Study of BGO/Collimator Optimization for PoGO

1
Study of BGO/Collimator Optimization for PoGO

• August 8th, 2005
• Tsunefumi Mizuno, Hiroshima University/SLAC
• mizuno_at_SLAC.Stanford.EDU
• History of changes
• August 12, 2005 updated by T. Mizuno

2
Contents

• Objective of this study (p. 3)
• Simulation (pp.4-9)
• Geometry (p.4)
• Simulation condition (p.5)
• Detector response (p.6)
• Event selection (p.7)
• Gamma-ray background model (p.8)
• BGO/Collimator optimization (pp.9-16)
• Side BGO length (p.9)
• Side/Bottom BGO thickness (p.10)
• Collimator Material (p.11)
• Fluorescence X-ray (p.12)
• Dual layer collimator (pp.13,14)
• Expected BG (pp.15,16)
• Summary (p.17)
• Appendix (p.18)

3
Objectives

• To find an optimum design of BGO and passive
  collimator regarding to background.
• Evaluate the background level with fluorescence
  X-rays and cosmic X-ray/gamma-ray background
  (here we call primary gamma) taken into account.

4
Simulated Geometry
fixed parameters

• Thickness of fast scint. 2.63cm
• (D 2.23cm)
• W (thickness of slow scint.) 0.2cm
• L1 (slow scint. length) 60cm
• L2 (fast scint. length) 20cm
• Thickness of btm BGO 2.68cm
• Gap between BGOs 0.5cm
• (including BaSo4 eflector)
• of units 217 (geometrical area of fast scint.
  not covered by slow scint. 934.4 cm2)

parameters studied here

• Length of btm BGO 3/4/5cm
• (not tapered in simulator for simplicity)
• Thickness of side Anti BGO 3/4/5cm
• Length of side Anti BGO 60/70/80cm
• Collimator material Sn/Pb
• single/dual layer collimator

5
Simulation Condition

• The same Crab spectrum as that used in Hiros
  EGS4 simulation was simulated here. That is,
• E-2.1 spectrum with 100mCrab intensity, 20-200keV
  (300.8 c/s/m2)
• 100 polarized, 6h exposure
• Attenuation by air of 4g/cm2 (atmospheric depth
  in zenith direction is 3g/cm2 and that in
  line-of-sight direction is 4g/cm2)
• Atmospheric downward/upward gamma and cosmic
  X-ray/gamma-ray background gamma (primary gamma)
  spectra for GLAST BFEM simulation were used as
  background.
• Use Geant4 ver5.1 with PoGO-fix for polarized
  Compton scattering.

6
Detector Resopnses

• The same detector responses as those used in
  Hiros EGS4 simulation
• If there is a hit in slow/anti/btm scintillators,
  event is rejected. (Threshold is 3 keV for
  anti/btm BGO and 30 keV for slow scintillator.
  Note that the position dependence has not taken
  into account yet.). Energy smearing and poisson
  fluctuation are not taken into account yet for
  veto scintillators.
• Assumed detector resposes
• 0.5 photo-electron/keV
• fluctuated by poisson distribution
• smeared by gaussian of sigma0.5 keV (PMT energy
  resolution)
• minimum hit threshold after three steps above is
  3 keV

7
Event Analysis

• The same as those of Hiros EGS4 Simulation
• Use events in which two or three fast
  scintillators detected a hit.
• The largest energy deposit is considered to be
  photo absorption
• The second largest energy deposit is considered
  to be Compton scattering.
• Smallest energy deposit (in case of three
  scintillators with hit) is ignored.
• Smear azimuth angle distribution with Hiros
  resolution function.
• No event selection on compton kinematics

8
Background gamma-ray spectra
atmospheric downward gamma (vertical)
primary gamma
atmospheric upward gamma (vertical)

• Atmospheric gamma spectral models are for
  Palestine, Texas.
• We have no data for atmospheric downward gamma
  below 1MeV, where primary gamma could be dominant.

