Radioactive isotope, number nuclei

In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bom-bardmeiit of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called nuclear fission, the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi s neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table. 

More than 1500 radioactive isotopes have been prepared in the laboratory. The number of such isotopes per element ranges from 1 (hydrogen and boron) to 34 (indium). They are all prepared by bombardment reactions in which a stable nucleus is converted to one... 

The number of protons is unique to the element but most elements can exist with two or more different numbers of neutrons in their nucleus, giving rise to different isotopes of the same element. Some isotopes are stable, but some (numerically the majority) have nuclei which change spontaneously - that is, they are radioactive. Following the discovery of naturally radioactive isotopes around 1900 (see Section 10.3) it was soon found that many elements could be artificially induced to become radioactive by irradiating with neutrons (activation analysis). This observation led to the development of a precise and sensitive method for chemical analysis. 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

All hydrogen atoms have 1 proton in their nucleus (otherwise they wouldn t be hydrogen), but most (99.985%) have no neutrons. These hydrogen atoms, called protium, have mass number 1. In addition, 0.015% of hydrogen atoms, called deuterium, have 1 neutron and mass number 2. Still other hydrogen atoms, called tritium, have 2 neutrons and mass number 3. An unstable, radioactive isotope, tritium occurs only in trace amounts on Earth but is made artificially in nuclear reactors. As other examples, there are 13 known isotopes of carbon, only 2 of which occur commonly, and 25 known isotopes of uranium, only 3 of which occur commonly. 

Because the emission of an a particle from a nucleus results in a loss of two protons and two neutrons, it reduces the mass number of the nucleus by 4 and reduces the atomic number by 2. Alpha emission is particularly common for heavy radioactive isotopes, or radioisotopes Uranium-238, for example, spontaneously emits an a particle and forms thorium-234. 

The isotopes of a particular element have the same number of protons in the nucleus but a different number of neutrons, giving them the same proton number (atomic number) but a different nucleon number (mass number, i.e. number of protons + number of neutrons). Isotopes may be stable or radioactive. Radioactive isotopes (radioisotopes) disintegrate spontaneously at random to yield radiation and a decay product. 

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. 

All 17 known isotopes of berkelium are radioactive. The most stable is berkelium-247. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element s name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. A radioactive isotope is one that breaks apart and gives off some form of radiation. 

In general, when the number of protons and neutrons in the nucleus are almost the same (i.e. differ by one or two), the isotopes are stable. As Z and N numbers become more dissimilar, isotopes tend to be unstable and break down by radioactive decay (usually liberating heat) to a more stable isotope. Unstable isotopes are called radioactive isotopes (see Section 2.8). 

Nuclei differ in their stability, and some are so unstable that they undergo radioactive decay. The ratio of the number of neutrons to number of protons (N/Z) in a nucleus correlates with its stability. Calculate the N/Z ratio for (a) Sm (b) Fe (c) °Ne (d) ° Ag. (e) The radioactive isotope decays in a series of nuclear reactions that includes another uranium isotope, and three lead isotopes, Pb, °Pb, and ° Pb. How many neutrons, protons, and electrons are in each of these fi ve isotopes ... 

Elements above lead (atomic numbers 83 and above) on the Periodic Table are radioactive (see Figure 9.2). Other elements may have one or more radioactive isotopes. Some elements occur naturally, while others are manmade. Each symbol on the Periodic Table represents one atom of that element. An atom is made up of a nucleus with varying numbers of electrons in orbits circling around the nucleus (see Figure 9.3). Located inside the nucleus are protons and neutrons. Protons in the nucleus of an atom represent the atomic number of that element. Neutron numbers may vary within the same type of element or from one element to another, but the number of protons must stay the same. The atom is the smallest part of an element that normally exists so any particle of an element that is smaller than an atom is commonly referred to as a subatomic particle. 

The decay of radioactive isotopes via electron emission, so-called beta decay, is a well-known phenomenon, hi this mode imstable nuclei that have an excessive number of neutrons, for example can emit fast electrons, particles, in order to attain a stable nuclear configuration. Nuclei with insufficient neutrons, such as can obtain stability by emitting fast positrons, particles (the anti-matter equivalents of electrons). Both processes are classified as radioactive f) decay. In each case, the mass munber of the nucleus remains constant but the atomic number changes. There exist several positron emitting isotopes, of which and in particular... 

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