Radiation and Radioisotopes
Atomic Structure
Radioactivity
Types of Emission
Energy
Types of Emmision
Activity
Decay Rate and Half-Life
Interactions with Matter
Radiation Quantities and Units
Characteristics of Commonly Used Radionuclides

Radiation is simply the movement of energy through space or another media in the form of waves, particles, or rays. Radioactivity is the name given to the natural breakup of atoms which spontaneously emit particles or gamma/X energies following unstable atomic configuration of the nucleus, electron capture or spontaneous fission.

The universe is filled with matter composed of elements and compounds. Elements are substances that cannot be broken down into simpler substances by ordinary chemical processes (e.g., oxygen) while compounds consist of two or more elements chemically linked in definite proportions. Water, a compound, consists of two hydrogen and one oxygen atom as shown in its formula H₂O. While it may appear that the atom is the basic building block of nature, the atom itself is composed of three smaller, more fundamental particles called protons, neutrons and electrons. The proton (p) is a positively charged particle with a magnitude one charge unit (1.602 x 10^-19 coulomb) and a mass of approximately one atomic mass unit (1 amu = 1.66x10^-24 gram). The electron (e-) is a negatively charged particle and has the same magnitude charge (1.602 x 10^-19 coulomb) as the proton. The electron has a negligible mass of only 1/1840 atomic mass units. The neutron, (n) is an uncharged particle that is often thought of as a combination of a proton and an electron because it is electrically neutral and has a mass of approximately one atomic mass unit. Neutrons are thought to be the "glue" which binds the nucleus together.

Combinations of the fundamental particles following certain strict natural laws result in the formation of atoms. In concept, the neutrons and protons form a dense, central core or nucleus around which the electrons rotate in various orbits. Nearly all of an atom's mass is located in the nucleus. Natural law specifies that each atom has the same number of protons as it has electrons. This means that the total positive charge in the nucleus is equal to the total negative charge of the orbiting electrons and this produces an electrically neutral atom.

Each element has a unique number of protons (and corresponding neutrons) that determine its chemical properties. The number of protons in an atom is its atomic number, represented by the symbol Z. Thus, for carbon, which has 6 protons, Z=6. When the chemical symbol for an element is used with its atomic number, the atomic number is subscripted, e.g., ₆C. Thus, all atoms with an atomic number of 1 are hydrogen atoms (₁H); 2 are helium atoms (₂He); 3 atoms are lithium (₃Li); 4 are beryllium atoms (₄Be); etc.

The chemical properties of an atom are determined by the number of protons contained in the nucleus. For example, every atom which has six protons in its nucleus is a carbon atom. However, while all the atoms of a particular element have the same number of protons, they may have different numbers of neutrons. For carbon, there can be five, six, seven, or eight neutrons. Each of these atoms is a different isotope of carbon. Figure 1 illustrates the isotopes of carbon. All the isotopes of carbon are chemically identical, because the chemical properties are dictated by the atomic number (number of protons) of the element. The term nuclide means any isotope of any element.

Figure 1. The Isotopes of Carbon

Naturally occurring elements often have several different isotopes. While most of these naturally occurring isotopes are stable, some are unstable. Usually an atom is unstable because the ratio of neutrons to protons produces a nuclear imbalance (i.e., too many protons or too many neutrons in the nucleus). These unstable atoms attempt to become stable by rearranging the number of protons and neutrons in the nucleus to achieve a more stable ratio. The excess energy from this rearrangement is ejected from the nucleus as kinetic energy. In this rearrangement, the isotope usually changes atomic number and sheds any excess energy by emitting secondary particles and or electromagnetic rays/photons. This change in the nucleus is called nuclear disintegration. The entire process of unstable isotopes disintegrating and emitting energy is called radioactive decay or decay and an isotope capable of undergoing radioactive decay is said to be radioactive.

Most of the isotopes encountered in nature are not radioactive. However, there are mechanisms to inject energy into an isotope's nucleus causing it to become unstable. To activate an isotope is to make it radioactive. This can be accomplished in a nuclear reactor where the nucleus can be bombarded by neutrons or in an accelerator where high speed electrons, protons, or larger particles can be injected into the nucleus. If something merely touches a radioisotope, it does not become radioactive, but it may become contaminated with radioactive material (see Chapter 6).

