Physics SS 3 Week 4
Topic: RADIOACTIVITY
RADIOACTIVITY
Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two stronger forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta and gamma radiation, but there are several other varieties of radioactive decay.
Alpha Radioactivity
Composed of two protons and two neutrons, the alpha particle is a nucleus of the element helium. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it has a very short range. It is not sustainable for radiation therapy since its range is less than a tenth of a millimeter inside the body. Its main radiation hazard comes when it is ingested into the body; it has great destructive power within its short range. In contact with fast-growing membranes and living cells, it is positioned for maximum damage.
Alpha particle emission is modeled as a barrier penetration process. The alpha particle is the nucleus of the helium atom and is the nucleus of highest stability.
Alpha Barrier Penetration
The energy of emitted alpha particles was a mystery to early investigators because it was evident that they did not have enough energy, according to classical physics, to escape the nucleus. Once an approximate size of the nucleus was obtained by Rutherford scattering, one could calculate the height of the Coulomb barrier at the radius of the nucleus. It was evident that this energy was several times higher than the observed alpha particle energies. There was also an incredible range of half lives for the alpha particle which could not be explained by anything in classical physics.
The resolution of this dilemma came with the realization that there was a finite probability that the alpha particle could penetrate the wall by quantum mechanical tunneling. Using tunneling, Gamow was able to calculate a dependence for the half-life as a function of alpha particle energy which was in agreement with experimental observations.
Alpha, Beta, and Gamma
Historically, the products of radioactivity were called alpha, beta, and gamma when it was found that they could be analyzed into three distinct species by either a magnetic field or an electric field.
Penetration of Matter
Though the most massive and most energetic of radioactive emissions, the alpha particle is the shortest in range because of its strong interaction with matter. The electromagnetic gamma ray is extremely penetrating, even penetrating considerable thicknesses of concrete. The electron of beta radioactivity strongly interacts with matter and has a short range.
Different radiations have different properties, as summarized below:
Properties of Radiation
| Radiation | Alpha (α) –particles | Beta particles | Gamma (λ) rays |
| Nature | Helium nuclei 42H | High energy electrons | Electromagnetic waves of very short wavelength |
| Velocity |
Charge
5-7% speed of light
+2e(+3.2 x 10-19 C)Travel at approx speed of light –e(-1.6 x 10-19 C)Travel at speed of light
Electrically Neutral Mass Relatively
Massive Relatively light Negligible Effect of magnetic field Slightly deflected in a magnetic field in a direction expected for a +ve charge Strongly deflected in a magnetic field in a direction expected for a -ve charge Small or no effect Ionizing power Large, cause heavy ionization Medium. About 0.1% of that of α-particles Small Penetrating power Little penetrating power e.g. by thin sheets of paper Good penetrating power in air, several mm of aluminium High penetrating power in air and in solid e.g. many cm of lead Flourescent Cause fluorescence in ZnSNo fluorescence in ZnS
Peaceful Uses of Radiation
Although scientists have only known about radiation since the 1890s, they have developed a wide variety of uses for this natural force. Today, to benefit humankind, radiation is used in medicine, academics, and industry, as well as for generating electricity. In addition, radiation has useful applications in such areas as agriculture, archaeology (carbon dating), space exploration, law enforcement, geology (including mining), and many others.
1. Medical Uses
2. Academic and Scientific Applications
3. Industrial Uses
4. Nuclear Power Plants
Medical Uses
Hospitals, doctors, and dentists use a variety of nuclear materials and procedures to diagnose, monitor, and treat a wide assortment of metabolic processes and medical conditions in humans. In fact, diagnostic x-rays or radiation therapy have been administered to about 7 out of every 10 Americans. As a result, medical procedures using radiation have saved thousands of lives through the detection and treatment of conditions ranging from hyperthyroidism to bone cancer.
The most common of these medical procedures involve the use of x-rays — a type of radiation that can pass through our skin. When x-rayed, our bones and other structures cast shadows because they are denser than our skin, and those shadows can be detected on photographic film. The effect is similar to placing a pencil behind a piece of paper and holding the pencil and paper in front of a light. The shadow of the pencil is revealed because most light has enough energy to pass through the paper, but the denser pencil stops all the light. The difference is that x-rays are invisible, so we need photographic film to “see” them for us. This allows doctors and dentists to spot broken bones and dental problems.
X-rays and other forms of radiation also have a variety of therapeutic uses. When used in this way, they are most often intended to kill cancerous tissue, reduce the size of a tumor, or reduce pain. For example, radioactive iodine (specifically iodine-131) is frequently used to treat thyroid cancer, a disease that strikes about 11,000 Americans every year.
