We describe the use of a miniature microwave Fiske cavity coupled to a Josephson junction oscillator to monitor the dynamic quasiparticle population created in the surrounding superconductor by incident x rays. An expression is obtained for the phase velocities of the TM modes supported by this structure. The design has the advantage of being inherently thick—sufficient to completely absorb low energy x rays and is potentially scalable to a pixilated device with a pixel size 50 &mgr;m in linear dimension. The device represents an alternative strategy to implement the dynamic microwave absorption approach suggested by Gulian and Van Vechten and subsequent variations. ©1996 American Institute of Physics.