Point-source dose rate, shielding attenuation, and stay-time — for health physics technicians, RSOs, and reactor operators. Every constant is drawn from published references and shown with the working.
Live visualization — gamma source → shield → detector, driven by the numbers below
Unshielded—Shielded—Transmission—
Source & geometry
Shielding
Stay-time (dose budget)
Shielding design (optional)
Leave blank to skip. Tells you how thick the selected material must be.
Results
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Unshielded dose rate at distance
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Dose rate from the bare source at your chosen distance.
Show the math ▾
Dose rate behind shield primary beam
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Show the math ▾
Shielding strength (this isotope + material)
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HVL = thickness that halves the beam. TVL = thickness that cuts it to one-tenth.
Show the math ▾
Allowed stay-time for your dose budget
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Based on the dose rate at your position (shielded if a shield is set).
Shielding required for your target
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Show the math ▾
Scatter buildup: the shielded value above is the primary (un-scattered) beam only. Real dose behind a shield is higher because scattered photons build up. For a safe shield design, use the target-thickness result — it adds Cember's one-HVL buildup margin.
Educational estimate — not a shielding design of record. This tool uses point-source, single-effective-energy physics with broad-beam design margins. It does not replace a qualified RSO, a facility ALARA review, or a full buildup/Monte-Carlo analysis (e.g. MCNP, ANSI/ANS-6.4.3). Verify against your site's radiation protection program before acting.
How to use this calculator
Estimate the gamma dose rate from a sealed point source, how much a shield cuts it, and how long you can safely stay.
1. Describe the source
Isotope — pick your source. Only isotopes with a published gamma constant are listed.
Activity — how strong the source is (Ci or Bq family). 1 Ci = 37 GBq.
Distance — from the source to where a person stands. Dose falls off as 1/distance², so doubling the distance quarters the dose.
2. Add a shield (optional)
Pick a material and thickness. The tool shows the dose behind it, plus the HVL (halves the beam) and TVL (cuts it to a tenth).
The shielded number is the primary beam only. Scattered photons add "buildup" on top, so treat it as a floor, not the real dose.
3. Pick your view
Health Physics view — adds a "shielding design" box: enter a target dose rate and it tells you how thick the material must be (with a one-HVL buildup margin, per Cember).
RO / SRO view — adds a stay-time box: enter a dose budget (e.g. 100 mrem) and it tells you how long you can work at that position. Units default to mrem/h.
Units
Switch the results between mSv/h, µSv/h, mrem/h and rem/h with the buttons above the results. 1 mSv = 100 mrem. 1 Sv = 100 rem.
The physics, briefly
Dose rate = Γ × A ÷ d² → behind shield × e^(−μ·x)
Γ is the isotope's gamma constant, A the activity, d the distance, μ the material's attenuation coefficient at the isotope's gamma energy, x the shield thickness. Every number and its source is in the footer.