Insights into positron annihilation lifetime spectroscopy by molecular dynamics simulations

The relationship between free-volume properties measured from positron annihilation lifetime spectroscopy (PALS) and calculated from molecular dynamics simulations has been investigated for glassy and liquid glycerol in the temperature range 150–400 K. A virtual probing procedure has been developed to retrieve information on the basic free-volume properties of the simulated microstructures, i.e. mean cavity volume and free-volume cavity fractions. Our data leads us to infer on the occurrence of experimentally non-detectable small cavities with mean equivalent radius of 1.8–1.9 Å between 250 and 275 K. The size of these limiting cavities is found to be temperature dependent, being smaller at low temperatures. At high temperatures, above a characteristic PALS temperature Tb2L , the formation of very large cavities is predicted. This finding suggests that, when the dimension of the holes in the system exceeds a given value, the PALS measurements become unable to catch the complete structural information and phenomena of dynamical origin enter into play in the PALS signal decay. The calculated number of cavities is found to be almost independent on the temperature from the glassy up to the liquid phase, thus furnishing a certain support to theoretical models proposed to evaluate the free-volume cavity fractions.     

Preliminary studies on a variable energy positron annihilation lifetime spectroscopy system

There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.     

Polydimethylsiloxane rubber gamma radiation effect studies by positron annihilation lifetime spectroscopy

Polydimethylsiloxane rubber was irradiated at various radiation doses up to 800 kGy in air. The lifetime and intensity of the long component τ₃ were obtained by positron annihilation lifetime spectroscopy (PALS). The crystallization property and the molecular flexibility were estimated using the differential scanning calorimetry and dynamic thermomechanic analysis. Thermal volatile property was determined by thermogravimetry analysis. It was proved that the cross-linking reaction made the chain flexibility of the rubber to reduce gradually during gamma radiation, which resulted in the reduction of the o-Ps intensity observed by PALS. Although the degradation effect existed during gamma radiation, it was still less significant than the cross-linking effect when the radiation dose was up to 800 kGy.     

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.     

多孔硅薄层的正电子湮灭寿命研究

用阳极氧化法按不同腐蚀条件制备的多孔硅，其薄层厚度仅为几十纳米，远小于^22Na源的正电子平均射程．文章提出了一种用控Na的正电子测量体寿命谱并扣除基片贡献的方法，来确定多孔硅薄层的平均孔径。     

A new positron source for positron annihilation lifetime experiments

A method using ion-implantation of Na²² to produce source-sample systems applicable to positron annihilation lifetime experiments at elevated temperatures in metals is presented.     

Digital positron lifetime spectroscopy at EPOS

As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means.Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with ⁶⁰Co), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.     

Defect structure studies using positron annihilation spectroscopy

The positron annihilation method is a new addition to the range of sensitive complementary nuclear techniques available for materials’ research. The preferential sensitivity of positrons towards micro-defect domains which are not assessable by other techniques makes it an attractive tool for many materials science problems. The present paper is intended as a brief introduction on the principle of measurements and its potential is exemplified with the help of results on some metallic and ceramic systems.     

Effect of rare earth substitutions on some physical properties of Mn-Zn ferrite studied by positron annihilation lifetime spectroscopy

Mn-Zn ferrite substituted with rare earth ions have been investigated by means of positron annihilation lifetime spectroscopy (PALS). The variations of lifetime parameters τ_av, I₂, and κ with ionic radius of rare earth ions, grain size and electrical resistivity for all samples have been studied. The intergranular pores increase with increasing the ionic radius of rare earth ions and grain size of the samples.     

SHI induced modification in CR-39 polycarbonate by positron annihilation lifetime studies

CR-39 is a polycarbonate widely used as SSNTD for recording nuclear charged particles and in other applications. Latent ion tracks produced in the polymers due to the damage produced by the passage of Swift Heavy Ions contain amorphous material with highest degree of disorder, changing the free volume properties which have strong correlation with the macroscopic properties of the material. Positron annihilation lifetime spectroscopy (PALS) provides direct information about the dimension, content and hole size distribution of free volume in polymers. The effect of irradiation of ⁴⁰Ar (14.9 MeV/n) ions on CR-39 polycarbonate by Positron Annihilation Lifetime Spectroscopy (PALS) is reported here. PALS provides a non-destructive and non-interfering probe, having high detection efficiency for free volume hole properties. From o-Ps lifetime mean free volume hole radius and average free volume of the micro-voids have been calculated. PAL measurement shows an increase in free volume on irradiation.     

The influence of geometry on the resolution of a positron annihilation lifetime spectrometer

A detailed study of the prompt resolution curve of a positron annihilation lifetime spectrometer shows that the geometry of the source-detector set-up has great influence on its shape.     

The influence of the source on the resolution of a positron annihilation lifetime spectrometer

A detailed study of the prompt resolution curve of a positron annihilation lifetime spectrometer shows that the isotope used as source has great influence on its shape.     

