Chimney Top Calculators

I-131 dose rate & shielding calculator

Dose rate from a Iodine-131 point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.22 R·m²/(Ci·h), half-life 8.025 days.

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
220 mR/h
Exposure rate
220 mR/h
Air-kerma rate
1.928 mGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
2.2 mSv/h · 220 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.22 R·m²/(Ci·h)

About Iodine-131

Thyroid therapy and imaging; environmental tracer after reactor releases.

Principal photons: 364.5 keV (81.5 %), 637.0 keV (7.2 %), 284.3 keV (6.1 %), 722.9 keV (1.8 %). Specific gamma-ray constant used here: 0.22 R·m²/(Ci·h) (2.2 R·cm²/(mCi·h)).

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)2.44 mR/h0.22 mR/h0.055 mR/h0.0088 mR/h0.0022 mR/h
1 GBq (27 mCi)66.1 mR/h5.95 mR/h1.49 mR/h0.238 mR/h0.0595 mR/h
1 Ci (37 GBq)2,440 mR/h220 mR/h55 mR/h8.8 mR/h2.2 mR/h

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

The formula

Ẋ [R/h] = 0.22 × 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 I-131 source at 2 m: Ẋ = 0.22 × 0.010 / 2² = 0.55 mR/h. Halving the distance to 1 m quadruples it to 2.2 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of I-131?
About 220 mR/h (0.22 R/h) for an unshielded point source, using Γ = 0.22 R·m²/(Ci·h). That is roughly 2.2 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of I-131 in SI units?
About 0.0595 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 2.2 R·cm²/(mCi·h).
What is the half-life of I-131?
8.025 days (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.
Is there shielding data for I-131?
The broad-beam table used by this site (NCRP Report 49) does not list I-131. Choose "Custom HVL" in the calculator and enter a half-value layer from a reference appropriate to its photon energies.

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. 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.)