Photocathodes—past performance and future potential

Light detection and imaging at very low intensities, even down to individual photon detection, has been made possible by the development of photomultiplier and imaging tubes. They have two main components: a photocathode detector and an electron multiplication section, but throughout their long history their performance has been limited by the response of the photocathodes. Reappraisal of the underlying science of cathode preparation emphasizes that they are still performing well below their full potential. The reasons for this are discussed and some indications of how improvements could be made are suggested. Since many of the potential advances are within the scope of current technology it is certainly feasible to achieve enhancements in performance by factors of two to ten across the blue to near-infrared spectral range. Higher improvement factors are implied but will be difficult to realize in the normal spectral range for wavelengths below 1 w m. Significant gains in sensitivity, and perhaps in response speed, will open many new opportunities for low light level sensors and the prediction is that they will be an enabling technology for developments in biological and medical applications, among others.     

Determination of neptunium in plutonium materials by ICP-MS

An ICP-MS analytical method as an alternative to the current radiochemical method was developed to analyze trace level ²³⁷Np in bulk plutonium materials. In this method, ²³⁷Np is determined together with a suite of trace elements during a single analysis using one dissolution solution. Method validation was achieved through precision examination, spike recovery study, detection limit determination, comparison of results with the radiochemical method, and laboratory intercomparison studies on Pu metals. The ICP-MS method significantly reduced the analysis cost, the sample amount, consumption of chemicals and waste generation, as well as the sample turnaround time.     

Six-coordinate uranium complexes featuring a bidentate anilide ligand

The synthesis of a new bidentate anilide ligand and four uranium amide complexes utilizing the ligand are reported. The secondary aniline HN[R]Ar_MeL (R = C(CD₃)₂CH₃, Ar_MeL = 2-NMe₂-5-MeC₆H₃) is prepared by condensation of H₂NAr_MeL and acetone-d₆ followed by alkylation of the resulting imine with MeLi. The ligand precursors (Et₂O)Li(N[R]Ar_MeL) and K(N[R]Ar_MeL) are prepared through deprotonation of HN[R]Ar_MeL with n-BuLi and KH, respectively. Treatment of UI₃(THF)₄ with (Et₂O)Li(N[R]Ar_MeL) (2 equiv) provides the uranium(III) -ate complex Li[I₂U(N[R]Ar_MeL)₂] (Li[1]), while treatment of UI₃ with three equiv. of K(N[R]Ar_MeL) provides the neutral uranium(III) complex U(N[R]Ar_MeL)₃ (2). Both uranium(III) complexes are susceptible to 1e oxidation, as is demonstrated by the syntheses of the uranium(IV) derivatives I₂U(N[R]Ar_MeL)₂ (1) and [U(N[R]Ar_MeL)₃][OTf] ([2][OTf]; OTf = CF₃SO₃). The spectroscopic and X-ray structural characterization of all four uranium complexes is described. The structures of 2 and [2][OTf] exhibit a large degree of steric pressure about the uranium center, effectively preventing the [2]⁺ ion from achieving a seven-coordinate structure.     

New images evoke FAScinating questions

Two recent papers in Science reported the X-ray structures of the large, organizationally distinct animal and fungal fatty acid synthases at 5 A. These new structural insights have unexpected implications for enzyme function for the other "iterative" and "assembly line" megasynthases.     

Thermoluminescence studies of thermally treated CaB4O7:Dy

Borate based thermoluminescence dosimeters (TLD) show high sensitivity and good TL characteristics. One of the promising material amongst the dosimeters is Dy doped CaB₄O₇. Spectrally resolved thermoluminescence of Dy doped CaB₄O₇ shows three glow peaks at about 50 °C, 240 °C and 380 °C, the intensity of the 240 °C glow peak being the maximum. All TL experiments were conducted on a high sensitivity TL spectrometer at Sussex University with a heating rate of 50 °C min^?1. Two main emissions associated with the Dy dopant are observed at ～480 and 580 nm. The samples were subjected to a series of treatments including excitation by X-rays and UV laser radiation. As part of the present research CaB₄O₇:Dy materials were subjected to two different heat treatments; quenching and slow cooling in order to investigate the changes in TL characteristics.     

