Geant4 simulation for the study of origins of the background of 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
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Geant4 simulation for the study of origins of the background of the X-ray CCD camera onboard the Suzaku satellite ○ Takayasu Anada, Masanobu Ozaki, Tadayasu.
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✴ 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.