X-ray machines have also been connected to computers in machines called computerized axial tomography (CAT) or computed tomography (CT) scanners. These instruments provide doctors with color images that show the shapes and details of internal organs. This helps physicians locate and identify tumors, size anomalies, or other physiological or functional organ problems.
In addition, hospitals and radiology centers perform approximately 10 million nuclear medicine procedures in the United States each year. In such procedures, doctors administer slightly radioactive substances to patients, which are attracted to certain internal organs such as the pancreas, kidney, thyroid, liver, or brain, to diagnose clinical conditions.
Academic and Scientific Applications
Universities, colleges, high schools, and other academic and scientific institutions use nuclear materials in course work, laboratory demonstrations, experimental research, and a variety of health physics applications. For example, just as doctors can label substances inside people’s bodies, scientists can label substances that pass through plants, animals, or our world. This allows researchers to study such things as the paths that different types of air and water pollution take through the environment. Similarly, radiation has helped us learn more about the types of soil that different plants need to grow, the sizes of newly discovered oil fields, and the tracks of ocean currents. In addition, researchers use low-energy radioactive sources in gas chromatography to identify the components of petroleum products, smog and cigarette smoke, and even complex proteins and enzymes used in medical research.
Archaeologists also use radioactive substances to determine the ages of fossils and other objects through a process called carbon dating. For example, in the upper levels of our atmosphere, cosmic rays strike nitrogen atoms and form a naturally radioactive isotope called carbon-14. Carbon is found in all living things, and a small percentage of this is carbon-14. When a plant or animal dies, it no longer takes in new carbon and the carbon-14 that it accumulated throughout its life begins the process of radioactive decay. As a result, after a few years, an old object has a lower percent of radioactivity than a newer object. By measuring this difference, archaeologists are able to determine the object’s approximate age.
Industrial Uses
We could talk all day about the many and varied uses of radiation in industry and not complete the list, but a few examples illustrate the point. In irradiation, for instance, foods, medical equipment, and other substances are exposed to certain types of radiation (such as x-rays) to kill germs without harming the substance that is being disinfected — and without making it radioactive. When treated in this manner, foods take much longer to spoil, and medical equipment (such as bandages, hypodermic syringes, and surgical instruments) are sterilized without being exposed to toxic chemicals or extreme heat. As a result, where we now use chlorine — a chemical that is toxic and difficult-to-handle — we may someday use radiation to disinfect our drinking water and kill the germs in our sewage. In fact, ultraviolet light (a form of radiation) is already used to disinfect drinking water in some homes.
Similarly, radiation is used to help remove toxic pollutants, such as exhaust gases from coal-fired power stations and industry. For example, electron beam radiation can remove dangerous sulphur dioxides and nitrogen oxides from our environment. Closer to home, many of the fabrics used to make our clothing have been irradiated (treated with radiation) before being exposed to a soil-releasing or wrinkle-resistant chemical. This treatment makes the chemicals bind to the fabric, to keep our clothing fresh and wrinkle-free all day, yet our clothing does not become radioactive. Similarly, nonstick cookware is treated with gamma rays to keep food from sticking to the metal surface.
The agricultural industry makes use of radiation to improve food production and packaging. Plant seeds, for example, have been exposed to radiation to bring about new and better types of plants. Besides making plants stronger, radiation can be used to control insect populations, thereby decreasing the use of dangerous pesticides. Radioactive material is also used in gauges that measure the thickness of eggshells to screen out thin, breakable eggs before they are packaged in egg cartons. In addition, many of our foods are packaged in polyethylene shrink-wrap that has been irradiated so that it can be heated above its usual melting point and wrapped around the foods to provide an airtight protective covering.
All around us, we see reflective signs that have been treated with radioactive tritium and phosphorescent paint. Ionizing smoke detectors, using a tiny bit of americium-241, keep watch while we sleep. Gauges containing radioisotopes measure the amount of air whipped into our ice cream, while others prevent spillover as our soda bottles are carefully filled at the factory.
Engineers also use gauges containing radioactive substances to measure the thickness of paper products, fluid levels in oil and chemical tanks, and the moisture and density of soils and material at construction sites. They also use an x-ray process, called radiography, to find otherwise imperceptible defects in metallic castings and welds. Radiography is also used to check the flow of oil in sealed engines and the rate and way that various materials wear out. Well-logging devices use a radioactive source and detection equipment to identify and record formations deep within a bore hole (or well) for oil, gas, mineral, groundwater, or geological exploration. Radioactive materials also power our dreams of outer space, as they fuel our spacecraft and supply electricity to satellites that are sent on missions to the outermost regions of our solar system.
