Constantinos Nomicos

X-ray luminescence of ZnSCdS:Au,Cu phosphor using X-ray beams for medical applications

Abstract The purpose of the present study was to evaluate the X-ray luminescence and imaging performance of phosphor screens prepared from ZnSCdS:Au,Cu. Absolute efficiency, X-ray luminescence efficiency, detector optical gain, and gain transfer function were experimentally determined. Theoretical models were also employed to fit experimental data and to determine optical properties of the phosphor material. Additionally, the emission spectrum of ZnSCdS:Au,Cu was measured and its compatibility with the spectral sensitivity of radiographic optical detectors (films, photodiodes) was determined. Results showed that ZnSCdS:Au,Cu is an efficient phosphor exhibiting high intrinsic X-ray to light conversion efficiency (0.17) and an excellent spectral compatibility (0.9) with amorphous silicon photodiodes, used in optical detectors of modern digital radiography systems.

Linking electromagnetic precursors with earthquake dynamics: An approach based on nonextensive fragment and self-affine asperity models

Understanding the earthquake (EQ) preparation process in terms of precursory electromagnetic (EM) emissions has been an evolving field of multi-disciplinary research. EM emissions in a wide frequency spectrum ranging from kHz to MHz are produced by opening cracks, which can be considered as precursors of general fracture. An important feature, observed on both laboratory and geophysical scale, is that the MHz radiation systematically precedes the kHz one. Yet, the link between an individual EM precursor and a distinctive stage of the EQ preparation comprises a crucial open question. A recently proposed two-stage model on preseismic EM activity suggests that the MHz EM emission is due to the fracture of the highly heterogeneous system that surrounds the fault. The finally emerged kHz EM emission is rooted in the final stage of EQ generation, namely, the fracture of entities sustaining the system. In this work we try to further penetrate and elucidate the link of the precursory kHz EM activity with the last stage of EQ generation building on two theoretical models for EQ dynamics. First, the self-affine model states that an EQ is due to the slipping of two rough and rigid fractional Brownian profiles, one over the other, when there is an intersection between them. Second, the fragment–asperity model, rooted in a nonextensive Tsallis framework starting from first principles, consists of two rough profiles interacting via fragments filling the gap. In the latter approach, the mechanism of triggering EQ is established through the interaction of the irregularities of the fault planes and the fragments between them. This paper shows that these models of EQ dynamics can be linked with the detected kHz EM emission. In this framework of analysis of preseismic EM activity, we identify sufficient criteria that offer the possibility to discriminate whether a seismic shock is sourced in the fracture of fragments filling the gap between the rough profiles or in the fracture of “teeth” distributed across the fractional Brownian profiles that sustain the system.

Critical features in electromagnetic anomalies detected prior to the L’Aquila earthquake

Earthquakes (EQs) are large-scale fracture phenomena in the Earth’s heterogeneous crust. Fracture-induced physical fields allow a real-time monitoring of damage evolution in materials during mechanical loading. Electromagnetic (EM) emissions in a wide frequency spectrum ranging from kHz to MHz are produced by opening cracks, this can be considered as the so-called precursors of general fracture. We emphasize that the MHz radiation appears earlier than the kHz on both laboratory and geophysical scales. An important challenge in this field of research is to distinguish characteristic epochs in the evolution of precursory EM activity and identify them with the equivalent last stages in the EQ preparation process. Recently, we proposed the following two-stage model. (i) The first epoch, which includes the initial emergent MHz EM emission, is thought to be due to the fracture of a highly heterogeneous system that surrounds a family of large high-strength asperities distributed along the activated fault sustaining the system. (ii) The second epoch, which includes the emergent strong impulsive kHz EM radiation, is due to the fracture of the asperities themselves. A catastrophic EQ of magnitude Mw=6.3 occurred on 6 April, 2009 (06/04/09) in central Italy. The majority of the damage occurred in the city of L’Aquila. Clear kHz–MHz EM anomalies had been detected prior to the L’Aquila EQ. Here, we investigate the seismogenic origin of the MHz part of the anomalies. The analysis, which is in terms of intermittent dynamics of critical fluctuations, reveals that the candidate EM precursor (i) can be described as analogous to a thermal continuous phase transition and (ii) has anti-persistent behavior. These features suggest that this candidate precursor was triggered by microfractures in the highly disordered system that surrounded the backbone of asperities of the activated fault. A criterion for underlying strong critical behavior is introduced. In this field of research, reproducibility of results is desirable; and is best done by analyzing a number of precursory MHz EM emissions. We refer to previous studies of precursory MHz EM activities associated with nine significant EQs that have occurred in Greece in recent years. We conclude that all the MHz EM precursors studied, including the present one, can be described as analogous to a continuous second-order phase transition having strong criticality and anti-persistent behavior.

