I. Natural Radioactivity
Radioactivity is a natural phenomenon occurring in isotopes of elements with atomic numbers 20 and higher. This takes place in isotopes of elements with unstable atomic nuclei. Radiation-emitting isotopes are called radioisotopes. When an atom has an unstable nucleus, the number of protons and neutrons are not balanced and there are either too many protons or there are too few of it. This imbalance causes the nuclei to spontaneously decompose to form a more stable nucleus. During this process, energy or particles are released and this is known as radiation.
There are three types of radiation. These are either in the form of alpha (α) and beta (β) particles, positrons (β+), and gamma (γ) rays. An alpha particle is a heavy, short-ranged particle which is similar to helium. It has a mass number of 4 and a charge of +2. Generally, alpha particles are not able to penetrate the skin and are easily blocked off by paper or clothing. However, alpha-emitting particles such as radium (typical source), radon, and uranium can still be harmful to humans if they are inhaled, swallowed, or absorbed through the skin through open wounds. On the other hand, beta (β) particle is a high energy electron with a charge of -1 and mass number of 0. It is formed when a neutron in an unstable nucleus changes to a proton and an electron. Beta radiation is a light, short-range particle which is actually an ejected electron. Because of its very small mass, they move faster and farther than alpha particles. External exposure to beta particles can burn the surface of the skin. To prevent this, protective clothing such as lab gowns are used when handling beta-emitting radiation.
A positron, represented as β+ is similar to a beta particle with a mass number of 0 but with a +1 charge. It is produced from an unstable nucleus when a proton is transformed into a neutron. A positron and is an example of antimatter, which is the exact opposite of an electron.
When an electron and its antimatter, the positron, collide their very small masses are completely converted to energy in the form of gamma (γ) rays. Gamma rays are high-energy radiation released when an unstable nucleus stabilizes to a more stable configuration. It is highly penetrating and can travel as far as 500 meters in the air and 50 centimeters or more through the human tissues. Only very dense shielding such as lead or concrete can protect against gamma radiation. The most common source of gamma radiation is Technetium-99.
The cells most sensitive to ionizing radiation are the ones undergoing rapid division such as those of the bone marrow, skin, reproductive organs, and intestinal lining. Pregnant women are also at risk because a developing fetus with actively dividing cells might be at risk for birth defects. Protection against radiation can be done through shielding, limiting the time of exposure, and increasing distance from radioactive materials.
II. Radiation Measurement
The amount of radiation is usually measured through the use of a Geiger Counter. Nuclear radiation in the form of alpha particles, beta particles, or gamma rays are measured by current pulses produced often displayed by a needle or audible clicks.
Inside a Geiger Counter is a metal tube filled with inert gas such as argon. When radiation enters a window on the end of the tube, argon atoms form ions which then produce electric current. This burst of current is amplified to give a click and a reading on a meter. Figure 1 shows a schematic representation of how a Geiger Counter works:
Figure 1. Schematic Representation of a Geiger Counter (http://en.wikipedia.org/wiki/Geiger_counter)
Several units are being used to measure radioactivity. The curie (CI) is the original unit of activity which is equal to 3.7 X 1010 disintegrations per second. This is equivalent to the number of disintegrations of 1 gram of radium per second. The SI unit of radiation activity is the Becquerel (Bq), which is equal to 1 disintegration per second.
When radiation is applied to the human body, the rad (radiation absorbed dose) is the unit used to measure the amount of radiation absorbed by a gram of material such as body tissue. The SI unit for absorbed dose is the gray (Gy), defined as the joules of energy absorbed by 1 kilogram of body tissue. It is equivalent to 100 rad.
The human body is exposed to different low level radiations from naturally occurring radioactive sources. Examples of these radioisotopes commonly found in air and food are carbon-14, radon-222, strontium-90, and iodine-131. Radiation from the sun is also another source. People in higher altitudes are more exposed to this type of radiation.
A person may also receive a minimal amount of radiation if he is living near a nuclear power plant. Occasional sources of radiation from medical examinations such as dental and chest x-rays add up to radiation exposure. High doses of radiation received at one time may cause a temporary decrease in white cells. If a person is exposed to more than 100 rem of radiation, he may experience the symptoms of radiation sickness such as nausea, vomiting and fatigue.
