Springham, Stuart Victor

A 3 kJ plasma focus was operated with a 3He-D2 gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1:1 for 3He and D atoms. The fusion reactions D(3He,p)4He and D(d,p)3H were measured simultaneously using CR-39 polymer nuclear track detectors placed inside a pinhole camera positioned on the forward plasma focus axis. A sandwich arrangement of two 1000 μm thick CR-39 detectors enabled the simultaneous registration of two groups of protons with approximate energies of 16 MeV and 3 MeV arising from the D(3He,p)4He and D(d,p)3H reactions, respectively. Radial track density distributions were obtained from each CR-39 detector and per-shot average distributions were calculated for the two groups of protons. It is found that the D(3He,p)4He and D(d,p)3H proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(3He,p)4He fusion is concentrated close to the plasma focus pinch column, while the D(d,p)3H fusion occurs relatively far from the pinch. The relative absence of D(d,p)3H fusion in the pinch is one significant reason for concluding that the D(3He,p)4He fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(3He,p)4He fusion is due to energetic 3He2+ ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(3He,p)4He and D(d,p)3H fusion in the plasma focus are discussed.

A study of the body of experimental results as well as modelling indicates that all plasma focus machines big and small operate with the same energy density both in the axial and radial phases related to an almost constant current per unit radius for all machines. This results in operational speeds and compressed plasma temperatures which are the same for all machines. It is proposed that the existing network of 3 kJ machines may be the most efficient and effective way to produce a comprehensive experimental picture of the plasma focus, apart from 'large machine' effects such as the neutron 'saturation' effect. Operational benchmarks are established for these 3 kJ machines for the purpose of standardisation to increase the effectiveness of comparative studies using different measurements obtained from different, though nearly identical machines.

There are several definitions of Problem Based Learning (PBL). For example, those of the Basudur Simplex Model, Kaufman and Swartz. The common features are: 1) Find and define the problem; 2) Examine facts and possibilities; 3) Consider alternative solutions; 4) Implement the best solution and 5) Problems should be related to the “real world”. However, in the natural sciences and mathematics, one often proceeds from “real world” problems to the conceptualisation of the abstract. Conceptualisation of the abstract is one of the tenets of the natural sciences and mathematics. Perhaps it is required less in the biological sciences, but it is increasingly required in physics and almost entirely in mathematics. The usual definitions of PBL have to be adapted to take into account the fact that conceptualisation of the abstract, rather than solving “real world” problems, is the end-product of many problems in the scientific disciplines. We give examples and counter-examples of the applicability of PBL integrated with information technology in our disciplines.

The NX2 device, a low energy plasma focus, at the Nanyang Technological University in Singapore, was used as a soft X-ray (SXR) source for micromachining. The gas used was neon which produced SXRs in a narrow spectral range of 0.9 - 1.6 keV. The SXR yield from repetitive operation of the NX2 device was monitored and measured using a cost effective multi-channel SXR spectrometric system. The system consists of filtered BPX65 PIN diodes, with the associated electronics --- an integrator, sample and peak holder, analogue switch, an A/D converter and a microcontroller. The system enables easy shot-to-shot statistical analysis under repetitive operation at adjustable preset trigger frequencies. A total of 4000 shots were fired at 0.5 Hz, using the same gas filling. The SXR production was at an average yield of 60 J/shot and a maximum single-shot yield of more than 100 J. The SXRs emitted by the NX2 device was used for contact micromachining, producing structures with an excellent aspect ratio of up to 20:1 on 25 μm SU-8 resist.

A modified Problem Based Learning (PBL) approach has been implemented and evaluated for the laboratory sessions of the Thermal Physics first year undergraduate module in academic year 99/00. The principal objective of this PBL approach is to get away from the 'cookbook' approach to teaching laboratories, and instead transform them into micro-research projects which demand higher order cognitive involvement of the students. This modified PBL approach involved conducting two hour laboratory sessions for each experiment: one hour from each of two consecutive weekly classes. The first week's session was rounded off with a question and answer session aimed at promoting higher order cognitive involvement by the students in the experimental work. The students conducted and discussed the experiments in groups of 3 to 4, while I acted as facilitator. One observation is that the weaker students in the class are uncomfortable with the lack of a detailed experimental demonstration. Comparing the student reports for academic year 99/00 (PBL approach) with those from 97/98 (conventional approach), indicates that student's have higher cognitive understanding of the experiments enabling them to answer questions more competently, and their integration and retention of knowledge is enhanced. Moreover the students were more involved and devoted more energy and enthusiasm to the experimental work when the modified PBL approach was employed in comparison to the more conventional approach to laboratory work.

Here we report the emission properties of perovskite quantum dots and two- dimensional perovskites under high energy excitation from X-, gamma-ray, alpha particle and thermal neutron, aimed towards fast-timed imaging and particle detection applications.

Two-dimensional (2D) hybrid lead halide perovskites are potential candidates for high light yield scintillators as they have small band gaps between 3 and 4 eV and large exciton-binding energy. Here, we discuss the scintillation properties from a total of 11 organic/inorganic hybrid perovskite crystals with two already reported crystals, (PEA)₂PbBr₄ and (EDBE)PbBr₄. Their photoluminescence and X-ray luminescence (XL) spectra are dominated by narrow and broad band emissions, and they correspond to free exciton and self-trapped exciton, respectively. The lifetimes derived from time-resolved XL strongly vary from 0.6 to 17.0 ns. These values make this type of compound among the fastest scintillators. For the light yield derived from the XL, we found that only (PEA)₂PbBr₄, (EDBE)PbBr₄, and (BA)₂PbBr₄ crystals have light yields between 10,000 and 40,000 photons/MeV. The mechanisms for thermal quenching and afterglow are discussed in order to optimize the light yields. With gamma-ray excitation, we reported the best energy resolution of 7.7% at 662 keV with excellent proportionality. Finally, this study paves the way toward the ultimate high light yield and fast scintillators for medical and homeland security applications.

CsPbBr3 quantum dots (QDs) have recently gained much interest due to their excellent optical and scintillation properties and their potential for X-ray imaging applications. In this study, we blended CsPbBr3 QDs with resin at different QD concentrations to achieve thick films and to protect the CsPbBr3 QDs from environmental moisture. Then, their scintillation properties are investigated and compared to the traditional commercial scintillators, CsI:Tl microcolumns, and Gadox layers. The CsPbBr3 QD-resin sheets show a high light yield of up to 21 500 photons/MeV at room temperature and a relatively small variation in light yield across a wide temperature range. In addition, the CsPbBr3 QD-resin sheets feature a small scintillation afterglow. The CsPbBr3 QD-resin sheets show a negligible trap density for the concentration below 50% weight, indicating that traps might arise from the aggregation of the QDs. The CsPbBr3 QD-resin sheets are also very stable at low irradiation intensities and relatively stable at higher intensities, with higher CsPbBr3 QD concentrations being more stable. Gamma-ray-excited-time-resolved emission measurements at 662 keV showed that the CsPbBr3 QD-resin sheets have an average scintillation decay time between 108 and 176 ns, which are still 10 000 and 6000 times faster than CsI:Tl and Gadox, respectively. Imaging tests show that the CsPbBr3 QD-resin sheets have a mean transfer function of 50% at 2 lp/mm and 20% at 4 lp/mm, comparable to that of commercial Gadox layers. This feature makes CsPbBr3 QD-resin sheets a good candidate for the low-cost, flexible X-ray imaging screens and γ-ray applications.