Advanced · 25 min

Synthesising element 118 one atom at a time

Walk a superheavy ion through the whole facility — source, accelerator, target, separator, detector — and see why discovering a new element takes weeks of beam for a handful of atoms.

Goal

Reconstruct the complete fusion–evaporation production route for a superheavy element and understand why yields are counted in atoms, not grams.

Apparatus and reagents

A $\ce{^{48}Ca}$ beam, a $\ce{^{249}Cf}$ target, a recoil separator and a position-sensitive silicon detector — the same combination that produced $\ce{^{294}Og}$ at Dubna.

Procedure

Start at the ion source and accelerator stages; note that the beam energy must sit in a narrow window to fuse without destroying the product.
At the rotating target, picture the compound nucleus $\ce{^{297}Og}$ * evaporating three neutrons to reach $\ce{^{294}Og}$ .
Follow the recoils into the separator: magnetic fields bend away the intense beam and let only fusion recoils through.
At the detector, read the identification logic: a chain of α decays ending in spontaneous fission, each with a known half-life, fingerprints the new element.

What to observe

Only a tiny fraction of projectiles produce a survivor — fusion cross-sections for Z = 118 are below a picobarn.
The separator discards the overwhelming beam (~10¹⁸ ions) so the detector only sees the few recoils that fused.
Identification is statistical: a correlated α chain of the right energies and times is the certificate of a new element.

Explanation

Superheavy elements are made by complete fusion: $\ce{^{48}Ca + ^{249}Cf -> ^{297}Og^{*} -> ^{294}Og + 3n}$ . The projectile must tunnel the Coulomb barrier yet stay cold enough that the fragile compound nucleus cools by neutron evaporation rather than fissioning — the famous “survival probability” bottleneck. Because capture, fusion and survival probabilities multiply, production rates for the heaviest elements are a few atoms per week. The chemistry of these actinide-region products is then probed single-atom-at-a-time, before the isotope decays.

Chemists behind it

Glenn T. Seaborg