Nuclear Instruments and Methods in Physics Research A 436 (1999) 74}78 Quantum e$ciency of the CCD camera (XIS) for the ASTRO-E mission H. Katayama *, M. Shouho, T. Kohmura, K. Katayama, K. Yoshita, H. Tsunemi, S. Kitamoto, K. Hayashida, E. Miyata, K. Hashimotodani, K. Koyama, G. Ricker, M.W. Bautz, R. Foster, S. Kissel Department of Earth & Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan Department of Physics, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake-Cho, Sakyo-ku, Kyoto 606-8502, Japan Center of Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA CREST, Japan Science and Technology Corporation (JST), 4-1-8 Honmachi, Kawaguchi, Saitama 332, Japan Abstract We measured the optical and the X-ray transmission of the optical blocking "lters for the X-ray Imaging Spectrometers (XISs) which are the X-ray CCD cameras of the ASTRO-E satellite. We conclude that the oxidation of the aluminum reduces the optical transmission down to &60}70% of the theoretical value of the aluminum. We achieved optical transmission below 5 10 in the range from 4000 to 9500 As by using aluminum thickness of 1200 As, while the theoretical calculation requires 800 As. The measurement of absolute quantum e$ciency of XIS is also performed at several particular energies. We con"rmed 20% quantum e$ciency at 0.5 kev for the XIS engineering model (XIS EM). 1999 Elsevier Science B.V. All rights reserved. PACS: 95.55 Keywords: CCD; Calibration; Quantum e$ciency; ASTRO-E 1. Introduction ASTRO-E is the Japanese "fth X-ray astronomy mission scheduled to be launched in early 2000. Four X-ray CCD cameras (X-ray Imaging Spectrometer; XIS) are installed at the focal planes of the four X-ray telescopes. Schematic view of the XIS is shown in Fig. 1. The details of the XIS has been published in Hayashida et al. [1]. * Corresponding author. An optical blocking "lter is installed at the front of the CCD in order to prevent the optical and ultra-violet photon. The "lter is a polyimide "lm coated by aluminum on the both sides. This "lter is required for low optical transmission and high soft X-ray transmission. In this paper, we describe the calibration system for the XIS constructed at Osaka in Section 2. In Section 3, we present result of optical and X-ray transmission measurement on both the engineering model of the optical blocking "lter (OBF-EM) and the #ight model (OBF-FM). Brief discussion of 0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8-9 0 0 2 ( 9 9 ) 0 0 6 0 0-2
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 436 (1999) 74}78 75 Fig. 1. Schematic view of the XIS. Fig. 2. The side view of the silicon K-edge spectrometer. oxidation of the aluminum is presented in this section. In Section 4, we show the preliminary results on the quantum e$ciency of the XIS engineering model (XIS-EM). 2. Experimental setup Our calibration system for the XIS is shown in Fig. 2. This system consists an electron-impacted X-ray source and a grating spectrometer [2]. The X-ray source is the Manson Model 2 Ultrasoft X-ray source. A cathode "lament made of tungsten and an electron-impacted target of silver are used. We nominally set an anode voltage at 5 kv, "lament current at 6 A and electron emission current at 0.75 ma. The spectrometer is the Si K-edge Spectrometer Model FFS-II (SES) supplied by Hettrick Scienti"c inc. The SES is equipped with an adjustable entrance slit and a cylindrical mirror to focus X-ray beam. The collected X-ray beam is exposed to a variable line-space grating in which the groove density is 480 grooves/mm (grating; SA) at its center. Between the entrance slit and the cylindrical mirror, an Al "lter of 3000 As thickness is set to block optical light. This spectrometer system makes a #at focal plane at a position 775 mm away from the grating center. We adjusted to place the CCD chip at that focal plane. The energy range of SECTION I.
76 H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 436 (1999) 74}78 system is from about 0.25 to 2.2 kev, though X-rays up to 3 kev or higher are available by using higher order lights. Fig. 3 shows the inside view of the vacuum chamber. There are three motor-controlled stages which slide vertically. One of these is equipped with a plate which is connected to a liquid N trap through copper straps. The XIS camera is mounted on it. Various devices are set on the other two motor stages, according to the purpose of experiment. For example, when we measure the quantum e$ciency, a slit and a proportional counter are set on either of the two. Figs. 4 and 5 show the optical transmission and X-ray transmission of the OBF-EM respectively. The thickness of the layers reported by LUXEL Co. LTD are 355$ 36 As /1065$100 As /423$ 42 As. As shown in Fig. 4, we found unexpected high optical transmission. If we simulate the optical transmission changing the aluminum thickness, the thickness is roughly 60% of the LUXEL value. In the "gure, an example of the simulation is plotted, where aluminum/polyimide/aluminum "240 As / 1100 As /240 As. 3. X-ray and optical transmission of the OBF The X-ray transmission of the OBF was measured in the calibration system described in the previous section. We also measured the optical transmission by using optical light monochromized with a grating [3]. The OBF is made of polyimide with aluminum coating on both sides. The design thickness of the OBF-EM is aluminum/polyimide/aluminum "300 As /1000 As / 500 As, which was determined by the simulations of the optical and X-ray transmission. Our goals are the optical light transmission is below 5 10 in the optical range (from 400 to 950 As ) and the X-ray transmission is about 50% at O-K α emission line energy. Fig. 4. Optical transmission of the OBF-EM. Fig. 3. The chamber for the XIS calibration. The XIS camera body and the PC are shown. Fig. 5. X-ray transmission of the OBF-EM.
