How is thorium extracted from its ore

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Thorium is found in nature, it occurs mainly in the ore monazite, a complex phosphate made of thorium, uranium, cerium and lanthanides. With a half-life of 14 billion years, it is the starting isotope of the 4n + 0 decay cascade. For details of the decay, see below.

Thorium is sometimes still found in incandescent stockings (Auer stockings), such as those used in camping gas lamps. Non-burned-in mantles are well suited for recording the complex γ spectrum of thorium and its decay products. Incandescent mantles that have already been operated in a lamp are brittle and quickly disintegrate into radioactive powder, which makes handling very difficult. Depending on the manufacturing process, the thorium isotopes Th-232 and Th-228 are present in freshly manufactured incandescent mantles, the remaining elements of the decay chain only form over time. Because of the two isotopes, the secondary products below Th-228 do not simply arise exponentially. Corresponding to the half-life of thorium-228 of approx. 2 years and the half-life of radium-228 of almost 6 years, this results in a complicated course of the intensities.

The thorium oxide in the incandescent mantles ensures a particularly high light yield, as it is not a black body, but only emits little thermal infrared radiation and therefore emits more visible light (selective emitter). The radioactivity of the thorium is not relevant for the light yield. Because of the radioactive problem, newer mantles are mainly made of yttrium oxide, which is not radioactive but has a slightly less bright glow.

γ spectrum of a 15 year old thorium source

In the γ spectrum of thorium one finds lines of different isotopes of the decay chain. The following lines are clearly visible here, each from excited states of a nucleus after β-decay.

Th-228 *: 338.32 keV, 911.204 keV

Bi-212 *: 238.632 keV

Pb-208 *: 583.191 keV, 860.564 keV

There is also an overlay of Kα-Lines in the range of 80 - 90 keV. In contrast to radium, one can see an overlay of at least two lines here, since in addition to the component at 80 keV there is also one at 90 keV. Since the core de-excitation takes place via internal conversion in competition with the γ-emission, the γ-emitting isotopes are also good candidates for KαEmissions. The energies are 74.96 keV (Pb-208), 77.1 keV (Bi-212) and 93.3 keV (Th-228).

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