Unstable nuclei are radioactive. Unlike chemical processes which occur at the electron level and can be affected by external forces like heat, there is no known way to alter radioactive decay. Radioactive decay cannot be artificially accelerated or slowed down because radioactive instability involves extremely strong nuclear forces. Each isotope decays at a rate that is unique among all other nuclides. Additionally, the type and magnitude of the radioactive energy emitted depends upon the isotope. Thus, there are three parameters that uniquely identify any radionuclide: the type(s) of energy emitted, the magnitude of the energy, and the rate at which the isotope decays.

When a radioisotope decays it normally emits one or more of four basic types of radiations: alpha particles, beta particles, X- or gamma rays, and neutrons. These radiations interact with atoms and molecules in the environment and deposit their energy. Table 1, at the end of this section, presents some properties of these decay types.

An alpha (a) particle is a massive particle on the atomic scale. It consists of 2 neutrons and 2 protons and carries an electrical charge of +2. It is identical to a helium nucleus. Because the alpha particle is massive and highly charged it has a very short range and travels less than 5 cm in air or 0.044 cm in tissue before expending its energy, stopping, picking up two electrons to become a stable helium atom. Thus alpha particles are generally not a hazard to workers unless they get inside the body where they may cause much greater cellular damage than beta or gamma radiation.

A beta (ß) particle is a fast electron with a single charge. Depending on the isotope and mechanism of decay, the beta particle can have a negative or positive charge. The positively charged beta particle is called a positron (+ß). It usually results when the neutron:proton ratio is too low but when the alpha emission is not energetically possible. Positron emission produces a daughter nucleus which has the same atomic mass but is one less atomic number. The negatively charged beta particle is properly called an electron (e-) and, in every day usage the term beta radiation usually refers to the negative type, -ß. Beta (-ß) emission occurs when the neutron:proton ratio is too high.

Figure 2. Beta Decay Spectrum

Because beta radiation is a small particle with only a single charge, a beta particle has a much greater range than an alpha particle with the same energy. Low energy beta particles, energies less then 200 keV, are easily shielded and only pose a potential hazard if they get inside the body. Thus, the beta particle emitted from ³H with a maximum energy of 18 keV only travels about 6 mm in air and less than 0.00052 cm in tissue. Beta particles with energy less than 70 keV will not penetrate the protective layer of skin. Of the beta particles emitted from ¹⁴C or ³⁵S (Emax ~ 160 keV), only 11% are capable of penetrating the dead layer of skin (0.007 cm thick). On the other hand, high energy beta particles have longer ranges. The range of beta particles in air is approximately 12 ft per MeV. Therefore, the beta from ³²P ejected with a maximum energy of 1.7 MeV could travel up to 20 feet in air and 95% of the beta particles can penetrate the dead layer of skin, so ³²P may pose a potential radiation hazard even from outside the body. Shielding large quantities of high energy beta particle emitters is usually done with plastic or plexiglass because when the beta particles are shielded with dense materials like lead, bremsstrahlung xrays (see Chapter 5) are produced.

A gamma (?) ray is an electromagnetic ray emitted from the nucleus of an excited atom following radioactive disintegrations. Unlike beta particles, which are emitted in a spectrum of energies, gamma rays are emitted at discrete energies and provide a mechanism for the excited nucleus to rid itself of the residual decay energy that was not carried off by the particle emitted in its decay. Thus, many isotopes which decay by beta emission also have gamma rays (or photons) associated with the disintegration. Gamma rays are similar to light but of shorter wavelength and higher energy. Consider activities greater than 1 mCi a radiation hazard and shield with thick, dense material such as lead.

