.
New concepts coming out of ALICE and STAR experience.
“Stacked” events are not so big problem (STAR and ALICE):
Independent event vertex.
Confirmation by “fast detector” or at least “different” detector.
Ion field distortion is a “manageable” correction (STAR)
New device (ALICE):
Gate-less design using gain stage w/ intrinsically low Ion Back Flow (IBF).
Continuous readout electronics (define event boundaries offline).
Micro Pattern Gas Detector
SAMPA Chip
Tracking Systems

5. sPHENIX TPC @ EIC??? Fee: September 2016 1.6 m 2.11 m
onboard
offboard
2.11 m
Quad-GEM Gain Stage
low IBF
SAMPA: zero-suppression, untriggered
Update: Combine DAM and EBDC into single PCIe card
72 modules 2(z), 12(f), 3(r)
Full detector requires 72 modules in three sizes plus spares and prototypes.
Module production will be carried out in parallel at three institutions: WIS, Vanderbilt, PNPI.
Cost & Schedule includes factory commissioning & certification.
Plan to run the SAMPA chip as:
DSP on for zero suppression.
Untriggered.
Lowest power, highest density solution
Aggregate Data:
Aligns & Throttles data (dump if no trigger active)
Clustering for data size reduction
Compress for bandwidth reduction
Felix
Exact size & shape as BeAST (coincidence?)
Useful at EIC?...maybe…must follow details.

6. Space Charge: The Boogeyman? (even @ EIC?)
Distortions can be ENORMOUS:
STAR: 10 cm peak distortion corrected to 400 microns (0.4 %)
Corrects for primary charge only (gated TPC ~no Ion Backflow from gain stage).
Uses analytical phi-symmetric correction driven by current luminosity.
ALICE: 20 cm distortion corrected to 200 microns. (0.1%)
Ion Back Flow = 20X larger than primary charge.
Continuous readout determines INSTANTANEOUS space charge (every 5 msec)
3D correction since IBF determined by GAIN (gaps, dips, horror).
Advantages with a fresh design:
Only the physics of the effect is given.
Smart design can minimize the impact
ALICE
Turn all knobs to 11

7. Space Charge Content
1% IBF
Primary
IBF
𝜌 𝑟,𝑧 𝛼 𝐼𝑜𝑛𝑖𝑧𝑎𝑡𝑖𝑜𝑛 ∗ 𝑀𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑖𝑡𝑦 ∗𝑅𝑎𝑡𝑒 𝑣 𝐼𝑂𝑁 1− 𝑧 𝐿𝑒𝑛𝑔𝑡ℎ +𝐶 𝑟 2
Ionization: Use Low-Z Primary Gas (Ne)
Rate: Crossing Angle?
vION: Use Low-Z Primary Gas
IBF:
0% IBF
Primary
IBF
electrons
Central Membrane
Avalanche Mods
positive ions
ALICE
Tracking Systems

8. Knobs 1 & 2: Positive Ion Velocity
If the ions are fast, they leave the volume quickly.
1 𝐾 𝑡𝑜𝑡 = 𝑓 1 𝐾 𝑓 2 𝐾 𝑓 3 𝐾 3 +…
Use a low mass primary gas component to maximize the ion mobility.
Choose as large drift field as possible:
Drift Field
Mobility
Mobility
Mass
Knob 1: Ne-based mixture
Knob 2: 400 V/cm drift field
Bad for EIC…low ionization
Tracking Systems

9. Knob 3: GEM-stack Operating Point
ALICE R&D on Quad Gem-stacks shows illustrates compromise between dE/dx resolution (NOT sPHENIX spec) and Ion Back Flow.
We are free to choose operation point at very low IBF if we give up on dE/dx resolution.
OK for sPHENIX
Terrible for EIC
Knob 3: Choose lowest IBF operating point.

