Ortho-positronium Annihilation Rate Research Articles

ESR and PALS detection of the dynamic crossover in the supercooled liquid states of short and medium-sized n-alkanes

A joint study of the spin probe TEMPO dynamics by ESR and the annihilation rate of ortho-positronium by PALS in four short-and medium-sized n-alkanes is presented. In addition to the usually observed changes in both the reorientation dynamics and size of free volumes at the temperature of melting, Tm, and solid-solid phase transition, Tss, an additional coincidence between the characteristic ESR and PALS temperatures TX1fast ≅ Tb1sol < Tm, Tss was found. The phenomenological analysis of the viscosity data of n-alkanes using the power law equation indicates a presence of locally disordered regions in which the dynamic change occurs at the crossover temperature TX ≅ TX1fast ≅ Tb1sol.

Spin conversion and pick-off annihilation of ortho-positronium in gaseous xenon at elevated temperatures

We present experimental results for the annihilation rate of ortho-positronium in gaseous xenon at 250 kPa using a digital-oscilloscope-based positron annihilation lifetime spectrometer as a function of temperature (300--623 K). Owing to the Zeeman mixing of positronium in a magnetic field, the annihilation rate is divided into two components: one is due to ortho-para postronium spin conversion induced by spin-orbit interaction, while the other is due to pick-off annihilation. The spin-conversion annihilation rate is proportional to ${T}^{2.1}$, where $T$ is the temperature. We attributed this dependence to the fact that the spin conversion occurs only in $p$-wave scattering during positronium--xenon collisions and to the fact that, after thermalization of the ortho-positronium atoms, their velocity profile follows a Maxwell-Boltzmann distribution. The results also show that the pick-off annihilation rate is almost linear in temperature. This increase is a dependence exceptionally stronger than that observed in other gases.

Orthopositronium annihilation rates in gaseous halogenated methanes

The normalized orthopositronium quenching rate, ${}_{1}{Z}_{\mathrm{eff}}$, has been measured for CH${}_{3}$F, CH${}_{3}$Cl, CH${}_{3}$Br, and CH${}_{3}$I in the gas phase. Silica aerogel was used as the positronium formation medium and the microchambers. The ${}_{1}{Z}_{\mathrm{eff}}$ values obtained are ${}_{1}{Z}_{\mathrm{eff}}({\mathrm{CH}}_{3}\mathrm{F})=0.46\ifmmode\pm\else\textpm\fi{}0.03$, ${}_{1}{Z}_{\mathrm{eff}}({\mathrm{CH}}_{3}\mathrm{Cl})=0.58\ifmmode\pm\else\textpm\fi{}0.05$, ${}_{1}{Z}_{\mathrm{eff}}({\mathrm{CH}}_{3}\mathrm{Br})=0.70\ifmmode\pm\else\textpm\fi{}0.04$, and ${}_{1}{Z}_{\mathrm{eff}}({\mathrm{CH}}_{3}\mathrm{I})=2.1\ifmmode\pm\else\textpm\fi{}0.2$. The ${}_{1}{Z}_{\mathrm{eff}}$ value for CH${}_{3}$I is significantly larger than expected for pick-off quenching. It is probably caused by spin-conversion quenching through spin-orbit interaction.

Positronium as a probe of transient paramagnetic centers ina−SiO2

A laser-pump, positron-probe technique has been developed for measuring the dynamics of laser-induced paramagnetic centers in porous materials. The basis of the technique is the fact that the annihilation rate of ortho-positronium may increase in the presence of unpaired spins. By using an intense positron pulse to create positronium atoms in a porous silica sample we have used this effect as a probe to measure the time dependence of the recombination of laser-induced paramagnetic centers. We have observed nonexponential postirradiation decay kinetics extending over ten orders of magnitude that are similar to those previously observed in silica glasses.

Annihilation rates of ortho—positronium in organic liquids: correlation with liquid compressibility and polarization ratio

The bubble model of ortho—positronium annihilation rates customarily employs the value of the planar-surface tension in the estimation of the (surface) energy required to expand a normal interstitial cavity to form the bubble. However, bubble radii are estimated to be only about 0.4 nm; the surface of such a small bubble is expected to have a tension different from that of the planar surface. We have replaced the surface tension parameter by the isothermal compressibility, which is thought not to have a nanostructure limitation. The model has been further improved by introducing the polarization ratio (n 2 D − 1)/(n 2 D + 2) as a parameter, n D being the refractive index of the organic liquid. Calculations are presented for 20 liquids.

