The application of artificial neural networks to the inversion of the positron lifetime spectrum

A new method of processing positron annihilation lifetime spectra is proposed. It is based on an artificial neural network （ANN）-back propagation network （BPN）. By using data from simulated positron lifetime spectra which are generated by a simulation program and tested by other analysis programs, the BPN can be trained to extract lifetime and intensity from a positron annihilation lifetime spectrum as an input. In principle, the method has the potential to unfold an unknown number of lifetimes and their intensities from a measured spectrum. So far, only a proof-of-principle type preliminary investigation was made by unfolding three or four discrete lifetimes. The present study aims to design the network. Besides, the performance of this method requires both the accurate design of the BPN structure and a long training time. In addition, the performance of the method in practical applications is dependent on the quality of the simulation model. However, the chances of satisfying the above criteria appear to be high. When appropriately developed, a trained network could be a very efficient alternative to the existing methods, with a very short identification time. We have used the artificial neural network codes to analyze data such as the positron lifetime spectra for single crystal materials and monocrystalline silicon. Some meaningful results are obtained.

Investigation of Subboundaries Evolution in Superplastically Deformed NiAl by Positron Lifetime

In order to study the subboundaries evolution in superplastically deformed NiAl, the positron lifetime change during superplastic deformation process was measured. It is shown that the superplastic deformation of NiAl has not influence on its T2, the newly recrystallized grain boundaries formed during entire superplastic deformation process belong to the calegory of subboundaries and have not contribution to the superplastic strain.

Self-consistent field method and non-self-consistent field method for calculating the positron lifetime

Many methods are used to calculate the positron lifetime, these methods could be divided into two main types. The first method is atomic superposition approximation method and the second one is the so called energy band calculation method. They are also known as the non-self-consistent field method and self-consistent field method respectively. In this paper, we first introduce the two basic methods and then, we take Si as an example and give our calculation results, these results coincide with our latest experimental results, finally, we discuss the advantages and disadvantages of the two methods.

Effect of Annealing Temperature on Magnetic Aftereffect and Positron Lifetime in Fe_(73.5)Cu_1Nb_3Si_(13.5)B_9 Alloy

The magnetic aftereffect (MAE) and the positron lifetime were measured at room temperature on the Fe_73.5Cu_1 Nb_3Si_13.5 B_9 alloy in as-cast and after annealing at temperature T_a in the range from 450 to 750℃ It was found that both the MAE and the positron lifetime decrease with increasing T_a when T_≤600℃. While Ta≥650℃, MAE is essentially suppressed, and two positron lifetimes appear.

Positron Lifetime Study of Feromagnetism-Paramagnetism Phase Transition of Gadolinium

Positron lifetime spectra were measured near the Curie temperature of gadolinium metal. The lifetime data obtained were analysed as three component lifetime spectra with the Positronfit program. It was found that the change of bulk lifetime τ 1 was comparatively great near T <20 ℃(Gd Curie temperature), but τ 1 kept almost unchanged under temperature other than this temperature range.

Relationship between positron bulk lifetime and lattice constants-research on NaCl-type crystals

The positron lifetimes of some compounds with NaCl-type crystal structure are calculated with the method of atomic superposition approximation （ATSUP） based on the theories of local-density-approximation （LDA） and general- gradient-approximation （GGA）. The systematical results are fitted to a curve as a function of lattice constants. The positron bulk lifetimes of some other compounds with NaCl-type crystal structure, which are deduced from the systematical results, are in agreement with the experimental results given in other literature.

A Review of Studies of Polymeric Membranes by Positron Annihilation Lifetime Spectroscopy

A review is presented of studies of polymeric membranes by applying positron annihilation lifetime spectroscopy （PALS）. PALS has been used to study subnanometer-sized holes, to determine their size distribution and free-volume fractions, and to probe molecular-sized vacancies in glassy polymers. At present, PALS is believed to be a highly effective physical method for the examination of polymeric membranes.

Precipitation Study in Inconel 625 Alloy by Positron Annihilation Spectroscopy

Precipitation in Inconel 625 alloy has been studied by positron annihilation spectroscopy and electron microscopy. The observed dependence of annihilation characteristics on aging time is attributed to the change of the positron state due to the increase and decrease of the density and size of the 7' precipitates. Hardness measurements and lifetime measurements are in good agreement.