9
Side BGO Length

• 100mCrab vs. background spectrum
• Passive collimator Sn 100um
• Side/Bottom BGO thickness 3cm

atmospheric downward gamma
atmospheric upward gamma
100mCrab (incident)
100mCrab (detected)
BG due to atmospheric gamma, Side BGO
length60cm/70cm/80cm

• No sinificant difference in summed BG below 40keV
  and above 100keV
• Longer BGO reduces the background in 50-100 keV.
  (Pb collimator can also do. See p. 11)

10
Side/Bottom BGO Thickness

• 100mCrab vs. background spectrum
• Passive collimator Sn 100um
• Side BGO length 60cm

atmospheric downward gamma
atmospheric upward gamma
100mCrab (incident)
100mCrab (detected)
BG due to atmospheric gamma, Side/Btm BGO
thicknss3cm/4cm/5cm

• No sinificant difference in summed BG below 70 keV

11
Collimator Material
atmospheric downward gamma

• 100mCrab vs. background spectrum
• Side BGO length 60cm
• Side/Btm BGO thickness 3cm
• Standard process (no fluorescence X-ray)

primary gamma
100mCrab (incident)
100mCrab (detected)
atmospheric upward gamma
BG due to gamma, Collimator Pb 50um/Sn100 um

• Pb collimator reduces summed BG above 50 keV

12
Effect of Fluorescence X-ray
atmospheric downward gamma

• 100mCrab vs. background spectrum
• Side BGO length 60cm
• Side/Btm BGO thickness 3cm
• Low energy process (fluorescence X-ray)

primary gamma
100mCrab (incident)
100mCrab (detected)
atmospheric upward gamma
BG due to gamma, Collimator Pb 50um/Sn100 um

• BG below 30 keV for Pb collimator is worse than
  that for Sn collimator, due to fluorescence
  X-rays from Pb.

13
Dual Layer Collimator (1)

• Due to fluorescence X-rays, BG level for Pb
  collimator becomes higher than that for Sn
  collimator below 30keV.
• Dual layer collimator could reduce the BG outer
  collimator (Pb) eliminates contamination from
  primary gammas and downward atmospheric gammas,
  and inner collimator (Sn) eliminates fluorescence
  X-rays from Pb collimator.
• We tested two configurations. The idea of
  shortened Pb collimator is, to make the pass
  length in Sn collimator long enough to absorb
  fluorescent X-rays from Pb.

normal configuration
shortened Pb collimator
a long pass length
Pb collimator (50um, 50cm) Sn collimator (50um,
60cm)
Pb collimator (50um, 60cm) Sn collimator (50um,
60cm)
b short pass length
Fast/slow scintillator
14
Dual Layer Collimator (2)
atmospheric downward gamma

• BGO configuration is the same as p.12

primary gamma
100mCrab (incident)
100mCrab (detected)
atmospheric upward gamma
BG due to gamma, Pb collimator, standard
process(solid line) lowE
process(dotted line) Dual layer collimator,
normal configuration
shortened Pb collimator

• Dual collimator reduces BG below 30 keV. No
  significant difference in summed BG between
  normal configuration and shortened Pb collimator
  below 60 keV. (see next)

15
Expected BG (1)

• No significant difference among Sn and dual layer
  collimators below 60 keV.
• Dual collimator with shortened Pb gives the
  lowest BG in high energy.

primary gamma downward/upward atmospheric gamma
100mCrab (incident)
100mCrab (detected)
BG due to gamma, Pb 50um/60cm Sn 100um/60cm Pb
50um/60cm Sn 50um/60cm Pb 50um/50cm Sn
50um/60cm
16
Expected BG (2)
Contribution of each component is shown here.
Shortened Pb collimator with Sn collimator inside
100mCrab (incident)
100mCrab (detected)
BG due to gamma, Total primary gamma atmospheric
downward gamma atmospheric upward gamma
17
Summary

• BG dependence on BGO length/thickness and
  collimator configuration are studied.
• 3 components of gamma-ray background (primary,
  atmospheric downward/upward) and fluorescence
  X-rays are taken into account.
• Longer side BGO reduces BG above 50 keV (p.9). Pb
  collimator instead of Sn can also do this. (p.11)
• Thicker side/bottom BGO reduces BG above 80 keV.
  (p.10)
• Dual layer collimator with shortened Pb gives the
  lowest BG. Below 60keV, there is no significant
  difference among Sn collimator and dual
  collimators (normal configuration and shortened
  Pb). (pp.11-15)

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Appendix Energy of incident gamma which
contribute to BG
atmospheric downward gamma

• Energy distribution of incident gamma clearly
  shows the process how they contribute to BG.
• Pb collimator of 50um is assumed here

primary gamma
2 or 3 fast scintillators have a hit
atmospheric upward gamma
Events that contribute BG

• Contamination in FOV.
• Penetrate BGO without interaction, hit fast
  scintillators and absorbed by collimator.