Download Astronomical Observation with a Nb-Al-AlOX-Al

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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.
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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
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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
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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.
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