Positron lifetime calculations of defects in fusion irradiated beryllium

Numerical quantum-mechanical positron lifetime calculations were performed for mono-vacancies, di-vacancies, tri-vacancies and small nano-voids containing helium and hydrogen in neutron irradiated beryllium. Helium and hydrogen atoms in the sample after the irradiation are considered as atoms forming interstitial O-type loops. Spherical clusters of vacancies are included in the calculations as a reference. It was found that the presence of He and H atoms significantly changes the positron lifetime in irradiated beryllium. A correlation between the positron lifetime and mutual position of vacancies in nano-voids and interstitial loops was established.     

Transport coefficients for positron swarms in nitrogen

Monte Carlo simulations of positron transport in nitrogen have been performed based on a complete set of cross sections that has been updated as a result of both experimental and theoretical advances. As the positronium formation cross section becomes dominant at larger mean energies in nitrogen than in argon and hydrogen the effect is much smaller in the case of nitrogen. Nevertheless, the differences between bulk and flux transport coefficients, even in nitrogen, exceed those for electrons, providing another example of the very special case of transport of charged particles under the influence of strong non-conservative collisions.     

The effect of alloying elements on the vacancy defect evolution in electron-irradiated austenitic Fe-Ni alloys studied by positron annihilation

The vacancy defect evolution under electron irradiation in austenitic Fe-34.2 wt% Ni alloys containing oversized (aluminum) and undersized (silicon) alloying elements was investigated by positron annihilation spectroscopy at temperatures between 300 and 573 K. It is found that the accumulation of vacancy defects is considerably suppressed in the silicon-doped alloy. This effect is observed at all the irradiation temperatures. The obtained results provide evidence that the silicon-doped alloy forms stable low-mobility clusters involving several Si and interstitial atoms, which are centers of the enhanced recombination of migrating vacancies. The clusters of Si-interstitial atoms also modify the annealing of vacancy defects in the Fe-Ni-Si alloy. The interaction between small vacancy agglomerates and solute Al atoms is observed in the Fe-Ni-Al alloy under irradiation at 300-423 K.     

Molecular model for annihilation rates in positron complexes

The molecular approach for positron interaction with atoms is developed further. Potential energy curves for positron motion are obtained. Two procedures accounting for the nonadiabatic effective positron mass are introduced for calculating annihilation rate constants. The first one takes the bound-state energy eigenvalue as an input parameter. The second is a self-contained and self-consistent procedure. The methods are tested with quite different states of the small complexes HPs, e⁺He (electronic triplet) and e⁺Be (electronic singlet and triplet). For states yielding the positronium cluster, the annihilation rates are quite stable, irrespective of the accuracy in binding energies. For the e⁺Be states, annihilation rates are larger and more consistent with qualitative predictions than previously reported ones.     

Radiation damage in a-SiO2 exposed to intense positron pulses

In addition to its numerous technological applications amorphous silica (a-SiO₂) is also well suited to the creation and study of exotic atoms such as positronium (Ps) and muonium. In particular, a dense Ps gas may be created by implanting an intense positron pulse into a porous a-SiO₂ sample. However, such positron pulses can constitute a significant dose of radiation, which may damage the sample. We have observed a reduction in the amount of Ps formed in a thin film of porous a-SiO₂ following irradiation by intense positron pulses, indicating the creation of paramagnetic centers. The data show that the primary effect of the irradiation is the inhibition of Ps formation, with no significant change in the subsequent Ps lifetime, from which we deduce that damage centers are created primarily in the bulk material and not on the internal surfaces of the pores, where they would be accessible to the long-lived Ps. We find that the damage is reversible, and that the system may be returned to its original state by heating to 700 K. The implications of these results for experiments with dense Ps in porous materials are discussed.     

Characterization of defect accumulation in neutron-irradiated Mo by positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy measurements were performed on neutron-irradiated low carbon arc cast Mo. Irradiation took place in the high flux isotope reactor, Oak Ridge National Laboratory, at a temperature of 80 ± 10 °C. Neutron fluences ranged from 2 × 10²¹ to 8 × 10²⁴ n/m² (E > 0.1 MeV), corresponding to displacement damage levels in the range from 7.2 × 10⁻⁵ to 2.8 × 10⁻¹ displacements per atom (dpa). A high density of submicroscopic cavities was observed in the neutron-irradiated Mo and their size distributions were estimated. Cavities were detected even at a very low-dose of ∼10⁻⁴ dpa. The average size of the cavities did not change significantly with dose, in contrast to neutron-irradiated bcc Fe where cavity sizes increased with increasing dose. It is suggested that the in-cascade vacancy clustering may be significant in neutron-irradiated Mo, as predicted by molecular dynamics simulations.     

