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LETTER PASJ: Publ. Astron. Soc. Japan 56, L19–L23, 2004 August 25 c 2004. Astronomical Society of Japan. Astronomical Observation with a Nb-Al-AlOX -Al-Nb STJ Single Photon Detecor for Optical Wavelengths Shigetomo S HIKI , Hiromi S ATO, Yoshiyuki TAKIZAWA, Masahiko K URAKADO, and Hirohiko M. S HIMIZU Image Information Research Unit, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198 [email protected] (Received 2004 June 14; accepted 2004 June 29) Abstract A test observation using a Nb-Al-AlOX -Al-Nb superconducting tunnel junction detector was carried out using a small commercial optical telescope. Single photons of visible wavelengths were detected from α Lyr, α Boo, and Jupiter, and pulse-height distributions were obtained. The observed spectra roughly agree with predictions. Key words: instrumentation: detectors — techniques: photometric — techniques: spectroscopic 1. Introduction A superconducting tunnel junction (STJ) device is a candidate detector for next-generation optical astronomy, because it enables us to detect energy of a single visible photon. Because of their fast response time and sensitivity to the energy of a single photon, STJ detectors will make possible multi-color time-resolved imaging observations. An STJ device consists of a thin-film sandwich of a superconductor, an insulator, and a superconductor. When a photon is absorbed by a superconducting film, a number of Cooper pairs are broken into quasi-particles (charge carriers), because the binding energy of a Cooper pair (∼ 10−3 eV) is very much smaller than the energy of the incident photon, even at visible wavelengths. One can find the energy of the incident photon by measuring the number of tunneled charge carriers.√ The expected energy resolution is δE(FWHM) = 2.35 × E(F + G), where is 1.7∆ (Kurakado 1982), 2∆ is the binding energy of a Cooper pair, F is the Fano factor of around 0.2, G is the statistical fluctuation of the tunneling process and G ∼ 1 in the case of a high tunneling probability (Goldie et al. 1994). An energy resolution of ∼ 0.2 eV is expected for a photon energy of 2.5 eV using a niobium-STJ detector. The first detection of a single optical photon was obtained by the ESTEC group using niobium STJ (Perryman et al. 1993). Currently, better energy resolution is being achieved using tantalum or aluminum STJ detectors (Peacock et al. 1998; Wilson et al. 2002; Brammertz et al. 2004), and the spatial coverage and the energy resolution are being developed with the STJ or TES (superconducting transition edge sensor) detector (Savu et al. 2004; Martin et al. 2004; Burney et al. 2004). Observations were carried out using a 36-pixel tantalum STJ array on the 4.2-m William Herschel Telescope for the dynamic spectrum of the Crab pulsar as well as the direct measurements of redshifts, eclipsing binaries, and the stellar effective temperature (Perryman et al. 1999, 2001; de Bruijne et al. 2002; Reynolds et al. 2003; Steeghs et al. 2003). We are developing a niobium STJ detector because niobium has a fast quasi-particle recombination time, allowing us to achieve the best time resolution. For practical applications of STJ detectors, the response time is an important parameter because it limits the maximum event rate, the time resolution and the maximum brightness of the observable flux. Currently, the best-studied materials for STJ detectors are niobium, tantalum, and aluminum, with measured response times of ∼ 1µs, ∼ 10µs, and ∼ 100µs, respectively (Verhoeve et al. 1996; Peacock et al. 1998; Wilson et al. 2001). Applications of a Nb-STJ detector have been limited. The detection of an optical single photon was reported previously (Perryman et al. 1993; Shiki et al. 2004), but no observation was made of a celestial object. We have fabricated Nb-Al-AlOX -Al-Nb STJ devices (Sato et al. 2000; Takizawa et al. 2002; Otani et al. 2002) and succeeded in detecting a single optical photon (Shiki et al. 2004). In this study, we used our STJ detector for astronomical observations of spectroscopic standard stars to demonstrate its performance. 2. Observation The observations were carried out on 2003 March 26 from the top of the Main Research Building at the Institute of Physical and Chemical Research (RIKEN). A schematic picture of the telescope system is shown in figure 1. The telescope system consisted of a φ = 130 mm, f = 1000 mm telescope (TOA-130, Takahashi) on an equatorial mount (NJP, Takahashi). Tracking was automatically corrected with a φ = 78 mm, f = 630 mm telescope and a cooled CCD camera (BJ-40, Bitran). A focal-plane image was transferred with an optical-fiber array (N = 6000, φ = 2mm, l = 3m). The other end of the optical-fiber array was focused on the STJ detector using a relay lens. An STJ detector was installed in a 3 He cryostat, and light reached it through three windows. A CaF2 window is maintained in a vacuum. An IR-cut glass window with cutoff at 1 µm was kept at 77 K and a quartz window was kept at 4.2 K to reduce thermal radiation. The image on the opticalfiber array and the STJ device was monitored using a chrome cube half mirror and a cooled CCD camera (SV-16, Koeisha). A typical image from the chip monitor is shown in figure 2. L20 S. Shiki et al. [Vol. 56, Fig. 1. Telescope system of the observation. Fig. 2. Typical image on the monitor. A stellar image was seen on an STJ detector. Stellar light illuminated its front side, and the STJ chip was illuminated from the back side to determine the position of the detector. Fig. 4. Typical output signal of a single