Chimney Top Calculators

Ra-226 dose rate & shielding calculator

Dose rate from a Radium-226 point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.825 R·m²/(Ci·h), half-life 1,600 years.

Calculator

Calculated — choose the unit for the result.

Calculated — choose the unit for the result.

When solving for shield thickness this is the target dose rate behind the shield.

Calculated — choose the unit for the result.

Results

Dose rate
825 mR/h
Exposure rate
825 mR/h
Air-kerma rate
7.231 mGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
8.25 mSv/h · 825 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.825 R·m²/(Ci·h)
  • Ra-226: Classic radium standard: Ra-226 in equilibrium with its short-lived daughters, filtered by 0.5 mm platinum.

About Radium-226

Historical luminous paint and brachytherapy sources; legacy check sources; NORM contamination.

Principal photons: Many lines from daughters Pb-214 and Bi-214 (e.g. 351.9 keV, 609.3 keV, 1764.5 keV). Specific gamma-ray constant used here: 0.825 R·m²/(Ci·h) (8.25 R·cm²/(mCi·h)). Classic radium standard: Ra-226 in equilibrium with its short-lived daughters, filtered by 0.5 mm platinum.

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)9.17 mR/h0.825 mR/h0.206 mR/h0.033 mR/h0.00825 mR/h
1 GBq (27 mCi)248 mR/h22.3 mR/h5.57 mR/h0.892 mR/h0.223 mR/h
1 Ci (37 GBq)9,170 mR/h825 mR/h206 mR/h33 mR/h8.25 mR/h

Multiply mR/h by 10 for an approximate µSv/h figure (1 R ≈ 10 mSv for photons).

Shield thickness for a given reduction

MaterialHVLTVL÷10÷100÷1000
Lead1.66 cm5.5 cm5.5 cm11 cm16.5 cm
Steel (iron)2.2 cm7.4 cm7.4 cm14.8 cm22.2 cm
Concrete6.9 cm23.4 cm23.4 cm46.8 cm70.2 cm

Thicknesses use the tenth-value-layer model; values are broad-beam figures from NCRP Report 49.

The formula

Ẋ [R/h] = 0.825 × A [Ci] × T / d² [m²]

T is the shield transmission (1 with no shield). See the main calculator page for the full derivation and assumptions.

Worked example

A 10 mCi Ra-226 source at 2 m: Ẋ = 0.825 × 0.010 / 2² = 2.06 mR/h. Halving the distance to 1 m quadruples it to 8.25 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of Ra-226?
About 825 mR/h (0.825 R/h) for an unshielded point source, using Γ = 0.825 R·m²/(Ci·h). That is roughly 8.25 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of Ra-226 in SI units?
About 0.223 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 8.25 R·cm²/(mCi·h).
What is the half-life of Ra-226?
1,600 years (ENSDF evaluated value via NuDat 3). Use the decay calculator to correct a source's activity to today's date before computing a dose rate.
How much lead does it take to shield Ra-226?
Using broad-beam values from NCRP Report 49, one tenth-value layer of lead is 5.5 cm and one half-value layer is 1.66 cm. Reducing the dose rate 100-fold takes about 11 cm of lead.

Sources

  1. U.S. Department of Health, Education, and Welfare (1970). Radiological Health Handbook, revised edition. Public Health Service, Bureau of Radiological Health.
  2. Cember, H. & Johnson, T. E. (2009). Introduction to Health Physics, 4th edition. McGraw-Hill. (Chapters on external radiation safety and shielding.)
  3. NCRP Report No. 49 (1976). Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies up to 10 MeV. National Council on Radiation Protection and Measurements. (Broad-beam half-value and tenth-value layers for gamma sources.)
  4. National Nuclear Data Center, Brookhaven National Laboratory. NuDat 3 (ENSDF evaluated half-lives and photon emission data). www.nndc.bnl.gov
  5. ICRU Report 85 (2011). Fundamental Quantities and Units for Ionizing Radiation. (Mean energy per ion pair in dry air, W/e = 33.97 J/C.)