An experimental method for the determination of spatial-frequency-dependent detective quantum efficiency (DQE) of scintillators used in X-ray imaging detectors

Abstract The spatial-frequency-dependent detective quantum efficiency (DQE) of imaging scintillators was studied independently of the optical detector (film, photocathode, or photodiode) employed in medical imaging devices. A method was developed to experimentally determine the scintillator DQE in terms of its luminescence efficiency (LE), quantum detection efficiency, modulation transfer function, and light emission spectrum. Gd2O2S : Tb, La2O2S : Tb, Y2O2S : Tb and ZnSCdS : Ag scintillating screens were prepared in laboratory and were excited to luminescence by a medical X-ray tube. Maximum DQE values varied between 0.13 and 0.33 depending on the scintillator material, the screen coating weight, and the tube voltage; Gd2O2S : Tb was superior to La2O2S : Tb followed by ZnSCdS : Ag and Y2O2S : Tb. This ranking was maintained at frequencies up to 100 cycles/cm. Considering the same material, DQE of thin screens was found superior to DQE of thicker screens at medium-to-high frequencies. The proposed method allows for the comparison of imaging characteristics of scintillating materials without the inclusion of optical detector imaging properties.

Self-organised critical features in soil radon and MHz electromagnetic disturbances: Results from environmental monitoring in Greece

This paper addresses the issues of self-organised critical behaviour of soil-radon and MHz-electromagnetic disorders during intense seismic activity in SW Greece. A significant radon signal is re-analysed for environmental influences with Fast Fourier Transform and multivariate statistics. Self-organisation of signals is investigated via fractal evolving techniques and detrended fluctuation analysis. New lengthy radon data are presented and analysed accordingly. The data did not show self-similarities. Similar analysis applied to new important concurrent MHz-electromagnetic signals revealed analogous behaviour to radon. The signals precursory value is discussed.

Radon-222: A Potential Short-Term Earthquake Precursor

This paper attempts to survey and catalog published short-term pre-earthquake precursors based on radon gas emissions. A series of papers were searched to collect relevant data, such as the epicentral distance, the extent, time and duration of the radon disturbance and to analyze the precursory value of each observable. In general, enhanced radon emissions have been observed prior to earthquakes and this has been recorded all over the world. The abnormal radon exhalation from the interior of earth has been associated with earthquakes and is considered an important field of research. The proposed physical models attempt to relate the observed radon disturbances with deformations occurring in the earths crust prior to forthcoming earthquakes. While the models provide some physical explanations, there are many parameters that require further investigation.

Long-range memory patterns in variations of environmental radon in soil

This paper addresses issues of long-range memory and self-organisation in variations of radon in soil in Greece. The methods of rescaled-range, roughness-length, variogram, fractal dimension and block entropy were employed through lumping. Sliding was utilised with the wavelet spectral fractal technique. Antipersistent Hurst exponents in the range 0 < H < 0.5 were mainly identified. Persistent exponents (0.5 < H < 1) were also detected. Switching between persistency and antipersistency was observed and considered consistent with an underlying geo-environmental long-memory self-organisation. Fractal dimensions were in the range 1.2 < D < 2. The anomalous parts of the 2008 radon signal presented significantly lower fractal dimensions. Value ranges of Shannon, Shannon-per-letter, conditional, Tsallis and normalised Tsallis block entropies were 0.67 ≤ H(n) ≤ 2.73, 0.2 ≤ h(n) ≤ 0.7, 0.2 ≤ h(n) ≤ 0.6, 0.36 ≤ Sq ≤ 1.11, 0.50 ≤ Ŝq ≤ 9.55 respectively. The entropy values were affected by the block-size n. The entropic index values of the radon anomalies were significantly lower indicating long-memory underlying patterns. Underlying sources are discussed. The asperity-model is proposed.