Exploring positron characteristics utilizing two new positron–electron correlation schemes based on multiple electronic structure calculation methods

We make a gradient correction to a new local density approximation form of positron-electron correlation. The positron lifetimes and affinities are then probed by using these two approximation forms based on three electronic-structure calculation methods, including the full-potential linearized augmented plane wave（FLAPW） plus local orbitals approach,the atomic superposition（ATSUP） approach, and the projector augmented wave（PAW） approach. The differences between calculated lifetimes using the FLAPW and ATSUP methods are clearly interpreted in the view of positron and electron transfers. We further find that a well-implemented PAW method can give near-perfect agreement on both the positron lifetimes and affinities with the FLAPW method, and the competitiveness of the ATSUP method against the FLAPW/PAW method is reduced within the best calculations. By comparing with the experimental data, the new introduced gradient corrected correlation form is proved to be competitive for positron lifetime and affinity calculations.

Variation of Radiation Damage with Irradiation Temperature and Dose in CLAM Steel

The dependences of radiation induced defects on irradiation temperature up to 700℃ at 15 dpa and on irradiation dose up to 85 dpa at room temperature have been investigated by the heavy ion irradiation and the positron annihilation lifetime spectroscopy for the CLAM. A void size peak is observed at -500℃ where the vacancy cluster contains 9 vacancies and has an average diameter of 0.59 nm. The size of the vacancy clusters increases with the increase of irradiation dose at room temperature, and the vacancy cluster at 85 dpa consists of 9 vacancies and reaches a size of 0.60 nm in diameter. The absolute values of the void size at the peak and the increase of void size with dose in the CLAM steel are negligible compared to those of the normal stainless steels, indicating that the CLAM steel has good radiation resistant property.

The effects of fast neutron irradiation on oxygen in Czochralski silicon

The effects of fast neutron irradiation on oxygen atoms in Czochralski silicon （CZ-Si） are investigated systemically by using Fourier transform infrared （FTIR） spectrometer and positron annihilation technique （PAT）. Through isochronal annealing, it is found that the trend of variation in interstitial oxygen concentration （[Oi]） in fast neutrons irradiated CZ-Si fluctuates largely with temperature increasing, especially between 500 and 700℃. After the CZ-Si is annealed at 600℃, the V4 appearing as three-dimensional vacancy clusters causes the formation of the molecule-like oxygen clusters, and more importantly these dimers with small binding energies （0.1-1.0eV） can diffuse into the Si lattices more easily than single oxygen atoms, thereby leading to the strong oxygen agglomerations. When the CZ-Si is annealed at temperature increasing up to 700℃, three-dimensional vacancy clusters disappear and the oxygen agglomerations decompose into single oxygen atoms （O） at interstitial sites. Results from FTIR spectrometer and PAT provide an insight into the nature of the [Oi] at temperatures between 500 and 700℃. It turns out that the large fluctuation of [Oi] after short-time annealing from 500 to 700℃ results from the transformation of fast neutron irradiation defects.

Design of a high-sampling-rate electronic module for array-detector positron annihilation lifetime measurements

Introduction In this study,a high-time-resolution electronic module with a high channel density and low power consumption was designed for the measurement of the multi-detector array positron annihilation lifetimes.This electronic module consisted of 32 input channels,and each channel provided a high sampling rate up to 5.12 GSPS based on a Domino Ring Sampler 4(DRS4)chip.Compared to the high-speed flash analog-digital converter(FADC),DRS4 chip has a higher channel density with an affordable lower price and power consumption.Methods The developed electronic module was also capable of real-time data analysis for directly extracting the time information of input signals at the data acquisition site,thereby significantly decreasing the data rate.The digital constant fraction discriminator(DCFD)algorithm was implemented in the field programmable gate array(FPGA)for performing the time pick-up.Results The coincidence time resolution of the electronic module was measured,and the test results revealed a value of 26 ps.A prototypical 16-pixel detector module of the multi-detector system was evaluated using this electronic module,and the coincidence time resolution of the prototypical module was 411.84 ps.Conclusions The electronic module was confirmed to satisfy the severe requirements of the multi-array-detector positron annihilation lifetime measurement system.It was also suitable for other high-time-resolution,high-channel-density,costeffective,and low-power-consumption applications.

