Kazuko Kubo

Radiation Effects in LiF Crystals

Recovery behaviors of lattice expansion and optical absorption of neutron-irradiated LiF crystals have been measured as a function of annealing temperature. Remarkable correspondence between the two properties leads an interpretation of steps or humps and reduction in recovery curves of lattice constant respectively by formation of colloidal centers which are probably coagulations of vacancies and recombination of vacancies and interstitials. Optical microscopic observations reveal the fact that the square cavities, considered to be coagulations of vacancies remaining after displacement of atoms by neutron irradiation, preferentially formed on dislocation lines, along which F 2 gas escapes from the crystals. It is suggested that the colloidal centers are formed by clustering of point defects and some of them, which probably consist of vacancies, grow to square cavities during heat treatment.

Identification of Dislocations in NaF Crystals by Electrolytic Coloration and Chemical Etching

Several states of dislocations in nominally pure NaF single crystals are identified by means of two different techniques. The electrolytic coloration is caused by formation of metal Na colloidal particles, which decorate dislocations in the NaF. The chemical etchings, with two different etchants, reveal the dislocations of different characteristics. The two techniques are proposed as the methods for definite observation of the dislocations. The phenomena related to slip, climb and precipitation in the early stage are discussed in connection with the observed dislocations.

Effects in the Proton Bombardment of NaF Crystal. I. Measurement of the Average Projected Range

NaF crystals were bombarded with protons of energy from 0.6 to 2 MeV at somewhat high temperatures, to have Na colloid in the proton-penetrated zone. The average projected range of protons was measured by thickness of the colloid layer. The range-energy relation obtained in the present study has little effect of the proton channeling and seems to be that for random directions. The microscopic cleavage profile in the beam direction included colored zone, separate colloid layers, microcracks, and turning of the cleavage steps, which appears to have close relation with the effects of resonant 19 F(p, α) 16 O reactions as well as the proton penetration itself. Their locations coincide with the range ends of the protons of incident and resonant energies and of the α-particles produced at 340.5 keV resonance. Hydrogen and oxygen with protons and α-particles may contribute to the occurrence of a local strain to initiate cracks.

COLLOIDAL Li METAL IN NEUTRON-IRRADIATED LiF CRYSTALS

The appearance of colloidal particles of lithium metal in lithium fluoride single crystals after thermal neutron irradiation and thermal annealing is discussed. The dependence of particle formation on neutron dose, annealing temperature, and irradiation temperature is discussed. (R.E.U.)

Observations of Dislocation Network in Neutron-Irradiated LiF Crystal

Phase-contrast microscopy observations are described for a LiF crystal irradiated to 2 x 10/sup 17/ not and annealed at 400 to 800 deg C. Previous reports that square cavities with faces parallel to (100) and colored by the interference of light were found densely near cracks and subboundaries while sparsely in subgrnins are confirmed. Other observations are reported. Closely formed cavities decorate subboundaries but few are observed within 30 mu of outer surfaces of subboundaries. There are isolated dislocation lines as well as isolated cavities in subgrains, visually observable by having light scatterers. A dislocation network exists near the spot with dense concentration of cavities. Dense arrangements of dislocation lines become observable on subboundaries and near cracks when the crystal is annealed at about 700 deg C for a short time. (T.R.H)

Effects of Proton Bombardmemt on CaF2 Crystals

New features were observed in synthetic CaF 2 crystals which were colloidally colored by intense proton bombardmemt. There are (1) a surface of discontinuity at the end of the proton range, (2) a colloidally colored thin layer between the bombarded plane and the range end, and (3) the presence of plane where the dislocations were most decorated. These phenomena were explained to result from the inhomogeneous ionization and heat release during the proton penetration, and the atomic displacements and hydrogen introduction upon stopping of the protons in the crystal.

Depths of the Regions Damaged by Protons, Deuterons and α-Particles in Lithium Fluoride Single Crystal

The depths of regions damaged by 0.6∼2.0 MeV protons, deuterons or α-particles penetrating the LiF single crystal were optically measured by utilizing the diffuse reflection of light from the metallic colloids formed by bombardment. The results were compared with the range-energy relations theoretically calculated on the Bethe formula. The depth data obtained agree well with the theoretical ranges with Z =10 or 12, depending on the bombarding ions, not with Z =6 which is usually taken for LiF. The rough approximation is given by the formulae with simple power of the energy value. As the sources of discrepancy between the depth of damaged region and the ion range, the following are possible: Channeling of the ions during penetration in the crystal and diffusion of the ion-induced defects.

Dislocations Observed in NaF Crystals. I. Rosette Structure and Its Changes under External Stress

Three dimensional observation of individual dislocation lines constituting indentation rosette was made by electrolytic decoration. The majority of lines belonged to systems of loop families of {110} and {112}. From the structure, the formation of rosette was inferred to consist of the initial stage where the elemental glides operate, and the rearrangement stage where cross glide, climb, multiplication and network formation also operate. Upon applying external stress by bending and compression the rosette structure transformed to an arrangement more complicated than that expected. Causes of the complication are the plastic deformation and residual strains around indent. Mechanical and electrostatic motives may explain the characteristic transformation.

Defect Formation by N++ Ion Bombardment in LiF and NaF

The F-light absorption was measured in LiF and NaF single crystals bombarded by 0.7 MeV N ++ ions from a Van de Graaff accelerator. The growth curves showed a maximum, from which the F-center concentration at the saturation was estimated to be 7.6 and 3.8×10 19 F-centers/cm 3 in LiF and NaF, respectively. The rates of production, 3020 and 1210 F-centers/N ++ ion, and the formation energies of an F-center, 232 and 578 eV, were determined from the initial portions of the growth curves in LiF and NaF, respectively. The formation of metallic film on the bombarded surface and other imperfections as the result of radiation damage are discussed, referring also to the micrographic observations. It is thus concluded that the damage by N ++ ions is fairly severe; the double charge is possibly responsible for this phenomenon.

Dislocations Observed in NaF Crystals. II. Nucleation Sources on Cleavage Surfaces

Dislocation loop families beneath the cleavage face have been studied three dimensionally by means of the electrolytic coloration. Emerge planes are {110} and (001). Dislocation at shallow levels are strongly influenced by image force, i. e. freed from the face or changed their shape. The nucleation sources of dislocation are found at structural inhomogeneities where stress concentration occurs during the cleavage and electrolysis. Simple loop families are relatively isolated from each other, while complicated ones are observed to be close together with the multiform neighborhood. The mechanisms of nucleation and complication are discussed for each dislocation loop family, from simple to complicated ones including newly found chevron patterns.