10. Knob 4: Field Cage geometry
Our space charge calculations follow the 3D-model from Rossiger’s thesis (developed in close consultation with Jim Thomas who did same for STAR)
Distortions maximize at each field cage (inner cage is worse).
Note: IBF is implemented in simulation as:
A smear proportional to distortion
A shift proportional to distortion
Proportionality constants from ALICE/STAR experience…
“Tunable” (and tested) to much more pessimistic assumptions.
Dodged the space charge bullet?
sPHENIX
~8mm
~3mm
EIC
Note: Because of the 1.5 T field, the rdf distortion in sPHENIX is comparable to dr.
Knob 4: Move the inner field cage to R=20 cm

11. Is space charge an issue for EIC?
Consensus without calculation:
No it is not an issue.
If sPHENIX TPC works, EIC is easier.
sPHENIX optimization for heavy ions inappropriate for EIC.
sPHENIX emphasizes position resolution and low Ion Back Flow at a loss of dE/dx resolution.
EIC requires dE/dx as the principal deliverable of the TPC.
Quantitative comparison of sPHENIX and EIC:
Question: Is the sPHENIX TPC EIC?
dE/dx requires higher ionization density
Machine Conditions
Positive Ion Mobility
EIC-TPC cannot ignore r<30 cm.
EIC must operate avalanche w/ good dE/dx resol!
After optimizing for dE/dx; EIC-TPC space charge is challenging!

12. New Concept: Passive IBF grid.
Questions:
What drives the “Universal” IBF curve?
Can we do better?
In ALICE mode, the 1st GEM is coupled DIRECTLY to the TPC volume.
Ions produced in the 1st avalanche stage are coupled directly into volume.
High gain in the 1st GEM means large IBF.
Low gain in the 1st GEM means large gain fluctuations.
New concepts:
Mesh with asymmetric entrance/exit field:
Transparency near 100% preserves dE/dx resolution.
Imposes additional positive ion shielding.
Bad dE/dx
Bad Space Charge
Proposed to EIC R&D
R&D program will fully optimize the IBF grid design for the EIC-TPC.
Possible 4-5X IBF reduction with ~no loss in dE/dx resolution!
Perfect e-Transmission
Perfect Ion-Blocking

13. Mixed-gain hybrid.
Standard MMG+2 GEMs (not rotated foils) setup, IROC ALICE TPC size (“HIROC”) X-ray gun X,Y scan with 1” step, ~1. cm diameter ionization spot. Ne+CO2+N2 ( )
, nA
Cathode current / Anode current, “20” means 0.2%
Ongoing Yale enhanced by recent addition for forces from Saclay/SBU
Ne + CO2 (10%)
MMG+2 GEMs. IBF vs E-resolution
Red: standard setup (NIM A834, p149)
Blue: with R-layers, #1 need increase voltage
on MMG and GEMs
Black: with R-layers, #2

14. Yeah, but there is more to a TPC than IBF!
PID aspect of TPC comes from a measurement of dE/dx
Question: Can Cluster Counting work in a TPC in the presence of diffusion?
Ionization process involves two stages:
Release of primary electron (pure Poisson statistics).
Production of secondaries (smeared by E(primary) distribution.
Non-intuitive:
“Cluster counting” of primaries has better PID separation power.

15. Cluster Counting with Smearing…
Timepix Track
In this example calculation (not EIC configuration), cluster counting is superior to dE/dx until the counting efficiency.
Cluster counting techniques:
Hyper-pixelated with high gain (demonstrated using TIMEPIX - $$$).
Moderately pixelated by using TIMEPIX connected to pads.
DRAMATIC reduction in thickness of electronics as compared to standard (e.g. SAMPA) readouts.
Requires VERY high gain to include single electron clusters: DEMONSTRATED with large number of GEMS in a stack
4 Gev K-p
Taming the channel count
Bonus to EIC TPC:
Significantly extended reach in PID.
Significantly lower thickness to endcap.
Proposed to EIC R&D this year

16. Alternative to TPC?
Panda Straw Tube Tracker for FAIR uses straws 2 atm pressure.
Doubles ionization density.
Low mass ends since pressure generates wire tension.
Short drift so clusters don’t diffuse much.
QUESTION: Can you get enough gain from a wire to count single electron clusters?

17. Summary
Continuous readout TPC seems like a winning choice for a volume EIC.
Optimization VERY different from ongoing efforts in ALICE or sPHENIX.
Nonetheless, the challenges are on a good path to mitigation:
New electrode structures during electron transport for passive IBF reduction.
New hybrid gain stages w/ mMEGAS and GEMs
Cluster-counting superior to dE/dx via high but not ultra-high pixel density (TIMEPIX).
Significant material endcaps via TIMEPIX.
Other ideas (high pressure straws) may also be a viable choice w/ R&D.
Proposed EIC R&D
R&D ongoing…Yale/SBU/Saclay
Proposed EIC R&D
Proposed EIC R&D
Should be proposed