🌀Principles of Physics III
🌀principles of physics iii review
9.3 Radioactivity and Decay Processes
4 min read•Last Updated on August 16, 2024
Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei. It's a natural process that transforms one element into another, discovered by Henri Becquerel in 1896 while studying uranium salts.
This section explores the types of radioactive decay: alpha, beta, and gamma. We'll learn about their characteristics, how to balance nuclear equations, and the concept of decay series. Understanding these processes is crucial for nuclear physics applications.
Radioactivity and its origins
Discovery and fundamental concepts
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- Radioactivity describes spontaneous emission of radiation from unstable atomic nuclei transforming one element into another
- Henri Becquerel discovered radioactivity in 1896 while studying uranium salts
- Marie and Pierre Curie conducted further investigations expanding knowledge of radioactive elements
- Natural radioactivity occurs in elements with atomic numbers greater than 83 (uranium, thorium, radium)
- Artificial radioactivity induced in stable nuclei through nuclear reactions (technetium-99m, cobalt-60)
Causes and characteristics of radioactivity
- Instability in radioactive nuclei arises from imbalance in proton-neutron ratio or excess nuclear energy
- Half-life characterizes rate of radioactive decay unique to each radioisotope
- Half-life remains independent of external factors (temperature, pressure)
- Radioactive decay follows first-order kinetics decay rate proportional to number of radioactive nuclei present
- Decay constant (λ) relates to half-life through equation:
Types of radioactive decay
Alpha decay
- Emission of alpha particle (two protons and two neutrons) from parent nucleus
- Reduces atomic number by 2 and mass number by 4
- Alpha particles highly ionizing but least penetrating (stopped by sheet of paper)
- Energy spectrum discrete peaks due to quantized nuclear energy levels
- Example decay equation:
Beta decay
- Three forms beta-minus (β⁻), beta-plus (β⁺), and electron capture
- β⁻ decay emits electron and antineutrino (neutron converts to proton)
- β⁺ decay emits positron and neutrino (proton converts to neutron)
- Electron capture inner orbital electron captured by nucleus converting proton to neutron
- Beta particles moderately ionizing and penetrating (stopped by thin aluminum sheet)
- Energy spectrum continuous due to energy sharing between emitted particles
- Example β⁻ decay:
Gamma decay
- Emission of high-energy photons (gamma rays) from excited nucleus
- No change in atomic number or mass number
- Gamma rays least ionizing but most penetrating (requires thick lead shielding)
- Energy spectrum discrete peaks corresponding to nuclear energy level transitions
- Often accompanies alpha or beta decay as nucleus de-excites
- Example gamma decay:
Balancing nuclear decay equations
General principles
- Nuclear decay equations must balance atomic number (Z) and mass number (A) on both sides
- Conservation of electric charge and nucleon number fundamental to balancing equations
- Use correct superscript and subscript notation for each particle (mass number top, atomic number bottom)
- Account for all emitted particles including neutrinos and antineutrinos in beta decay
Specific decay equations
- Alpha decay equation:
- Beta-minus decay equation:
- Beta-plus decay equation:
- Electron capture equation:
- Gamma decay equation:
Decay series and daughter nuclides
Decay series concepts
- Decay series describes sequence of radioactive decays from parent nuclide to stable daughter nuclide
- Common naturally occurring decay series uranium series, thorium series, actinium series
- Each series starts with long-lived parent nuclide ends with stable isotope of lead
- Branching decay occurs when parent nuclide decays via multiple modes with specific branching ratios
- Example uranium-238 decay series involves 14 steps before reaching stable lead-206
Daughter nuclides and equilibrium
- Daughter nuclides products of radioactive decay may be radioactive or stable isotopes
- Secular equilibrium arises when parent nuclide half-life much longer than daughter nuclides
- In secular equilibrium ratio of parent to daughter nuclides remains constant over time
- Equation for secular equilibrium:
- Where λ₁, N₁ are decay constant and number of atoms of parent, λ₂, N₂ for daughter
Applications of decay series
- Radiometric dating determines age of geological samples (carbon-14 dating, uranium-lead dating)
- Nuclear forensics analyzes radioactive materials to determine origin and history
- Understanding behavior of naturally occurring radioactive materials (NORM) in environment
- Radon gas mitigation in buildings based on understanding uranium decay series
- Radioactive tracers in medical imaging (technetium-99m from molybdenum-99 decay)
Key Terms to Review (20)
Radiation dose: Radiation dose refers to the amount of energy deposited by ionizing radiation in a given mass of tissue, commonly expressed in units like grays (Gy) or sieverts (Sv). It is a crucial measure in understanding the biological effects of radiation exposure on living organisms and helps in assessing the potential risks associated with various sources of radiation, including medical imaging and radioactive materials.
Radiation exposure: Radiation exposure refers to the amount of ionizing radiation energy that an individual or an object absorbs from radioactive materials, including natural sources and artificial radiation. This exposure can lead to various biological effects, depending on the dose received and the type of radiation involved, and is a critical aspect in understanding radioactivity and decay processes.
Radioactive isotopes: Radioactive isotopes are variants of chemical elements that have an unstable nucleus and emit radiation as they decay into more stable forms. These isotopes are essential in understanding various decay processes, as they undergo transformations that can result in the release of alpha, beta, or gamma radiation, ultimately leading to a different element or isotope altogether.
Nuclear binding energy: Nuclear binding energy is the energy required to hold the protons and neutrons together within an atomic nucleus. This energy is a crucial factor in understanding the stability of nuclei, as it indicates how tightly the particles are bound. A higher binding energy generally means a more stable nucleus, while lower binding energy can lead to instability and various decay processes.