The determination of arsenic and selenium in standard reference materials using sector field ICP-MS in high resolution mode

Sector field ICP-MS was used to analyse As and Se in a range of standard reference materials (NIST 1643d Water, NIST 1573a Tomato Leaves, NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine Level 2). A spectral resolution of m/Δm = 7500 enabled ⁷⁵As and ⁷⁷Se to be separated from problematic ArCl interferences. Following microwave acid digestion, solid samples were typically diluted 1 + 99 prior to analysis, while the urine sample was diluted 1 + 9. The water sample was analysed undiluted and diluted 1 + 9. Despite near baseline spectral separation, ⁷⁵As and ⁷⁷Se were still found to be influenced by ArCl at high Cl concentrations, the effect being most pronounced for ⁷⁷Se. When necessary ⁸²Se was also monitored to determine the accuracy of the ⁷⁷Se results. Detection limits (LOD, based on 3σ of 10 replicates) for ⁷⁵As, ⁷⁷Se and ⁸²Se in ultra-pure water, 1% (w/w) HNO₃ and 1% (w/w) HCl were ～?0.1, ～?0.2 and ～?0.5 ng g^–1, respectively. Although signal intensities when using high resolution were ～?1% of that found when using low resolution mode (m/Δm = 300), measured As concentrations and certified values were found to agree to within ± 11% for all samples analysed. The concentration of Se in NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine were found to be in agreement with certified values to within ± 15– 20%, as measured by ⁷⁷Se. However, closer agreement (± 5%) was found when these samples were analysed using ⁸²Se. The Se concentration in NIST 1643d Water was found to agree to within ± 5% of the certified value (depending on dilution factor). Due to the low concentration of Se in NIST 1573a Tomato Leaves, quantitation was not possible (below LOQ, 10σ). As a consequence of the lower ion transmission when using resolution 7500, analytical precisions were found to be elevated over that normally observed using low resolution mode, typically ± 5–20% (depending on analyte concentration and isotopic abundance).     

Excess Si concentration dependence of the photoluminescence of Si nanoclusters in SiO2 fabricated by ion implantation

A method for the fabrication of luminescent Si nanoclusters in an amorphous SiO₂ matrix by ion implantation and annealing, and the detailed mechanisms for the photoluminescence are reported. We have measured the implanted ion dose, annealing time and excitation energy dependence of the photoluminescence from implanted layers. The samples were fabricated by Si ion implantation into SiO₂ and subsequent high-temperature annealing. After annealing, a photoluminescence band below 1.7 eV has been observed. The peak energy of the photoluminescence is found to be independent of annealing time and excitation energy, while the intensity of the luminescence increases as the annealing time and excitation energy increase. Moreover, we found that the peak energy of the luminescence is strongly affected by the dose of implanted Si ions especially in the high dose range. These results indicate that the photons are absorbed by Si nanoclusters, for which the band-gap energy is modified by the quantum confinement effects, and the emission is not simply due to direct electron–hole recombination inside Si nanoclusters, but is related to defects probably at the interface between Si nanoclusters and SiO₂, for which the energy state is affected by Si cluster–cluster interactions. It seems that Si nanoclusters react via a thin oxide interface and the local concentrations of Si nanoclusters play an important role in the peak energy of the photoluminescence.     

A particle-hole calculation for pion production in relativistic heavy-ion collisions

A differential cross section for π-meson production in peripheral heavy-ion collisions is formulated within the context of a particle-hole model in the Tamm-Dancoff approximation. This is the first attempt at a fully quantum-mechanical particle-hole calculation for pion production in relativistic heavy-ion collisions. The particular reaction studied is an ¹⁶O projectile colliding with a ¹²C target at rest. In the projectile we form a linear combination of isobar-hole states, with the possibility of a coherent isobar giant resonance. The target can be excited to its giant M1 resonance (J^π = 1⁺, T = 1) at 15.11 MeV, or to its isobar analog neighbours, ¹²B at 13.4 MeV and ¹²N at 17.5 MeV. The theory is compared to recent experimental results.     

Enhanced anti-diastereo- and enantioselectivity in alcohol-mediated carbonyl crotylation using an isolable single component iridium catalyst

The cyclometalated iridium complex (S)-I derived from [Ir(cod)Cl](2), 4-cyano-3-nitrobenzoic acid, allyl acetate, and (S)-SEGPHOS is conveniently isolated by precipitation or through conventional silica gel flash chromatography. This single-component precatalyst allows alcohol mediated carbonyl crotylations to be performed at significantly lower temperature, resulting in enhanced levels of anti-diastereo- and enantioselectivity. Most significantly, the chromatographically isolated precatalyst (S)-I enables carbonyl crotylations that are not possible under previously reported conditions involving in situ generation of (S)-I.     

Luminescence spectra of doped and undoped lead tungstate crystals

TL spectra of undoped lead tungstate crystals exhibit glow peaks at 30 v K and 85 v K centred at 440 v nm, plus a peak at 50 v K at 530 v nm in an annealed sample. Annealing adds a 170 v K peak. Trivalent dopants of La 3+ and Y 3+ reduce the green luminescence, and Nb 5+ introduces a peak near 100 v K centred at 530 v nm; Sb introduces features between 40 v K and 90 v K and 150 and 180 v K. The luminescence emissions around 50 v K may be attributed to complex intrinsic defect centres, including (WO 4 ) m 3 . Of the four dopants studied in the present research, Sb +5 has the highest luminescence intensity. CL spectra show interesting anomalies near 170 v K which are linked to a phase change of water/ice nanoparticles trapped at dislocations.