Analysis of astronomical data from optical superconducting tunnel junctions

Superconducting tunnel junctions (STJ) have now reached a state of maturity where small arrays have for the first time been used in practical applications in ground based astronomy. Future generations of devices are now under construction to enhance the current capability. For example larger format arrays, higher readout speeds, and improved spectral resolution are all issues being addressed. In this paper we present specifically the performance at ultraviolet wavelengths from 100 - 500 nm of the current generation of STJ arrays based on tantalum. Recent results on such tantalum based photon counting superconducting tunnel junctions (STJ) which are now suitable for use as broad-band low resolution spectrometers for ultraviolet astronomy are presented. These STJs, operated at a temperature of approximately 0.3 K, have demonstrated a limiting resolution of approximately 8 nm at 200 nm. These devices can be designed to be extremely linear in response with photon energy while measuring the individual photon wavelength and arrival time. The quantum efficiency for single photons is over approximately 50% at approximately 200 nm. Such an STJ has been packaged into a small prototype 6 X 6 array and shown to have good uniformity of response across all pixels. Larger arrays are under development which could contribute significantly to many fields of ultraviolet astronomy being able to provide efficiently and simultaneously the broad band spectrum and photon arrival time history of every single object in the field over a very wide dynamic range. With lower energy gap superconducting materials the resolution should become higher and possibly for hafnium based devices as high as approximately 1 nm at 200 nm. Is such devices can be developed then imaging spectrometers which can simultaneously record the image and spectra of objects in a large field and therefore provide an object's red-shift may become a reality.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Superconducting tunnel junctions (STJs) have been exten- sively investigated as photon detectors. They combine intrinsic energy resolution with good detection efficiency and high responsivity, thus pro- viding energy-resolved photon-counting performance in a wide energy range, from the near IR (NIR) to the x rays. On this basis, STJs offer advantages with respect to alternative photon detection systems, such as an intrinsic energy resolution, higher speed, and when compared to wavelength dispersive systems, increased detection efficiency. We over- view the STJ development in the Astrophysics Division of the European Space Agency (ESA), including STJ fabrication and operation, spectro- scopic performance of single STJs, small arrays, and STJ-absorber structures in the UV-NIR and x-ray bands. As a first application we de- scribe S-Cam, a cryogenic camera for ground-based optical astronomy exploiting a 636 array of Ta STJs. This camera has undergone four campaigns at the 4.2-m William Herschel Telescope at La Palma (Spain), and future generations of the camera are under active develop- ment. For the soft x-ray band (50 to 3000 eV) a STJ-based instrument with an active area of 73 7m m 2 is proposed as part of the payload of the X-ray Evolving Universe Spectroscopy (XEUS) mission, which is cur- rently under study at ESA. Future developments include devices based on lower Tc superconductors for improved energy resolution as well as larger format detector arrays combined with alternative readout schemes. © 2002 Society of Photo-Optical Instrumentation Engineers.

Some recent results associated with the development of tantalum based photon counting superconducting tunnel junctions (STJ) suitable for use as broad-band low resolution spectrometers for optical and ultraviolet astronomy are presented. A m square tantalum based STJ, operated at a temperature of 0.3 K, has demonstrated a limiting resolution of nm at 200 nm and nm at 1000 nm. The device is extremely linear in response with photon energy, and covers the waveband from 200 nm to while measuring the individual photon wavelength and arrival time. The short wavelength limit is currently constrained by the current experimental configuration (a fibre optic) as well as to some extent the sapphire substrate. The estimated quantum efficiency for single photons is over between 200 and 700 nm with a maximum of at 550 nm. Such an STJ when packaged into an array could contribute significantly to many fields of near infrared, optical and ultraviolet astronomy being able to provide efficiently and simultaneously the broad band spectrum and photon arrival time history of every single object in the field over a very wide dynamic range.

Superconducting tunnel junction (STJ) detectors are in common use for high-resolution soft X-ray spectroscopy at high count rates. Each quasiparticle in superconductor should be treated as quantum superposition of electron-like and hole-like excitations. This duality nature of quasiparticle leads to multitunneling in STJ. Because of the multitunneling process, one quasiparticle can transfers into heat the energy that equals to the difference in quasiparticle energy between two electrodes more than once. As a result, the energy transferred into heat in STJ detector is several times grater the energy of X-ray quantum. In this work, the theory of branching cascade processes is applied to the process of energy transfer caused by quasiparticle multitunneling. The mean and the variance of STJ temperature shift from the substrate temperature caused by heating of STJ by X-rays flux were derived.

Superconducting tunnel junction (STJ) detectors have excellent energy resolution, high detection efficiency, and high counting rate in the soft X-ray energy range. These properties are suitable for synchrotron radiation facilities. However, in order to achieve a high throughput analysis for dopants with a lower concentration, the pixel number of STJ array detectors should be further increased to enlarge the sensitive area. As a first step, we have fabricated 400-pixel STJ array detectors and checked the operation yield and the uniformity of leakage current (\(I_{leak}\)) values. Fabricated STJs showed excellent current-voltage curves having low subgap current. The operation yield of STJs with \(I_{leak}\) values of less than 10 nA was 96 \(\%\). The average \(I_{leak}\) of 3.6 nA is roughly equivalent to that of the 100-pixel STJ array detector, which is open to users at synchroton radiation beam lines and have 12 eV energy resolution at 400 eV.

