Nuclide reference

What is on each sheet, and how it is worked out

Each sheet gives the half-life, decay mode and specific activity, the photon, beta and alpha lines that the evaluated data records, and — where there are photons above the cutoff — the air kerma rate constant and the dose rate a given activity produces at a metre. None of it is copied from a published table of constants. The emission data come from the IAEA evaluated file and the photon interaction coefficients from NIST, and every derived quantity is recomputed from those two.

Emission probability is not dose contribution

A line's emission probability says how often it appears per hundred decays. Its share of the dose rate also carries its energy and the absorption coefficient of air at that energy, so the two rankings differ — a weak, hard line can outrank a strong, soft one. Each sheet prints both columns side by side and says how many lines it takes to account for ninety per cent of the dose rate. For most nuclides the answer is one or two out of a list of twenty or more.

Half-value layers are solved for the spectrum, not for one energy

The usual shortcut is to pick a representative energy, look the attenuation coefficient up, and divide. That is exact only for a nuclide with a single line. When there are several, each one is attenuated at its own rate, and the thickness that halves the total is not the thickness that halves any one of them. Sodium-22 shows the size of the error plainly: read from its strongest line at 1275 keV the lead half-value layer looks like 10.5 mm, but that line carries only 55% of the kerma and the rest is 511 keV annihilation radiation, so the real figure is 6.5 mm.

These sheets solve for the thickness that leaves half — then a tenth — of the whole spectrum's air kerma rate, line by line. The figures are narrow beam: scattered radiation is not added back, so a shield designed to these numbers will let more through than they suggest. The gamma shielding calculator takes a buildup factor when the answer has to be a design figure.

When half-value layers stop being additive

For a single energy the tenth-value layer is always 3.32 times the half-value layer, and stacking three half-value layers leaves an eighth. Neither holds for a broad spectrum. The first layer removes the soft part cheaply and what comes through is harder than what went in, so the second layer removes less than the first. Tin-113 is the extreme case here, with a tenth-value layer eighty-four times its half-value layer. Every sheet prints the ratio so the reader can see when the rule of thumb has stopped working.

Nuclides with no photon above the cutoff

51 of the 147 nuclides here have no photon line above the conventional 20 keV cutoff, so their air kerma rate constant is zero and no external dose rate can be quoted. They are not rare or exotic — they include tritium, carbon-14, nickel-63, strontium-90, technetium-99 and iodine-129, which is to say most of what matters in waste characterisation and decommissioning. Calculators built around a table of gamma constants either omit them or return zero without saying why. Their sheets give what does apply: beta endpoint and mean, range in common absorbers, bremsstrahlung yield against atomic number, and specific activity.

Beta shielding is a low-atomic-number job

Stopping a beta is a matter of enough material, and the range follows from the endpoint energy. What changes with the choice of material is how much of the beta energy leaves again as bremsstrahlung, which rises with atomic number. Shielding a strong beta emitter with lead converts a particle you could have stopped into a photon you now have to stop. Each beta sheet prints the yield in acrylic and in lead so the comparison is in front of the reader.

What these sheets do not tell you

Every figure here is an external quantity for a bare point source in air. Nothing on these sheets is an internal dose: committed effective dose per becquerel taken in depends on chemical form and biokinetics, and this site does not compute it. A real source has self-absorption, a capsule and a geometry, and each of those lowers what a meter would actually read. Shielding thicknesses are narrow beam, so scatter is not added back and a design needs more than they say. Where a nuclide emits betas, the range is an empirical fit and skin dose from a contamination is a different calculation again — it needs source geometry and a point kernel, which this site does not do. Each sheet repeats only the part of this that its own numbers make specific.