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George Szekeres was a distinguished Hungarian-Australian mathematician, who worked in many different areas of mathematics, and with many collaborators. He was born in Budapest in 1911. His youth between the two World Wars was spent in Hungary, a country that, as a result of historical events, went through a golden age and produced a great number of exceptional intellects; his early mathematical explorations were in the company of several of these. However, for family reasons, he trained as a chemist rather than a mathematician. From 1938 to 1948, he lived in Shanghai, China, another remarkable city, where he experienced the horrors of persecution and war but nevertheless managed to prove some notable mathematical results. In 1948, he moved to Australia, as a lecturer, then senior lecturer, and finally reader, at the University of Adelaide, and then in 1964 he took up the Foundation Chair of Pure Mathematics at the University of New South Wales; in Australia he was able to bring his mathematical talents to fruition. After many years in Sydney, he returned to Adelaide, where he died in 2005. We discuss his early life in Hungary, his sojourn in Shanghai, and his mature period in Australia. We also discuss some aspects of his mathematical work, which is extraordinarily broad.

In this article, the authors present a method for quickly and easily constructing test phantoms for PET and SPECTquality assurance. As a demonstration, they constructed a complex prototype test phantom, showing the strengths of the construction method. Images taken using a PET/CT and a SPECT scanner are presented, along with a qualitative evaluation of PET/CT using the test phantom. The construction technique provides a quick, easy, and cost effective means of constructing a phantom for use in nuclear medicine imaging.

The flat-panel detector (FPD) is the state-of-the-art detector for digital radiography. The FPD can acquire images in real-time, has superior spatial resolution, and is free of the problems of x-ray image intensifiers-veiling glare, pin-cushion and magnetic distortion. However, FPDs suffer from poor signal to noise ratio performance at typical fluoroscopic exposure rates where the quantum noise is reduced to the point that it becomes comparable to the fixed electronic noise. It has been shown previously that avalanche multiplication gain in amorphous selenium (a-Se) can provide the necessary amplification to overcome the electronic noise of the FPD. Avalanche multiplication, however, comes with its own intrinsic contribution to the noise in the form of gain fluctuation noise. In this article a cascaded systems analysis is used to present a modified metric related to the detective quantum efficiency. The modified metric is used to study a diagnostic x-ray imaging system in the presence of intrinsic avalanche multiplication noise independently from other noise sources, such as electronic noise. An indirect conversion imaging system is considered to make the study independent of other avalanche multiplication related noise sources, such as the fluctuations arising from the depth of x-ray absorption. In this case all the avalanche events are initiated at the surface of the avalanche layer, and there are no fluctuations in the depth of absorption. Experiments on an indirect conversion x-ray imaging system using avalanche multiplication in a layer of a-Se are also presented. The cascaded systems analysis shows that intrinsic noise of avalanche multiplication will not have any deleterious influence on detector performance at zero spatial frequency in x-ray imaging provided the product of conversion gain, coupling efficiency, and optical quantum efficiency are much greater than a factor of 2. The experimental results show that avalanche multiplication in a-Se behaves as an intrinsic noise free avalanche multiplication, in accordance with our theory. Provided good coupling efficiency and high optical quantum efficiency are maintained, avalanche multiplication in a-Se has the potential to increase the gain and make negligible contribution to the noise, thereby improving the performance of indirect FPDs in fluoroscopy.

The past decade has seen the swift development of the flat-panel detector (FPD), also known as the active matrix flat-panel imager, for digital radiography. This new technology is applicable to other modalities, such as fluoroscopy, which require the acquisition of multiple images, but could benefit from some improvements. In such applications where more than one image is acquired less radiation is available to form each image and amplifier noise becomes a serious problem. Avalanche multiplication in amorphous selenium (a-Se) can provide the necessary amplification prior to read out so as to reduce the effect of electronic noise of the FPD. However, in direct conversion detectors avalanche multiplication can lead to a new source of gain fluctuation noise called depth dependent avalanche noise. A theoretical model was developed to understand depth dependent avalanche noise. Experiments were performed on a direct imaging system implementing avalanche multiplication in a layer of a-Se to validate the theory. For parameters appropriate for a diagnostic imaging FPD for fluoroscopy the detective quantum efficiency (DQE) was found to drop by as much as 50% with increasing electric field, as predicted by the theoretical model. This drop in DQE can be eliminated by separating the collection and avalanchemore » regions. For example by having a region of low electric field where x rays are absorbed and converted into charge that then drifts into a region of high electric field where the x-ray generated charge undergoes avalanche multiplication. This means quantum noise limited direct conversion FPD for low exposure imaging techniques are a possibility.« less

