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Chapter 10:Radioactivity and Nuclear Processes

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RADIOACTIVE NUCLEI• In 1896 Henri Becquerel, a French physicist, discovered

that uranium compounds emitted rays that could expose and fog photographic plates wrapped in lightproof paper.

• Research showed that the penetrating rays originate from changes that occur in the nuclei of some atoms.

• Radioactive nuclei are nuclei that undergo spontaneous changes and emit energy in the form of radiation.

• The emission of radiation by radioactive nuclei is often called radioactive decay.

• The intensity of radiation is unaffected by factors that normally influence the rates of chemical reactions: the temperature, pressure, and type of uranium compound used.

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• Radiation emitted by uranium, and by other radioactive elements, can be separated into three types by an electrical or magnetic field.

• The three types had different electrical charges: One was positive (alpha rays), one was negative (beta rays), and one carried no charge (gamma rays).

• Today it is known that other types of radiation (e.g. neutrons and positrons) are also emitted by radioactive nuclei.

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ALPHA RADIATION• Alpha radiation consists of a stream of particles called

alpha particles. Alpha particles are identical to helium-4 nuclei; they consist of two protons and two neutrons.

BETA RADIATION• Beta radiation consists of a stream of beta particles that

are identical to electrons. They are created in the nucleus of radioactive atoms when a neutron is converted into a proton and an electron.

GAMMA RADIATION• Gamma radiation consists of high energy rays similar to X

rays, but with a higher energy.

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NEUTRON RADIATION• Neutron radiation consists of a stream of high energy

neutrons emitted from the nuclei of radioactive nuclei.

POSITRON RADIATION• Positron radiation consists of a stream of positrons which

are identical to electrons except they have a positive electrical charge. Positrons are created in the nucleus of radioactive atoms when a proton is converted into a neutron and a positron.

ELECTRON CAPTURE• While electron capture does not emit a stream of particles,

it is a mode of decay for some unstable nuclei in which an electron from outside the nucleus is drawn into the nucleus, where it combines with a proton to form a neutron.

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CHARACTERISTICS OF NUCLEAR RADIATION• The characteristics of the common types of radiation along

with the symbols used to represent them are summarized in the following table:

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EQUATIONS FOR NUCLEAR REACTIONS• In nuclear reactions, specific isotopes of an element may

behave differently. • For that reason, all particles involved in nuclear reactions

are designated by the notation , where X is the symbol for the particle, A is the particle mass number, and Z is the particle atomic number or electrical charge.

• In the notation, the symbol for the element is used as X when the nucleus of the element is involved in the radioactive decay.

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• The equation for a nuclear reaction is balanced when the sum of the atomic numbers of the particles on the left side of the equation equals the sum of the atomic numbers of the particles on the right side, and the sum of the mass numbers on the left side equals the sum of the mass numbers on the right side.

• When an unstable nucleus undergoes radioactive decay and the nucleus of a new element is produced as a product, the nucleus of new element that is produced is called a daughter nucleus.

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EXAMPLES OF NUCLEAR REACTIONS• Example 1: Bromine-84 decays by emitting a beta particle.

What is the symbol for the daughter produced?

• Solution: The symbol for bromine-84 is . A beta particle has a mass number (the upper number) of 0, and a charge (the lower number) of -1. Thus, the daughter must have a mass number of 84 and an atomic number of 36. The element with an atomic number of 36 is krypton with a symbol of . The balanced equation is:

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• Example 2: When samarium-148 undergoes radioactive decay, the daughter produced is neodymium-144. What kind of radiation is emitted during the decay?

• Solution: The daughter has a mass number of 144, so the emitted radiation must have a mass number of 4. The difference between the atomic numbers is 2. Therefore, it is an alpha particle. The balanced equation is:

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• Example 3: Tungsten-175 decays by drawing in an electron from outside the nucleus in a process called electron capture. Write a balanced nuclear equation for the process.

• Solution: The daughter must have a mass number of 175, because the mass number of an electron is 0. The atomic number of the daughter must be 73 because the charge on the electron is -1. The balanced equation is:

and the daughter is seen to be tantalum-175.