First temperature stage evolution of irradiation-induced defects in tungsten studied by positron annihilation spectroscopy

The behaviour of vacancy like implantation-induced defects created in the track region of 800 keV ³He ions in polycrystalline tungsten was studied by Doppler broadening spectroscopy as a function of annealing temperature. A slow positron beam, coupled with a Doppler broadening spectrometer, was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in tungsten samples implanted at different fluences from 10¹⁴ to 5 × 10¹⁶ cm⁻². The behaviour of the S(E), W(E) and S(W) plots with the annealing temperature clearly indicates that the irradiation-induced vacancy like defects begin to evolve between 523 and 573 K, whatever the implantation fluence. This first temperature stage evolution corresponds to the migration of the monovacancies created during implantation to form larger vacancy like defects of which depth profile is different from the initial radiation-induced defects one.     

The influence of f-type function in positron-He/positron-H2 scattering with the Schwinger multichannel method

In this paper we present results for positron-Helium and positron-H₂ scattering with the inclusion of the f-type Cartesian Gaussian functions in our computer codes of the Schwinger multichannel method (SMC). The effects of this modification can be noticed in the integral cross-section for both studied targets, with our new curves being closer to the most recent experimental measurements. The inclusion of the f-type function in the scattering wave function expansion also helped us to obtain a better set of results with the SMC method for the annihilation parameter. Data for differential cross-section (DCS) for helium is presented as well as our improvement in the DCS data in the forward scattering angles for the hydrogen molecule.     

正电子寿命谱仪的最佳分辨率

能窗的选择对正电子寿命谱仪的分辨率有决定性的影响。本文描述了一种获得谱仪最佳分辨率的方法。将正电子寿命谱仪的可工作能区分成42个小块。测量每小块能区的分辨率(FWHM)、计数率和峰位“走动”。从中选出分辨率小、计数率高和“走动”小的好能区。能窗是由若干个最佳小能区组成。谱仪分辨率的理论计算值和实验测量值符合得很好。     

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe₅₀Al₅₀ alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.     

Positron annihilation study and computational modeling of defect production in neutron-irradiated reactor pressure vessel steels

Positron annihilation spectroscopy (PAS) and a computer simulation were used to investigate a defect production in reactor pressure vessel (RPV) steels irradiated by neutrons. The RPV steels were irradiated at 250 °C in a high-flux advanced neutron application reactor. The PAS results showed that mainly single vacancies were created to a great extent as a result of a neutron irradiation. Formation of vacancies in the irradiated materials was also confirmed by a coincidence Doppler broadening measurement. For estimating the concentration of the point defects in the RPV steels, we applied computer simulation methods, including molecular dynamics (MD) simulation and point defect kinetics model calculation. MD simulations of displacement cascades in pure Fe were performed with a 4.7 keV primary knock-on atom to obtain the parameters related to displacement cascades. Then, we employed the point defect kinetics model to calculate the concentration of the point defects. By combining the positron trapping rate from the PAS measurement and the calculated vacancy concentrations, the trapping coefficient for the vacancies in the RPV steels was determined, which was about 0.97 × 10¹⁵ s⁻¹. The application of two techniques, PAS and computer simulation, provided complementary information on radiation-induced defect production.     

Annihilation characteristics of positrons in free-standing thin metal and polymer films

Annihilation characteristics of positrons and positronium (Ps) in thin metal and polymer films were studied. Monoenergetic positrons were implanted into free-standing thin W and Au films and the annihilation γ-rays of positron-electron pairs were measured as a function of the incident energy of positrons. At the front-side surfaces of the films, an emission of Ps into vacuum and a resultant self-annihilation of ortho-Ps (o-Ps) were observed. At the backside surfaces, the Ps emission was found to be enhanced by an increase in the numbers of epithermal positrons and/or secondary electrons introduced by the impact of energetic positrons. For thin polymer films (polyester and polystyrene), the emission rate of o-Ps from the backside surfaces was higher than that from the metal films, which was attributed to the out-diffusion of o-Ps formed in the films. Those results suggested that the emission rate of Ps into vacuum was sensitive to the Ps formation process in the bulk and at the surface.     

Positron annihilation characteristics of ODS and non-ODS EUROFER isochronally annealed

Yttrium oxide dispersion strengthened (ODS) and non-ODS EUROFER produced by mechanical alloying and hot isostatic pressing have been subjected to isochronal annealing up to 1523 K, and the evolution of the open-volume defects and their thermal stability have been investigated using positron lifetime and coincidence Doppler broadening (CDB) techniques. Transmission electron microscopy (TEM) observations have also been performed on the studied samples to verify the characteristics of the surviving defects after annealing at 1523 K. The CDB spectra of ODS EUROFER exhibit a characteristic signature that is attributed to positron annihilation in Ar-decorated cavities at the oxide particle/matrix interfaces. The variation of the positron annihilation parameters with the annealing temperature shows three stages: up to 623 K, between 823 and 1323 K, and above 1323 K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T ? 623 K. In the temperature range 823-1323 K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323 K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523 K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.     

