Positron Annihilation Lifetime Measurements Research Articles

Correlation between positron annihilation lifetime and photoluminescence measurements for calcined Hydroxyapatite

Hydroxyapatite (HAp) Ca10(PO4)6(OH)2 is a compound that has stable chemical properties, composition, and an affinity for human bone. As a result, it can be used in odontology, cancer treatment, and orthopedic grafts to repair damaged bone. To produce calcined HAp at 600 °C with different pH values, a wet chemical precipitation method was employed. All synthesized HAp samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), photoluminescence (PL), Zeta potential, and positron annihilation lifetime spectroscopy (PALS). The XRD results revealed that all calcined HAp samples were formed in a hexagonal structure with a preferred (002) orientation at different pH values. The crystal size of the samples was determined using the Scherrer equation, which ranged from 16 to 25 nm. The SEM and TEM results showed that the morphology of the samples varied from nanorods to nanospheres and rice-like structures depending on the pH value of the sample. The PL measurements indicated that the blue and green emission peaks of HAp were due to defects (bulk, surface, and interface) in the samples, which created additional energy levels within the band gap. According to Zeta potential measurements, the charge carrier changed from a positive to negative value, ranging from 3.94 mV to − 2.95 mV. PALS was used to understand the relationship between the defects and the photoluminescence (PL) properties of HAp. Our results suggest that HAp nanoparticles have excellent potential for developing non-toxic biomedical and optical devices for phototherapy.

Investigation of vacancy-type defects in diamond-like carbon films using slow positron beam

Positron annihilation spectroscopy was used to investigate vacancy-type defects in diamond-like carbon (DLC) films. From Doppler broadening measurements of the γ-rays produced by annihilation and positron annihilation lifetime (PAL) using a slow positron beam on DLC films deposited by various deposition methods and conditions, it was found that there is a good correlation between the S parameter obtained by Doppler broadening and PAL. The result of PAL correlates well with film density and hardness, indicating that PAL measurements can be used as an indicator of these film properties. The hydrogen content in the DLC film was roughly proportional to the PAL. However, there were also cases in which PAL differed greatly even with almost the same hydrogen content, and the sp 2/(sp 2+sp 3) ratio of carbon in DLC films has no direct relationship with PAL.

Effect of electron beam irradiation on hole structure of cellulose acetate membrane

Cellulose acetate (CA) is one of the most popular biomaterials used for filtration membranes in water treatment.In this study, the CA membranes immersing in water were irradiated by electron beam, and the hole structure of CA membranes at different irradiation doses was investigated. The SEM pictures show the combination of micro-meter sized pores in CA membrane during the irradiation process. The electrochemical impedance spectroscopy(EIS) results reveal the membrane resistance decreases with the irradiation dose, which implies easier ion permeation. The positron annihilation lifetime measurement (PAL) results indicate the mean free-volume hole size becomes smaller with increasing irradiation dose. The above results demonstrate that electron beam irradiation would lead to chain scission of CA, and then the formation of new pores and pores combination, which provides more channels for ion permeation. FTIR results suggest stronger hydrogen bonding in CA after the irradiation in water. The differential scanning calorimetry (DSC)results indicate that the crystallinity has been enhanced with the irradiation dose up to 10 kGy and reduces in the range from 10 kGy to 80 kGy. The results could provide the clues explaining the performance deterioration of CA membrane during treating radioactive waste water.

Extremely Low Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Performance in α-Cu2Se through the Incorporation of Porous Carbon.

In this work, a series of Cu2Se/x wt % porous carbon (PC) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) composite materials were synthesized by ball milling and spark plasma sintering (SPS). The highly ordered porous carbon was synthesized by a hydrothermal method using mesoporous silica (SBA-15) as the template. X-ray diffraction results show that the incorporation of porous carbon induces a phase transition of Cu2Se from the β phase to the α phase. Meanwhile, the addition of porous carbon reduces the carrier concentration from 2.7 × 1021 to 2.45 × 1020 cm-3 by 1 order of magnitude. The decrease of the carrier concentration leads to the reduction of electrical conductivity and the increase of the Seebeck coefficient, which results in the enhancement of the power factor. On the other hand, the incorporation of porous carbon into Cu2Se increases the porosity of the composites and also introduces more interfaces between the two materials, which is evidenced by positron annihilation lifetime measurements. Both pores and interfaces greatly enhance phonon scattering, leading to extremely low lattice thermal conductivity. In addition, the decrease of electrical conductivity also causes a sufficient reduction in electronic thermal conductivity. Due to the above synergistic effects, the thermoelectric performance of the Cu2Se/PC composite is significantly enhanced with a maximum ZT value of 0.92 at 403 K in the Cu2Se/1 wt % PC composite, which is close to that of the Bi2Te3-based materials. Our work shows that α-Cu2Se has great potential for near-room-temperature thermoelectric materials.

