Papers by Keyword: Doppler Broadening Spectroscopy

Abstract: The PULSTAR nuclear reactor at North Carolina State University is a 1-MWth open-pool reactor and a center for irradiation and examination of materials. Among the different facilities based on this reactor, an intense positron beam facility has been established and operational since 2009, providing a positron beam reaching 6 × 10⁸ e⁺/s and two positron beam spectrometers. This facility has been recently upgraded in several aspects in terms of sample manipulation and heating/cooling capabilities, as well as coincidence Doppler Broadening Spectroscopy (CDBS) capability, enabled by a new sample changer and target chamber with new digital MCA systems. Currently, both beam and bulk spectrometers can perform CDBS and Positron Annihilation Lifetime Spectroscopy (PALS) with varying temperature settings. In addition, environmental control has also been added to the bulk system, where the sample pressure and humidity can be adjusted together with temperature. These systems have been utilized to study a variety of nuclear-related materials, such as metal alloys and oxide compounds, that had undergone surface and bulk damages induced by radiation. Such materials are widely used as structural and sensor materials related to nuclear reactors where microstructural damage is critical in determining their performance and failure modes. Some of the studies also demonstrated the unique advantage of Positron Annihilation Spectroscopy (PAS) over other traditional characterization techniques.

Abstract: The high sensitivity of positrons to directly probe atomic scale defects revealing their structure and characteristics makes it a unique tool in materials science research covering all types of materials from hard to soft matter. This review focuses on applications of positron annihilation spectroscopy (PAS) in hard materials. However, it is not intended as a comprehensive review of the foundations of positron annihilation spectroscopy and description of its techniques. These exist in numerous publications cited in this review. Instead, the aim here is to facilitate employing PAS and interpretation of its measurements by illustrating the advantages, limitations, and challenges and guiding the reader on how to overcome technical problems and how to interpret PAS results in meaningful ways. Applications of PAS in electronic and photonic materials, nuclear and irradiated materials, and engineering materials are discussed. Examples are given to guide the reader on how PAS can be combined with complementary methods to uncover the fundamentals of defect physics and reveal interesting new phenomena in condensed matter.

Abstract: Utilizing positron annihilation spectroscopy for studying the energetics, kinetics or charge states of open volume point defects in semiconductors is seldom straight forward. Although obtaining usable experimental results with the technique is usually fairly easy, designing a suitable experiment for a specific case and/or interpreting the results in an unambiguous manner can be challenging. The goal of this lecture is to give advice and suggestions on what to consider when planning experiments with Positron Annihilation Spectroscopy (PAS) in semiconductors, through various example cases. This contribution is not meant to be scientific, rather educational.

Abstract: The experimental study of the structure of commercially pure titanium after saturation with hydrogen from the gas phase by means of positron lifetime spectroscopy (PLS) and Doppler broadening spectroscopy (DBS) was carried out. In the result of penetration and accumulation of hydrogen, significant changes of annihilation characteristics occurred due to the defect structure changing. The investigated samples contained hydrogen in concentrations varying from 0 to 0.961 wt.%. Several stages of hydrogen interaction with the metal structure were revealed.

Abstract: This study was focused on commercial oxide-dispersion strengthened (ODS) steels - MA 956 (20%Cr), PM 2000 (19%Cr), ODM 751 (16%Cr) and MA 957 (14%Cr) developed for fuel cladding of GEN IV reactors. The ODS steels are described in order to comparison of their microstructure features. Vacancy defects were observed by Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). Residual stress proportional to all kinds of defects was investigated by Magnetic Barkhausen Noise (MBN) measurement. The highest presence of open volume defects was found in MA 956 and the lowest defect concentration in MA 957, although this steel contains the largest defects (six-vacancies together with dislocations). Other investigated steels demonstrated probably three- or four-vacancy clusters. Further, results from positron technique indicated proportionality of chromium content to defect concentration. Magnetic Barkhausen noise results also showed that Hpeak value (describing grain size) increased with growth of chromium content. However residual stress was independent on chromium level.