Radioactive Transitions
Perry Sprawls, Ph.D.

Online Textbook

Table of Contents

 

INTRODUCTION AND OVERVIEW
ISOBARIC TRANSITIONS
   Beta Emission
   Positron Emission
   Electron Capture
ISOMERIC TRANSITIONS
   Gamma Emission
   Internal Conversion
ALPHA EMISSION
PRODUCTION OF RADIONUCLIDES
Radionuclides Used in Nuclear Medicine (Table)

 

INTRODUCTION AND OVERVIEW

CONTENTS

   In the previous section we showed that certain nuclei are not completely stable and eventually undergo an internal change that will produce a more stable nuclear structure. This spontaneous change is a radioactive transition. In some older literature this event is called a nuclear disintegration. The terminology is misleading because the nucleus does not disintegrate; it simply undergoes a slight change. This event is illustrated in the figure below. The original nucleus is designated the parent, and the nucleus after the transition is designated the daughter. In radioactive transitions, energy is emitted as radiation. The types of radiation encountered in nuclear medicine are shown below. The radiation is in the form of either energetic particles or photons.


Various Radiations Produced by Radioactive Transitions

Various Radiations Produced by Radioactive Transitions

   Most radionuclides emit a combination of radiations; the types depend on the physical characteristics of the nucleus and are considered in more detail later. The daughter nucleus can be either stable or a radioactive or metastable nucleus that will undergo another transition in the future.

   In most in-vivo nuclear medicine procedures it is desirable to use a radionuclide that emits photons in the range of 100 to 500 keV. The penetrating ability of photons is related to their energy. Many photons in this energy range can emanate from the body, but not penetrate through the detector and be lost. Particle radiation is not useful in most diagnostic procedures. In fact, it is usually undesirable because it deposits its energy in the body close to the site of origin and can contribute significantly to patient dose without contributing to diagnostic information. With many radionuclides, particle radiation is a byproduct of the transition required for desirable photon emissions.

   It is often useful to construct a diagram to show changes in the nucleus and the radiation emitted during a radioactive transition. A transition diagram, sometimes referred to as a decay scheme, is shown below. Two types of changes occur within a nucleus: loss of energy and, possibly, a change in atomic number in an isobaric transition. In the transition diagram, relative nuclear energy is represented by vertical distance and relative atomic number by horizontal distance. The actual scales are not usually shown as they are in the figure below. The position of the nucleus before and after the transition is represented by horizontal lines in the diagram. In the figure below the image of a nucleus is resting on these lines. This is not generally found in the conventional diagram but is here to help us follow the transition. The steps in the transition are represented by lines running downward. The transition always moves downward because the nucleus is decreasing its energy by emitting radiation. If the atomic number changes (isobaric transition), the transition line will slant to the right or left.


Diagram Showing Radioactive Transition

Diagram Showing Radioactive Transition

   The vertical distance between the parent and daughter positions represents the total transition energy. This value is always specific for the transition associated with each radioactive nuclide. But all nuclei of the same nuclide do not necessarily go from the parent to daughter state in the same way. The significance of this is discussed in detail later.

ISOBARIC TRANSITIONS

CONTENTS

  Most radioactive transitions have several steps. For most radionuclides, the first step is an isobaric transition usually followed by an isomeric transition and interactions with orbiting electrons. The three types of isobaric transitions of interest to us are (1) beta emission, (2) positron emission, and (3) electron capture.

   In nuclear stability, the neutron-proton ratio (N/P) is crucial. If it is too low or too high, the nucleus will eventually rearrange itself into a more stable configuration. Beta radiation, which is the emission of energetic electrons, results when an N/P ratio is too high for stability; positron emission or electron capture occurs when it is too low for stability. These two conditions are represented by specific areas of the nuclide chart shown below. Beta emitters are above the stable nuclides, and positron emitters and electron capture nuclides are below.


Nuclide Chart Showing the Relationship between Radioactive and Stable Nuclides

 
Nuclide Chart Showing the Relationship between Radioactive and Stable Nuclides

   Beta Emission

CONTENTS

   A beta transition is illustrated below. We should recall that the nuclear N/P is too high for stability. During the transition, this condition is relieved by the conversion of an internal neutron into a proton, accompanied by the emission, from the nucleus, of an electron. The electron, or beta particle, has two functions in this transition.


