Alpha particle Argon

The alkali metals are not found free in nature, because they are so easily oxidized. They are most economically produced by electrolysis of their molten salts. Sodium (2.6% abundance by mass) and potassium (2.4% abundance) are very common in the earth s crust. The other lA metals are quite rare. Francium consists only of short-lived radioactive isotopes formed by alpha-particle emission from actinium (Section 26-4). Both potassium and cesium also have natural radioisotopes. Potassium-40 is important in the potassium-argon radioactive decay method of dating ancient objects (Section 26-12). The properties of the alkali metals vary regularly as the group is descended (Table 23-1). 

The noble gases are in scarce supply on Earth. Argon is the most abundant because it is the product of the beta decay of potassium-40. Alpha particles are just helium nuclei and are the principal source of helium that concentrates in natural gas deposits. Although not retained by Earth s gravitational field, helium is the second most abundant element in the cosmos and plays a central role in the proton-proton cycle of stars and nucleosynthesis. 

The affects of photosensitive dopants added to the pure liquids have also been measured, both with electrons and alpha particles. We observe an improved energy resolution only in the case of alpha particles. In pure liquid argon, the 5.5 MeV alpha peak was measured with about 5Z fwhm at a field of 38 kV/cm. In argon doped with 14 ppm of allene, the same resolution could be observed already with a field of less than 20 kV/cm. 

The Geiger counter, invented in 1928 and named after one of its two inventors, H. Geiger and E.W. MuUer, counts particles emitted by radioactive nuclei in a non-reactive noble gas, like argon. Alpha and beta particles are detected this way. 

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