Sulfur-35
S-35 · Sulfur, Z = 16, A = 35
At 1.58e+15 Bq/g — 4.27e+4 Ci/g — a gigabecquerel of S-35 amounts to 632 ng, which is why activity rather than mass is how anyone states the quantity. Sulfur-35 decays by beta-minus decay with a half-life of 87.37 days, falling to 78.82% of today's activity in a month and 5.51% in a year.
The beta endpoint is 0.167 MeV, mean 0.0488 MeV over 1 branch. That endpoint stops in 0.268 mm of acrylic or 0.127 mm of glass. Of the beta energy, 0.04% turns into X-rays in acrylic and 0.48% in lead.
Decay is the only handle: a month leaves 78.82% of today's activity and a year leaves 5.51%, with ten half-lives at 2.39 years and a mean life of 126 days per atom.
Half-life, specific activity and beta endpoint
| Half-life | 87.37 days (7.549e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.58e+15 Bq/g (4.27e+4 Ci/g) |
| Beta endpoint / mean | 0.167 MeV / 0.0488 MeV |
0.268 mm of acrylic stops the 0.167 MeV endpoint
Katz–Penfold fit to the 0.167 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.
| Absorber | Range for the endpoint (mm) |
|---|---|
| acrylic | 0.268 |
| aluminum | 0.117 |
| water | 0.316 |
| glass | 0.127 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.04% |
| lead (Z = 82) | 0.48% |
Activity over months and years
Ten half-lives is 2.39 years — a storage problem rather than a disposal one, with 5.51% of today's activity still there after a year. The mean life 1/λ is 126 days.
| 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 | 290 days |
| Time to fall to 1 % | 1.59 years |
| Time to fall to 0.1 % | 2.38 years |
Limits of these range figures
- 0.268 mm of acrylic for the 0.167 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Beta and decay calculators for S-35
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.