This course introduces concepts of radioactivity, and covers the following topics:

  • measurement of radioactivity
  • radioactive waste disposal.

Radioactivity is a spontaneous process by which unstable atoms of an element lose excess energy by emitting that energy in the form of particles or electromagnetic waves to gain stability.

Isotopes of an element differ in their physical properties but are similar in their chemical properties. Radioactive isotopes are called radioisotopes and have many applications in various field of study. For example, they are used as  radiopharmaceuticals when tagged to other molecules to detect metabolic pathways, to study the kinetics of enzyme reactions, or to diagnose, and treat diseases.

Radioactivity

Radioactivity is spontaneous disintegration of atomic nuclei. The nucleus emits α particles, β particles, or electromagnetic rays during this process. Approximately 3000 unstable nuclides have been discovered so far.. They undergo spontaneous fission to achieve stability. During this process, the parent nuclide transforms into an atom of a different type, called the daughter nuclide.

Figure 1. Diagrammatic Representation of Process of Radioactivity
Figure 1. Diagrammatic Representation of Process of Radioactivity

Units of Radioactivity

Radioactivity is measured in curie. It is disintegration rate of 1 g radium that is 3.7 × 1010 disintegrations per second. SI unit for radioactivity is Becquerel (Bq), which is defined as one disintegration per second.

1 Curie (Ci) = 3.7 × 1010 disintegrations per second (dps)

                     = 2.22 × 1012 disintegrations per minute (dpm)

1 millicurie (mCi) = 3.7 × 107 dps

1 microcurie (μCi) = 3.7 × 104 dps

1 Becquerel (Bq) = 1dps = 2.7 × 10-11 Ci

1 Kilobecquerel (KBq) = 103 dps = 2.7 × 10-8 Ci

Radiation dosimetry is calculation of the absorbed dose. The following three units are related with radiation dosimetry

  • Roentgen(R) for exposure
  • Rad (radiation absorbed dose) for absorbed dose
  • Rem (roentgen equivalent man) for dose equivalent

The roentgen is defined as the amount of x or γ radiation that produces ionization of one electrostatic unit of positive or negative charge per cubic centimeter of air at 0°C and 760 mmHg (STP)

1R = 2.58 × 10-4 C/kg

Where, R = Roentgen

               C = 1.6 × 10-19 Coulomb (C) or 4.8 × 10-10 electrostatic units.

The rad is used to measure the absorbed dose. It is a universal unit. It is a measure of the energy deposited in unit mass of any material by any form of radiation or

1 rad = 100erg/g absorber or 1 rad = 10-2 J/kg

Since, 1 joule = 107 erg.

The SI unit of radiation dose is gray (Gy) that is equal to 100 rad i.e.,

                            1 Gy = 100 rad or 1 J/kg absorber

                            1 rad = 0.01 Gy

Different materials that receive the same exposure may not absorb the same amount of energy. In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose. To gauge biological effects, the dose in rads is multiplied by a quality factor, which is dependent upon the type of ionizing radiation.

The rem is dose equivalent is a measure of the differences in the effectiveness of different radiations causing biological damages. The dose equivalent relates the absorbed dose to the biological effect of that dose. The SI unit of dose equivalent is the sievert (SV), i.e.,

                                                      1 sievert (Sv) = 100 rem

In radiobiology, the rem is defined as,

                                                       rem = rad × RBE

Where, RBE = relative biological effectiveness of the radiation.

In radiation protection, rem is defined as,

                                                     rem = rad × QF × N

Where, Q = quality factor 

 N = modifying factor of the radiation being studied or in question.

The United States Nuclear Regulatory Commission uses the units curie, rad, and rem as part of the Code of Federal Regulations 10CFR20.

Radioactive Decay

Fission is a process of nuclear reaction by which  a heavier or larger nucleus breaks down into smaller fragments (Figure 1). The fragments or fission products are nearly equal to half of the original mass. In this process, two or three neutrons are also emitted with the mean energy of 1.5 MeV and releases 200 MeV energy, mainly in the form of heat. In other words, it is an exothermic process. In 1940, Soviet physicist Georgy Flyorov and Konstantin Petrzhak discovered the process of spontaneous fission.

