Alpha-Decay

An alpha particle (a) is a helium ion. Specifically, it is the +2 ion of the helium isotope He. Consisting of a bare nucleus that contains two protons and two neutrons, an alpha particle has a charge of +2 and a mass number of +4. 

Alpha decay tends to occur mostly among the heavy elements in the periodic table, elements with atomic numbers greater than 82 (lead). Because the number of protons in the parent isotope decreases by two, the atomic number of the daughter isotope (the decay product) is two units less than the atomic number of the parent isotope, and the daughter corresponds to an element that lies two squares to the left of the position occupied by the parent isotope. Because the number of neutrons also decreases by two, the mass number of the daughter isotope is four units less than the mass number of the parent. 

In alpha decay, the atomic number of the element decreases by two and the mass number decreases by four. Because its atomic number changes, the identity of the element changes. 

These equations illustrate two important conservation laws that must be obeyed when writing equations for nuclear reactions. Just as we do in an ordinary chemical reaction, we balance the number of atoms of each element on both sides of the equation. In a nuclear reaction, we also balance subscripts and superscripts on both sides of the equation. (Unlike the practice in an ordinary reaction, we do not concern ourselves with balancing charges in a nuclear reaction since in a nuclear reaction we are ignoring the electrons anyway and only indicating the nuclei of the atoms.) 

If the species in the reaction are nucUdes of elements, the subscripts represent the elements atomic numbers, or, equivalently, the charges on their nuclei. A fundamental physical law is the law of conservation of charge, which must be obeyed in all chemical and physical processes. 

Details of the decay of radionuclides are recorded in the form of decay schemes, in which the energy levels are plotted and the half-lives, the nuclear spins, the parity and the transitions are indicated. Nuclei with higher atomic numbers are put to the right, and energies are given in MeV. As an example, the decay scheme of is plotted in Fig. 5.1. 

As indicated in Table 5.1, jHe nuclei are emitted by a decay, and so the atomic number decreases by two units and the mass number by fom units (first displacement law of Soddy and Fajans). 

The energy A of a decay can be calculated by means of the Einstein formula AE = Am cP  

All a particles originating from a certain decay process are monoenergetic, i.e. they have the same energy. The energy of the decay process is spht into two parts, the kinetic energy of the a particle, E, and the kinetic energy of the recoiling nucleus, Fn  

Because the mass of heavy nuclei is appreciably higher than lhal of an a parliclc (wn m-y), is only about 2 /o smaller than AZ . 

An alpha particle is an He-4 nucleus, and the emission of this particle is commonly the preferred mode of decay at high atomic numbers, Z 83. In losing an alpha particle, the nucleus loses four units of mass and two units of charge  

Typical is the decay of the most common isotope of radium  

The product in this case is the most common isotope of radon, Rn (usually just called radon and which incidentally is responsible for the largest radiation dose from a single nuclide to the general population). A fixed quantity of energy, g, equal to the difference in mass between the initial nuclide and final products, is released. This energy must be shared between the Rn and the He in a definite ratio because of the conservation of momentum. Thus, the alpha-particle is mono-energetic and alpha spectrometry becomes possible. In contrast to beta decay, there are no neutrinos to take away a variable fraction of the energy. 

In many cases, especially in the lower Z range of a decay, the emission of an alpha particle takes the nucleus directly to the ground state of the daughter, analogous to the pure-(3 emission described above. However, with 

The energy that becomes available as a result of the emission of the alpha particle is called the decay energy Q, defined by 

Obviously, for a decay to occur, should be greater than zero. Therefore, a decay is possible only when 

If the daughter nucleus is left in its ground state, after the emission of the alpha, the kinetic energy of the two products is (from Eq. 3.40), 

In many cases, the daughter nucleus is left in an excited state of energy E,-, where i indicates the energy level. Then, Eq. 3.44 becomes 

Example 3.7 What are the kinetic energies of the alphas emitted by 

Many radioisotopes exist, but not all radioisotopes are created equal. Radioisotopes break down through three separate decay processes (or decay modes) alpha decay, beta decay, and gamma decay. The following sections show you equations detailing each type of decay. Note The symbols showing the isotope notation for each radioactive isotope cire as follows or 2 Y, where 

The first type of decay process, called alpha decay, involves emission of an alpha pcirticle by the nucleus of an unstable atom. An alpha particle (a particle) is nothing more exotic than the nucleus of a helium atom, which is made of two protons and two neutrons. Emitting an alpha 

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The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cms. 

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

An alplia p uticle is an energetic helium nucleus. The alplia particle is released from a radioactive element witli a neutron to proton ratio tliat is too low. The helium nucleus consists of two protons and two neutrons. The alplia particle differs from a helimn atom in that it is emitted witliout any electrons. The resulting daughter product from tliis tj pe of transformation lias an atomic number Uiat is two less tluin its parent and an atomic mass number tliat is four less. Below is an e. aiiiple of alpha decay using polonium (Po) polonium has an atomic mass number of 210 (protons and neutrons) and atomic number of 84. 

Plutonium has a much shorter half-life than uranium (24.000 years for Pu-239 6,500 years for Pu-240). Plutonium is most toxic if it is inhaled. The radioactive decay that plutonium undergoes (alpha decay) is of little external consequence, since the alpha particles are blocked by human skin and travel only a few inches. If inhaled, however, the soft tissue of the lungs will suffer an internal dose of radiation. Particles may also enter the blood stream and irradiate other parts of the body. The safest way to handle plutonium is in its plutonium dioxide (PuOj) form because PuOj is virtually insoluble inside the human body, gi eatly reducing the risk of internal contamination. 