Experimental studies on positronium negative ions

Recently, a new stage of experimental studies on positronium negative ions has begun. This is due to the development of a new detection method and the discovery of the ion formation at metal surfaces. In the present paper, experimental studies are reviewed and future experiments are proposed.     

Calculations for very low energy scattering of positrons by molecular hydrogen

We give a progress report on ongoing calculations of phase shifts for very low energy elastic scattering of positrons by molecular hydrogen, using the generalised Kohn variational method. Further, provisional calculations of Z_eff for molecular hydrogen at low energies are presented and discussed. The preliminary nature of the work is emphasised throughout.     

Positron annihilation and X-ray diffraction studies on tin oxide thin films

Positron annihilation spectroscopy along with glancing incidence X-ray diffraction have been used to investigate tin oxide thin films grown on Si by pulsed laser deposition. The films were prepared at room temperature and at 670 K under oxygen partial pressure. As-grown samples are amorphous and are found to contain large concentration of open volume sites (vacancy defects). Post-deposition annealing of as-grown samples at 970 K is found to drastically reduce the number of open volume sites and the film becomes crystalline. However, film grown under elevated temperature and under partial pressure of oxygen is found to exhibit a lower S-parameter, indicating lower defect concentration. Based on the analysis of experimental positron annihilation results, the defect-sensitive S-parameter and the overlayer thickness of tin oxide thin films are deduced. S-W correlation plots exhibit distinct positron trapping defect states in three samples.     

Influence of different chemical elements on irradiation-induced hardening embrittlement of RPV steels

Fe–Cu binary alloys are often used to mimic the behaviour of reactor pressure vessel steels. Their study allows identifying some of the defects responsible for irradiation-induced hardening. But recently the influence of manganese and nickel in low-Cu steels has been found to be important as well. In contrast with existing models found in the literature, which predict that hardening saturates after a certain dose, Fe alloys containing nickel and manganese irradiated in a material test reactor (BR2) show a continuous increase of hardening, up to doses equivalent to about 40 years of operation. Considerations based on positron annihilation spectroscopy analyses suggest that the main objects causing hardening in Cu-free alloys are most probably self-interstitial clusters decorated with manganese. In low-Cu reactor pressure vessel steels and in Fe–CuMnNi alloys, the main effect is still due to Cu-rich precipitates at low doses, but the role of manganese-related features becomes predominant at higher doses.     

调试正电子寿命谱仪能窗的一种简便方法

最近几年,恒比微分甄别器己广泛用于正电子湮没寿命谱仪中,使该装置简化到可以不用符合方式,只使用阳极引出。以往调试能窗是利用打拿极输出做能谱显示来解决。本文总结了在不用打拿极的情况下也可达到这个目的的经验。它的主要思想是:利用恒比微分甄别器(ORTEC 583)的输出随阈值或能窗的微分变化来确定511 keV康峰的边沿和选出具有最佳效率的能窗。方法的可靠性用直接显示的能谱做了检验,也用示波器测定了脉冲的幅度,     

Development of a method for measuring the ortho-positronium quenching rate in low vapor-pressure gases

A new measurement and analysis method for determining the ortho-positronium collisional quenching rate for low vapor-pressure gases is developed. It uses hydrophobic silica aerogels as highly efficient positronium formation media and microchambers. The value of the normalized positronium quenching rate, ¹Z_eff, for CH₃Br measured by this method is 0.70 ± 0.04.     

Microstructure evolution of Ge implanted silicon oxide thin films upon annealing treatments

The structural evolution of silicon oxide films with Ge⁺ implantation was traced with a positron beam equipped with positron annihilation Doppler broadening and lifetime spectrometers. Results indicate that the film structure change as a function of the annealing temperature could be divided into four stages: (I) T < 300 °C; (II) 300 °C ? T ? 500 °C; (III) 600 °C ? T ? 800 °C; (IV) T ? 900 °C. In comparison with stage I, the increased positron annihilation Doppler broadening S values during stage II is ascribed to the annealing out of point defects and coalescence of intrinsic open volumes in silicon oxides. The obtained long positron lifetime and high S values without much fluctuation in stage III suggest a rather stable film structure. Further annealing above 900 °C brings about dramatic change of the film structure with Ge precipitation. Positron annihilation spectroscopy is thereby a sensitive probe for the diagnosis of microstructure variation of silicon oxide thin films with nano-precipitation.