Tan Tuck Lee, Augustine

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.

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.

The ability to produce scientific explanations is an important learning goal in our local physics curriculum. Yeo and Gilbert (2014) showed that its production entails attending to its function, form (structural organisation of meanings) and level (of precision [model used], abstractness and complexity [use of representations]). These multi-dimensions might explain why students find producing an “acceptable” scientific explanation difficult. There are also few studies that examines the kinds of explanations students need to construct, the process of constructing one and the difficulties encountered. There is also an absence of a comprehensive framework for teachers to assess students’ explanations. The goals of this study are thus to (a) develop a framework of scientific explanations that is comprehensive and parsimonious, and (b) produce a characterisation of the process that students undergo in producing scientific explanations.

This paper reports the temperature-dependent tailoring of acceptor defects in oxygen rich ZnO thin films, for enhanced p-type conductivity. The oxygen rich p-type ZnO thin films were successfully grown by pulsed laser deposition on silicon substrate at different postdeposition annealing temperatures (500–800 °C). The oxygen rich ZnO powder was synthesized by wet chemical method using zinc acetate dihydrate [Zn(CH3COO)2·2H2O] and potassium hydroxide (KOH) as precursors. The powder was then compressed and sintered to make pellets for pulsed laser deposition system. The x-ray diffraction analysis exhibits an improved crystallinity in thin films annealed at elevated temperatures with a temperature-dependent variation in lattice constants. An analysis of Auger Zn L 3 M 4,5 M 4,5 peak reveals a consistent decrease in interstitial zinc (Zn i ) exhibiting its temperature-dependent reversion to zinc lattice sites. Room temperature photoluminescence of the p-type ZnO shows a dominant deep level emission peak at ∼3.12 eV related to oxygen interstitials (acceptors). The relative concentration of oxygen interstitials (O i ) increases with increase in annealing temperature, resulting in enhanced hole carrier concentration. The maximum hole carrier concentration of 6.8 × 1014 cm−3 (indicating p-type conductivity) was estimated using Hall probe measurements for the thin film sample annealed at 700 °C.

A total of 11 fundamental and 3 overtone bands of the formaldoxime isotopologue 13CD2NOH were identified using its Fourier transform infrared (FTIR) spectra which were recorded with a low resolution (0.50 cm−1) in the 500–4000 cm−1 region, and high resolution (0.00096 cm−1) in the 280–500 cm−1 region. Their relative infrared (IR) band intensities were also measured. Furthermore, a rovibrational analysis of the IR transitions of the band of 13CD2NOH was carried out using its high-resolution FTIR spectrum which was recorded at the Australian Synchrotron. A total of 1077 IR transitions of the C-type band were assigned and fitted using the Watson's A-reduced Hamiltonian in the Ir representation to derive its band center and the = 1 state rovibrational constants up to all 5 quartic centrifugal distortion terms for the first time, with a root-mean-square (rms) deviation of 0.00044 cm−1. The band center of the band of 13CD2NOH were found to be 391.054446(36) cm−1. The ground state rovibrational constants up to all 5 quartic terms were determined for the first time by fitting 407 ground state combination differences (GSCDs) derived from the assigned IR transitions of the band of 13CD2NOH of this work. The rms deviation of the GSCD fit was 0.00040 cm−1. Additionally, all 3 rotational constants and 5 quartic centrifugal distortion terms of the ground state and 3 rotational constants of the = 1 state of 13CD2NOH were computed from theoretical anharmonic calculations at two different levels of theory, B3LYP and MP2 with the cc-pVTZ basis set, for comparison with the experimental results. Close agreement was found for the calculated and experimental rovibrational constants of 13CD2NOH for both ground and = 1 states. The vibrational anharmonic frequencies of the 12 fundamental bands of 13CD2NOH in the 280–4000 cm−1 region, and their IR band intensities were also calculated using B3LYP and MP2 with the cc-pVTZ basis set, and they were compared with the respective experimental data. Finally, the ground state rotational constants and the band center of the band of the cis conformer of 13CD2NOH were calculated and compared with those of the trans conformer of this work.

This study reports the enhanced ferromagnetic ordering in ZnO:Mn nanoparticle thin films, grown at different substrate temperatures using pulsed laser deposition. The optimum growth conditions were deduced from X-ray, photoemission and magnetic measurements. The X-ray measurements reveal that there was an optimum substrate temperature where the thin films showed relatively stronger texture, better crystallinity and lower strain. Substrate temperature tuned the deep level recombination centers in ZnO:Mn, which changed the optical quality by altering the electronic structure. The M-H curves, in the present study, revealed superior ferromagnetic response of 20-nm sized particles in ZnO:Mn thin film grown at a substrate temperature of 450 °C. Ferromagnetic ordering becomes weaker at higher/lower substrate temperatures due to the activation of native defects in ZnO host matrix.

This paper investigates the ferromagnetism in ZnO:Mn powders and presents our findings about the role played by the doping concentration and the structural defects towards the ferromagnetic signal. The narrow-size-distributed ZnO:Mn nanoparticles based powders with oxygen rich stoichiometery were synthesized by wet chemical method using zinc acetate dihydrate and manganese acetate tetrahydrate as precursors. A consistent increase in the lattice cell volume, estimated from x-ray diffraction spectra and the presence of Mn 2p3/2 peak at 640.9 eV, in x-ray photoelectron spectroscopic spectra, confirmed a successful incorporation of manganese in its Mn2+ oxidation state in ZnO host matrix. Extended deep level emission spectra in Mn doped ZnO powders exhibited the signatures of oxygen interstitials and zinc vacancies except for the sample with 5 at. % Mn doping. The nanocrystalline powders with 2 and 5 at. % Mn doping concentration were ferromagnetic at room temperature while the 10 at. % Mn doped sample exhibited paramagnetic behavior. The maximum saturation magnetization of 0.05 emu/g in the nanocrystalline powder with 5 at. % Mn doping having minimum defects validated the ferromagnetic signal to be due to strong p-d hybridization of Mn ions.

The effect of varied concentrations of deuterium-krypton (D 2 –Kr) admixture on the neutron emission of a fast miniature plasma focusdevice was investigated. It was found that a judicious concentration of Kr in D 2 can significantly enhance the neutron yield. The maximum average neutron yield of (1±0.27)×10 4  n/shot for pure D 2 filling at 3 mbars was enhanced to (3.14±0.4)×10 5  n/shot with D 2 +2% Kr admixture operation, which represents a >30 -fold increase. More than an order of magnitude enhancement in the average neutron yield was observed over the broader operating range of 1–4 mbars for D 2 +2% Kr and D 2 +5% Kr admixtures.