Alpha carbon atoms, 348 Alpha decay, 417, 443 Alpha particle, 417 scattering, 245 Aluminum boiling point, 365 compounds, 102 heat of vaporization, 365 hydration energy, 368 hydroxide, 371 ionization energies, 269, 374 metallic solid, 365 occurrence, 373 properties, 101 preparation, 238. 373 reducing agent, 367 Alums, 403 Americium... 

Such alpha-recoil plays a fundamental role in fractionating the nuclides from one another in the low-temperature environment. During igneous processes, on the other hand, alpha recoil is probably not important in the generation of disequilibria ( °Th, Ra, and Pa). Beattie (1993) pointed out that the time scale of annealing of alpha decay damage at high temperatures was much shorter than the time scale of decay of these nuclides. 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Th, Th and Po, all decay by alpha emission and are thus measurable by isotope dilution and alpha spectrometry (Ivanovich and Murray 1992). However, " Th is produced by the alpha decay of and in turn decays by beta emission to via the short-lived intermediate " Pa (half-life 1.18 m) ... 

Kramish, A. Spontaneous fission versus alpha decay. Physic. Rev. 88, 1201... 

The recoil factors r define the probability of whether an attached radioactive atom desorbs from the particle surface in consequence of an alpha decay or not. Mercer and Strowe (1971) found a recoil factor = 0.81 in their chamber studies in contradiction to the value of ri 0.4 measured by Kolerski et al. (1973). No other results about the recoil factor are available in the literature. 

Most dosimetry models have incorporated the so-called Weibel A airway dimensions (Weibel, 1963) in order to calculate aerosol deposition, clearance and the density of alpha-decays per unit surface... 

It is seen that the diameters of bronchioles (averaged over generations 11 - 15) vary little with age. The increase in bronchial size is greater, but still less than might be expected if airways are simply scaled for overall body dimensions (illustrated by the dashed curves in Figure 9, which are functions of body weight W). Since bronchiolar diameter does not change much with age it is likely that the thickness of bronchiolar epithelium is also relatively constant. However, in the case of the bronchi, it is reasonable to assume that epithelial thickness is proportional to bronchial diameter. Thus, it is necessary to use age dependent conversion factors between the surface density of alpha-decays and dose to cells. 

The alpha particle is a helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions. Alpha decay often occurs among nuclei that have a favorable neutron/proton ratio, but contain too many nucleons for stability. The alpha particle is a massive particle consisting of an assembly of two protons and two neutrons and a resultant charge of +2. 

Alpha (a.) decay. As we shall see later, the alpha particle, which is a helium nucleus, is a stable particle. For some unstable heavy nuclei, the emission of this particle occurs. Because the a particle contains a magic number of both protons and neutrons (2), there is a tendency for this particular combination of particles to be the one emitted rather than some other combination such as s3Li. In alpha decay, the mass number decreases by 4 units, the number of protons decreases by 2, and the number of neutrons decreases by 2. An example of alpha decay is the following ... 

Radon-222 undergoes alpha decay according to the following balanced equation ... 

Sometimes it is difficult to predict if a particular isotope is stable and, if unstable, what type of decay mode it might undergo. All isotopes that contain 84 or more protons are unstable. These unstable isotopes will undergo nuclear decay. For these large massive isotopes, we observe alpha decay most commonly. Alpha decay gets rid of four units of mass and two units of charge, thus helping to relieve the repulsive stress found in the nucleus of these isotopes. For other isotopes of atomic number less than 83, we can best predict stability by the use of the neutron to proton (n/p) ratio. 

Know that nuclear stability is best related to the neutron-to-proton ratio (n/p), which starts at about 1/1 for light isotopes and ends at about 1.5/1 for heavier isotopes with atomic numbers up to 83- All isotopes of atomic number greater than 84 are unstable and will commonly undergo alpha decay. Below atomic number 84, neutron-poor isotopes will probably undergo positron emission or electron capture, while neutron-rich isotopes will probably undergo beta emission. 

In the second of their 1915 papers (Harkins and Wilson 1915b), Harkins and Wilson note from their study of the light elements (up to atomic number 27) that the main isotopic species had atomic masses which are integral multiples of 4. They concluded from this that, for those light nuclei, an important constituent must be the alpha particle just as it must be in the heavier radioactive nuclei which undergo alpha decay. In order to rationalize all the nuclei, including their nuclear charges, they... 

Po-209, the most stable isotope of polonium, decays into lead-205 by alpha decay. It costs about 3,000 per microcurie, which is a very small amount of polonium. 

ISOTOPES All 41 isotopes of astatine are radioactive, with half-lives ranging from 125 nanoseconds to 8.1 hours. The isotope As-210, the most stable isotope with an 8.1-hour half-life, is used to determine the atomic weight of astatine. As-210 decays by alpha decay into bismuth-206 or by electron capture into polonium-210. 

Helium in the Earth is replaced by the decay of radioactive elements in the Earths crust. Alpha decay produces particles f He ) known as alpha particles, which can become helium atoms after they capture two electrons. This new helium works its way to the surface of the Earth and escapes into the atmosphere where, in time, it escapes into space. 

ISOTOPES There are 37 isotopes of radon. All are radioactive. None are stable. They range in mass numbers from Rn-196 to Rn-228. Their half-lives range from a few microseconds to 3.8235 days for Rn-222, which is the most common. It is a gas that is the result of alpha decay of radium, thorium, or uranium ores and underground rocks. 

Actinium is an extremely radioactive, silvery-white, heavy metal that glows in the dark with an eerie bluish hght. It decays rapidly which makes it difficult to study, given that it changes into thorium and francium through electron capmre and alpha decay. Its melting point is 1,051°C, its boding point is 3,198°C, and its density is 10.07g/cm. ... 

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