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