Pulse shape analysis for γ-ray tracking (Part II): Fully informed particle swarm algorithm applied to AGATA

AGATA is an array of highly segmented HPGe detectors using the technique of g \gamma -ray tracking to reconstruct the scattering path of g \gamma -rays interacting within the active material. A basic requirement therefore is a precise reconstruction of the individual interaction locations within the detectors. This is possible through the use of pulse shape analysis (PSA) which, however, has to be conducted in real time due to the high data rates expected. We report on the use of a fully informed particle swarm (FIPS) algorithm for PSA. The achieved position resolution, in the order of a few millimeters, is within the requirements of a g \gamma -ray tracking array. The energy resolutions in Doppler-corrected spectra of a simulated in-beam experiment are comparable to those of a non-real-time extensive PSA based on a full grid search.     

The effect of side-chain length of cellulose fatty acid esters on their thermal,barrier and mechanical properties

Currently, long-chain cellulose esters are not produced commercially because of high price, and since their preparation typically requires a large quantity of chemicals. To reduce the chemical consumption, cellulose reactivity needs to be increased without losing its quality. One way to increase the reactivity of cellulose is to decrease its molar mass in a controlled manner. In this study, we have synthesized cellulose esters with different side-chain length (C6–C18) in a homogeneous system using ozone molar mass-controlled cellulose. The target was to keep the degree of substitution as low as possible while still ensuring the suitability of cellulose esters for solvent casting. Thermal, barrier and mechanical properties were studied depending on cellulose fatty acid ester side-chain length. All our molar mass-controlled cellulose esters form optically transparent, flexible and heat-sealable films with good water barrier properties and are processable without the addition of an external plasticizer. Furthermore, the films have mechanical properties comparable to some generally used plastics. These good properties suggest that our molar mass-controlled cellulose esters could be potential candidates for various applications such as films and composites.     

Advances in wide bandgap SiC for optoelectronics

Silicon carbide (SiC) has played a key role in power electronics thanks to its unique physical properties like wide bandgap, high breakdown field, etc. During the past decade, SiC is also becoming more and more active in optoelectronics thanks to the progress in materials growth and nanofabrication. This paper will review the advances in fluorescent SiC for white light-emitting diodes, covering the poly-crystalline doped SiC source material growth, single crystalline epitaxy growth of fluorescent SiC, and nanofabrication of SiC to enhance the extraction efficiency for fluorescent SiC based white LEDs.     

The Turkish accelerator complex IR FEL project

Turkish accelerator complex (TAC) project was approved by State Planning Organization (DPT) of Turkey in 2006. The complex will contain a linac-ring type electron–positron collider as a particle factory and different accelerator based light sources. As a first step to the national center, the construction of an IR FEL facility is planned until 2011. It is also planned that the technical design report for TAC will be completed in 2010. The TAC IR FEL facility will consist of an electron linac in the range of 15–40 MeV energy to obtain FEL in 2–185 microns range. In this study, the preliminary parameters of TAC IR FEL facility were presented. The possible using of the obtained FEL in material science, nonlinear optics, semiconductors, biotechnology, medicine and photochemical processes were discussed.     

A mean-field theory for self-propelled particles interacting by velocity alignment mechanisms

A mean-field approach (MFA) is proposed for the analysis of orientational order in a two-dimensional system of stochastic self-propelled particles interacting by local velocity alignment mechanism. The treatment is applied to the cases of ferromagnetic (F) and liquid-crystal (LC) alignment. In both cases, MFA yields a second order phase transition for a critical noise strength and a scaling exponent of 1/2 for the respective order parameters. We find that the critical noise amplitude η_c at which orientational order emerges in the LC case is smaller than in the F-alignment case, i.e. η^LC_C<η^F_C. A comparison with simulations of individual-based models with F- resp. LC-alignment shows that the predictions about the critical behavior and the qualitative relation between the respective critical noise amplitudes are correct.     

Search for exotics in γD↦φnp reactions near threshold<Superscript>⋆</Superscript>

We discuss coherent and incoherent φ -meson photoproduction off the deuteron at low energy and small momentum transfer with the aim to check whether the recent experimental data need for their interpretation an inclusion of exotic channels. Our analysis of the differential cross-section and spin-density matrix elements shows that new data on the γD → φX reaction at E _γ ∼ 2 GeV may be understood on the basis of conventional dynamics.     

Test of pulse shape analysis using single Compton scattering events

Procedures developed to separate single- and multiple-site events in germanium detector are tested with specially selected event samples provided by an 18-fold segmented prototype germanium detector for phase II of the germanium detector array, GERDA. The single Compton scattering, i.e. single-site, events are tagged by coincidently detecting the scattered photon with a second detector positioned at a defined angle. A neural network is trained to separate such events from events which come from multi-site dominated samples. Identification efficiencies of ≈80% are achieved for both single- and multi-site events. PACS 23.40.-s; 14.60Pq; 29.40.-n     

Influence of pressure on the relative population of the two lowest vibrational levels of the C <Superscript>3</Superscript> Π<Subscript>u</Subscript> state of nitrogen for electron beam excitation

Continuous and pulsed 12 keV electron beams were used to excite nitrogen within a gas cell at pressures ranging from 10 to 1400 hPa. The pressure dependence of the ratio of photon fluxes for emission from vibrational levels v'=0 and 1 of the C ³Π _u state has been studied. The results confirm the presence of a collisional excitation mechanism populating v'=0, 1 in addition to electron impact excitation. Rate constants of (1.27 ±0.04)×10^-11 cm³s^-1 [ v'=0] and (2.68 ±0.08)×10^-11 cm³s^-1 [ v'=1] were measured for C ³Π _u quenching by ground state nitrogen. For electron beam conditions relative excitation efficiencies of 1:0.59:0.22 for vibrational levels 0, 1 and 2 were calculated. The recorded flux ratios are compared with the predictions given by a vibrational relaxation model.