Beta-plus decay

Beta-plus In beta-plus decay, a proton in the nucleus decays into a neutron, a positron ( Je), and a tiny, weakly interacting pcirticle called a neutrino (v). This decay decreases the atomic number by 1. The mass number, however, does not change. Both protons and neutrons cire nucleons (pcirticles in the nucleus), after all, each contributing 1 atomic mass unit. The general pattern of beta-plus decay is shown here ... 

Beta-minus Beta-minus decay essentially mirrors beta-plus decay. A neutron converts into a proton, emitting an electron and an anftneutrino (which has the same symbol as a neutrino except for the line on top). Particle and antiparticle pairs such as neutrinos and antineutrinos are a complicated physics topic, so we ll keep it basic here by saying that a neutrino and an antineutrino would annihilate one another if they ever touched, but they re otherwise very similar. Again, the mass number remains the same after decay because the number of nucleons remains the same. However, the atomic number increases by 1 because the number of protons increases by 1 ... 

Note that the nuclear reaction is balanced, as previously described. It turns out that there are two types of beta decay, beta-plus decay (which produces a positron) and beta-minus decay (which produces an ordinary electron). Note, however, that this electron, although indistinguishable from any other electron, is the product of the decay, or the falling apart, of a nucleus. Beta-minus and beta-plus decays are denoted by placing a )8 or a over the arrow. Tritium decays by beta-minus decay, as shown in Equation (10.16) ... 

The half-lives of uranium-238 with respect to alpha decay, tritium with respect to beta-minus decay, and boron-8 with respect to beta-plus decay are 4.51 X 10 (4.51 billion) years, 12.3 years, and 0.77 years, respectively. 

Note that argon is in significantly greater supply than the others. Why should this be The answer lies in the amount of argon-40 produced by the beta-plus decay of potassium-40. Recall (see p. 343) that this decay scheme has been employed to indirectly determine the age of various hominids like the Australopithecus afarensis called Lucy. Although argon gas remains trapped in some rock samples and is therefore useful for chronometric determinations, most of it escapes into the open atmosphere. 

Neutrinos are created in nuclear processes and in various elementary particle interactions. The most familiar process is nuclear beta-decay, in which an unstable nucleus simultaneously emits an electron (beta-ray) and a neutrino. This process may be visualized as an unstable nucleus radiating its energy by creating a pair of leptons a neutrino and an electron. It is referred to as beta-minus decay when an electron (e ) is emitted with an antineutrino Ve) or beta-plus decay when a positron (e+) is emitted with a neutrino (Vg). In another beta-decay process, called electron capture, one of the orbital electrons in an atom is absorbed by the nucleus and a neutrino is emitted. Examples of these processes are... 

Neutrino (V)—A neutral particle of infinitesimally small rest mass emitted during beta plus or beta minus decay. This particle accounts for conservation of energy in beta plus and beta minus decays. It plays no role in damage from radiation. 

The second type of decay, called beta decay (fi decay), comes in three forms, termed beta-plus, beta-minus, and electron capture. All three involve emission or capture of an electron or a positron (a pcirticle with the tiny mass of an electron but with a positive chcirge), and all three also change the atomic number of the daughter atom. 

Positron emission—A proton turns into a neutron plus a positively charged electron known as a positron or beta-plus particle. As with electron emission there s another particle included this time a neutrino instead of an antineutrino. An isotope of fluorine decays into oxygen as follows ... 

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... 

Radionuclides are, by definition, unstable and decay by one, or more, of the decay modes alpha, beta-minus, beta-plus, electron capture or spontaneous fission. Although strictly speaking a de-excitation rather than a nuclear decay process, we can include isomeric transition in that list from the mathematical point of view. The amount of a radionuclide in a sample is expressed in Becquerels -numerically equal to the rate of disintegration - the number of disintegrations per second. We refer to this amount as the activity of the sample. Because this amount will change with time we must always specify at what time the activity was measured. 

An example of an isotope that decays by beta-plus emission is boron-8, the longest-lived radioactive isotope of boron. The reaction for its decay is represented in Equation (10.17) ... 

Potassium-40 decays via both beta-minus and beta-plus emission. Write a nuclear equation for each process. 

The longest-lived radioactive isotope of oxygen is 0, which decays via beta-plus emission with a half-life of 124 s. Write a nuclear equation for this decay process. 

Other applications of these elements take advantage of their nuclear properties. For example, one prevalent method of establishing the age of early humanoids is the potassium-argon dating procedure developed in the 1950s. Potassium-40 has a half-life of 1.3 billion years and decays by either beta-minus or beta-plus emission, as shown in Equations (12.22) and (12.23) ... 

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

Beta particles are emitted when a neutron is converted to a proton plus an electron and the electron is lost. Unlike the discrete energy emissions from the decay of alpha particles, beta particles are emitted along a spectrum of energies, because energies are shared between positive and negative electrons. Positrons are emitted when a proton becomes a neutron and decays by beta emission or an electron is captured. These are competing processes, and both occur with about the same frequency (Harley, 2001, 2008). 

Write a nuclear reaction to represent cobalt-60 decaying to nickel-60 plus a beta particle plus a gamma ray. 

Each radionuclide among the more than one thousand that are known has a unique decay scheme by which it is identified. For this reason, among others, researchers have studied decay schemes over the years and their reported information has been compiled and periodically updated. The compiler surveys the reported information for each radionuclide and attempts to select the most reliable information for constructing a self-consistent decay scheme. The fraction of beta particles that feed an excited state must match the fraction of gamma rays plus conversion electrons emitted by the excited state. The energy difference between any two states must be consistent with the energies of the transition radiations plus the recoil energy of the atom that emitted the radiations. 

The Sr decay scheme in Fig. 9.3 is simple. For practical purposes, the radionuclide emits a beta particle group with a maximum energy of 1495 keV (plus the associated neutrino group discussed in Section 2.2.2). The half-life is 50.5 days. An obscure beta particle group of 590 keV maximum energy, followed by gamma rays of 909 keV with an intensity of about 0.01%, is of no practical consequence in radioanalytical chemistry. 

A beta particle, (3 , is an electron in all respects it is identical to any other electron. Following on from Section 1.1, the sum of the masses of the "Ni plus the mass of the (3 , and i>, the anti-neutrino, are less than the mass of "Co. That mass difference drives the decay and appears as energy of the decay products. What happens during the decay process is that a neutron is converted to a proton within the nucleus. In that way the atomic number increases by one and the nuclide drops down the side of the valley to a more stable condition. A fact not often realized is that the neutron itself is radioactive when it is not bound within a nucleus. A free neutron has a half-life of only 10.2 min and decays by beta emission ... 

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