We performed macroscopic experiments on the motion of a sphere through an array of obstacles that highlight the deterministic nature of the lateral displacements that lead to particle separation in microfluidic systems. The motion of the spheres is irreversible and displays directional locking. The locking directions can be predicted with a single parameter that distinguishes between reversible and irreversible particle-obstacle collisions. These results stress the need to incorporate irreversible interactions to predict the movement of a non-Brownian sphere passing through a periodic array.
When energy is deposited in a thin-film cryogenic detector, such as from the absorption of an x-ray, an important feature that determines the energy resolution is the amount of athermal energy that can be lost to the heat bath prior to the elementary excitation systems coming into thermal equilibrium. This form of energy loss will be position dependent and therefore can limit the detector energy resolution. An understanding of the physical processes that occur when elementary excitations are generated in metal films on dielectric substrates is important for the design and optimization of a number of different types of low-temperature detectors. We have measured the total energy loss in one relatively simple geometry that allows us to study these processes and compare measurements with calculation based upon a model for the various different processes. We have modeled the athermal phonon energy loss in this device by finding an evolving phonon distribution function that solves the system of kinetic equations for the interacting system of electrons and phonons. Using measurements of device parameters such as the Debye energy and the thermal diffusivity we have calculated the expected energy loss from this detector geometry, and also the position-dependent variation of this loss. We have also calculated the predicted impact on measured spectral lineshapes and have shown that they agree well with measurements. In addition, we have tested this model by using it to predict the performance of a number of other types of detector with different geometries, where good agreement is also found.
The ideal X-ray camera for astrophysics would have more than a million pixels and provide an energy resolution of better than 1 eV FWHM for energies up to 10 keV. We have microfabricated and characterized thin-film magnetic penetration thermometers (MPTs) that show great promise towards meeting these capabilities. MPTs operate in similar fashion to metallic magnetic calorimeters (MMCs), except that a superconducting sensor takes the place of a paramagnetic sensor and it is the temperature dependence of the superconductor's diamagnetic response that provides the temperature sensitivity. We present a description of the design and performance of our prototype thin-film MPTs with MoAu bilayer sensors, which have demonstrated an energy resolution of ∼2 eV FWHM at 1.5 keV and 4.3 eV FWHM at 5.9 keV.
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