19.1 Nuclear Stability and Radioactive Decay

• Nuclear stability can be considered from both a kinetic and a thermodynamic point of view

  • Thermodynamic stability refers to the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons

• Radioactive nuclei can undergo decomposition in various ways

• These decay processes fall into two categories: those that involve a change in the mass number of the decaying nucleus and those that do not

• A-particle production involves a change in A for the decaying nucleus; B-particle production has no effect on A

• The most common decay process in which the mass number of the decaying nucleus remains constant is B-particle production

• Positron (01e) production

• G rays are usually produced in a radioactive decay event

  • A decay series involves several radioactive decays to finally reach a stable nuclide

  • Radioactive decay follows first-order kinetics

  • Half-life of a radioactive sample: the time required for half of the nuclides to decay

• The transuranium elements (those beyond uranium in the periodic table) can be synthesized by particle bombardment of uranium or heavier elements

19.2 The Kinetics of Radioactive Decay

• The rate of decay is the negative of the change in the number of nuclides per unit of time

• The half-life of a radioactive sample is defined as the time required for the number of nuclides to reach half the original value

  • If the half-life of a radioactive nuclide is known, the rate constant can be easily calculated, and vice versa.

  • The half-lives of radioactive nuclides vary over a tremendous range

19.3 Nuclear Transformations

• Many other nuclear transformations have been achieved, mostly using particle accelerators

• The linear accelerator employs changing electric fields to achieve high velocities on a linear pathway

• Neutrons are often employed as bombarding particles to effect nuclear transformations

  • Because neutrons are uncharged and thus not repelled electrostatically by a target nucleus, they are readily absorbed by many nuclei, leading to new nuclides.

  • By using neutron and positive-ion bombardment, scientists have been able to extend the periodic table

• In the years since 1940, the elements with atomic numbers 93 through 112, called the transuranium elements, have been synthesized

• As a result, only a few atoms have ever been formed

19.4 Detection and Uses of Radioactivity

• **** Geiger counters are often called survey meters in the industry

• This instrument takes advantage of the fact that the high-energy particles from radioactive decay processes produce ions when they travel through matter.

• The probe of the Geiger counter is filled with argon gas, which can be ionized by a rapidly moving particle.

• The formation of ions and electrons produced by the passage of the high-energy particle allows a momentary current to flow.

  • Electronic devices detect this current flow, and the number of these events can be counted

• Another instrument often used to detect levels of radioactivity is a scintillation counter, which takes advantage of the fact that certain substances

• Archeologists, geologists, and others involved in reconstructing the ancient history of the earth rely heavily on radioactivity to provide accurate dates for artifacts and rocks

• Radioactive nuclides are often called radionuclides. Carbon dating is based on the radionuclide 14/6C.

• One drawback of radiocarbon dating is that a fairly large piece of the object must be burned to form carbon dioxide

• Radio tracers provide sensitive and noninvasive methods for learning about biological systems, for detection of disease, for monitoring the action and effectiveness of drugs, and for early detection of pregnancy

19.5 Thermodynamic Stability of Nucleus

• It compares the mass of a nucleus to the sum of the masses of its component nucleons

• The energy changes associated with normal chemical reactions are small enough that the corresponding mass changes are not detectable

• The difference between the sum of the masses of the component nucleons and the actual mass of a nucleus (called the mass defect) can be used to calculate the nuclear binding energy

19.6 Nuclear Fission and Nuclear Fusion

• Fusion: The process of combining two light nuclei to form a heavier, more stable nucleus

• Fission: The process of splitting a heavy nucleus into two lighter, more stable nuclei

• Current nuclear power reactors employ controlled fission to produce energy

  • For the fission process to be self-sustaining, at least one neutron from each fission event must go on to split another nucleui

  • If on average, less than one neutron causes another fission event, the process dies out and the reaction is said to be subcritical

  • If exactly one neutron from each fission event causes another fission event, the process sustains itself at the same level and is said to be critical

  • If more than one neutron from each fission event causes another fission event, the process rapidly escalates and the heat buildup causes a violent explosion.

• Control rods: Composed of substances that absorb neutrons and regulate the power level of the reactor

• At present, many technical problems remain to be solved, and it is not clear which method will prove more useful or when fusion might become a practical energy source

19.7 Effects of Radiation

• Radiation can cause direct (somatic) damage to a living organism or genetic damage to the organism’s offspring

• The biological effects of radiation depend on the energy, the penetrating ability, the ionizing ability of the radiation, and the chemical properties of the nuclide producing the radiation

• Both the energy dose of the radiation and its effectiveness in causing biological damage must be taken into account

• The reason for the subtlety of radiation damage is that even though high-energy particles are involved, the quantity of energy actually deposited in tissues per event is quite small

• Somatic damage: Damage to the organism itself,

• Genetic damage: Damage to the genetic machinery

• Two models of radiation damage are the linear model and the threshold model

  • The linear model postulates that damage from radiation is proportional to the dose, even at low levels of exposure.

  • The threshold model, on the other hand, assumes that no significant damage occurs below a certain exposure

  • If the threshold model is correct, a certain level of radiation exposure beyond natural levels can be tolerated