Alpha-Decay Induced Shortening of Fission Tracks Simulated by in Situ Ion Irradiation

The diffusion of defects (e.g., vacancies and interstitials) and elements used for dating (e.g., He and Pb) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of 238U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils from the alpha-decay of 238U and 232Th. We simulate alpha-decay induced track-shortening by combining ion accelerator irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of self-irradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology. © 2021 Elsevier Ltd This work is supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070201), the National Second Expedition to the Tibetan Plateau (2019QZKK0707), and National Natural Science Foundation of China (NSFC 41673062). The electron microscopy with in situ ion irradiation was accomplished at Argonne National Laboratory at the IVEM-Tandem Facility, a U.S. Department of Energy Facility funded by the DOE Office of Nuclear Energy, operated under Contract No. DEAC02-06CH11357 by U. Chicago Argonne, LLC. The authors thank warmly M. Kirk, E. Ryan and P. Baldo from Argonne National Laboratory for assistance with the in situ TEM experiments. We thank David Shuster and three anonymous reviewers for their suggestions and comments to improve this paper.