1. GEANT Simulations and Track Reconstruction
for the Super-Bigbite Spectrometer Project
SBS Technical Review
L.Pentchev
Jan 22, 2010

2. Outline
GEANT simulations of GEp(5) background and GEM photon detection efficiency
Experimental verification of MC simulations
Description of track search algorithm
Efficiency of track reconstruction

3. GEANT3 Simulations of Background Rates
SBS high background rates require GEM capability
Anticipated GEM rates are orders of magnitude lower than the ones demonstrated at CERN
DAQ and tracking require knowledge of background rates for design

4. GEANT3 Simulations of Background Rates
GEANT3 code with 100 keV threshold used for background simulations
Includes description of target, scattering chamber, magnet, SBS detectors (with COMPASS type GEM), BigCal, beamline, beam dump
Magnetic fields from separate calculations included with magnet volumes
Several configurations tested to study background contributions
Several configurations studied:
Target only
Target + scattering chamber
+ magnet, field clamps, lead shielding
+ beam line and beam dump

5. GEANT3 Simulations of Background Rates
Low energy (<1 GeV) charged particles swept by the magnet: rates on First Tracker dominated by photon flux, while rear tracker rates dominated by e- produced by target photons in analyzers
Two step simulations for photons: background rates simulations with 100 keV threshold and GEM photon detection simulations with 10 keV threshold 5

6. GEM Photon Detection Probability
Separate g efficiency study by E.Cisbani using Geant4 with 10 keV threshold
“standard” COMPASS-type GEM
Photon Energy, MeV
Chamber
<g eff> (%) 1
0.31 2
0.54 3
0.63 4
0.73 5
0.76 6
0.79
Photon spectrum on front GEM chamber
From GEANT3
Material thickness (~photon eff.) will be reduced by about 30% compared to COMPASS GEMs 6

7. MC Background Results: First GEM chamber
Initial soft electrons swept by the magnet
Photons originate from the target or from electrons hitting material in front of the magnet
First GEM chamber:
Configuration
Ebeam = 11 GeV, 75 mA
rates,
MHz/cm2
g induced hits,
Charged rates, MHz/cm2
Target
114
0.350
1,010
Target + Scatt. chamber
118
0.361
Target + Scatt. Ch. + Magnet
143
0.437
0.119

8. BigCal Coordinate Detector
Background Results: All Trackers
For full configuration of GEp(5):
Hit rate, kHz/cm2
First Tracker
556
Second Tracker
359
Third Tracker
125
BigCal Coordinate Detector
130
2nd tracker
CH2
CH2
1st tracker
3rd tracker
BigCal Coordinate Detector
Al absorber

9. MC and experimental Background Rate Comparison
Transversity Experiment E06-010
5.9 GeV beam, BigBite at 300,
40cm 3He polarized target
3 MWDC behind the BigBite magnet
Luminosity with H2: cm-2 s-1;
11.8 mA current
GEANT3 Calculation
Simplified target description
100 keV threshold
Hadron rates from DINREG
Differential measurements of MWDC rates with - without H2 in target cell results:
Data
Simulation (g + hadron)
6.1 MHz
4.8 MHz
20% of rate for filled target configuration are “room” background
MC explains 80% of the experimental rates
This is encouraging level of agreement
Dedicated studies with simpler geometry and detector shielding could provide more confidence in MC

10. Tracking Detector Configuration
Lead
(X,Y) x2
(U,V) x2
(X,Y) x2
(U,V) x2
(X,Y) x2
(X,Y) x2
HCAL
CH2
CH2
(U,V) x2
1st tracker
2nd tracker
3rd tracker
CH2
Number of Layers
Area (cm2)
Pitch (mm)
Channels
Front Tracker 6
40x150
0.4
49k
2nd Tracker 4
50x200
1.6
13.5K
3rd Tracker
13.5k
BigCal 2
80x300
1.0
12k
(X) x2
Al absorber
BigCal

11. Track Reconstruction: Step 1
From the BigCal hit, reconstruct e- position (vertical only) on BigCal Coordinate Detector
Elastic kinematics correlate e- and p tracks
3rd
MC for elastic events
2nd
1st tracker p e
BigCal
beam

12. Track Reconstruction: Step 1
BigCal hit constrains proton search region to 0.2 * 18cm2 (x * y) and 0.7 * 30 mrad2 (Qx * Qy)

13. Track Reconstruction: Step 2
Tracking in First Tracker: For each possible hit on each GEM plane, starting from the front, by using angular constraints, small region on next GEM chamber is defined

14. Track Reconstruction: Step 3
Proton is likely to be only scattered in first or second analyzer
HCAL + front tracker constrains 75 cm2 search region in 3rd tracker 14

15. Track Reconstruction: Step 4
Front and 3rd tracker constrains 2nd tracker search region to 2 x 0.9 cm2 areas

16. Track Reconstruction Inefficiency due to random rates
Using calculated full setup rates:
Tracker
Search Area
Rate
Pitch
Occupancy
Pseudo-Track
[cm2]
[kHz/cm2]
[mm]
[%]
[per event]
Front
3.6
556
0.4
14.5
0.007
2nd
0.8
359
1.6
18.8
0.015
3rd
72.4
125
7.1
0.001
BigCal
4.5
130
1.0
2.3
0.026
Pseudo-tracks represents tracking inefficiency
Total inefficiency about 5% combining all trackers
Future steps: full simulations including dead zones, amplitude distribution of signals, MC simulation of tracking

17. Summary
MC demonstrates that rates are well below the maximum capability of GEMs
The trackers are capable of operation at projected luminosity with 5% losses due to pseudo-tracks
Presently, there is agreement at 20% level, between MC and experimental rates in a BigBite experiment
Additional experimental studies and MC simulations will be performed