X-ray detectors based on superconducting tunnel junctions (STJs) have demonstrated good energy resolution in the soft
X-ray energy range 0.1-6 keV. In particular DROIDS (Distributed Read Out Imaging Devices), consisting of a
superconducting absorber strip with superconducting tunnel junctions as read-out devices on either end, could combine
this high resolving power with a large sensitive area and good soft X-ray detection efficiency.
In this paper we present results on the spectroscopic performance of Al and Ta/Al DROIDs with different
absorber materials (Ta, Re) and with variations in absorber configurations: our standard absorber integrated with the
read-out structure is compared with absorbers deposited after definition of the read-out structure. The latter allows
maximising the detection efficiency through thicker layers and different absorber materials. Also, absorbers which are
electrically coupled to the readout structure are compared to insulated absorbers which couple to the readout structure by
phonon exchange across a thin dielectric layer. New process routes have been designed for all new configurations.
Whilst not all these structures have been fabricated successfully yet, our integrated absorber sofar exhibits the best
performance, with 2.45 eV FWHM at 400 eV and 16.6 eV FWHM at 5.9 keV.
Results are presented on the development of a portable detector suitable for detection of individual thermal neutrons. The
device is based on direct absorption of neutrons in an absorber film containing 10B. The resultant charge arising from the
capture products is detected by a p-n junction partly formed from this absorber and internal to the device. When a small
bias voltage is applied (typically a few volts) a current pulse is observed due to the movement of this charge in the
electric field of the p-n junction. For each detected neutron the charge pulse height, rise time and time of detection are
recorded. Device performance, in terms of efficiency and spectral response, is explored as a function of neutron
absorber thickness, geometry and overall diode electrical characteristics and validated against neutron source
measurements at the UK National Physical Laboratory (NPL). The diodes have a natural background suppression
capability through traditional pulse height and pulse rise time discrimination. The manufacturing process permits
fabrication of arrays of diodes, with typical areas of ~15 mm2, thus increasing the collecting area and the signal to noise
ratio, albeit with increased readout complexity. The associated multi-channel readout electronics is standard, however,
and commonly used in existing X-ray sensors. Simple portable sensors based on these heterodiodes are expected to have
applications in the detection of nuclear materials in a variety of security related situations.
In this paper we present the preliminary results from experiments with Distributed Read Out Imaging Devices (DROIDs) in the optical and in the X-ray regime. For the optical results DROIDs of different lengths ranging from 200 to 700 μm have been used with an STJ lay-up of Ta/Al/AlOx/Al/Ta with thicknesses of 100/30/1/30/100 nm. With this data the behavior with different absorber length has been investigated to determine an optimal absorber size for a DROID array to be used in the optical wavelength regime. The optimum absorber size has been found to be 30x400 um. The X-ray data has been obtained with a similar device structure but with 60 nm aluminium trapping layers to increase the trapping of quasiparticles in the STJs. In this paper we only present the data obtained with the standard DROID size of 400μm. With this device an extensive set of measurements have been performed which involves; a scan in photon energy ranging from 50eV to 1900 eV, a scan in temperature and a scan in bias voltage. We report here only results from the preliminary analysis of the data obtained with readout electronics comprising the normal preamplifier and subsequent shaping stage. For the final analyzes the pulses resulting from the STJs have been digitized and are ready to be analyzed. The pulses have been used to estimate the decay time of the STJs which appear to be very short. This is probably caused by the poor trapping of quasiparticles. Detailed results on this process will be presented however at a later date.
The requirement on energy resolution for detectors in future X-ray satelite missions such as XEUS (X-ray Evolving
Universe Spectroscopy mission) is <2eV in the soft x-ray range of 50-2000 eV, with a detection efficiency >80%. In
addition, the requirements for field of view and angular resolution demand a detector array of typically 150x150 micron
sized pixels in a 30x30 pixel format. DROIDs (Distributed Read Out Imaging Devices), consisting of a superconducting
absorber strip with superconducting tunnel junctions (STJs) as read-out devices on either end, can fulfill these
requirements. The amplitudes of the two signals from the STJs provide information on the absorption position and the
energy of the incoming photon in the absorber. In this paper we present the development status of Ta/Al 1-D DROIDs, as
well as the the short term development program that should result in a full size XEUS array.
