W. Triftshäuser

A pulsing system for low energy positrons

Abstract A pulsing system for a low energy positron beam of variable energy was developed. The system consists of a beam chopper and a beam buncher. The performance of the system was tested in various positron lifetime measurements. A time resolution of 135 ps was achieved for the pulsing system at positron energies in the range between 0.5 keV and 28 keV.

An improved pulsed low-energy positron system

A pulsed low-energy positron system for positron-lifetime spectroscopy has been up-graded and modified with regard to its three main components. A new moderator preparation chamber has been added, the radiofrequency pulsing concept has been modified using a pre-buncher and a pre-chopper in front of the existing chopper-buncher section. Furthermore, a new target station allowing measurements at variable temperatures has been incorporated. The lifetime spectra now reveal a strongly increased ratio of peak to background and of peak to satellite peak.

Annealing behavior of vacancies and Z1/2 levels in electron-irradiated 4H–SiC studied by positron annihilation and deep-level transient spectroscopy

Annealing behavior of vacancies and the Z1/2 levels in n-type 4H–SiC epilayers after 2 MeV electron irradiation has been studied using positron annihilation and deep-level transient spectroscopy. Isochronal annealing studies indicate that silicon vacancy-related defects are primarily responsible for positron trapping. The Z1/2 levels are the predominant deep centers after irradiation and subsequent annealing at 1200 °C. Both the positron-trapping rate at vacancies and the Z1/2 concentration decrease in a similar manner while annealing from 1200 to 1500 °C. It is thus proposed that the Z1/2 levels originate from silicon vacancy-related defects.

Defect identification in GaAs grown at low temperatures by positron annihilation

We use positron annihilation to study vacancy defects in GaAs grown at low temperatures (LT–GaAs). The vacancies in as-grown LT–GaAs can be identified to be Ga monovacancies, VGa, according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged AsGa+ antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is consistent with the data. The density of VGa is related to the growth stoichiometry in LT–GaAs, i.e., it increases with the As/Ga beam equivalent pressure (BEP) and saturates at 2×1018 cm−3 for a BEP⩾20 and a low growth temperature of 200 °C. Annealing at 600 °C removes VGa. Instead, larger vacancy agglomerates with a size of approximately four vacancies are found. It will be shown that these vacancy clusters are associated with the As precipitates formed during annealing.

Evidence for negatively charged vacancy defects in 6H-SiC after low-energy proton implantation

We have used pulsed-slow-positron-beam-based positron lifetime spectroscopy to investigate the nature of acceptors and charge states of vacancy-type defects in low-energy proton-implanted 6H-SiC(H). We can infer from the temperature dependence of the lifetime spectra that neutral and negatively charged vacancy clusters exist in the track region. Depending on annealing, they give rise to positron lifetimes of 257±2, 281±4, and 345±2 ps, respectively. The 281 ps cluster likely has an ionization level near the middle of the band gap. By comparison with theory, the 257 and 280 ps are identified as (VC–VSi)2 and (VC–VSi)3 clusters, respectively. In addition, other acceptors of ionic type act as strong trapping centers at low temperature (T<150 K). Neutral monovacancy-like complexes are also detected with a lifetime of 160±2 after 900 °C annealing.

First platinum moderated positron beam based on neutron capture

Abstract A positron beam based on absorption of high energy prompt γ-rays from thermal neutron capture in 113 Cd was installed at a neutron guide of the high flux reactor at the ILL in Grenoble. Measurements were performed for various source geometries, dependent on converter mass, moderator surface and extraction voltages. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. The positron source consists of platinum foils acting as γ−e + e − -converter and positron moderator. Due to the negative positron work function moderation in heated platinum leads to emission of monoenergetic positrons. The positron work function of polycrystalline platinum was determined to 1.95(5) eV. After acceleration to several keV by four electrical lenses the beam was magnetically guided in a solenoid field of 7.5 mT leading to a NaI-detector in order to detect the 511 keV γ-radiation of the annihilating positrons. The positron beam with a diameter of less than 20 mm yielded an intensity of 3.1×10 4 moderated positrons per second. The total moderation efficiency of the positron source was about e =1.06(16)×10 −4 . Within the first 20 h of operation a degradation of the moderation efficiency of 30% was observed. An annealing procedure at 873 K in air recovers the platinum moderator.

Positron annihilation studies of neutron irradiated and thermally treated reactor pressure vessel steels

Abstract Positron annihilation lifetime measurements using the pulsed low energy positron system (PLEPS) were applied for the first time for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation thermal treatment can be detected. The samples originated from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range from 7.8×1023 to 2.5×10 24 m −2 .

Identification of getter defects in high-energy self-implanted silicon at Rp/2

Abstract A strong gettering effect appears after high-energy Si self-implantation and subsequent annealing in two zones at the projected range of the silicon ions (Rp) and in a region at about Rp/2. The defects responsible for the impurity gettering at Rp/2 were studied by means of positron annihilation. It was found that diffusing Cu impurities were captured by small vacancy agglomerates. Monoenergetic positron beams with improved depth resolution were used to characterize the defects. Excellent depth resolution was obtained when samples were wedge-shaped polished and studied using the Munich Scanning Positron Microscope.

A Pulsed Positron Microbeam

The first pulsed microbeam for positrons in the keV energy range is described. The principle of operation and the construction details are given. The performance achieved is: 20 μm (FWHM) spot diameter; 350 ps (FWHM) time resolution; 5000 e+ /(s mCi) at 5 keV final energy.

Helium irradiated nickel investigated by positron annihilation

Abstract Nickel specimens have been homogeneously implanted with helium ions at about 325 K to a total dose of 500 appm. Distinct defect structures are observed in the irradiated specimen during isochronal annealing. Positron lifetime and Doppler broadening techniques were applied to study the evolution of the defect configurations. The influence of helium on the positron annihilation characteristics is clearly visible. The nucleation of helium bubbles could be determined. At higher annealing temperatures two distinct positron bound states are found. For both states the temperature dependence of the annihilation characteristics is similar to that reported for voids. Clear differences in the annihilation parameters have been established for empty and helium-decorated vacancy clusters.