Parallel Magnetic Field Spatial Resolution Micromegas Weekly - - PowerPoint PPT Presentation

Garfield Simulation: Electron Diffusion in a Parallel Magnetic Field – Spatial Resolution

Micromegas Weekly 10.09.13

10.09.13 1 Jona Bortfeldt

Jona Bortfeldt – LMU Munich

Reminder: Influence of parallel B-field on Cluster Width? (-> Spatial Resolution, Discharges, … )

10.09.13 Jona Bortfeldt 2

• B-field parallel to drift field „guides“ the

electrons and suppresses transverse diffusion

• how large is the influence on the cluster width?
• in NSW B < 0.3T

simple Garfield simulation:

• 2 dimensional; 5mm drift region, 128μm

amplification region, homogeneous readout plane

• mesh replaced by wires, no penning transfer

→ gas gain approximate

• planar diffusion correct
• 1000 pions with 130GeV at perpendicular

incidence

• two methods to determine the event width:
• last hit strip – first hit strip

→ total width

• Gaussian fit to resulting charge distribution

→ standard deviation

Calculation of Spatial Resolution

• assign charge detected within 250µm to a single strip
• calculate charge-weighted mean:

𝑡𝑢𝑠𝑗𝑞 ×𝑑ℎ𝑏𝑠𝑕𝑓(𝑡𝑢𝑠𝑗𝑞) 𝑏𝑚𝑚 𝑡𝑢𝑠𝑗𝑞𝑡 𝑑ℎ𝑏𝑠𝑕𝑓(𝑡𝑢𝑠𝑗𝑞) 𝑏𝑚𝑚 𝑡𝑢𝑠𝑗𝑞𝑡

• calculate residual = reconstructed hit – expectation
• fit residual distribution from 1000 tracks with single Gaussian

what this simulation can and cannot do:

 determine influence of diffusion on spatial resolution  show influence of B field on diffusion X model electron mesh transparency (relevant for small and very large drift fields) X determine influence of gas gain = pulse height on spatial resolution (not a large influence anyway) X model capacitive coupling between strips and charge distribution (charge distribution

• n strips not as smooth as in reality)

Comparison of Simulation and Experiment – Spatial Resolution for Ar:CO2 85:15

Bortfeldt et al. NIM A 718 406-408

measured spatial resolution

• similar trends visible except for small fields
• deviation: mesh transparency not modeled correctly
• ffset: capacitive/inductive coupling not considered

Comparison of Simulation and Experiment – Spatial Resolution for Ar:CO2 93:7

• similar behavior visible
• ffset: capacitive/inductive coupling not considered

Simulated Spatial Resolution for different B fields

consider spatial resolution at fixed drift field (no change in mesh transparency) → reduced diffusion improves spatial resolution 10% reduction for B ≤ 0.4T 15% reduction for B ~ 0.6T 25% reduction for B ~ 1T

Simulated Spatial Resolution for Different Strip Periodicities

• for larger strip pitch, reduced

diffusion also improves spatial resolution

• „jump“ between 400µm pitch

and 500µm pitch due to not considered capacitive/inductive coupling

Summary

• spatial resolution has been determined from simple Garfield simulation
• trends for spatial resolution vs drift field for Ar:CO2 93:7 and 85:15 are reproduced

by simulation

• influence of B field on spatial resolution can be extracted from simulation
• parallel B field → reduced diffusion → improved spatial resolution
• deviation: mesh transparency, gas gain, capacitive/inductive coupling not modeled

correctly

• results should be cross checked with a more advanced simulation

Thank you!

backup: simulated cluster width for different magnetic fields

10.09.13 Jona Bortfeldt 9

total width sigma from Gaussian fit for B≤0.4T: ~10% variation, for 0.6T ≤ B ≤ 0.8T: ~15% reduction, for B=1T: ~20% reduction

backup

10.09.13 Jona Bortfeldt 10

transverse diffusion for angle(E,B) = 0 degrees