[en] A cascaded linear system model that includes incomplete charge collection and interaction-depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency, DQE(0), of a direct conversion x-ray image detector. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) addition of electronic noise. The primary x-ray photon interaction and also the secondary K-fluorescent photon interaction are included in determining the interaction-depth dependent conversion gain across the photoconductor. We examine DQE(0) of a-Se detectors for fluoroscopic applications as a function of photoconductor thickness with varying amounts of electronic noise and x-ray exposure under (a) constant field, and (b) constant voltage operating conditions. We show that there is an optimum photoconductor thickness, which maximizes DQE(0) under a constant voltage operation. The optimum thickness depends on the added electronic noise, x-ray exposure, charge collection efficiency and bias voltage. For the quantities mentioned above that are appropriate for a-Se detectors and fluoroscopic applications, the optimum a-Se thickness is ∼700 μm and the corresponding DQE is ∼0.4. It is shown that the DQE depends strongly on the charge transport properties of the photoconductors. With the radiation-receiving electrode negatively biased, the DQE is more dependent on electron lifetime (τ_e) than hole lifetime (τ_h). Full electron trapping, (τ_e=0) reduces the DQE by about 73.3% at the detector thickness of 1000 μm whereas full hole trapping (τ_h=0) reduces the DQE by about 43.7%. The DQE for the negative bias is lower than for the positive bias, and the difference in DQE, as expected, increases with the photoconductor thickness because of the asymmetric transport properties of holes and electrons in a-Se. The present results show that the DQE generally does not continue to improve with greater photoconductor thickness because of charge carrier trapping effects. The DQE of a polyenergetic x-ray beam is only slightly lower than a monoenergetic x-ray beam with the same average photon energy. The theoretical model shows a very good agreement with the experimental DQE versus exposure characteristics published in the literature

[en] An analytical expression for calculating the modulation transfer function (MTF) due to distributed carrier trapping in the bulk of the photoconductor of a direct conversion pixellated x-ray image detector is derived using the trapped charge distribution across the photoconductor. The analytical expressions of trapped charge distributions are also derived by solving the continuity equation for both types of carriers (electrons and holes). The MTF of photoconductive x-ray detectors is analysed in terms of normalized parameters, namely (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness) and (b) normalized carrier schubwegs (schubweg/thickness). Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the carriers that move away from the pixels improves the sharpness of the x-ray image. The MTF model is applied to polycrystalline CdZnTe detectors and is fitted to recently published experimental results. The theoretical model shows very good agreement with reported experimental data

[en] We have examined the effect of high-dose x-ray irradiation on electron transport in stabilized amorphous selenium (a-Se) x-ray photoconductive films (of the type used in x-ray image detectors) by measuring the electron lifetime τ_e through interrupted-field time-of-flight experiments. X-ray induced effects have been examined through two types of experiments. In recovery experiments, the a-Se was preirradiated with and without an applied field (5 V/μm) during irradiation with sufficient dose (typically ∼20 Gy at 21 °C) to significantly reduce the electron lifetime by ∼50%, and then the recovery of the lifetime was monitored as a function of time at three different temperatures, 10 °C, 21 °C, and 35 °C. The lifetime recovery kinetics was exponential with a relaxation time τ_r that is thermally activated with an activation energy of 1.66 eV. τ_r is a few hours at 21 °C and only a few minutes at 35 °C. In experiments examining the irradiation induced effects, the a-Se film was repeatedly exposed to x-ray radiation and the changes in the drift mobility and lifetime were monitored as a function of accumulated dose D. There was no observable change in the drift mobility. At 21 °C, the concentration of x-ray induced deep traps (or capture centers), N_d, increases linearly with D (N_d ∼ D) whereas at 35 °C, the recovery process prevents a linear increase in N_d with D, and N_d saturates. In all cases, even under high dose irradiation (∼50 Gy), the lifetime was recoverable to its original equilibrium (pre-exposure) value within a few relaxation times