Diagram of a Transition That Produces Beta Radiation

Diagram of a Transition That Produces Beta Radiation

   One function is to carry away from the nucleus a one-unit negative charge so that a neutron (no charge) can be converted into a proton with a one-unit positive charge. A fundamental principle of physics is that electrical charge cannot be created or destroyed. The only way to change the charge on an object, such as a nucleus, is to transfer electrons to or from the object. The emission of a beta particle causes the number of protons, and therefore the atomic number, of the nucleus to increase by one unit. Since the mass number, or total number of neutrons and protons, is not changed, the transition is isobaric.

   The second function of the electron is to carry off a portion of the energy given up by the nucleus. The energy is carried as kinetic energy by the electron. But the energy carried by a beta particle is usually less than the total transition energy given up by the nucleus. The remaining portion is removed from the nucleus by the emission of a very small particle known as a neutrino. In each transition, the sum of the beta and neutrino energy is equal to the transition energy for the nuclide. Unlike the beta particle, the neutrino is very penetrating and carries the energy out of the patient's body.

   A typical energy spectrum for a beta-emitting nuclide is shown below. The maximum energy corresponds to the transition energy. This is the energy a beta particle would have in the few instances no neutrino is emitted. The average energy value indicates the radiation dose or energy deposited in the body by the beta radiation. The shape of the beta energy spectrum varies from nuclide to nuclide. The relationship between average energy and transition energy depends on the value of the transition energy and the atomic number of the nuclide. For most radionuclides encountered in nuclear medicine, the average beta energy is usually between 25% and 30% of the maximum energy.


Spectrum of Beta Radiation Energy


Spectrum of Beta Radiation Energy

 

   Positron Emission

CONTENTS

   Two types of transitions can occur when the nuclear N/P is too low for stability. One is positron emission. A positron is a small particle that has essentially the same mass as an electron but has a positive rather than negative electrical charge. The nuclear transition resulting in the emission of a positron is illustrated below. The transition energy is shared between the positron and a neutrino.


Diagram of a Transition That Produces Positron Radiation

Diagram of a Transition That Produces Positron Radiation

   In this transition a proton is converted into a neutron as the positron particle is formed. Since a neutron is heavier than a proton, energy is required for this conversion. The energy equivalent to the mass difference between a neutron and a proton plus the energy equivalent of the positron mass is approximately 1.8 MeV. This means that the total transition energy must be at least 1.8 MeV for positron emission to occur.

   The positron is the antiparticle of an electron and will enter an annihilation reaction when the two particles meet. Since electrons are normally abundant in material, positrons are annihilated soon after their emission. Positron emitters are useful in nuclear medicine because of the radiation produced when the positron is annihilated. The total masses of the positron and the electron are converted into energy according to the relationship

E = MC2.

   The energy produced is 1.022 MeV emitted as a pair of photons, each with an energy of 511 keV. Therefore, the radiation from a positron-emitting material is photons with a characteristic energy of 511 keV. The pair of photons leave the site traveling in opposite directions. This is useful in imaging, because it allows the annihilation site to be precisely determined.

 

   Electron Capture

CONTENTS

   A nucleus can also relieve a low neutron-proton ratio by capturing and absorbing an electron from a shell. Since most electrons are captured from the K shell, this process is sometimes referred to as K-capture. Capture from the L and M shells is possible under some conditions, but does not occur so frequently as from the K shell. The electron capture process is illustrated below. When the negative electron enters the nucleus, the positive charge of one proton is canceled and the proton is converted into a neutron. This results in the reduction of the atomic number by one unit. Since the mass number does not change, electron capture is an isobaric transition. Electron capture often competes with positron emission; if a nuclide is a positron emitter, some nuclei will emit positrons and some will capture electrons. The ratio between the two processes is specific for each nuclide.


Diagram of a Transition That Produces Electron Capture

Diagram of a Transition That Produces Electron Capture

   In an electron capture transition, radiation is not emitted directly from the nucleus but results from changes within the electron shells. Electron capture creates a vacancy in one shell, which is quickly filled by an electron from a higher energy location. As the electron moves down to the K shell, it gives off an amount of energy equivalent to the difference in the binding energy of the two levels. This energy is emitted from the atom in either characteristic x-ray photons or Auger electrons. Auger electrons are produced when the energy given up by the electron filling the K-shell vacancy is transferred to another electron, knocking it out of its shell. Most Auger electrons have relatively low energies.