Figure 3. Gamma Ray Decay

An x-ray is an electromagnetic ray, identical to a gamma ray in all respects except for point of origin. Gamma rays are emitted from the nucleus as part of the nuclear decay process. X-rays originate from outside the nucleus normally as part of electron orbital changes. A neutron (n) is an elementary nuclear particle with a mass approximately the same as that of a proton and is electrically neutral. Normally the neutron decays to a proton. Except when bound in the nucleus, the neutron is not a stable particle. A free neutron decays to a proton with the emission of a -ß and an antineutrino. This decay process takes on the average about 12 minutes. There are few naturally occurring neutron emitters. Aside from nuclear-fission reactions, the only way to produce neutron sources is through bombardment of the nuclei with high energy radiation (both particles and rays). Because a neutron is uncharged, it easily passes through the electron cloud and can interact with the nucleus of the atom, often making the atom radioactive.

Table 1. Decay Types

The energy carried away by the radiation is expressed in units of electron volts (eV). The electron volt is a very small quantity of energy (1.6x10-19 Joule). Most radiations are ejected with energies of many thousand or millions of electron volts, listed as either keV (kiloelectron volts - 1000 eV) or MeV (megaelectron volts - 1,000,000 eV). The amount of energy involved and the type of radiation emitted determines the penetrability of the radiation and consequently the shielding thickness and type required to protect workers from radiation. All things being equal, the higher the energy, the more penetrating the radiation. For example, gamma rays have higher decay energy and need more shielding than alpha or beta particles which have lower decay energies.

The decay of a radioactive sample is statistical. Just as it is not possible to change a specific isotope's rate of decay, it is impossible to predict when a particular atom will disintegrate. Rather, one measures activity as the number of radioactive nuclei that change or decay per unit time (e.g., second). The special unit of activity is the curie (Ci) where 1 curie represents 37 billion (3.7 x 1010) nuclear disintegrations (decays) per second (dps) or alternately, 2.22x1012 disintegrations per minute (dpm). Sub-multiples of the curie are the millicurie (mCi) which represents one-thousandth (3.7 x 107 dps) of a curie and the microcurie (µCi) which represents one-millionth (3.7 x 104 dps) of a curie. As will be discussed later in this section, everywhere except in the U.S., the curie unit has been replaced by the bequerel (Bq) unit where 1 Bq is 1 nuclear disintegration or decay per second.

Radioisotopes are always in a state of decay, emitting energy. Therefore, the amount of radioactivity remaining is continually decreasing. Each radioisotope has a unique decay rate and that decay rate is the physical half-life (T_1/2) of the radioisotope. Half-life describes the length of time required for the amount of radioactivity present to decrease to half of its original amount. This decrease in material is not linear. Figure 4 shows a plot of the decay of a radioisotope having a half-life of 1 day so you can see how rapidly the amount decreases.

Figure 4. HALF-LIFE

Referring to Figure 4, if we start with 100 microcuries of material, by the end of the second day (two half-lifes) we will have only 25 microcuries of material, and at the end of the fourth day only 6.25 microcuries will remain. However, there are still over one quarter million disintegrations per second remaining, even after four half-lives.

From Figure 4 you can see that the greater the number of radioactive atoms that are initially present, the greater the number of nuclei that will decay during a half-life (e.g., if 100 radioactive atoms are present, 50 will decay in one half-life, if 1000 radioactive atoms are present, on average 500 will decay in one half life). The decay rate or activity of a radioactive sample is proportional to the number of unstable nuclei that are initially present. This relationship is expressed by the universal decay equation where A0 is the original radioactivity of the sample, At is the amount of radioactivity remaining after the elapsed time, t, and _1/2 is the (physical) half-life of the radioisotope. The decay constant, ?, expresses the rate of decay as a factor of the radioactive half-life (T_1/2, where ? = ln 2/T_l/2.

Figure 5. Universal Decay Equation

When alpha, beta or gamma radiation passes through matter it interacts with atoms and molecules, depositing energy in the matter until it has spent its kinetic energy and comes to rest or is absorbed. Ionizing radiation interacts at the orbital electron level and results in ionization and excitation of the atoms in the matter. Ionization (Figure 6) is the process where the electrons are knocked out of their orbits producing ion pairs (i.e., a free electron and a positively charged atom or molecule). If sufficient energy is deposited an orbital electron may be excited and on returning to ground state, emit low energy radiation.

