Speaker
Description
In astrophysics, isotopically enriched magnesium is studied to better understand each isotope’s individual role in stellar fusion reactions. To fully understand the modes by which these reactions occur, accurate calculations of nuclear cross sections for each isotope are necessary [1–3]. Accurate nuclear cross sections are reliant on uniformity of the magnesium foil, high isotopic purity of the sample, and two inversely proportional parameters—either a long beam capture volume (or thick foil) or an extended time in beam.
Magnesium metal is infamous, even in commercial production environments [4], for its low sublimation temperature under high vacuum, poor workability, high reactivity with air, and low density. These characteristics make it a challenging candidate for thick, self-supporting foils. Most magnesium foils produced for use in beamlines are made via evaporation. Unfortunately, this process results in high loss of the extremely expensive starting material [4]. The targets also have an upper limit of thickness at ~200 µg/cm2 [5], after which the metal deforms and begins flaking during deposition. If a user requests material thicker than this limit, then multiple foils must be fabricated and stacked which compounds uncertainty due to each individual foil’s inherent defects.
The production of thick (i.e., <3 mm thick and >500 µg/cm2) magnesium foils are described herein (Figure 1), beginning with the initial distillation of the metal, through melting, hot rolling, and final cold rolling to illustrate the niche techniques needed to overcome its challenging material properties.
[1] O. Straniero, G. Imbriani, F. Strieder, et. al. Impact of a Revised 25Mg(p, γ)26Al Reaction Rate on the Operation of the Mg-Al Cycle. The Astrophysical Journal 763:100 1-10 (2013).
[2] C. Massimi, P. Koehler, S. Bisterzo, et. al. Resonance Neutron-Capture Cross Sections of Stable Magnesium Isotopes and Their Astrophysical Implications. Physical Review 85 044615 (2012).
[3] C. Chen, Y.J. Li, H. Zhang, Z. H. Li. Preparation of Large-Area Isotopic Magnesium Targets for the 25Mg(p, γ)26Al Experiment at JUNA. Nuclear Science and Techniques 31 (2020).
[4] Q. Shen, Y. Ba, P. Zhang, J, Song, F. Pan. Recent progress in the research on magnesium and magnesium alloy foils: A short review. International Journal of Minerals, Metallurgy and Materials 5, 842-854 (2024).
[5] G.E. Thomas. Isotope Targets Preparation By Vapor Deposition. Nuclear Instruments and Methods 200 (1982) 27-31.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Isotope R&D and Production