Radionuclide Guide
What changes when a new radionuclide arrives
Operational profiles for the radionuclides coming into theranostics after Lu-177: the decay chain, which instruments see it, shielding, contamination control, waste and licensing. Written for RSOs, physicists and technologists, with every number cited.
- Pb-212Lead-212 · half-life 10.64 hours
Beta emitter whose short-lived progeny give one alpha particle per decay
One alpha particle per Pb-212 decay, from Bi-212 or Po-212.
Read the profile - Tb-161Terbium-161 · half-life 6.953 days
Beta emitter with abundant conversion and Auger electrons
Handled much like Lu-177, with the same chemistry and a similar half-life.
Read the profile - Ac-225Actinium-225 · half-life 9.92 daysProfile in preparation
Alpha emitter with four alpha particles in its decay chain
A profile is in preparation. The Ac-225 Decay Explorer already covers its daughters.
Ac-225 Decay Explorer - At-211Astatine-211 · half-life 7.214 hoursProfile in preparation
Alpha emitter, produced in a cyclotron
A profile is in preparation.
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.
General guidance for planning. Your license, your procedures and your Radiation Safety Officer decide how a radionuclide is handled at your facility.
Adding a new radionuclide to your program?
License amendments, shielding evaluation, procedures and staff training, with a medical physicist.