Positron lifetime calculations of defects in chromium containing hydrogen or helium

Helium behavior in ferritic/martensitic steels irradiated in spallation target

“…It is also well known, that the presence of helium and/or hydrogen atoms in the vacancy clusters shortens the lifetime of the corresponding component. Calculations of Troev et al [21] for vacancy-helium complexes in chromium show that, for instance, a V 2 He 1 cluster is characterized by a positron lifetime of 218 ps and a positron lifetime of 252 ps can be attributed to a cluster of 6 vacancies with 3 helium atoms (V 6 He 3 ).…”

“…With increasing He-to-vacancy ratio, a cluster becomes less effective in the positron trapping [21,30]. In the work of Morishita et al [7], authors show that the binding energies of an interstitial helium atom, an isolated vacancy and a self-interstitial iron atom do not much depend on the size of the cluster but rather on the He-to-vacancy ratio of the cluster.…”

“…Particularly, two techniques, namely positron lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS), have been used in the nuclear material research in the last decade [11][12][13][14]. Much effort in both the theoretical and experimental fields was devoted to the investigation of the helium effect in structural materials [12,13,[15][16][17][18][19][20][21]. Most of the published papers, however, discuss helium-implanted materials with extremely high He-to-dpa ratio (>1000 appm He/dpa).…”

PLEPS study of ion implanted and annealed Fe-11.62%Cr alloys

“…Specimens implanted at a level of 0.3 C/cm 2 showed significant decrease of mean positron lifetime at temperature of 600 ºC (figure 1). Analysis based on the MLT at this temperature, at the depth of main damage caused by He ions (figure 2), could be interpreted as extensive decrease of defect size to monovacancies or small vacancy clusters [8,9]. Components reflecting positron lifetimes in bulk and defects showed a decrease.…”

Microstructural study of He‐implanted Fe‐Cr alloys with the use of conventional lifetime technique and pulsed low energy positron beam

“…In the zone of maximum damage in the implanted specimens, two different defect lifetimes have been observed. The shorter lifetime between 240 ps and 300 ps could be assigned to small vacancy clusters of size less than 6 neighbouring monovacancies or larger clusters filled with helium [10]. The longer lifetime between 400 ps and 500 ps corresponds to annihilation in large voids (> 1 nm) [11].…”