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

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<mark>Journal publication date</mark>04/2006
<mark>Journal</mark>Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Issue number2
Number of pages3
Pages (from-to)689-691
Publication StatusPublished
<mark>Original language</mark>English


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 μm2 and 20×200 μm2 absorbers (including 50×50 μm2 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.