Coincidence Doppler Broadening Research Articles

동시계수 양전자 소멸 측정에 의한 양성자 조사된 BaSrFBr:Eu 박막 특성

동시 계수 도플러 넓어짐 양전자 소멸 분광법과 양전자 소멸 수명 측정법으로 BaSrFBr:Eu의 박막 시료에 0, 3, 5, 7.5 MeV 에너지의 양성자 조사에 의한 결함을 측정하여 박막구조 특성에 대하여 조사하였다. 양전자와 전자의 쌍소멸로 발생하는 511 keV 감마선 스펙트럼의 수리적 해석 방법인 S-변수를 사용하여, 박막 구조 변화를 측정하였다. 본 연구에서 측정된 S-변수가 박막에 조사된 양성자의 빔 에너지에 따라 변하지 않고 거의 일정한 값을 보였다. 따라서 양성자 조사에너지의 세기 변화에 따라 결함이 증가하지 않았으며, 그 이유는 양성자 조사 에너지에 따른 Bragg 피크 때문에 박막 시료의 특정 깊이에 결함을 형성하여 박막전체의 결함으로 잘 나타나지 않기 때문으로 판단된다. 향후, 박막의 두께에 따른 결함의 분포를 측정하기 위해서는 양전자 선원 Na의 사용 대신 양전자 빔을 이용하여야 한다. Enhance signal-to-noise ratio, Coincidence Doppler Broadening positron method has been applied to study of characteristics of BaSrFBr:Eu film sample. In this investigation the numerical analysis of the Doppler spectra was employed to the determination of the shape parameter, S, defined as the ratio between the amount of counts in a central portion of the spectrum and the total counts of whole spectrum. The films were exposed by 0, 3, 5, and 7.5 MeV proton beams ranging from 0 to <TEX>$10^{13}$</TEX> ptls. The S-parameter values were increased as increasing the exposed time and the energies, that indicated the defects generate more.

Quenched‐in vacancies in Fe‐Al alloys

AbstractQuenched‐in vacancies in Fe3Al‐based intermetallics were studied in this work. A stoichiometric Fe3Al alloy was compared with non‐stoichiometric specimens either with a deficiency or with an excess in Al content. Vacancies in specimens quenched from the disordered A2 phase were investigated by three independent techniques of positron annihilation spectroscopy: positron lifetime (LT) studies, slow positron implantation spectroscopy (SPIS) with a continuous slow positron beam, and coincidence Doppler broadening (CDB). It was found that the combination of LT and SPIS enables to determine reliably even very high concentrations of vacancies. Infor‐mation about the local chemical environment of quenched‐in vacancies was obtained from CDB measurements. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

He bubble formation and emission of He in irradiated Fe

AbstractThe formation and stability of He bubbles in Fe irradiated with 2.5 MeV He to a dose of 1.0×1020 ions/m2 were investigated. The results of positron lifetime and coincidence Doppler broadening (CDB) measurements showed that He bubbles were formed in the He‐ion‐irradiated Fe. The growth of the He bubbles by the movement and coalescence of bubbles at temperatures higher than 773 K was detected. The He bubbles remained even at the high temperature of 1373 K. In addition, He atoms were also trapped by dislocation loops formed by the irradiation. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cover Picture: Phys. Status Solidi C 6/11

AbstractThe GiPS facility (Gamma‐induced Positron Spectroscopy) has been set up for user operation at the Research Center Dresden‐Rossendorf, Germany, in 2009. A bunched electron beam from a superconducting linear accelerator (LINAC) in cw mode (bunch length &lt;5 ps; repetition time 38 ns) generates bremsstrahlung photons up to 20 MeV. The photon beam hits the sample and positrons are generated by pair production in the whole sample volume. All important positron techniques are applied to study the real structure of bulky samples (coarse powder, samples for non‐destructive testing, liquids, activated samples). Due to the unique time structure, positron lifetime and AMOC (Age–Momentum Correlation) measurements are possible. Coincidence Doppler Broadening Spectroscopy (CDBS) is also available. The spectra of all techniques were proven to exhibit extraordinary quality, as is reported by Reinhard Krause‐Rehberg et al. on p. 2451 ff (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 °C annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600 °C annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Positron lifetime and Doppler broadening study of defects in hydrogen charged Al–Mg alloys

