Protactinium-231

Pa-231 · Protactinium, Z = 91, A = 231

A gigabecquerel of Pa-231 is 569 mg of material, which follows from a specific activity of 1.76e+9 Bq/g (0.0475 Ci/g). Protactinium-231 decays by alpha decay with a half-life of 32570 years; forty years leaves 99.91% of today's activity and ten thousand years leaves 80.83%.

Alpha emission is led by 5.014 MeV at 25.4%, one of 12 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 99.91% of today's activity and ten thousand years leaves 80.83%, with ten half-lives at 325,693 years and a mean life of 46,988 years per atom.

Half-life 32570 years, 1.76e+9 Bq/g, and no line recorded

Half-life	32570 years (1.028e+12 s)
Decay mode	alpha decay
Specific activity	1.76e+9 Bq/g (0.0475 Ci/g)

12 alpha lines, strongest 5.014 MeV

12 recorded lines from 4.681 to 5.059 MeV, the strongest 5.014 MeV at 25.4% of 99.07% total alpha emission.

Energy (MeV)	Emission probability (%)
5.0138	25.4
4.9513	22.8
5.0284	20
5.0586	11
4.7360	8.4
4.9340	3
5.0320	2.5
4.6810	1.5
4.8530	1.4
4.9860	1.4

Activity over millennia

Ten half-lives is 325,693 years. On any timescale a facility can be planned over the activity is constant — 99.91% is left after forty years — and the mean life 1/λ is 46,988 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	108,193 years
Time to fall to 1 %	216,386 years
Time to fall to 0.1 %	324,579 years

Limits — intake, not external dose

Alpha reaches nothing through skin — Pa-231 is an intake problem, not an external one.
What every sheet leaves out, internal dose included.

Decay and mass calculators for Pa-231

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