Radiation Shielding Calculator
Calculate the thickness of shielding material needed to reduce gamma radiation from an initial intensity to a safe target level. Essential for designing radiation barriers in nuclear facilities, medical imaging rooms, and industrial radiography.
Last updated: September 2026
Formula below · 2 sources (nrc.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Shield thickness is estimated with broad-beam tenth-value layers (TVLs): each TVL of material cuts the dose rate tenfold, including the scattered photons (buildup) that a narrow-beam μ-based estimate ignores. Required thickness = TVL × log₁₀(I₀ / I). TVLs from NCRP Report 49: for photons up to 0.662 MeV (Cs-137) lead 2.1 cm, steel 5.3 cm, concrete 15.7 cm; for 0.662–1.33 MeV (Co-60) lead 4.0 cm, steel 6.9 cm, concrete 20.6 cm. Water is scaled from concrete by density (37 and 48 cm). Using the Cs-137 values below 0.662 MeV and the Co-60 values up to 1.33 MeV is conservative (lower energies need less shield). Above 1.33 MeV the table does not apply and the calculator says so. Results are for a point source and primary-beam shielding; have a qualified expert confirm any design.
How to use
Suppose a Co-60 source (1.25 MeV) gives 1,000 mR/hr and you want 2.5 mR/hr behind a lead shield. The reduction is 1,000 / 2.5 = 400, i.e. log₁₀(400) = 2.60 tenth-value layers (8.6 halvings). Thickness = 2.60 × 4.0 cm = 10.4 cm of lead. In concrete it would be 2.60 × 20.6 = 53.6 cm. For a Cs-137 source (0.662 MeV) the same reduction needs 2.60 × 2.1 = 5.5 cm of lead.
Frequently asked questions
What is a half-value layer and how is it used to design radiation shielding?
A half-value layer (HVL) is the thickness of a material that halves the intensity; a tenth-value layer (TVL) cuts it tenfold (narrow-beam TVL = 3.32 HVL). Narrow-beam values from μ (HVL = 0.693/μ) ignore scattered photons, so for real shields broad-beam values measured with scatter included are used. This calculator uses broad-beam TVLs: thickness = TVL × log₁₀(reduction). For Co-60, lead's broad-beam TVL is 4.0 cm (about 1.2 cm per HVL).
Why does gamma-ray energy affect the required shielding thickness?
The interaction cross-section between photons and matter changes dramatically with energy. At low energies (< 0.5 MeV) the photoelectric effect dominates, providing very efficient attenuation—especially in dense materials like lead. At higher energies (1–10 MeV) Compton scattering dominates and attenuation coefficients decrease, requiring more material for the same dose reduction. Pair production becomes significant above 1.022 MeV. This is why a Co-60 source (1.17 & 1.33 MeV) needs considerably more lead shielding than an I-131 source (0.364 MeV).
Which shielding material is best for gamma radiation protection?
Lead is the classic choice because its high atomic number (Z = 82) and density (11.34 g/cm³) give it excellent attenuation per unit thickness, and it is easily formed into sheets and bricks. Concrete is preferred for large structural shields due to low cost and structural integrity. Tungsten is used when space is critically constrained (e.g., medical collimators). Water and polyethylene are efficient for neutron shielding but less so for gamma rays. In practice, the best material depends on energy spectrum, available space, cost, and whether neutron shielding is also required.