Figure 2. Radioactive decay process. The nucleus of an uranium 235 atom splits into smaller isotopes krypton and barium, producing free neutron, gamma rays and releasing a large amount of energy.
Figure 2. Radioactive Decay Process.

In practice, however, spontaneous fission is energetically feasible for nuclei with atomic masses 220 u (atomic mass unit). The chances of fission increase with the increase in mass number of the heavy nuclei. The half-life (t1/2) for spontaneous fission of U235 is 2 x 1017 years. Neutrons produced by the cosmic rays induced the first fission reaction.

For example, a uranium-235 breaks apart (fission), yielding energy and small fragments (daughter nuclei), the mass of which is less than the mass of the parent nuclei (Figure 2).

Figure 3. Diagrammatic Representation of Fission reaction of U 235 nuclei
Figure 3. Diagrammatic Representation of Fission Reaction of U-235 Nuclei

Types of Radioactive Decay

Alpha (α) Decay

It is a type radioactive decay, in which atomic number of the parent nuclide is reduced to 2 and the mass number by 4 as the parent nuclei emits an alpha particle. Heavy nuclei such as uranium, neptunium, and radon undergo decay by emission of α-particle. For example,                  

Figure 4. Diagrammatic Representation of Alpha Decay
Figure 4. Diagrammatic Representation of Alpha Decay

The protons carrying same number of positive charges inside the nucleus of an atom tend to repel each other. This repulsive force between protons is usually overcome by strong force, which keeps them bound together. However, when there is large number of protons, electrostatic force between protons becomes too large. The strong electrostatic force is not able to hold the protons together, resulting in an alpha decay. An alpha particle (helium nucleus containing 2 protons and 2 neutrons) is emitted during this process. The radioactive isotope (parent nuclei) changes into another element (daughter nuclei).

Figure 5. Diagrammatic representation of alpha decay
Figure 5. Diagrammatic representation of alpha decay

Beta (β) Decay

Beta decay is a radioactive process in which an electron is emitted from the nucleus of a radioactive atom. When a nucleus is neutron rich i.e., it has higher N/Z ratio compared to the stable nucleus), it decays with β particle and antineutrino (ν).  An antineutrino (ν), is a small entity without mass or charge that is needed to conserve energy in radioactive decay process (Figure 6).

Beta particle is high-speed electron released from degenerating radioactive nucleus emitted with variable energy. Decay or transition energy is the difference in energy between parent and daughter nuclide. Out of all types of radiation, beta particle has medium penetrating and ionizing power i.e., most beta particles can be stopped by a few millimeters of aluminum. Example of beta decay is shown below.

Figure 6. Diagrammatic representation of Beta Decay
Figure 6. Diagrammatic representation of Beta Decay

Interaction of β particles with matter

βparticles emitted by radionuclides interact with surrounding material (medium) and they produce the bremsstrahlung (i.e. “braking radiation” or “deceleration radiation”). Electrons passing through matter are decelerated in columbic field of atomic nuclei and produces x-rays continuously, known as bremsstrahlung (Figure 7). Bremsstrahlung has continuous spectrum with a characteristic profile and energy cutoff. Nikola Tesla discovered this phenomenon. Since electrons are much lighter than protons, electron bremsstrahlung is more common.

Figure 7. Bremsstrahlung produced by interaction of electrons to matter
Figure 7. Bremsstrahlung produced by interaction of electrons to matter

Gamma (γ) Decay

Gamma decay is a type of radioactive decay in which atomic nucleus (parent) emit gamma rays without change in its atomic mass or number. Gamma rays are also known as gamma radiation. These are electromagnetic radiation of high frequency and have no charge or mass. It is a form of ionizing radiation.

Gamma rays are high-energy photons, particle analogue of an electromagnetic wave (Figure 8). The gamma decay also includes two other electromagnetic processes, i.e. internal pair production and internal conversion. It is the most useful radiation that is used for diagnostic and therapeutic imaging in medical science.

