1. Simulation of Properties of COMPASS Drift-Chamber Prototypes
Ran Bi
University of Illinois at Urbana-Champaign
DNP Oct 2013

2. Overview
New drift chamber (DC5) will be designed and built for COMPASS-II
Goal: attain 200 µm position resolution
Two prototypes (PTA & PTB) have been built at UIUC
Requirements for the charge threshold needed for the DC5 FEE were simulated with GARFIELD and confirmed experimentally with test beam measurements using PTA at DESY.

3. Steps in GARFIELD Simulations
Gas composition
High voltage
Arrival time distributions
Induced signals
Position resolution

4. High Voltage/Gas Mixture
Cathode planes: 0 V
Field wires: −300 V
Sense wires: V
Gas composition: 45% Ar, 50% C2H6, 5% CF4
Equipotential lines
Electron drift velocity vs. Electric field

5. Arrival Time Distribution
For each electron from a fixed track
Arrival time distribution
Sense wire

6. Position Resolution
Use RMS of the arrival time distribution of the nth electron multiplied with vDrift to estimate the position resolution.
Need to detect the 5th electron or earlier to reach 200 µm

7. Induced Signals vs Charge Sensitivity of FEE
Induced signal is integrated till the arrival time of the 5th electron
4 fC threshold is required to achieve 200 µm position resolution

8. Test of PTA with CMAD-based FEM in test beam at DESY

9. Garfield vs DESY Test Beam Results with DC Prototype A: R-T RelationDD
R – T relation from Garfield
PTA: scatter plot
Garfield: solid line

10. DESY Test Beam Results: Position Resolution vs Position in Drift Cell
Position resolution vs. location of electron beam track between sense and field wires
200 µm
Drift Time [ns]

11. Summary
Position resolution of 200 µm can be achieved with a FEE that has a 4 fC threshold
Results from DESY test beam agree with GARFIELD simulations