Astronomy of radioactivity

Unlike stellar spectroscopy, the analysis of meteoritic grains and inclusions can provide an extremely precise isotopic breakdown. The weak point of this technique, however, is that the exact characteristics of the stars from which the grains formed can only be inferred. When we detect fight, we can deduce its celestial source by extending back its fine of incidence and we can determine the composition of the source from the spectral lines it contains. But we do not know where the meteorite grains came from, and only their composition can tell us anything of their origins. 

Each known type of grain is made from a particularly refractory form of material. Themselves born in extreme heat conditions, these grains survived the formation of the Solar System without the least difficulty. They have been able to carry down the isotopic composition of their source quite intact, throughout the whole prehistory of the Sun. But their message has not yet been perfectly decoded. The story of this star dust will therefore be continued, especially as it is radioactive and can be identified by its gamma emissions. 

The best illustration of radioactive astronomy is titanium-44. We shall take it as the archetype of a good radioactive isotope. It is relatively abundant and has a reasonable lifetime of around 100 years, neither too long, nor too short. Only aluminium-26 can rival it in this respect and nuclear gamma astronomy has already reaped some of the rewards (see Fig. 4.4). 

Calcium-44 bears the same relation to titanium-44 as a grandson to his grandfather. The radioactive affiliation is 

Titanium-44 transmutes to scandium-44 by emitting two gamma rays, at 68 and 78 keV. The new nucleus then transmutes to calcium-44 but not before emitting a gamma ray at 1.157 MeV. 

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