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ISOTOPE HALF-LIFE• The half-life of an isotope is the time required for one-half

of the radioactive nuclei in a sample of the isotope to undergo radioactive decay.

• The half-life of an isotope is used to indicate stability. The longer the half-life, the more stable the isotope is.

EXAMPLES OF HALF-LIVES

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HALF-LIFE CALCULATIONS• Half-lives can be determined by measuring the number of

times a sample of radioactive material is reduced by 1/2 in a measured amount of time.

• The fraction of original sample remaining as a function of time is shown graphically in the following diagram:

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• Example 1: Iodine-132 decays by beta emission and forms krypton gas. A 100 mg sample of solid iodine-132 has a mass of 12.5 mg after decaying for 6.9 hours. Assume the krypton gas formed by the decay all escapes into the air, and determine the half-life of iodine-132.

• Solution: After one half-life passed, the sample would weigh 50 mg. After another half-life passes, the 50 mg would have become 25 mg, and after a third half-life had passed, the 25 mg would have become 12.5 mg. Thus, 3 half-lives passed in 6.9 hours, so one half-life is equal to 6.9hr/3 or 2.3 hours.

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• Example 2: After decaying for 80 hours, only 1/16 of the original sample of a radioactive isotope remains. What is the half-life of the isotope?

• Solution: After decaying for one half-life, ½ the original sample would remain. After a second half-life passes, ½ of the ½ (or ¼) would remain. After a third half-life, ½ of the ¼ (or 1/8) would remain, and after a fourth half-life, ½ of the 1/8 (or 1/16) of the original sample would remain. Thus, four half-lives passed in 80 hours, and the half-life of the isotope is 20 hours.

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THE HEALTH EFFECTS OF RADIATION• The greatest danger to living organisms of exposure to

long-term, low-level radiation is the ability of high-energy or ionizing radiation to dislodge electrons from molecules and generate highly reactive particles called radicals or free radicals.

• Free radicals are very reactive and may cause reactions to occur among stable materials in the cells of organisms such as genes and chromosomes. Such reactions might lead to genetic mutations, cancer, or other serious conditions.

• Short-term exposure to intense radiation results in tissue destruction in the exposed area and causes the symptoms of acute radiation syndrome.

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PROTECTION AGAINST RADIATION EXPOSURE• The use of shielding or distance are effective ways to

prevent or minimize the exposure of individuals to harmful radiation.

• Shielding involves the placement of dense absorbing materials such as lead or concrete between the radiation source and individuals.

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• Distance involves the use of the inverse square law of radiation which is a mathematical way of saying that the intensity of radiation is inversely proportional to the square of the distance from the source of the radiation.

• The mathematical equation is:

• In this equation, Ix is the radiation intensity at a distance dx from the radiation source, and Iy is the radiation intensity at a distance dy from the radiation source.

• According to this equation, a doubling of the distance from a radiation source will decrease the intensity of the radiation to ¼ the intensity at the original distance.

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MEASUREMENT UNITS FOR RADIATION• Quantities of radiation are measured using two types of

units, physical units and biological units.

PHYSICAL UNITS OF RADIATION• Physical units of radiation indicate the activity of a radiation

source in terms such as the number of nuclear decays that occur per minute. Two common units are the Curie and the Becquerel. Both units are named after scientists who did significant amounts of work with radioactivity.

BIOLOGICAL UNITS OF RADIATION• Biological units of radiation indicate the damage caused by

the radiation in living tissue. Some biological units are the Roentgen, the Rad, the Gray, and the Rem.

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PHYSICAL UNITS OF RADIATION

CURIE• The Curie (Ci) is a physical unit that is equal to 3.7x1010

nuclear decay reactions per second. It is a large unit, so fractions such as millicurie (mCi), microcurie (µCi), and picocurie (pCi) are often used.

BECQUEREL• The Becquerel (Bq) is a physical unit that is equal to one

nuclear decay reaction per second.