DAMAGE OF SILICONE RUBBER INDUCED BY PROTON IRRADIATION

In this paper, the damage to methyl silicone rubber induced by irradiation with protons of 150 keV energy wasstudied. The surface morphology, tensile strength, Shore hardness, cross-linking density and glass transition temperaturewere examined. Positron annihilation lifetime spectrum analysis (PALS) was perfomed to reveal the damage mechanisms ofthe rubber. The results showed that tensile strength and Shore hardness of the rubber increased first and then decreased withincreasing irradiation fluence. The PALS characteristics τ_3 and I_3, as well as the free volume V_f, decreased with increasingirradiation fluence up to 10^(15) cm^(-2), and then increased slowly. It indicates that proton irradiation causes a decrease of freevolume in the methyl silicone rubber when the fluence is less than 10^(15)cm^(-2), while the free volume increases when thefluence is greater than 10^(15)cm^(-2). The results on cross-linking density indicate that the cross-linking induced by protonirradiation is dominant at smaller proton fluences, increasing the tensile strength and Shore hardness of the rubber, while thedegradation of rubber dominates at greater fluence, leading to a decrease of tensile strength and Shore hardness.

Interstitial Solution Carbon Concentration and Defects of Ti+Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti＋Nb bearing ULC- BH （ultra-low carbon bake hardening） steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy （PALS） and coincidence Doppler broadening spectroscopy （CDBS）. The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the sam- ples under different conditions is analyzed. A correlation of lifetime component v~ values with interstitial solution car- bon concentrations in the samples for different continuous annealing processes is established, while a correlation of li- fetime component r2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that in- ternal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti＋ Nb bearing ULC-BH steel.

Atmospheric CO_(2) capture and photofixation to near-unity CO by Ti^(3+)-V_(o)-Ti^(3+) sites confined in TiO_(2) ultrathin layers

To realize efficient atmospheric CO_(2) chemisorption and activation,abundant Ti^(3+) sites and oxygen vacancies in TiO_(2) ultrathin layers were designed.Positron annihilation lifetime spectroscopy and theoretical calculations first unveil each oxygen vacancy is associated with the formation of two Ti^(3+)sites,giving a Ti^(3+)-V_(o)-Ti^(3+) configuration.The Ti^(3+)-V_(o)-Ti^(3+) sites could bond with CO_(2) molecules to form a stable configuration,which converted the endoergic chemisorption step to an exoergic process,verified by in-situ Fourier-transform infrared spectra and theoretical calculations.Also,the adjacent Ti^(3+)sites not only favor CO_(2) activation into COOH*via forming a stable Ti^(3+)–C–O–Ti^(3+) configuration,but also facilitate the rate-limiting COOH^(*)scission to CO^(*)by reducing the energy barrier from 0.75 to 0.45 e V.Thus,the Ti^(3+)-V_(o)-TiO_(2) ultrathinlayers could directly capture and photofix atmospheric CO_(2) into near-unity CO,with the corresponding CO_(2)-to-CO conversion ratio of ca.20.2%.

Effects of alloying elements Mn,Mo,Ti,Si,P and C on the incubation period of void swelling in austenitic stainless steels

Void swelling,which induces the degradation of the original properties of nuclear materials under high-energy particleirradiation,is an important problem.The incubation period,a transient stage before the steady void growth,determines the duration of service of nuclear materials.Several experimental studies have been performed on void observations by transmission electron microscopy(TEM),which,however,has a resolution limit for the size of defect clusters.Positron annihilation lifetime spectroscopy(PALS)enables the detection of small vacancy clusters,single vacancies,dislocations and precipitates.The use of these two methods provides complementary information toward detecting defect information in the incubation period.Here,defect structures during the incubation period in austenitic stainless steels,by means of PALS and TEM are reviewed.The role of alloying elements into determining the period is explained.Furthermore,the existing problems and research directions in this field are presented.