Chimney Top Calculators

Cs-137 dose rate & shielding calculator

Dose rate from a Cesium-137 point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.33 R·m²/(Ci·h), half-life 30.08 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
330 mR/h
Exposure rate
330 mR/h
Air-kerma rate
2.892 mGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
3.3 mSv/h · 330 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.33 R·m²/(Ci·h)
  • Cs-137: Includes the 662 keV gamma of its Ba-137m daughter (85.1 % per decay) in secular equilibrium.

About Cesium-137

Portable moisture/density gauges, level and thickness gauges, calibration sources, blood irradiators.

Principal photons: 661.7 keV (85.1 %, via Ba-137m). Specific gamma-ray constant used here: 0.33 R·m²/(Ci·h) (3.3 R·cm²/(mCi·h)). Includes the 662 keV gamma of its Ba-137m daughter (85.1 % per decay) in secular equilibrium.

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)3.67 mR/h0.33 mR/h0.0825 mR/h0.0132 mR/h0.0033 mR/h
1 GBq (27 mCi)99.1 mR/h8.92 mR/h2.23 mR/h0.357 mR/h0.0892 mR/h
1 Ci (37 GBq)3,670 mR/h330 mR/h82.5 mR/h13.2 mR/h3.3 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
Lead0.65 cm2.1 cm2.1 cm4.2 cm6.3 cm
Steel (iron)1.6 cm5.3 cm5.3 cm10.6 cm15.9 cm
Concrete4.8 cm15.7 cm15.7 cm31.4 cm47.1 cm

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

The formula

Ẋ [R/h] = 0.33 × 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 Cs-137 source at 2 m: Ẋ = 0.33 × 0.010 / 2² = 0.825 mR/h. Halving the distance to 1 m quadruples it to 3.3 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of Cs-137?
About 330 mR/h (0.33 R/h) for an unshielded point source, using Γ = 0.33 R·m²/(Ci·h). That is roughly 3.3 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of Cs-137 in SI units?
About 0.0892 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 3.3 R·cm²/(mCi·h).
What is the half-life of Cs-137?
30.08 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 Cs-137?
Using broad-beam values from NCRP Report 49, one tenth-value layer of lead is 2.1 cm and one half-value layer is 0.65 cm. Reducing the dose rate 100-fold takes about 4.2 cm of lead.

Sources

  1. Unger, L. M. & Trubey, D. K. (1982). Specific Gamma-Ray Dose Constants for Nuclides Important to Dosimetry and Radiological Assessment. ORNL/RSIC-45/R1, Oak Ridge National Laboratory. www.osti.gov
  2. U.S. Department of Health, Education, and Welfare (1970). Radiological Health Handbook, revised edition. Public Health Service, Bureau of Radiological Health.
  3. Cember, H. & Johnson, T. E. (2009). Introduction to Health Physics, 4th edition. McGraw-Hill. (Chapters on external radiation safety and shielding.)
  4. 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.)
  5. National Nuclear Data Center, Brookhaven National Laboratory. NuDat 3 (ENSDF evaluated half-lives and photon emission data). www.nndc.bnl.gov
  6. 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.)