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BIOLOGICAL UNITS OF RADIATION

ROENTGEN• The Roentgen (R) is a biological unit used with X rays or

gamma rays. One Roentgen is the amount of radiation that will generate 2.1x109 ion pairs in 1cm3 of dry air at normal temperature and pressure. The effect of X rays and gamma rays is greater when they pass through tissue. One Roentgen generates about 1.8x1012 ion pairs per gram of tissue.

RAD• The Rad (D) is a biological unit expressing the amount of

energy transferred to tissue. One Rad of radiation transfers 1x10-2 joules or 2.4x10-3 calories of energy to each kg of tissue through which it passes.

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GRAY• The Gray (Gy) is a biological unit expressing the amount of

energy transferred to tissue. One Gray of radiation transfers 1 joule of energy to each kg of tissue through which it passes.

REM• The rem (roentgen equivalent in man) is a biological unit

that has the same health effect on an individual as 1 roentgen of X rays or gamma rays, independent of the type of radiation involved. It is especially useful for individuals exposed to numerous types of radiation that have different health effects.

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UNITS FOR MEASURING RADIATION

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METHODS OF DETECTING RADIATION• Film badges are commonly used by people working in

environments where they might be exposed to radiation.• The extent of exposure of the film increases with the

amount of radiation.• The film is developed after a specific amount of time, and

the degree of exposure indicates the radiation dose that has been absorbed during that time.

• Scintillation counters are radiation-detection devices that operate on the principle that phosphors give off light when struck by radiation.

• Geiger-Müller tubes are radiation-detection devices operating on the principle that ions form when radiation passes through a tube filled with low-pressure gas.

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GEIGER-MÜLLER COUNTER

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MEDICAL USES OF RADIOISOTOPES• Radioactive isotopes can be detected easily in the body by

using radiation detectors. • Radioactive isotopes and nonradioactive isotopes of the

same element undergo the same chemical reactions in the body.

• These two characteristics make radioactive isotopes useful as tracers in diagnostic medical work and as therapeutic agents in some medical treatments.

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TRACERS• Tracers are radioisotopes used medically because their

progress through the body or their localization in specific organs can be readily followed.

• Radioisotopes used as tracers should have as many of the following five characteristics as possible:

• Tracers should have short half-lives so they will decay while the diagnosis is being done but will give off as little radiation as possible after the diagnosis is completed.

• The daughter produced by the decaying tracer should be nontoxic and give off little or no radiation of its own. Ideally, it should be stable.

• The tracer should have a long enough half-life to allow it to be prepared and administered conveniently.

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• If possible, the radiation given off by the tracer should be gamma rays because they penetrate tissue well and can be detected readily by detectors located outside the body.

• The tracer should have chemical properties that make it possible for the tissue being studied to either concentrate it in diseased areas and form a hot spot or essentially reject it from diseased areas and form a cold spot.

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THERAPEUTIC USE OF RADIOISOTOPES• Radioisotopes administered internally for therapeutic use

should have as many of the following four characteristics as possible:

• The isotope should emit less penetrating alpha or beta radiation to restrict damage to the target tissue.

• The isotope half-life should be long enough to allow sufficient time for the desired therapy to be completed.

• The daughter of the isotope should be nontoxic and should give off little or no radiation.

• The target tissue should be able to concentrate the isotope to restrict the radiation damage to the target tissue.

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EXAMPLES OF MEDICALLY USEFUL ISOTOPES

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NONMEDICAL USES OF RADIOISOTOPES• Many applications of radioisotopes have been made in

diverse areas, including scientific studies, industry, and archeology.

SCIENTIFIC STUDIES• The study of photosynthesis in plants has been aided by

the use of radioisotopes. During photosynthesis, plants combine carbon dioxide gas with water to form carbohydrates like starch and cellulose. Energy to drive the process is obtained from sunlight. The study of the chemical pathways followed by the carbon of CO2 in photosynthesis has been greatly aided by using CO2 that contains radioactive carbon-14.

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INDUSTRY• Radioactive compounds can be used in a thin plug of liquid

separating different petroleum products moving in a pipeline. When a detector on the outside of the pipe detects the radioactive plug, a valve is opened to send the new product coming through to its proper destination.