photon event. The STJ detector was illuminated using a 470 nm LED. Single-photon events were clearly detected at t = 0 µs and t = 300 µs. Fig. 3. Total efficiency of the telescope system. Fig. 5. Correlation between the pulse height and the energy of a photon. The line was obtained by a linear fitting to the observed data. Astronomical Observation with Nb-STJ Detector L21 Fig. 6. Pulse-height distribution of celestial objects; (a) α Lyr, (b) α Boo, and (c) Jupiter. The error bars indicate 1 σ . Any event of a pulse height lower than 60 ADU was discarded because of the noise of the electrical circuit. The background was electrical noise, which was observed when the telescope was closed. The spectral efficiency of the system — including absorption and blocking from the optical fiber, transmittance of windows, and reflectance at the surface of the niobium — is shown in figure 3. The reflection at the lens, loss due to the alignment of the optics, and atmospheric absorption were not included. The efficiency contains a scaling error of about 30% originating from the uncertainty of the transmittance of the optical-fiber array. We used an STJ detector that was 20 µm × 20 µm operated at 0.35 K with a 3 He cryostat. A magnetic field of about 10 mT was applied to reduce the Josephson current using a superconducting magnet set inside the cryostat. The leakage current at a bias voltage of 0.2 mV was 10 nA. The signal of the STJ detector was read using a charge-sensitive amplifier (A250, Amptek) with a feedback capacitance of 2 pF operated at room temperature. The signal from the charge-sensitive amplifier was shaped with a time constant of 1.0 µs using a shaping amplifier (570, Ortec). The pulse heights of each pulse were obtained and integrated using a multichannel analyzer (MCA7700, Seiko EG&G). A typical output signal of a single photon from the shaping amplifier is shown in figure 4. The observed correlation between photon energy and pulse height is shown in figure 5. An energy resolution was 1.1 eV FWHM for a 2.6 eV (470 nm) visible photon, which is five-times the predicted value. The uncertainty of the energy calibration was about 12%. The stability of the energy calibration was monitored during the period in every change of objects using a 470 nm LED source during our 3 hr observing run. The mean pulse height was 88.9 ADU and the standard deviation of the mean pulse height was 0.8 ADU. 3. Results and Discussion We observed α Lyr (A0 V), α Boo (K1 III), and Jupiter. The stellar image was de-focused to an image diameter of 2 mm at the surface of the optical-fiber array, because otherwise the event rate would completely saturated the STJ detector. With respect to the pixel size of 20 µm × 20 µm and the image size of 3.2 mm φ on the STJ chip, the flux was reduced to 5 × 10−5 times for original intensity. The pulse-height distributions of the objects and background are shown in figure 6. We could clearly discern the response of the STJ detector to the celestial objects. The response of the STJ detector was fast. The observed event rate was 1.7 kcps for Jupiter and no two-photon event signatures were seen. In another laboratory experiment, 15% of all detections were two-photon events at the incidence rate of 70 kcps using another niobium STJ detector and a continuous LETTER No. 4] L22 S. Shiki et al. [Vol. 56, Fig. 8. (a) Observed spectra, (b) simulated spectra, (c) overlayed spectra of α Lyr, (d) overlayed spectra of α Boo. calibrated using three LEDs (λ = 370nm, 470 nm, and 525 nm). We found that the peak pulse height of two spectra were clearly different. The shift is caused by the different colors of two objects and is not observable with other conventional detectors without additional wavelength-dispersing elements. We simulated the spectra of α Lyr and α Boo using published spectra (Tüg et al. 1977; Burnashev 1985). The simulated spectra are shown in figure 8b. We should mention that the efficiency of the observing system has a scaling error of 30%, and contains some uncertainties. The observed profiles match the simulated spectra well, including the peak shift, which was seen in the observed spectra. However, comparing the observed and simulated spectra of the same object, the profiles were slightly different (figures 8c, d). The reason for the difference would be the uncertainty of the energy calibration. Fig. 7. Pulse-height distribution at the high count-rate of 70 kcps with continuous illumination by a 370 nm LED. A single photon peak is seen around 75 ADU and a double photon event is seen at around 150 ADU. LED light source (figure 7). The light spectra were obtained for α Lyr and α Boo (figure 8a) after calibrating the pulse-height spectra by using the factor of de-focusing and the aperture of the telescope. The relationship between the energy and the pulse height was The authors wish to thank Hideaki Fujiwara and Daisuke Miyamoto for their kind help with these observations. The authors thank to Dr. J.H.J. de Bruijne for fruitful discussions. This study was supported by a special post-doctoral fellowship at RIKEN. Optical constants were obtained using the facilities of the Advanced Technology Center in the National Astronomical Observatory of Japan. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France, which is mirrored by the Astronomical Data Analysis Center in the National Astronomical Observatory of Japan. Astronomical Observation with Nb-STJ Detector L23 References Brammertz, G., Peacock, A., Verhoeve, P., Martin, D., & Venn, R. 2004, Nucl. 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