Displacement of cobalt impurities in molybdenum (cobalt) studied by X-ray excitation with channeled protons

Proton induced X-ray excitation (PIXE) and ion channeling have been applied to study the location of Co atoms with respect to the host lattice in a dilute Mo(Co) alloy single crystal prepared by implantation of 250 keV Co+ ions in the [111] direction and subsequent annealing at 1400 K. The yields of proton induced K-X-rays from the cobalt impurity, of K- and L-X-rays and of backscattered protons from the molybdenum host were measured simultaneously as a function of the crystal orientation with respect to the analysing 600 keV proton beam. The experimental yields around the [111], [110] and [100] axes were compared with theoretical yields calculated on the basis of Monte Carlo computer simulations. Experimental scans for the Mo matrix are in good agreement with calculated scans for a perfect crystal, provided the X-ray absorption in the target, the energy and impact parameter dependence of the X-ray production cross section is taken into account in the calculations. For the Co foreign atoms it is shown that more than 95% are initially substitutional. After irradiation with 600 keV protons at 50 K a considerable fraction of Co atoms is displaced from substitutional to interstitial lattice sites. The measurements are compatible with a displacement by 0.105 nm into the [110]-direction being the dominant configuration. In addition a small fraction of Co atoms, displaced by 0.14 nm into the [100]-direction, can be identified at higher irradiation fluences. The fluence dependence of the displaced fractions is in good agreement with the unsaturable trap model, when the [110]-displacement is attributed to the trapping of a single Mo interstitial atom to form a '[110]-mixed dumbbell' with the Co, and when the [110]-displacement is assigned to the trapping of two Mo interstitials at the Co impurity. (orig.)