Nuclear Power Plants
Electricity produced by nuclear fission — splitting the atom — is one of the greatest uses of radiation. As our country becomes a nation of electricity users, we need a reliable, abundant, clean, and affordable source of electricity. We depend on it to give us light, to help us groom and feed ourselves, to keep our homes and businesses running, and to power the many machines we use. As a result, we use about one-third of our energy resources to produce electricity.
Electricity can be produced in many ways — using generators powered by the sun, wind, water, coal, oil, gas, or nuclear fission. In America, nuclear power plants are the second largest source of electricity (after coal-fired plants) — producing approximately 21 percent of our Nation’s electricity.
The purpose of a nuclear power plant is to boil water to produce steam to power a generator to produce electricity. While nuclear power plants have many similarities to other types of plants that generate electricity, there are some significant differences. With the exception of solar, wind, and hydroelectric plants, power plants (including those that use nuclear fission) boil water to produce steam that spins the propeller-like blades of a turbine that turns the shaft of a generator. Inside the generator, coils of wire and magnetic fields interact to create electricity. In these plants, the energy needed to boil water into steam is produced either by burning coal, oil, or gas (fossil fuels) in a furnace, or by splitting atoms of uranium in a nuclear power plant. Nothing is burned or exploded in a nuclear power plant. Rather, the uranium fuel generates heat through a process called fission.
Nuclear power plants are fueled by uranium, which emits radioactive substances. Most of these substances are trapped in uranium fuel pellets or in sealed metal fuel rods . However, small amounts of these radioactive substances (mostly gases) become mixed with the water that is used to cool the reactor. Other impurities in the water are also made radioactive as they pass through the reactor. The water that passes through a reactor is processed and filtered to remove these radioactive impurities before being returned to the environment. Nonetheless, minute quantities of radioactive gases and liquids are ultimately released to the environment under controlled and monitored conditions.
Radioactive Decay; Half-Life; Decay Constant
Radioactivity is a spontaneous process. It goes on independent of external control. it is not affected by temperature or pressure or by chemical treatment . It is also a random process as no one can predict which atom will disintegrate at a given time.
Experiments have shown that each radioactive element has a definite rate of decay which can be characterized by its Half-life.
Half-Life of a radioactive element is the time taken for half of the atoms initially present in the element to decay.
If the half-life of an element is T years, it means that after T years, 1 gm of the element will have a mass of 1/2gm, after 2T years, the mass of the element will be 1/4gm (or ½ of ½ gm) and so on.
Thus if we have 1000 atoms of a radioactive element initially, whose half-life is 10 years, then after 10 years, 500 atoms will remain ; after 20 years, 250 atoms will be left and after 30 years, 125 atoms will be left undecayed and so on.
Decay Constant, λ
The rate of decay of radioactive elements is found to be proportional to the number of atoms of the material present. Suppose there are N atoms of a radioactive element present at a time, t, then the probable number of disintegrate per unit time or activity can be expressed by –dN/dt (The minus sign arises from the fact that N is decreasing with time). Since the rate of disintegration is proportional to the number of atoms present at a given time, we have
– dN/dt α N or dN/dt = -λN
where λ is a constant of proportionality called the Decay Constant of the element.
From the above equation we have
λ = -1/N(dN/dt)
Hence Decay Constant is defined as the instantaneous rate of decay per unit atom of a substance OR
no. of atoms disintegrating per second/no. of atoms in the source at the time = λ
Also by interpreting the first equation, we have that
N = N0e-λt
where N0 is the number of atom present at time t = 0 (i.e. at the time when observations of decay were begun) and N is the number of atoms present at time t.
We obtain the time required for half of the atoms to disintegrate (half-life) by substituting N = 1/2N0 into this equation N = N0e-λt and eliminating N0 we have N0/2 = N0e-λt
½ = e –λt
Taking the natural or Naperian logarithm of both
loge ½ = – λt
But loge ½ = loge 1 – loge 2 = 0 – loge 2 = -0.693
Hence, -0.693 = -λt
t = 0.693/λ
Questions
1. Radioactivity refers to the particles which are emitted from nuclei as a result of
A. nuclear stability B. nuclear instability C. Uranium decrement D. Proton Increment
2. Which of these is not part of radioactive emission?
A. Alpha B. Gamma C. Electron D. Beta
3. Complete this statement, The rate of decay of radioactive elements is found to be ………………………….. to the number of atoms of the material present.
A. Proportional B. Inverse C. indirect D. direct
4. Half-Life of a radioactive element is the time taken for half of the atoms initially present in the element to …………………………
A. form B. compose C. decay D. stimulate
5. A certain radioactive element has a half-life of 10 years. How long will it take to loose 7/8 of its atoms originally present.
A. 20 B. 30 C. 25 D. 35
Answer
1. B 2. C 3. A 4. C 5. B