Not all radiation is bad. In fact, small doses of radiation are being used to improve the shelf-life of food. The US Food and Drug Administration (FDA) has approved the use of 0.3-1.0 kGy of ionizing radiation from Cobalt-60 or Cesium-137 for the treatment of foods. Some fruits and vegetables are irradiated to kill harmful bacteria that cause spoilage and other food-borne illnesses.
III. Half-Life of a Radioisotope
The half-life of a radioisotope is defined as the amount of time necessary for one half of the quantity of nuclide to decay. The half-life for a given isotope is constant. Naturally occurring isotopes of the elements such as Carbon-14 (5730y), Radium-226 (1600y) and Uranium-238 (4.5X109y) have long half-lives. They disintegrate slowly and produce radiation over long periods of time. It is for this reason that they are being used to estimate or “date” old artifacts and other archaeological finds. On the other hand, radioisotopes used in nuclear medicine such as Technetium-99 disintegrate rapidly and are totally eliminated from the body in two days. The continuous decay of radioisotopes until they are all converted into their most stable form is represented in a decay curve. A decay curve is a diagram that shows the decay of each radioisotope for each half-life as shown in Figure 1.
Figure 1. Decay Curve of Iodine-131 (Timberlake, 2011)
Carbon dating is a very popular concept used to estimate the age of organic materials such as human bones or paper. There are three naturally occurring carbon isotopes namely C-12, C-13, and C-14. The C-12 and C-13 radioisotopes are basically stable, but C-14 decays to N-14 with a half-life of approximately 5730 years. Carbon radioisotopes are taken in by plants from the atmosphere through photosynthesis. Herbivores eat plants which introduces carbon into their bodies. When the organism dies, the C-14 continues to decay. Measurement of C-12 is done by burning a sample and estimating the amount of C-12 to determine how much radiocarbon has decayed (“Carbon-14 Dating”). The computation for half-life may be represented through the Exponential Decay Formula:
Eq (1):
A=A0*2(-t/k)
where:
A is the present amount of the radioisotope
A0 is the original amount of the radioisotope
t is the time it takes to reduce the original amount to the present amount
k is the half-life of the isotope
IV. Medical Applications Using Radioactivity
Radioactivity has many applications especially in the field of medicine. Radioisotopes may be used to provide images of body organs which are used by doctors to examine the condition of that body organ without performing an actual operation. For example, Au-198 is used in liver imaging, Ce-141 in gastrointestinal tract diagnosis, I-125 in the treatment of thyroid and Ga-68 in the detection of pancreatic cancer.
Another method in nuclear medicine is the Positron Emission Tomography (PET). This is used in brain scanning of people with neurodegenerative diseases such as Alzheimer’s disease. Positron emitters with short half-lives are used in this method. For example, fluorine-18 combined with glucose in the body is used to study brain function, metabolism, and blood flow. The positrons emitted by these radioisotopes combine with electrons to produce gamma rays which are detected by the computer to create an image of the organ.
The Magnetic Resonance Imaging (MRI) is a powerful imaging technique which is the least invasive. MRI is based on the principles of nuclear magnetic resonance (NMR). The human body is made up of approximately 63% hydrogen atoms. Since hydrogen nuclei have an NMR signal, these are being detected to form images which are based on the changes in the absorption energy when the protons in hydrogen are excited by a stronger magnetic field. The energies absorbed are calculated and converted to color images of the body.
An internal form of radiation therapy called “brachytherapy” is used to treat cancer cells. High dose of radiation is delivered directly to a cancerous area, minimizing damage to normal cells. Because higher doses of radiation are used, fewer treatments over a shorter periods are made. Brachytherapy has been used in the treatment of prostate and cancers.
References:
Hornak, Joseph. The Basics of MRI. Rochester Institute of Technology, 2011. Web. 17 Apr 2012.
Timberlake, Karen. Chemistry: An Introduction to General, Organic, and Biological Chemistry, 11th Edition. Illinois: Prentice Hall, 2012. Print.
“Carbon-14 Dating”. NDT Resource Center. 2010. Web. 17 Apr 2012.
http://en.wikipedia.org/wiki/Geiger_counter
