* Corresponding author. Tel.: #31-71-565-5380; fax: #31- 71-565-4690. E-mail address: rdhartog@astro.estec.esa.nl (R. den Hartog). Nuclear Instruments and Methods in Physics Research A 444 (2000) 278}282 An X-ray photon-counting imaging spectrometer based on a Ta absorber with four superconducting tunnel junctions Roland den Hartog*, P. Verhoeve, D. Martin, N. Rando, A. Peacock, M. Krumrey, D.J Goldie Astrophysics Division, Space Science Dept. of the European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, D-10587 Berlin, Germany Oxford Instruments Scientixc Research Division, Newton House, Cambridge Business Park, Cambridge CB4 4WY, UK Abstract We present the "rst results obtained with a two-dimensional X-ray imaging spectrometer consisting of a 200200 m Ta absorber and read out by four Ta/Al superconducting tunnel junctions (STJs). A preliminary image reconstruction algorithm allows the visualisation of the di!raction pattern from a 5 m pinhole illuminated with 10 keV X-rays. The image suggests a spatial resolution better than 10 m. The algorithm does not take into account quasi-particle losses in the absorber. Hence, the pulse-height reconstruction is not optimal and the energy resolution varies signi"cantly across the absorber. The best energy resolution is obtained for a 2020 m area in the centre of the absorber, and amounts to &77 eV at a photon energy of 5895 eV, with a 70 eV electronic noise contribution.  2000 Elsevier Science B.V. All rights reserved. PACS: 85.25.cp; 29.30.kv; 74.76.-w 1. Introduction Compared to Si-based CCDs, superconductive detectors such as microcalorimeters and super-con- ducting tunnel junctions (STJs) o!er a superior energy resolution, a very broad energy range (from &1 to '10 keV), and the possibility of single- photon counting with high rates (up to several tens of kHz). However, the requirement of individual biasing and read-out of these devices impedes the development of the equivalent of a 1000 pixel imaging array [1,2]. A possible solution is o!ered by distributed read-out devices (DRODs), in which photons are absorbed in a single crystal of large dimensions [3] or a smaller poly-crystalline layer [4}6], and detected by STJs located at the corners or the edges of the absorber. Compared to an array of STJs, such a detector geometry provides a large sensitive area with a considerable reduction in the number of read-out connections. By time-coinci- dent event measurement it is possible to recon- struct both the absorption position and the energy of the incoming photons. Disadvantages of this approach are the relatively low sustainable count rate and the sensitivity of the absorber to #ux trapping. In this paper we discuss a DROD which com- bines the advantages of a high-quality epitaxial Ta absorber with a more moderate size of 200200 m, read out by 50 m square STJs. We present the results of two experiments, which 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 9 5 - 9