By separating two thin layers of superconductor with an insulating barrier, scientists at the European Space Agency have created an optical detector that can detect a signal as faint as a single photon and determine its wavelength. That combination of abilities is far beyond current astronomical detectors, and laboratories are now vying to fabricate large arrays of these devices, called superconducting tunnel junctions (STJs), which could capture complete images. If these efforts succeed--cooling the devices and taming their electronic noise may be stumbling blocks--STJs could revolutionize the search for faint galaxies, planets around other stars, and pulsars.

Characterization of an Nb/Al/AlO x/Al/Nb superconducting tunnel junction detector with a very high spatial resolution in the soft X-ray range

Structural analysis of a cryogen-free refrigerator for space

Thermal conductivity of several fibre-reinforced composites between 2 K and 300 K

We present the development of Superconducting Tunnel Junction (STJ) detectors as a far-infrared single photon spectrometer, which is motivated for an application to a search for the radiative decay of the cosmic neutrino background (CνB). The photon energy spectrum from the radiative decays of CνB is expected to have a sharp edge at high energy end in a far-infrared region ranging from 14 meV to 25 meV (from 50 µm to 90 µm in wavelength) in the cosmic infrared background and the overwhelming infrared foreground from the zodiacal emission. Thus, the detector is required photon-by-photon detection with sufficiently high energy resolution, in order to gain the best signal-to-noise ratio as well as to identify the edge structure. The following two types of photon detectors are under consideration: an array of niobium/aluminum STJ (Nb/Al-STJ) pixels with a diffraction grating, and STJ using hafnium (Hf-STJ). Each Nb/Al-STJ pixel is required to be capable of detecting single photons in the far-infrared region, and the pixel array measures the photon wavelength spectrum which the diffraction grating creates. Hf-STJ is expected to achieve 2% energy resolution for single photon of 25 meV due to very small gap energy of hafnium.

Superconducting tunnel junction (STJ) detectors have potential as ultra-high-energy resolution detectors. Recently, single-junction detectors, where radiation is directly absorbed and detected by an STJ, have shown much higher energy resolutions than semiconductor detectors even at high count rates of over 104 counts per second, e.g. resolutions of 12 eV for 5.9 keV x-rays, 6 eV for 277 eV x-rays and 0.2 eV for 2 eV optical photons. The energy resolutions of series-junction detectors, where radiation is absorbed by a single-crystal substrate and the resulting non-thermal phonons are detected by STJs connected in series on the substrate, are not yet better than those of semiconductor detectors but are improving rapidly. This paper briefly introduces some principles of STJ detectors and reviews their recent developments. Copyright © 2000 John Wiley & Sons, Ltd.

Multiple tunneling of quasiparticles in a superconducting tunnel junction (STJ) detector increases the fluctuations in the measured charge. To optimize the energy resolution of an STJ detector, it is necessary to know the time dependence of the integrated charge and its noise. In this work, the theory of branching cascade processes was applied to the process of quasiparticle multitunneling. The duality nature of quasiparticles and the coupling of the quasiparticle and the phonon subsystems were taken into account. The formulae for time dependences of the mean value and the relative variance of the integrated charge were derived. Influence of a shaping amplifier on the relative variance of an STJ detector was also considered.

We are developing a cryogen-free Ta-based superconducting tunnel junction (STJ) detector for soft X-ray spectroscopy at synchrotrons. With an energy resolution 10 times higher than conventional solid-state X-ray detectors and count-rate capabilities above 5 kHz/pixel, STJ detectors offer potentially increased sensitivity for fluorescence-yield X-ray absorption spectroscopy (FY-XAS). We have developed 36-pixel arrays of 208 \(\times \) 208 \(\upmu \)m Ta STJs with an energy resolution of \(\sim \)9 eV FWHM at the 525 eV oxygen K line. Compared to earlier Nb-based STJs, Ta-STJs offer improved energy resolution and absorption efficiency and extend the operating range to several keV. Here we describe the integration of the 36-pixel arrays into a cryogen-free, user-friendly X-ray spectrometer. A computer-controlled adiabatic demagnetization refrigerator coupled to a two-stage pulse tube refrigerator allows operation below 100 mK. The detector chip is located at the end of a 42 cm shielded snout for insertion into the analysis chamber. The system is currently being commissioned at the Advanced Light Source synchrotron.

There are a number of processes in superconducting tunnel junction (STJ) detectors that lead to peak broadening. In many papers, semi-empirical approach is used to take into account the contribution of one or another factor to the signal variance of an STJ detector. The correct formula for the signal variance cannot be semi-empirical at the researcher’s option. It must result strictly from exact mathematical description of the particle registration by detector. In this work, using the theory of branching cascade processes the general expression for the energy resolution of STJ detector for optical photons and soft X-rays was derived. It was shown that the general formula encloses all specific contributions of the peak broadening published in literature and has additional contributions to the energy resolution caused by fluctuations of photon energy and of absorber temperature.

This work pays attention to the inconsistency of the commonly used formulae for the internal gain, and the multitunneling noise factor in superconducting tunnel junction (STJ) detectors with experimental data. The origin of this inconsistency is in the fact that these formulae give the mean value and the relative variance of the number of crossings of the insulating barrier by a quasiparticle, but not the net charge transferred through the barrier that actually determines the signal of an STJ detector. In this work, the theory of branching cascade processes was applied to the process of multiple tunneling in STJ. The duality nature of quasiparticles and the coupling of the quasiparticle and phonon subsystems were taken into account. The general formulae for time dependences of the moments of the charge multiplication factor were derived.