Emission of positronium in a nanometric PMMA film

Positron beam experiments have been performed for the first time on a self-supporting polymethyl metacrylate (PMMA) film of 310 nm-thick made by spin coating. The positronium (Ps) emission from the PMMA surface is studied as a function of the positron implantation energy by using Doppler profile spectroscopy and Compton-to-peak ratio analysis. When the sample and the Ge-detector are perpendicular to the positron beam, the emission of para-positronium (p-Ps) is detected as a narrow central peak. By rotating the sample 45° with respect to the beam, the emission of p-Ps is detected as a blue-shifted fly-away peak. The bulk Ps fraction, the efficiency for the emission of Ps by picking up an electron from the surface, and the diffusion lengths of positrons (thermal and or epithermal), p-Ps and ortho-positronium (o-Ps) are obtained.     

Production and use of low-energy,monoenergetic positron beams from electron LINACS

Low-energy intense positron beams derived from pair production can be made at high-energy electron linacs and such beams are in operation or under installation at several linac facilities. Using a pulsed position beam made at a 100 MeV electron linac, we have measured the intensity and velocity distribution of positronium emitted from materials by measuring the time-of-flight of annihilating positronium. The time-of-flight data are augmented by positron lifetime and angular correlation measurements performed with the beam. Positronium spectra have been measured for a number of metallic samples. Several new observations have been made including details of the energy distribution of positronium emission formed by a thermalized positron and a conduction electron and the production of positronium from energetic positrons scattered out of the sample.This paper is based upon an invited talk given by R.H.H. at the International Symposium Production of Low-Energy Positrons with Accelerators and Applications Justus-Liebig-Universität Giessen, FRG (25–27 September, 1986)     

Positron annihilation study of voids in a neutron irradiated aluminum single crystal

We have measured the lifetimes of positrons in an aluminum single crystal which was irradiated to a fast neutron fluence of 1.5·10²¹ n/cm² (>0.18 MeV) at 50°C. These irradiation conditions produced 4.2·10¹⁴ voids/cm³ with a mean diameter of 330 ?, as determined by both small-angle x-ray scattering and transmission electron microscopy. The positron lifetime spectra were resolved into three lifetime components of 100, 300, and 500 ps. The short lifetime component is a result of fast trapping of positrons by the voids; the long lifetime components result from annihilations within the voids. The intensity of the long lifetime components increases with temperature in the range 80 to 300 K and supports the model of a positron state at the void surface. The positron diffusion coefficient appears to have aT ^1/2-dependence. A magnetic quenching experiment shows no indication of positronium formation. Finally, an isochronal heat treating sequence shows that the voids anneal out between 200 and 300°C, and that the lifetime spectrum after annealing is described by a single component of 170 ps, the observed lifetime in unirradiated aluminum. Research sponsored in part by the U.S.Atomic Energy Commission under contract with the Union Carbide Corporation.     

Direct measurement of the parapositronium lifetime in alpha-SiO2

The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.     

Electric-field effect on positronium formation and decay in some polymers and fused quartz

The effect of static electric field on the formation and annihilation of positronium in polyethylene, polystyrene, teflon, quartz, nylon and perspex has been investigated employing a fast timing spectrometer with prompt resolution (fwhm) ∼200 ps at the positron experimental settings. No significant variation in the lifetime of the long lived pick-off component with electric field has been observed. The intensity of the long-lived pick-off component decreases with increase of the field in all the cases investigated except in nylon and perspex where the intensity remains constant with the field. The results have been discussed in the frame work of the spur reaction model proposed by Mogensen and also in the light of the Ore gap model, as discussed by Brandt and Wilkenfeld. Another interesting correlation seems to exist between positronium formation and dielectric loss factor; in case of increasing loss factor there is a decrease in the effect of electric field. Work performed at Saha Institute of Nuclear Physics, Calcutta 700009, India.     

Positron annihilation study of voids in a neutron irradiated aluminum single crystal

We have measured the lifetimes of positrons in an aluminum single crystal which was irradiated to a fast neutron fluence of 1.5·10²¹ n/cm² (>0.18 MeV) at 50°C. These irradiation conditions produced 4.2·10¹⁴ voids/cm³ with a mean diameter of 330 Å, as determined by both small-angle x-ray scattering and transmission electron microscopy. The positron lifetime spectra were resolved into three lifetime components of 100, 300, and 500 ps. The short lifetime component is a result of fast trapping of positrons by the voids; the long lifetime components result from annihilations within the voids. The intensity of the long lifetime components increases with temperature in the range 80 to 300 K and supports the model of a positron state at the void surface. The positron diffusion coefficient appears to have aT ^1/2-dependence. A magnetic quenching experiment shows no indication of positronium formation. Finally, an isochronal heat treating sequence shows that the voids anneal out between 200 and 300°C, and that the lifetime spectrum after annealing is described by a single component of 170 ps, the observed lifetime in unirradiated aluminum.     

On the annihilation rate of positronium in dense rare gases

The dependence of the positronium decay rate on the gas density is considered. It is known that the positronium can be localized in a dense gas with the formation of a cavity “bubble” around the positronium. It results in a significant delay of the annihilation. The paper containes the quantitative theory of this phenomenon. A comparison is made with the experimental data for He⁴, He³, Ne.     

Positron annihilation in alpha- and beta-cyclodextrin

We have measured positron annihilation lifetimes in α- and β-cyclodextrin samples both in air and in vacuum. The existence of different hollow spaces in these clathrates offers a good test of the “free volume” model of positronium decay in molecular solids. Supported in part by the Research Corporation     

Positron annihilation study on nanometer cavities in porous silicon

Measurements were carried out on the positron lifetime and the Doppler broadening of its annihilation radiation in porous silicon. A very long lifetime of a few tens of nanoseconds was found. TheS parameter increased upon annealing in vacuum at 350 °C. It is pointed out that positron/positronium spectroscopy is very useful for the study of physical and chemical properties of porous silicon.     

