On the limited amplitude resolution of multipixel Geiger-mode APDs

The limited number of active pixels in a Geiger-mode Avalanche Photodiode (G-APD) results not only in a non-linearity but also in an additional fluctuation of its response. Both these effects are taken into account to calculate the amplitude resolution of an ideal G-APD, which is shown to be finite. As one of the consequences, the energy resolution of a scintillation detector based on a G-APD is shown to be limited to some minimum value defined by the number of pixels in the G-APD.Comment: 5 pages, 3 figure

Energy Relaxation at a Hot-Electron Vortex Instability

At high dissipation levels, vortex motion in a superconducting film has been observed to become unstable at a certain critical vortex velocity v*. At substrate temperatures substantially below Tc, the observed behavior can be accounted for by a model in which the electrons reach an elevated temperature relative to the phonons and the substrate. Here we examine the underlying assumptions concerning energy flow and relaxation times in this model. A calculation of the rate of energy transfer from the electron gas to the lattice finds that at the instability, the electronic temperature reaches a very high value close to the critical temperature. Our calculated energy relaxation times are consistent with those deduced from the experiments. We also estimate the phonon mean free path and assess its effect on the flow of energy in the film.Comment: 8 pages, 7 figure

Vortex instability in molybdenum-germanium superconducting films

We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov (LO) picture with a quasiparticle-energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity, NDC, non-monotonic IV curveComment: 5 pages, 3 figure

Dynamics of vortex penetration, jumpwise instabilities and nonlinear surface resistance of type-II superconductors in strong rf fields

We consider nonlinear dynamics of a single vortex in a superconductor in a strong rf magnetic field $B_0\sin\omega t$. Using the London theory, we calculate the dissipated power $Q(B_0,\omega)$, and the transient time scales of vortex motion for the linear Bardeen-Stephen viscous drag force, which results in unphysically high vortex velocities during vortex penetration through the oscillating surface barrier. It is shown that penetration of a single vortex through the ac surface barrier always involves penetration of an antivortex and the subsequent annihilation of the vortex antivortex pairs. Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex velocities $v(t)$ results in a jump-wise vortex penetration through the surface barrier and a significant increase of the dissipated power. We calculate the effect of dissipation on nonlinear vortex viscosity $\eta(v)$ and the rf vortex dynamics and show that it can also result in the LO-type behavior, instabilities, and thermal localization of penetrating vortex channels. We propose a thermal feedback model of $\eta(v)$, which not only results in the LO dependence of $\eta(v)$ for a steady-state motion, but also takes into account retardation of temperature field around rapidly accelerating vortex, and a long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance $R_s$ calculated as a function or rf frequency and field. It is shown that trapped flux can result in a temperature-independent residual resistance $R_i$ at low $T$, and a hysteretic low-field dependence of $R_i(B_0)$, which can {\it decrease} as $B_0$ is increased, reaching a minimum at $B_0$ much smaller than the thermodynamic critical field $B_c$.Comment: 18 figure

Long-term operation of a multi-channel cosmic muon system based on scintillation counters with MRS APD light readout

A Cosmic Ray Test Facility (CRTF) is the first large-scale implementation of a scintillation triggering system based on a new scintillation technique known as START. In START, the scintillation light is collected and transported by WLS optical fibers, while light detection is performed by pairs of avalanche photodiodes with the Metal-Resistor-Semiconductor structure operated in the Geiger mode (MRS APD). START delivers 100% efficiency of cosmic muon detection, while its intrinsic noise level is less than 10^{-2} Hz. CRTF, consisting of 160 START channels, has been continuously operated by the ALICE TOF collaboration for more than 25 000 hours, and has demonstrated a high level of stability. Fewer than 10% of MRS APDs had to be replaced during this period.Comment: Proceedings of NDIP-2008. 8 pages, 8 figures, 6 reference

Performance Studies of Prototype II for the CASTOR forward Calorimeter at the CMS Experiment

We present results of the performance of the second prototype of the CASTOR quartz-tungsten sampling calorimeter, to be installed in the very forward region of the CMS experiment at the LHC. The energy linearity and resolution, as well as the spatial resolution of the prototype to electromagnetic and hadronic showers are studied with E=20-200 GeV electrons, E=20-350 GeV pions, and E=50,150 GeV muons from beam tests carried out at CERN/SPS in 2004. The responses of the calorimeter using two different types of photodetectors (avalanche photodiodes APDs, and photomultiplier tubes PMTs) are compared.Comment: 16 pages, 22 figs., submitted to EPJ-