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8 keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25 {mu}m a-Se layer with a maximum gain of {approx}300. Changing the energy of the incident x ray influenced the range of the primary photoelectron andmore » noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.« less

The use of low-mobility photoconductors in x-ray imaging, especially amorphous selenium (a-Se), is currently of great interest. Detailed measurements of the photoconversion gain factor W± and gain fluctuations σ have been obtained to facilitate applications using a-Se and to provide a better understanding of the charge formation process. The measurements were made with an x-ray spectrometer constructed using a-Se as the x-ray absorber. Pulse height spectra of known monoenergetic x-ray sources were measured to determine the response of the a-Se. Both the gain and fluctuations calculated from the data were found to depend on the x-ray energy e, and the applied field E, indicating the recombination of a larger amount of charge that was created initially.

The use of photoconductors, especially amorphous selenium ( a -Se), in x-ray imaging is currently of interest. A critical performance parameter of an imaging detector is the Swank factor for degradation of the signal to noise ratio, or DQE(0), due to variations in the detector response. The Swank factor is evaluated from measured pulse height spectra generated by the absorption of monoenergetic x-rayphotons. The spectra show an additional width over previous theoretical expectations, but the Swank factor is still close to the high values previously predicted theoretically.

The flat-panel detector (FPD) is the state-of-the-art detector for digital radiography. The FPD can acquire images in real-time, has superior spatial resolution, and is free of the problems of x-ray image intensifiers-veiling glare, pin-cushion and magnetic distortion. However, FPDs suffer from poor signal to noise ratio performance at typical fluoroscopic exposure rates where the quantum noise is reduced to the point that it becomes comparable to the fixed electronic noise. It has been shown previously that avalanche multiplication gain in amorphous selenium (a-Se) can provide the necessary amplification to overcome the electronic noise of the FPD. Avalanche multiplication, however, comes with its own intrinsic contribution to the noise in the form of gain fluctuation noise. In this article a cascaded systems analysis is used to present a modified metric related to the detective quantum efficiency. The modified metric is used to study a diagnostic x-ray imaging system in the presence of intrinsic avalanche multiplication noise independently from other noise sources, such as electronic noise. An indirect conversion imaging system is considered to make the study independent of other avalanche multiplication related noise sources, such as the fluctuations arising from the depth of x-ray absorption. In this case all the avalanche events are initiated at the surface of the avalanche layer, and there are no fluctuations in the depth of absorption. Experiments on an indirect conversion x-ray imaging system using avalanche multiplication in a layer of a-Se are also presented. The cascaded systems analysis shows that intrinsic noise of avalanche multiplication will not have any deleterious influence on detector performance at zero spatial frequency in x-ray imaging provided the product of conversion gain, coupling efficiency, and optical quantum efficiency are much greater than a factor of 2. The experimental results show that avalanche multiplication in a-Se behaves as an intrinsic noise free avalanche multiplication, in accordance with our theory. Provided good coupling efficiency and high optical quantum efficiency are maintained, avalanche multiplication in a-Se has the potential to increase the gain and make negligible contribution to the noise, thereby improving the performance of indirect FPDs in fluoroscopy.

The past decade has seen the swift development of the flat-panel detector (FPD), also known as the active matrix flat-panel imager, for digital radiography. This new technology is applicable to other modalities, such as fluoroscopy, which require the acquisition of multiple images, but could benefit from some improvements. In such applications where more than one image is acquired less radiation is available to form each image and amplifier noise becomes a serious problem. Avalanche multiplication in amorphous selenium (a-Se) can provide the necessary amplification prior to read out so as to reduce the effect of electronic noise of the FPD. However, in direct conversion detectors avalanche multiplication can lead to a new source of gain fluctuation noise called depth dependent avalanche noise. A theoretical model was developed to understand depth dependent avalanche noise. Experiments were performed on a direct imaging system implementing avalanche multiplication in a layer of a-Se to validate the theory. For parameters appropriate for a diagnostic imaging FPD for fluoroscopy the detective quantum efficiency (DQE) was found to drop by as much as 50% with increasing electric field, as predicted by the theoretical model. This drop in DQE can be eliminated by separating the collection and avalanche regions. For example by having a region of low electric field where x rays are absorbed and converted into charge that then drifts into a region of high electric field where the x-ray generated charge undergoes avalanche multiplication. This means quantum noise limited direct conversion FPD for low exposure imaging techniques are a possibility.

