Positron Decay

Effects of different modes of radioactive decay on the position of an isotope on the Chart of the Nuclides. Beta-decay, which changes a neutron to a proton, moves the nuclide up and to the left. Positron decay or electron capture, which changes a proton into a neutron, moves the nuclide down and to the right. And -decay, which is the emission of a 4He nucleus, moves the nuclide down and to the left. 

E.C. = electron capture, 3 = beta decay, T = positron decay, a = alpha decay, S.F. = spontaneous ... 

This reaction produces two groups of neutrinos, one in which the ground state of 7Li is populated (90% abundance) and one in which the 0.477-MeV excited state is populated (10% abundance). The source 8B refers to the positron decay... 

Note (3 decay (without a superscript) always means the ejection of a fT particle. Positron decay is always denoted y3+. 

Nieminen, R M. (1980). Nonlinear density dependence of the positron decay rate in helium. Phys. Rev. A 21 1347-1349. 

Just like (3 decay, the mass numbers of the parent and daughter in positron decay are identical but the atomic numbers are different. However, in positron decay, the daughter has a Z that is one less than the parent nucleus. Positron decay becomes energetically possible only when the decay energy exceeds... 

Electron Capture Decay. Electron capture decay is a competing process to positron decay and thus results in an increase in the neutron-to-proton ratio in the nucleus. In this process, a bound, inner orbital electron is captured by the nucleus, resulting in the conversion of a proton into a neutron, the emission of a neutrino, and, if the daughter nucleus is left in an excited state, the emission of one or more gamma rays. The net reaction is shown below ... 

Beta-Minus Decay 370 Positron Decay 3 72 Electron Capture Decay 3 72 Gamma Ray Emission 373... 

Two different stable nuclides, related by two steps of positron decay, occur with the same mass number of 36 ... 

Another mode of decay is possible for man-made proton-rich nuclei. It has been shown that certain of such nuclei can capture one of their orbital electrons, lowering the atomic number by one unit and leaving the mass number unchanged. The net change is the same as that in positron decay, but orbital-electron capture or K capture occurs when the mass of the parent atom exceeds that of the daughter atom, but by less than 0.00110 atomic mass unit. A typical example is the conversion of Be7 (mass 7.01916) to Li7 (mass 7.01822) ... 

Positron decay occurs in proton-rich nuclei. In this case, the positron (or p+ particle) is originated by conversion of a proton into a neutron, along with the emission of a neutrino to conserve the energy. Positrons are the antiparticle of electrons. In a very fast process (10 12s), emitted positrons collide with an electron of a nearby atom and both particles disappear in a process called annihilation. The necessary conservation of mass and energy accounts for the transformation of the mass of both particles into energy, which is characteristically emitted in the form of two 511-keV photons almost in opposite directions. Consequently, positron emitters are used to label radiopharmaceuticals produced with diagnostic purposes by imaging. 

Br lodination of proteins may be replaced by bromation. The Br-label is considered to be more ble than those of the radionuclides of iodine It splits off by positron decay and has a short half-life (Table 1). Protdns are labeled enzymatically either by bromperoxidase from the microorganisms Pecillus capitatus and Bonnermisoma hamifera or by myeloperoxidase... 

Thus, besides a decay, fi decay and y transition, a fourth type of decay is known since 1982. In the meantime, further examples of proton decay have been discovered, all on the extreme proton-rich side of the chart of nuclides. In this region, proton emis-siori (p decay) competes with emission of positrons decay), and because in most cases decay is favoured, p decay is observed relatively seldom. 

In positron emission tomography (PET) the two SllkeV y-ray photons emitted simultaneously in opposite directions are registered by y-ray detectors, indicating that the positron decay must have occurred somewhere along the line between these two detectors. The same holds for other events of positron decay, and the radionuclide can be localized at the intersection of these lines. 

Oxidation reactions in the hot zone are probably responsible for the large yield of Cr(VI) obtained in the positron decay of Mn to Cr in crystals of cesium permanganate (11). Manganous ion in solution yielded only chromic ion, whereas in crystals of MnCOs, 30% of the chromium recoils appeared as Cr(VI). This probably occurred as a result of reactions between chromium recoils and debris oxygen atoms or carbonate ions. 

Figure 13-8 Schematic diagram of a PET imaging system with multiple scintillation detectors that localize the positron decay along a tine. By using multiple position-sensitive detectors around the patient, the annihilation photons are acquired along many parallel lines and many angles simultaneously with four rings of deteaors (only one ring shown). After use of reconstruction algorithms, the internal distribution of the radioactivity can be determined and displayed on a cathode ray tube.

EC, and IT refer to p-decay, positron decay, electron capture, and isomeric transition, respectively. Where a nudide is known to have more than one mode of decay, they are listed in the order of their prevalence. 

The simplest p-p chain begins with the p-p reaction already discussed. The rest mass of the proton is 1.00728 u, where u is the atomic mass unit, 1 /12th the mass of a 12C atom. The fusion of two protons, including a positron decay, yields a deuterium nucleus with rest mass 2.01355 u. The energy excess is thus... 

The positron decay of 8B( J7r = 2+) goes mainly to the r = 1.6 MeV broad excited state in 8Be at excitation energy Ex = 2.94 MeV (J7r = 2+) due to the selection rules (see Fig. 8). This excited state has very short lifetime and quickly decays into two a-particles. This completes the pp III part of the pp-chain. The average energy of the neutrinos from 8B reactions is E (SB) = 7.3 MeV. These neutrinos, having relatively high energy, play an important role in several solar neutrino experiments. The neutrino spectrum is not the same... 

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