Radioactivity (Molecular Biology)

Radioactivity is the property of certain elements to undergo spontaneous transformation of their atomic nuclei, the release of energy, and the formation of new elements (decay products). Radioactive nuclei are unstable and seek a more stable configuration by releasing energetic particles or photons of energy. These particles may include alpha particles, beta particles, and positrons. Photons include gamma rays, X rays, and neutrinos, which are discrete quanta of energy without mass or charge. These emissions impart energy to matter by creating tracks of ionized molecules. The emission of charged particles or gamma rays may be measured with radiation detectors. This property of radioactive materials makes them useful for a great number of practical applications in the physical and biomedical sciences.

Every chemical element has one or more radioactive isotopes, and the total number of known radioactive and stable isotopes is more than 1500. Radioactive isotopes (radioisotopes or radionuclides ) of a given element differ in the number of neutrons in the nucleus and, hence, in total atomic mass. A radioactive label (radiolabel) is a radionuclide or radioisotope in a chemical compound that replaces a stable isotope of the same element. It is used to mark the compound for detection by instruments that measure radioactivity. Radioactive labeling is useful for tracking the uptake, retention, metabolism, or clearance of chemical compounds, or for investigating metabolic pathways, enzyme kinetics, or chemical reactions. Radioactive labels are sometimes called radioactive tracers.

The radioactivity of a particular nuclide is determined by the configuration of its atomic nucleus and is independent of the chemical and physical state of the radioisotope and its environment (temperature and pressure). Radioactivity takes many different forms. The process of radioactive disintegration results in changes to the nucleus in atomic number and in its number of nucleons (protons plus neutrons), as indicated in Table 1.

Table 1. Changes in Atomic Number and the Number of Nucleons by Radioactive Transformation®

                                            Change in

Radiation Emitted	Atomic Number	Number of Nucleons
Alpha particle	-2	-4
Beta (minus) particle	+1	0
Beta (plus) particle (or positron)	-1	0
Gamma or X ray	0	0

An example of radioactive transformation is the decay of phosphorus-32 to sulfur-32 by emission of a beta (minus) particle (or electron), -:1-:

Beta decay and positron emission also are accompanied by emission of energy in the form of a nonionizing neutrino. Decay schemes for radionuclides may be complex.

The total energy of the gamma rays and charged particles emitted during radioactive decay is equivalent to the net decrease in the rest mass of the disintegrating atom as it changes from parent to decay product. Its energy, momentum, and electronic charge are conserved. The emitted energy is either kinetic energy of particles in motion or quantum energy of photons, each of which degrades into heat. The ionization of matter through which radiation passes may result in direct or indirect chemical changes and radiation damage.

1. Half-Life

The transformation kinetics of radioactive decay are unique to each radioisotope, and each has its own characteristic, constant decay rate. The time required for any given radioisotope to decay in amount to one-half of its original amount is a measure of the rate at which radioactive transformation takes place. The physical half-life (t1 / 2) of a radioactive atom may range from fractions of a second to billions of years and is unique to each radionuclide. Naturally existing radionuclides have long physical half-lives or are created by the decay of radioisotopes with long half-lives.

If N is the number of identical radioactive atoms and l is the radioactive decay constant (s-1; reciprocal seconds), then 1/l is the activity, and the rate of change in N at any time t is equal to the activity

Integrating from t = 0 (when N = N0), we obtain

Equation 6 describes the observed law of radioactive decay and is a formula that is useful for determining the amount of a radioactive material remaining after a period of time t (Fig. 1).

Figure 1. Semilog plot of the exponential decay of a radioactive material in terms of the number of half-lives that have transpired.

The unit of activity was originally the curie (Ci), the number of disintegrations per second taking place in 1 g of 226Ra, where

In recent years, the curie has been replaced with the special S.I. unit becquerel (Bq), which is one disintegration per second. However, the units of curie, millicurie, and microcurie are still commonly used.

2. Mean Life

The mean life of a radioactive material is the time required for an original amount N0 to decay to 1/e of the original amount. Thus

where t is the mean life (s) of the material:

3. Specific Activity

The specific activity (Bq/g) is the activity (Bq) of a radioactive material per unit mass (g) or volume. The mass or volume may refer to the element itself or to the medium in which the radioactive material is contained. For example, the specific activity of a carbon-14-labeled compound is the radioactivity (Bq) of the carbon-14 divided by the total mass of all the compound molecules in a given volume.

4. Serial Transformation

If the decay products of a radioactive material are themselves radioactive, a decay chain is said to exist. The ingrowth of the first decay product is dependent on the rate of decay of the parent, and so forth through each daughter-product decay, until a stable isotope finally ends the chain. Three natural radioactive series exhibit long decay chains of successive members: the thorium-232 chain, with 12 members, concludes with stable lead-208; the uranium-238 series, with 19 members, concludes with stable lead-206, and the uranium-235 series, with 14 members, concludes with stable lead-207. A fourth series starting with plutonium-241, with 15 members and concluding with bismuth-209, has been created artificially. Each radioactive isotope of a decay chain emits alpha or beta particles and possibly also gamma rays.

An example of serial transformation is given by the decay of krypton-90, a fission product of uranium-235. Each step involves beta (minus) decay:

The rate of formation of a daughter radionuclide, or the number of daughter atoms present (Nd) from its parent (Np), is equal to the rate of formation of the parent; however, each will have different decay rates. If the half-life of the parent (p) is much longer than that of the daughter (d) and only the parent is present at time zero (t = 0), the formation of the daughter product is described by secular equilibrium:

When transient equilibrium occurs, the daughter radioisotope undergoes radioactive transformation at the same rate as it is produced, decreasing in amount with time according to the decay rate of its parent, as shown in Figure 2.