The results of positron lifetime and Doppler broadening spectrum of defects in the hydrogen charged non-heat treatable 5xxx Al alloys are presented in this work. The yield stress of the sample was reduced for about 20 MPa after hydrogen was charged. A similar trend was observed in positron lifetime measurement, as the average lifetime τav descended remarkably to almost the level of Al matrix. The change in coincidence Doppler broadening (CDB) spectroscopy was also significant, exhibited by the characteristic change in CDB radio curves of a sample before and after hydrogen was charged. After hydrogen charging, there is an obvious enhancement in the high momentum region compensating dehancement in the low momentum region. This indicates the existence of hydrogen filling effect. The vacancies around the Mg atoms should be preferential filling sites for hydrogen because Mg has a strong affinity for hydrogen. The formation of an Mg–H bond parallel to a grain boundary is an important factor in weakening the grain boundary cohesion.

Probing Phase Evolution Behavior during Nanocrystallization of Metallic Glass Using Positron Annihilation Spectroscopy

Positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) techniques have been used to study the nanocrystallization behavior of soft magnetic materials, using Metglas 2826MB as an example. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterization techniques revealed the presence of two phases, γ-(Fe,Ni) and (Fe,Ni,Mo)23B6, as well as the size and volume fraction of the phases following predetermined thermal annealing. Two distinct positron lifetime components have been observed in both amorphous and nanocrystallized samples. In the nanocrystallized samples, it has been demonstrated unambiguously that small and large lifetime components were due to positron annihilation with crystalline nanophases and with amorphous-crystalline or intercrystalline interfaces, respectively. First-principle calculation of the positron lifetime and electron momentum distribution in crystalline phases, supplemented by TEM and XRD studies, helped in the unambiguous interpretation of the experimental observation. This study throws new insights into positron behavior in metallic glasses, especially in the presence of single or multiple nanophases embedded in the amorphous matrix.

Characterizing Native Point Defects in ZnO Bulk by Positron Annihilation Spectroscopy

Positron annihilation spectroscopy was employed to study the native point defects in ZnO single crystal, in combination with calculated results of positron lifetime and electron momentum distribution. The theoretical and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600oC annealing treatment. Oxygen vacancies have been characterized by coincidence Doppler broadening (CDB) spectroscopy, combined with calculated electron momentum distribution. CDB spectroscopies show that oxygen vacancies don't appear until 600oC annealing treatment, and increase due to ZnO decomposition with annealing temperature rising.

Defect Analysis of Phospher (Ba, Sr) FBr : Eu by X-Ray Irradiation

The mechanical property of a phosphore layer was investigated by measuring the resolution (LP/mm) and by positron annihilation spectroscopy and SEM. Image plate samples containing the phosphore layer were irradiated by X-rays in a hospital numerous times over a course of several years. The LP/mm values of a (Ba,Sr)FBr : Eu image plate irradiated by X-rays varied between 2.2 and 2.0 over a period of four years. Coincidence Doppler Broadening (CDB) positron annihilation spectroscopy was used to analyze defect structures. The S parameters of the samples from hospital use varied from 0.6219 to 0.6232. There was a positive relationship between the time of exposure to the X-rays and the S parameters. Most of the defects were found to have been generated by X-rays.

Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II

The Munich pulsed low energy positron beam system (PLEPS) is now installed at the high intensity positron source (NEPOMUC) at the Munich Research Reactor FRM-II. In order to enhance the performance of the system several improvements have been implemented: two additional collinear detector ports have been installed. Therefore in addition to the normal lifetime measurements it is now possible to simultaneously perform Doppler-broadening, coincident Doppler-broadening and age momentum correlation experiments. An additional chopper was included to periodically suppress pulses and therefore to extend the standard time window of 20 ns for precise measurements of longer lifetimes. First test-experiments have been performed in May and July 2007. With all pulsing components in operation we achieved a count-rate of 1.4 × 10 4 counts per second. The total time resolution (pulsing and detector) was about 240 ps (FWHM) with a peak to background ratio up to 6 × 10 3:1.

Status of an R&D project of a positron gun at “Horia Hulubei” NIPNE Bucharest

A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8–50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a 22NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi 22NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron.

