Radiation intensity and expansion saturation of the crystal lattice of irradiated diamond

Abstract

It is shown that the saturation of the expansion of the diamond crystal lattice during irradiation in a reactor depends not only on the temperature but also on the intensity of the irradiation. Expansion saturation decreases with decreasing irradiation intensity. The temperature influences saturation expansion indirectly via an increase of the effectiveness of the annealing of defects, and the intensity of irradiation stimulates additional annealing by increasing its duration. The same level of the saturation of expansion of the diamond lattice, specifically, 2.75%, can be attained at irradiation temperatures 80 and 300°C if the diamond is irradiated with neutron flux density 10 12 and 10 14 sec -1 ·cm -2 , respectively.One modern idea concerning change in the properties of materials irradiated in a nuclear reactor is based on the zone theory of the formation of defects. Zones with an elevated concentration of defects are formed at the end of the tracks of the first knocked-out atoms. Their size depends on the type of irradiated material. For example, the zone volume in diamond is about 5·10 -21 cm 3 . The displacement zones arising in irradiated material do not overlap initially, so that the number of zones, the number of defects, and the change of the properties, specifically, the expansion of the crystal lattice of diamond, increase at this stage in proportion to the irradiation dose. As the irradiation dose increases, some displacement zones start to overlap and the number of defects increases more slowly, destroying the linearity of the dose dependence of the expansion of the lattice. By this time, the new zones which arise result only in a replacement of old zones by new ones, the concentration of zones does not change, and the expansion of the lattice reaches saturation. Figure 1 shows that the character of the dependence agrees with the zone model: saturation (plateau) appears after the linear section. The dependence, presented in Fig. 1, of the expansion of the diamond lattice in percent on the neutron fluence F is described by the equationwhere 3.65 is the corresponding plateau in the expansion of the diamond lattice (saturation), %; 0.032 characterized the initial slope angle of the (1) dependence. It is assumed that the concentration of defects in any volume excited by displacements is not associated with the time elapsed after their formation. In reality, it does depend on the time, i.e., the "age" of the zone up to the moment of repeated excitation or termination of irradiation, and is determined by thermal annealing. When the structure of the material is photographed instantaneously during irradiation, sections (volumes) which have not been touched by displacements as well as volumes which have transitioned through the state of displacement one or more times can be seen in the photograph. The age of these volumes will be different, for example, the ones that arose at first and remained untouched by secondary excitation up to termination of irradiation will be older than the ones where the displacem...