1. MAP 2014 Spring Workshop Fermilab 27-31 May, 2014
Accelerator Physics Center
Computational Issues
Sergei Striganov
Fermilab
MAP 2014 Spring Workshop
Fermilab
27-31 May, 2014

2. Outline
MARS-EGS
Occupancy estimates
Conclusion

3. MARS vs MARS-EGS5
Background energy spectra
Background entrance into detector

4. Gamma flux: entrance to detector vs entrance to nozzle
Gamma flux: entrance to detector vs entrance to nozzle. Beam pipe – 5 cm radius, nozzle minimal radius – 2 cm
vertical coordinate
horizontal coordinate
Maximum at positive (negative) entrance to nozzle and negative (positive) entrance to detector – backscattering from nozzle jaws!

5. Electron flux: entrance to detector vs entrance to nozzle
Electron flux: entrance to detector vs entrance to nozzle. Beam pipe – 5 cm radius, nozzle minimal radius – 2 cm
vertical coordinate
horizontal coordinate
Maximum at positive (negative) entrance to nozzle and negative
(positive) entrance to detector – backscattering from nozzle jaws!

6. Nozzle geometry – 2 vertex setups
Maximum at entrance to nozzle = 2 cm, entrance to detector = -5cm
Maximum at positive (negative) entrance to nozzle and negative (positive) entrance to detector – backscattering from nozzle jaws!
Simulation of backscattering very sensitive to details!

7. Hit calculations
Hit definition: charged track left sensitive volume + charged track is stopped in sensitive volume. To estimate occupancy we need to perform simulation for chosen pixel size. Appropriate electron transport threshold should be determined as function of pixel size. Electron/positron from background files produced only 3% of background in vertex and tracker detectors. Above difference between MARS-EGS5 and MARS electron/positron simulation does not important for Higgs factory backgrounds calculation. In MARS minimal energy of produced δ-electron Ed= electron transport threshold. Number of produced δ-electron ~ 1/Ed .

8. Simple estimate of occupancy
Simulations were performed with MARS background files in EGS5
mode with 3, 10, 20, 30 keV thresholds. Number of charged tracks
leaving detector weakly depends on Ed , number of stopped tracks is
proportional ~ 1/Ed . Low energy δ-electron are produced with large
angle to δ-electron direction. Part of them is stopped in same pixel as
track going from this pixel. To avoid double counting we need to choose
adequate electron transport threshold. Electron ranges in silicon:
3 keV – 0.14 μm, 10 keV – 1.5 μm, 20 keV – 5 μm, 30 keV – 10 μm.
Probability to stop in neighbor pixel:
energy < 10 keV energy < 3 keV
5 μm % %
10 μm % %
20 μm % %
10 keV is close to estimated from above
20 kev is minimal estimate for 5 μm
30 kev is minimal estimate for 10 μm
10 keV estimate is only 30% large than 30 keV estimate in simulation. .

9. Occupancy in vertex detector (3 keV threshold)
name
hit/cm2
5x5 μm, %
10x10 μm,%
20x20 μm,%
Barrel 1
1.2
4.8
19.2
Barrel 2
0.11
0.44
1.8
Barrel 3
542
0.43
Barrel 4
559
0.45
Endcup 1
0.2
0.8
3.6
Endcup 2
0.57
2.26
Endcup 3
0.36
1.45
Endcup 4
0.5

10. Occupancy in vertex detector (10 keV threshold)
name
hit/cm2
5x5 μm, %
10x10 μm,%
20x20 μm,%
Barrel 1
0.66
2.64 11
Barrel 2
0.06
0.24
9.6
Barrel 3
300
Barrel 4
142
0.1
Endcup 1
0.49
1.96
Endcup 2
0.31
1.64
Endcup 3
0.20
0.80
Endcup 4
662
0.26

11. Occupancy in vertex detector (30 keV threshold)
name
hit/cm2
5x5 μm, %
10x10 μm,%
20x20 μm,%
Barrel 1
0.53
2.12
8.48
Barrel 2
0.05
0.2
0.8
Barrel 3
234
0.19
Barrel 4
111
0.09
Endcup 1
0.39
1.56
Endcup 2
0.24
0.94
Endcup 3
0.15
0.60
Endcup 4
581
0.23

12. Summary
There are significant differences in energy spectra and entrance point distribution of background electrons/positrons calculated using MARS-EGS and MARS.
Most of electrons coming to detector through beryllium beam pipe
are produced by backscattering of high energy electron on nozzle jaws. Simulation of backscattering is very delicate procedure and should be checked.
MARS-EGS and MARS results for gammas are in satisfactory agreement. About 97% of hits in vertex&tracker are produced by neutrals (gamma and neutrons), so problem with electron simulation does not important for this application.
Precise calculation of occupancy calculation impossible without detail simulation of detector. But it is possible to make estimate with about 30-50% accuracy.

13. Backup