Development of novel positron lifetime measurement system for potential liquid material inspection

β+-γ coincidence method and anti-coincidence method have great potential in positron annihilation lifetime (PAL) measurements of liquid material. Based on anti-coincidence method with the advantage of better picking out positron annihilation events in the sample, a novel PAL spectrometer, which can isolate the sample from the positron source and is suitable for liquid material inspection, is developed. The influences of the measuring geometry and the signal processing on the anti-coincidence effect are systematically studied. By comparing the PAL spectra of well annealed pure iron acquired by the new system with that acquired by the conventional system, the performance of system is evaluated. The analysis results of PAL spectra at different distances from the sample to the source and precise measurement of PAL in liquid water show that reliable values are obtained for liquid material inspection at an adjustable distance of less than 1.0 mm. This system is expected to be applied to liquid scintillators, oils or other special materials, which could easily contaminate the positron source.

Positron annihilation lifetime measurement with TOF-PET detectors: feasibility of Iodine-124 use

Positronium (Ps) imaging is getting attention for nuclear medicine applications, but appropriate radionuclides have not been evaluated systematically. This paper investigated the use of 124I, which is a positron emitter with 603 keV prompt gamma ray emission with a fraction of 11.7%. The accuracy of positron annihilation lifetime measurement with 124I was compared with 22Na, which is often used in positron annihilation lifetime measurement, for certified reference materials. Results obtained with TOF-PET detectors suggested that the accuracy of the lifetime value estimation was slightly worse than that for 22Na, while the positron annihilation lifetime measurement using 124I was a feasible choice.

Free volume study of sulfonated FEP proton exchange membrane

Changes in the free volume of poly(tetrafluoroethylene-co-hexafluoropropylene) proton exchange membrane caused by the grafting of styrene and incorporation of a sulfonic acid group (-SO3H) was investigated by using a positron annihilation lifetime measurement. The free volume size of the film decreases because of not only the incorporation of -SO3H but also structural changes in hydrogen bonds during proton transportation. The decrease of free volume size leads to the close distribution of -SO3H; therefore, the performance of the proton exchange membrane increases with the increase of the degree of grafting.

Super high counting rate of positron annihilation lifetime spectrometer achieved via multiple detectors

Positron annihilation lifetime (PAL) spectroscopy has long been considered as a powerful approach to characterize atomic-scale vacancy-type defects and supermicropores, which affect the macroscopic properties of materials. In standard PAL measurements, several million annihilation events are required to achieve accuracy. For a conventional PAL spectrometer which consists of two scintillation detectors and analytical electronics, the two detectors are usually perpendicularly positioned to diminish spectra distortions (caused by backscattering and pileup). The perpendicular placement of detectors leads to a counting rate of 100 – 300 counts/second (cps) while using a 30 μCi 22Na source (3.3 – 10 cps/μCi). Consequently, collecting one PAL spectrum with total counts of one million takes several hours. The long time limits its potential to observe changes in the microstructure. To increase the counting rate of PAL measurements, we propose a novel multi-detector PAL spectrometer, which includes three or four detectors, a digital oscilloscope, and an algorithm for any detector to record both start and stop signals. The spectrometer achieves a high counting rate of around 2144 cps (71.5 cps/μCi) with four detectors, which is about ten times that of a conventional one with two detectors perpendicularly positioned. This work enables us to perform in-situ PAL measurements to disclose the minute-scale evolution of microstructure more precisely during fast physical and chemical processes in the future.

Interface-Mediated Mechanoluminescence Enhancement from Heterojunction Phosphors: Experiment and Theory

Mechanoluminescence (ML) phosphors have made significant progress in various fields, such as artificial intelligence, the Internet of Things, and biotechnology. However, enhancing their weak ML intensity still remains a challenge. Here, we report a new series of Na1-xMgxNbO3:Pr3+ (x = 0.00, 0.10, 0.20, 0.40, 0.60, 0.80, and 1.00 mol %) heterojunction systems, which exhibit significant ML enhancement as compared with either the Pr3+-doped NaNbO3 or MgNbO3, and the physical mechanisms behind the ML enhancement have been explored comprehensively from both the experiment and theory points of view. Experimental tests, including thermoluminescence and positron annihilation lifetime measurements, combined with first-principles calculations, consistently indicate that the ML enhancement observed in these newly reported systems is due to the formation of heterojunctions, which plays a crucial role in modulating the defect configuration of the phosphors and facilitating efficient charge transfer. By controlling the Na/Mg ratio in conjunction with Pr3+ doping, continuous changes in the band offset and the concentrations of certain types of traps in the forbidden gap are achieved, leading to the optimum conditions in the 8/2 ratio samples. These findings demonstrate a novel type of ML phosphor and provide a theoretical basis for the design of high-performance ML phosphor.

