Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

96 CHAPTER 4
when an orbital electron is captured. It is thus seen that in all types of radioactive
decay involving either the capture or emission of an electron, a neutrino must be
emitted in order to conserve energy. However, in contrast to positron and negatron
(ordinary beta) decay, in which the neutrino carries off the difference between the
actual kinetic energy of the particle and the maximum observed kinetic energy and,
therefore, has a continuous energy distribution, the neutrino in orbital electron
capture is necessarily monoenergetic.
Whenever an atom is transformed by orbital electron capture, an X-ray, characteristic
of the daughter element, is emitted as an electron from an outer orbit
falls into the energy level that had been occupied by the captured electron. That
characteristic X-rays of the daughter should be observed follows from the fact that
the X-ray photon is emitted after the nucleus captures the orbital electron and is
thereby transformed into the daughter. These low-energy characteristic X-rays must
be considered by health physicists when they compute absorbed radiation doses
from internally deposited isotopes that decay by orbital electron capture.
Isomeric Transitions
Gamma Rays
Gamma rays are monochromatic electromagnetic radiations that are emitted from
the nuclei of excited atoms following radioactive transformations; they provide a
mechanism for ridding excited nuclei of their excitation energy without affecting
either the atomic number or the atomic mass number of the atom. Since the health
physicist is concerned with all radiations that come from radioactive substances and
since X-rays are indistinguishable from gamma rays, characteristic X-rays that arise
in the extranuclear structure of many nuclides must be considered by the health
physicist in evaluating radiation hazards. However, because of the low energy of
characteristic X-rays, they are of importance mainly in the case of internally deposited
radionuclides. Annihilation radiation—the gamma rays resulting from the mutual
annihilation of positrons and negatrons—are usually associated, for health physics
purposes, with those radionuclides that emit positrons. In considering the radiation
hazard from 22 Na, for example, two 0.51-MeV photons (the energy equivalent of
the two particles that were annihilated) from the annihilation process, which are
not shown on the decay scheme, must be considered together with the 1.277-MeV
gamma ray, which is shown on the decay scheme. The general rule in health physics,
therefore, is automatically to associate positron emission with gamma radiation in
all problems involving shielding, dosimetry, and radiation hazard evaluation.
Internal Conversion
Internal conversion is an alternative isomeric mechanism to radiative transition by
which an excited nucleus of a gamma-emitting atom may rid itself of excitation energy.
It is an interaction in which a tightly bound electron interacts with its nucleus,
absorbs the excitation energy from the nucleus, and is ejected from the atom. Internally
converted electrons appear in monoenergetic groups. The kinetic energy
of the converted electron is always found to be equal to the difference between the
energy of the gamma-ray emitted by the radionuclide and the binding energy of
the converted electron of the daughter element. Since electrons in the L energy