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 ○ 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 X-ray Satellite: Suzaku XRT X-Ray Telescope Launch Date July 10, 2005 Orbit LEO altitude ~550 km inclination ~ 31° Sensitive to not only X-rays but also charged particles → Background Reducing the background XIS X-ray Imaging Spectrometer HXD Hard X-ray Detector We study how the backgrounds are produced by Monte Carlo simulation Goal

XIS camera Suzaku satellite 2.5 cm 1.4 cm 2.5 cm focal length 4.75m 15 cm CCD chip 1M pixels X-ray telescope (XRT) XIS: X-ray Imaging Spectrometer ✴ Energy range: keV ✴ Located in the focal plane of XRT ✴ Energy range: keV ✴ Located in the focal plane of XRT Imaging Area close up X-ray sensitive CCD operated in a photon-counting mode position and energy of each photon are measured

X-ray CCD Detector 1) 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) depletion layer photoabsorption insulator gate Detection Mechanism

Background must be reduced We need to understand accurately how the background is produced in the camera Construct a Monte Carlo simulator, which can reproduce the background accurately. Importance of Background Reduction Capability required to the next generation X-ray CCD camera Energy Flux Background Source A 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

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 CCD 1. Incident particle generation CCD 4. Event extraction X-ray !! Geant4Geant

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 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/cm 2 Mass: ~ 5 kg these parameters are adjusted to the design value of XIS 2. Geometry of the Detector Simplify Spherical Shell Model Geant4Geant4 CCD CCD

Geant4Geant4 Cuts 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 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 2134 Aluminum 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 Note: an electron may lose most of its energy to produce an X-ray photon. Range in Aluminum 30 keV10 keV Electron8 μm6 μm X-ray18.5 mm750 μm X-ray Al electron X-rays have very long range compared with electrons. Total: 10 g/cm mm 18 mm 750 μm 1 μm very short range electrons must be created in outside layer Geant4Geant4 Cuts for electrons of each layer

succeeded in reducing the CPU time to simulate the interaction with housing Effectiveness of Setting Cuts per Region Comparison 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 cuts per region are optimized CPU time when 1,000 of 1 GeV protons are injected ) 1 min. Reduction in 1/25 ! 1) 25 min. Geant4Geant4

Spherical Shell Model is sufficiently good approximation in E > 10 keV Energy spectra of particles just entering the CCD are compared X-ray Proton Electron Effect of Housing Model Simplification BlueRed Geant4Geant4

Presence/Absence of the field-free region produces large difference in the image and the spectrum FI: X-rays enters the CCD through the gates BI: X-rays do not go through the gates BI FI Low Energy X-ray High Energy X-ray Low Energy X-ray High Energy X-ray There are two types of CCDs onboard the Suzaku satellite. CCD Structure Front-Illuminated CCD (FI) Back-Illuminated CCD (BI) Difference FI: exists BI: 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

Geant4Geant4 3. Charge Diffusion Model in CCD X-ray: Energy is deposited at a point Charged particle: Energy is deposited along the track in CCD Charge recombination is also included in this model. generate electrons along the track FI X-rayCharged particle Diffusion

FIBI 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) FI Simulation FI Real BI Real BI 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 high low 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 in FI: all energy range BI: energies higher than 4 keV blue: real data (XIS night earth) red: simulation Comparison of Spectra 2.4×10 8 cosmic-rays are injected X-ray: 2.3×10 8 Electron: 4.5×10 6 Proton: 5.0×10 6 ✤ Non-X-ray grade (largely spread events) FIBI

FI & BI: reproduced the continuum in all energy range ✤ X-ray grade (background events) 2.4×10 8 cosmic-rays are injected X-ray: 2.3×10 8 Electron: 4.5×10 6 Proton: 5.0×10 6 Examine the origins of these events FIBI Comparison of Spectra these particles are injected 3 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 background

X-ray884 (62%) electron452 (32%) proton39 (3%) others44 (3%) electron4967 (74%) X-ray 787 (12%) proton362 (5%) others570 (9%) FI BI Main source of background FI: X-rays BI: electrons Source of the Background The incident particles on CCD which produce the background events Background CCD

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 the interaction of cosmic-ray protons with surrounding materials FI BI electron depletion layer X-ray depletion layer recoil electron 1 pixel

Summary Successfully reproduced the background in XIS using Geant4 Successfully reproduced the background in XIS using Geant4 Origins: Cosmic X-rays High energy X-rays originated from cosmic-ray protons Electrons produced by cosmic-ray protons Application of this result: Background modeling Development of a low-background X-ray CCD camera FI BI

these particles are shot 3 times as many as the theoretical flux in order to conform the real data. FI: all energy range BI: reproduced the spectrum shape higher than 4 keV reproduced the continuum in all energy range blue: real data (XIS night earth) red: simulation FI BI Comparison of Spectrum ✤ X-ray grade (background events) 2.4×10 8 cosmic-rays are shot. X-ray: 2.3×10 8 Electron: 4.5×10 6 Proton: 5.0×10 6 ✤ Non-X-ray grade (largely spread events) Examine the origins of these events

X-ray884 (62%) electron452 (32%) proton 39 (3%) others 44 (3%) electron4967 (74%) X-ray 787 (12%) proton 362 (5%) others 570 (9%) FI BI Main source of background FI: X-ray BI: electron The Source of the Background The incident particles on CCD which made background events Background CCD

Spectrum of X-rays which made background by the Compton scattering at the moment of entering into CCD Recoil electrons produced by the Compton scattering of Cosmic X-rays or hard X-rays generated by cosmic-ray protons in the CCD Origins of the Background （ FI ）

background electron spectrum at the moment of entering into CCD Electrons produced by the ionizing process of electrons generated by cosmic-ray protons or cosmic-ray electrons in the CCD Origins of the Background （ BI ）

2. Charge Transfer and Readout CCD (Charge Coupled Device) : Semiconductor detector, alined mosaic-like MOS (Metal-Oxide-Semiconductor) Capacitors Imaging Area Framestore Output Node Serial Shift Registers Shield Framestore Imaging Area X-ray CCD Detector Transfer charges by changing node voltage regularly In order to prevent X-rays from entering during transfer, transfer the charges from Imaging Area to Framestore Area(exists Al shield above) quickly Get the amount of stored charges in each pixel (Image data) Despite so many pixels (about 1 million), only one output node is needed.

Output Parameters Position Direction Energy As for primary particles As for incident particles on CCD Incident Position Incident Energy Created Position Created Volume Name Created Physical Process Deposited energy for each step in CCD The physical process of the energy deposition Position for each step in CCD Primary particle Incident particle Step 3 Step 2 Step 1 Step 4