In this process, the nucleus captures an electron from an inner electron shell (K or L shell) resulting in the following transition:

p++ e2 ! n 1V (4.111)

The process is characterized as electron capture, EC decay, or EX decay. EC decay is energetically more desirable than positive beta decay since there is no beta particle emission in EC decay. The neutrinos formed in the electron capture are monoenergetic.

The electron capture is always followed by the emission of electromagnetic radi­ation because the orbital vacancy results in an excited electron state. When the vacancy in the K shell is filled with an electron from an outer, mainly L, shell, the difference between the K and L binding energies is emitted as characteristic X-ray radiation. It is emphasized here that the high-energy electromagnetic radiation is called “gamma radiation” if it is the result of nuclear transition, while if the source of the radiation is the transition of electrons between the extranuclear orbitals, it is called an “X-ray.”

Instead of X-ray radiation, the excitation energy can be transferred to another electron, which is then ejected from the atom. This second ejected electron is called an Auger electron. In this process, the produced nucleus has more than one positive charge, so it can react easily with other substances. The probability of the Auger effect decreases as the atomic number increases. As a result, the ratio of the gamma photons and the Auger electrons depends on the atomic number: for light elements, the Auger electrons are significant, while for heavy elements, the characteristic X-ray is dominant (Figure 4.12).

Furthermore, the electrons captured from the K and L shells, on their pathway toward the nucleus, lose their energy in the nuclear field. This process results in the emission of X-ray radiation called inner Brehmsstrahlung, the spectrum of which is continuous. Thus, as a result of electron capture, both characteristic and continuous X-ray radiations are emitted.

The electron capture results in excited nuclei. This excitation energy may be lost through either the emission of gamma photons or the transition of the excita­tion energy to an electron on the atomic orbital (mainly a K electron) of the same atom, followed by an electron emission. The latter process is called “internal con­version,” and the emitted electrons are conversion electrons. The kinetic energy of the conversion electron is equal to the energy of the gamma quantum reduced by

the binding energy of the electron. This means that the conversion electrons, similar to Auger electrons, have discrete energy.

In some cases, the energy of the electron capture can be measured by using the cyclic process, as shown by the following:

244Am ——! 244Pu (4.112)

a

240Np 244Am

523 MeV

в 0.36 Mevt і AEEC?

a

240U 244Pu

4.65 MeV

4.2.4 Proton and Neutron Decay

Proton decay can take place after positive beta radiation of the light elements, which is followed by proton emission. For example,

Neutron decay, or delayed neutron decay, may occur when a negative beta decay followed by neutron emission takes place. Neutron decay can be observed for the heavier nuclides too. For example,

17N —-— 17O* —-—— 16O (4.114)

Some fission products emit negative beta particles as well as neutrons, for example,

87Br -—— 87Kr* —-— 86Kr (4.115)

127I —-— 137Xe* —^— 136Xe (4.116)

These isotopes are significant in the neutron flux of nuclear reactors, especially when the power is decreasing.