* Corresponding author. Nuclear Instruments and Methods in Physics Research A 444 (2000) 28}32 The lateral proximity e!ect and long-range energy-gap gradients in Ta/Al and Nb/Al Josephson junctions Roland den Hartog*, A. Golubov, D. Martin, P. Verhoeve, A. Poelaert, A. Peacock, M. Krumrey Astrophysics Division, Space Science Department of the European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands University of Twente, Department of Applied Physics, P.O.Box 217, 7500 AE Enschede, The Netherlands Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, D-10587 Berlin, Germany Abstract We present two independent experiments, each of which suggests that the local energy gap in Ta (and Nb) has a lateral spatial variation on a scale of several m. The "rst experiment is a series of current}voltage characterizations of Nb/Al/AlO  and Ta/Al/AlO  Josephson junctions, which reveals a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap. An extended version of the current theory of the proximity e!ect could explain this phenomenon when a lateral coherence length is introduced, which is of the order of the bulk coherence length of the materials. The second experiment is a series of coincidence measurements of photon absorption events in a Ta absorber between two Ta/Al junctions. There is a clear distinction in the pulse-height characteristics between events detected in the absorber and the junctions. Interestingly, there are also events indicating the presence of a transition region between the absorber and the junction. Event statistics imply that this region has a size of &6 m, independent of photon energy, which is quite a bit larger than even the bulk coherence length in Ta. It is argued that an additional e!ect due to &smearing' by the relaxed quasi-particle cloud must also be present. These e!ects are interesting and intriguing; not only from a theoretical viewpoint, but also for energy-gap engineering of supercon- ducting materials for practical applications, e.g. in a variety of photon and particle detectors.  2000 Elsevier Science B.V. All rights reserved. PACS: 85.25.Cp; 29.30.kv; 74.50.#r; 74.76.!W Keywords: Josephson devices; X- and -ray detectors; Proximity e!ects; Tunneling phenomena; Superconducting "lms 1. Introduction When a superconducting material is brought into electrical contact with another (super-)con- ducting material, the energy gap   is locally a!ected. This is in essence the proximity e!ect. With the advent of modern theoretical descriptions ([1,2] and References therein), the proximity e!ect is now well enough understood to use calculated predictions in the design of superconducting devi- ces. Examples of this &energy-gap engineering' are given by Poelaert [3]. Up to now most treat- ments of the proximity e!ect have been strictly 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 2 1 - 2