Positrons precipitate

I2, T2) attributed to positrons trapped at the crystalline-amorphous interface had T2 0.32 ns and h exhibited a precipitous decrease from about 58% to about 50% at the yield point, followed by recovery back to about 58%. This phenomenon interpreted as indicating interfacial loss of defects occurs during the initial deformation process and then some unknown recovery process takes place subsequently. 

Hasegawa M, Nagai Y, Toyama T, Nishiyama Y, Suzuki M, Alamazouzi A, Walle E and Gerard R, Evolution of irradiation-induced Cu precipitation and defects in surveillance test specimens of pressure vessel steels of nuclear power reactors positron annihilation and 3 dimensional atom probe study , ibid. Paper No. 18. 

Phythian et al performed a series of isochronal annealing experiments on model Fe-Cu-Mn-Ti-N alloys. Positron annihilation lifetime measurements indicated the presence of a long lifetime component (-265ps) associated with very stable precipitates possibly also associated with C. Further interpretation of the results indicated a strong association between vacancy-type defects and Cu atoms. 

The relative values of the energy of the ground state of a delocalised positron differ in different materials. This makes it possible to trap a positron in a precipitate with low positron ground-state energy. 

If and only if the difference between the positron s affinity for the host matrix and for the precipitate is positive will the precipitate attract the positron, making localisation or trapping in the precipitate possible. Despite this, the size of the precipitate must be large enough for the positron to be trapped. In the case of a spherical potential, the minimum radius for a bound state is [106]... 

It should be pointed out that positrons can also be trapped by misfit defects at the matrix-precipitate interface or by defects inside a precipitate, despite the positron s affinity for the precipitate. 

Positron annihilation spectroscopy can provide essential information about the deterioration in the mechanical properties of RPV steels (microstructural defects and precipitates) during their irradiation, which is known as neutron embrittlement. Currently, there are three main techniques based on annihilation phenomena positron lifetime spectroscopy, Doppler-broadening spectroscopy and angular correlation measurements. 

In order to elucidate whether such a precipitate can trap positrons, the positron affinities A+ for the host material and the precipitate were calculated [154], The A+ values were found to be relatively high and the positron lifetimes very short for perfect MC carbides. This fact confirms that perfect MC (M s Cr, V, Ti, Mn, Fe, Zr, Nb) carbides are very dense materials that cannot trap positrons when embedded in the Fe matrix. In general, from a PAS point of view, radiation damage can be interpreted as a combination of radiation-induced point defects, dislocations and small vacancy clusters [129,130] that occur mainly in the region of the precipitate-matrix interface. 

Positron annihilation can cause an increase in vacancy type defects due to irradiation [149,157-168,180], but the loads used during the RPV s operational lifetime (10 n/m ) are too small to produce large changes in positron annihilation parameters. Irradiation temperatures of around 300 °C cause the partial annealing of small (vacancy) defects, so that the positron lifetime remains practically unchanged or decreases after the initial period (this probably occurs after Cu, P and/or carbide precipitation, and is connected to restructuring). For more information, see [181] and Figure 4.50. 

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