A = λn: The equation a = λn represents the relationship between the decay constant (a), the activity (λ), and the number of radioactive nuclei (n) in a sample. This equation is crucial for understanding how radioactive materials emit radiation over time and how their decay processes can be quantified. It connects the physical properties of radioactivity with the mathematical framework used to analyze decay processes in various contexts, including nuclear physics and medical applications.
Nuclear medicine: Nuclear medicine is a medical specialty that uses radioactive substances to diagnose and treat diseases. This field combines principles of physics and chemistry to create images of the body's internal structures and functions, often focusing on the detection of cancer and other serious conditions. It relies on radioactivity and decay processes, as well as nuclear reactions that release energy, to provide valuable information about how organs and tissues are functioning.
N(t) = n0 e^(-λt): The equation n(t) = n0 e^(-λt) describes the exponential decay of a radioactive substance over time, where n(t) is the number of radioactive atoms remaining at time t, n0 is the initial number of radioactive atoms, λ (lambda) is the decay constant specific to the substance, and e is the base of natural logarithms. This formula illustrates how the quantity of a radioactive material decreases as time progresses, emphasizing the randomness and statistical nature of decay processes.
Radiometric dating: Radiometric dating is a scientific method used to determine the age of materials by measuring the relative abundance of specific radioactive isotopes and their stable decay products. This technique relies on the principles of radioactivity and the predictable decay rates of isotopes, allowing scientists to estimate when certain events occurred in geological history.
Conservation of Nucleons: Conservation of nucleons refers to the principle that the total number of nucleons (protons and neutrons) in a closed system remains constant during nuclear reactions. This means that in processes such as decay, fission, and fusion, nucleons can be rearranged but cannot be created or destroyed. Understanding this principle is crucial for explaining the stability of atomic nuclei and the energy changes that occur during nuclear processes.
Law of radioactive decay: The law of radioactive decay describes the process by which unstable atomic nuclei lose energy by emitting radiation, leading to a decrease in the number of radioactive atoms over time. This law is foundational to understanding how and why certain isotopes transform into different elements or isotopes through processes such as alpha decay, beta decay, and gamma decay, each characterized by distinct emission patterns and half-lives.
Beta particle: A beta particle is a high-energy, high-speed electron or positron emitted during the radioactive decay of an atomic nucleus. This process occurs when a neutron in the nucleus transforms into a proton and emits an electron, or when a proton transforms into a neutron and emits a positron. The emission of beta particles is a crucial aspect of beta decay, which plays a significant role in the broader understanding of radioactivity and decay processes.
Alpha particle: An alpha particle is a type of subatomic particle consisting of two protons and two neutrons, essentially making it a helium nucleus. These particles are released during the process of alpha decay, which is one of the primary ways unstable atomic nuclei lose energy. Alpha particles are relatively heavy and carry a positive charge, making them significant in understanding radioactivity and the mechanisms behind nuclear reactions and energy release.
Gamma photon: A gamma photon is a high-energy electromagnetic radiation emitted from the nucleus of an atom during radioactive decay. Unlike alpha and beta particles, gamma photons carry no charge and have no mass, making them highly penetrating and capable of traveling significant distances through matter. This characteristic allows gamma photons to be used in various applications, including medical imaging and cancer treatment.
Non-ionizing radiation: Non-ionizing radiation refers to a type of electromagnetic radiation that does not carry enough energy to ionize atoms or molecules, meaning it cannot remove tightly bound electrons. This form of radiation includes low-energy waves such as radio waves, microwaves, and infrared radiation, which can be found in various applications including communication technologies and heating processes. While non-ionizing radiation is generally considered less harmful than ionizing radiation, understanding its properties and interactions is crucial for safety and health considerations.
Decay Constant: The decay constant is a probability measure that quantifies the rate at which a radioactive substance disintegrates over time. It is denoted by the symbol λ (lambda) and indicates how likely it is for a single atom to decay in a given time period. This concept is crucial for understanding both the behavior of radioactive materials and the calculations involved in determining half-lives and age estimations in radioactive dating.
Half-life: Half-life is the time required for half of the radioactive nuclei in a sample to decay into a different state or isotope. This concept is crucial for understanding how unstable isotopes transform over time, indicating their rate of decay. The half-life remains constant for a given isotope, regardless of the amount present, and is a fundamental aspect in fields like radioactive dating and applications involving nuclear physics.
Alpha decay: Alpha decay is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle, consisting of two protons and two neutrons, effectively reducing its atomic number by two and its mass number by four. This process transforms the original nucleus into a new element, leading to a decrease in nuclear stability and is a key aspect of understanding how elements change over time.
Gamma decay: Gamma decay is a type of radioactive decay in which an unstable nucleus releases energy in the form of gamma rays, resulting in a lower energy state without changing the number of protons or neutrons in the nucleus. This process is significant as it often accompanies other forms of decay, helping to stabilize the nucleus after alpha or beta decay. Gamma decay is crucial for understanding the behavior of radioactive materials and their detection.
Beta decay: Beta decay is a type of radioactive decay in which an unstable atomic nucleus transforms into a more stable one by emitting a beta particle, which can be either an electron or a positron. This process plays a crucial role in the stability of atomic nuclei and helps us understand radioactivity and decay processes, the half-life of isotopes, and the interactions among elementary particles.
Ionizing radiation: Ionizing radiation refers to high-energy radiation that carries enough energy to ionize atoms or molecules by dislodging electrons from them. This type of radiation can lead to chemical changes in matter and is significant in various fields such as medicine, nuclear energy, and environmental science, especially in understanding the effects of radioactive decay processes.