Positron annihilation spectroscopic investigation of Al-pillared montmorillonites

We report, for the first time, our result on characterisation of Al-pillared clay using positron annihilation spectroscopy (PAS) along with XRD and BET techniques. Positron life-time spectra for Al-pillared montmorillonite, as received and calcined at different temperature (100–500°C), along with the parent natural clay are measured. The annihilation rate of ortho-positronium (o-Ps) increases monotonically with calcination temperature up to 300°C and falls subsequently. Similar behaviour is seen for the fraction of o-Ps formed. We interpret the initial rise in annihilation rate of o-Ps to be due to the increase in Bronsted acidity with dehydroxylation of pillars. The reduction in annihilation rate on calcination at higher temperatures could be due to loss of acidity and/or due to mesopore formation following delamination of clay structure.

Magnetic effects on positronium formation and annihilation in organic liquids

It was recently found that an external magnetic field markedly enhances the orthopositronium annihilation rate in certain organic solutions above and beyond that expected for the Zeeman effect. Several solutions have been studied, but the effect has been found only for nitrobenzene in n-hexane. A mechanism for this is proposed and tested numerically with a kinetics scheme modified to include nonhomogeneous spur effects. The mechanism features the quenching of orthopositronium by a short-lived triplet solute species, the concentration of which is moderated by the applied magnetic field.

A multi-state model for the density dependence of ortho-positronium annihilation rate in gases

The authors present a new model for the calculation of the annihilation rate of ortho-positronium in gases. The measured positronium pick-off annihilation rate is identified with an expectation value over an ensemble of inhomogeneities of the fluid. A single density inhomogeneity is described with a predefined, suitably parametrised density profile and the expectation value is integrated over a reasonably selected volume in the parameter space. Comprehensive tests of the model are performed to determine the dependences on various model parameters and the results are compared with both experimental data and previously used theoretical models. Results for argon, xenon and methane are presented. The qualitative agreement with the experimental data is better throughout a wide density range than that obtained with previously used theoretical models.

An application of the simple model of positronium-induced cavities to orthopositronium annihilation rates in ethane gas

The authors have analysed the density and temperature dependences of orthopositronium annihilation rates in ethane gas following the simple model of positronium-induced cavities. The results obtained for the potential barrier, size of the cavity and positronium binding energy show that a bound state of positronium does not exist in this cavity. This is further supported by calculations presented for the changes in free energy due to positronium trapping following the self-consistent field model. An analysis of these annihilation rate data following a simple model of density fluctuations provides a reasonably good description of the observations for relatively small density fluctuations.

Pressure and temperature dependences of orthopositronium annihilation rates below the critical temperature of ethane

We report new results on the pressure and temperature dependences of the pickoff annihilation rates of orthopositronium in ethane below its critical temperature. These data enable a detailed study of the recently discovered enhancement in the annihilation rates of orthopositronium in ethane near 115 amagat at 305.45 K by presenting similar enhancements at several other temperatures below T c.

Density and temperature dependences of ortho-positronium annihilation rates in low temperature gaseous N 2

Deviations of the ortho-positronium annihilation rate from the linear dependence on density have been observed in N 2 gas at temperatures 100–170 K. The deviations observed at not so high densities can be reasonably explained by the simple density fluctuation model, but disagreement between this model and observed deviations is found at much higher densities.

Effect of gravity on density distributions and orthopositronium annihilation rates in ethane and methane near the critical point

The effect of gravity on density distributions has been studied in ethane and methane near their critical points using the linear-model parametric equation of state. The results obtained from this study are used to further understand the sensitivity of orthopositronium annihilation rates to density fluctuations in molecular gases. It is shown that the influence of gravity is too small to account for the recently observed dependence of orthopositronium annihilation rates on the density of ethane gas at 306.4 K. However, a significant variation in local density vs height is calculated at temperatures closer to the gas–liquid critical point. The density and temperature dependencies of the annihilation rates of orthopositronium atoms, recently observed in ethane and methane gases, are discussed in terms of the findings of this study.

Density corrections to the positron annihilation rate in moderately dense gases

The density and temperature dependences of the effective charge Zeff describing the free-positron annihilation rate in dense gases are investigated by means of the method of the temperature Green's functions. It is shown that the dependence is determined by correlations in positions of the atoms in the medium and by multiple scattering processes. The theoretical results are compared with the experimental data. The possibility of applying the proposed theory to the investigation of the orthopositronium annihilation rate in dense gases is discussed.

Light particle states in dense gases and mixtures

The states available to a light particle in models for gases and gas mixtures are investigated. The models are characterized by atoms (molecules) which mutually interact as hard spheres and which interact with a light particle via a short ranged repulsive interaction. The trends in the densities of states for the light particle, the highest energy for localization, and the number of localized states are pursued as a function of the physical parameters—the particle densities of the constituents, the scattering strengths, the packing fraction, and the relative atomic sizes—using a variational technique and numerical computations. Mean field effects and those due to fluctuations may be isolated. It is noted that these type of calculations provide a context within which experiments, such as those for ortho-positronium annihilation rates in gases, may be analyzed.

