Nuclear Transformation and Modes of Decay PDF

Nuclear Transformation and Modes of Decay Nuclear Transformation When an unstable (i.e., radioactive) atomic nucleus undergoes the spontaneous transformation, called radioactive decay, radiation is emitted. If the daughter nucleus is stable, this spontaneous transformation ends. If t...

Nuclear Transformation and Modes of Decay Nuclear Transformation When an unstable (i.e., radioactive) atomic nucleus undergoes the spontaneous transformation, called radioactive decay, radiation is emitted. If the daughter nucleus is stable, this spontaneous transformation ends. If the daughter is also unstable, the process continues until a stable nuclide is reached. Nuclear Transformation Most radionuclides decay in one or more of the following ways: (1) alpha decay (2) beta-minus emission (3) beta-plus (positron) emission, (4) electron capture (5) isomeric transition. Alpha Decay Alpha (α) decay is the spontaneous emission of an alpha particle (identical to a helium nucleus consisting of two protons and two neutrons) from the nucleus. Alpha decay typically occurs with heavy nuclides (A > 150) and is often followed by gamma and characteristic x-ray emission. These photon emissions are often accompanied by the competing processes of internal conversion and Auger electron emission Alpha Decay Alpha particles are the heaviest and least penetrating form of radiation. Alpha particles are not used in medical imaging because their ranges are limited to approximately 1 cm/MeV in air and typically less than 100 µm in tissue. Beta-Minus (Negatron) Decay Occurs with radionuclides that have an excess number of neutrons compared with the number of Protons (i.e., a high N/Z ratio). Results in the conversion of a neutron into a proton with the simultaneous ejection of a negatively charged beta particle and an antineutrino. Beta-Minus (Negatron) Decay With the exception of their origin (the nucleus), beta particles are identical to ordinary electrons. Beta decay increases the number of protons by 1 and thus transforms the atom into a different element with an atomic number Z + 1. Beta-Minus (Negatron) Decay Although the β- particles emitted by a particular radionuclide have a discrete maximal energy (Emax), almost all are emitted with energies lower than the maximum. The average energy of the β- particles is approximately 1/3 Emax. The balance of the energy is given to the antineutrino (i.e., Emax = E β- + Eύ ). beta-minus decay results is a polyenergetic spectrum of energies ranging from zero to Emax. Any excess energy in the nucleus after beta decay is emitted as gamma rays, internal conversion electrons. Beta Particles — The beta particle is an electron like particle. — Because of its smaller size; it does have a longer range than alpha — ~10-100 cm air — 1-2 cm tissue Beta-Plus Decay (Positron Emission) Occurs in “neutron-poor” radionuclides (i.e., those with a low N/Z ratio). The net result is the conversion of a proton into a neutron with the simultaneous ejection of the positron (β+) and a neutrino (ν). Positron decay decreases the number of protons (atomic number) by 1 (transforms the atom into a different element). Beta-Plus Decay (Positron Emission) The daughter atom, with one less proton in the nucleus, initially has too many orbital electrons and thus is a negative ion. The daughter quickly releases the extra orbital electron to the surrounding medium and becomes a neutral atom. Positron — Positron interacts with an electron in a process called annihilation. — Annihilation results in total conversion of positron and electron masses into energy and release of two photons with 511 Kev energy in angle of 180˚ in between. Electron Capture Decay An alternative to positron decay for neutron- deficient radionuclides. In this decay mode, the nucleus captures an orbital (usually a K- or L-shell) electron, with the conversion of a proton into a neutron and the simultaneous ejection of a neutrino. Electron capture can be described by the following equation: Electron Capture Decay — The capture of an orbital electron creates a vacancy in the electron shell, which is filled by an electron from a higher energy shell. Electron transition results in the emission of characteristic x-rays and/or Auger electrons. Isomeric Transition — Isomeric transition is a decay process that yields gamma radiation without the emission or capture of a particle by the nucleus. There is no change in atomic number, mass number, or neutron number The energy is released in the form of gamma rays, internal conversion electrons, or both.  Isomeric Transition Often during radioactive decay, a daughter is formed in an excited (i.e., unstable) state. Gamma rays are emitted as the daughter nucleus undergoes an internal rearrangement and transitions from the excited state to a lower energy state. Once created, most excited states transition almost instantaneously (on the order of 10-12 s) to lower energy states with the emission of gamma radiation. However, some excited states persist for longer periods, with half- lives ranging from nanoseconds (10-9 s) to more than 30 years. These excited states are called metastable or isomeric states and those with half-lives exceeding a millisecond (10-3 s) are denoted by the letter “m” after the mass number (e.g., Tc-99m). Electromagnetic Radiation Photon of energy — Pure energy, no mass — Travel at speed of light (C = 3 x 108 m/s) — Electric and magnetic properties — Called “photons”  Decay Schemes The majority of the information about the decay process and its associated radiation can be summarized in a line diagram called a decay scheme. Decay schemes identify the parent, daughter, mode of decay, energy levels including those of excited and metastable states, radiation emissions, and sometimes physical half-life and other characteristics of the decay sequence.  The top horizontal line represents the parent, and the bottom horizontal line represents the daughter. Horizontal lines between those two represent intermediate excited or metastable states. A diagonal arrow to the right indicates an increase in Z, which occurs with beta-minus decay.  A diagonal arrow to the left indicates a decrease in Z such as decay by electron capture. A vertical line followed by a diagonal arrow to the left is used to indicate alpha decay and in some cases to indicate positron emission when a radionuclide decays by both electron capture and positron emission. Vertical down pointing arrows indicate gamma ray emission, including those emitted during isomeric transition. Gamma ray transitions (TC-99m) There are three gamma ray transitions as Tc-99m decays to Tc-99. 1. The gamma 1 (γ1) transition occurs very infrequently because 99.2% of the time this energy is internally converted. After internal conversion, the nucleus is left in an excited state, which is followed almost instantaneously by gamma 2 (γ 2) transition to ground state. occurs 89.1% of the time. the principal photon imaged in nuclear medicine.  Like gamma 1, the gamma 3 transition occurs very infrequently relative to the probability of internal conversion electron emission. The vacancies created in orbital electron shells following internal conversion result in the production of characteristic x-rays and Auger electrons. F-18 decay F-18 decays by positron emission (represented by a solid vertical line followed by a diagonal arrow to the left) 97% of the time. Electron capture (represented by a diagonal arrow to the left) occurs 3% of the time.  the interaction of the positron with an electron results in the production of two 511-keV annihilation radiation photons. 18F is the most widely used radionuclide for PET imaging.