The design and construction of an experimental MgO cold cathode X-ray tube for use in XRF spectrometry.

Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.An introduction to the fundamental concepts of X-ray physics and X-ray tube design is given. This discussion also includes a brief description of various X-ray tube types available commercially for a number of different industrial applications. The design of a high-energy MgO cold cathode X-ray tube, which is to be used in an X-ray fluorescence (XRF) spectrometer, is described in detail with emphasis placed on the electron beam focusing mechanism and the theory of operation as well as the construction of the X-ray tube MgO cold cathode, which functioned as the electron emitter of the device. A detailed account is also given of the output characteristics of the X-ray tube power supply, which has a direct effect on the design requirements and consequently the performance of the X-ray tube. An investigation into the manufacture of the vacuum envelope with particular attention focused on the production of reliable metal-to-ceramic seals was performed. A number of tests were conducted especially with regard to the maximum temperature that such seals may withstand without becoming permanently damaged. These tests were essential, since high temperature gradients tend to develop in an X-ray tube during operation, which the metal-to-cerarnic seals of the tube must be capable of withstanding if damage to the device is to be avoided. The set-up of the XRF spectrometer in which the completed X-ray tube was tested is discussed, in which the X-ray current and voltage measuring techniques are described. Furthermore a detailed account of the operation of the X-ray detector system and the multichannel analyser is given, which was used to detect and record spectra of the sample elements excited by the primary radiation of the X-ray tube. Finally the measured X-ray tube performance characteristics are discussed and compared to the predicted results

Comparison of the performance of different instruments in the stray neutron field around the CERN Proton Synchrotron

This paper discusses an intercomparison campaign carried out in several locations around the CERN Proton Synchrotron. The locations were selected in order to perform the measurements in different stray field conditions. Various neutron detectors were employed: ionisation chambers, conventional and extended range rem counters, both commercial and prototype ones, including a novel instrument called LUPIN, specifically conceived to work in pulsed fields. The attention was focused on the potential differences in the instrument readings due to dead-time losses that are expected to affect most commercial units. The results show that the ionisation chambers and LUPIN agree well with the expected H*(10) values, as derived from FLUKA simulations, showing no relevant underestimations even in strongly pulsed fields. On the contrary, the dead-time losses of the other rem counters induced an underestimation in pulsed fields that was more important for instruments characterised by a higher dead tim

Quenching-assisted actuation mechanisms in core-shell structured BiFeO3-BaTiO3 piezoceramics

Electromechanical actuation in piezoceramics is usually enhanced by creating chemically homogeneous materials with structurally heterogeneous morphotropic phase boundaries, leading to abrupt changes in ion displacement directions within the perovskite unit cell. In the present study, an alternative mechanism to enhance electromechanical coupling is found in both chemically and structurally heterogeneous BiFeO3-BaTiO3 lead-free piezoceramics. Such a mechanism is observed in a composition exhibiting core-shell type microstructure, associated with donor-type substitution of Ti4+ for Fe3+, and is primarily activated by thermal quenching treatment. Here, we describe the use of in situ high-energy synchrotron X-ray powder diffraction upon the application of a high electric field to directly monitor the ferroelectric and elastic interactions between these composite-like components, formed as core and shell regions within grains. Translational short or long-range ordering is observed in the BiFeO3-depleted shell regions which undergo significant structural alterations from pseudocubic Pm3m relaxor-ferroelectric in slow-cooled ceramics to rhombohedral R3c or R3m with long-range ferroelectric order in the quenched state. The strain contributions from each component are calculated, leading to the conclusion that the total macroscopic strain arises predominantly from the transformed shell after quenching. Such observations are also complemented by investigations of microstructure and electrical properties, including ferroelectric behaviour and temperature-dependent dielectric properties

Significant benefits of AIP testing and clinical screening in familial isolated and young-onset pituitary tumors

Context Germline mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene are responsible for a subset of familial isolated pituitary adenoma (FIPA) cases and sporadic pituitary neuroendocrine tumors (PitNETs). Objective To compare prospectively diagnosed AIP mutation-positive (AIPmut) PitNET patients with clinically presenting patients and to compare the clinical characteristics of AIPmut and AIPneg PitNET patients. Design 12-year prospective, observational study. Participants & Setting We studied probands and family members of FIPA kindreds and sporadic patients with disease onset ≤18 years or macroadenomas with onset ≤30 years (n = 1477). This was a collaborative study conducted at referral centers for pituitary diseases. Interventions & Outcome AIP testing and clinical screening for pituitary disease. Comparison of characteristics of prospectively diagnosed (n = 22) vs clinically presenting AIPmut PitNET patients (n = 145), and AIPmut (n = 167) vs AIPneg PitNET patients (n = 1310). Results Prospectively diagnosed AIPmut PitNET patients had smaller lesions with less suprasellar extension or cavernous sinus invasion and required fewer treatments with fewer operations and no radiotherapy compared with clinically presenting cases; there were fewer cases with active disease and hypopituitarism at last follow-up. When comparing AIPmut and AIPneg cases, AIPmut patients were more often males, younger, more often had GH excess, pituitary apoplexy, suprasellar extension, and more patients required multimodal therapy, including radiotherapy. AIPmut patients (n = 136) with GH excess were taller than AIPneg counterparts (n = 650). Conclusions Prospectively diagnosed AIPmut patients show better outcomes than clinically presenting cases, demonstrating the benefits of genetic and clinical screening. AIP-related pituitary disease has a wide spectrum ranging from aggressively growing lesions to stable or indolent disease course