[en] Wideband quadrature frequency resolved spectroscopy (QFRS) of photoluminescence (PL) lifetime distributions from 2 ns to 160 s is shown to be very effective in elucidating the characteristic features of radiative transitions of Er³⁺ ions in GeGaSe and GeGaS chalcogenides glasses (ChGs). Undoped GeGaSe ChGs show triple-peak lifetime distributions of which two short-lifetimes are associated with singlet-triplet excitons and longest-lifetime, ∼20 s, with radiative tunnelling (RT) of distant-pairs (DPs). Er-doped GeGaSe and GeGaS ChGs exhibit a double-peak lifetime distribution, consisting of a peak at ∼3.3 and ∼5.3 ms, respectively, a characteristic of the Er³⁺ luminescence centre and another peak at ∼20 s, similar to that of undoped GeGaSe ChGs. It is shown that the QFRS can separate and analyse two mixed radiative transitions of Nd³⁺ ions, ⁴F_3/2V⁴I_15/2 and ⁴F_5/2,²H_9/2V⁴I_15/2 in GaLaS ChGs. From the QFRS results we can experimentally extract the branching ratio β_J and lifetime τ ∼ 77 (μs for 4 lasing transitions ⁴F_3/2→⁴I_J(J = 9/2, 11/2, 13/2, 15/2) of Nd³⁺ ions in GaLaS ChGs, in particular, the weakest transition ⁴F_3/2→⁴I_15/2.

[en] The domain structure of two selected single-crystal VO₂ microrods (MRs) with different morphologies was mapped by pixelated Raman imaging through the metal-to-insulator transition (MIT) during heating and cooling schedules through 68 °C. The results show that MIT does not occur simultaneously and uniformly throughout the whole MR, and instead, it proceeds through alternating metal–insulator domains. Each structural domain possesses its own MIT transition temperature and hysteresis width. The variations in MIT characteristics among different domains of a single MR are probably ascribed to structural nonuniformity. The observed overall MIT transition and hysteresis width of a given VO₂ single-crystal MR is an aggregate manifestation of the MIT properties of domains within the crystal. Graphic abstract:

[en] Electrical and optical properties Cs-doped a-Se_0.95As_0.05 (stabilized a-Se that has been alloyed with As) have been investigated. As expected there was no electron paramagnetic resonance signal on Cs-doped films or bulk samples, which put the spin-active defect concentrations below 10¹⁵ cm⁻³. The Cs-addition to a-Se_0.95As_0.05, leads to the n-type doping of a-Se_0.95As_0.05 in the sense that the undoped material has μ_hτ_h>> μ_eτ_e whereas the alkaline doped material has μ_eτ_e>> μ_hτ_h. The Cs addition also leads to a reduction of the refractive index n and a reduction of the glass transition temperature T_g, and affects the temporal relaxation behavior of a-Se film thickness after annealing and sequential quenching. We have measured the refractive index dispersion, n(λ) versus λ, bandgap (E_g) and Urbach width (ΔE) for undoped and Cs-doped films at room temperature and at a temperature just below the glass transition temperature. The photoluminescence (PL) experiments confirm earlier experiments that the PL emission is a broad emission spectrum with a significant Stoke’s shift following roughly the ~ E_g/2 empirical rule. The present work confirms that Cs-doped and As-stabilized a-Se is n-type.

[en] Although amorphous selenium (a-Se) has a long and successful history of application in optical and X-ray imaging, some of its fundamental properties are still puzzling. In particularly, the mechanism of carrier recombination following x-ray excitation and electric field and temperature dependences of the electron-hole pair creation energy (W_e_h_p) remain unclear. Using the combination of X-ray photocurrent and pulse height spectroscopy measurements, we measure W_e_h_p in a wide range of temperatures (218–320 K) and electric fields (10–100 V/µm) and show that the conventional columnar recombination model which assumes Langevin recombination within a column (a primary electron track) fails to explain experimental results in a wide range of electric fields and temperatures. The reason for the failure of the conventional model is revealed in this work, and the theory of the columnar recombination is modified to include the saturation of the recombination rate at high electric field in order to account for the experimental results in the entire range of fields and temperatures.

