The FLiNaK and FLiNaK-CsF (5 mol%) samples were made in laboratory, in a glove box filled with argon. The FLiNaK salt (an eutectic mixture of 46.5 mol% LiF, 11.5 mol% NaF, and 42 mol% KF) used in this study was sampled from a previously melted large quantity of salt (⇠20 kg). The high purity CsF (99.99% from Sigma Aldrich) was further dried in glove box by heating at 250 C for 3 hours prior to mixing with FLiNaK salt in the molar ratio of 5:95. The mixture was encapsulated in a glassy carbon crucible and heated at 700 C for 3 hours to fully melt the mixture. The cooled salt chunks were ground in a glove box using an agate mortar and pestle before loading a small quantity of the material into a specifically designed sample holder described below.