The ground calibration of the backside illuminated CCD

The ground calibration of the backside illuminated CCD camera of XIS onboard Astro-E 2 (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 | Event Detection | Charge Trailing | Characteristic of XIS-BI Grade method Charge trailing against the transfer. We developed “Charge Trail Correction”. | Onboard Calibration Comparison with Chandra/ACIS

1. Introduction Suzaku (Astro-E 2) XIS : Front-illuminated (FI) CCD× 3 + Back-illuminated (BI) CCD× 1 BI-CCD‥ High quantum efficiency for soft X-ray XIS-FI XIS-BI Quantum efficiency of XIS sensor

1. Introduction back surface X-ray Charge cloud spreads widely ↓ Energy resolution become worse ex. Chandra/ACIS ΔE(BI) ≒ 2×ΔE(FI) spread electrode Chemisorption charging process strengthen the electric field Collection efficiency of electrons were improved !! structure of XIS-BI

1. Introduction XIS-BI XIS-FI ΔE= 129 e. V (BI) 128 e. V (FI) @5. 9 ke. V counts/ke. V ΔE= 49 e. V (BI) 42 e. V (FI) @0. 53 ke. V Energy resolution of the XIS-BI is almost comparable with FI! Spectrum of O-K line

1. Introduction Ground Calibration Task Share Components Location X-ray Source Chip level CSR/MIT Fluorescent X-rays ACIS chips (C, O, F, Al, Si, P, Ti, Mn, Cu) calibrated at BESSY Osaka Univ. Grating Spectrometer 0. 2 -2. 2 ke. V Polypro-window Gas PC & XIS-EU Kyoto Univ. Fluorescent X-rays (Al, Cl, Ti, Mn, Fe, Zn, Se) Window-less SSD Synchrotron Facility Synchrotron X-rays + monochrometer (Transmission measurement with PIN diode) Camera without OBF +Flight Model AE OBF Camera onboard ISAS/JAXA the satellite 55 Fe QE reference

2. Event Detection Grade 02346 are used as X-ray event Grade 0 Grade 3 Grade 6 Grade 1 Grade 4 Grade 7 Grade 2 We analyzed BI data similarly to FI. Grade 5 ↓ split over 2 x 2 region Several problems were found. 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

Vertical 3. Charge Trailing Ground Calibration Imaging Area of XIS Uniform illumination of fluorescent X-ray transfer Read out node Distribution of Grade 7 events Several X-ray events escape to Grade 7? Counts Distribution of Grade 0, 2, 3, 4, 6 events not uniform! V (Vertical)

Grade 0 Grade 2 Contribute to the increasing Vertical 3. Charge Trailing PH Some charges are deposited during the transfer Grade 0 transfer Trailing charge transfer Grade 2

3. Charge Trailing Counts Distribution of Grade 7 events PH V (Vertical) transfer Grade 6 transfer Trailing charge transfer Grade 7

3. Charge Trailing extracted only Mn-K event Q’ (ADU) Trailing Charge ≡ Q’[ADU] V Q’ = C×N ; C = 6. 8× 10 -3 N (Number of transfer) spectrum of Mn-K Mean PHA ≡ Q[ADU] “Charge Trail Ratio” (CTR) ≡ the probability of charge trailing par 1 pixel transfer CTR [1/transfer] = C/Q CTR = 4. 5× 10 -6 @5. 9 ke. V (Mn-K)

3. Charge Trailing Relation of the CTR and the PHA is able to be expressed by the power-law function CTR depends on the PHA of event CTR = (1. 72× 10 -4)×(PHA[ADU])-0. 5 Q (PHA) We have developed “Charge Trail Correction”. V Before V After

3. Charge Trailing After the Charge Trail Correction … Counts Distribution of Grade 0, 2, 3, 4, 6 events not uniform! V (Vertical) Distribution of Grade 0, 2, 3, 4, 6 events becomes uniformly. Grade 7 events due to charge trail are successfully reduced. → The detection efficiency improve about 10 -20%.

4. Onboard Calibration XIS FI XIS BI ACIS BI Spectra of E 0102 -72 XIS keeps their performance even after the launch!!

Summary Suzaku/XIS is composed of 3 FI-CCD and 1 BI-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 proceeding now. |

4. Spilt Threshold Optimization Split threshold = 20 ADU (for XIS-FI) XIS-FI (Sp. Th. = 20 ADU) XIS-BI (Sp. Th. = 20 ADU) 20 ADU is not optimum value of the split threshold for BI?

4. Spilt Threshold Optimization O-K (0. 5 ke. V) Zn-K (8. 6 ke. V) ΔE (e. V) Efficiency Split Threshold (ADU) O-K line optimum split threshold Zn-K line Split Threshold (ADU) 10 ADU for 0. 5 ke. V 13 ADU for 8. 6 ke. V

4. Spilt Threshold Optimization Optimize the split threshold for each energy events The function for setting split threshold optimum split threshold [ADU] = 10. 359 + 2. 2075 log 10(E [ke. V] ) we make Grade classification using variable split threshold Energy resolution of the XIS-BI is almost comparable with FI XIS-FI (Sp. Th. = 20 ADU) ΔE= 49 e. V (BI) 42 e. V (FI) @0. 53 ke. V XIS-BI (Sp. Th. = 20 ADU) XIS-BI (variable Sp. Th. ) ΔE= 129 e. V (BI) 128 e. V (FI) @5. 9 ke. V