Vladislav Zakharov Monday February 10 th , 2020 2 Background & - PowerPoint PPT Presentation

1 Vladislav Zakharov Monday February 10 th , 2020

2 Background & Motivation: Time Projection Chamber (TPC) u Ø A type of detector Ø A type of capacitor Outer & Inner mandrel construction in our lab u To be used in sPHENIX u Can be used in Electron Ion Collider (EIC) u Vlad

3 Collider Experiments Charged particles, ions or leptops ( 𝑓 " ), u hit neutral atoms in fix target. They hit each other in beam-beam u Soon, ion-to-leptop in beam-beam EIC =) u Vlad

4 Overview of sPHENIX at RHIC Multiple different detectors, in layers on top of u each other, are needed to measure all the results. Tracking: u Ø High precision (and high cost) pixilated silicon detectors Ø TPC: measures tracks from charged particles with the help of a 𝐶 -field Energy Deposition: u Ø Electro-Magnetic Calorimeter: measures energy “showers” from electrons & photons Ø Hadronic Calorimeter: Energy from hadrons Other detectors u Ø Scintillators e.g. RICH, µ -detector, etc. Vlad

5 ATLAS Lego model at WIS J Vlad

6 TPC’s Operating Principle 1. Anode & Cathode separated by a dielectric fluid (usually gas; unless you’re looking for 𝜉 ). 2. Particle traverses the gas, ionizing it 3. Uniform 𝐹 -field drifts the resulting charges 4. Anode is segmented to see the track left by the particle ≈1.6m m 1 1 . 2 ≈ Vlad

7 Detection Stage Unique interleaving “Zig-Zag” pads u Maximize charge sharing through: u Ø max incursion of neighboring pads Ø Minimal tip-to-tip spacing Over a decade work minimizing Differential Non-Linearity (DNL) - u measures deviation from expected results across pads 𝜀 - ≈ 𝑇𝑗𝑕𝑜𝑏𝑚 𝑌~ ∑ 𝑟 * 𝑦 * 𝜀 - ≈ 𝑋 𝑂𝑝𝑗𝑡𝑓 ∑ 𝑟 * 12 W is width of the pad Charge clouds collected on multiple vs. a on single pad Vlad

8 Amplification Stage 1 electron doesn’t have enough charge to overcome electronics noise u Need to use gain: u Ø Gas Electron Multiplier (GEM) Ø Micro-Megas ( µ M) Ø Multi-Wire Proportional Chambers (MWPC) Create large local 𝐹 -field that accelerate the incoming electrons. The high-energy 𝑓 " then u hits the nearby neutral gas molecules and forces them to release multiple 𝑓 " . With a high enough 𝐹 -field, or several stages to cascade, the resulting electron cloud can u be reliably detected Vlad

9 Gas Electron Multiplier (GEM) About 2,000 gain. Quad-Stack pioneering by ALICE u Δ V = top to bottom of single foil, Δ V = between two GEMs u Ø Δ V and Δ V are comparable at ≈ 200—400V, but distance Δ d ≈ 2—4mm while Δ d ≈ 40—60µm! Ø 𝐹 ;<*=> = .4 kV/cm, 𝐹 ><?@A=B< ≈ 1s kV/cm, 𝐹 CDEB ≈ 10s kV/cm ΔV 1 ΔV 1 ΔV 2 ΔV 2 ΔV 3 ΔV 3 ΔV 4 ΔV 4 Pad Plane Vlad

10 Real GEM photos Vlad

11 0% IBF ρ(r,Z) [ fC/𝑑𝑛 n ] IBF & Space Charge (SC) Primary IBF c b" deafgh i j 10 𝜍 𝑠, 𝑨 ∝ UD@*V?>*D@ ∗ XYE>*ZE*[*>\ ∗]?>B < k ^ _`a z [m] SC is the enemy of resolution u Radius [m] 𝑤 *D@ ;<*=> = 𝐿𝐹 (large K {Ne}, large 𝐹 = 400𝑊/𝑑𝑛 ) ⃗ u 1% IBF Detector performance limited by the fluctuations in u deflections since SC is not continuous on average Minimize C: Bias Operating Point of Micro pattern Gas u 100 Detector (MPGD) for low IBF (such as was done by ALICE), Passive IBF shielding (topic for today’s talk) z [m] Radius [m] u At 2,000 gain & only 1% IBF , 20 ions are drift and only K (mobility) of Ne 1 is primary. This is 95% of the Space Charge! Vlad

Electron vs. Ion Transport in a Gas 12 Battling SC requires distinguishing between 𝑓 " and ion transport u Both obey the Langevin Equation for transport: u 𝑛 𝑒 ⃗ 𝑤 𝑒𝑢 = 𝑟𝐹 + 𝑟 ⃗ 𝑤×𝐶 − 𝜆 ⃗ 𝑤 Full characterization is VERY COMPLEX requiring calculations & measurements u Nonetheless, we can direct our calculations using simplified considerations u The basic “Langevin Distinctions” between 𝑓 " and ions are: u u Opposite q: Design Forward-Backward Asymmetry into electric fields u Different ⃗ 𝑤 : Typically opposite in direction, different in magnitude… Use 𝐶 to our advantage It is possible to design structures that utilize all these differences to: u Ø Minimize the amount of ions coming from the avalanche and reaching the main drift volume Ø Retaining high 𝑓 " transport to the avalanche zone Vlad

