Simulation study of the 511 keV annihilation line observation
with the Soft Gamma-ray Detector onboard ASTRO-H
Yuto Ichinohe ([email protected]), H. Odaka, T. Sato, S. Takeda, S. Watanabe,
M. Kokubun, T. Takahashi, H. Tajima, T. Tanaka, Y. Uchiyama, K. Nakazawa
1. Introduction
International Workshop on Positrons in Astrophysics
March 20-23, 2012 - Mürren, Switzerland
SGD performance at 511 keV
The 511 keV annihilation line is the one of the key targets of
the Soft Gamma-ray Detector (SGD) onboard ASTRO-H.
The soft gamma-ray band, including the 511 keV annihilation
line, is less explored than the hard X-ray or GeV gamma-ray
bands, due to 1) high in-orbit background and 2) low
detection efficiency. To overcome these observational
difficulties, the SGD adopts a new concept of “narrow fieldof-view (FOV) Compton camera”. In order to study the SGD
performance at 511 keV, we conducted detailed Monte-Carlo
simulations with full implementation of the detector mass
model and event analysis, considering realistic activation
background estimation based on the radiation environment
at the low Earth orbit.
Angular Resolution
Counts
7000
Energy Resolution
6000
9000
8000
7000
6000
5000
4000
Counts
Counts
-- Energy Resolution: 4.1 keV
-- Angular Resolution: ~5.0 deg
-- Effective Area: 2.7 cm2
-- Background: 2-4x10-5 counts/sec/keV
5000
9000
8000
5.0º (FWHM)
4000
7000
4.1 keV (FWHM)
6000
5000
3000
4000
3000
In the simulations, we assumed following parameters:
-- Obserbation time: 1Msec
-- Energy cut range: 511.0 ± 4.1 keV
-- Theta cut range: -5.0 - 5.0 deg
3000
2000
2000
2000
1000
0
500
505
510
515
1000
0
0
100
200
300
520
Energy (keV)
400
1000
500
Energy (keV)
0
-4
-2
0
2
4
ARM (deg)
→ H. Odaka’s talk on Friday
2. How to reduce the backgrounds?
There are many background / noise origins
-- Proton, CXB, etc... coming from outside of the line of sight toward the target
-- Photon escaped from the detector after Compton Scattered by the detector
-- Gamma-rays or electrons emitted by the radioisotopes generated inside the
detectors themselves due to radioactivation by trapped protons (the most
dominant background origin for the 511 keV observation)
Simulated Background Spectrum (SAA vs. non-SAA)
Counts/sec/keV
Soon after the satellite passes the SAA (South
Atlantic Anomaly), the detector material is
radioactivated, resulting in high background level
at 511 keV. In this simulation, in order to achieve
higher sensitivity, we selected the events 30000
sec after passage of the SAA.
SAA pass
non-SAA pass
10-4
10
-4
10-5
10
-5
300
350
400
450
500
550
600
650
700 480
Energy (keV)
490
500
510
520
530
540
Energy (keV)
(c) NASA
The SGD achieves high sensitivity by the powerful background rejection
Compton Camera
Compton Reconstruction
Incident Photon
Incident direction cut
FOV: 10ºx10º
Hit Data (E1, r1), (E2, r2), ...
Photon coming
from inside of
the detector
(Activation)
NO
Compton Reconstruction
→ Compton Cone
CdTe
(E1 , r1 )
Si
CdTe
10-3
10
10-4
10-4
③
Event
Background
2
me c
1
E2
10-5
1
E1 + E2
① Before anti-coincidence
10-6
3 hit
2 Hit
3 Hit
4 Hit
4 > Hit
56%
2 hit
Si
Si
CdTe
CdTe
CdTe
When the incident photon energy is high (>
400 keV), the ratio of the number of 3 or
more hit events to the number of 2 hit
events becomes larger. Currently, Si-SiCdTe (SSC) events and Si-CdTe-CdTe
(SCC) events are included in the analysis.
CdTe
Effective Area at 511 keV
The Si hit is regarded as the first
Compton scattering. 2 possibilities
Si-CdTe1-CdTe2 and
Si-CdTe2-CdTe1 are compared in the
point of view which is more physically
consistent. By reconstructing each
last two CdTe-CdTe events regarding
the first Si hit as the source, we can
calculate the Δθ (ARM). We choose
the possibility of which Δθ is smaller
than the other.
