Neutrons with the Nucleus

Immediately following the discovery of the neutron, it was realized that the new particle would be an ideal tool for inducing nuclear reactions, since 

A concept of great relevance in considering the interaction of neutrons of low or moderate energy with nuclei is that of the compound nucleus. When the neutron interacts with the nucleus, it is first captured by the nucleus (Z, N) to form the heavier nucleus (Z, N 4- 1). The lifetime of this compound nucleus (typically 10 s) is long on the nuclear time scale, i.e., it is much longer than the time which the neutron would have taken to travel through a distance equal to the nuclear diameter, which is of the order of 10 s for a 1-MeV neutron (of velocity approximately 10 ms ) incident on a nucleus of diameter 10 m. 

In practice, the resonance condition for a given level can be achieved over a finite spread of incident energy values, since the energies of the 

For higher neutron energies, ( 1 MeV) the ratio of level width to level spacing in the compound nucleus increases to such an extent that the concept of sharp resonance peaks is no longer relevant. 

Since the quantity F is inversely proportional to the time for which the compound nucleus exists before de-excitation, it is in fact a measure of the probability, per unit time, of the de-excitation taking place. In general, there will be a number of possible ways in which this may occur for example, the compound nucleus may emit the same, or another neutron, or a charged particle, or it may lose its excitation energy by the emission of a quantum of radiation y ray). Each of these possible processes is characterized by its own partial width, F, which is proportional to the relative probability of the particular reaction taking place. The parameter F is then the total level width, and is the sum of all the partial widths corresponding to the possible modes of break-up of the compound nucleus, i.e.. 

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The consequences of considering the interaction of the neutron with the nucleus as being able to be represented by a simple potential well was explored soon after the discovery of the neutron. The fact that 1 = 0 resonances occur every... 

In neutron activation analysis, an activation of a sample material is accomplished by placing the sample in some position within the neutron environment. At the time the sample is exposed to neutrons, a compound nucleus is formed as the result of the interaction of a neutron with the nucleus of a stable isotope of the element being determined. The compound nucleus i.e. the end-product of an excitation process caused by both the kinetic and binding energy of the neutron with the nucleus, instantaneously loses its excess energy by a transformation to a more stable isotope by emitting prompt radiations. As a result of this event, another stable nuclide or a radioactive isotope is formed. This radioisotope then becomes the activation, or isotopic, indicator of the element of interest. 

NAA is the most common form of activation analysis. The activation reaction is induced by the interaction of a neutron with the nucleus of an analyte element. Depending on the energy of the incident neutron and the reaction cross sections of the target elements, different types of reactions can take place, leading to activation products as described in O Sect. 30.2. This reaction is commonly followed by the measurement of a nuclide-characteristic de-excitation step (radioactive decay). It is this characteristic gamma-ray decay that is commonly used in the detection and determination of the element of interest. 

Nowadays, chemical elements are represented in abbreviated form [2]. Each element has its ovm symbol, which typically consists of the initial upper-case letter of the scientific name and, in most cases, is followed by an additional characteristic lower-case letter. Together with the chemical symbol, additional information can be included such as the total number of protons and neutrons in the nucleus, the atomic number (the number of protons in the nucleus) thus isotopes can be distinguished, e.g., The charge value and, finally, the number of atoms which are present in the molecule can be given (Figure 2-3). For example, dioxygen is represented by O2. 

Other elements have atoms that can have different ratios of protons to neutrons. Indeed, hydrogen actually consists of three types of atoms. All hydrogen atoms have the same number of protons (one for hydrogen), giving each a mass of 1 Dalton, but some atoms of hydrogen also contain one neutron in the nucleus as well as the proton (mass of 2 Da), while yet others have two neutrons with each proton (mass of 3 Da). Thus hydrogen has three naturally occurring isotopes of mass 1, 2, and 3 Da. Chemically, there are only small differences between the reactivities of the different isotopes for any one element. Thus isotopes of palladium aU react in the same way but react differently from all isotopes of platinum. 

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conseiwed. For example, the nucleus "" Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of Cf are reconfigured into other nuclei. Usually a few neu-... 

It is relatively easy to summarize how nuclear stability (and hence the attractive nuclear forces) depends upon the numbers of protons and neutrons in the nucleus. For atoms with atomic number less than 20, the most stable nuclei are those in which there are equal numbers of protons and neutrons. For atoms with atomic numbers between 20 and 83, the most stable nuclei have more neutrons than protons. For atoms of atomic number greater than 83, no nucleus can be considered stable by our definition. These... 

Nuclei that have too many protons relative to their number of neutrons correct this situation in either of two ways. They either capture one of their Is electrons or they emit a positron (a positively charged particle with the same mass as an electron). Either process effectively changes a proton to a neutron within the nucleus. 

