Pb-212 · Lead-212
Beta emitter whose short-lived progeny give one alpha particle per decay. Half-life 10.64 hours.
Pb-212 acts as an in vivo generator for targeted alpha therapy: it decays to Bi-212, and Bi-212 and Po-212 deliver one alpha particle per Pb-212 decay. One branch of the chain passes through Tl-208, which emits a 2.6 MeV gamma ray. That gamma ray, not the alpha, drives shielding and dose-rate planning.
What changes in practice
- One alpha particle per Pb-212 decay, from Bi-212 or Po-212.
- The 2.6 MeV gamma ray from Tl-208 is far more penetrating than Lu-177 or Tc-99m photons. Shields designed for those give much less protection here.
- Bi-212 and Tl-208 grow in within a few hours of separation, so vials, patients and waste carry the full chain, including the 2.6 MeV gamma.
- The half-life suits decay-in-storage, but a Th-228 or Ra-224 generator, if used, is long-lived and is never treated as decay waste.
Decay chain
Pb-212 · 10.64 h
Beta minus to Bi-212
Bi-212 · 60.54 min
Beta minus to Po-212 (64.07%), or alpha to Tl-208 (35.93%)
Po-212 · 0.300 µs
Alpha (8.785 MeV) to stable Pb-208
Tl-208 · 3.058 min
Beta minus to stable Pb-208, with a 2.6 MeV gamma
Main emissions
| Type | From | Energy | Per Pb-212 decay |
|---|---|---|---|
| Alpha | Po-212 | 8.785 MeV | 64.07% |
| Alpha | Bi-212 | about 6.1 MeV | 35.93% |
| Gamma | Pb-212 | 238.6 keV | 43.6% |
| Gamma | Pb-212 | 300.1 keV | about 3.2%* |
| Gamma | Bi-212 | 727.3 keV | 6.65% |
| Gamma | Tl-208 | 583.2 keV | about 30.5%* |
| Gamma | Tl-208 | 2614.5 keV | about 35.8%* |
* Worked out from the cited values, at equilibrium with the progeny:
- 300.1 keV: 7.3 relative to the 238.6 keV line × 43.6%
- 583.2 keV: 85.2 relative × 0.99755 normalisation × 35.93% branch
- 2614.5 keV: 99.76% per Tl-208 decay × 35.93% branch
Detection and measurement
- Dose rate: an ionization chamber or energy-compensated meter. Check the meter is calibrated for, or has a known response to, high-energy photons.
- Surface contamination: an alpha/beta probe (dual-phosphor or gas proportional) sees the alphas; a GM pancake sees the betas. Hold an alpha probe close, since alphas travel only a few centimetres in air.
- Wipe tests: count in an alpha/beta counter, or a gamma counter with a window set for Pb-212 or its progeny.
- Dose calibrator: use a setting established for Pb-212 at a stated time after separation. The reading changes while the progeny grow in.
Shielding
- Have a physicist evaluate vial, syringe, waste and room shielding for the 2.6 MeV line before first use.
- Distance and time do more than usual, because thin lead does little at 2.6 MeV.
- A physical 0.5 mm layer of pure lead transmits about 97.6% of the uncollided 2.615 MeV photons (NIST narrow-beam model). This is not a prediction for a lead-equivalent apron; garment performance requires energy-specific data.
- Alphas need no shielding outside the body; the hazard is intake.
Contamination control
- Intake is the main hazard from an alpha emitter. Double gloves, no skin contact, covered cuts, and no eating or drinking in the area.
- Survey hands and clothing with an alpha-capable probe before leaving the area.
- Ask the RSO whether bioassay is needed after a spill or suspected intake.
Waste and decay-in-storage
- Ten half-lives is about 4.4 days, well within the NRC 120-day limit for decay-in-storage (10 CFR 35.92). Agreement States may differ.
- Survey without shielding before release, with an instrument that responds to the progeny emissions.
- If Pb-212 comes from a Th-228 or Ra-224 generator, check the supplier certificate for breakthrough. Th-228 (1.9 years) is not suitable for decay-in-storage.
Licensing
- The license must authorize Pb-212 in the form used. A license amendment and Authorized User review may be needed before the first patient.
Compared with Lu-177
- Alpha therapy through progeny, versus Lu-177’s beta.
- Hours instead of days: 10.64 h against 6.65 d, so deliveries and scheduling are tighter.
- A 2.6 MeV gamma (about 36% of decays) against Lu-177’s 113 and 208 keV photons. Shielding is a different problem.
- Contamination surveys need alpha capability, not only a GM pancake.
Calculated pure-lead thickness for 90% attenuation of selected photon lines
A tenth-value layer (TVL) reduces the uncollided photon fluence at one energy to 10% of its incident value. The Ac-225 and Pb-212 rows use photons from their daughters, Bi-213 and Tl-208.
Narrow-beam calculation; not a wall-design specification or a comparison of total dose.
- Tc-99m≈ 0.86 mm140.5 keV
- Lu-177≈ 2.23 mm208.4 keV
- Ac-225≈ 10.2 mmBi-213 daughter · 440.4 keV
- Pb-212≈ 46.6 mmTl-208 daughter · 2614.5 keV
These are representative lines, not the most energetic photons in each decay spectrum. For example, Lu-177 also emits a weaker 321.3 keV line, and Bi-213 has weaker lines above 1 MeV. Their contribution can matter after substantial shielding.
