Nucleus a-particle

James Chadwick was happy to return to England in 1917. He had been studying in Germany at the outbreak of World War 1 and had been imprisoned there for four years. He was broke but alive. Fortunately, his old mentor Ernest Rutherford took him in. His job was to search for the neutral particle that Rutherford believed must exist in the atomic nucleus, a particle he called a neutron. 

Droplet nucleus A particle consisting of dried mucus in which micro-organisms are embedded. 

A nucleus that consists of neutrons and protons can be stable or unstable. Understanding of the factors that affect the nuclear stability is beyond the scope of this discourse, and it is not necessary to really understand for our purpose here. However, we have the facts i.e., which nuclei (isotopes) are stable and which are unstable. The unstable isotopes would not remain as such and spontaneously change into stable isotopes. In this process, the unstable nucleus emits particles and/or energy in the form of radiation (gamma (y)-ray). Particles that are emitted include electron (called i-particle), and helium nucleus (a-particle). Hence, these unstable isotopes are called radioactive, emitting a, p, and/or y radiation. There are other kinds of radiation, as well, but these three are the major ones. 

A positively charged subatomic particle equivalent to a helium nucleus (a). 

OC-Decay. In a-decay the parent atom of atomic number Z and mass M emits an a-particle, a He nucleus having Z = 2 and A = 4 and becomes an atom having atomic number Z — 2 and mass A — 4. From the conservation of energy, the energy of the a-particle is... 

In this decay process, only one particle is emitted and, because energy is conserved, for each level in the daughter nucleus there is a unique a-particle energy. This means that a measurement of the differences in the energies of the a-particles emitted in a radioactive decay gives expHcidy the differences in the energies of the levels in the daughter nucleus. 

The recoiling a-particle ( He) and lithium nucleus (]U) between them carry 2.4 MeV of energy and this is shed within just a few m, the a-particle travelling about 9fim and the Li nucleus about S.S im in the opposite direction. The radiation damage is thus confined within the cancerous tissue alone. 

The roots of the LBNL can be traced back to the 1920s, and the pursuit of the secrets of the nucleus. Ernest O. Lawi ence, built the first large cyclotron (a particle accelerator) on the Berkeley campus of the University of California in 1931. Unlike most the other labs, LBNL s beginnings depended on the support of philanthropists who saw the promise in Lawrence s work. Seeking private sector support, an... 

Nuclei suitable for fusion must come near each other, where near means something like the nuclear radius of 10" cm. For positively charged nuclei to make such a close approach it requires large head-on velocities, and therefore multimillion-degree Celsius temperature. In contrast, fission can occur at normal temperatures, either spontaneously or triggered by a particle, particularly an uncharged neutron, coming near a fissionable nucleus. 

This means that most of the mass of the atom must be furnished by the nucleus. However, the mass of the nucleus is not determined by the number of protons alone. For example, a helium nucleus has two protons and a hydrogen nucleus has one proton. Yet a helium atom is measured to be four times heavier than a hydrogen atom. What can be the composition of the helium nucleus A partial answer to this problem was obtained when a third particle, the neutron, was... 

You have been told that the atomic nucleus bears a positive charge and is surrounded by a number of negatively charged particles called electrons. Also, the nucleus is supposed to contain most of the mass of the atom and to be made of protons and neutrons, each of which has nearly two thousand times the mass of the electron. How do we know that atoms are built this way How do we know that there is such a particle as an electron Again, weight relations associated with chemical reactions provide key evidence. 

The products are an a-particle (a helium nucleus), and a thorium isotope that is unstable and that rapidly decays by emitting successively two electrons ... 

Alpha particles (a particles), which are equivalent to the nucleus of a helium atom (two neutrons and two protons)... 

An electron in an atom is like a particle in a box, in the sense that it is confined within the atom by the pull of the nucleus. We can therefore expect the electron s wavefunctions to obey certain boundary conditions, like the constraints we encountered when fitting a wave between the walls of a container. As we saw for a particle in a box, these constraints result in the quantization of energy and the existence of discrete energy levels. Even at this early stage, we can expect the electron to be confined to certain energies, just as spectroscopy requires. 

When Rutherford allowed the radiation to pass between two electrically charged electrodes, he found that one type was attracted to the negatively charged electrode. He proposed that the radiation attracted to the negative electrode consists of positively charged particles, which he called a particles. From the charge and mass of the particles, he was able to identify them as helium atoms that had lost their two electrons. Once Rutherford had identified the atomic nucleus (in 1908, Section B), he realized that an a particle must be a helium nucleus, He2+. An a particle is denoted or simply a. We can think of it as a tightly bound cluster of two protons and two neutrons (Fig. 17.5). 

FIGURE 17.5 An a particle has two positive charges and a mass number of 4. It consists of two protons and two neutrons and is the same as the nucleus of a helium-4 atom. 

The most common types of radiation emitted by radioactive nuclei are a particles (the nuclei of helium atoms), /3 particles (fast electrons ejected from the nucleus), and 7 rays (high-frequency electromagnetic radiation). 

FIGURE 17.7 When a nucleus ejects an a particle, the atomic number of the atom decreases by 2 and the mass number decreases by 4. The nucleons ejected from the upper nucleus are indicated by the blue boundary. 

Fig. 17.7), is therefore the nucleus of an atom of a different element. For example, when a radon-222 nucleus emits an a particle, a polonium-218 nucleus is formed. In this case, a nuclear transmutation, the conversion of one element into another, has taken place. Another important difference between nuclear and chemical reactions is that energy changes are very much greater for nuclear reactions than for chemical reactions. For example, the combustion of 1.0 g of methane produces about 52 kj of energy as heat. In contrast, a nuclear reaction of 1.0 g of uranium-235 produces about 8.2 X 10 kj of energy, more than a million times as much. 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

C22-0054. Identify the compound nucleus and final product resulting from each of the following nuclear reactions (a) carbon-12 captures a neutron and then emits a proton (b) the nuclide with eight protons and eight neutrons captures an a particle and emits a y ray and (c) curium-247 is bombarded with boron-11, and the product loses three neutrons. 

The symbols and are, respectively, the laplacian operators for a single nucleus and a single electron. The variable is the distance between nuclei a and / , Vai the distance between nucleus a and electron i, and the distance between electrons i and j. The summations are taken over each pair of particles. The quantity e is equal to the magnitude of the electronic charge e in CGS units and to e/(47reo) / in SI units, where eo is the permittivity of free space. 

Biolistics involves bombarding plant cells with tiny (4- am) microprojectiles made of gold or tungsten. These microprojectiles are coated with DNA and are propelled at high velocity from a particle gun or gene gun into plant tissue or cells. In this method, the projectile penetrates the cell wall and carries the transgene into the cell nucleus. 

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