Atomic and Nuclear PhysicsDOE-HDBK-1019/1-93MODES OF RADIOACTIVE DECAYRev. 0Page 25NP-01Figure 9 Orbital Electron CapturePositively charged electrons (beta-plus) are known as positrons. Except for sign, they are nearlyidentical to their negatively charged cousins. When a positron, represented as e, , or simply as+1 +10 0e or , is ejected from the nucleus, the atomic number is decreased by one and the mass number+ +remains unchanged. A proton has been converted to a neutron. An example of a typical positron(beta-plus) decay is shown below.ElectronCapture(EC,K-capture)Nuclei having an excess of protons may capture an electron from one of the inner orbits whichimmediately combines with a proton in the nucleus to form a neutron. This process is called electroncapture(EC). The electron is normally captured from the innermost orbit (the K-shell), and,consequently, this process is sometimes called K-capture. The following example depicts electroncapture.A neutrino is formed at the sametime that the neutron is formed, andenergy carried off by it serves toconserve momentum. Any energythat is available due to the atomicmass of the product beingappreciably less than that of theparent will appear as gammaradiation. Also, there will alwaysbe characteristic x-rays given offwhen an electron from one of thehigher energy shells moves in to fillthe vacancy in the K-shell.Electron capture is showngraphically in Figure 8.Electron capture and positronemission result in the production ofthe same daughter product, andthey exist as competing processes.For positron emission to occur, however, the mass of the daughter product must be less than themass of the parent by an amount equal to at least twice the mass of an electron. This mass differencebetween the parent and daughter is necessary to account for two items present in the parent but notin the daughter. One item is the positron ejected from the nucleus of the parent. The other item isthat the daughter product has one less orbital electron than the parent. If this requirement is not met,then orbital electron capture takes place exclusively.
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