Plutonium-242

Pu-242 · Plutonium, Z = 94, A = 242

Plutonium-242 (Pu-242) is slow enough to be handled as a material rather than as a trace: 1.47e+8 Bq/g, or 0.00396 Ci/g, puts a gigabecquerel at 6.83 g. Decay is by alpha decay, half-life 3.73 × 10⁵ years, and shedding even one per cent of the activity takes 5,408 years.

Alpha emission is led by 4.902 MeV at 76.5%, one of 3 recorded lines. None of it reaches through skin, so the limit here is intake, not external dose.

Decay is the only handle: forty years leaves 99.99% of today's activity and ten thousand years leaves 98.16%, with ten half-lives at 3.73 million years and a mean life of 538,114 years per atom.

Half-life 3.73 × 10⁵ years, 1.47e+8 Bq/g, and no line recorded

Half-life	3.73 × 10⁵ years (1.177e+13 s)
Decay mode	alpha decay
Specific activity	1.47e+8 Bq/g (0.00396 Ci/g)

3 alpha lines, strongest 4.902 MeV

3 recorded lines from 4.756 to 4.902 MeV, the strongest 4.902 MeV at 76.5% of 99.93% total alpha emission.

Energy (MeV)	Emission probability (%)
4.9023	76.5
4.8582	23.4
4.7562	0.0304

Activity over geological time

Ten half-lives is 3.73 million years. On any timescale a facility can be planned over the activity is constant — 99.99% is left after forty years — and the mean life 1/λ is 538,114 years.

Elapsed	Fraction remaining
1 half-life	50.0 %
2 half-lives	25.0 %
5 half-lives	3.13 %
10 half-lives	0.0977 %
Time to fall to 10 % of today's activity	1.24 million years
Time to fall to 1 %	2.48 million years
Time to fall to 0.1 %	3.72 million years

Limits — intake, not external dose

Alpha reaches nothing through skin — Pu-242 is an intake problem, not an external one.
What every sheet leaves out, internal dose included.

Decay and mass calculators for Pu-242

Other Plutonium nuclides: Pu-238, Pu-239, Pu-240, Pu-241, Pu-244

Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.