Reactions gamma emissions

Unstable niobium isotopes that are produced in nuclear reactors or similar fission reactions have typical radiation hazards (see Radioisotopes). The metastable Nb, = 14 yr, decays by 0.03 MeV gamma emission to stable Nb Nb, = 35 d, a fission product of decays to stable Mo by... 

Again, both mass and charge are conserved. Gamma emission often accompanies both alpha and beta decay, but because gamma emission does not change the parent element it is often emitted when writing nuclear reactions. 

The fifth type of radioactive emission, gamma radiation, does not result in a change in the properties of the atoms. As a result, they are usually omitted from nuclear equations. Gamma emissions often accompany other alpha or beta reactions—any decay that has an excess of energy that is released. For example, when a positron collides with an electron, two gamma rays are emitted, a phenomenon usually referred to as annihilation radiation. 

All the nuclear reactions that have been described thus far are examples of radioactive decay, where one element is converted into another element by the spontaneous emission of radiation. This conversion of an atom of one element to an atom of another element is called transmutation. Except for gamma emission, which does not alter an atom s atomic number, all nuclear reactions are transmutation reactions. Some unstable nuclei, such as the uranium salts used by Henri Becquerel, undergo transmutation naturally. However, transmutation may also be forced, or induced, by bombarding a stable nucleus with high-energy alpha, beta, or gamma radiation. 

The emission of a gamma ray does not change the elemental identity of the nucleus, even though energy is released. In that sense, a gamma emission, by itself, is not a nuclear change. Therefore, for the remainder of this chapter we will consider only alpha and beta emissions in nuclear reactions. 

There are two primary types of reactions between neutrons and nuclei (1) absorption (a ) in which a neutron enters a nucleus and other particles or photons leave the nucleus and (2) scattering (ag) in which a neutron enters a nucleus, may transfer some of its energy to the nucleus, and exits the nucleus as a free neutron. A.bsorption is also referred to as capture, Capture can have differing results. For example, when a thermal neutron is captured by a U235 nucleus, 85x of the time fission (Uf) results and 15 of the captures result in gamma emission (a ) from the excited U236 nucleus but no fission. 

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

Am is produced when 239Pu is exposed to neutrons, such as may occur in nuclear reactors. (239Pu, it should be noted, is produced when uranium 238 [238U], is exposed to neutrons.) The reaction sequence involves the successive absorption of neutrons and emission of gamma rays, written as (n,y) and the emission of a beta particle, (3. ... 

Radioactivity—Spontaneous nuclear transformations that result in the formation of new elements. These transformations are accomplished by emission of alpha or beta particles from the nucleus or by the capture of an orbital electron. Each of these reactions may or may not be accompanied by a gamma photon. 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

The y particle is emitted virtually instantaneously on the capture of the neutron, and is known as a prompt y - it can be used analytically, in a technique known as prompt gamma neutron activation analysis (PGNAA), but only if such y s can be measured in the reactor during irradiation. Under the conditions normally used it would be lost within the nuclear reactor. In this reaction, no other prompt particle is emitted. The isotope of sodium formed (24Na) is radioactively unstable, decaying by beta emission to the element magnesium (the product nucleus in Figure 2.13), as follows ... 

Make sure that in alpha, beta, gamma, and positron emission the particle being emitted is on the right-hand side of the reaction arrow. In electron capture, the electron should be on the left side of the arrow. 

The nuclear reaction in the hydrogen bomb that produced fermium was the result of the acquisition of 17 neutrons by uranium from the explosion resulting in uranium-255 and some gamma radiation. U-255 decays by (3-electron emission to form fermium-255, as depicted in the equation as follows ... 

The fact that neutrinos are emitted during the transformation provides an opportunity for direct observation of the reactions taking place at the heart of the Sun. Note that antimatter is produced in this strange reaction, in the form of the positton or antielectron e+. The positrons generated immediately annihilate with electrons in the surrounding medium with subsequent emission of gamma rays. 

Another important characteristic is that ion beams can produce a variety of the secondary particles/photons such as secondary ions/atoms, electrons, positrons. X-rays, gamma rays, and so on, which enable us to use ion beams as analytical probes. Ion beam analyses are characterized by the respectively detected secondary species, such as secondary ion mass spectrometry (SIMS), sputtered neutral mass spectrometry (SNMS), electron spectroscopy, particle-induced X-ray emission (PIXE), nuclear reaction analyses (NRA), positron emission tomography (PET), and so on. 

Although the nucleus is normally found in its lowest energy state, it may be produced as the result of a nuclear reaction, or tlirougli radioactivity in a number of excited states whose detailed properties may differ quite markedly from the lowest state, if formed in an excited state, it will decay, normally by the emission of electromagnetic radiation (gamma rays) lo the lowest state, or by the emission of particles to another nucleus. 

Positrons can be produced by either nuclear decay or the transformation of rhe energy of a gamma ray into an electron-positron pair. In nuclei that are proton-rich, a mode of decay that permits a reduction in the number of protons with a small expenditure of energy is positron emission. The reaction taking place during decay is... 

---