J. K. Wigmore

Lattice dynamics of a disordered solid-solid interface

Generic properties of elastic phonon transport at a disordered interface are studied. The results show that phonon transmittance is a strong function of frequency and the disorder correlation length. At frequencies lower than the van Hove singularity the transmittance at a given frequency increases as the correlation length decreases. At low frequencies, this is reflected by different power laws for phonon conductance across correlated and uncorrelated disordered interfaces which are in approximate agreement with the perturbation theory of an elastic continuum. These results can be understood in terms of simple mosaic and two-color models of the interface.

Picosecond dynamics of hot carriers and phonons and scintillator non-proportionality

We have developed a model describing the non-proportional response in scintillators based on non-thermalised carrier and phonon transport. We show that the thermalization of e-h distributions produced in scintillators immediately after photon absorption may take longer than the period over which the non-proportional signal forms. The carrier and LO-phonon distributions during this period remain non-degenerate at quasi-equilibrium temperatures far exceeding room temperature. We solve balance equations describing the energy exchange in a hot bipolar plasma of electrons/holes and phonons. Taking into account dynamic screening, we calculate the ambipolar diffusion coefficient at all temperatures. The non-proportional light yields calculated for NaI are shown to be consistent with experimental data. We discuss the implications of a non-equilibrium model, comparing its predictions with a model based on the transport of thermalised carriers. Finally, evidence for non-equilibrium effects is suggested by the shape...

Heat pulse scattering at rough surfaces: reflection

We have modelled the scattering of heat pulses from rough surfaces, as observed in reflection experiments. The effect of long-range irregularities was calculated in the eikonal approximation. For diffusive scattering from short-range irregularity, an analytic expression was obtained which is valid in the most common experimental arrangements. The model was used to interpret data obtained in heat pulse experiments on buried interfaces in silicon.

Matter at low temperatures

This book meets the need for a clear and unified introduction to the physics of matter at low temperatures. It is particularly suitable for advanced undergraduate and postgraduate students, but will also be useful for researchers newly entering this field. The authors discuss the diverse phenomena that occur at low temperatures, some of which have no analogue in the everyday world at room temperature, and indicate the fundamental significance that many of them carry for basic physics. They explain the general principles involved in the design and use of low temperature systems, and how experimental measurements can be achieved in practice. The final chapter presents a brief account of the more important applications of low temperatures that are now being realized. There is a bibliography at the end of each chapter. Contents: Introduction, Solids: structure and phonons. Solids: electronic properties. Superconductivity. Helium·4. Helium·3. Obtaining low temperatures. Applications.

Resolution limitation due to phonon losses in superconducting tunnel junctions

We report greatly enhanced energy resolving power of Ta∕Al superconducting tunnel junction (STJ) photon detectors. The improvement in resolution has led to the observation of features in the energy dependence of the resolving power, which are not predicted by the currently prevailing formulas. We show that these effects are related to phonon noise generated during energy down-conversion in the superconducting electrodes, and the dependence of phonon energy loss on the distance of a photon absorption site from the escape interface. The predictions of the model agree well with experimental data in the energy range 0.6–6eV.

Polarization effects in thallium bromide x-ray detectors

We present the results of a detailed experimental study of polarization effects in thallium bromide planar x-ray detectors. Measurements were carried out in the range 10–100 keV by scanning a highly focused x-ray beam, 50 μm in diameter, from a synchrotron source across the detector. Above a certain radiation threshold the detector response showed a systematic degradation of its spectroscopic characteristics, peak channel position, peak height, and energy resolution. Using a pump-and-probe technique, we studied the dynamics of spectral degradation, the spatial extent and relaxation of the polarized region, and the dependence of the detector response on bias voltage and temperature. For comparison, we modeled polarization effects induced by the charging of traps by both electrons and holes using a model based on recent theoretical work of Bale and Szeles. We calculated the charge collection efficiency and spectral line shapes as functions of exposure time, beam position, count rate, and photon energy, and ...

Analytic model for the spatial and spectral resolution of pixellated semiconducting detectors of high-energy photons

We report the development of a general analytic method for describing the responsivity and resolution for a pixellated semiconductor detector structure in terms of device and material properties. The method allows both drift and diffusive transport to be modelled, for which previously only Monte Carlo techniques have been available. We obtain a general solution, and show specific results for an array of square pixels, illustrating the device constraints required to optimize spatial and spectral resolution.

Hard x-ray response of a CdZnTe ring-drift detector

We present the results of an experimental study of a special type of CdZnTe detector of hard x and γ rays—a ring-drift detector. The device consists of a double ring electrode structure surrounding a central point anode with a guard plane surrounding the outer anode ring. The detector can be operated in two distinctively different modes of charge collection—pseudohemispherical and pseudodrift. We study the detector response profiles obtained by scanning the focused x-ray beam over the whole detector area, specifically the variations in count rate, peak position, and energy resolution for x rays from 10 to 100 keV. In addition, at 662 keV the energy resolution was shown to be 4.8 keV, more than a factor of 2 better than for CdZnTe coplanar grid detectors. To interpret the experimental data, we derive an analytical expression for the spatial distribution of the electric field inside the detector and neglecting carrier diffusion, and identify carrier collection patterns for both modes of operation within the...

Time dependence of tunnel statistics and the energy resolution of superconducting tunnel junctions

Multiple tunneling of quasiparticle charge carriers in a superconducting tunnel junction (STJ) enhances the signal generated by a photon absorption event. It is also an additional source of noise, responsible for a substantial degradation of the energy resolution. Although tunneling is a binomial chance process, governed by a constant tunneling probability, the resulting cumulative statistics of tunnelled quasiparticles depend on time. In particular, the variance of the total number of tunneled quasiparticles reaches a minimum after a finite integration time, corresponding to a minimum in the spectral linewidth. Since the intrinsic energy resolution of the present generation of STJs is mainly limited by the scatter on the number of tunneled quasiparticles, the improvement of the tunnel noise can be experimentally tested by variation of the pulse integration time. An analytical theory is developed that describes the relation between the tunnel noise and the transfer function of the pulse integration hardwa...

Local trap spectroscopy in superconducting tunnel junctions

We show that thermal activation of quasiparticles from local traps is responsible for the temperature variation of responsivity observed for some superconducting tunneling junction photon detectors. With this model, the depth of the local traps in two different proximized Ta structures was found to be the same, 0.20±0.02 meV.