Mass change, from radioactive decay

Radioactive decay is a nuclear process from an intrinsically unstable nucleus that emits alpha particles, beta particles and gamma rays. The loss of mass from the nucleus changes the element to one of a lower mass. Carbon dating uses the decay of the 14C nucleus, a heavy and unstable isotope of carbon, to become the stable 14N isotope. The overall process is written ... 

Often the expected change in 210Pb concentration with depth (obtained from the logarithmic plot of unsupported activity as a function of the overlying mass of dry sediment accumulated) shows variations which are outside the analytical errors expected from the measurement of radioactive decay. Possible reasons for these discrepancies are ... 

Aluminum also has 14 radioactive isotopes. A radioactive isotope gives off either energy or subatomic particles in order to reduce the atomic mass and become stable. When the emission produces a change in the number of protons, the atom is no longer the same element. The particles and energy emitted from the nucleus are called radiation. The process of decaying from one element into another is known as radioactive decay. 

Radioactivity is characterized by the emission of energy (electromagnetic or in the form of a particle) from the nucleus of an atom, usually with associated elemental conversion. There are four basic types of radioactive decay (Table 5.4), of which alpha (a) and beta (p ) decay are most common in nature. Alpha emission is the only type of decay that causes a net mass change in the parent nuclide by loss of two protons plus two neutrons. Because two essentially weightless orbiting electrons are also lost when the equivalent of a helium nucleus is emitted, the parent nuclide transmutes into a daughter element two positions to the left on the periodic table. Thus decays by ot... 

In naturally occurring uranium minerals we should and do find some lead-206 isotopes formed by radioactive decay. Assuming that no lead was present when the mineral was formed and that the mineral has not undergone chemical changes that would allow the lead-206 isotope to be separated from the parent manium-238, it is possible to estimate the age of the rocks from the mass ratio of 2 Pb to 23 u. The above equation tells us that for every mole, or 238 g, of manium that undergoes complete decay, 1 mole, or 206 g, of lead is formed. If only half a mole of manium-238 has undergone decay, the mass ratio Pb-206/U-238 becomes... 

Budget methods. Vertical exchange rates and turbulent diffusivities Kz can be calculated from the heat balance or the mass balance of tracers for which transformation rates are known. Assuming horizontal homogeneity, the temporal change of tracer mass below a given depth z must be the sum of the net vertical mass flux through the cross-section at z and all sources and sinks of tracer mass below z. In the case of conservative tracers sources and sinks below z must be mass fluxes across the sediment-water interface. In the case of H, radioactive decay is an additional sink. In the case of He, tritium decay represents a source. If the increase of mass due to all sources and sinks, Sm, is known, the net mass flux can be calculated ... 

Beta particles are high-energy electrons which are ejected from the nucleus. Since there are not normally any electrons in the nucleus, the beta particles must have been produced in the nucleus during radioactive decay. It is now known that during this process, a neutron changes into a proton and an electron. From this it follows that the atomic number (the number of protons) of the nuclide increases by one, while the mass number (the number of protons and neutrons) remains the same. The electron escapes from the nucleus and is now called a beta particle. The general equation is - e-... 

The second type of corpuscular radiation in radioactive decay is B"-radiation. In this case, negatively charged electrons are ejected from the radionuclide. The atomic mass number of the radionuclide does not change. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

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