The ground calibration of the back-side illuminated CCD camera of XIS

onboard Astro-E2 (Suzaku)

H. Yamaguchi, H. Nakajima, H. Matsumoto, T. G. Tsuru, K. Koyama (Kyoto Univ., Japan), D. Matsuura, T. Miyauchi, S.Katsuda, M. Namiki, K. Torii, K. Hayashida, H. Tsunemi (Osaka Univ., Japan), and XIS team

Contents

Introduction Characteristic of XIS-BI

Event Detection Grade method

Charge Trailing Charge trailing against the transfer.

We developed “Charge Trail Correction”.

Onboard CalibrationComparison with Chandra/ACIS

1. IntroductionSuzaku (Astro-E2)

XIS sensor

XIS : Front-illuminated (FI) CCD×3 + Back-illuminated (BI) CCD×1

BI-CCD ‥ High quantum efficiency for soft X-ray

Quantum efficiency of XIS

XIS-FIXIS-BI

X-ray

electrode

1. Introduction

Chemisorptioncharging process

strengthen the electric field

Collection efficiency ofelectrons were improved !!

structure of XIS-BI

back surface

spread

Charge cloud spreads widely↓

Energy resolution become worse

ex. Chandra/ACISΔE(BI) 2×ΔE(FI)≒

1. Introduction

Energy resolution of the XIS-BI is almost comparable with FI!

ΔE= 49eV (BI) 42eV (FI) @0.53keV

ΔE= 129eV (BI) 128eV (FI) @5.9keV

cou

nts/

keV

Spectrum of O-K line

XIS-BIXIS-FI

Components Location X-ray Source QE reference

Chip level CSR/MITFluorescent X-rays (C,O,F,Al,Si,P,Ti,Mn,Cu)

ACIS chips calibrated at BESSY

Camera without OBF

+Flight Model AE

Osaka Univ.Grating Spectrometer

0.2-2.2keVPolypro-window Gas PC & XIS-EU

Kyoto Univ.Fluorescent X-rays (Al,Cl,Ti,Mn,Fe,Zn,Se)

Window-less SSD

OBFSynchrotron Facility

Synchrotron X-rays + monochrometer

(Transmission measurement with PIN diode)

Camera onboard the satellite

ISAS/JAXA 55Fe

1. IntroductionGround Calibration Task ShareGround Calibration Task Share

2. Event DetectionGrade02346 are used as X-ray event

The center pixel

A pixel whose PH is larger than split threshold and added to the PHA (= summed PH)

A pixel whose PH is larger than split threshold but NOT added to the PHA

Grade0

Grade1

Grade2

Grade3

Grade4

Grade5

Grade6

Grade7

split over 2x2 region

We analyzed BI data similarly to FI.↓

Several problems were found.

3. Charge TrailingV

erti

cal Imaging Area

of XIS

Uniform illumination of fluorescent X-ray

Read out node

Distribution of Grade0,2,3,4,6 events

V (Vertical)

Cou

nts

Distribution of Grade7 events

Cou

nts

V (Vertical)

not uniform!

Ground Calibration

transfer

Several X-ray events escape to Grade7?

3. Charge Trailing

transfer transfer transfer

Trailing charge

Some charges are deposited during the transfer.

Grade0 Grade2

Ver

tica

lContribute to the increasing

Grade0 Grade2

PH

3. Charge Trailing

transfer transfer transfer

Trailing chargeGrade6 Grade7

Distribution of Grade7 events

Cou

nts

V (Vertical)

PH

3. Charge Trailing

Q

’ (A

DU

)

extracted only Mn-K event

CTR = 4.5×10-6 @5.9keV (Mn-K)

“Charge Trail Ratio” (CTR) ≡ the probability of charge trailing par 1 pixel transfer

N (Number of transfer)

V

Trailing Charge ≡ Q’[ADU]

Q’ = C×NQ’ = C×N ; C = 6.8×10-3

Mean PHA ≡ Q[ADU]

spectrum of Mn-K

CTR [1/transfer] = C/Q

3. Charge Trailing

C

TR

Q (PHA)

CTR depends on the PHA of event

Relation of the CTR and the PHA is able to be expressed by the power-law function

CTR = (1.72×10-4)×(PHA[ADU])-0.5

We have developed “Charge Trail Correction”.

Before AfterV V

3. Charge TrailingAfter the Charge Trail Correction …

Distribution of Grade0,2,3,4,6 events

not uniform!

Cou

nts

V (Vertical)

Distribution of Grade0,2,3,4,6 events becomes uniformly.Grade7 events due to charge trail are successfully reduced.

→ The detection efficiency improve about The detection efficiency improve about 10-20%.10-20%.

4. Spilt Threshold OptimizationSplit threshold = 20ADU (for XIS-FI)

XIS-FI (SpTh.= 20ADU)

XIS-BI (SpTh.= 20ADU)

20ADU is not optimum value of the split threshold for BI?

O-K line

Zn-K lineoptimum split threshold

Split Threshold (ADU) Split Threshold (ADU)

Split Threshold (ADU)

ΔE (eV)

Efficiency

O-K (0.5keV) Zn-K (8.6keV)

4. Spilt Threshold Optimization

10ADU for 0.5keV13ADU for 8.6keV

4. Spilt Threshold Optimization

The function for setting split threshold optimum split threshold [ADU] = 10.359 + 2.2075 log10(E [keV] )

Optimize the split threshold for each energy events

we make Grade classification using variable split threshold

XIS-FI (SpTh.= 20ADU)

XIS-BI (SpTh.= 20ADU)

XIS-BI (variable SpTh.)

Energy resolution of the XIS-BI is almost comparable with FI

ΔE= 49eV (BI) 42eV (FI) @0.53keV

ΔE= 129eV (BI) 128eV (FI) @5.9keV

5. Onboard Calibration

XIS FIXIS BI

XIS keeps their performance even after the launch!!

ACIS BI

Spectra of E0102-72

Summary Suzaku/XIS is composed of 3 FI-CCD and 1 B

I-CCD. Good energy resolution of BI was achieved by

chemisorption charging process. We developed new analysis method, “Charge

Trail Correction”. → Detection efficiency improved. More detailed onboard calibration is proceedin

g now.