Atomic and Nuclear Physics DOE-HDBK-1019/1-93 MODES OF RADIOACTIVE DECAY Rev. 0 Page 25 NP-01 Figure 9   Orbital Electron Capture Positively charged electrons (beta-plus) are known as positrons.  Except for sign, they are nearly identical to their negatively charged cousins.  When a positron, represented as     e,        , or simply as +1 +1 0 0 e   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. Electron Capture (EC, K-capture) Nuclei having an excess of protons may capture an electron from one of the inner orbits which immediately combines with a proton in the nucleus to form a neutron.  This process is called electron capture  (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 electron capture. A  neutrino  is  formed  at  the  same time that the neutron is formed, and energy  carried  off  by  it  serves  to conserve momentum. Any energy that is available due to the atomic mass of the product being appreciably  less  than  that  of  the parent    will    appear    as    gamma radiation.  Also, there will always be  characteristic  x-rays  given  off when an electron from one of the higher energy shells moves in to fill the     vacancy     in     the     K-shell. Electron capture is shown graphically in Figure 8. Electron    capture    and    positron emission result in the production of the   same   daughter   product,   and they exist as competing processes. For positron emission to occur, however, the mass of the daughter product must be less than the mass of the parent by an amount equal to at least twice the mass of an electron.  This mass difference between the parent and daughter is necessary to account for two items present in the parent but not in the daughter.  One item is the positron ejected from the nucleus of the parent.  The other item is that the daughter product has one less orbital electron than the parent.  If this requirement is not met, then orbital electron capture takes place exclusively.