Nuclear positron decay

The energy distribution of beta particles emitted in negatron or positron decay is continuous (Fig. 19.3). The maximum energy associated with the distribution is called max and is characteristic of the particular nuclear transformation. At energies less than this, part of the energy resides in the neutrino or antineutrino emitted with the beta particle the sum of the two energies is equal to the characteristic maximum energy. 

Write complete nuclear equations for the following processes (a) tritium, H, undergoes decay, (b) Pu undergoes a-particle emission, (c) l undergoes fj decay, (d) Cf emits an a particle, and (e) B undergoes positron decay. 

Writing Nuclear Equations for Positron Decay (Section 17.3)... 

Neutrinos are created in nuclear processes and in various elementary particle interactions. The most familiar process is nuclear beta-decay, in which an unstable nucleus simultaneously emits an electron (beta-ray) and a neutrino. This process may be visualized as an unstable nucleus radiating its energy by creating a pair of leptons a neutrino and an electron. It is referred to as beta-minus decay when an electron (e ) is emitted with an antineutrino Ve) or beta-plus decay when a positron (e+) is emitted with a neutrino (Vg). In another beta-decay process, called electron capture, one of the orbital electrons in an atom is absorbed by the nucleus and a neutrino is emitted. Examples of these processes are... 

The emission of y rays follows, in the majority of cases, what is known as P decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which prompdy relaxes by giving offy rays. This is illustrated in Figure 2. 

X 109 years and decays by positron (+°P) emission. Write the equation for this nuclear reaction. 

Electron capture accomplishes the same end result as positron emission, but because the nuclear charge is low, positron emission is the expected decay mode in this case. Generally, electron capture is not a competing process unless Z 30 or so. 

The positron has a short life and will quickly be annihilated in a reaction with an electron, producing y-photons of characteristic energy (0.51 MeV). In addition, the basic nuclear process itself is usually accompanied by the emission of y-radiation. As in the case of negatron decay a complete energy... 

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. 

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... 

Decay. The diminution of a radioactive substance due to nuclear emission of alpha or beta particles, gamma rays or positrons. 

Our straightforward bookkeeping has shown us that for (3+ decay, the difference between the initial and final nuclear masses, must be at least 2m0c2 (1.02 MeV) for the decay to be energetically possible. This energy represents the cost of creating the positron. 

Another trend is that radioactive nuclei with higher neutron/proton ratios (top side of the band) tend to emit j8 particles, while nuclei with lower neutron/proton ratios (bottom side of the band) tend to undergo nuclear decay by positron emission, electron capture, or a emission. This makes sense if you think about it The nuclei on the top side of the band are neutron-rich and therefore undergo a process that decreases the neutron/proton ratio. The nuclei on the bottom side of the band, by contrast, are neutron-poor and therefore undergo processes that increase the neutron /proton ratio. (Take a minute to convince yourself that a emission does, in fact, increase the neutron/proton ratio for heavy nuclei in which n > p.)... 

Gamma Rays (Nuclear-decay y-Rays, 0.5-m.e.v. Photons from Annihilation of Positrons or X-rays). The development of large sodium iodide crystal 7-ray spectrometers (13) has made possible high detection efficiencies (close to 100% for some 7-ray energies). Also, whole-body counters utilizing large cylindrical liquid scintillators provide a detection efficiency of 15% for the 7-rays emitted from potassium-40 in the human body (23). 

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