Figure 6. Ionization

A quantity is some physically measurable entity (e.g., length, mass, time electric current, etc.) that needs to be measured. A unit is the amount of quantity to be measured. Units for various quantities are formulated when needed by national or international organizations such as the National Institute of Science and Technology (NIST) or the international General Conference on Weights and Measures (CGPM). Traditional units were formulated by these organizations.

Traditional units used to measure and quantify radioactive materials were developed during late 1800's during early research into radioactive materials. That early research showed that the number of ion pairs produced in a physical substance is related to the amount of radiation energy deposited in the substance. The oldest, still used, radiation unit, the roentgen (R), is based on the number of ion pairs produced in a volume of air traversed by x- or gamma radiation . This unit of x-/? radiation exposure in air, is defined to be the collection of 2.58x10-4 coulombs per kilogram of air. Since each electron carries a charge of 1.6x10-19 coulomb, this represents 1,610,000,000,000,000,000 ion pairs in a kilogram of air. Submultiples, the milliroentgen (mR) and the microroentgen (µR) are also frequently used.

Early radiation researchers also investigated the effects of radiation energy on matter. Initially the roentgen was widely used, but because it is limited to x-/? radiation in air, a second unit, the rad, was defined to be the unit of absorbed dose in any matter. The rad equals 100 ergs of energy deposited per gram of matter. Although the roentgen is a unit of radiation exposure in air and a rad is a unit of exposure in tissue, the two units are very close in magnitude. The are correlated by the fact that 1 roentgen produces 0.96 rad in biological tissue. An easy method for defining think of it as the acronym for radiation absorbed dose.

Investigating the effects of radiation at the cellular level, researchers found that for the same quantity of absorbed dose, different types of radiations produced different amounts of cellular damage. For example, the cellular damage from an absorbed dose of 100 rad from alpha particles was significantly more severe than the damage caused by 100 rad from gamma rays. The "quality" of the alpha particle's deposited energy, at the cellular level, is greater than the "quality" of the gamma ray's deposited energy. The rem is the unit of radiation dose equivalence used to equalize the biological effectiveness of the various types of radiation. The radiation absorbed dose in rad, multiplied by the radiation's quality factor, Q, which varies from 1-20 (Table 3), produces the dose equivalent, in rem, of the radiation exposure (i.e., rem = rad x Q). Thus, an absorbed dose of 1 rad to tissue from a radionuclide deposition produces a dose equivalence of 1 rem if the radionuclide is ß/? emitter and a dose equivalence of 20 rem if the radionuclide is an a emitter. An easy method for defining a rem is to think of it as the acronym for roentgen equivalent man.

In the 1970's an international agreement was developed to make all physical constants convertible by multiplying by units of "1" alone. This system of measurement is called the international system of units or SI (Le Système International d' Unitès). SI units are based on the MKS (meters, kilogram, second) system. Thus, the unit of (radio)activity was changed from the curie which is 3.7 x 1010 nuclear disintegrations per second (dps) to the becquerel (Bq) which is 1 dps. The traditional unit of absorbed dose, the rad, was replaced by a new unit of absorbed dose, the gray (Gy), which is defined as 1 Joule per kilogram, and is equal to 100 rad. Similarly, the unit of dose equivalent was replaced by the sievert (Sv) which is equal to 100 rem. Table 3 shows the relationship between the traditional and new (SI) units. However, the U.S. has not yet converted to the SI units and all domestic regulations continue to use the traditional (special) units of curie, rad and rem.

Radioisotope use at the University typically consists of small quantities of liquid materials, but some users have laboratories where relatively large quantities of radioactive material are used. To ensure worker safety and to prevent accidental exposure, all labs where radioactive materials may be used or stored are conspicuously posted with "Caution - Radioactive Materials" signs. To reduce radiation exposure, workers in these posted areas must understand the characteristics of the radioisotopes. Characteristics of commonly used radioisotopes are listed in Table 5. This table includes: isotope and chemical symbol (e.g., ³ H, 32^P); half-life (T½); and, most importantly, energy of the major radiations emitted. Because ß particles are emitted in a wide spectrum of energies, the energy listed is the maximum energy that the emitted beta particle can possess.