质子辐照空间级硅橡胶的正电子淹没寿命谱研究

 用正电子淹没寿命谱方法（PALS）研究了质子辐照对空间级硅橡胶KH-L-Y微观结构的影响。试验结果表明,PALS谱所揭示的最长寿命成分的t₃, I₃及自由体积分数Vf随辐照剂量的增加开始明显下降;而当辐照剂量大于10¹⁵cm^-2后,随剂量的增加平缓上升。辐照剂量小于10¹⁵cm^-2时,质子辐照使硅橡胶自由体积减小,分子链间堆砌紧密;辐照剂量大于10¹⁵cm^-2时,质子辐照使硅橡胶自由体积增大。交联密度及DMA测试结果同样表明,质子辐照在剂量较小时硅橡胶的交联密度及玻璃化转变温度增加,辐照以交联效应为主;而剂量较大时辐照降解占优势。     

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.     

Detection of hydrogen in neutron-irradiated nickel using positron lifetime spectroscopy

Hydrogen in nano-voids in neutron-irradiated nickel has been detected using positron annihilation lifetime spectroscopy (PALS). As positron lifetime is greatly affected by nano-voids bound with hydrogen, special attention was paid to the analysis. The positron lifetime of neutron-irradiated nickel at higher irradiation doses increased with the dose, which is an indicator for vacancy cluster (nano-void) formation in the lattice. The introduction of hydrogen in well-annealed nickel by electrical charging also resulted in an increase in positron lifetime due to vacancy formation. In neutron-irradiated nickel specimens, hydrogen charging shortened the positron lifetime from 456 to 334 ps (irradiation dose: 3 × 10^?3 dpa). Isochronal annealing behaviour of hydrogen-charged nickel and neutron-irradiated nickel was also studied. Positron trapping rate was calculated using a simple trapping model. Thermal desorption spectroscopy was used for the investigation of hydrogen behaviour in non-irradiated hydrogen-charged nickel.     

Defects in electron irradiated GaP studied by positron lifetime spectroscopy

Defects induced by electron irradiation were investigated in GaP. The irradiation was performed at 15 K with an incident electron energy of 2 MeV and a fluence of 10¹⁸cm^–2. Annealing experiments were carried out in the temperature range between 100 and 1000 K.Ga and P vacancies were detected after electron irradiation and the different annealing behaviour of the two types of vacancies was observed. The recovery stage between 100 and 400 K was attributed to the annealing of Ga vacancies and the recovery at temperatures above 900 K to the annealing of P vacancies.We also performed Hall measurements to determine the location of the Fermi level in the bandgap during the annealing. Two different ionization levels of the P vacancy were found which can be attributed to the transitions V _P ⁺ /V _P ^o and V _P ^o /V _P ^– .Temperature-dependent measurements were performed to study the effect of shallow positron traps.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Application of positron annihilation spectroscopy on the ion implantation damaged Fe-Cr alloys

Four different Fe-Cr binary alloys with Cr content 2.5-11 wt% were studied in details using various methods. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were applied to obtain basic information, required for standard positron annihilation lifetime spectroscopy (PALS) spectra analysis. Additionally, PALS measurements were performed on as-received state as well as on helium implanted specimens. The He implantation was proposed for simulation of radiation damage and obtain high doses even in near surface areas (up to 1 μm). The implantation was based on the SRIM code simulation and next DPA calculations. Final concentration of vacancy type defects were calculated for 250 keV He²⁺ beam and the maximum was determined in 600 μm depth. Such specimens are very suitable for positron beam study of vacancy type defect mobility as a result of thermal treatment, which will be performed simultaneously in the future.     

Studying the recovery of as-received industrial Al alloys by positron annihilation spectroscopy

Positron annihilation lifetime (PAL) spectroscopy, Doppler broadening of annihilation radiation (DBAR) spectroscopy and Vickers microhardness (Hv) measurements were performed to study the micro- and macro-structure variations during isochronal annealing from room temperature (RT) to 500 °C of commercial pure Al (1 1 0 0), Al-Mn-Mg (3 0 0 4) and Al-Mg-Si (6 2 0 1) alloys. Three annealing stages of microstructures have been identified as recovery, partial recrystallization and complete recrystallization followed by grain growth. A positive correlation between the macroscopic mechanical properties (Hv) and positron annihilation parameters has been achieved for the three samples under investigation.     

Effective concentration of electrons in metals upon measurements via positron annihilation spectroscopy

Calculations are performed together with the summarization of published data on the measurement of positron annihilation rates in a number of simple substances. The information potential of different methods underlying positron annihilation spectroscopy is analyzed as applied to the study of condensed matter. The features of the mechanism of positron annihilation in metals are discussed. The possibility of investigating the electronic and defect structures of metals and alloys using the method of the time distribution of annihilation photons is considered.