At equilibrium,and the activities of each radioactive member are equalA state of transient equilibrium exists when the half-life of the parent is somewhat greater than that of the daughter and both half-lives are relatively short. An example is the ingrowth of technetium-99 m (the most widely used radioisotope in diagnostic nuclear medicine) from parent molybdenum-99 (Fig. 2). The number of daughter atoms present at any time t is given by

Figure 2. Radioactive decay of molybdenum-99 and ingrowth of technetium-99 m with time (an example of transient equilibrium). The amount of radioactivity from each isotope is plotted as a function of time.

5. Alpha Particles

An alpha particle is equivalent to a helium nucleus (two protons plus two neutrons). Alpha decay usually occurs in nuclei of heavy radioactive atoms. An example of alpha decay is the transformation of radium-226 to radon-222 with a half-life of 1600 years:

The range of alpha particles in media other than air may be estimated from

where A is the atomic number of the absorbing medium and Rair is the range of the alpha particle in

Alpha particles are essentially monoenergetic. During formation of radon-222, two electrons are ejected from the outermost electron shell. The +2 charged helium (He ) nucleus is ejected along a straight path, giving up energy and producing ion pairs along the way. When it comes to rest, it captures two electrons from its environment and becomes a stable, neutral helium atom. Most of the 4.78 MeV is given up to the absorbing medium as kinetic energy. During 95% of the time there is emission of a 4.70 MeV alpha particle and 0.08 MeV as radon-222 recoil energy. In the remaining 5% of the time there is emission of a 4.6-MeV alpha particle, and 0.18 MeV is given off as gamma rays.

The range of the alpha particle in a unit-density medium is only a few micrometers. The thickness of skin or a sheet of paper is sufficient to prevent most alpha-particle penetration. The range, R, of an alpha particle in air at 0°C and 760 mmHg pressure may be estimated from air (cm).

Alpha-particle tracks may give rise to low energy, secondary electron tracks, called delta rays, which radiate outward to distances of tens of nanometers from the primary particle track.

6. Beta Particles

Beta particles are electrons that are ejected from the nucleus of an unstable, beta-emitting atom. Beta particles carry a single negative charge and small mass (only about 1/1800th) that of a proton or neutron). Beta emission appears as the change in a nucleus of one neutron into a proton, and occurs among radionuclides with greater numbers of neutrons than protons in the nucleus. An example was given in equation 1 above.

Beta particles are not monoenergetic, but rather are emitted with a continuous energy distribution, ranging from near zero to the theoretical maximum energy. To comply with the law of conservation of energy, each beta particle is accompanied by emission of a neutrino, whose energy makes up the difference between the theoretical maximum energy of the beta particle and its observed kinetic energy. Gamma rays may accompany beta emission in order to reach the ground energy state of the daughter product. For example, potassium-42 decays 82% of the time by a maximum energy of 3.55 MeV to calcium-42, and 18% of the time by a maximum energy of 2.04 MeV, together with a gamma ray of 1.53 MeV.

When beta particles slow to rest, they transfer a negative charge to the absorber. Their range is a few millimeters in unit-density tissue. The beta energy range may be measured by adding successively thicker absorbers until a count rate cannot be detected. An absorber that stops one-half the beta particles is about one-eighth the range of beta particles.

7. Positrons

Positrons are positively charged electrons that are emitted from atomic nuclei where the neutron:proton ratio is low and sufficient energy is not available for alpha-particle decay. Positron emission represents the transformation within the nucleus of a proton into a neutron. In other ways, the emission of positrons is similar to that of beta (minus) particles, which have similar mass and range in tissue. When a positron comes to rest, it quickly combines with an electron, and the two particles annihilate and give off two gamma-ray photons, whose energies are equal to the mass equivalent of the positron plus the electron (two photons of 0.511 MeV). An example of positron decay is the transformation of sodium-22 to neon-22:

When an atom decays by electron capture, a characteristic X ray (photon) is emitted as an electron from an outer orbit falls into the energy level of the captured electron.

8. Electron Capture

Some radionuclides decay by the process of electron capture, in which a K orbital electron is captured into the nucleus, uniting with a proton or hydrogen nucleus and changing it into a neutron:

 

9. Gamma Rays

Gamma rays are monoenergetic photons or quanta of energy with discrete frequency that are emitted from the nucleus during radioactive decay to remove excess energy. Like X rays, gamma rays are highly penetrating electromagnetic photons of energy. Gamma rays usually accompany beta decay and always accompany positron decay. Gamma rays are attenuated by matter, and the efficiency of the shielding increases with atomic number.

10. Internal Conversion

Radioactive decay by internal conversion takes place when an unstable nucleus of a gamma-emitting nucleus gives off excess excitation energy by imparting energy to an orbital K- or L-shell electron, ejecting it from the atom. Characteristic X rays are emitted as outer-shell orbital electrons collapse inward to fill vacant energy levels produced by ejected electrons. If the characteristic X rays are absorbed by an inner orbital electron, internal conversion may take place, ejecting the electron (called an Auger electron).

11. Radiation Absorbed Dose

Radiation absorbed dose is the energy deposited by radiation per unit mass of the absorbing medium. The radiation absorbed dose in units of gray (Gy) to a unit-density medium containing an alpha-emitter is

where Ea is the average alpha-particle energy, Na is the number of alpha particles emitted in the medium, and g is the mass of the medium.

The radiation absorbed dose rate (D°, in grays per second) from a beta-emitting radioisotope under conditions of charged-particle equilibrium can be estimated from

where Eb is the average beta-particle energy per disintegration and Nb is the number of beta-particle disintegrations taking place per second, all per gram of medium. If the mass of the medium is small, some of the beta energy may escape, and conditions for charged-particle equilibrium may not be met.