Quasiparticle diffusion, edge losses, and back-tunneling in superconducting tunnel junctions under x-ray irradiation
Journal of Applied Physics 86:8 (1999) 4580-4587
Abstract:
The efficiency of the charge collecting mechanism of a radiation detector based on superconducting tunnel junctions depends strongly on the competition between tunneling and losses of quasiparticles in the electrodes of the junction. In this article we discuss the loss of quasiparticles, both in the bulk and at the edges of the electrodes. The quasiparticle loss taking place during their diffusion in the junction limits the amount of charge carriers tunneling and thereby the energy resolution of the detector. A charge amplification process caused by multiple tunneling of quasiparticles is beneficial to higher signal to noise ratios. Such multiple tunneling is always present in the case of junctions based on symmetrical electrodes. In this article we present a theory for the detector response which takes into account both multiple tunneling and the diffusion of quasiparticles. A model is presented for the spectral response of the detector based on four parameters one of which is the degree of back-tunneling present in the device. Analytical solutions of the model for rectangular and circular junctions are discussed. The predictions of the theory are compared with an experimental pulse height spectrum obtained on a Nb/Al2O3/Nb junction. © 1999 American Institute of Physics.High-resolution x-ray spectroscopy with superconducting tunnel junctions
X-RAY SPECTROM 28:5 (1999) 309-311
Abstract:
An energy resolution of Delta E = 12 eV (FWHM) was measured for the Mn-55 K alpha(1) line (E = 5.90 keV) using a single superconducting Al/AlxOy/Al tunnel junction. The total detector area of 100 x 100 mu m was illuminated, while a slit mask of about 150 mu m width was used to shadow the detector leads and substrate area from the impinging x-rays. The total electronic noise contribution to the energy resolution was measured as Delta E-elec = 7 eV (FWHM). The process of tunnel junction fabrication was optimized towards a reproducible and high-quality growth of the tunnel barrier. High-quality aluminium films promote long quasi-particle lifetimes and ensure reproducible tunnel barrier growth. The residual resistance ratio of a 1 mu m thick aluminium test film was about 100, Copyright (C) 1999 John Wiley & Sons, Ltd.Determination of quasidiffusive phonon propagation in BaF2 using pulse shape analysis and possible implications for particle detection
PHYSICA B 263 (1999) 610-612
Abstract:
We have shown that for quasidiffusive phonon propagation in an isotropic crystal the diffusion coefficient D = KL8/9, where K is a material constant and L is the distance between the phonon source and point of detection. We have demonstrated this is the case for BaF2 bombarded by alpha-particles. This implies that pulse shape analysis may provide a mechanism to achieve position sensitivity in phonon mediated detectors. (C) 1999 Elsevier Science B.V. All rights reserved.Propagation of non-thermal phonons induced by α-particle bombardment in BaF2
Journal of Applied Physics 85:3 (1999) 1302-1310
Abstract:
We have measured the time dependence of the flux of non-thermal phonons generated by α particles in a BaF2 single crystal of mass 1 g using a Series Array of Superconducting Tunnel Junctions. Taking advantage of the almost perfect elastic symmetry of BaF2, we approximate the equations of quasidiffusive propagation to give a simple diffusion equation characterized by an effective diffusion coefficient D = KL8/9, where K is a material constant and L the distance between the phonon source and the point of detection. We use this model to perform pulse-shape analysis on our data and show that agreement is excellent, and highly preferred over a ballistic pulse shape. The distributions of values of K determined for different lengths L are found to overlap strongly, in support of the law D=KL8/9. We suggest that pulse-shape analysis may provide an alternative to time-difference measurements to achieve position sensitivity in a phonon-mediated detector based on a BaF2 absorber. © 1999 American Institute of Physics.The CRESST dark matter search
Astroparticle Physics 12:1-2 (1999) 107-114