Precautions
Protactinium is both toxic and highly radioactive and thus all manipulations with it are performed in a sealed glove box. Its major isotope 231Pa has a specific activity of 0.048 Ci/gram and primarily emits alpha-particles of the energy 5 MeV, which can be stopped by a thin layer of any material. However, it slowly decays, with the half-life of 32,760 years, into 227Ac, which has a specific activity of 74 Ci/gram, emits both alpha and beta radiation, and has a much shorter half-life of 22 years. 227Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA), as summarized in the table below.
| Isotope | 231Pa | 227Ac | 227Th | 223Ra | 219Rn | 215Po | 211Pb | 211Bi | 207Tl |
|---|---|---|---|---|---|---|---|---|---|
| SA (Ci/g) | 0.048 | 73 | 3.1×104 | 5.2×104 | 1.3×1010 | 3×1013 | 2.5×107 | 4.2×108 | 1.9×108 |
| Decay | α | α, β | α | α | α | α | β | α, β | β |
| Half-life | 33 ka | 22 a | 19 days | 11 days | 4 s | 1.8 ms | 36 min | 2.1 min | 4.8 min |
As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air. Only about 0.05% of ingested protactinium is absorbed into the blood and the remainder is excreted. From the blood, about 40% of the protactinium deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas in other organs the kinetics has a fast and slow component. So in the liver 70% of protactinium have a half-life of 10 days and 30% remain for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In all these organs, protactinium promotes cancer via its radioactivity. The maximum safe dose of Pa in the human body is 0.03 µCi that corresponds to 0.5 micrograms of 231Pa. This isotope is 2.5×108 times more toxic than hydrocyanic acid. The maximum allowed concentrations of 231Pa in the air is 3×10−4 Bq/m3.
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