Beta particle | EPFL Graph Search

A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons respectively. Beta particles with an energy of 0.5 MeV have a range of about one metre in the air; the distance is dependent on the particle energy. Beta particles are a type of ionizing radiation and for radiation protection purposes are regarded as being more ionising than gamma rays, but less ionising than alpha particles. The higher the ionising effect, the greater the damage to living tissue, but also the lower the penetrating power of the radiation. β− decay An unstable atomic nucleus with an excess of neutrons may undergo β− decay, where a neutron is converted into a proton, an electron, and an electron antineutrino (the antiparticle of the neutrino): _neutron → _proton + _electron + _electron antineutrino This process is mediated by the weak interaction. The neutron turns into a proton through the emission of a virtual W− boson. At the quark level, W− emission turns a down quark into an up quark, turning a neutron (one up quark and two down quarks) into a proton (two up quarks and one down quark). The virtual W− boson then decays into an electron and an antineutrino. β− decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons also decay via this process. Both of these processes contribute to the copious quantities of beta rays and electron antineutrinos produced by fission-reactor fuel rods. Positron emission Unstable atomic nuclei with an excess of protons may undergo β+ decay, also called positron decay, where a proton is converted into a neutron, a positron, and an electron neutrino: _proton → _neutron + _positron + _electron neutrino Beta-plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is greater than that of the parent nucleus, i.

Creep Properties and Related Microstructural Changes of a Lamellar Titanium Aluminide Alloy before and after Helium-Implantation

Per Magnusson

The creep properties of a cast gamma-alpha-2 lamellar titanium aluminide of type Ti-46Al-2W-0.5Si (at. %) containing beta-particles was investigated using both helium-implanted and non-implanted material. Creep tests were performed in vacuum under constant stress using small miniaturized dog-bone shaped samples with gauge dimensions of 10 × 2 × 0.2 mm3. Creep temperatures ranged from 700 °C to 1000 °C and applied stresses ranged from 75 MPa to 400 MPa. Helium implantation was carried out with 0-24 MeV helium ions homogeneously implanting the miniaturized samples from 6.3 to 1333 appm He, at temperatures of 150 °C and 630 to 1000 °C. Non-implanted samples showed very good creep resistance up to temperatures of roughly 850 °C; at temperatures above 900 °C a decrease in creep resistance was observed. For temperatures up to 900 °C the activation energy of creep was determined to 405 kJ/mol and the stress exponent of 7.2. Size and geometry effects of miniaturized samples were observed in form of large scatter in creep strain rates and lower strains at fracture. Electron microscopy showed that the initial microstructure was unstable and that the alloy had a tendency to decompose the alpha-2 phase and form beta-gamma structures, loss of initial microstructure was accelerated with increasing temperature. The creep deformation mechanism was determined to be a combination of stress-induced phase change and dislocation creep. A gradual change in fracture behavior was observed, at lower temperatures fracture was brittle with little necking, at the highest temperature the fracture was completely ductile. Material crept and implanted at 700 and 800 °C exhibited embrittlement at helium contents above 10 appm, characterized by a strong loss of ductility and creep lifetime. Samples crept and implanted at 900 °C experienced embrittlement for all tested helium contents. Investigations with transmission electron microscopy revealed a substantial clustering and growth of helium bubbles at interfaces in the alpha-2 phase. An average bubble size for embrittlement between 5.3 and 6.7 nm was determined. The behavior and evolution of helium bubbles were studied with post implantation annealing experiments. Samples were implanted at 150, 630, 800 and 1000 °C and subsequently annealed at temperatures from 600 to 900 °C. Material implanted at 150 °C showed no bubbles before annealing, only after post-implantation annealing was widespread clustering observed. Material implanted at temperatures of 630 °C or higher, exhibited bubbles before post-implantation annealing. Depending on the implantation temperature, two different branches were observed in the size and distribution of helium bubbles even after post-implantation annealing. Samples implanted at 150 or 630 °C and then annealed at higher temperatures had high bubble density with small bubbles found in the entire microstructure. Samples implanted at 800 or 1000 °C had low bubble densities and the bubbles were only observed at alpha-2/beta and alpha-2/gamma interfaces, with sizes larger than the average bubble radius for embrittlement. Based on the post-implantation annealing experiments, a temperature limit for the onset of helium embrittlement between 630 and 700 °C is predicted. The helium embrittlement, the loss of initial microstructure and decreased creep resistance at temperatures above 900 °C suggests a limited applicability of lamellar TiAl in advanced nuclear reactor environments.

EPFL2012

Creep Properties and Related Microstructural Changes of a Lamellar Titanium Aluminide Alloy before and after Helium-Implantation

Per Magnusson

EPFL2012