Chimney Top Calculators

Am-241 dose rate & shielding calculator

Dose rate from a Americium-241 point source at any distance, with shielding — or solve for activity, distance or shield thickness. Γ = 0.013 R·m²/(Ci·h), half-life 432.6 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
13 mR/h
Exposure rate
13 mR/h
Air-kerma rate
113.9 µGy/h
Dose-equivalent rate (≈, 1 R ≈ 1 rem)
130 µSv/h · 13 mrem/h
Activity
1 Ci · 37 GBq
Distance
1 m · 3.281 ft
Γ used
0.013 R·m²/(Ci·h)
  • Am-241: Computed from the 59.5 keV gamma (35.9 %) with NIST air energy-absorption data. Neptunium L x-rays (13–21 keV) are excluded because typical source encapsulation absorbs them; bare-source values in some tables are several times higher.

About Americium-241

Smoke detectors, Am-241/Be neutron sources in moisture/density gauges and well logging, thickness gauges.

Principal photons: 59.5 keV (35.9 %), plus Np L x-rays. Specific gamma-ray constant used here: 0.013 R·m²/(Ci·h) (0.13 R·cm²/(mCi·h)). Computed from the 59.5 keV gamma (35.9 %) with NIST air energy-absorption data. Neptunium L x-rays (13–21 keV) are excluded because typical source encapsulation absorbs them; bare-source values in some tables are several times higher.

Unshielded exposure rate by distance

Activity0.3 m1 m2 m5 m10 m
1 mCi (37 MBq)0.144 mR/h0.013 mR/h0.00325 mR/h5.2 × 10⁻⁴ mR/h1.3 × 10⁻⁴ mR/h
1 GBq (27 mCi)3.9 mR/h0.351 mR/h0.0878 mR/h0.0141 mR/h0.00351 mR/h
1 Ci (37 GBq)144 mR/h13 mR/h3.25 mR/h0.52 mR/h0.13 mR/h

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

The formula

Ẋ [R/h] = 0.013 × 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 Am-241 source at 2 m: Ẋ = 0.013 × 0.010 / 2² = 0.0325 mR/h. Halving the distance to 1 m quadruples it to 0.13 mR/h.

Frequently asked questions

What is the dose rate at 1 meter from 1 Ci of Am-241?
About 13 mR/h (0.013 R/h) for an unshielded point source, using Γ = 0.013 R·m²/(Ci·h). That is roughly 0.13 mSv/h with the 1 R ≈ 10 mSv photon convention.
What is the gamma constant of Am-241 in SI units?
About 0.00351 µSv/h at 1 m per MBq (approximate dose-equivalent rate), or 0.13 R·cm²/(mCi·h).
What is the half-life of Am-241?
432.6 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.
Is there shielding data for Am-241?
The broad-beam table used by this site (NCRP Report 49) does not list Am-241. Choose "Custom HVL" in the calculator and enter a half-value layer from a reference appropriate to its photon energies.

Sources

  1. National Nuclear Data Center, Brookhaven National Laboratory. NuDat 3 (ENSDF evaluated half-lives and photon emission data). www.nndc.bnl.gov
  2. Hubbell, J. H. & Seltzer, S. M. (2004). Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients (version 1.4). NIST Standard Reference Database 126. www.nist.gov
  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.)