Magnetic quenching of positronium in thorium phosphate: an experimental approach to p-Ps pick-off lifetime ?

The effects of magnetic field (B) quenching on positronium (Ps) are studied in solid thorium phosphate by using both the lifetime spectroscopy (LS) and Doppler broadening (DBARL) techniques, in view of gaining information on the pick-off decay rate constant of positronium singlet state (p-Ps) and compare it to that of the triplet state (o-Ps). At B=0, the longest lifetime is [MATH] 700 ps, with an intensity of some 23 %. The effects of B on LS parameter R (normalized integral of counts in a time window) show that [MATH] is ascribable to o-Ps. Fitting the variations with B of both R and [MATH]4 (lifetime of the m=0 o-Ps substate) requires two phenomenological parameters : (i) the contact density parameter, [MATH]0.22 and (ii) either an additional contact density parameter, [MATH]0.36, or a p-Ps pick-off decay constant, [MATH], different from that of o-Ps, [MATH]. Resolving the deconvoluted DBARL spectra to derive the intensity of the intrinsic decay component at B=0 gives a better agreement with the first ([MATH]) than with the second ([MATH]) of these possibilities

Physicochimie du positonium et chimie sous rayonnement

Un bref exposé est fait des expériences les plus marquantes, démonstratives des liens existant entre la formation du positonium dans les liquides, telle qu’étudiée par les techniques d’annihilation des positons, et diverses données de la radiolyse pulsée. Quelques voies actuelles de recherche sont également délinéées

Current Problems and Unknowns in Positronium Dynamics

A survey is made on positronium dynamics in liquids. From literature data, it is shown that applying conventional kinetics to positronium reactions leads to quite satisfactory results in a large variety of solvents, solutes, reaction types and temperatures. On the opposite, applying the theoretically well established concepts of positronium bubble state and time-dependent reaction rate coefficient would imply the use of unrealistic solute radii in the related kinetic equations, with possible exception of large solutes (micelles). Some solutes displaying unexpectedly high reaction rate constants are shown to be refractory to any of the above quantitative approaches. The particular case of reversible positronium bound-state formation is further examined, the most consistent results arising, again, from the conventional analysis of the data

Positronium as a probe in natural polymers: decomposition in starch

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I3, remains constant at 26.2% while its lifetime, $\tau$3, is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, Eact = (1.52 ± 0.05) eV, and a frequency factor, ln(k0, s-1) = 25.3 ± 0.4. In the decomposition region, the PALS data show the unexpected correlation ($\tau$3n)3 = I3n, linking the normalised values of $\tau$3, $\tau$3n, and of I3, I3n. This is explained by considering that the changes in I3 with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in 3 reflect the shrinking of the radius of the free volumes, the latter decreasing in proportion to p1/3. Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, Eact = (1.53 ± 0.03) eV, and frequency factor, ln(k0, s-1) = 25.4 ± 0.2, in excellent agreement with the values derived from TGA

Size of Sodium Dodecyl Sulfate Micelles in Aqueous Solutions as Studied by Positron Annihilation Lifetime Spectroscopy

The micelles formed by sodium dodecyl sulfate (SDS) are studied in aqueous solutions at 294 K as a function of nominal SDS concentration (C) using positron lifetime spectroscopy. In a four-component analysis of the data, it appears that the lifetime (τ3) of the positronium triplet state (o-Ps) in the aqueous phase decreases significantly with increasing C. Preliminary results on sodium methanesulfonate solutions show that this compound does not result in any significant Ps lifetime quenching. Thus, the changes in the lifetime are attributed to the trapping of Ps from the aqueous to the organic micellar phase. On the basis of simple diffusion-controlled kinetics, the observed linearity of 1/τ3 with C indicates that the ratio of the micelle core radius (Rmic) to the aggregation number (Nag) is about constant over the concentration range investigated, in agreement with information from other studies. Quantitatively, on the hypothesis of a constant value of Nag = 64, the use of the simple model, or of a more rigorous one, for Ps diffusion and trapping leads to Rmic = 1.50 and 1.19 nm, respectively. Both values are lower than 1.84 nm as expected from other studies on the total SDS aggregate radius at 298 K. However, taking account of the size of the polar sulfate groups and of some slight variation of Nag with C, the most complete model leads to Rmic = 1.75 nm, in satisfactory agreement with the 1.84 nm value. Examination of the intensities of the two longest-lived components, related to o-Ps in the aqueous and micellar phases, shows that, as was the case for reverse micelles, most of the Ps formation occurs in the former. The aggregated SDS molecules display a very low inhibiting power toward Ps formation, with an inhibition constant of 0.01 M-1, in agreement with the low ability of alkyl sulfates and sulfonates to scavenge electrons. The trapping of Ps in the micelles is confirmed by the continuous increase with C of the intensity of the longest-lived component, related to o-Ps in the organic phase

Effect of water-soluble electron scavengers on positronium formation in AOT/water/isooctane microemulsions

In a previous work reporting on positron annihilation lifetime spectroscopy (LS) measurements in AOT/water/isooctane microemulsions, a model was established leading to the satisfactory description of both triplet positronium (o-Ps) formation and decay. The model proposes that a large proportion of Ps is formed in the aqueous pseudophase, a fraction of which can diffuse out to the organic phase. It thus predicts that an electron scavenger, soluble in the aqueous pseudophase only, should have an inhibiting action on both those species decaying in the aqueous (o-Psaq) and organic (o-Psorg) phases. To assess this prediction, measurements are performed with efficient inhibitors of Ps formation, incorporated into the reverse micelles: Te(OH)6, KNO3, and HgCl2. The last solute is also known as a Ps quencher in water. In all three cases, the lifetime results show that the radii of the water cores remain unaltered by the presence of the additives. As expected, the intensities of both o-Psaq and o-Psorg decrease with increasing solute concentration. However, the quenching power of HgCl2 is suppressed in the water aggregates; this is probably due to the association of the Hg2+ cations with the sulphonate groups of AOT. On this basis, the inhibition of HgCl2 is attributed to positron capture by the Cl- ions. The inhibition constant of Te(OH)6, KNO3 and Cl- are all found to be about 65% of what they are in pure water. Qualitatively, this is ascribable to the existence of mixed water-isooctane positron spurs.http://www.sciencedirect.com/science/article/B6TFM-3X64HXF-H/1/bf6610292275cc80b4c67fa7c003528

Captures successives de particules par des composés halogénés et détachement d’halogénure dans la grappe du positon

HgCl2 inhibits positronium formation in methanol by quasi - free electron scavenging. Combining the techniques of lifetime spectroscopy and Doppler broadening of the annihilation radiation lineshape, it is shown that this process is followed by a rapid detachment of Cl- ions which react with the positron to form a bound state. These findings have the potential of giving access to some as yet unknown characteristics of the positron spur, such as the distribution and number of the reactive species