A SiPM-based dual and triple coincidence system for positron annihilation lifetime measurement

Positron annihilation lifetime (PAL) spectroscopy is a powerful non-destructive method to study defects in materials. Silicon photomultipliers (SiPMs) have emerged as promising photodetector alternatives for PAL systems. However, further research is needed to fully optimize SiPM arrays for PAL applications. In this study, a dual and triple coincidence system has been developed based on four 1 × 4 SiPM arrays with resistor-chain readout, and initial results of PAL measurement have been obtained. A pixel-by-pixel time shift correction method was proposed and successfully applied using triple γ coincidence for summing 128 PAL spectra in one measurement. The system provides position, timing, and energy information for both 1275 and 511 keV γ-rays. The use of SiPM arrays and the pixel-by-pixel time shift correction method have enabled precise and reliable measurements of PALs in single-crystal silicon and yttria-stabilized zirconia samples. The developed SiPM-array-based PAL system also offers a technical solution to measure angular correlations of positron annihilation radiation, with the potential for further improvements.

Evaluation of a resistor-chain-based SiPM detector for positron annihilation lifetime measurement

Silicon photomultipliers (SiPMs) have great potential to replace photomultiplier tubes (PMTs) and exceed their performance in positron annihilation parameter-measurement systems. However, due to the limited active area of a single SiPM, it only covers a small γ-ray emission acceptance range and provides position information over a very small region. Using a SiPM array is a good way to overcome these problems. In this study, a resistor-chain-based SiPM detector was designed and evaluated for positron annihilation lifetime (PAL) measurements. This detector uses a 1 × 4 scintillator array, a 1 × 4 SiPM array, and a resistor-chain readout circuit to obtain position, energy, and timing information about 1275 and 511 keV γ-rays, with the readout pulses serving as both the position, energy, and timing signals. A comprehensive evaluation was conducted to assess the detector in terms of position profile, energy resolution, and time resolution. The position profiles showed that all the crystal elements in the lutetium fine silicate scintillator array were clearly resolved. The detector had an energy resolution of 11.3±0.9% and a time resolution of 198±9 ps. A total of 32 PAL spectra were obtained using two SiPM array detectors. The PAL in yttria-stabilized zirconia (YSZ) sample was accurately verified using the summed PAL spectrum. The developed SiPM detector provides good performance for PAL measurements and could be used to build a high-performance PAL spectrometer.

Estimation of lattice defect density by positron annihilation lifetime measurement in a short time

Estimation of lattice defect density by positron annihilation lifetime measurement in a short time

Giant Reduction in the Thermal Conductivity of Porous β-Ga2O3 with Multiple Heat-Blocking Mechanisms

In this work, multiple strategies have been adopted to suppress the lattice thermal conductivity in β-Ga2O3 by point defects, grain boundaries, secondary phases, and pores. Porous β-Ga2O3 was prepared by a convenient calcination method together with spark plasma sintering technology. Positron annihilation lifetime and cathodoluminescence measurements reveal the existence of small micropores and various vacancies in the as-sintered β-Ga2O3. Macropores with sizes on the order of hundreds of nanometers are also observed by scanning electron microscopy. Due to these pores, the as-sintered β-Ga2O3 shows a high porosity of 63.4% and possesses abundant boundaries between the solid matrix and the air. In addition, the secondary phase of ε-Ga2O3 is also revealed by X-ray diffraction. The above microstructures cause strong enhancement in phonon scattering. As a result, an extremely low thermal conductivity of 0.30 W m–1 K–1 is obtained at room temperature in the as-sintered β-Ga2O3, which is far below the amorphous limit (1.15 W m–1 K–1) and is the lowest value reported so far for β-Ga2O3. Our work provides an avenue to reduce the thermal conductivity for the β-Ga2O3 materials and is believed to be more widely applicable for many other materials.

