It was shown that γ-ray induced photocurrent in PbI2 films is determined by the competition of two processes: electron–hole pair generation due to X-ray photon absorption and their recombination on surface defects. As a result of these processes, PbI2 films of (94±4) μm in thickness have maximal charge collection properties. Electron–hole pair creation energy was estimated using the I–t and I–V characteristics of the films. The values obtained with these methods, (8.8±0.5) and (3.4±1.4) eV, respectively, are close to the theoretical prediction.

The response of TlBr crystals (both undoped and doped with either In1+ or Pb2+) to X-ray radiation was studied. It was experimentally observed that the response of TlBr, TlBr:Pb2+ and TlBr:In1+ crystals to X-ray irradiation strongly depends on the polarity of irradiated contact. A simple model based on X-ray and charge carriers’ interaction qualitatively explained the effect. The Tl1+ ions’ X-ray-induced mobility of 6.6×10−14cm2/Vs and diffusion coefficient of 1.7×10−15cm2/s at room temperature were estimated from X-ray-induced current–time measurements of TlBr and TlBr:Pb2+ crystals using suggested model.

PbI2 thick films were coated with different materials (Parylene C, HumiSeal and PbBr2) in order to improve the films’ surface properties. It was shown that Parylene C and HumiSeal coating decreased the dark current of PbI2 films and changed the shape of the I–V and I–t curves. X-ray-irradiated films demonstrated a significant increase in signal-to-noise ratio due to a decrease in the dark current, a slightly increased rise time, and a decreased decay time compared to non-coated PbI2 films. In addition, Pd atoms from the electrical contact easily penetrated the Parylene C protective layer over a period of 1–2h. The Pd diffusion coefficient and Parylene C dark resistivity estimated from these experiments were DPd=(3.6±2.1)×10−13cm2/s and ρParylene C=(130±70)TΩcm, respectively. Only the PbBr2 coating led to decreased rise and decay times compared to non-coated PbI2 films.

PbI2 polycrystalline films were grown on glass substrates with an indium-tin-oxide coating by a physical vapor deposition method using PbI2 raw materials purified by a zone refining technique.Numerous dark current-voltage, dark current-temperature, and dark current-time measurements were taken to study the bulk and surface electrical properties of the film. The impurity level which is responsible for dark current behavior of PbI2 films, E-a = 0.79+/-0.11 eV in energy gap, was calculated from dark current temperature dependencies. The motions of ions (impurities) determine a film's dark bulk and surface conductivity. The suggested polarization model, based on orientational polarization explains the films dark current decay over time. A surface defects model, which attributes the film's surface as the source and sink of defects, describes the features of PbI2 films I-V curves behavior. These physical hypotheses are in good agreement with currently available experimental data. The instability of the measured film's properties is determined by surface morphology that leads to the immense free surface of PbI2 films. (C) 2008 Elsevier B.V. All rights reserved.

An experimental method for determining the range of alpha particles in films based on I-V(s) analysis has been suggested. The range of 5.5 MeV alpha particles in PbI(2) films determined by this technique is 30+/-5 microm, and this value is in agreement with the value calculated by SRIM (the stopping and range of ions in matter), r=24 microm in PbI(2). More than 100 I-V(s) of PbI(2) films with different thicknesses and quality have been analyzed, and the influence of alpha particle radiation on PbI(2) I-V(s) curves has been studied. Developed analytical methods (dependence of current density on electric field and conception of surface defects) were used, and the method limitations are discussed. It was shown that I-V(s) demonstrate the tendency to obey Ohm's law under alpha radiation. On the other hand, dark conductivity of the lead iodide films shows a typical impure character that can lead to an overestimation of the alpha particles' range in PbI(2) films. After films were exposed to alpha radiation, the dark resistivity and I-V shape of some films improved. Also, a weak decrease of the charge carrier concentration, due to a decrease of the "surface defect" concentration ("surface refining"), was registered after successive measurements of I-V(s).

Polycrystalline PbI2 films were grown on glass substrates with a conductive indium–tin-oxide coating using a physical vapor deposition method. Numerous dark current–voltage and current–time dependencies were recorded in order to study the electrical and detector properties of the films. A very poor correlation between the physical properties of the films and technological parameters was observed. A decrease in the distance between the crucible and the substrate lead to an increase in film density. High density (about 5g/cm3) and good morphology films have been grown using a large crucible diameter, film growth rate of 2–4μm/h, a heater temperature of approximately 250°C, and a substrate-to-crucible distance of about 15–20mm. No real correlation between the mechanical and detector properties of the films was found. This lack of correlation could be explained by a contradiction between the physical properties of the PbI2 film (polarization, long relaxation time, charge accumulation effect) and the experimental setup (irradiation of full film surface and short term measurement). The linear dependence of film sensitivity to X-ray irradiation on dark current density was observed, and a simple physical model explaining such behavior is suggested. The multi-measurement technique is also suggested as a method for obtaining the quasi-equilibrium state of films.

