Amplification of terahertz radiation in carbon nanotubes

We investigate theoretically the feasibility of amplification of terahertz radiation in aligned achiral carbon nanotubes, a zigzag (12,0) and an armchair (10,10) in comparison with a superlattice using a combination of a constant direct current (dc) and a high-frequency alternate current (ac) electric fields. The electric current density expression is derived using the semiclassical Boltzmann transport equation with a constant relaxation time. The electric field is applied along the nanotube axis. Analysis of the current density versus electric field characteristics reveals a negative differential conductivity behavior at high frequency, as well as photon assisted peaks. The photon assisted peaks are about an order of magnitude higher in the carbon nanotubes compared to the superlattice. These strong phenomena in carbon nanotubes can be used to obtain domainless amplification of terahertz radiation at room temperature.     

A quick method for the determination of the Al/Si weight ratio in alumino-silicates using an Am-Be neutron source

An irradiation procedure with fast and thermal neutrons from a 5 Ci Am-Be isotopic neutron source irradiation facility in combination with a 3'3' NaI(Tl) detector system has been used to determine Al/Si weight ratios in alumino-silicates. Samples were irradiated with and without Cd cover for 10 minutes and counted for 10 minutes after a waiting time of 1 minute. The peak area analysis of the 1779 keV gamma-ray line of ²⁸Al product radionuclide produced via ²⁷Al(n,γ)²⁸Al and ²⁸Si(n,p)²⁸Al reactions in combination with the neutron flux parameter at the irradiation site and nuclear data were used to determine Al/Si ratios. Due to discrepancy in literature data, Am-Be neutron source spectrum averaged cross sections of (n,p) reactions on ²⁷Al, ²⁸Si and ⁵⁶Fe were determined by the activation technique using ¹¹⁵In(n,n')^115mIn as the fast neutron flux monitor reaction. The method was tested using mixtures of high-purity Al₂O₃ and SiO₂ with known weight ratios of Al/Si and validated by a certified reference material BCS-CRM 348 (Ball Clay). Results are presented for bentonite, kaolin, bauxite, feldspar and ball clay samples from Nigeria. The method is non-destructive, rapid and suitable for use in-situ for large-scale exploration works and industrial process control. This revised version was published online in July 2006 with corrections to the Cover Date.     

In situ gamma spectrometry of terrestrial naturally occurring radioactive materials and radiation hazard assessment of Kebbi State,Nigeria

Journal of Radioanalytical and Nuclear Chemistry - Environmental monitoring was performed in order to measure the activity concentrations of U-238, Th-232 and K-40. In situ gamma spectrometric...     

Synthesis and thermal characterization of NZP compounds Na1?xLixZr2(PO4)3 (x?=?0.00–0.75)

Sodium zirconium phosphate (NZP) composition Na_1−x Li _x Zr₂(PO₄)₃, x = 0.00–0.75 has been synthesized by method of solid state reaction method from Na₂CO₃·H₂O, Li₂CO₃, ZrO₂, and NH₄H₂PO₄, sintering at 1050–1250 °C for 8 h only in other to determine the effect on thermal properties, such as the phase formation of the compound. The materials have been characterized by TGA and DTA thermal analysis methods from room temperature to 1000 °C. It was observed that the increase in lithium content of the samples increased thermal stability of the samples and the DTA peaks shifted towards higher temperatures with increase in lithium content. The thermal stability regions for all the sample was observed to be from 640 °C. The sample with the highest lithium content, x = 0.75, exhibited the greatest thermal stability over the temperature range.     

Assessing the potential impact of immunity waning on the dynamics of COVID-19 in South Africa: an endemic model of COVID-19

We developed an endemic model of COVID-19 to assess the impact of vaccination and immunity waning on the dynamics of the disease. Our model exhibits the phenomenon of backward bifurcation and bi-stability, where a stable disease-free equilibrium coexists with a stable endemic equilibrium. The epidemiological implication of this is that the control reproduction number being less than unity is no longer sufficient to guarantee disease eradication. We showed that this phenomenon could be eliminated by either increasing the vaccine efficacy or by reducing the disease transmission rate (adhering to non-pharmaceutical interventions). Furthermore, we numerically investigated the impacts of vaccination and waning of both vaccine-induced immunity and post-recovery immunity on the disease dynamics. Our simulation results show that the waning of vaccine-induced immunity has more effect on the disease dynamics relative to post-recovery immunity waning and suggests that more emphasis should be on reducing the waning of vaccine-induced immunity to eradicate COVID-19.