Development of a small and light portable positron annihilation lifetime measurement equipment for on-site measurements

Positron annihilation lifetime measurement has been a destructive analysis method in the sense that it requires cutting the specimen in two pieces, but the anti-coincidence method we developed enables the measurement of positron annihilation lifetime without cutting. In this study, by applying this anti-coincidence method, we developed a small and light portable positron annihilation lifetime measurement equipment for on-site measurements. In order to make the equipment small and light, we used a small photomultiplier tube for the γ-ray detectors, and we determined the detector configuration by simulation and experiment. To evaluate the performance of the developed equipment, we conducted positron annihilation lifetime measurements on shot-peened stainless steel and compared the results to those by the desktop anti-coincidence equipment. Just like the desktop equipment, the developed equipment revealed the increase in the defect density by shot-peening as the change in the positron lifetime, verifying the effectiveness of this equipment.

Free volume and gas transport properties of hydrolyzed polymer of intrinsic microporosity (PIM-1) membrane studied by positron annihilation spectroscopy

In this work, a series of hydrolyzed polymers of intrinsic microporosity (PIMs) membranes were successfully synthesized under alkaline conditions. Positron annihilation lifetime measurement is employed to analyze the pore structure of the hydrolyzed PIMs. In the original PIM-1, there are two kinds of pores, i.e. micropores with radius of 4.32 Å and ultramicropores with radius of 2.58 Å. The size of all the pores shows a continuous decrease with increasing hydrolysis time, which is confirmed by the reduced chain-to-chain spacing in the hydrolyzed PIMs measured by X-ray diffraction. Meanwhile, the fractional free volume also shows decrease after the hydrolysis process, while the relative number of ultramicropores increases. Benefiting from the smaller pore size in the hydrolyzed PIMs membrane, the gas molecules with smaller size (such as CO 2 ) can be separated more efficiently. The selectivity of CO 2 /CH 4 of hydrolyzed PIMs reaches up to 30.05 with a relatively high CO 2 permeability of 140.58 Barrer, which exceeds the Robeson’s 1991 upper bound. Our results indicate that the hydrolyzed polymers of intrinsic microporosity membranes are promising candidates for gas separation membrane in the future. • A series of hydrolyzed PIMs membranes were synthesized under alkaline conditions. • The process shortens the reaction time and improves the gas separation performance. • The logarithm of P shows a linear relationship with 1/FFV. • Positron lifetime measurements indicate the free volume adjusts the selectivity.

Positron annihilation study of lattice defects and nanoporous structures in Mn4+ doped K2SiF6 nanophosphors exhibiting high quantum yield

The present paper reports an application positron annihilation spectroscopy (PAS) to systematically study the formation of lattice defects, nanoporous structures and sites of Mn4+ ions doped into the K2SiF6 (KSFM) nanophosphors prepared using four different solvents. Three main techniques of PAS, including positron annihilation lifetime (PAL), Doppler broadening (DB) and electron momentum distribution (EMD) measurements, were simultaneously carried out under the same vacuum condition. The structural characteristics and quantum yield (QY) of KSFM nanophosphors were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), coupled plasma mass spectroscopy (ICP-MS), photonic multichannel analyzer-spectrophotometer (PMAS) and fluorescence spectrophotometer (PLS). The obtained results indicated that decreasing the dielectric constant of the solvents led to the reduction of the particle sizes in KSFM. The occupation of Mn4+ to the sites of Si4+ in the SiF62– octahedron structures of KSFM as well as the high concentration of lattice defects and nanopores (average size of 0.390–0.621 nm) in the doped materials that used isopropanol and diethyl ether solvents were indicated by PAS. Moreover, increasing the concentration of doped Mn4+ caused a decrease of the occupation of Mn4+ to the adjacent Si-vacancy and K-divacancy and at the same time, the number of Si-vacancies in KSFM increased. Combining PAS and QY measurements, we found that the QY of KSFM nanophosphors depended on not only the porous structures (nanopores) but also the concentration of doped Mn4+ in the sites of Si as well as the concentration of Si-vacancies.

Microstructural and defects characteristics of xV2O5–(1-x)(0.35MoO3–0.65ZnO) glass nanocomposites utilizing positron annihilation spectroscopy and correlated experimental methods

