VISIBLE RADIATION DAMAGE EFFECTS OF 40-MEV ALPHA PARTICLES ON SODIUM CHLORIDE CRYSTALS

摘要

The bombardment of sodium chloride crystals with high fluxes (>1014 particles/cm2) of 40-Mev alpha particles, whose computed range is 0.72 milli meter, produces a damaged planar region normal to the alpha beam near the end of the range. The irradiated, crystals are colored, throughout the portion between the incident surface and a plane parallel to this surface at a depth of 0.73+0.01 millimeter. The damaged region is characterized by (l) a line of discontinuity of the surface cleavage pattern at the coloration limit on sur faces parallel to the beam, (2) a plane of "easy" cleavage at various depths between 0.43 and 0.62 millimeter, and (3) crystallographic cavities that appear after the crystals are heated to 400° C. These cavities form in planes at the coloration limit, at the easy cleavage plane, and sometimes in planes at slightly shallower depths. The heating suffices to bleach completely the F-centers produced by the irradiation, and, during this time, the helium gas from the stopped alpha particles escapes from the crystals. Chemical analyses show that some "free" chlorine and "free" sodium atoms are present in the ir radiated colored crystals, but that the irradiated bleached, crystals contain no free chlorine or sodium.