Gamma-ray emission from molecular clouds: A probe of cosmic-ray origin and propagation

Cosmic rays up to at least 10¹⁵ eV (PeV) are believed to be emitted by Galactic sources, such as supernova remnants. However, no conclusive evidence of their acceleration has been found yet. A trace of ongoing cosmic-ray acceleration is the gamma-ray emission produced by these highly energetic particles when they scatter off the interstellar medium gas, mainly atomic and molecular hydrogen. Whereas the atomic hydrogen is uniformly distributed in the Galaxy, the molecular hydrogen is usually aggregated in dense clouds, and the gamma-ray emission from such clouds is particularly intense and localised. A multi-frequency approach, which combines the data from the upcoming and future gamma-ray emissions with the data from the submillimeter and millimeter surveys of the molecular hydrogen, is therefore crucial to probe the Galactic cosmic-ray flux. In order to fully exploit this multi-frequency approach, one needs to develop predictions of the expected emission. Here we will discuss the GeV to TeV emission from runaway CRs penetrating molecular clouds close to the young supernova remnant RX J1713-3946 and in molecular clouds illuminated by the background cosmic-ray flux.     

Capped semiconductor nanocrystals for device applications

For device purpose, our main aim is to synthesise material which is chemically and thermally stable, as well as enhancement in luminescence properties followed with matching lattice parameters. This can be achieved by precisely controlling the size of semiconductor nanocrystals which can create an opportunity for producing functional materials with new properties. Here we showed advantages of using both organic and inorganic capping agents. We reported two synthesis routes, one will lead to nanocomposites and other to Core/Shell nanostructures. Our mechanism consists of two stages: core nanoparticle formation and shell growth. Gibbs free energy of hydration of Zn+2 gives more clarity for shell growth over core rather than ion displacement from core. Colloidal films comprising of nanocrystalline CdS/ZnS were fabricated by the dip coating method. A blue shift in energy level at the nanoscale is demonstrated by optical absorption. Electron microscopy studies with an SEM and TEM show a particle size of 10 nm and diffraction patterns show a crystalline nature. Absence of lattice mismatching is one of the important parameter for device fabrication, which is confirmed by Raman spectroscopy. Overall reduction in optical absorption due to blue shift is expected to result in higher performance, especially in short-circuit currents in CdS/CdTe solar cells.     

Controllable spontaneous emission from a double Λ-type four-level atom in an anisotropic photonic crystal

The spontaneous emission behavior of a double Λ-type four-level atom embedded in a photonic crystal (PC) with anisotropic dispersion relations is investigated. It is shown that some interesting phenomena such as the spectral-line enhancement, the spectral-line narrowing, the spectral-line suppression and the spontaneous emission cancellation can be observed by adjusting the density of states of the photonic-band-gap (PBG) reservoir and the combination of two pump intensities. The proposed scheme can be achieved by use of coherent pump fields into the transitions from the upper levels to the ground level in rubidium atom confined in a PC. These theoretical investigations may provide more degrees of freedom to manipulate the atomic spontaneous emission.     

~ 1.2 μm near-infrared emission and gain anticipation in Ho doped heavy-metal gallate glasses

Ho³⁺-doped low-phonon-energy heavy-metal gallate glasses (LKBPBG) have been prepared and efficient 1.199 μm emission originating from the ⁵I₆ → ⁵I₈ radiative transition has been observed under 900 nm excitation. The spontaneous emission probability and the maximum stimulated emission cross-section were derived to be 294.31 s^− 1 and 3.46 × 10^− 21 cm², respectively. The ratio of quantum yields between ~ 1.2 and ~ 2.0 μm emissions was identified to be 16%, demonstrating that the ⁵I₆ → ⁵I₈ transition is favorable for optical amplification. The maximum gain coefficient of 1.84 dB/cm at 1.199 μm wavelength was anticipated in the ideal status. These results indicate that the Ho³⁺-doped LKBPBG glasses have a promising potential for the development of ~ 1.2 μm signal amplifier devices.     

Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV-Visible diode array and excitation emission matrix fluorescence

Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000 Da. Both the number-average (M_n) and weight-average (M_w) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M_n range of 0.8–1.1 kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S_R = S_275–295:S_350–400) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40 nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition.     

The “Average” Work Function and Emission Area in the Fowler-Nordheim Equation

Derivation of the elementary Fowler-Nordheim equation is based on several strong physical assumptions (e.g. smooth flat surface and uniform work function across emitting surface, constant emission area and a uniform distribution of the applied field over it). A real emitter, however, definitely does not fulfill these assumptions. In spite of it the total emission current follows the FN equation and is frequently used for an estimation of the "average" work-function. The physical basis of this procedure and of the terms "average" work function and "emission area" are analyzed from the experimentalist's point of view. Thus some of the older - more or less intuitive - conclusions are substantiated.     

Spectroscopic and photoluminescence studies on optically transparent magnetic nanocomposites based on sol–gel glass: Fe3O4

Sol–gel glasses with Fe₃O₄ nanoparticles having particle sizes laying in the range 10–20 nm were encapsulated in the porous network of silica resulting in nanocomposites having both optical and magnetic properties. Spectroscopic and photoluminescence studies indicated that Fe₃O₄ nanocrystals are embedded in the silica matrix with no strong Si–O–Fe bonding. The composites exhibited a blue luminescence. The optical absorption edge of the composites red shifted with increasing concentration of Fe₃O₄ in the silica matrix. There is no obvious shift in the position of the luminescence peak with the concentration of Fe₃O₄ except that the intensity of the peak is decreased. The unique combinations of magnetic and optical properties are appealing for magneto–optical applications.     

Time resolution of the bunch-shape detector with a thin-wire emission target

The time resolution of a new bunch-length detector based on the radio frequency (RF) scanning of the produced secondary electrons is calculated. A dedicated Monte-Carlo code by means of SIMION software is developed for accurate simulation of spread in the investigated secondary electrons transit times and image width. In calculation the initial energy distribution of electrons and the actual structure of accelerating and focusing electric fields were considered. It is shown that by using a thin-wire emission target a femtosecond time precision could be achieved using moderate applied voltages.     

Green and red up-conversion emissions and thermometric application of Er^3＋-doped silicate glass

The green and red up-conversion emissions centred at about 534, 549 and 663 nm of wavelength, corresponding respectively to the ^2H11/2 → ^4I15/2, ^4S3/2 → ^4I15/2 and ^4F9/2 → ^4I15/2 transitions of Er^3＋ ions, have been observed for the Er^3＋-doped silicate glass excited by a 978 nm semiconductor laser beam. Excitation power dependent behaviour of the up-conversion emission intensity indicates that a two-photon absorption up-conversion process is responsible for the green and red up-conversion emissions. The temperature dependence of the green up-conversion emissions is also studied in a temperature range of 296-673 K, which shows that Er^3＋-doped silicate glass can be used as a sensor in high-temperature measurement.     

Electron field emission characteristics of nano-catkin carbon films deposited by electron cyclotron resonance microwave plasma chemical vapour deposition

This paper reported that the nano-catkin carbon films were prepared on Si substrates by means of electron cyclotron resonance microwave plasma chemical vapour deposition in a hydrogen and methane mixture. The surface morphology and the structure of the fabricated films were characterized by using scanning electron microscopes and Raman spectroscopy, respectively. The stable field emission properties with a low threshold field of 5V/μm corresponding to a current density of about 1μA/cm^2 and a current density of 3.2mA/cm^2 at an electric field of 10V/μm were obtained from the carbon film deposited at CH4 concentration of 8%. The mechanism that the threshold field decreased with the increase of the CH4 concentration and the high emission current appeared at the high CH4 concentration was explained by using the Fowler-Nordheim theory.