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Half-value layer

Dose rate behind solid shielding, calculated from the half-value layers (HVL) of the isotope in lead, steel or concrete.

Ci
m
mm
The same thickness applies to all three materials.

Without shielding—mSv/h
Lead—mSv/h
Steel—mSv/h
Concrete—mSv/h

Method

Ḣ = Ḣ₀ × 2^(−t / HVL)

Ḣ₀ = A × 0.037 × Γ / d²

  • Ḣ₀ — dose rate without shielding in mSv/h
  • t — shield thickness in mm
  • HVL — half-value layer in mm for the material
  • Γ — gamma constant in mSv·m²/(h·TBq)
  • d — source-to-shield distance in m

HVL values used — Ir-192: lead 4 mm, steel 13 mm, concrete 60 mm. Se-75: 1 / 8 / 30 mm. Co-60: 13 / 20 / 100 mm. These are approximations for a broad beam: scatter and build-up are ignored, so the real dose rate behind the shield is understated. Measure before working behind shielding.

Indicative result — check it against your authorisation and your dosimetry.