Potassium-38, radioactive decay

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). 

Write the nuclear equation for the radioactive decay of potassium-40 by beta emission. Identify the parent and daughter nuclides in the decay. 

Potassium has three naturally occurring isotopes 39K (93.08%), 40K (0.01%), and 41K (6.91%). The radioactive decay of 4,)K to argon (40Ar), half-life of 109 years, makes it a useful tool for geological dating. Some physical properties of potassium are summarized in Table 1 (1—3). 

Geothermal heat that warms the Earth from underground originates from the radioactive decay of atoms such as uranium, thorium, radium, potassium... and provides significantly milder conditions than those that would result from the mere heat balance between the heat flux from the sun and the heat loss by radiation to the deep black space. 

We know that nuclear weapons are capable of mass destruction, yet radiation therapy, shown in Figure 4.18, is a proven cancer fighter. Smoke detectors, required by law in all homes, rely on the radioactive decay of americium-241. The human body itself is radioactive, due to the presence of radioactive isotopes including carbon-14, phosphorus-32, and potassium-40. Most people view radioactivity and nuclear reactions with a mixture of fascination, awe, and fear. Since radioactivity is all around us, it is important to understand what it is, how it arises, and how we can deal with it safely. 

Radioactive substances are widely distributed on the earth. Some are found in the atmosphere, but the major part is present in the lithosphere. The most important ones are the ores of uranium and thorium, and potassium salts, including the radioactive decay products of uranium and thorium. Uranium and thorium are common elements in nature. Their concentrations in granite are about 4 and 13mg/kg, respectively, and the concentration of uranium in seawater is about 3 pg/l. Some uranium and thorium minerals are listed in Table 1.1. The most unportant uranium mineral is pitchblende (UsOs). Uranium is also found in mica. The most important thorium mineral is monazite, which contains between about 0.1 and 15% Th. 

Potassium-40, which has a half-life of 1.28 billion years, represents only about 0.012% of the potassium present in Earth today. Potassium-40 is useful for dating ancient rocks and minerals. Potassium-40 produces two different isotopes in its radioactive decay. About 11% of the potassium-40 in a mineral decays to argon-40 by emitting a positron. 

Potassium forms 2.50% of earth s crust and was first isolated in 1807. It is one of the most reactive metals. Radioactive decay of to " °Ar is used as a tool in geological dating. 

Where the number of both protons and neutrons in an atom is known we are able to identify a specific isotope of a specific element and this is termed a nuclide. Some naturally occurring elements are radioactive and specific isotopes of these elements are called radionuclides. This term implies that their nuclei are unstable and spontaneously decay, transforming the nucleus into that of a different element. Radioactive decay is written in equations that look a little like those for chemical reactions, but they need to express the atomic mass of the elements involved and the type of rotation emitted. A number of modes of radioactive decay are possible, and here we outline some of the common ones. The decay of potassium (40K) can be written ... 

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). 

With its predictable and unchanging rates, radioactive decay has provided scientists with a technique for determining the age of fossils, geological formations, and human artifacts. Using a knowledge of the half-life of a given radioisotope, one can estimate the age of an object in which the iso- tope is found. Four different isotopes are commonly used for dating objects carbon-14, uranium-238, rubidium-87, and potassium-40. Now look at one of these techniques in more detail. 

Potassium is also famous for one of its isotopes, radioactive potassium-40, which has a long half-life of 1.25 billion years. (Half-life refers to the amount of time it takes for half of the elements atoms to disintegrate.) Potassium-40 occurs naturally and is used by researchers to determine the age of rocks. As potassium-40 decays, it becomes a noble gas called argon. By determining how much argon is present in a rock, researchers can estimate the rock s age. Using this technique, scientists have estimated some rocks on Earth to be as old as 3.8 billion years. 

---