SiLi Advanced Compton Telescope

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Si(Li) Advanced Compton Telescope Richard
Kroeger Jim Kurfess, Eric Grove, Neil Johnson,
Bernard Phlips, Mark Strickman Naval Research
Laboratory Ethan Hull, Paul Luke, Richard Pehl,
Craig Tindall Lawrence Berkeley National
Laboratory
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1999 GRAPWG Report
Advanced Compton Telescope (ACT) The HIGHEST
PRIORITY major mission recommended by the GRAPWG
is ACT, a high-technology MeV line and continuum
Compton Telescope mission operating in the 500
keV to 30 MeV range.
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Compton Scattering
L1
L2
L3
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1 MeV in Silicon ?
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3-Compton Efficiency
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Detection Efficiency vs. active material fraction
0.95 0.90 0.80
Coherent scattering has the same effect as
passive material
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Doppler Broadening
Silicon is the idea choice 4 keV for 40 degree
scatter
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Silicon Doppler Correction
12 keV FWHM Using knowledge of point source
position
19 keV FWHM Source position unknown
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Battling Doppler
Source

• Total energy absorption E0 computed from summing
  energy losses. Doppler does not contribute to
  energy uncertainty, only the angular uncertainty.
• Known source position Doppler contribution in
  first scatter estimated from two ways of
  determining E1 one using L1, and the other using
  L2. Clear improvements are realized.
• Four or more interactions Doppler contribution
  in upstream scatter estimated. Subject of
  ongoing work.

E0
L1
E1
L2
E2
Good energy and spatial resolution are essential
L3
Doppler broadening degrades background rejection
using point source discrimination method.
E3 etc
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Ge Laboratory measurement
Ge strip det.
Ge strip det.aray
PMT
?
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Event simulations
4 interactions with energy loss
9 interactions
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Event reconstruction

• Events with 4 or more interactions
• Number of interactions 4 5 6 7 more
• Number of sequences 24 120 720 5040 lots more

1. Find a sequence of 3 that works, compute
probabilities
2. Check that the 4th interaction is possible
3. Repeat process for all possible sequences,
determine most probable of the valid choices
4. Easily generalizes to 5 or more interactions
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ACT Mission Profile Considerations

• Equatorial Orbit
• Reduce cosmic ray and SAA background
• Wide Field of view, on the order of ?60
• Cover 70 of sky each orbit
• Achieve 106 s observations for most of sky in 1
  mo.
• Include CZT top layer and coded mask for 10-100
  keV
• meet EXIST science objectives
• Undertake as MIDEX opportunity
• Competitive opportunity every 2 years

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Baseline Advanced Compton Telescope (ACT)
1 m2 frontal area 25 layers 7-mm thick
double-sided silicon strip detectors, each 10x10
cm active area 42 g/cm2 thick 420 kg
silicon Broad FoV (? 60-75 deg)
1.5 mm thick silicon crossed strip detector
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ACT Sensitivity
Broad Line (30 keV) sensitivity (106 s)
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SN science
SN Ia rates (per year) for pairs of models per
year would be distinguished
Chandrasekhar Sub-Chandrasekhar
normal
Super lum.
Vela 26Al Simulation
Sub lum.
Milne et al, 2000
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Gamma Ray Lines of GRAPWG recommendation