A nuclear magnetic resonance study of decomposition in neutron-irradiated LiF

Abstract

The decomposition of LiF produced by neutron irradiation in a reactor has been studied by nuclear magnetic resonance (NMR) techniques. Several sets of crystals and powdered samples have been irradiated at a variety of temperatures for total neutron doses between 10 15 and 2 × 10 19 n/cm 2. In several cases the temperature of the sample during irradiation has been measured by means of thermocouples. The temperature of a sample during the irradiation usually lies in the region of 120 to 300°C. After irradiation, distinctive lines were found superimposed upon the characteristic NMR spectrum of LiF. Through a detailed study of their properties, the additional lines have been shown to be due to F 19 nuclei in a molecular fluorine phase probably trapped in small cavities in the crystals, and to Li 7 nuclei in a metallic lithium phase. The concentration of molecular fluorine in the samples has been found to increase with dose when the specimen temperature during irradiation ( T s ) does not rise above approximately 300°C, reaching an ultimate concentration of from one to two percent of all fluorine in the sample at doses above 10 18 n/cm 2. However, the concentration is a strong function of T s , if T s , is near or above 300°C. The concentration of lithium metal observable by NMR may be even more sensitive to T s . No mass loss due to escape of fluorine is observed when T s , is kept below 300°C. Disagreements between measurements of lattice expansion in neutron-irradiated LiF by X-ray and density studies may arise from the heterogeneous nature of the damage; conflicting results from measurements of linear expansion and density (on different specimens) may arise from different values of T s .