• A tracer added to fluids moving in pipes makes it easy to detect leaks.

• The effectiveness of lubricants has been studied by the use of radioactive metal isotopes as components of metal parts. As the metal part wears, the isotope shows up in the lubricant and the amount of wear can be determined.

• The thickness of aluminum foil can be continuously monitored during production. A radioactive source positioned on one side of a moving strip of foil sends radiation through the foil to a detector located on the opposite side of the foil. The detected intensity is an indication of the thickness of the foil.

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ARCHEOLOGY• The use of carbon dating to determine the age of artifacts

is indispensable in some archeology studies.• Radioactive carbon-14 forms naturally in the atmosphere.

The radioactive carbon is converted to CO2 gas which becomes incorporated into the cellulose of plants by photosynthesis.

• In living plants an equilibrium exists in which the plants contain the same fraction of radioactive carbon as the air. When the plant is cut down, carbon dioxide intake stops and the radioactive carbon in the plant begins to decay.

• The amount of carbon-14 in an artifact can be compared to the amount in air. The difference and the known half-life of carbon-14 (5600 years) allows the time the carbon-14 has been decaying to be calculated, which is the age of the artifact.

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• Carbon-14 dating can only be used on objects less than about 50,000 years old.

• Other dating methods have developed.• Potassium-40 undergoes electron capture to produce

argon-40.• The half-life of potassium-40 is 1.3 x 109 years.• By determining the amount of argon-40 in a potassium-

containing mineral, it is possible to estimate the age of the mineral.

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INDUCED NUCLEAR REACTIONS• Induced nuclear reactions are reactions that take place

when nuclei are bombarded with subatomic particles such as alpha particles or neutrons.

• An example of an induced nuclear reaction is the one that produces radioactive carbon-14 in the atmosphere. This process takes place when a nitrogen-14 atom is struck by a cosmic ray neutron.

• The reaction is:

• Induced nuclear reactions have been used to produce all of the elements in the periodic table with atomic numbers greater than 92.

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• The particles used to bombard nuclei may be charged or uncharged. Charged particles can be accelerated to high speeds before the bombardment occurs by using particle accelerators such as the cyclotron.

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• A variety of nuclear reactions can be induced by bombarding either stable or naturally radioactive nuclei with high-energy particles.

• These artificially induced reactions may lead to one of four results:

• a new stable nucleus is formed.

• a new unstable nucleus is formed and undergoes decay.

• nuclear fission occurs.

• nuclear fusion occurs.

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NUCLEAR ENERGY• Nuclear energy is released in large amounts by the

processes of nuclear fission or nuclear fusion.

NUCLEAR FISSION• Nuclear fission is a process in which large nuclei split into

smaller, approximately equal-sized nuclei when hit by neutrons.

• During nuclear fission reactions, the total mass of the products of the reaction is less than the total mass of the reactants. The mass difference appears as energy in agreement with Einstein's famous equation E=mc2, where the mass difference is m and c is the velocity of light.

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• A chain reaction is a nuclear reaction in which the products of one reaction cause a repeat of the reaction to take place.

• An expanding or branching chain reaction is a reaction in which the products of one reaction cause more than one more reaction to occur.

• A critical reaction is a constant rate reaction.• A supercritical reaction is a branching chain reaction

that will lead to an explosion.

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• A moderator is a material capable of slowing down neutrons that pass through them.

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• A critical mass is the minimum amount of fissionable material needed to sustain a critical chain reaction at a constant rate.

• A supercritical mass is the minimum amount of fissionable material that must be present to cause a branching chain reaction to occur.

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• A breeder reaction is a nuclear reaction in which isotopes that will not undergo spontaneous fission are changed into isotopes that will.

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NUCLEAR FUSION• Nuclear fusion is a process in which small nuclei combine

or fuse to form larger nuclei.• As in nuclear fission reactions, the total mass of the

reactants is greater than the total mass of the products, and the mass difference appears a energy in agreement with Einstein's equation.

• A thermonuclear reaction is a nuclear fusion reaction that requires a very high temperature to start.

• The overall reaction for the energy output of the sun is: