Measurement of the ortho-positronium confinement energy in mesoporous thin films

In this paper, we present measurements of the ortho-positronium emission energy in vacuum from mesoporous films using the time of flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50-400 K. The sample with smaller pore size exhibits a higher minimal energy ($73\pm$5 meV), compared to the sample with bigger pores ($48\pm$5 meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-positronium being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.Comment: 8 pages, 9 figures *Added references. * Corrected typos and Fig. 3 label. * Revised argument in section IV B abd C, results unchanged

Mesoporous silica films with varying porous volume fraction: direct correlation between ortho-positronium annihilation decay and escape yield into vacuum

accepted for publication in Applied Physics LettersThe behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction

Positronium reemission yield from mesostructured silica films

The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142 ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2gamma and 3gamma energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for Ps annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in CTACl-TEOS films

MOX assay using He-4 scintillation detectors

A neutron spectroscopic technique for plutonium content measurement is described. The technique exploits the kinematic cuto of neutron emission from (α, n) reactions on oxygen. The Watt spectrum of ssion neutron emission extends to higher energies without such a cuto. 4He scintillation detectors were calibrated with an energy cut to reject neutrons of low energies, thereby making the detectors sensitive only to ssion neutrons but not to neutrons from the (α, n) reaction on oxygen. Experimental results are presented. Simulations are discussed to evaluate possible self shielding eects. Furthermore, numerous factors in uencing gamma rejection are discussed

4He detectors for Mixed Oxide (MOX) fuel measurements

A neutron spectroscopic technique for plutonium content measurement is described. The technique exploits the kinematic cutoff of neutron emission in (α,n) reactions on oxygen. The Watt spectrum of fission neutron emission extends to higher energies without such a cutoff. 4He scintillation detectors were calibrated with an energy cut to reject neutrons of low energies, thereby making the detectors sensitive only to fission neutrons but not to neutrons from the (a,n) reaction on oxygen

Positron annihilation in latex-templated macroporous silica films: Pore size and ortho-positronium escape

Depth profiling 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 ortho-positronium (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 re-emission yield decreases from ~0.25 to ~0.11 as the pore size increases from 32 to 75 nm. The o-Ps re-emission 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−2. 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 efficiently 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 becomes small), this capture into the micropores assists the capture into the macropores.ISSN:1367-263