Investigation of Positron Moderator Materials for Electron-Linac-Based Slow Positron Beamlines

Abstract

Positron re-emission properties were studied on moderator materials in order to improve the positron moderation system of electron-linac-based intense slow positron beamlines. The re-emitted positron fraction was measured on tungsten, SiC, GaN, SrTiO3, and hydrogen-terminated Si with a variable-energy pulsed positron beam. The results suggested that tungsten is the best material for the primary moderator of the positron beamlines while epitaxially grown n-type 6H–SiC is the best material for the secondary moderator. Defect characterization by monoenergetic positron beams and surface characterization by Auger electron spectroscopy were carried out to clarify the mechanism of tungsten moderator degradation induced by high-energy electron irradiation. The characterization experiments revealed that the degradation is due to both radiation-induced vacancy clusters and surface carbon impurities. For the restoration of degraded tungsten moderators, oxygen treatment at ∼900°C is effective. Furthermore, it was found that oxygen at the tungsten surface inhibits positronium formation; as a result, it can increase the positron re-emission fraction.

Structural and defect characterization of Gd-doped GaN films by X-ray diffraction and positron annihilation

“…Positron annihilation lifetime spectra for Samples A and B obtained at E = 5 keV. The higher background for the spectrum of Sample A is due to the longer positron diffusion length, expected from the S -E curves, as well as to the relatively high positron re-emission efficiency for GaN system [9].…”

“…Before installing to the system, the moderator was annealed in vacuum by electron-bombardment heating with a maximum electric power of 1 kW. The annealing procedure was as follows: 1) evacuating the chamber to get 2 × 10 −5 Pa, 2) introducing air into the chamber with a pressure of 1 × 10 −4 Pa, 3) annealed for 1 min at 1400 • C to remove carbon impurities from the tungsten surface as carbon monoxide gas [9,10], 4) evacuating the chamber again, and annealed at 1700 • C for a few seconds at 3 × 10 −5 Pa to eliminate tungsten oxide on the mesh surface [9], 5) repeating Step 1-4 for 3 cycles to eliminate carbon impurities in the tungsten mesh, 6) finally, annealed at 2300 • C for 1 min at 7 × 10 −5 Pa, and allowed to cool in vacuum with a cooling rate of roughly 50 • C/s.…”

Experimental investigation of slow-positron emission from 4H-SiC and 6H-SiC surfaces

“…Tungsten (W) is, however, the most commonly used moderator because of its general stability and relatively high efficiency ∼10 −3 under non-UHV conditions [2]. In recent years, positron emission has been observed from the surfaces of various wide-band-gap semiconductors (for example, GaN [5,6], SiC [5][6][7][8][9] and diamond [10]). These wide-band-gap materials having a high electron mobility and a negative positron work function are potential substrate materials for fabricating field-assisted (FA) moderators in which an internal electric field present in the moderator causes the implanted positrons to drift to the emitting surface [11].…”

“…For the case of epitaxial materials, at room temperature, the positron diffusion length was reported as 348 nm [19] and 250 nm [20] (calculated from the measured values of D e f f ∼ 3.5 cm 2 s −1 and τ ave ∼ 180 ps in [20]). The n-type SiC materials are also known to emit positrons with energies in the 2.1-3.0 eV range [5][6][7][8][9]. The breakdown electric field of this material is as high as 2 × 10 6 V cm −1 [21] and can probably sustain a positron velocity of ∼10 7 cm s −1 before electrical breakdown occurs-similar to the electron saturated velocity [21].…”

Positron annihilation and cathodoluminescence study on inductively coupled plasma etched GaN

“…The mean implantation depth of positrons is indicated on the upper horizontal axis. The increase in S at low E (0.1 keV) in all the S–E curves is a result of positron annihilation at the sample surface (). The constant S values for E 10 keV correspond to positron annihilation in the bulk of the sample.…”