Imaging spectroscopy with Ta/Al DROIDs, performance for different Al trapping layer thicknessess
Publication date: 16 April 2006
Authors: Hijmering, R.A., et al.
Journal: Nuclear Instruments and Methods in Physics Research Section A
Volume: 559
Issue: 2
Page: 689
Year: 2006
Copyright: Elsevier
To overcome the limited field of view which can be achieved with single STJ arrays, DROIDs (Distributed Read Out Imaging Devices) 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 in the STJs. The trapping will change with thicker Al layers because the energy gap of the devices will decrease. A series of 50x200µm and 20x200µm absorbers (including 50x50µm STJs) and different Al trapping layer thicknesses, ranging from 65 to 130nm, 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 quasiparticles into the STJ and extra losses to the absorber.
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