0.030cm2
0.592cm2 0.520cm2
Total: 2.7 cm2
0.494cm2
1.052cm2
480
490
500
510
520
530
540
Energy (keV)
4. SGD Sensitivity for the 511 keV Line
The sensitivity of the SGD for the 511 keV
annihilation line reaches to the 5-7x10-5 ph/
sec/cm2 with exposure of 1 Msec (depends
on the systematic error of the background).
511 keV candidates
3 or more hit Compton Events
-6
One β+ decay positron annihilates into two 511
keV lines. Background rejection efficiency of the
active shields for such events is better than that
for fluorescence lines or Gamma decay lines.
Background is suppressed by 1/20
with Compton Reconstruction
Source
Si-CdTe-CdTe
event reconstruction
10
103
Energy (keV)
102
511 keV Flux
(10-5 ph/cm2/sec)
GC, bulge
100
GC, Disk
47
LMXB
~1
BH Binary
40 (Cyg X-1)
Nova
10
511
511 keV
keV Line
Line Sensitivity
Sensitivity (5σ)
(5σ)
7.0E-05
2.0E-04
511 keV
Flux
(ph/sec/cm2)
511
keVPhoton
Photon
Flux
(ph/sec/cm2)
29%
5%
-5
③ After initial direction cut
Number of hits per event at 511keV
11%
10
② After anti-coincidence
By using the information on deposited energy
and interacted positions, we can limit the incident
direction of the photon that is consistent with the
Compton Kinematics (Compton Cone). If this
direction is not in the BGO FOV, this event will be
rejected as background.
3. To Improve the Detection Efficiency
Inter-CC
-3
507 keV (121Te)
YES
NO
When SGD detects an
event including some BGO
hits, this event is rejected
as background.
511 keV (β+ decay)
BGO
cos = 1
BGO
10-2
(E2 , r2 )
β+ Decay
Annihilation
Outside of
the BGO FOV ?
1SGD
ARMcut 5deg
systematic error
of the background
1.8E-04
6.0E-05
systematic error of
the background
1.6E-04
5.0E-05
1.4E-04
1.5%
3%
1.5%
5%
3.0%
5.0%
1.2E-04
4.0E-05
1.0E-04
8.0E-05
3.0E-05
6.0E-05
2.0E-05
4.0E-05
2.0E-05
1.0E-05
2E+06
3E+06
4E+06
5E+06
6E+06
7E+06
8E+06
9E+06
1E+07
0E+001E+06
0E+00 1E+06 2E+06 3E+06 4E+06 5E+06 6E+06 7E+06 8E+06 9E+06 1E+07
Exposure Time (sec)
Exposure Time (sec)
non-SAA pass
Energy cut range 511.0 ± 4.1 keV
(based on 1 Msec simulation data)
100
Case: Galactic Center
Counts/keV
Escaping
photon
10-2
Photon coming from the
outside of the BGO FOV
YES
Hits at BGO?
②
Si
Compton Cone
①
Proton, CXB,
etc...
Counts/sec/keV
BGO Active Shield
Anti-Coincidence
Simulated Background Spectrum (non-SAA pass)
Counts/sec/keV
Narrow FOV
90
Observed Spectrum
Signal
Background
80
70
2Hit Si-Si
2Hit Si-CdTeBottom
2Hit Si-CdTeSide
3Hit SSC
3Hit SCC
Effective Area:
2.7 cm2 (Total)
Effective area is highly improved
by using the 3 hit Compton Events
Galactic Center Bulge Region
Radius: 3 deg
Flux: 1x10-3 ph/sec/cm2
Line width: 2.4 keV (FWHM)
Exposure: 1 Msec
60
50
40
30
20
24σ Detection
10
0
480
490
500
510
520
530
540
Energy (keV)
・The Background rejection technique for the SGD reduces the activation background count by 1/100.
5. Conclusions
・The SGD will enable precise measurement of 511 keV line flux from the GC bulge.
・The SGD 1Msec sensitivity for the 511 keV annihilation line will be ~5x10-5 ph/cm2/sec.