Chimney Top Calculators

Tc-99m dose rate & shielding calculator

Dose rate from a Technetium-99m point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.078 R·m²/(Ci·h), half-life 6.007 hours.

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
78 mR/h
Exposure rate
78 mR/h
Air-kerma rate
683.6 µGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
780 µSv/h · 78 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.078 R·m²/(Ci·h)
  • Tc-99m: Includes the 18–21 keV Tc K x-rays; the 140.5 keV gamma alone gives about 0.06 R·m²/(Ci·h).

About Technetium-99m

The most widely used diagnostic nuclear-medicine isotope (bone, cardiac and thyroid scans).

Principal photons: 140.5 keV (89 %), Tc K x-rays 18–21 keV. Specific gamma-ray constant used here: 0.078 R·m²/(Ci·h) (0.78 R·cm²/(mCi·h)). Includes the 18–21 keV Tc K x-rays; the 140.5 keV gamma alone gives about 0.06 R·m²/(Ci·h).

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)0.867 mR/h0.078 mR/h0.0195 mR/h0.00312 mR/h7.8 × 10⁻⁴ mR/h
1 GBq (27 mCi)23.4 mR/h2.11 mR/h0.527 mR/h0.0843 mR/h0.0211 mR/h
1 Ci (37 GBq)867 mR/h78 mR/h19.5 mR/h3.12 mR/h0.78 mR/h

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

The formula

Ẋ [R/h] = 0.078 × 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 Tc-99m source at 2 m: Ẋ = 0.078 × 0.010 / 2² = 0.195 mR/h. Halving the distance to 1 m quadruples it to 0.78 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of Tc-99m?
About 78 mR/h (0.078 R/h) for an unshielded point source, using Γ = 0.078 R·m²/(Ci·h). That is roughly 0.78 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of Tc-99m in SI units?
About 0.0211 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 0.78 R·cm²/(mCi·h).
What is the half-life of Tc-99m?
6.007 hours (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 Tc-99m?
The broad-beam table used by this site (NCRP Report 49) does not list Tc-99m. 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. National Nuclear Data Center, Brookhaven National Laboratory. NuDat 3 (ENSDF evaluated half-lives and photon emission data). www.nndc.bnl.gov
  4. 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.)