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 (τ 3 ) 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/τ 3 with C indicates that the ratio of the micelle core radius (R mic ) to the aggregation number (N ag ) is about constant over the concentration range investigated, in agreement with information from other studies. Quantitatively, on the hypothesis of a constant value of N ag ) 64, the use of the simple model, or of a more rigorous one, for Ps diffusion and trapping leads to R mic ) 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 N ag with C, the most complete model leads to R mic ) 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.

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Reverse micelles are studied in sodium AOT/water/isooctane mixtures as functions of AOT concentration (C AOT ), water to AOT mole ratio (w 0 ) and temperature (T), from 294 to 333 K, using positron annihilation lifetime spectroscopy (LS). By four-component analysis of the spectra, it is possible to extract the LS parameters (intensities, I i , and lifetimes, τ i ) of the triplet positronium (o-Ps) present in the aqueous (I 3 , τ 3 ) and organic (I 4 , τ 4 ) phases. The latter lifetime is constant and corresponds with the value measured in pure isooctane, while τ 3 is remarkably lower than the value for pure water. This difference is attributed to the out-diffusion of o-Ps from the water cores to isooctane. The relevant rigorous diffusion equations imply two fitting parameters, the radius of the water aggregates (r 0 ) and a transmission factor (h). Fixation of r 0 ) 3.6 nm for C AOT ) 0.1 M, w 0 ) 20, and T ) 294 K, as known from previous work, allows the quantitative derivation of the r 0 values for all other conditions. The water spheres appear to present some permeability to o-Ps, with a transmission factor h ) 0.12 nm -1 . The sphere radius increases smoothly with C AOT and w 0 and, more importantly, with T. The changes with w 0 give r 0 ) 0.181w 0 and 0.186w 0 nm at 294 and 298 K, respectively, and are in excellent agreement with previous proposals. The sum of the intensities, I tot ) I 3 + I 4 , is much lower than the o-Ps intensity in pure isooctane. In particular, at C AOT > 0.04 M, I tot appears very close to the value found for pure water. The possibility of a strong inhibition of Ps formation due to the micelles, as proposed in previous work, is ruled out because of the negligible electron or positron scavenging ability of alkyl sulfonates. It is thus concluded that Ps formation occurs primarily in the aqueous part of the micelles, the water aggregates representing efficient traps for the positrons while they are slowing down in the solutions. IntroductionPositronium (Ps), the bound state of a positron (e + ) with an electron (e -), has been used increasingly in recent decades as an efficient probe of the physical and chemical properties of matter. 1 The most commonly used positron annihilation technique (PAT) is lifetime spectroscopy (LS), which allows us to obtain both the lifetimes (τ i ) and relative abundances (or intensities, I i ) of the various positron species. Normally, in order of increasing lifetimes, these are singlet Ps (p-Ps, i ) 1), free positron (i ) 2), and triplet Ps (o-Ps, i ) 3). Because of its distinct long lifetime, the latter appears to be the most useful probe. The o-Ps decay occurs with the emission of two γ-rays through the pick-off process, i.e., the annihilation of the positron with one of the (bound) electrons of the surrounding medium. Information is obtained through two distinct, independent processes, Ps formation and annihilation, both of which depend on various characteristics of the systems under investigation. In liquids, Ps formation may occur on a very short time ...

Positron annihilation techniques are used for the structural investigation of solids but the interpretation of results in grainy and porous media is still unclear. A unique picture can be obtained assuming that the dominant process is Ps trapping in competing "extended free volume" sites. In samples with a large amount of free volumes near-saturation Ps trapping will rule the lifetime pattern, and very long lifetimes of over 100 ns might arise from o-Ps trapped in mesopores. It is shown that lifetime parameters must be corrected for the 3γ/2γ counting efficiency ratio. The results demonstrate the high sensitivity of Ps to mesopores in zeolites but also that Ps-trapping poses limitations on the applicability of lifetime to structural investigation in porous systems. The evolution of the lifetime spectra upon changes in the sample and measuring conditions should be considered in a complex way, observing not only changes in some selected components but in the whole lifetime pattern simultaneously.

In this study we present experimental and simulated results from a first prototype of a positron emission tomography (PET) system based on the resistive plate chamber (RPC) technology -a gaseous particle detector developed for high energy physics. The prototype is aimed at validating the expectations, derived from simulations, of a system with sub-millimeter spatial image resolution and no parallax error, which may be useful for the imaging of small animals.By imaging a point-like 22 Na source in the transaxial plane, an intrinsic spatial resolution of 0.52 mm FWHM was demonstrated for a system diameter of 60 mm. A corresponding image spatial resolution of 0.5 mm FWHM was obtained using the standard algorithm of filtered back projection and 0.4 mm FWHM after reconstruction by an ML-EM type algorithm. An optimized complete system was simulated to evaluate sensitivity and system count rate performance for the imaging of mice, suggesting a sensitivity up to 2.1 % at the centre of the field of view and a peak NEC rate up to 320 Kcps for a total activity of 100MBq in the phantom.