   Many radionuclides that undergo electron capture are used in nuclear medicine because the energy of characteristic x-ray photons is ideal for in-vivo studies.

 

ISOMERIC TRANSITIONS

CONTENTS

   After a radioactive nucleus undergoes an isobaric transition (beta emission, positron emission, or electron capture), it usually contains too much energy to be in its final stable or daughter state. Nuclei in these intermediate and final states are isomers, since they have the same atomic and mass numbers. Nuclei in the intermediate state will undergo an isomeric transition by emitting energy and dropping to the ground state.

 

   Gamma Emission

CONTENTS

   In most isomeric transitions, a nucleus will emit its excess energy in the form of a gamma photon. A gamma photon is a small unit of energy that travels with the speed of light and has no mass; its most significant characteristic is its energy. The photon energies useful for diagnostic procedures are generally in the range of 100 keV to 500 keV.

   The energy of a gamma photon is determined by the difference in energy between the intermediate and final states of the nucleus undergoing isomeric transition. This difference is the same for all nuclei of a specific nuclide. However, many nuclides have more than one intermediate state or energy level. When this is the case, a radionuclide might emit gamma photons with several different energies. This is illustrated in below.


Relationship of Nuclear Energy Levels to the Energy Spectrum of Gamma Photons

Relationship of Nuclear Energy Levels to the Energy Spectrum of Gamma Photons

   The nuclide used in this illustration has two intermediate states or energy levels. One has an energy 500 keV above the daughter level, and the other is 300 keV higher than the first. When there are several different intermediate energy levels, it is common for some nuclei to go to one level and other nuclei to go to another level during isobaric transition. This is usually indicated on the transition diagram by showing the percentage of nuclei that go to each energy level. In the illustration considered, 80% of the nuclei go directly to intermediate energy level number 1, and 20% go directly to level number 2. The gamma photons are emitted when the nuclei move from these intermediate energy levels down to the daughter nuclide level.

   Nuclei that have gone to a specific intermediate energy level might then go directly to the daughter level or to a lower intermediate level. With this in mind we can predict the gamma photon energy spectrum produced by our example nuclide. The spectrum will consist of three discrete energies as shown above. Sixty percent of the parent nuclei will go to intermediate energy level number 1 and then directly to the daughter level 800 keV below. Therefore, 60% of the transitions will give rise to an 800-keV photon. Twenty percent of the nuclei that go to energy level 1 will then go to intermediate level number 2 by emitting a 300-keV photon. Forty percent of the nuclei will go through intermediate energy level number 2, either directly from the parent or from intermediate level number l. When these nuclei drop to the daughter energy level, a 500-keV gamma photon will be emitted. It is the combination of different energy levels and different transition routes that gives rise to the different energies in the typical gamma spectrum. For most radionuclides, one or two gamma energies will account for the vast majority of transitions.

   For most nuclides, the time spent by the nucleus in the intermediate state is extremely short and the isomeric transition appears to coincide with the isobaric transition. In some nuclides, however, the nuclei remain in the intermediate state for a longer time. In this case, the intermediate state is referred to as a metastable state. Metastable states are of particular interest in nuclear medicine because they make possible the separation of electron and photon radiation. In a diagnostic procedure it is
undesirable to have electron radiation in the body because it contributes to radiation dosage but not to image formation. By using a nuclide that has already undergone an isobaric (electron-emitting) transition and is in a metastable state, it is possible to have a radioactive material that emits only gamma radiation.

   Technetium-99m is a nuclide used in the metastable state. The parent nuclide is molybdenum, which undergoes an isobaric transition to technetium-99 in the metastable state. The technetium-99m will later undergo an isomeric transition to technetium-99.

 

   Internal Conversion

CONTENTS

   Under some conditions, the energy from an isomeric transition can be transferred to an electron within the atom. This energy supplies the binding energy and expels the electron from the atom. This process is known as internal conversion (IC) and is an alternative to gamma emission. In many nuclides, isomeric transitions produce gamma photons and IC electrons. When an electron is removed from the atom by internal conversion, a vacancy is created. When the vacancy is filled by an electron from a higher energy level, energy must be emitted from the atom as a characteristic x-ray photon or an Auger electron.

