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Geant4 simulation for the study of origins of the
backgroundof the X-ray CCD camera
onboard the Suzaku satellite
Geant4 simulation for the study of origins of the
backgroundof the X-ray CCD camera
onboard the Suzaku satellite
○ Takayasu Anada, Masanobu Ozaki, Tadayasu Dotani, Hiroshi Murakami, Junko
Hiraga, Yoshinori Ichikawa, Satoshi Murasawa, Masaki Kitsunezuka(ISAS/JAXA),
and the Suzaku XIS team
○ Takayasu Anada, Masanobu Ozaki, Tadayasu Dotani, Hiroshi Murakami, Junko
Hiraga, Yoshinori Ichikawa, Satoshi Murasawa, Masaki Kitsunezuka(ISAS/JAXA),
and the Suzaku XIS team
The Suzaku satellite and the onboard X-ray CCD camera
Background simulation and a comparison with the flight data
Origins of the background
Contents
The Suzaku satellite and the onboard X-ray CCD
camera
The Suzaku satellite and the onboard X-ray CCD
camera
The X-ray Satellite: Suzaku
XRTX-Ray Telescope
Launch Date
July 10, 2005
OrbitLEO
• altitude ~550 km• inclination ~ 31°
Sensitive to not only X-rays but also charged particles→ Background
Reducing the backgroundXIS
X-ray Imaging Spectrometer
HXDHard X-ray Detector
We study how the backgrounds are produced by Monte Carlo simulation
Goal
XIS cameraXIS cameraSuzaku satelliteSuzaku satellite
2.5 cm
1.4
cm
2.5
cm
focal length4.75m
15 cm
CCD chipCCD chip
1M pixelsX-ray
telescope(XRT)
XIS: X-ray Imaging Spectrometer
✴ Energy range: 0.4 - 12 keV✴ Located in the focal plane of XRT
✴ Energy range: 0.4 - 12 keV✴ Located in the focal plane of XRT
Imag
ing
Are
a
close up
close up
X-ray sensitive CCD operated in a photon-
counting mode
position and energy of each photon are measured
X-ray CCD Detector1) An X-ray photon enters into CCD and is absorbed in the depletion layer with creating a photoelectron.
2) The photoelectron excites surrounding material and produces an electron cloud.
3) The electrons drift toward the gates and are stored in the transfer channel.
4) Readout.
Sensitive to not only X-rays but also charged particles
→ “Background Events”
1 pixel 24μm
X-ray
(~1,600e- for 6 keV X-ray)
depletionlayer
photoabsorption
insulatorgate
Detection Mechanism
Background must be reducedWe need to understand accurately how the background is produced in the camera
Construct a Monte Carlo simulator, which can reproduce the background
accurately.
Construct a Monte Carlo simulator, which can reproduce the background
accurately.
Importance of Background ReductionCapability required to the next generation X-ray CCD
cameraCapability required to the next generation X-ray CCD camera
Energy
Flu
x
Background
SourceA future X-ray CCD camera needs to cover higher energy band to adapt the development of the so-called super-mirror.Because the background becomes dominant at higher energies, it needs to be reduced to achieve high sensitivity.
Background simulation and a comparison with XIS flight data
Background simulation and a comparison with XIS flight data
drift
X-ray
2. Interaction of the incident particles with the detector
3. Electron diffusion
Cosmic X-ray(10 keV - 6 MeV)
Cosmic-ray Electron( 100 keV – 200
GeV)
Cosmic-ray Proton(30 MeV – 200 GeV)
Flow of the Simulation
1.1. 2.2.
CCD
1. Incident particle generation
CCD4. Event extraction
X-ray !!
Geant4Geant4Geant4Geant4
4.4.3.3.
Cosmic X-ray(10 keV - 6MeV)
Cosmic-ray Electron( 100 keV – 200GeV )
Cosmic-ray Proton(30MeV – 200GeV)
Cosmic-ray Spectrum
•Use the cosmic-ray spectrum appropriate for the altitude of the Suzaku satellite
•X-rays, protons, and electrons are considered
•Assume that cosmic-rays come from the entire solid angle
•Cut-off rigidity is 8.4 GV
1. Cosmic-ray Spectrum Model
T. Mizuno et. al. 2004, APJ
XIS ModelXIS
Model • Simple structure(constructed from Aluminum shell with gold surface inside)• Window on the line of sight• Easy to optimize the production cuts per region
• Reproduce the materials and their configuration accurately• Time-consuming when tracking < 10 keV electrons and photons(difficult to optimize the production cuts per region)
Thickness: 10 g/cm2
Mass: ~ 5 kg
these parameters are adjusted to
the design value of XIS
2. Geometry of the Detector
Simplify
Spherical Shell ModelSpherical
Shell Model
Geant4Geant4Geant4Geant4
CCDCCD
CCDCCD
Geant4Geant4Geant4Geant4Cuts per Region
Sensitive energy range of XIS is from 0.4 to 12 keV→ low energy particles need to be created
(down to 250 eV, lower energy limit of Geant4 output)However, it’s time-consuming if such particles are produced in all
volumes Set the production cuts per
regionSet the production cuts per
region
Low energy electrons
are also produced (down to 250 eV)
Low energy electrons are
discarded
Outside:
Inside:
Aluminum
Gold
The concept of the setting:
Cuts per Region
2 134Aluminum
Gold
1. lower limit (Gold)
• 1 μm (Al)• 6 μm (Al)• 8 μm (Al)
optimize the production cuts for the region divided into four
layers
optimize the production cuts for the region divided into four
layers
Note: an electron may lose most of its energy to produce an X-ray
photon.