One binary and a few ternary glass nanocomposite systems of the general composition formula xV2O5–(1-x)(0.35MoO3–0.65ZnO), where x varied from 0.0 to 0.95, were prepared via the melt quenching technique and characterized through Transmission electron microscopy and X-ray diffraction method. Positron annihilation lifetime and coincidence Doppler broadening spectroscopy techniques were also utilized to detect various types of defects formed within the samples. The patterns of the X-ray diffraction method revealed distinct diffraction peaks specifying the formation of nanocrystallites in x = 0.0, 0.15 and 0.35 samples.While for x = 0.55, 0.75 and 0.95 samples, significantly broadened patterns were observed, indicating predominant amorphous characteristics. Different shapes and sizes of so-formed nanocrystallites were also confirmed and measured by Transmission electron microscopy images. Moreover, selected area electron diffraction patterns revealed a few bright spots superimposed over diffuse rings, specifying the presence of nanocrystallites over the amorphous glass matrix. Positron annihilation lifetime measurements for x = 0.0, 0.15, and 0.35 samples revealed that positrons were trapped in the intermediate gaps around the nanocrystallites and vacancy types of defects. Positrons were trapped within the porous type defects inside the amorphous glass matrix and in vacancy clusters for x = 0.55, 0.75, and 0.95 samples. Further, coincidence Doppler broadening studies established the formation of vacancy types of defects due to cation non-stoichiometry and amorphousness of the glass nanocomposites.

HTDMA-modified bentonite clay for effective removal of Pb(II) from aqueous solution

This work studies the Pb(II) removal onto bentonite clay modified by hexadecyl trimethyl ammonium bromide (HDTMA). Characterizations of the unmodified and modified materials were performed by using XRD, SEM, TG-DSC, FT-IR, and BET surface area analyses. Factors influencing the uptake of Pb(II) from aqueous solution, such as pHsolution, ion strength, uptake time, adsorbent dosage, and initial Pb(II) concentration, were examined. The obtained results showed that bentonite clay was successfully modified by HDTMA, resulting in an increase in its surface area by about 70 %. The Pb(II) adsorption onto modified bentonite clay reached equilibrium at pH = 5.0 after 120 min. Studies within the isotherm and kinetic models demonstrated that the adsorption followed the Sips isotherm and pseudo-second-order kinetic models. The maximum monolayer adsorption capacity calculated from the Langmuir model at 30 °C was 25.8 mg/g, which is much higher than that obtained for the unmodified sample (18.9 mg/g). The FT-IR and TG-DSC analyses indicated that the formation of inner-sphere complexes plays a fundamental role in the mechanism of Pb(II) uptake onto HDTMA-bentonite clay. This mechanism of Pb(II) adsorption was further investigated, for the first time, by using the positron annihilation lifetime (PAL) and electron momentum (EMD) measurements. The PAL and EMD analyses indicated that the existence of Al and Si mono-vacancies in the HDTMA-bentonite should have essential contributions to the adsorption mechanism. In particular, we found a very interesting mechanism that the Pb(II) adsorption should occur inside the interlayer spaces of the HDTMA-bentonite.

Effect of gamma-ray on producing induced colour centres and on positron annihilation lifetime of bismuth-doped zinc sodium borate glasses

Abstract A melt quenching method have been used for preparing a glass composition of the form (70-x)%B2O3-15%ZnO-15%Na2O-x%Bi2O3 (where x = 0, 5, 10, 15, 20, 25 mol %). The prepared glass was studied by FTIR and Positron annihilation lifetime measurements. FTIR analysis after 80 kGy -irradiation of bismuth-doped glass samples, show the dependence of the number of structural units BO3 and BO4 on the -irradiation dose. FTIR absorption spectra revealed that gamma irradiation produces a degree of a disorder or defects of the glass network which producing colour centres. The objective FTIR spectra were used to estimate the N4 ratio and its dependence on the composition. Positron annihilation lifetime measurements were carried out for all samples before and after 80 kGy -irradiation. Irradiated samples show a colour change, which is most likely due to the formation of colour centres. Such colour changes due to -irradiation allow estimating that the prepared glass samples can be used as a shielding material or as a radiation indicator sensor.

Comparison of Hydrogen Thermal Desorption Analysis Curves of Electron-Irradiated F82H and Creep-Ruptured Pure Fe Obtained by Experiments and Simulations

Herein, we compared thermal desorption analysis (TDA) curves obtained by conducting experiments and simulations. In addition, we discussed the validation of our simulations and trapping sites of hydrogen atoms. In as-received F82H, when the samples contained solute atoms, grain boundaries, dislocations, and precipitates, the experimental curve corresponded to the simulated curve. In positron annihilation lifetime (PAL) measurements, di-vacancies were detected in the electron-irradiated F82H. When we changed the growth and the concentration of vacancy-type defects during temperature increase using the rate theory, the simulation results agreed with experiment results. In creep-ruptured Fe, only dislocations were detected by the PAL measurements. However, the existence of a type of defect, which was related to grain boundaries, must be assumed to fit the simulation curve to the experimental one. In the next step, the diffusion of hydrogen atoms on grain boundaries should be added to simulation program.