Effect of thermal neutron irradiation on the elastic moduli and structure of boron containing crystals

The effect of thermal neutron irradiation on the elastic moduli and structure of boron containing tourmaline, danburite and kurnakovite crystals has been studied. The elastic properties of well-defined tourmalines, danburite and hambergite crystals have been studied. Some of the related data and the results prior to irradiation are published in the Journal of Applied Physics 50, 6005(1979). Ultrasonic measurements indicate that the elastic constants of tourmalines are little affected by neutron irradiation up to about 8 x 1017 n/cm2. Detectable damage on tourmalines starts to appear at a fluence of 1 x 1018 n/cm2, with a rather rapid decrease of the elastic constants being observed at higher fluences. Above 2 x 1018 n/cm2 cracks are generated and tourmalines become rather brittle. Radiation damage mechanisms of the B(n,α)Li reaction suggest that cracks and brittle fracture are due to anisotropic stress concentrations around the displacement cascades and helium bubbles. Rapid decrease of the elastic constants of tourmaline in similar to the observed decrease of the metamict minerals, except that in tourmaline the damage rate is not uniform throughout the sample due to absorption and attenuation of neutrons by B10. Although the positions and intensities of the Bragg peaks and the background scattering are affected by irradiation, in the short range the crystalline nature of tourmalines is preserved up to the observed flux of 7-8 x 1018 n/cm2. Danburite crystals become brittle and they are found to be less stable at a fluence of 8 x 1017 n/cm2. This is attributed to their composition, which contains more boron, and to their structure, which is easily cleaved. Kurnakovite becomes amorphous at a fluence of 2.5 x 1017 n/cm2