Positron Annihilation in Carbon Nanotubes Studied by Coincidence Doppler Broadening Spectroscopy

In order to assign the sites of positron annihilation, coincidence Doppler broadening spectra were measured for a highly oriented pyrolytic graphite crystal, graphite powder, multi-walled carbon nanotubes (MNTs) and cupstacked carbon nanotubes (CNTs). The spectrum for graphite powder normalized to that for highly oriented pyrolytic graphite (HOPG) is almost flat in the momentum region from 7 × 10−3 to 13 × 10−3mec, having a ratio close to unity. The flat spectrum demonstrates that positrons injected into graphite powder annihilate in the interlayer spaces of piled graphite sheets, in the same manner as positrons in highly oriented pyrolytic graphite annihilate in the bulk. The coincidence Doppler broadening spectra for MNTs and CNTs are quite different from that for highly oriented pyrolytic graphite, which indicates that positrons injected into MNTs and CYTs annihilate not in the bulk, but on surface. The positron lifetime spectrum for multi-walled carbon nanotubes is analyzed in terms of a single component due to surfacetrapped positrons, while that for CNTs is decomposed into three components attributable to para-positronium surface-trapped positrons and orthopositronium. The difference between the coincidence Doppler broadening spectrum for CNTs and that for MNTs is explained in terms of positron annihilation on zigzag surfaces of CNTs which are composed of both graphitesheet and graphite-edge planes.

저에너지 양전자 소멸 분광법을 이용한 MgB2박막 구조 특성

저속 에너지 도플러 넓어짐 양전자 소멸 분광법으로 <TEX>$MgB_2$</TEX> 박막내의 원자 크기 정도 고체 구조 특성에 대하여 조사하였다. 양전자와 전자의 쌍소멸로 발생하는 511keV 감마선 스펙트럼의 수리적 해석 방법인 S-변수를 사용하여, 상전이 근처 온도에서 박막의 구조 변화를 측정하였다. 비등방성 구조로 된 <TEX>$MgB_2$</TEX> 박막에서 초전도 특성을 갖는 상전이 온도 근처에서 S-변수를 측정하였다. 양전자의 입사 에너지 10keV에서 측정된 S-변수의 최고치는 박막의 온도가 30K에서 0.567이고, 50 K에서는 0.570로 큰 변화는 없었다. 이 결과로부터 양전자가 Boron 층의 초 전자와 소멸하기 보다는 Mg층 근처의 상 전자와 소멸하는 것으로 판단된다. <TEX>$MgB_2$</TEX>의 박막의 외층은 Mg층으로 이루어졌다고 할 수 있다. The Characterization of <TEX>$MgB_2$</TEX> Thin Film by Slow Positron Annihilation Spectroscopy Enhance signal-to-noise ratio, slow positron coincidence Doppler Broadening method has been applied to study of characteristics of <TEX>$MgB_2$</TEX> superconductor film, which were performed at 30 K and 50 K sample temperature near Tc of it. In this investigation the numerical analysis of the Doppler spectra was employed to the determination of the shape parameter, S, defined as the ratio between the amount of counts in a central portion of the spectrum and the total counts of whole spectrum. The S-parameter values were increased then decreased while the positron implantation energies were increasing, that indicated the diffusion into the samples. The S-parameters of the anisotropic 1 <TEX>${\mu}m$</TEX> <TEX>$MgB_2$</TEX> thin film which were implanted by positrons at 10 keV are 0.567 at 30 K and 0.570 at 50 K. It is believed that the positrons annihilate with normal-electrons instead of super-electrons in the <TEX>$MgB_2$</TEX> superconductor.

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 1523K, 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 1523K. 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 623K, between 823 and 1323K, and above 1323K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T⩽623K. In the temperature range 823–1323K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.

Development of Transition Edge Sensor Gamma-Ray Detectors for Positron-Annihilation-Spectroscopy

We are beginning development of a Transition Edge Sensor (TES) based gamma-ray detector for use in Coincidence-Doppler-Broadening (CDB) Positron-Annihilation-Spectroscopy (PAS) studies. We are specifically motivated to asses and study damage to nuclear Reactor Pressure Vessels (RPVs) which results from neutron irradiation during reactor operation. In this paper we provide a brief introduction to CDB-PAS and discuss our detector design.

