Half-Value Layer (HVL) Calculator
HVL is the thickness of a given material that drops the gamma intensity by half. It is the number a radiographer pencils on a survey map when laying out boundaries, and it is the same number a regulator audits when reviewing a posted dose-rate. This tool tabulates published HVL and TVL values for the three sources in everyday industrial use — Ir-192, Co-60, and Se-75 — across lead, steel, tungsten, and concrete.
How it works
Gamma attenuation through a uniform absorber follows I = I₀ × e^(-μx) where μ is the linear attenuation coefficient. Setting I/I₀ = 0.5 and solving gives HVL = ln(2)/μ. The same algebra with I/I₀ = 0.1 gives the tenth-value layer (TVL). μ depends on photon energy and absorber atomic number, so each source-material pair has its own HVL. Industry shielding design uses tabulated values from NCRP Report 49 and ANSI N43.3 rather than re-deriving from cross-sections.
Formula
HVL = ln(2) / μ ; TVL = ln(10) / μ
HVL = ln(2) / μ ; TVL = ln(10) / μWorked example
Iridium-192 emits an average gamma energy of 380 keV. The linear attenuation coefficient in lead at that energy is ~1.44 cm⁻¹, giving HVL = ln(2)/1.44 = 0.48 cm = 4.8 mm. Ten HVLs (48 mm) drops intensity to ~0.1% — which is why a 50 mm lead pot is the working benchmark for an Ir-192 source projector storage container.
| Variable | Value |
|---|---|
| input: source | Ir-192 |
| input: material | Lead |
| output: hvl_mm | 4.8 |
| output: tvl_mm | 16.0 |
When to use this tool
Use when laying out a radiography boundary, sizing a temporary lead blanket wall, sanity-checking a shielding calculation submitted by a vendor, or briefing a permit-issuing authority on dose rates outside a controlled area.
Limitations
Where this calculator stops being accurate:
- Values assume narrow-beam, mono-energetic gamma — broad-beam scatter buildup adds 10-40% to required thickness.
- Ir-192 spectrum has 8 distinct gamma lines (296–612 keV). Tabulated HVL is the effective average.
- Concrete HVL assumes density 2.35 g/cm³; barite or heavy concrete differs significantly.
- Does not account for source decay over time — pair with the source-decay calculator for current activity.
- For X-ray tubes the HVL depends on tube kV and inherent filtration; use a kV-specific table from NCRP 49.
Frequently Asked Questions
Why is HVL different for Ir-192 versus Co-60?
Cobalt-60 emits at 1.17 and 1.33 MeV — much higher energy than the 296–612 keV spread of Ir-192. Higher-energy photons interact less per centimetre of lead, so HVL in lead rises from ~4.8 mm for Ir-192 to ~12 mm for Co-60. The practical consequence: a Co-60 source projector needs roughly 2.5× the lead shielding of an Ir-192 unit, which is why portable Co-60 cameras are rare and most fieldwork uses Ir-192 or Se-75.
Can I add HVL values to design a shield from multiple materials?
For first-cut estimates yes — convert each layer to its own HVL count and sum them. A 25 mm steel plate plus a 10 mm lead sheet for Ir-192 is roughly 1.7 HVL (steel) + 2.1 HVL (lead) = 3.8 HVL total, which is about a 14× attenuation. For permit-grade calculations you must apply buildup factors per NCRP 49 because scattered photons from an upstream layer behave differently than primary photons entering the next layer.
How many HVLs do I need for a controlled-area boundary?
10 CFR 20.1301 limits public dose to 2 mrem in any one hour outside a restricted area. Starting from a 100 Ci Ir-192 source the unshielded dose rate at 1 m is ~50 R/h = 50,000 mrem/h. To reach 2 mrem/h at 1 m you need ~14 HVLs (50,000 / 2¹⁴ ≈ 3). In practice the radiographer combines distance (inverse square) with shielding, so the HVL count drops; the safe-distance calculator handles the inverse-square half.
Why list both HVL and TVL?
HVL is convenient for verbal communication ("two more HVLs and we are good") but design calculations using percent attenuation are easier in tenth-value layers — each TVL is a factor of 10. A 4-TVL shield drops intensity to 0.01% which is most regulators' boundary criterion. Field log sheets often track both: HVL for quick sign-off, TVL for the engineered shielding letter that goes in the permit packet.
References & Standards Cited
- 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
- ANSI N43.3-2008 General Radiation Safety for Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources
- ASNT CP-189 (2020), Standard for Qualification and Certification of NDT Personnel — Radiographic Testing
- 10 CFR 20.1301 — Dose Limits for Individual Members of the Public
- IAEA Safety Reports Series No. 13 (1999), Radiation Protection and Safety in Industrial Radiography
Founder of NDT Connect and Atlantis NDT. 15+ years in industrial inspection across oil & gas, petrochemical, and offshore. ASNT Level III certified across five methods. Drives platform standards for the NDT Connect marketplace.