Distributed Read-Out Imaging Devices Research Articles

Experimental and theoretical response of distributed read-out imaging devices with imperfect charge confinement

We present a model to describe the responsivity of distributed read-out imaging devices following photon absorption in the absorber or in the base or top film of the superconducting tunnel junctions at either end of the absorber. The model describes the processes most relevant for photon detection, taking into account diffusion of quasiparticles across the absorber and imperfect confinement in the superconducting tunnel junctions via exchange of quasiparticles between absorber and the junction. It incorporates diffusion mismatch between superconducting tunnel junction and absorber, possible asymmetry between the two junctions and asymmetry between base and top electrodes within each junction. We have conducted dedicated experiments in which different experimental conditions were varied in order to test the model. A good agreement was found between the experimental results and model predictions.

First results of a cryogenic optical photon-counting imaging spectrometer using a DROID array

Context. In this paper we present the first system test in which we demonstrate the concept of using an array of Distributed Read Out Imaging Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam 3 detector the next step in the development of a cryogenic optical photon counting imaging spectrometer under the S-Cam project is to increase the field of view using DROIDs. With this modification the field of view of the camera has been increased by a factor of 5 in area, while keeping the number of readout channels the same. Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel Junction array for a 3x20 DROID array. The extra data reduction needed with DROIDs is performed offline. Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV) of the current array is too low for direct astronomical applications, the imaging quality is already good enough for pattern detection, and will improve further with increasing responsivity. Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.

Position Resolution in DROIDs

Since the very beginning, Distributed Read-Out Imaging Devices (DROIDs) were proposed to achieve both good position and energy resolutions. In DROIDs, the absorption of primary particle energy occurs in a long superconductive strip. Quasiparticles produced in the absorber diffuse along the strip and counted by the superconductive tunnel junctions positioned at the two ends of the strip. In this paper the formula for estimation DROID’s position resolution from experimental data was derived. This formula takes into account correlation between fluctuations of signals of DROID’s detectors.

Injection-Detection Experiments in All Aluminum 1-D Imaging Spectrometers Based on Superconducting Tunnel Junctions

We report on a class of low temperature radiation detectors based on superconducting tunnel junctions (STJs) in which the incoming radiation is absorbed in a long superconducting strip while the readout operation occurs at the two ends of the strip, where two STJs are laterally positioned. These Distributed Read-Out Imaging Devices, or DROIDs, provide spectroscopy, 1-D imaging, single-photon sensitivity, and high quantum efficiency, all in one device. Typically these devices are realized by using Tantalum for the absorber strip and Aluminum for the two STJs. In this way the quasi-particles are created in the Tantalum and subsequently trapped in the Aluminum. As illustrated here, it is possible to fabricate a DROID using a single superconducting material. This choice gives up the trapping effect but has the advantage of eliminating the interface between different superconducting materials. Such a device combines the best quality STJs, large diffusion and lifetime values, with low energy gap for improved energy and position resolution. We report on measurements of current injection done on prototype devices, which demonstrates that STJs can serve as quasi-particle sinks and facilitate charge division in DROIDs. For sufficiently high tunneling rates, DROIDs based on a single material may be able to obtain performances comparable to DROIDs based on two materials.

Characterisation and modelling of transition edge sensor distributed read-out imaging devices

We report on the experimental characterisation and modelling of Transition Edge Sensor (TES)-based Distributed Read-Out Imaging Devices (DROIDs), for use as position-sensitive detectors in X-ray astronomy. Latest experimental results from prototype DROIDs using Ir TESs with Au absorbers are reported. Through modelling and the development of signal processing algorithms we are able to design the DROID for optimum spectral and spatial resolution depending upon application.

Signal processing for distributed readout using TESs

We describe optimal filtering algorithms for determining energy and position resolution in position-sensitive Transition Edge Sensor (TES) Distributed Read-Out Imaging Devices (DROIDs). Improved algorithms, developed using a small-signal finite-element model, are based on least-squares minimisation of the total noise power in the correlated dual TES DROID. Through numerical simulations we show that significant improvements in energy and position resolution are theoretically possible over existing methods.

Imaging spectroscopy with Ta/Al DROIDs: Performance for different absorber lengths

To overcome the limited field of view that can be achieved with single Superconducting Tunneling Junctions (STJ) arrays, Distributed Read Out Imaging Devices (DROIDs) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position and the sum signal is a measure for the energy of the absorbed photon. To produce a large field of view with the least number of connection wires possible, the size of the DROID is an important parameter. A set of devices with different lengths, ranging from 200 to 700 μm, has been tested at optical wavelengths. The widths of the DROIDs are 30 μm with 30×30 μm 2 STJs Ta/Al STJs on either side. With 30 nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300 mK. All devices have been processed on a single wafer and are located on the same chip. Thus the STJs are all identical and any variation in response can be attributed to a difference in geometry. The position resolution is found to be degraded for shorter absorbers due to cross-talk between the two STJs. The charge output of the different devices decreases with length due to reduced tunnel probability and losses in the absorber.

Imaging spectroscopy with Ta/Al DROIDs: Performance for different Al trapping layer thicknesses

To overcome the limited field of view, which can be achieved with single superconducting tunneling junction (STJ) arrays, distributed read-out imaging devices (DROIDs) are being developed. DROIDs consist of a superconducting absorber strip with proximized STJs on either end. The ratio of the two signals from the STJs provides information on the absorption position, and the sum signal is a measure for the energy of the absorbed photon. In our devices, the absorber is an epitaxial Ta strip that extends underneath the Ta/Al read-out STJs. Thus, the bottom electrode of the STJs is an integral part of the absorber. Due to the proximity effect, the STJs have a lower energy gap than the absorber, causing trapping of quasiparticles (QPs) in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50×200 μm 2 and 20×200 μm 2 absorbers (including 50×50 μm 2 STJs) and different Al trapping layer thicknesses, ranging from 65 to 130 nm, have been tested. The devices have been illuminated with 6 keV 55Fe photons. The position resolution is found to improve with increasing Al thickness. It is found that the current model needs to be adapted for DROIDs to account for different injection of QPs into the STJ and extra losses to the absorber.