Two-dimensional model of a strip detector with two tunnel junctions (Q694303)

The paper presents a 2D model that examines how the energy resolution is affected by the geometrical dimensions of the absorber strip, quasiparticle losses in the absorber strip and its lateral boundaries, the efficiency of quasiparticle capture in the tunnel junction and also self-recombination of quasiparticles. First, at the statement of the problem, the cloud of non-equilibrium quasiparticles in the absorber strip is characterized by concentration, satisfying an equation of parabolic type with homogeneous boundary conditions of third kind. In the model, quasiparticles reaching the boundaries leave the strip and enter the trap, where they tunnel producing detector signals. As a result, these signals and the spectral line shape are defined by the strip size, quasiparticle capture efficiency in the tunnel junction, the quasiparticle loss parameter on the absorber lateral boundaries, and the quasiparticle recombination parameter. For a point source without quasiparticle recombination, the problem is solved by the separation of variables method. To investigate the effect of above parameters on the detector characteristics the detector signals are calculated for various photon absorption coordinates, numerically. To allow for recombination losses, the initial diffusion equation is solved and the trapped charges are calculated by numerical methods. In this case, the initial quasiparticle distribution is defined by the Gaussian function and the calculations are carried out by grid methods. The differential operators are replaced by difference operators, the Laplace operator is approximated on a five-point stencil, the derivatives with respect to the spatial variables at a point are replaced with the second difference derivative and the time derivative is replaced with the first difference derivative. As, it is shown by computations, the effect of self-recombination of non-equilibrium quasiparticles on detector signals is similar to the effect of boundary losses. At the following discussion of the basic idea of strip detectors, it is obtained an integral expression for the detector spectral line. Finally, numerical results show that boundary losses and quasiparticle recombination distort the spectral line shape and lower the energy resolution of the superconducting tunnel junction detectors.