Chimney Top Calculators

Ir-192 dose rate & shielding calculator

Dose rate from a Iridium-192 point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.48 R·m²/(Ci·h), half-life 73.83 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
480 mR/h
Exposure rate
480 mR/h
Air-kerma rate
4.207 mGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
4.8 mSv/h · 480 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.48 R·m²/(Ci·h)
  • Ir-192: Radiography references often quote 0.5–0.59 R·m²/(Ci·h) depending on encapsulation; 0.48 is the handbook value for an unfiltered point source.

About Iridium-192

Industrial gamma radiography of welds and castings; high-dose-rate brachytherapy.

Principal photons: 316.5 keV (82.8 %), 468.1 keV (47.8 %), 308.5 keV (30.0 %), 296.0 keV (28.7 %) and weaker lines. Specific gamma-ray constant used here: 0.48 R·m²/(Ci·h) (4.8 R·cm²/(mCi·h)). Radiography references often quote 0.5–0.59 R·m²/(Ci·h) depending on encapsulation; 0.48 is the handbook value for an unfiltered point source.

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)5.33 mR/h0.48 mR/h0.12 mR/h0.0192 mR/h0.0048 mR/h
1 GBq (27 mCi)144 mR/h13 mR/h3.24 mR/h0.519 mR/h0.13 mR/h
1 Ci (37 GBq)5,330 mR/h480 mR/h120 mR/h19.2 mR/h4.8 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
Lead0.6 cm2 cm2 cm4 cm6 cm
Steel (iron)1.3 cm4.3 cm4.3 cm8.6 cm12.9 cm
Concrete4.3 cm14.7 cm14.7 cm29.4 cm44.1 cm

Thicknesses use the tenth-value-layer model; Ir-192 has a spread of photon energies; its steel values are the ones used in industrial-radiography practice.

The formula

Ẋ [R/h] = 0.48 × 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 Ir-192 source at 2 m: Ẋ = 0.48 × 0.010 / 2² = 1.2 mR/h. Halving the distance to 1 m quadruples it to 4.8 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of Ir-192?
About 480 mR/h (0.48 R/h) for an unshielded point source, using Γ = 0.48 R·m²/(Ci·h). That is roughly 4.8 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of Ir-192 in SI units?
About 0.13 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 4.8 R·cm²/(mCi·h).
What is the half-life of Ir-192?
73.83 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.
How much lead does it take to shield Ir-192?
Using broad-beam values from NCRP Report 49, one tenth-value layer of lead is 2 cm and one half-value layer is 0.6 cm. Reducing the dose rate 100-fold takes about 4 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. 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.)