Ortho-positronium lifetime measurement – positron source activity and statistics

Thraenert, S.; Hassan, E.M.; Krause‐Rehberg, R.

doi:10.1016/j.nimb.2006.04.167

Cited by 4 publications

(3 citation statements)

References 7 publications

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“…Measurements continued until 10 6 lifetime counts were collected using a ∼22 μCi 22 Na source contained between two 7 μm thick Kapton foils. Ideally, 4 × 10 6 counts are collected to provide excellent statistics yet balance long counting times . As a control, fresh CBV28014 PALS results were analyzed after 1 × 10 6 , 5 × 10 6 , and 1 × 10 7 counts.…”

Section: Methodsmentioning

confidence: 99%

Catalyst Deactivation Probed by Positron Annihilation Spectroscopy

Taylor

Urban-Klaehn

et al. 2021

ACS Catal.

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There is a widespread need to understand and improve the aging characteristics and properties of catalysts that support essential chemical reactions, including methanol-to-hydrocarbons (MTH) conversion. Here, we describe pilot measurements of a zeolite ZSM-5 catalyst with MTH as a model reaction using positron annihilation spectroscopy, which was able to identify unique signatures for different components in the catalyst mixture (i.e., zeolite and silica gel) and track the evolution of microstructures in different stages of catalyst deactivation. Samples were analyzed with both positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) measurements to investigate and characterize the aging process of zeolite catalysts. PALS results show a distinct lifetime related to the presence of zeolite (active catalyst) and silica gel (diluent). Tracking these characteristic lifetimes as the catalyst deactivates shows that zeolite nanopores decrease in size as the reaction progresses, while the mesopores show an increase in size probably due to degassing. Analysis of CDB ratio curves reveals a distinct change in the defect structure of the sample that occurs between 47 and 23% conversion of the catalyst. Further examination of the shaping parameters of the Doppler broadened positron annihilation spectra suggests that the complexity of microstructures increases as the zeolite ages; notably, there is an increase in pore number density with a concomitant decrease in the pore size that suggests that catalyst coking fractionates larger pores into numerous smaller pores. Our findings suggest that the sensitivity of PALS and CDB techniques to different pore structures can potentially be useful as a characterization method for heterogeneous catalysis studies.

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Section: Methodsmentioning

confidence: 99%

Catalyst Deactivation Probed by Positron Annihilation Spectroscopy

Taylor

Urban-Klaehn

et al. 2021

ACS Catal.

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“…The activity of the used positron source was 0.12 × 10 6 Bq (~ 3.2 µCi). Such a weak source is required to obtain the lifetime τ 4 in a sufficient way [7]. All PALS spectra of the CPG samples consist of four different lifetime components which have its origin in the annihilation of the p-Ps (τ 1 = 0.125 ns), the annihilation of free positrons (τ 2 ~ 0.5 ns), the annihilation of the o-Ps in the amorphous region of the glass (τ 3 ~ 1.5 ns) and the annihilation of o-Ps in the pores of the glass (τ 4 ).…”

Section: Introductionmentioning

confidence: 99%

Verifying the RTE model: ortho‐positronium lifetime measurement on controlled pore glasses

Thraenert

Hassan

Enke

et al. 2007

Phys. Status Solidi (c)

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In porous media, the vacuum lifetime of ortho-positronium (o-Ps) of τ = 142 ns can be reduced markedly by pick-off annihilation (interaction with electrons of the host material). So the o-Ps lifetime is determined by the pore size which can be extracted by utilising approved models like the Tao-Eldrup model for pore sizes smaller than 1 nm and the Tokyo model or RTE model for larger pore sizes. The RTE model contains an explicit temperature dependence of the o-Ps lifetime. Experiments on controlled pore glasses (CPG) with different pore sizes (2 -70 nm) at different temperatures (50 -500 K) were performed to verify the RTE model. A general agreement for T = 300 K could be found. The temperature dependence of the lifetime, especially for low temperatures, could not be approved sufficiently. 1 Introduction The growing interest of science and industry in the development of porous glass for various applications (low-dielectric thin films, catalysis, molecular filter) necessitate an adequate characterisation of its properties, especially of the pore size. Positronium annihilation lifetime spectroscopy (PALS) allows its non-destructive measurement. In dielectric amorphous material a part of the positrons is formed to a bound state of a positron and an electron which is called positronium (Ps) [1]. There are two spin states of Ps. The singlet spin state (para-positronium, p-Ps) has a very short self-annihilation lifetime of τ S = 0.125 ns and is not suitable to deliver information about the pore size. The long vacuum lifetime of the triplet spin state (ortho-positronium, o-Ps) of τ T = 142 ns is, however, an adequate sensor for measuring the pore size. The vacuum lifetime of o-Ps can be reduced markedly by pick-off annihilation (interaction with electrons of the host material). So the annihilation lifetime contains information about the pore size which can be extracted by calculating the annihilation rate of o-Ps:

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“…A stronger limitation of the counting rate is due to the high variance of the measured lifetimes of o-Ps because of statistical uncertainty caused by random coincidences. This effect was investigated in [5]. The measured lifetime spectra N(t) were analyzed by means of the decomposition on four exponential components in the following way:…”

Section: Resultsmentioning

confidence: 99%

Positron Lifetime Spectroscopy of Silicon Nanocontainers for Cancer Theranostic Applications

Akmalova¹,

Dubov²,

Степанов³

et al. 2018

KEn

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Biocompatibility and biodegradability of porous silicon (por-Si) nanoparticles (NPs), as well as the fact that they can selectively accumulate in tumor tissues, allow using them as containers for delivery of diagnostic markers or drugs for therapy of cancer tumors. Advantages of por-Si NPs as carriers of drugs are also favorable due to the high surface area and large pore volume. To apply por-Si NPs as nanocontainers it is necessary to have the comprehensive information about their porosity. In our work we use the positron annihilation lifetime (PAL) spectroscopy for porosity investigation. Samples of por-Si were prepared by electrochemical etching of heavily boron doped crystalline Si wafers in a hydrofluoric acid solution. The prepared por-Si films were dried and mechanically milled to obtain powder of NPs, which was pressed into tablets for PAL investigation. Ortho-positronium components of the measured positron lifetime spectra allowed us to evaluate the pore size distribution in por-Si NPs as continuous bimodal one with two peaks near 1 nm and 3 nm.

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Ortho-positronium lifetime measurement – positron source activity and statistics

Cited by 4 publications

References 7 publications

Catalyst Deactivation Probed by Positron Annihilation Spectroscopy

Catalyst Deactivation Probed by Positron Annihilation Spectroscopy

Verifying the RTE model: ortho‐positronium lifetime measurement on controlled pore glasses

Positron Lifetime Spectroscopy of Silicon Nanocontainers for Cancer Theranostic Applications

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