Émission de positron

Résumé

L'émission de positron ou désintégration β+ est un type de désintégration radioactive β dans laquelle un proton est converti en neutron, avec émission d'une particule β+ (positron) et d'un neutrino: : p^+ ~\rightarrow~ n+e^++{\nu}_e Mécanisme d'émission Les protons et neutrons ne sont pas des particules élémentaires, mais sont chacun constitués de trois quarks : un proton est constitué de deux quark up de charge +2/3 et d'un quark down de charge −1/3 (uud), ce qui lui confère une charge +1 ; un neutron est constitué de deux quarks down et un quark up (udd), d'où une charge nulle. Ces quarks sont capables de changer de nature, en particulier de up à down. C'est ce phénomène qui produit une radiation β : en l'occurrence, dans le cas d'une désintégration β+, un quark up se change en quark down, changeant un proton en neutron avec émission d'un boson W+ : : u ~\rightarrow~ d + W^+ Cette réaction est immédiatement suivie par la désintégration du b

Source officielle

https://fr.wikipedia.org/wiki/%C3%89mission_de_positron

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A high-energy (0.5-3.0 TeV centre of mass) electron-positron Compact Linear Collider (CLIC) is being studied at CERN as a new physics facility. The design study has been optimized for 3 TeV centre-of-mass energy. Intense bunches injected into the main linac must have unprecedentedly small emittances to achieve the design luminosity 1035cm-2s-1 required for the physics experiments. The positron and electron bunch trains will be provided by the CLIC injection complex. This thesis describes an optics design and performance of a positron damping ring developed for producing such ultra-low emittance beam. The linear optics of the CLIC damping ring is optimized by taking into account the combined action of radiation damping, quantum excitation and intrabeam scattering. The required beam emittance is obtained by using a TME (Theoretical Minimum Emittance) lattice with compact arcs and short period wiggler magnets located in dispersion-free regions. The damping ring beam energy is chosen as 2.42 GeV. The lattice features small values of the optical functions, a large number of compact TME cells, and a large number of wiggler magnets. Strong sextupole magnets are needed for the chromatic correction which introduces significant nonlinearities, decreasing the dynamic aperture. The nonlinear optimization of the lattice is described. An appropriate scheme of chromaticity correction is determined that gives reasonable dynamic aperture and zero chromaticity. The nonlinearities induced by the short period wiggler magnets and their influence on the beam dynamics are also studied. In addition, approaches for absorption of synchrotron radiation power produced by the wigglers are discussed. Realistic misalignments of magnets and monitors increase the equilibrium emittance. The sensitivity of the CLIC damping ring to various kinds of alignment errors is studied. Without any correction, fairly small vertical misalignments of the quadrupoles and, in particular, the sextupoles, introduce unacceptable distortions of the closed orbit as well as intolerable spurious vertical dispersion and coupling due to the strong focusing optics of the damping ring. A sophisticated beam-based correction scheme has been developed in order to bring the design target emittances and the dynamic aperture back to the ideal value. The correction using dipolar correctors and several skew quadrupole correctors allows a minimization of the closed-orbit distortion, the cross-talk between vertical and horizontal closed orbits, the residual vertical dispersion and the betatron tune coupling. The small emittance, short bunch length, and high current in the CLIC damping ring could give rise to collective effects which degrade the quality of the extracted beam. A number of possible instabilities and an estimate of their impact on the ring performance are briefly surveyed. The effects considered include fast beam-ion instability, coherent synchrotron radiation, Touschek scattering, intrabeam scattering, resistive-wall wake fields, and electron cloud.

EPFL2006

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Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

Shuo Tong, Wei Wang, Jieping Zhu

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Delta lambda, 77-101 nm) with quantum yields (phi(FL)) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (phi(PL)) and lifetime (tau(p) up to 251 mu s). The presence of intramolecular H-bonds and negligible pi-pi stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, pi-pi and C-H center dot center dot center dot pi interactions could be responsible for the observed RTP of iodine containing phosphors.

WILEY-V C H VERLAG GMBH2022

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Blumino is the first analog silicon photomultiplier with integrated amplifier, comparator and time-to-digital converter (TDC). The combination of a photodetector together with on-chip readout circuitry enables system-level advantages, such as internal parasitic reduction, compactness and simplicity. The analog silicon photomultiplier has a third output, called fast terminal (FT), in addition to the anode and cathode, which is used for timing measurements. The analog silicon photomultiplier presents excellent photon detection efficiency greater than 40% at 420 nm, making it suitable for positron-emission tomography. Measurement results of the TDC indicate a resolution of 128 ps least significant bit (LSB) with a differential nonlinearity and integral nonlinearity of -1/+5 LSB and -2.4/+0.9 LSB, respectively. The discriminator comprises two preamplifier stages followed by a complementary self-biased differential amplifier stage which is coupled to the analog silicon photomultiplier's FT through a decoupling capacitor. The sensor is also fully backward-compatible through the standard output which can be coupled to dedicated ASICs and standard readout integrated circuits. In addition to the electrical, radiation, and optical performance, the integration of a custom CMOS analog silicon photomultiplier process with standard CMOS process was investigated.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

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EPFL2006