224 research outputs found
Spatial confinement of muonium atoms
We report the achievement of spatial confinement of muonium atoms (the bound
state of a positive muon and an electron). Muonium emitted into vacuum from
mesoporous silica reflects between two SiO confining surfaces separated by
1 mm. From the data, one can extract that the reflection probability on the
confining surfaces kept at 100 K is about 90% and the reflection process is
well described by a cosine law. This technique enables new experiments with
this exotic atomic system and is a very important step towards a measurement of
the 1S-2S transition frequency using continuous wave laser spectroscopy.Comment: 5 pages, 6 figure
Positronium production in cryogenic environments
We report measurements of positronium (Ps) formation following positron irradiation of mesoporous SiO2 films and Ge(100) single crystals at temperatures ranging from 12–700 K. As both of these materials generate Ps atoms via nonthermal processes, they are able to function as positron-positronium converters at cryogenic temperatures. Our data show that such Ps formation is possibly provided the targets are not compromised by adsorption of residual gas. In the case of SiO2 films, we observe a strong reduction in the Ps formation efficiency following irradiation with UV laser light (λ=243.01 nm) below 250 K, in accordance with previous observations of radiation-induced surface paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser light (λ=532 nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous SiO2 films and Ge crystals were found to produce Ps efficiently in cryogenic environments. Accordingly, these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold Ps atoms
A new path to measure antimatter free fall
We propose an experiment to measure the free fall acceleration of neutral antihydrogen atoms. The originality of this path is to first produce the Hbar+ ion
Positron and positronium annihilation patterns in zeolites and bulk ceramics
Positron lifetime (LT) measurements have revealed that in porous media the annihilation pattern is probably dominated by a near-saturation level trapping both for the positron and the positronium as well, which masks the contribution from the ldquofree annihilationrdquo, i.e., annihilation from delocalised positrons. In this work we compare positron lifetime spectra of crystalline zeolites with bulk ceramic materials fabricated by sintering from the same constituents as the zeolites and analyse common features and differences between them. For the dense ceramic samples the contribution from long-living components to the annihilation pattern is substantially lower than that for the zeolites. The results are analysed with the aim to extract new knowledge for the zeolites and for the sensitivity limits for free volume studies in ceramics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Positron annihilation in latex templated macroporous silica films: pore size and ortho-positronium escape
International audienceDepth profling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that o-Ps escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps reemission yield decreases from ~ 0:25 to ~ 0:11 as the pore size increases from 32 to 75 nm. The o-Ps reemission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9:1±0:4 g cm-1. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2)ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1:4 ± 4 nm) and from macropores with large effective size (~ 32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19±2 ns. Our results imply that o-Ps effciently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores become small), this capture into the micropores assists the capture into the macropores
On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range
Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ~1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.http://www.sciencedirect.com/science/article/B6THY-4SHF49N-1S/1/3eb43650299e0466e76cbbbfdaca9fa
Muonium emission into vacuum from mesoporous thin films at cryogenic temperatures
We report on Muonium (Mu) emission into vacuum following {\mu}+ implantation
in mesoporous thin SiO2 films. We obtain a yield of Mu into vacuum of (38\pm4)%
at 250 K temperature and (20\pm4)% at 100 K for 5 keV {\mu}+ implantation
energy. From the implantation energy dependence of the Mu vacuum yield we
determine the Mu diffusion constants in these films: D250KMu = (1.6 \pm 0.1)
\times 10-4 cm2/s and D100KMu = (4.2\pm0.5)\times10-5 cm2/s. Describing the
diffusion process as quantum mechanical tunneling from pore-to-pore, we
reproduce the measured temperature dependence T^3/2 of the diffusion constant.