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 436 (1999) 74}78 77 We derived the thickness of aluminum and polyimide by "tting a simple layer-model to the X-ray data, because the X-ray transmission depends on only the total thickness of each material. The result is 612$35 and 1240$35 As, for aluminum and polyimide, respectively. Those are not consistent with the LUXEL values. Casteil et al. [4] pointed out that the unexpected high optical transmission is due to the oxidation of aluminum surface. We tried to "t the X-ray transmission data with Al O #Al# polyimide model. The data shows signi"cant oxidation layer; 72$19 As. The thickness of the polyimide and the aluminum are 1105$52 As and 581$30 As, respectively. The di!erence from the LUXEL value becomes small. This result directly supports the oxidation explanation [3]. Therefore we changed the design value for the OBF-FM by increasing the total aluminum thickness, i.e., aluminum/polyimide/aluminum "1000 As /1000 As / 200 As. The measured values by LUXEL are 1000$50 As /1050$100As /203 $ 25 As. Figs. 6 and 7 show the optical transmission and X-ray transmission of the OBF-FM, respectively. There is little di!erence of the X-ray transmission between the OBF-EM and the OBF-FM, while the optical transmission of the OBF-FM is improved. Thus the OBF-FM satis"es our goal. In Fig. 6, an example of the simulation is also plotted, where Fig. 7. X-ray transmission of the OBF-FM. aluminum/polyimide/aluminum"700 As /1000 As / 140 As. Therefore, in the optical range, the e!ective thickness of the aluminum is roughly 70%. The X-ray data were also "tted by a simple two-layer (aluminum#polyimide) model. The derived thicknesses were 874$33 and 1677$42 As for aluminum and polyimide, respectively, and this is again inconsistent with the LUXEL values. If we used the three-layer model (aluminum#al O #polyimide) model, the best "t values are 693$ 39 As /296$ 30 As /1086$74 As, and we got better agreement with the LUXEL values. 4. Quantum e7ciency of the XIS Fig. 6. Optical transmission of the OBF-FM. We use the proportional counter (PC) as a reference to measure the quantum e$ciency of the XIS. The PC is a gas #ow type using P10 gas (90% Ar#10% CH ) and the window of the PC is polypropylene (1.65 μm) coated with carbon (0.23 μm) and attached to a supporting mesh. We had measured the window transmission as a function of energy in the same way as the OBF. We use the e$ciency of the proportional counter calculated from geometry and the measured window transmission. Fig. 8 shows the quantum e$ciency of the engineering model of XIS (XIS-EM). The e$ciency SECTION I.
78 H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 436 (1999) 74}78 5. Conclusion We measured the optical and X-ray transmission of the OBF as the "rst step to measure the quantum e$ciency of the XIS. We conclude that oxidation of the aluminum signi"cantly change the optical transmission, and the e!ective thickness of the aluminum for the optical transmission is 70}60%. We also measured quantum e$ciency of the XIS-EM by using a PC as a reference. In this work, we established the method of measurement of quantum e$ciency of the XIS. References Fig. 8. Quantum e$ciency of the XIS for each segment. includes the transmission of the OBF-EM. The chip has four read out nodes, so there are four data points (Segments A}D) at each energy. We can con"rm there is no signi"cant di!erence of quantum e$ciency between segments. The solid line is a model with a slab structure of poly-silicon (gate) and silicon dioxide (cover and insulator) for the dead layer and silicon depletion layer. The thickness of the materials were not derived from the "tting, but adopted as a reasonable choice (e.g Ref. [5]). [1] K. Hayashida et al., Proc. SPIE 3445 (1998) 278. [2] K. Hashimotodani, T. Toneri, S. Kitamoto, H. Tsunemi, K. Hayashida, E. Miyata, K. Katayama, T. Kohmura, R. Asakura, K. Koyama, K. Yamamoto, K. Miyaguchi, H. Suzuki, Rev. Sci. Instr. 69 (1998) 392. [3] T. Kohmura, K. Katayama, R. Asakura, S. Kitamoto, H. Tsunemi, K. Hayashida, E. Miyata, K. Hashimotodani, H. Katayama, M. Shouho, K. Koyama, S. Kissel, G. Ricker, M. Bautz, R. Foster, XIS team, Property of the optical blocking "lter of the XIS for the ASTRO-E mission, Proceedings of the Broad Band X-Ray Spectra of Cosmic Sources, COSPAR, Nagoya, 1998, in press. [4] C.M. Casteil, D.J. Watson, A. Wells, B.J. Kent, M. Barbera, A. Collira, M. Bavdaz, SPIE 3114 (1997) 384. [5] M. Pivovaro!, S. Jones, M. Bautz, S. Kissel, G. Prigozhin, G. Ricker, H. Tsunemi, E. Miyata, IEEE 45 (2) (1998) 164.