Artificial formation of alpha recoil tracks using an americium source

Alpha recoil track (ART) is a lattice defect formed by the alpha decay of 238U, 235U, 232 Th, and their daughter products. The age since the start of ART accumulation can be estimated using the number of ARTs and the uranium and thorium concentrations, given the known decay constants of uranium and thorium. It is necessary to investigate mineral surface properties, appropriate etching conditions, and the ART annealing characteristics upon heating to apply ART observation to the dating and thermal history study of various minerals. This study aims to develop a method for artificially forming ART so that the shape and properties of etched ART in various minerals can be studied in the future. A 300 Bq americium source was used to irradiate muscovite, in which the ARTs were well observed in previous studies, for time intervals of 1 h, 3 h, 6 h, 12 h, 2 days, 4 days, and 1 week after annealing treatment to erase all naturally occurring ARTs, and the samples were observed using phase contrast microscopy after chemical etching. The ART areal density formed on the sample surface and the irradiation time has a linear relationship for the samples with irradiation time up to 12 h. However, the size distribution of the formed artificial ARTs showed a different distribution from that of the naturally observed ARTs. Several samples of muscovite irradiated for 3 h were prepared and annealed at different temperatures (100 °C, 150 °C, and 200 °C) and times (30 min, 1 h, 3 h, 5 h, 10 h, 20 h, 100 h, and 352 h) to determine the stability of ARTs under geological conditions. Given the uniform size distribution of natural ARTs, the results of this annealing experiment indicate that the smaller size of natural ARTs may be indicative of annealing at ambient temperature for an extended period (108–109 yrs), or ARTs may have been annealed in the recent past at a slightly higher temperature (150 °C–200 °C). We cannot rule out the possibility that the observed natural ART-like tracks were formed by the bombardment of smaller nuclei (e.g., move of major atoms by cosmic ray irradiation), assuming that the track size reflects the energy and mass of nuclei. Further detailed studies of annealing behavior and track formation processes by various atoms are required to reach a definitive understanding.