Superconducting tunnel junctions are being developed for application as photon detectors in astronomy. We present the latest results on the development of very high quality, very low critical temperature junctions, fabricated out of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic demagnetisation refrigerator. The contacts to the top and base electrodes of these junctions are fabricated either out of Nb or Ta, which has strong implications on the loss time of the quasiparticles. The Nb contacted junctions show quasiparticle loss times varying between 5 and 80 μsec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 μV, because of the set-in of strong non-equilibrium quasiparticle multiplication currents at higher bias voltages. The Ta-contacted junctions, on the other hand, show quasiparticle loss times in excess of 200 μsec. These long loss times lead to very strong quasiparticle multiplication, which prevents the stable biasing of the junctions even at very low bias voltages. Junction fabrication and characterisation are described, as well as the response of the detectors to monochromatic light with wavelengths varying from 250 to 1000 nm. The energy resolution of the detectors is discussed.
We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), 1- and 2-D photon-counting imaging spectrometers, based on Ta/Al-based STJs placed on a Ta absorber. Results obtained with highly collimated illumination with 10 keV X-ray photons clearly demonstrate the imaging capabilities of 2-D DROIDs. The derived spatial FWHM resolution is 7 micrometers for a 200 X 200 micrometers <SUP>2</SUP> absorber. With a 1-D DROID we have measured an intrinsic energy resolution of 15 eV FWHM for 6 keV photons. At high energies (E > 6 keV) the resolution is limited by spatial fluctuations in the qp recombination rate.
The next generation of superconducting tunnel junctions based on lower critical temperature superconductors such as hafnium are now under development. Such a material with a bandgap well below a meV has the potential to provide very high wavelength resolution at soft x-ray wavelengths. In this paper we report the first results on the characteristics of hafnium thin films deposited on r-plane sapphire. The physical properties of these films together with the electrical and superconducting characteristics are described. Currently the electrical conductivity of these films are limited by scattering from the films columnar grain structure. The superconducting transition temperature has been found to vary from approximately 137-200 mK, somewhat higher than that in the bulk, while the critical magnetic field applied in a direction parallel to the film is determined to be approximately 750 gauss far, larger than that observed in bulk hafnium.
Arrays of superconducting tunnel junctions (STJs) provide the possibility to perform high-resolution imaging spectrophotometry at x-ray wavelengths. We describe the applications of STJ arrays to x-ray astronomy, and present measurements on a 3 by 3 test array of Nb/Al-based STJs, operated at a temperature of 1.2 K, which illustrate the current photometric and spectroscopic capabilities of such devices. These results demonstrate the basic experimental feasibility of STJ arrays and indicate that there are no fundamental problems to be expected in the development of large-format x-ray detector arrays based on STJs.
Nb/Al/AlO<SUB>x</SUB>/Al/Nb superconducting tunnel junctions (STJs) have been studied extensively as photon detectors, because of their intrinsic capabilities in terms of charge output and energy resolving power. A critical element in such an STJ is the aluminum layer which separates the superconductive Nb from the AlO<SUB>x</SUB> tunnel barrier. In this paper, the role of this Al layer is investigated. The behavior of high quality STJs, differing by the Al thickness only, is analyzed. Five thicknesses ranging between 5 nm and 120 nm are considered. The charge output, the energy linearity and resolution for the case of 6 keV x-ray photons are discussed.
Measurements of the x ray response of niobium-based superconducting tunnel junctions with Al trapping layers are presented. Signal amplitudes equivalent to as many as 2.3 multiplied by 10<SUP>8</SUP> tunneled electrons for a 5.9 keV photon are observed, corresponding to an amplification factor of approximately 100 per initially created quasiparticle in niobium. The detectors, however, exhibit a significant non-linearity in their energy response. The energy resolution is approximately 140 eV FWHM at 5.9 keV.
X-ray spectra at 6 keV from niobium based superconducting tunnel junctions with highly transmissive tunnel barriers are presented. Signals from the two films can clearly by discriminated by their different temporal and pulse height characteristics. Levels of tunneled charge as high as 2.7 X 10<SUP>6</SUP> electrons at 5.9 keV are observed. The best energy resolution obtained at T equals 1.2 K is 53 eV FWHM including electronic noise, for a 50 X 50 micrometers <SUP>2</SUP> device in a configuration where the x-ray source is collimated to selectively illuminate the center part of the device. Non-linearity is observed which appears dependent on film volume, implying that self recombination may play an important role in these devices. The energy resolution is found to degrade with increasing magnetic field. The spectra from the polycrystalline top film appear significantly degraded if magnetic flux is deliberately trapped in the device.