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How were einsteinium and fermium isotopes produced in underground nuclear explosions?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

Einsteinium and fermium isotopes are produced in nuclear explosions through a process known as rapid-neutron capture, or the r-process.1

In these events, a uranium target is subjected to an intense bombardment of fast (high-energy) neutrons for a very small fraction of a second. This rapid influx of neutrons allows the target nuclei to capture multiple neutrons in quick succession, forming heavier isotopes before they have the opportunity to undergo beta decay.12

During underground nuclear detonations conducted in the late 1960s, this method resulted in the creation of significant quantities of einsteinium and fermium isotopes. Once produced, these isotopes were recovered from the surrounding rock debris through specialized mining techniques and subsequent chemical processing. The heaviest isotope typically produced and identified through this process is fermium-257.1