Discrimination between pre-seismic electromagnetic anomalies and solar activity effects

Laboratory studies suggest that electromagnetic emissions in a wide frequency spectrum ranging from kilohertz (kHz) to very high megahertz (MHz) frequencies are produced by the opening of microcracks, with the MHz radiation appearing earlier than the kHz radiation. Earthquakes are large-scale fracture phenomena in the Earths heterogeneous crust. Thus, the radiated kHz–MHz electromagnetic emissions are detectable not only in the laboratory but also at a geological scale. Clear MHz-to-kHz electromagnetic anomalies have been systematically detected over periods ranging from a few days to a few hours prior to recent destructive earthquakes in Greece. We should bear in mind that whether electromagnetic precursors to earthquakes exist is an important question not only for earthquake prediction but mainly for understanding the physical processes of earthquake generation. An open question in this field of research is the classification of a detected electromagnetic anomaly as a pre-seismic signal associated with earthquake occurrence. Indeed, electromagnetic fluctuations in the frequency range of MHz are known to be related to a few sources, including atmospheric noise (due to lightning), man-made composite noise, solar–terrestrial noise (resulting from the Sun–solar wind–magnetosphere–ionosphere–Earths surface chain) or cosmic noise, and finally, the lithospheric effect, namely pre-seismic activity. We focus on this point in this paper. We suggest that if a combination of detected kHz and MHz electromagnetic anomalies satisfies the set of criteria presented herein, these anomalies could be considered as candidate precursory phenomena of an impending earthquake.

Fractal evolution of MHz electromagnetic signals prior to earthquakes: results collected in Greece during 2009

This paper addressed a fractal evolution of 11 one-month lasting MHz electromagnetic disturbances, recorded in Greece prior to nine significant earthquakes of 2009. Time-space wavelet-based power spectral techniques were employed in the analysis. All investigated signals evolved naturally to epochs of fractal organization in space and time. Continuous organization was detected in seven signals. Significant number of successive () power-law -values were observed lying between 1.5 and 3.0 or above. The majority of fractal segments exhibited anti-persistent () or persistent () behaviour. Switching between persistency and anti-persistency was also found. Locality and sensitivity were traced. Findings were considered indicative of self-organized critical states of the last stages of preparation of the investigated earthquakes. Results implied fractional Brownian modelling. Explanations were proposed in view of the asperity model. Persistent–anti-persistent MHz anomalies were due to self-organized micro-cracking of the heterogeneous medium of the earths crust which may have led the systems evolution towards global failure. The precursory value of the signals was discussed.

Europium-activated phosphors for use in X-ray detectors of medical imaging systems

Three europium-activated phosphors Y2O2S:Eu, Y2O3:Eu, and YVO4:Eu emitting red light were studied to investigate their suitability for radiographic cassettes or digital imaging systems. Screens were prepared from phosphor powders with various coating thicknesses by sedimentation. To assess phosphor light producing efficiency in relation to patient dose, each screen was X-rayed using 40–120 kVp and the number of light photons emitted per X-ray incident was experimentally and theoretically evaluated. Additionally, the capability of the emitted light to sensitize films or to generate electrons in silicon photodiodes used in digital imaging systems was examined. Y2O2S:Eu screens were most efficient in light emission, and when combined with either red sensitive films or Si photodiodes, they were found superior to Y2O3:Eu or YVO4:Eu screens in film grain or electron signal generation. In many cases they were also found superior to terbium-activated phosphors. Provided that several problems related to industrial production (special dyes, reflective backing, crossover effects) are dealt with, those europium-activated screens could be employed in low-tube-voltage radiographic applications.