Advanced Compton Telescope

1
Advanced Compton Telescope 3-Compton
w/semiconducting detectors Richard Kroeger Naval
Research Laboratory
2
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.
3
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

4
Compton Scattering
5
Three Gamma Interaction Technique
E1

?1

E2

?2
E3
?3
E4

• Incident gamma ray energy determined with partial
  energy loss
• Only three interactions required
• Dramatic improvement in efficiency
• New alternative Silicon only Compton telescope

6
Three Gamma Interaction Technique
Errors in E1 and ?1
7
Three Gamma Interaction Technique
Differential cross section for double scattering
at angles ?1 and ?2
8
New Germanium Technology
Detector arrays
a-Ge contacts
9
Laboratory measurement
True 3-Compton measurement Knowledge of
source position and energy not used PSPMT
positions and metrology need improvement
40 keV
accidentals
no Compton shelf!
?
23 cm
23 cm
137Cs
Ge strip det. array
Ge strip det.
PSPMT
10
Laboratory measurement
3-Compton measurement Source position used to
determine energy NO PSPMT Metrology needs
improvement
40 keV
no Compton shelf!
23 cm
137Cs
?
Ge strip det. array
Ge strip det.
11
1 MeV in Silicon ?
12
Energy Spectra (1 MeV)
13
Si(Li)

• Thick silicon (gtgt 6mm) possible using Si(Li)
• 6 mm requires less electronics than thin devices,
  but needs 3-D readout
• 1-2 mm does not require 3-D readout
• Requires 1-2 mm crossed-strip segmentation
• (Einstein and BBXRT flew segmented Li contacts)
• Promise of high yield of good devices using 1
  kW-cm material
• Challenge
• Develop segmentation technique (ultrasonic
  machining)
• - or -
• Develop a-Si contacts (vapor deposition contacts)
• - or -
• Develop 2 mm thick intrinsic devices (very high
  purity material needed)
• Radiation damage concern at warm temperatures

14
Baseline Advanced Compton Telescope (ACT)
1 m2 frontal area 30 layers 6-mm thick
double-sided silicon strip detectors 42 g/cm2
thick 420 kg silicon Broad FoV (? 60-75
deg) Charged particle anti-coincidence
1.5 mm thick silicon crossed strip detector
15
3-Compton Efficiency
16
Event reconstruction
4 interactions with energy loss
9 interactions
17
Event reconstruction

• What is the correct sequence of interactions?
• Consider events with 3 interactions
• There are six possible sequences
• Lets just try them all and see which ones work

2
3
4
5
6
1
18
Event reconstruction
Search all permutations for possible physical
solutions Event eloss1 0.548, eloss2 0.136,
eloss3 0.142 Order Qual ein cth1 loss2
cth2 eout cth3 0 1 2 0 1.000 0.381
0.136 0.515 0.174 -0.316 0 2 1 2
0.835 -0.167 0.142 -0.736 0.010 1 0 2
2 0.877 0.893 0.548 -0.959 0.051 -6.377 1
2 0 6 0.423 0.429 0.142 -0.736 -0.403
5.784 2 0 1 2 0.883 0.889 0.548 -0.959
0.057 -5.293 2 1 0 6 0.594 0.730 0.136
0.515 -0.232 4.821 red invalid
solution Only ONE solution!
19
Event reconstruction
Search all permutations for possible physical
solutions Event eloss1 0.075, eloss2 0.354,
eloss3 0.025 Order Qual ein cth1 loss2
cth2 eout cth3 0 1 2 0 1.000 0.959
0.354 0.657 0.546 0.960 0 2 1 7
0.168 -1.463 0.025 -0.988 -0.286 10.280 1 0 2
0 0.544 -0.754 0.075 -0.764 0.090 -0.219 1 2
0 7 0.447 -3.364 0.025 -0.988 -0.007 82.383
2 0 1 6 0.214 0.691 0.075 -0.764 -0.240
7.588 2 1 0 0 0.950 0.986 0.354 0.657
0.495 0.864 red invalid solution Three
solutions, but two are wrong
20
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
2. Check that the 4th interaction is possible
3. Repeat process for all possible sequences
4. Easily generalizes to 5 or more interactions
21
Event reconstruction
Three interactions How many valid
solutions? One 12 Two 43 Three 41 Four 4 (ave
rage 2.4) We know more The first interaction
was the largest energy loss in 60 of the events.
Four interactions How many valid
solutions? Zero 7 close events One 75 Two 12 Th
ree 2 Four 4 We know more The first
interaction was the largest energy loss in 63 of
the events.
22
Detection Efficiency vs. active material fraction
0.95 0.90 0.80
Coherent scattering has the same effect as
passive material
23
Backgrounds

• No 3-Compton background rejection simulations to
  report on at this time
• Speculate that b-g decays and diffuse sky bkg
  will not be rejected
• b-g-g decays may be highly rejected

• 80 cm x 80 cm area
• 27 layers, 6 mm thick silicon
• 37 g/cm2 thick
• 241 kg silicon
• 1 mm thick Pb shield
• 8300 cnts/sec bkg rate
• Accidental rates (1 ms window)
• 2-way 138 Hz
• 3-way 2.3 Hz
• 4-way 0.04 Hz

Singles bkg. est.
24
Backgrounds
Ge D1/D2 ACT background estimate 28 degree orbit
_at_ 450 km 2 of instrument background is
consistent with point source position This is a
2-interaction (coincidence) background rate that
we have studied using a Monte Carlo (EGS-4) with
detailed gamma decay schemes for activation
radioactivities.
We presume the 3-Compton background is
similar -yet- The detection efficiency is an
order of magnitude higher
25
ACT Sensitivity
Broad Line (30 keV) sensitivity (106 s)
26
3-D locations in GSDs
Momayezi, Warburton and Kroeger (SPIE, 1999) have
shown 3-D positioning capability in germanium
strip detectors. X-Y positions are determined
from the electron and hole signals induces on
orthogonal anode strips. Z-location is
determined from the relative arrival times of the
signals at the anode and cathode.
27
3-D Locations in GSDs
Momayezi, Warburton and Kroeger (SPIE, 1999) have
shown 3-D positioning capability in germanium
strip detectors. Shown here is response to a
Compton scattering event within a single GSD.
Note the possible use of adjacent Spectator
strip signals to refine the X-Y position.
28
Electron Tracking
e
e
silicon
847 keV
29
Polarization measurements using GSDs

• Compton telescopes make natural polarimeters.

Polarization likelihoods may assist in event
reconstruction and restrict possible source
locations in the primary event cone.
30
Technology/Studies Required

• Thick double-sided silicon strip detectors (or
  thin Ge)
• cost/yield, radiation damage, operating
  temperature
• Low-power spectroscopy ASICs
• 3-D capability
• Minimize passive mass in detector volume
• Simulations
• performance vs. ?E and ?x
• detector material
• impact of Doppler broadening
• event reconstruction
• electron tracking enhancements
• polarization enhancements
• performance vs. fraction of passive material in
  detector
• sensitivity/performance vs. size/configuration
  of detector
• sensitivity vs. orbit

31
Gamma Ray Lines of GRAPWG recommendation
32
ACT-3 Scientific Capabilities

• Supernovae
• Novae
• Compact Galactic Objects
• Diffuse Galactic Emissions
• Active Galactic Nuclei
• Gamma Ray Bursts
• Cosmic Gamma-ray Background
• Solar Activity
• Polarized emission