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Imaging spectroscopy with Ta/Al DROIDs, performance for different absorber lengths

Imaging spectroscopy with Ta/Al DROIDs, performance for different absorber lengths

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: 692-694
Year: 2006

Copyright: Elsevier

To overcome the limited field of view that 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. 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 700µm, has been tested at optical wavelengths. The widths of the DROIDs are 30µm with 30x30µm STJs Ta/Al STJs on either side. With 30nm layer thickness of Al the trapping of quasiparticles in the STJ is not optimal, but the devices can comfortably be operated at 300mK. 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.

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