Range in Aluminum 30 keV 10 keV
Electron 8 μm 6 μm
X-ray 18.5 mm 750 μm
X-ray
Al
electron
X-rays have very long range
compared with electrons.
Total: 10 g/cm2
17.75 mm
18 mm
750 μm
1 μmvery short range
electrons must be created
in outside layer
Geant4Geant4Geant4Geant4
Cuts for electrons of each layer
succeeded in reducing the CPU time to simulate the interaction
with housing
Effectiveness of Setting Cuts per RegionComparison of the CPU time between the pre-
optimized and the optimized models in cuts per regionComparison of the CPU time between the pre-
optimized and the optimized models in cuts per region
electrons with energy
> 250 eV are created in all
regions
1 GeV proton 1 GeV proton
cuts per region are optimized
CPU time when 1,000 of 1 GeV protons are
injected1. 2.
2) 1 min.
Reduction in 1/25 !
1) 25 min.
Geant4Geant4Geant4Geant4
Spherical Shell Model is sufficiently good approximation in
E > 10 keV
Energy spectra of particles just entering the CCD are compared
Energy spectra of particles just entering the CCD are compared
X-rayX-ray ProtonProtonElectronElectron
Effect of Housing Model Simplification
Blue Red
Geant4Geant4Geant4Geant4
Presence/Absence of the field-free region produces large difference in the image and the spectrum
FI: X-rays enters the CCD through the gatesBI: X-rays do not go through the gates
BIFILow Energy
X-rayHigh Energy
X-rayLow Energy
X-rayHigh Energy
X-ray
There are two types of CCDs onboard the Suzaku satellite.
CCD Structure
• Front-Illuminated CCD (FI) • Back-Illuminated CCD (BI)
DifferenceDifference
FI: existsBI: removed
→ make largely spread events
the side with the gate structure is called front
side
in which electron cloud diffuse largely
Gate structure Field-free region
Geant4Geant4Geant4Geant4
3. Charge Diffusion Model in CCD
X-ray: Energy is deposited at a pointCharged particle: Energy is deposited along the track in CCD
Charge recombination is also included in this model.
generate electrons
along the track
FIX-ray Charged particle
DiffusionDiffusion
FI BI
BI : no spread events.
Frame Image (Flight Data)
This difference is caused by the presence/absence of the field-free
region
FI : largely spread events(dozens of pixels)
(1 GeV proton into the CCD)
FISimulation
FIReal
BIRealBI
Simulation
Simulation Real Data
Reproduce the image well
dead layer
depletion layer
dead layer
depletion layer
field free region
0.7μm
70μm
545μm
0.7μm
45μm
Comparison of Image when a cosmic-ray proton enters into CCD
Note that this method remove most of the non-X-ray background events,
but some of them still remain in the X-ray grade
X-ray grade
Non-X-ray grade
PH highlow
4. Event Extraction (the same process on the
satellite)
ー the Grade Method ーlocal peak of the pulse height
significant charges were detected
Spread within 2x2 pixels
Spread exceeding 2x2 pixels
Pick up charge clouds and distinguish X-ray
events from charged particle events by the
charge split pattern Sum of white and orange pixels’ pulse height becomes the event’s
energy
these particles are injected
3 times as many as the model flux in order to conform to the real
data.
The simulation is successful inFI: all energy range
BI: energies higher than 4 keV
blue: real data (XIS night earth)red: simulation
Comparison of Spectra2.4×108 cosmic-
rays are injected
X-ray: 2.3×108
Electron: 4.5×106
Proton: 5.0×106
✤Non-X-ray grade (largely spread events)FI BI
FI & BI: reproduced the continuum in all energy range
✤X-ray grade (background events)
2.4×108 cosmic-rays are injected
X-ray: 2.3×108
Electron: 4.5×106
Proton: 5.0×106
Examine the origins of these
events
FI BI
Comparison of Spectrathese particles are
injected3 times as many as the model flux in order to conform to the real
data.
blue: real data (XIS night earth)red: simulation
Origins of XIS backgroundOrigins of XIS background
X-ray 884 (62%)electron 452 (32%)
proton 39 (3%)
others 44 (3%)
electron
4967 (74%)
X-ray 787 (12%)proton 362 (5%)others 570 (9%)
FI
BI
Main source of backgroundFI: X-rays BI: electrons
Source of the BackgroundThe incident particles on CCD which produce the background
eventsThe incident particles on CCD which produce the background
events
BackgroundCCD
Origins of the Background in detail
Recoil electrons produced in CCD
by the Compton scattering of cosmic X-rays or high energy X-
rays originated from cosmic-ray
protons
Electrons generated by theinteraction of cosmic-ray protons
with surrounding materials
FI
BIelectro
n
depletionlayer
X-ray
depletionlayer
recoil electron
1 pixel
1 pixel
SummarySuccessfully reproduced the background
in XISusing Geant4
Successfully reproduced the background in XIS
using Geant4
Origins:Origins:•Cosmic X-rays •High energy X-rays originated from cosmic-ray protons
•Electrons produced by cosmic-ray protons
Application of this result:Application of this result:
• Background modeling• Development of a low-background X-ray CCD camera
FIFI
BIBI