The carrier “antibinding” in quantum dots: a charge separation effect

We show that the carrier “antibinding” observed recently in semiconductor quantum dots, i.e., the fact that the ground state energy of two electron-hole pairs goes above twice the ground-state energy of one pair, can entirely be assigned to a charge separation effect, whatever its origin. In the absence of external electric field, this charge separation comes from different “spreading-out” of the electron and hole wavefunctions linked to the finite height of the barriers. When the dot size shrinks, the two-pair energy always stays below when the barriers are infinite. On the opposite, because barriers are less efficient for small dots, the energy of two-pairs in a dot with finite barriers, ends by behaving like the one in bulk, i.e., by going above twice the one-pair energy when the pairs get too close. For a full understanding of this “antibinding” effect, we have also reconsidered the case of one pair plus one carrier. We find that, while the carriers just have to spread out of the dot differently for the “antibinding” of two-pairs to appear, this “antibinding” for one pair plus one carrier only appears if this carrier is the one which spreads out the less. In addition a remarkable sum rule exists between the “binding energies” of two pairs and of one pair plus one carrier.     

A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE

We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Some aspects of positronium physics

Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps → invisible) ≃ 10⁻⁸–10⁻⁷ are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. The text was submitted by the authors in English.     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

Structure of quantum-mechanical relativistic two-body interactions for spinning particles

Relativistic constraint mechanics yields consistent systems of coupled Dirac equations for pairs of spinning particles. We explicitly connect these equations to the Bethe-Salpeter equation of quantum field theory and to the interactions of classical Fokker-Tetrode dynamics (and hence to classical relativistic field theory) to obtain versions of these equations governed by systems of (possibly noncoulombic) relativistic potentials whose detailed structures contain important relativistic effects like correct Darwin interactions. We recast the defining pair of Dirac equations in a number of equivalent but important forms—“external potential,” Sazdjian, hyperbolic, and Breit— and examine their interconnection. Since the potentials in these equations are no more singular than — 1/4r² we are able to solve appropriate versions of them nonperturbatively for the qˉq system to obtain a very good fit to the entire meson spectrum and for the e ⁺ e ⁻ system to calculate the positronium spectrum of QED correct through order α ⁴ .     

Kinetics of positron annihilation in ionic crystals

A kinetic scheme of decay of positron and positronium states for the general case — a real ionic crystal with point lattice defects and a developed surface — is examined. The properties of positron and positronium states in the volume of the crystal, positron and positronium color centers, and surface positron and positronium states, are analyzed. It is shown that all the available experimental data qualitatively confirm the conclusion of an annihilation mechanism based on the postulated kinetic scheme of positron annihilation in real ionic crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–55, September, 1977.     

Cellular Biology in Terms of Stochastic Nonlinear Biochemical Dynamics: Emergent Properties,Isogenetic Variations and Chemical System Inheritability

Based on a stochastic, nonlinear, open biochemical reaction system perspective, we present an analytical theory for cellular biochemical processes. The chemical master equation (CME) approach provides a unifying mathematical framework for cellular modeling. We apply this theory to both self-regulating gene networks and phosphorylation-dephosphorylation signaling modules with feedbacks. Two types of bistability are illustrated in mesoscopic biochemical systems: one that has a macroscopic, deterministic counterpart and another that does not. In certain cases, the latter stochastic bistability is shown to be a “ghost” of the extinction phenomenon. We argue the thermal fluctuations inherent in molecular processes do not disappear in mesoscopic cell-sized nonlinear systems; rather they manifest themselves as isogenetic variations on a different time scale. Isogenetic biochemical variations in terms of the stochastic attractors can have extremely long lifetime. Transitions among discrete stochastic attractors spend most of the time in “waiting”, exhibit punctuated equilibria. It can be naturally passed to “daughter cells” via a simple growth and division process. The CME system follows a set of nonequilibrium thermodynamic laws that include non-increasing free energy F(t) with external energy drive Q _hk ≥0, and total entropy production rate e _p =−dF/dt+Q _hk ≥0. In the thermodynamic limit, with a system’s size being infinitely large, the nonlinear bistability in the CME exhibits many of the characteristics of macroscopic equilibrium phase transition.     

Theory of positronium formation on solid surfaces: I. General theory and its application to positronium formation probability

We consider the micro-processes of positronium formation on surfaces with a new Hamiltonian, in which we treat the positron as a quantum particle. First we calculate the temperature dependence and the positronium work function dependence of the positronium formation probability in the case of no surface binding states. We succeeded in deriving a slightly increasing profile of the positronium fraction at low temperatures, as was found in the experiment of Lynn et al. We also calculated the probability of the positronium formation at excited levels and of the formation of the negative positronium ion. Furthermore, the positronium fraction is shown to be proportional to the length at which the surface electron density drops to half the maximum value.     

An intense pulsed positron beam

A positron pulsing system for an intense positron beam generated by an electron linac is reported. The pulsing system generates an intense pulsed positron beam of variable energy and variable pulse period. The pulsed positron beam is used as a non-destructive probe for various material research. In this paper, we also discuss applications of the pulsed positron beam: positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique.     

Resonant positronium formation in atomic gases

A method is presented for calculation of quasistationary excited states of positronium ions. These states are treated as bound positron states in the quadrupole field of an excited valence electron, which are capable of decaying both with positron emission into the continuous spectrum, and with positronium emission. The existence of such states should manifest itself in resonances in the positronium formation section upon motion of positrons with energy in the eV range. The resonant section parameters are calculated for a number of atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 6–12, March, 1984.