[en] Previous work has demonstrated that fluorophosphate (FP) glasses doped with trivalent samarium (Sm³⁺) can be used as a dosimetric detector in microbeam radiation therapy (MRT) to measure high radiation doses and large dose variations with a resolution in the micrometer range. The present work addresses the use of intense optical radiation at 405 nm to erase the recorded dose information in Sm³⁺-doped FP glass plates and examines the underlying physics. We have evaluated both the conversion and optical erasure of Sm³⁺-doped FP glasses using synchrotron-generated high-dose x-rays at the Canadian Light Source. The Sm-ion valency conversion is accompanied by the appearance of x-ray induced optical absorbance due to the trapping of holes and electrons into phosphorus-oxygen hole (POHC) and electron (POEC) capture centers. Nearly complete Sm²⁺ to Sm³⁺ reconversion (erasure) may be achieved by intense optical illumination. Combined analysis of absorbance and electron spin resonance measurements indicates that the optical illumination causes partial disappearance of the POHC and the appearance of new POEC. The suggested model for the observed phenomena is based on the release of electrons during the Sm²⁺ to Sm³⁺ reconversion process, the capture of these electrons by POHC (and hence their disappearance), or by PO groups, with the appearance of new and/or additional POEC. Optical erasure may be used as a practical means to erase the recorded data and permits the reuse of these Sm-doped FP glasses in monitoring dose in MRT

[en] Microbeam radiation therapy (MRT) utilizes highly collimated synchrotron generated x-rays to create narrow planes of high dose radiation for the treatment of tumors. Individual microbeams have a typical width of 30–50 µm and are separated by a distance of 200–500 µm. The dose delivered at the center of the beam is lethal to cells in the microbeam path, on the order of hundreds of Grays (Gy). The tissue between each microbeam is spared and helps aid in the repair of adjacent damaged tissue. Radiation interactions within the peak of the microbeam, such as the photoelectric effect and incoherent (atomic Compton) scattering, cause some dose to be delivered to the valley areas adjacent to the microbeams. As the incident x-ray energy is modified, radiation interactions within a material change and affect the probability of interactions, as well as the directionality and energy of ionizing particles (electrons) that deposit energy in the valley regions surrounding the microbeam peaks. It is crucial that the valley dose between microbeams be minimal to maintain the effectiveness of MRT. Using a monochromatic x-ray source with x-ray energies ranging from 30 to 150 keV, a detailed investigation into the effect of incident x-ray energy on the dose profiles of microbeams was performed using samarium doped fluoroaluminate (FA) glass as the medium. All dosimetric measurements were carried out using a purpose-built fluorescence confocal microscope dosimetric technique that used Sm-doped FA glass plates as the irradiated medium. Dose profiles are measured over a very a wide range of x-ray energies at micrometer resolution and dose distribution in the microbeam are mapped. The measured microbeam profiles at different energies are compared with the MCNP6 radiation transport code, a general transport code which can calculate the energy deposition of electrons as they pass through a given material. The experimentally measured distributions can be used to validate the results for electron energy deposition in fluoroaluminate glass. Code validation is necessary for using transport codes in future treatment planning for MRT and other radiation therapies. It is shown that simulated and measured micro beam-profiles are in good agreement, and micrometer level changes can be observed using this high-resolution dosimetry technique. Full width at 10% of the maximum peak (FW@10%) was used to quantify the microbeam width. Experimental measurements on FA glasses and simulations on the dependence of the FW@10% at various energies are in good agreement. Simulations on energy deposited in water indicate that FW@10% reaches a local minimum around energies 140 keV. In addition, variable slit width experiments were carried out at an incident x-ray energy of 100 keV in order to determine the effect of the narrowing slit width on the delivered peak dose. The microbeam width affects the peak dose, which decreases with the width of the microbeam. Experiments suggest that a typical microbeam width for MRT is likely to be between 20–50 µm based on this work. (paper)

[en] Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function (MTF) of amorphous selenium (a-Se) based active matrix flat panel imagers (AMFPIs) are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions: The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons (electrons remain in deep traps due to their long release times compared with the time scale of the experiments) and the deep trapping of drifting holes and electrons.