Nuclear and radiochemistry

Synthesis of New Elements

How accelerator beams and radioactive actinide targets push the boundary of the periodic table toward the island of stability through heavy-ion fusion-evaporation.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: fusing nuclei against enormous repulsion

Every atomic nucleus carries a positive electric charge. Bringing two heavy nuclei close enough for the short-range strong nuclear force to bind them requires overcoming a formidable Coulomb barrier. When they do fuse, the resulting compound nucleus is so intensely excited that it almost always tears itself apart by fission. Surviving by boiling off a few neutrons to yield a single stable superheavy atom is an extraordinarily rare event.

Interactive scheme comparing hot fusion (48Ca + 249Cf -> 294Og via 3n evaporation followed by alpha decays to 282Cn and spontaneous fission) and cold fusion (70Zn + 208Pb -> 277Cn via 1n evaporation down to 257No).

SchoolSchool level: transuranic elements and fusion-evaporation

Definition: Compound nucleus and fusion-evaporation

When an accelerated projectile nucleus completely coalesces with a target nucleus, they form a transient compound nucleus that shares all mass, charge, and excitation energy. To reach the ground state without fissioning, the excited nucleus boils off one or more neutrons (xn channel) and radiates excess energy as gamma photons.

Following neptunium (Z=93) and plutonium (Z=94), elements up to fermium (Z=100) were produced through multiple neutron capture in nuclear reactors and identified in thermonuclear debris (such as the 1952 Ivy Mike test for einsteinium and fermium). Beyond Z=100, short half-lives prevent accumulation of targets, requiring heavy-ion fusion: accelerated projectile beams (such as 48Ca or 70Zn) collide with heavy targets (208Pb, 209Bi, or transuranic actinides).

\ce{^{48}_{20}Ca + ^{249}_{98}Cf -> [^{297}_{118}Og^*] -> ^{294}_{118}Og + 3 ^1_0n}

Milestones in the synthesis of transuranic and superheavy elements
Element	Z	Year	Reaction / Method	Facility / Team
Neptunium (Np)	93	1940	Neutron capture: $\ce{^{238}U(n,\gamma) ->[\beta^-] ^{239}Np}$	Berkeley 60-inch cyclotron (McMillan & Abelson)
Plutonium (Pu)	94	1940–1941	Deuteron beam: $\ce{^{238}U + ^2H -> ^{238}Np + 2n}$	UC Berkeley (Seaborg, McMillan, Kennedy, Wahl)
Bohrium to Copernicium	107–112	1981–1996	Cold fusion on $\ce{^{208}Pb}$ / $\ce{^{209}Bi}$ (1n channel)	GSI Darmstadt SHIP (Hofmann, Münzenberg)
Nihonium (Nh)	113	2004–2012	Cold fusion: $\ce{^{70}_{30}Zn + ^{209}_{83}Bi -> ^{278}_{113}Nh + n}$	RIKEN Nishina Center, GARIS (Morita et al.)
Flerovium to Oganesson	114–118	1999–2010	Hot fusion: $\ce{^{48}Ca}$ on actinides (3n–4n channels)	FLNR Dubna DGFRS + LLNL + ORNL (Oganessian et al.)

Example: Genetic decay chain identification of a single atom

In heavy-ion experiments, production rates are so low that a single atom must be identified unambiguously. How does a recoil separator and position-sensitive detector prove that an atom of element 118 was created?

Solution

The recoil separator filters out unreacted beam ions in microseconds and implants the fusion product into a silicon strip detector. The detector records the exact pixel coordinate and implantation time. In that same pixel, consecutive alpha decays are observed: 294Og decays by alpha (11.65 MeV) to 290Lv, which decays by alpha (10.84 MeV) to 286Fl, then alpha to 282Cn, which terminates in spontaneous fission. Because spatial correlation and lifetimes match known daughter isotopes, the genetic chain anchors the identity beyond doubt.