We extract a potential barrier of (-0.3 \pm 0.1) eV which is consistent with
our computed Mu work-function in SiO2 of [-0.3,-0.9] eV. The high Mu vacuum
yield even at low temperatures represents an important step towards next
generation Mu spectroscopy experiments.Comment: 5 pages, 5 Figure
Health-related quality of life impact of cobimetinib in combination with vemurafenib in patients with advanced or metastatic BRAFV600 mutation-positive melanoma
Background: In the coBRIM study, cobimetinib plus vemurafenib (C+V) significantly improved survival outcomes vs placebo and vemurafenib (P+V) in patients with advanced/metastatic BRAF(V600)-mutated melanoma. An analysis of health-related quality of life (HRQOL) from coBRIM is reported.Methods: Patients completing the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (QLQ-C30) at baseline and >= 1 time point thereafter constituted the analysis population. Change from baseline >= 10 points was considered clinically meaningful.Results: Mean baseline scores for all QLQ-C30 domains were similar between arms. Most on-treatment scores for QLQ-C30 domains were also comparable between arms. A transient deterioration in role function in cycle 1 day 15 (C1D15; -14.7 points) in the P+V arm and improvement in insomnia in the C+V arm at C2D15 (-12.4 points) was observed. Among patients who experienced a >= 10-point change from baseline (responders), between-group differences were greatest for insomnia (16%), social functioning (10%), fatigue (9%) and pain (7%), all favouring C+V. Diarrhoea, photosensitivity reaction, pyrexia, and rash did not meaningfully affect global health status (GHS). Serous retinopathy was associated with a transient decrease in GHS at C1D15 assessment.Conclusions: In patients with advanced/metastatic BRAFV600-mutated melanoma, treatment with C+V maintained HRQOL compared with P+V, with superior efficacy
Development of a PbWO4 Detector for Single-Shot Positron Annihilation Lifetime Spectroscopy at the GBAR Experiment
We have developed a PbWO4 (PWO) detector with a large dynamic range to measure the intensity of a positron beam and the absolute density of the ortho-positronium (o-Ps) cloud it creates. A simulation study shows that a setup based on such detectors may be used to determine the angular distribution of the emission and reflection of o-Ps to reduce part of the uncertainties of the measurement. These will allow to improve the precision in the measurement of the cross-section for the (anti)hydrogen formation by (anti)proton-positronium charge exchange and to optimize the yield of antihydrogen ion which is an essential parameter in the GBAR experiment
Photoperiod affects the phenotype of mitochondrial complex I mutants
Plant mutants for genes encoding subunits of mitochondrial Complex I (CI, NADH:ubiquinone oxidoreductase), the first enzyme of the respiratory chain, display various phenotypes depending on growth conditions. Here, we examined the impact of photoperiod, a major environmental factor controlling plant development, on two Arabidopsis thaliana CI mutants: a new insertion mutant interrupted in both ndufs8.1 and ndufs8.2 genes encoding the NDUFS8 subunit, and the previously characterized ndufs4 CI mutant. In long day (LD) condition, both ndufs8.1 and ndufs8.2 single mutants were indistinguishable from Col-0 at phenotypic and biochemical levels, whereas the ndufs8.1 ndufs8.2 double mutant was devoid of detectable holo-CI assembly/activity, showed higher AOX content/activity and displayed a growth-retardation phenotype similar to that of the ndufs4 mutant. Although growth was more affected in ndufs4 than ndufs8.1 ndufs8.2 under short day (SD) condition, both mutants displayed a similar impairment of growth acceleration after transfer to LD as compared to the WT. Untargeted and targeted metabolomics showed that overall metabolism was less responsive to the SD-to-LD transition in mutants than in the WT. The typical LD acclimation of carbon, nitrogen-assimilation and redox-related parameters was not observed in ndufs8.1 ndufs8. Similarly, NAD(H) content, that was higher in SD condition in both mutants than in Col-0, did not adjust under LD. We propose that altered redox homeostasis and NAD(H) content/redox state control the phenotype of Complex I mutants and photoperiod acclimation in Arabidopsis
- …