How the values are calculated
T(E, x) = exp[−(μ/ρ)(E) · ρ · x]
TVL [mm] = 10 · ln(10) / [(μ/ρ)(E) · ρ]
Here x is in cm, μ/ρ is in cm²/g, and lead density ρ = 11.35 g/cm³. Values use log–log interpolation between NIST total mass-attenuation coefficients, including coherent scattering. The factor 10 converts cm to mm. Values are approximate; μ/ρ is not the mass energy-absorption coefficient μen/ρ.
| Parent / emitter | Energy (keV) | μ/ρ (cm²/g) | TVL (mm) | Transmission through ⅛-inch lead sheeting (3.175 mm) |
|---|---|---|---|---|
| Tc-99mEmitter: Tc-99m | 140.511 | 2.3714 | 0.855 | 0.019% |
| Lu-177Emitter: Lu-177 | 208.366 | 0.91103 | 2.227 | 3.8% |
| Ac-225Emitter: Bi-213 | 440.440 | 0.19851 | 10.219 | 48.9% |
| Pb-212Emitter: Tl-208 | 2614.511 | 0.043567 | 46.565 | 85.5% |
For Tl-208: μ/ρ ≈ 0.043567 cm²/g; μ ≈ 0.494484 cm⁻¹; TVL ≈ 46.57 mm. Substituting this unrounded thickness gives T = 0.100. Through ⅛ inch (3.175 mm) of pure lead, approximately 85.5% of this line remains uncollided.
Lead sheeting used in construction
The comparison uses ⅛-inch lead sheeting (3.175 mm), a thickness available for lead-lined construction. It models the pure-lead layer alone, not a complete wall assembly. This is an illustrative thickness, not a standard requirement for every room. Actual barrier design depends on source activity, workload, distance, occupancy, the full spectrum, scatter, and continuity at joints and penetrations. A qualified shielding designer determines the required assembly. A garment’s lead-equivalent rating is a separate, energy-dependent specification.
Daughter activity changes with time
The 2614.511 keV line has 99.755% yield per Tl-208 decay. Bi-212 feeds Tl-208 through a 35.93% branch, giving approximately 35.84 photons per 100 completed Pb-212 decay chains when progeny are retained. That integrated yield is not a fixed instantaneous photon rate per unit Pb-212 activity: daughter ingrowth, separation and redistribution matter.
Do alpha therapies require additional shielding?
Not necessarily. Existing shielding may be adequate, but each therapy should be assessed before clinical use. Photon emissions from the radionuclide and its daughters, activity, workload, and room layout determine whether upgrades are needed. A qualified medical physicist or radiation protection specialist should evaluate the facility.
The selected-photon calculations on this page explain attenuation through pure lead. They do not determine whether a facility needs additional shielding or specify a wall thickness.
Why one TVL does not establish the full-spectrum dose reduction
For uncollided collision air kerma, each photon line is weighted by its emitting nuclide’s activity Aj(t), photon yield yji, energy Eji, and the air mass energy-absorption coefficient. At a fixed source–detector geometry:
RK(x,t) = Σ wji(t) exp[−μPb(Eji)x] / Σ wji(t)
wji(t) = Aj(t) yji Eji (μen/ρ)air(Eji)
Solve RK = 0.1 using the complete spectrum and time-dependent source composition for an uncollided collision-air-kerma TVL. A dose-equivalent calculation requires the relevant fluence-to-dose coefficients instead. Broad-beam scatter/buildup, secondary radiation, source and shield geometry, self-attenuation, and beta/bremsstrahlung contributions require additional modeling. This page does not compute those quantities and is not a clinical shielding design.
Data sources & emission-yield convention
- NIST mass attenuation coefficients for lead (Z = 82)
- NIST elemental densities (lead: 11.35 g/cm³)
- LNHB/DDEP Tc-99m evaluated decay table
- LNHB/DDEP Lu-177 evaluated decay table
- LNHB/DDEP Bi-213 evaluated decay table
- LNHB/DDEP Tl-208 evaluated decay table
- LNHB/DDEP Bi-212 branching evaluation
Selected-line yields per 100 decays of the actual emitter: Tc-99m 88.5; Lu-177 10.425; Bi-213 26.1; Tl-208 99.755. Yields do not enter the monoenergetic TVL equation; they do enter source-rate and spectrum-weighted calculations. The comparison uses 100–4000 keV, above the lead K edge. Tables checked October 18, 2026.
Sources
- IAEA Decay Data (DDEP evaluation), Pb-212
- IAEA Decay Data (DDEP evaluation), Bi-212
- IAEA Decay Data (DDEP evaluation), Tl-208
- IAEA Decay Data (DDEP evaluation), Po-212
Decay data checked against these sources in October 2026. Practice notes are general guidance for planning; your license, your procedures and your Radiation Safety Officer decide how Pb-212 is handled at your facility.
Preparing to use Pb-212?
License amendment, shielding evaluation, procedures and staff training, with a medical physicist.