Positron annihilation study of Mo dispersion in MoO3/Al2O3 catalysts

A series of MoO3/Al2O3 catalysts was prepared by the impregnation method. The positron annihilation lifetime spectra(PALS)and coincidence Doppler broadening (CDB) spectra of the samples were measured to study the pore structure and dispersion of MoO3. The PALS result shows that there are two types of pores with different size in the Al2O3 support. After MoO3 was loaded in the support, Mo species are mainly dispersed inside the large pores of the support, and thus decrease the volume of the large pores. The CDB spectrum shows significant change while MoO3 loading is just 3%. This indicates that the Mo species are dispersed into the pores, and cause the change of the electron momentum distribution seen by positrons. As the MoO3 loading exceeds 18%, the size of the large pores is decreased to the similar size of the small pores, and the positron annihilation parameters show nearly no change with further Mo loading, which indicates saturation of the dispersion of Mo species.

Relaxation of Free Volume in Zr&lt;SUB&gt;50&lt;/SUB&gt;Cu&lt;SUB&gt;40&lt;/SUB&gt;Al&lt;SUB&gt;10&lt;/SUB&gt; Bulk Metallic Glasses Studied by Positron Annihilation Measurements

Relaxation of free volume in Zr 50 Cu 40 Al 10 bulk metallic glass during isothermal annealing below T g (glass transition temperature) has been investigated by using positron annihilation lifetime and coincidence Doppler broadening (CDB) techniques. The mean positron lifetime decreases with increasing annealing time at each annealing temperature. The decrease in lifetime is due to shrinkage and annealing out of the free volume. The relaxation kinetics of free volume obeys the stretched exponential function (KWW: Kohlrausch-Williams-Watts law). An adjustable parameter of the KWW exponent β depending on temperature was determined. The electron momentum distribution around free volume derived from a CDB spectrum during annealing showed no appreciable change at each temperature. These results suggest that long-range atomic diffusion and rearrangement, particularly around the free volume, do not occur essentially during relaxation below Tg.

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB2

Abstract Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB 2 samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB 2 show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB 2 sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron–phonon coupling constant.

The precipitation process in Mg–Ca–(Zn) alloys investigated by positron annihilation spectroscopy

Abstract Coincidence doppler broadening (CDB) spectroscopy has been applied to study the precipitation process induced by aging in Mg–1.0 wt.% Ca and Mg–1.0 wt.% Ca–1.0 wt.% Zn alloys. In addition positron lifetime experiments and microhardness measurements have been performed. A peak centered at ∼11.5 × 10 −3 m 0 c is found in the CDB ratio spectra of the alloys aged at 473 K. It is attributed to annihilations with the core electrons of Ca. The results indicate the formation of a particle dispersion that hardens the alloys. This dispersion is correlated with the appearance of the peak attributed to Ca atoms. Zn atoms in the Mg matrix inhibit the formation of quenched-in vacancies bound to Ca atoms in the aged ternary alloy producing the dispersion refinement.

Positron annihilation study of neutron irradiated pure Fe and Fe‐Cu binary alloys

AbstractThe hardening and embrittlement of Reactor Pressure Vessel (RPV) steels is of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation‐induced damage, like vacancies, interstitials, solutes and their clusters. Fe‐Cu binary alloys are often used to mimic the behaviour of such steels. Their study allows identifying some of the defects responsible of the hardening, especially when compared to pure iron or C‐micro‐alloyed iron. Owing to their self‐seeking and selective trapping, positrons are used to determine the nature of these defects. Recently, at SCK·CEN, a new Positron Annihilation Spectroscopy (PAS) setup has been built, calibrated and optimized to measure the Coincidence Doppler Broadening (CDB) and Lifetime (LT) of neutron irradiated materials. This set‐up has been used to measure the CDB and LT of n‐irradiated pure Fe and Fe–Cu alloys. It is found that the clustering of Cu take place at the very early stages of irradiation using the CDB while LT measurement are showing much more vacancy clustering for low Cu alloys than in the higher ones. Increasing the neutron dose up to 1.3 × 1020 n/m2, allows the follow up of the kinetic of Cu and V‐clustering especially in Fe‐Cu alloys. It is found that both copper and carbon decrease the size of vacancy‐cluster, when added to iron. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)