Figure 8. Diagrammatic representation of gamma decay
Figure 8. Diagrammatic representation of gamma decay

Positron (β) Decay

Proton rich or neutron deficient nuclei (which has N/Z ratio less than that of the stable nuclei) decay by emitting positron and neutrino. The daughter nuclide has an atomic number 1 less than the parent nuclide. Positron decay only occurs when the difference in energy is larger than 1.02MeV between a parent and a daughter nuclide.

Positron is sometimes referred to as beta plus. 11C, 40K, 13N, 15O, 18F, or 121I decay by emitting positron. The equation shown below describes beta plus decay of 11C emitting 11B, a positron and a neutrino.

These isotopes are used in positron emission tomography (PET), a technique used in imaging. Positron is a positively charged particle, having same mass as beta particle or electron. Positrons emitting nuclides do not exist in nature. When positron combines with electron, annihilation occurs that gives rise to two photons emitted in opposite direction and with 511keV of energy (Figure 9).

Figure 9. Diagrammatic representation of Electron-Positron Annihilation Process
Figure 9. Diagrammatic representation of Electron-Positron Annihilation Process

Electron Capture (EC)

Electron capture is a process of decay for an atom that has too many electrons (nucleus with smaller N/Z ratio as compared to stable nucleus) and insufficient energy to emit a positron (Figure 10). It is also known as inverse beta decay. Gian-Carlo Wick gave the theory of EC in 1934, which was later developed by Hideki Yukawa and other scientists. EC occurs (usually but not necessarily), when the difference in energy is less than 1.02MeV between a parent and a daughter nuclide. An example of EC is shown below.

 

EC involves capture of electrons in the orbit of an atom, usually from the K or L electron shell, by proton present in the nucleus, forming a neutron or a neutrino. It is also called K-electron capture, or K-capture, and L-electron capture, or L-capture, respectively. EC detector (ECD) detects the phenomenon of electron capture. The electron capture detector is one of the most sensitive gas chromatography detectors available.

Figure 10. Diagrammatic representation of Process of Electron Capture
Figure 10. Diagrammatic representation of Process of Electron Capture

Many different radioisotope sources have been proposed and used for ECD however, most commonly used sources are 3H and 63Ni. Lovelock and co-workers have described two alternative modes of ECD operation., the coulometric mode, where electrons react quantitatively with the test molecule, and electron capture spectroscopy, where the energy of the electrons is modified by the application of the radio-frequency field and the response is measured as a function of the electron energy.

Isomeric Transition (IT)

Nucleus of an atom can remain at several excited energy states above the ground state. All these excited states are called isomeric states, which decays to ground state for stability (Figure 11). Time taken for these isomeric states of nuclei to reach ground stable state varies from picoseconds to several years. The long-lived isomeric state is called metastable state. Metastable state is denoted by “m”. For example, metastable state of technetium is denoted as 99mTc.

Figure 11. Diagrammatic Representation of Isomeric Transition of Technetium
Figure 11. Diagrammatic Representation of Isomeric Transition of Technetium

In most isomeric transitions, a nucleus emits its excess energy in the form of gamma rays. The gamma rays that are useful for the diagnostic procedures are generally in the range of 100 keV to 500 keV. Energy of a gamma photon is determined by difference in energy between intermediate and final states of the nucleus undergoing isomeric transition.

Internal Conversion (IC)

Internal conversion is a type of radioactive decay process in which electrons in one of the lower atomic orbital interact with an excited nucleus causing emission of the electron from an atom. A high-energy electron is emitted from the radioactive atom without beta decay. So, the high-speed electrons emitted by internal conversion are not beta particles. It is an alternative to gamma emission.

In internal conversion, the gamma ray (photon) emerges from the nucleus only to interact with one of the innermost orbital electrons and, as a result, the energy of the photon is transferred to the electron. The conversion electron is ejected from the atom with kinetic energy equal to the gamma energy minus the binding energy of the orbital electron. The orbital electron reaches the lower energy state emitting x-rays or an Auger electron.