The sub-micro level cannot easily be seen directly, and while its principles and components are currently accepted as tme and real, it depends on the atonuc theory of matter. The scientific definition of a theory can be emphasised here with the picture of the atom constantly being revised. As Silberberg (2006) points out, scientists are confident about the distribution of electrons but the interactions between protons and neutrons within the nucleus are still on the frontier of discovery (p. 54). This demorrstrates the dynamic and exciting nature of chemistry. Appreciating this overview of how scierrtific ideas are developing may help students to expand their epistemology of science. 

C02-0077. Write the correct elemental symbols for the following nuclei (include the atomic number subscript) (a) helium with the same number of neutrons and protons (b) tungsten with 110 neutrons (c) the nucleus with Z = 28 and N — 32 and (d) the nucleus with 12 protons and 14 neutrons. 

After Chadwick s discovery, scientists knew the three components of an atom protons and neutrons in the nucleus with electrons hovering outside. The masses and charges of these constituents are shown in Table 3.1. Chemists have developed a system to describe the elements based on their atomic makeup. The atomic number of an atom is the number of protons in the nucleus. This number is usually represented by the letter Z. Thus, for hydrogen Z = 1, for helium Z = 2, and so on. 

A glance at the periodic table (which will be covered in detail in Chapter 5) shows a list of elements with numbers that are not as neat as those for carbon. Iron, for instance, has an atomic mass of 55.845. Could an atom have a fractional proton or neutron Of course not. An element must have a fixed number of protons. That is what defines it as an element. However, the number of neutrons in the nucleus of an element can vary. Carbon, for instance, has two prominent forms. Carbon 12 has 6 protons and 6 neutrons whereas carbon 14 has 6 protons and 8 neutrons. 

Isotopes Atoms with the same number of protons and electrons but with a different number of neutrons in the nucleus. Isotopes of an element act the same chemically but differ in mass. 

The behavior of a multi-particle system with a symmetric wave function differs markedly from the behavior of a system with an antisymmetric wave function. Particles with integral spin and therefore symmetric wave functions satisfy Bose-Einstein statistics and are called bosons, while particles with antisymmetric wave functions satisfy Fermi-Dirac statistics and are called fermions. Systems of " He atoms (helium-4) and of He atoms (helium-3) provide an excellent illustration. The " He atom is a boson with spin 0 because the spins of the two protons and the two neutrons in the nucleus and of the two electrons are paired. The He atom is a fermion with spin because the single neutron in the nucleus is unpaired. Because these two atoms obey different statistics, the thermodynamic and other macroscopic properties of liquid helium-4 and liquid helium-3 are dramatically different. 

Tritium—The hydrogen isotope with one proton and two neutrons in the nucleus (Symbol 3H). It is radioactive and has a physical half-life of 12.3 years. 

The substances we call elements are composed of atoms. Atoms in turn are made up of neutrons, protons and electrons neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus. Nuclide is the general term referring to any nucleus along with its orbital electrons. The nuclide is characterized by the composition of its nucleus and hence by the number of protons and neutrons in the nucleus. All atoms of an element have the same number of protons (this is given by the atomic number) but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic weight of the element). Atoms with different atomic mass but the same atomic numbers are referred to as isotopes of an element. 

EXAMPLE 22.8. What small particle(s) must be produced with the other products of the reaction of a neutron with a nucleus by the following reaction ... 

Except for the simplest hydrogen atom with a single proton as its entire nucleus, all atoms contain neutrons (particles that are electrically neutral) in addition to protons. For most of the light elements, the numbers of protons and neutrons in the nucleus are nearly equal. Table 3-2 shows the most common nucleus for each element with the atomic weight rounded to the nearest integer. You can see that the rounded-off atomic weights are the sum of the protons and neutrons for each atom. The sum of the protons and neutrons is the mass number of an atom. 

The other approach, proposed slightly later by Hund[9] and further developed by Mulliken[10] is usually called the molecular orbital (MO) method. Basically, it views a molecule, particularly a diatomic molecule, in terms of its united atom limit . That is, H2 is a He atom (not a real one with neutrons in the nucleus) in which the two positive charges are moved from coinciding to the correct distance for the molecule. HF could be viewed as a Ne atom with one proton moved from the nucleus out to the molecular distance, etc. As in the VB case, further adjustments and corrections may be applied to improve accuracy. Although the imited atom limit is not often mentioned in work today, its heritage exists in that MOs are universally... 

The most basic unit of a chemical element that can undergo chemical change is an atom. Atoms of any element are identified by the number of protons and neutrons in the nucleus. The number of protons in the nucleus of an element is given by the atomic number. Hydrogen has one proton in its nucleus so its atomic number is one. The atomic number of carbon is six, because each carbon atom contains six protons in its nucleus. Besides protons, the nucleus contains neutrons. The number of protons plus the number of neutrons is the mass number of an element. A standard method of symbolizing an element is to write the elements with the mass number written as a superscript and the atomic number as a subscript. Carbon-12 would be written as... 

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