Forward-Backward Asymmetry 13 E drift core core electrons forward ions backward E hole core core halo halo halo halo E transfer Garfield & Magboltz simulation of charge The classic GEM picture with 𝐹 ><?@A=B< > 𝐹 ;<*=> u dynamics of 2 e - arriving in a GEM hole. e - Only a fraction of the transfer field lines originate in the drift volume paths are yellow, ion paths are red. Green u spots at ionization locations. Effective transparency difference for forward-backward: u Bohmer et al. – SC Effects in an Ungated GEM-based v gwxayzew Ø Driving characteristic is the field ratio: TPC v {w_zg Ø Most electrons get through (and avalanche), while many Ions are blocked Vlad

14 GEM Quad-Stack Data Energy Resolution 1 2 3 4 Odd & even GEMs are: 2014-03-03 TDR for the Upgrade of the ALICE TPC 1) Aligned but vary in pitch u Fundamental tradeoff of IBF efficacy vs. Energy Resolution: 2) Rotated with respect to u Gain biased toward last GEM(s) [nearest pads] è Low IBF each other u Gain biased away from first GEM(s), coupled to gas è Gain This reduces the chances of fluctuations… decreased resolution an ion from the pad plane floating to the gas volume Vlad

15 Hybrid: Dual-GEM and microMegas (µM) µ M zoom’ed in S. Aiola et al – Combination of dual-GEM and μM as gain elements for a TPC V. Manuel et al – A Radiation Imaging Detector Made by Post processing a Standard CMOS Chip u Nothing beats µM for Field Ratio u Most extreme by lowering 𝐹 *@;Y[>*D@ Ø Mid GEM lowers the induction field for the v |e}`~ concept, but eats 𝑓 " v x|`•e Ø Top GEM provides some gain to compensate for 𝑓 " loss in Mid GEM Vlad

16 Data from µ Megas Raw Better… but still competition: IBF vs Resol. Measurements of IBF vs. field ratio for a 1,500 lpi (lines per inch) micromesh. Done with an intense (10mA-10keV) X-ray gun. S. Aiola et al – Combination of dual-GEM and μM as gain elements for a TPC Colas P. et al - IBF in the Micromegas TPC for the Future Linear Collider Vlad

IBF reduction without 𝑓 " Resolution Loss? 17 In any multi-stage gain structure, a low gain stage makes irreducible contributions to gain u fluctuations. The first (early) stage(s) of 4G and 2G-µM must have low gain since they are coupled u strongly to the gas. v gwxayzew Nonetheless, the field ratio principle (large è low IBF) applies even without gain. u v {w_zg Therefore, a passive structure generating a field ratio can lower IBF with little or no loss in u energy resolution. Vlad

Passive Mesh Calculations/Simulations 18 Ideal 5X better Ideal 3X better 2X better (than 25% at a ratio of 1) Full Garfield transport calculations u Drift Field is fixed to sPHENIX (400 V/cm) u Transfer Field is scanned: E d , 2E d , 3E d , 4E d , 5E d , 6E d (from sublime to ridiculous) u Magnetic field is scanned (relevant for low E T ) 0T , 0.5T , 1.0T , 1.5T , 2.0T , 2.5T u Ideal result would be 100% 𝑓 " transparency and 100% ion-blocking u Vlad

Among the Best Studied: Passive Meshes 19 “Etched” Mesh A simple mesh should lighten the burden and Transfer field 2-3x E drift (reasonable) u improve performance on any 4G or 2G-µMEGAS structure. However, an improvement of only 2- IBF improvement factors 2-3x (excellent) u 3x in ion blocking would mean that IBF is still 𝑓 " Transmission 90-98% u the dominant source of SC Vlad

20 Bi-Polar Gating Grid Blocks ions by collecting them on negative wires… but blocks 𝑓 " with positive wires u Active gating: Since ions & 𝑓 " have different drift velocities and hence different drift times, u turn the voltages off to allow 𝑓 " to pass and then turn them back on to collect the ions. Ø Creates dead time while waiting for ions to be collected. Potentially huge data loss in high luminosity experiments. Vlad

21 But ions are The 𝑓 " are drifting back from coming the gain stage We still need the signal from 𝑓 " Vlad

What about the Magnetic Field Term? 22 𝑛 𝑒 ⃗ 𝑤 𝑒𝑢 = 𝑟𝐹 + 𝑟 ⃗ 𝑤×𝐶 − 𝜆 ⃗ 𝑤 Negligible for SLOW ions… not negligible for 𝑓 " u ‚ƒ @A , B = 1.4 Tesla ⁄ Ø In sPHENIX: 𝑤 ;<*=> ≈ 80 Traditionally, one attempts to zero this term to avoid extra distortions by making 𝐹 ∥ 𝐶 u 𝑤×𝐶 kick that only 𝑓 " feel Nonetheless, one can make a localized ⃗ u This concept is discussed in detail in Blum’s book v Question: Can the magnetic field aid electrons in passing through an otherwise closed gate? Vlad

Introduction of Magnetic Field: 23 𝐶 = 0 Magnetic Field brings electrons through. u Ions remain blocked. u Vlad

Simulations of the Bi-Polar Wires 24 e - transparency not perfect (70%) • Ne:CF 4 (90:10), 𝐹 ><?@A=B< = 600V/cm, 𝐹 ;<*=> = 300V/cm, Wire pitch = 1mm • But all the ions are still blocked Vlad

25 WIS Data Vlad