Sensitivity of Orthopositronium Annihilation Rates to Density Fluctuations in Ethane Gas

Orthopositronium annihilation rates have been measured in ethane gas for densities in the range 1.8 to 286 amagats at 306.4 K. The behavior of the measured annihilation rates at densities up to about 70 amagats can be explained in terms of a simple density fluctuation model. New features are observed at higher densities of ethane at 306.4 K.

Density and temperature dependencies of ortho-positronium annihilation rates in ethane gas

Results are reported for the density and temperature dependencies of the annihilation rates of o-Ps in ethane gas. The pickoff quenching rate of o-Ps in ethane is observed to vary with temperature at low densities of the gas. The observed o-Ps annihilation behavior with density and temperature of ethane is interpreted in terms of density fluctuations in the gas.

Dependence of ortho-positronium pick-off annihilation lifetime on free volume in organic molecular liquids calculated by Kincaid Eyring method, and positronium bubble formation

Sizes of free volume were quantitatively calculated for 42 organic liquids by means of the Kincaid and Eyring method. The results were discussed in relation to the pick-off annihilation lifetime of ortho-positronium in molecular liquids. The presence of strong correlation was found between them, this fact made us conclude that the free volume size was the most essential physical parameter, which determines the pick-off annihilation rate of ortho-positronium. The problem that the calculated free volume was too small for the existence of ortho-positronium could be eliminated by the equilibristic formation of a bubble around ortho-positronium. Correlation was shown between the free volume radii and bubble radii. Our discussion will likely apply to a wider area for the analysis of empirical data on pick-off rate of ortho-positronium in organic liquids and to the experimental investigation of the pick-off process of ortho-positronium annihilation.

Dependence of orthopositronium annihilation rates on density fluctuations in methane gas

Recent experiments have shown that the annihilation rate of positrons which form orthopositronium in methane gas varies linearly with gas density at low densities, but significantly deviates from linearity at high densities. We present evidence that this annihilation behavior in methane results from density fluctuations that are characteristic of an imperfect gas.

Orthopositronium decay in gaseous, liquid, and solid helium

Orthopositronium annihilation rates in liquid and solid helium up to 140 atm and in the liquid along the vapor-pressure curve up to the critical temperature have been measured and are analyzed in terms of the cavity model with a pressure-dependent radius. Two interdependent parameters of the model are the effective number of singlet annihilation electrons per helium atom $^{1}Z_{\mathrm{eff}}$ for orthopositronium decay in helium gas and the orthopositronium---helium-atom scattering length ${a}_{s}$. Measurements of $o\ensuremath{-}\mathrm{P}\mathrm{s}$ in low-density helium gas at 77 K yielded $^{1}Z_{\mathrm{eff}}=0.116\ifmmode\pm\else\textpm\fi{}0.004$ which is reasonably consistent with recently reported measurements at 300 K. With a low-density limit of $^{1}Z_{\mathrm{eff}}=0.125$, our pressure-dependent data are fit very well with an ${a}_{s}\ensuremath{\simeq}1.15 {a}_{B}$, while we do not obtain a good fit to the vapor-pressure measurements with the approximations used in the analysis. Sensitivity to the details of the model limits the accuracy of this result as a determination of ${a}_{s}$. We have also observed a nonlinearity in the $o\ensuremath{-}\mathrm{P}\mathrm{s}$ annihilation rate in He gas, and we have found ${Z}_{\mathrm{eff}}$ for free-positron decay to be 3.86 \ifmmode\pm\else\textpm\fi{} 0.04 in the gas.

Self-trapping of ortho-positronium in dense fluid helium

Previous theoretical work by the author, on electron self-trapping, is extended to positronium in helium. Results are obtained for several values of the positronium-helium $s$-wave scattering length ($a$), in order to determine the validity of published analyses of data on the dependence of the ortho-positronium annihilation rate on the average density and temperature of its fluid helium host. It is concluded that the recently reported value of $^{1}Z_{\mathrm{eff}}=0.129\ifmmode\pm\else\textpm\fi{}0.006$ is not contaminated by self-trapping effects or models, while the value reported for $a=0.77\ifmmode\pm\else\textpm\fi{}0.04$ \AA{} results from an invalid analysis. It is suggested that a value for $a$ in the neighborhood of 0.8 \AA{} may be reasonably inferred from available experimental data and the calculations presented here, but that the precision of this determination is not very high.