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8 keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25 microm a-Se layer with a maximum gain of approximately 300. Changing the energy of the incident x ray influenced the range of the primary photoelectron and noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness d(Se) and the applied electric field E(Se) of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the E(Se) dependence of both avalanche gain and optical quantum efficiency of an 8 microm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of E(Se): (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8 microm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

This paper reports on the initial development of an ablation gauge designed to measure the surface recession rate of carbon char. The recession of carbon char is typically associated with the ablation of carbon-carbon and carbon phenolic heat shields. Preliminary testing of the gauges has been conducted in the University of Queensland superorbital expansion tubes at flight speeds of up to 9 km/sec at conditions simulating entry of the Pioneer-Venus Multiprobe mission at the conditions corresponding to the maximum heat transfer rate in flight. Due to high heat transfer rates in the stagnation region, gauge recession is rapid and completed before the establishment of steady flow. Further testing has been conducted with a lower enthalpy condition. Ablation rates are measured by monitoring the resistance of the ablation gauge during testing and a transient heating technique for preheating the gauge surface to ablative temperatures has been developed.

Thirteen previously unpublished critical measurements on enriched (93.16 wt% 235U) uranium metal spheres, which are radially but not axially centered in cylinders of enriched (93.16 wt% 235U) urani...

Minimum critical masses are calculated for arrays of fissile metal and oxide rods or metal plates spaced in water. The composition of the fissile materials investigated were 96% 239Pu and 4% 240Pu ...

The effects of varying energy group cross-section sets and the space-angle mesh must be understood to evaluate critical parameters computed by the discrete ordinates Sn method. In this paper, these...

In evaluating fissile-material recovery operations involving meted salvage immersed in a reagent, a criticality safety engineer must be able to identify systems of minimum critical Mass. Further, he must know the effect on the reproduction factor caused by changes in process variables such as container size or the fissile concentration of the reagent. No new experimental results are reported but studies of the criticality aspects of fissile-metal immersion by analyzing the most applicable of the existing measurements are presented. The results are expressed in terms of the critical mass of the metal region (excluding the mass of fissile material in solution) as a function of the fissile concentration and dimensions of the liquid cylinder. The analysis indicates that the critical mass of practical combinations of uranium metal and uranium solution always exceeds that of an 18.7-g/cm/sub 3/, 93.2% /sup 235/U-enriched uranium sphere centered in a 300-g/liter metal-water mixture. The corresponding conservative approximation for plutonium systems holds for a 19.7-g/cm/sup 3/, 95% /sup 239/Pu sphere in a 200g/liter metal-water mixture. The upper limit of applicability of these results is 500 g/liter for both plutonium and uranium systems. The calculational techniques described underestimate critical masses of uranium by ~5%; the calculated massesmore » of plutonium are sufficiently overestimated to be conservative in practical applications. (auth)« less

The results of criticality measurements on enriched (93.16% 235U) uranium metal spheres symmetrically immersed in enriched (93.18% 235U) uranyl nitrate solution cylinders are reported. The solution...

A method for the determination of ochratoxin A in pig's kidneys is described. The detection limit is less than 1 µg kg–1. By using enzymic digestion concurrently with dialysis and by controlling the pH conditions, the sample is extracted and interfering co-extracts removed without the use of column chromatography. The final determination employs reversed-phase high-performance liquid chromatography with either a phthalimidopropylsilane or a C22(docosyl) bonded column, and the sensitivity of the fluorescence detector is increased ten-fold by the formation of a post-column derivative with ammonia solution.

The thermal economy in the F region of the atmosphere is investigated using the the barometric equation and the continuity equation for thermal energy. It is concluded that photoionization is certainly a major source, and possibly the predominant source, of F-region heating. The assumptions and approximations are discussed and are shown not to invalidate the conclusion. The heat flux due to photoionization in the F region at low and middle latitudes at sunspot maximum is estimated to lie between 0.5 and 2.0 ergs/cm2 sec, with a most probable value of about 1.0 erg/cm2 sec.