Quasiparticle Diffusion in Al Films Coupled to Tungsten Transition Edge Sensors

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We report recent results obtained from several W/Al test devices on Si wafers fabricated specifically to better understand energy collection in phonon sensors used for the Cryogenic Dark Matter Search (CDMS) experiment. The devices under study consist of three different lengths of 250 μm-wide by 300 nm-thick Al absorber films, coupled to 250 μm x 250 μm (40 nm thick) W-TESs at each end of the Al film. An 55Fe source was used to excite a NaCl reflector producing 2.6 keV Cl X-rays that were absorbed in our test device after passing through a collimator. The impinging X-rays broke Cooper pairs in the Al film, producing quasiparticles that we detected after they propagated into the W-TESs. We studied the diffusion of these quasiparticles in the Al, trapping effects in the Al film, and energy transmission at the Al/W interfaces.