The various isobaric and isomeric transitions give rise to a combination of both photon and particulate radiations. The radiations encountered in clinical procedures are summarized below. Nuclei with a high N/P generally produce beta radiation; those with a low N/P produce either positrons or electron capture. All transitions are usually followed by either gamma or internal conversion electron emission. Internal conversion and electron capture lead to x-ray or Auger electron emission.


Composite Diagram Showing the Various Nuclear Transitions That Produce Radiation

Composite Diagram Showing the Various Nuclear Transitions That Produce Radiation


ALPHA EMISSION

CONTENTS

   Some radioactive materials emit alpha particles during their transformation. An alpha particle consists of two neutrons and two protons. Because of their size and electrical charge, alpha particles are not good penetrators and deposit their energy very close to their origin. Because of this, and the fact that alpha particles generally have more energy than other radiation forms, significant tissue doses can result from an alpha emitter within the body. Alpha emitters are generally not used in clinical medicine. An exception is radium, which is often used in therapeutic procedures. In these procedures the radium is contained in sealed metal capsules that absorb alpha radiation and let gamma radiation through to the tissue.

   The transitions and principal radiations emitted by many nuclides used in clinical medicine are shown in the following table.

Radionuclides Used in Nuclear Medicine
Element A Z T1/2

Transition

Radiation Yield Energy (keV)
Hydrogen 3 1 12.3 yr Beta Beta 1.0 57
Carbon 11 6 20.3 min Positron Positron 1.0 394
Annihilation 2.0 511
Nitrogen 13 7 10 min Positron Positron 1.0 488
Annihilation 2.0 511
Carbon 14 6 5730 yr Beta Beta 1.0 49
Oxygen 15 8 2 min Positron Positron 1.0 721
Annihilation 2.0 511
Fluorine 18 9 109 min Positron Positron 0.97 250
EC (3%) Annihilation 1.94 511
Chromium 51 24 27.7 d EC Gamma 0.10 320
Cobalt 57 27 270 d EC Gamma 0.86 122
Gamma 0.10 136
Iron 59 26 45 d Beta Beta 0.52 150
Beta 0.46 81
Gamma 0.55 1099
Gamma 0.44 1292
Gallium 67 31 78.1 hr EC Gamma 0.38 93
Gamma 0.24 185
Gamma 0.16 300
IC Electron 0.28 84
Zinc 69m 30 13.8 hr Isomeric Gamma 0.95 440
Selenium 75 34 120 d EC Gamma 0.16 121
Gamma 0.54 136
Gamma 0.57 265
Gamma 0.24 280
Gamma 0.12 400
Strontium 85 38 65.1 d EC Gamma 0.99 514
Strontium 87m 38 2.8 hr EC and Isomeric Gamma 0.83 388
IC electron 0.14 372
Technetium 99m 43 6.0 hr Isomeric IC electron 0.09 119
Gamma 0.88 141
Indium 111 49 2.8 d EC Gamma 0.90 172
Gamma 0.94 247
Iodine 123 53 13 hr EC IC electron 0.13 127
Gamma 0.84 159
X-Ray 0.71 27
Iodine 125 53 60.2 d EC X-Ray 1.15 27
Iodine 131 53 8.0 d Beta Beta 0.90 192
Gamma 0.82 364
Gamma 0.07 637
Xenon 133 54 5.31 d Beta Beta 0.98 101
IC electron 0.53 45
Gamma 0.36 81
Ytterbium 169 70 32 d EC IC electron 0.47 108
IC electron 0.15 196
Gamma 0.45 63
Gamma 0.11 130
Gamma 0.17 177
Gamma 0.26 198
X-Ray 0.78 51
Mercury 197 80 65 hr EC IC electron 0.56 64
IC electron 0.19 75
Gamma 0.25 77
X-Ray 0.36 69
Gold 198 79 2.69 d Beta Beta 0.99 316
Gamma 0.96 412


PRODUCTION OF RADIONUCLIDES

CONTENTS

Some radionuclides occur in nature but are generally not suitable for clinical studies. Most are made by bombarding a nucleus with a particle such as a neutron or a proton. Beta emitters are created by neutron bombardment, and positron emitters and nuclides that undergo electron capture are created by bombardment with positive particles such as protons. Neutrons can be obtained from nuclear reactors or accelerators. Positive particles are obtained from accelerators, usually cyclotrons.