How RadMetric calculates photon shielding

Material composition and XCOM coefficients

For a mixture specified by elemental mass fractions, the mixture mass attenuation coefficient is formed from the mass-fraction-weighted elemental coefficients at each photon energy:

Here, w_j is the mass fraction of element j, and the fractions sum to 1. Compound formulas such as H₂O can be represented by their elemental mass fractions. XCOM supplies photon interaction coefficients for elements, compounds, and mixtures; RadMetric combines them with the entered bulk density and layer thickness.

Data source: NIST XCOM Photon Cross Sections Database.

Primary-photon transmission

For a straight path through shielding, the uncollided primary-photon transmission at energy E is estimated with Beer–Lambert attenuation:

The index k identifies each layer, ρ_k is its density, and x_k is its thickness along the path. The reported percentage is the share of the incident primary photons that remain in the direct beam without interacting. The complement is the share that has interacted; it is not a measurement of energy absorbed or dose.

For multiple photon energies with line intensities I_i, spectrum transmission is the fluence-weighted sum of the line transmissions:

HVL and TVL

The half-value layer (HVL) is the thickness where primary-photon transmission reaches 50%. The tenth-value layer (TVL) is where it reaches 10%. For a multi-energy spectrum, RadMetric solves for thickness using the summed line transmission; the effective HVL and TVL can therefore differ from the values for any single line as the spectrum hardens through the shield.

These values characterize attenuation of the direct photon beam for the specified material and spectrum. They are not dose-equivalent thicknesses and do not include scattered photons returning to a detector.

Air kerma rate estimate

For isotope sources, RadMetric evaluates eligible photon lines individually. For monoenergetic input, it assumes one emitted photon per decay. The air-kerma-rate constant uses each line’s energy and photon yield per decay with the NIST dry-air mass energy-transfer coefficient:

The implementation follows the NISTIR 7092A definition with a 10 keV photon cutoff and uses dry-air coefficients from NISTIR 4812. The tabulated coefficients are interpolated log-log quadratically at photon energies between reference points, so the calculated value is an estimate. For each photon line, the calculation applies its NIST XCOM narrow-beam transmission through the configured shield, then applies the entered activity and inverse-square point-source distance. The displayed air kerma rate is therefore the estimated rate after shielding for the configured geometry. In monoenergetic mode, the entered activity is interpreted with the one-photon-per-decay assumption. A Monte Carlo transport analysis can provide a higher-fidelity model of scattering, buildup, and complex geometry; it must still use appropriate inputs and validation.

Data and method references: NISTIR 7092A, NISTIR 4812, and Rafiei & Parsaei (2021), which reproduces the NIST/Higgins coefficient table. Kato (2014) is included as an independent coefficient study for comparison. Shield transmission uses NIST XCOM; see also NIST kerma coefficient definitions. Evaluated isotope photon emissions are sourced from IAEA LiveChart / ENSDF.