UndergraduateUndergraduate level: reaction kinematics, Coulomb barrier, and excitation energy

In heavy-ion fusion, projectile nuclei of mass A1 and charge Z1 collide with target nuclei (A2, Z2). The electrostatic potential energy reaches a maximum at contact radius R = r0 (A1^(1/3) + A2^(1/3)). Cold fusion combines heavy projectiles (50Ti, 54Cr, 58Fe, 62Ni, 64Ni, 70Zn) with spherical closed-shell targets (208Pb, 209Bi), forming compound nuclei with low excitation energy E ~ 10-15 MeV where only 1 or 2 neutrons evaporate. Hot fusion combines doubly magic 48Ca projectiles with deformed actinide targets (238U, 242Pu, 243Am, 245Cm, 248Cm, 249Bk, 249Cf), operating at E ~ 30-35 MeV with 3n or 4n evaporation channels. Notable recoil separators include SHIP and TASCA at GSI, GARIS at RIKEN, and DGFRS at Dubna.

V_C \approx \frac{Z_1 Z_2 e^2}{4\pi\varepsilon_0 \, r_0 (A_1^{1/3} + A_2^{1/3})} \approx \frac{1.44 \, Z_1 Z_2}{r_0 (A_1^{1/3} + A_2^{1/3})} \;\mathrm{MeV}

Example: Coulomb barrier and center-of-mass kinematics for 48Ca on 249Cf

Estimate the contact Coulomb barrier for 48Ca (Z1 = 20, A1 = 48) fusing with 249Cf (Z2 = 98, A2 = 249) using r0 = 1.30 fm. If the center-of-mass reaction energy is E_cm = 252 MeV and the reaction Q-value is -217 MeV, calculate the excitation energy E of the 297Og compound nucleus.

Solution

First compute radii: 48^(1/3) = 3.634 and 249^(1/3) = 6.292, giving sum 9.926. With r0 = 1.30 fm, denominator is 1.30 9.926 = 12.90 fm. The Coulomb barrier is V_C = 1.44 20 98 / 12.90 = 218.7 MeV. The excitation energy of the compound nucleus is E = E_cm + Q = 252 + (-217) = 35 MeV. This ~35 MeV is dissipating by boiling off 3 neutrons (binding energy ~7 MeV each plus kinetic energy ~2-3 MeV each) and gamma cascade to form 294Og.

AdvancedAdvanced: microscopic shell corrections and the island of stability

According to the classical liquid drop model, the fissility parameter x = (Z^2 / A) / (Z^2 / A)_crit implies that any nucleus with Z >= 104 has zero fission barrier (B_fiss = 0) and should disintegrate instantly within 10^-22 seconds. Superheavy nuclei exist entirely because of microscopic quantum shell corrections delta U_shell. Strong bunching of single-particle energy levels creates an artificial potential well that restores a fission barrier of 5 to 8 MeV. Theory predicts an island of enhanced stability centered near the spherical neutron shell closure N = 184 and proton shells at Z = 114, 120, or 126.

B_{\mathrm{fiss}}(Z, N) = B_{\mathrm{macro}}(Z, N) + \delta U_{\mathrm{shell}}(Z, N) + \delta P_{\mathrm{pair}}(Z, N)

ResearchResearch frontier: targeting elements 119 and 120 with titanium-50 beams

Current campaigns as of 2025 at RIKEN (Japan), the Superheavy Element Factory at FLNR Dubna (Russia), and GSI (Germany) focus on reactions such as 50Ti + 249Bk -> 299(119) and 50Ti + 249Cf -> 299(120), as well as 54Cr + 248Cm. Cross sections for these asymmetric reactions drop into the tens to hundreds of femtobarns, pushing separator transmission, target cooling wheels, and detector electronics to extreme physical limits.

References

Super-heavy element research · Y. T. Oganessian, V. K. Utyonkov, 2015
Experiment on the Synthesis of Element 113 in the Reaction 209Bi(70Zn,n)278113 · K. Morita et al., 2004
Toward the Discovery of New Elements: Production of Livermorium (Z=116) with 50Ti · J. M. Gates et al., 2024