A. G. Kozorezov

Hard X-ray spectroscopy using a small-format TlBr array.

We report X-ray measurements on a prototype 3 � 3 TlBr pixel array, produced to assess the technological feasibility of making a Fano limited imager, which can operate near room temperature. The device was fabricated on monocrystalline material of size 2.7 � 2.7 � 1.0 mm 3 . It has a pixel size of 350 � 350mm 2 and pitch of 450mm. Measurements were carried out on all pixels over the energy range 5.9–662 keV using radioactive sources. The leakage currents were found to be low enough to allow room-temperature operation, with typical energy resolutions of B20 keV FWHM at 59.95 keV under full-area illumination. At a reduced detector temperature of –301C, these fell to B4 keV FWHM. Although the spectral performance of the present array is currently impaired by material limitations, its spectral acuity was found to be greatly enhanced by the small pixel effect. Additional photon metrology was carried out at the Hamburger Synchrotron-strahlungslabor radiation facility. Under monochromatic pencil beam illumination, the measured energy resolutions at 20 keV were B3 keV FWHM at � 301C. The spatial uniformity of the array was measured using a 50 � 50mm 2 , 20 keV monoenergetic X-ray beam, raster scanned over the entire active area. The response, in terms of count rate, gain and energy resolution was found to be uniform at the few percent level, consistent with statistics. It was observed during these measurements, that the X-ray response of the pixels was unstable, showing time-dependent gain shifts indicative of polarization effects. The magnitude of the effect was proportional to the total energy deposition per unit time. Lastly, the use of TlBr arrays in nuclear medicine applications is discussed with particular emphasis on radio-guided surgical probes. Recommendations for an optimized design are given. r 2002 Elsevier Science B.V. All rights reserved.

The X-ray response of TlBr.

We present the results of a series of X-ray measurements on several prototype TlBr detectors. The devices were fabricated from mono-crystalline material and were typically of size 2.7×2.7×0.8 mm3. The material is extremely pure, having impurity concentrations <100 ppm. The measured electron and hole mobility–lifetime products were found to be 3×10−4 and 1×10−5 cm2 V−1, respectively, which are about an order of magnitude higher than previously reported values. Three detectors were fabricated and extensively tested over the energy range 2.3–100 keV at three synchrotron radiation facilities: the Physikalisch-Technische Bundesanstalt (PTB) laboratory at the Berliner Elektronenspeicherring fur Synchrotronstrahlung (BESSY II), the European Synchrotron Radiation Research Facility (ESRF) and the Hamburger Synchrotron-strahlungslabor (HASYLAB) radiation facility. Room temperature energy resolutions under full-area illumination of 1.8 and 3.3 keV FWHM have been achieved at 5.9 and 59.95 keV, respectively. At reduced detector temperatures of −30°C, these fall to 800 eV and 2.6 keV FWHM, respectively. Under monochromatic pencil beam illumination, the measured energy resolutions at 6 and 60 keV were 664 eV and 3 keV FWHM at the same temperature. For energies <20 keV, the measured spectra display symmetric photopeaks. However, the peaks become increasingly tailed at higher energies. At the highest energies, the energy-losses due to the electrons and holes are clearly separated. Whilst the detectors gave reproducible results over 12 months of operation, it was observed that for synchrotron beam measurements above 45 keV, they were unstable, showing rate dependent gain shifts and polarization effects. These were not observed at lower energies. The spatial uniformity of the detectors was measured using a 50×50 μm2, 12 keV mono-energetic X-ray beam, raster scanned over the forward active area. Whilst two detectors were spatially uniform to a level commensurate with statistics, the third was not. In all cases, evidence was found for charge collection problems caused by field fringing.

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.

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.

Weak-Link Phenomena in AC-Biased Transition Edge Sensors

It has been recently demonstrated that superconducting transition edge-sensors behave as weak-links due to longitudinally induced superconductivity from the leads with higher