Superconducting tunnel junctions are promising detector elements in next generation x-ray, UV, and optical detectors. Quasiparticles which approach the neighborhood of the normal core may scatter and emit a phonon, thus being trapped in the Abrikosov vortex (AV). In this way the existence of AVs reduces the lifetime of quasiparticles. In this experiment excess quasiparticles in aNb/AlOx/Nbjunction are created by means of a low temperature scanning electron microscope. The trapping rate of quasiparticles into AVs is determined by measuring the quasiparticle lifetime as a function of the AV concentration. This results in an effective trapping radius of the AV of14.8±1.4 nm,which is slightly less than theoretical expectations. ©1997 American Institute of Physics.