Editing Age of Earth (section)

===Invention of radiometric dating===
Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of [[radiometric dating]].
[[File:Ernest Rutherford 1908.jpg|thumb|[[Ernest Rutherford]] in 1908]]
[[Ernest Rutherford]] and [[Frederick Soddy]] jointly had continued their work on radioactive materials and concluded that radioactivity was caused by a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or [[Gamma ray|gamma]] radiation in the process. They also determined that a particular isotope of a radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "half-life", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product".

Some radioactive materials have short half-lives; some have long half-lives. Uranium and thorium have long half-lives and so persist in Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a [[stable element]]. These "[[decay chain]]s", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity and provided a basis for constructing techniques of radiometric dating.

The pioneers of radioactivity were chemist [[Bertram B. Boltwood]] and physicist Rutherford. Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904,<ref>{{cite book
 | first=E. | last=Rutherford | date=1906
 | title=Radioactive Transformations
 | url=https://archive.org/details/radioactivetran01ruthgoog | publisher=Charles Scribner's Sons
 | location=London }} Reprinted by Juniper Grove (2007) {{ISBN|978-1-60355-054-3}}.</ref> Boltwood was inspired to describe the relationships between elements in various decay series. Late in 1904, Rutherford took the first step toward radiometric dating by suggesting that the [[alpha particle]]s released by radioactive decay could be trapped in a rocky material as [[helium]] atoms. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms, but he would prove the connection four years later.

Soddy and Sir [[William Ramsay]] had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique. Rutherford wrote of addressing a meeting of the [[Royal Institution]] in 1904:

{{quote|I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the Earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye, and cock a baleful glance at me! Then a sudden inspiration came, and I said, "Lord Kelvin had limited the age of the Earth, provided no new source was discovered. That prophetic utterance refers to what we are now considering tonight, radium!" Behold! the old boy beamed upon me.<ref>{{cite book
 | first=Arthur Stewart | last=Eve | date=1939
 | title=Rutherford: Being the life and letters of the Rt. Hon. Lord Rutherford, O. M.
 | url=https://archive.org/details/rutherfordbeingl0000evea | url-access=registration | publisher=[[Cambridge University Press]]
 | location=Cambridge }}</ref>}}

Rutherford assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate and that helium did not escape from the sample over time. Rutherford's scheme was inaccurate, but it was a useful first step. Boltwood focused on the end products of decay series. In 1905, he suggested that lead was the final stable product of the decay of radium. It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with lead, and speculated that the radium–lead decay chain could be used to date rock samples. Boltwood did the legwork and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in 1907.<ref name="Boltwood" />

Boltwood's paper pointed out that samples taken from comparable layers of strata had similar lead-to-uranium ratios, and that samples from older layers had a higher proportion of lead, except where there was evidence that lead had [[Leaching (chemistry)|leached]] out of the sample. His studies were flawed by the fact that the decay series of thorium was not understood, which led to incorrect results for samples that contained both uranium and thorium. However, his calculations were far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Boltwood's 26 samples of 410 million to 2.2 billion years.<ref name="Boltwood" />