Argon radioactivity

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). 

Naturally occurring argon is a mixture of three isotopes. Twelve other radioactive isotopes are known to exist. 

From the radioactive decay constants and measurement of the amount of argon in a rock sample, the length of time since formation of the rock can be estimated. Essentially, the dating method requires fusion of a rock sample under high vacuum to release the argon gas that has collected through radioactive decay of potassium. The amount of argon is determined mass spectrometrically,... 

Argon-40 [7440-37-1] is created by the decay of potassium-40. The various isotopes of radon, all having short half-Hves, are formed by the radioactive decay of radium, actinium, and thorium. Krypton and xenon are products of uranium and plutonium fission, and appreciable quantities of both are evolved during the reprocessing of spent fuel elements from nuclear reactors (qv) (see Radioactive tracers). 

Helium is the second most abundant element in the universe (76% H, 23% He) as a result of its synthesis from hydrogen (p. 9) but, being too light to be retained by the earth s gravitational field, all primordial helium has been lost and terrestrial helium, like argon, is the result of radioactive decay ( He from a-decay of heavier elements, " °Ar from electron capture by... 

Finally, P also differs from other elements in that it is overwhelmingly dominated by a single, stable isotopic form containing 15 protons and 16 neutrons. There are only two naturally occurring radioactive forms of P P and P, which are produced in the atmosphere by nuclear reactions with argon. A small amount of P is... 

If the amount of a radioactive nuclide in a rock sample is N, the sum of this amount plus the amount of its product nuclide is A/q. For argon dating, Nq is the sum of potassium-40 and argon-40 present in a sample of rock. Assuming that Ar gas escapes from molten rock but is trapped when the rock cools and solidifies, the lifetime obtained by substituting these values into Equation is the time since the rock solidified. Such analyses show that the oldest rock samples on Earth are 3.8 X 10 years old. 

The electron capture detector is another type of ionization detector. Specifically, it utilizes the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the carrier gas molecules, thus generating electrons that constitute an electrical current. As an electrophilic component, such as a pesticide, from the separated mixture enters this detector, the electrons from the carrier gas ionization are captured, creating an alteration in the current flow in an external circuit. This alteration is the source of the electrical signal that is amplified and sent on to the recorder. A diagram of this detector is shown in Figure 12.13. The carrier gas for this detector is either pure nitrogen or a mixture of argon and methane. 

Krypton is expensive to produce, which limits its use as an inert gas. It is used in a mixture with argon to fill incandescent light bulbs, fluorescent lamps, lasers, and high-speed photography lamps. Radioactive Kr-85 is used as a source of radiation to measure the thickness of industrial materials. It is also used to test for leakage of scientific instruments. 

Plutonium is by far one of the most toxic radioactive poisons known. The metal, its alloys, and its compounds must be handled in a shielded and enclosed glove box that contains an inert argon atmosphere. It is a carcinogen that can cause radiation poisoning leading to death. 

Historically, chlorine was the first target used to trap neutrinos. Chlorine-37 is mainly sensitive to high-energy neutrinos emanating from marginal fusion reactions (2 out of 10000) which lead to production of boron-8. On rather rare occasions, under the impact of neutrinos, chlorine-37 is transformed into radioactive argon-37 which is easy to detect by its radiation. However, the myriads of low-energy neutrinos completely escape its notice. 

The element was discovered by Lord Raleigh and Sir William Ramsay in 1894. Argon is the third most abundant element in the atmosphere. Its concentration in air is 0.934% by volume. Also, it occurs in earth s crust at a concentration of 3.4 mg/kg, and in the sea water at 4.3 pg/L. It was most likely formed in earth crust by radioactive decay of K-40 and seeped out into the... 

Actinide metal samples are characterized by chemical and structure analysis. Multielement analysis by spark source mass spectrometry (SSMS) or inductively coupled argon plasma (ICAP) emission spectroscopy have lowered the detection limit for metallic impurities by 10 within the last two decades. The analysis of O, N, H by vacuum fusion requires large sample, but does not distinguish between bulk and surface of the material. Advanced techniques for surface analysis are being adapted for investigation of radioactive samples (Fig. 11) ... 

---