Thallium bromide (TlBr) is a compound semiconductor with high density, high atomic numbers and wide bandgap. In addition, recent results indicate that the mobility-lifetime product of electrons can be quite high, approaching the values for CdTe and CZT. These properties make TlBr a very promising material for nuclear radiation detector at room temperature. In this paper we report on our investigation of the performance of planar TlBr detector under high flux x-rays irradiation. This study proposes an alternate contact method that reduces the polarization effects, and the afterglow for a wide range of high flux applications.

TlBr doped with In and Pb was synthesized and grown using a vertical Bridgman method. The segregation coefficients of both dopants in TlBr were calculated using the different experimental optical and electrical characteristics of (TlBr)0.973–(InBr)0.027 and (TlBr)0.972–(PbBr2)0.078 crystals. The solubility of In and Pb in the liquid phase is higher than their solubility in solid TlBr: ks(In)=1.06 and ks(Pb)=1.3–1.6. Doping with both impurities leads to degradation of the electrical, optical, and detector properties of TlBr.

Thallium Bromide is a wide band gap, high Z, high density semiconductor that has been investigated as a nuclear radiation detector. In this paper we present experimental results on material purification, crystal growth, detector fabrication and detector characterization. TlBr material was purified by both distillation and zone refining. Crystals with 10-mm and 16-mm diameter were grown by the vertical Bridgman method. Single element devices and arrays were fabricated and tested. Mobility-lifetime products were found to increase as a result of purification. Potential applications of TlBr detectors are medical imaging and homeland security.

Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 μm. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposition invariably leads to non-uniform electronic properties, which is challenging to both model and predict. To overcome this, tighter control of deposition parameters is required during the slow growth process (<10 μm/hour). Lead iodide films are characterized in forms of planar devices deposited onto conductive glass and active pixel arrays deposited onto a-Si TFT arrays1. Electronic properties (e.g. leakage current, gain) show little variation that can be traced to substrate choice. Films generally provide less than 100 pA/mm2 leakage current as they show saturation in gain (at approximate fields of 1 V/μm). We recently modified our readout electronics to accept positive bias. Using positive bias on the top electrode provides better charge collection for the lower mobility electrons and (despite process variability) better quality films can provide sensitivities greater than 6 μC/R*cm2, with only partial x-ray absorption, and show less than 20 pA/mm2 dark current.

Polycrystalline lead iodide (PbI2) is one of only a few materials commonly mentioned as a potential direct converter for digital X-ray sensors. Previous evaluations have noted higher than desirable leakage currents and commented on imaging characteristics from sensors constructed with PbI2 films deposited onto amorphous Si thin film transistor arrays. Changes in film growth parameters show a significant reduction in leakage current, to 10's of pA/mm2 (or less than 1pA/pixel). Sensitivity remains good but is limited by incomplete X-ray absorption and lag. Image samples are derived from two different a-Si TFT designs, demonstrating high resolution and good contrast.

The significant reduction of the back scattering cross section of bounded layer composed of chaotically distributed resistive metallic film scatterers compared to the regular system of the same scatterers has been shown The theoretical estimates and the experimental measurements are in a good agreement and approach the experimental data for analogous bounded sand slab for nearly grazing angle of incidence.

AbstractAn adiabatic formula for the contracted Hamiltonian in a reference space containing bound‐state eigenfunctions of degenerate energy levels embedded in the continuum is derived. A general factorization theorem for the dynamic operatorSα(0, – ∞/λ) is proved, and the cancellation of the pole singularities in the perturbation series of the contracted Hamiltonian in adiabatic form is discussed.

AbstractAdiabatic formulae for secular operators and contracted Hamiltonians in an arbirary combination of degenerate or quasidegenerate subspaces are derived. A detailed consideration of the adiabatic limit in the power series is given, and “stability” of proper linear combinations with respect to a transformation produced by the Sα‐matrix is proved.

AbstractA general variational principle for transition and density matrices is proposed. The principle is closely related to Rowe's variational treatment of the equations‐of‐motion method. It permits the simultaneous construction of coupled approximations for two eigenstates, and it is a straightforward extension of the usual variational method.