Part 5

[13]For more on this family of elements, see _Rare Earths, The Fraternal Fifteen_, a companion booklet in this series.

[14]For a fuller explanation of the fission process, see _Our Atomic World_, another booklet in this series.

[15]Neutrons that have had their speed reduced by passing through a moderator (graphite, for example) which is built into every reactor to accomplish this very thing. For more about how this is done, see _Nuclear Reactors_ and _Research Reactors_, companion booklets in this series.

[16]Note that some radionuclides sometimes decay by one method, sometimes by another. For example, 98.8% of the nuclei of actinium-227 emit a beta particle to form thorium-227; the remaining 1.2% emit an alpha particle to form francium-223; both of these daughter products decay to radium-223.

[17]Some of the protactinium (0.12%) changes by an intermediate step, known as isomeric transition, in which its nucleus shifts to a lower energy state. The process does not alter the remaining parent-daughter progression in the series.

[18]Undergoes both alpha and beta decay, in definite proportion of decay events, as shown.

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